Comparative characteristics of dental bridges (stamped-soldered, slip-cast, combined)

Comparative characteristics of dental bridges (stamped-soldered, slip-cast, combined). Laboratory stages of manufacture of dental bridges. Possible complications

Bridge prostheses.

     Bridge prostheses are most common category of prostheses in partial included defects of the dentitions.

     The dental arch consists of two symmetrical halves and in loss of one of them another one can take its function. It is considered that the periodontium of each tooth, because of the special reserve possibilities, is capable of maintaining dual masticatory load. The construction of any bridge-shaped prosthesis is based on this principle, when two and more crowns bear intermediate part, body on themselves, which compensates the defect of the absent tooth or teeth. When the tooth load, i.e., width of the intermediate part is more than the reserve possibilities of the periodontium of the teeth, on which abutment crowns are located, it will give arise to physiological irritation, the injury, which will lead to the teeth loosening.

     The loss of teeth on one of the jaws frequently leads to the reconstruction of the bone tissues of the alveolar processes, protrusion of the teeth- antagonists on the opposite jaw, contributing to the formation of block in the sagittal direction (Popov -Godon phenomenon). To prevent these phenomena it is necessary to make bridge prostheses as early as possible with the restoration of the lost function of the dentition.

     All bridge prostheses can be divided regarding the material into: plastic, metallic, combined. Metallic and combined dentures are also divided into: soldered (at first supporting crowns are made, then the intermediate part, later on all elements are soldered by the solder) and whole cast (at first all parts of the prosthesis are made of wax, and then wax is substituted by the metal by the method of precision casting). The soldered bridge prostheses become the thing of the past, preserving the major advantages – cheapness, easiness of construction, abrading of insignificant quantity of hard tissues of the teeth. The whole cast prostheses with the wide acceptance of precision casting and its significant development acquire greater popularity in dentists and dental technicians.

The following postulate is the more generalized formulation of indications to the bridge prosthesis: the partial defects of the dentition when the sum of the coefficients of the masticatory effectiveness of supporting teeth according to Agapov is more or equal to the sum of the coefficients of the masticatory effectiveness of the absent teeth.

Coefficients of masticatory effectiveness according to Agapov:

Teeth       1   2   3   4   5   6   7	Total
the upper 2   1   3   4   4   6   5 jaw	25 un.
the lower 2   1  3   4   4   6   5 jaw	25 un.

 

By Oxman

Teeth       1   2   3   4   5   6   7   8	Total
the upper 2   1   2   3   3   6   5   3 jaw	25 un.
the lower 1   1   2   3   3   6   5   4 jaw	25 un.

 

Kennedy’s classification and classification by Kulazhenko.

Kennedy’s classification by classes:

1 bilateral end defect of the dentition.

2 unilateral end defect of the dentition.

3 intermediate defect in the lateral parts of the dentition.

4 intermediate defect in the region of the anterior part.

 

CLASSIFICATION BY V.I.KULAZHENKO

I class.The defect of the dentition is limited by one tooth – the continuous shortened dentition without the distal support (according to Kennedy – II class).

II class. Two defects, limited by two teeth – the shortened dentition with the bilateral defects without the distal support (according to Kennedy – I class).

III class.Two defects, limited by three teeth – bilateral defects, limited by three teeth, one defect without the distal support (on Kennedy – II class, I sub-class).

IV class.Two defects, limited by four teeth – bilateral defects with the distal supports (by Kennedy – III class, I subclass).

       In the presence of additional defects besides basic ones – these cases make the subclass of the basic class. The absence of front teeth with the presence of the lateral ones is also II class, but with the distal support, hence the construction of the denture will be another in this case.

All classifications proposed are characterized only by topography of the dentition.

     Local and general influence of defects on the organism. This pathology leads to the disturbance of the mastication function and it is the cause for cosmetic defects, produces a change in the person’s appearance, and loss of the last pair of the antagonists makes the person look older than his age. The loss of even one tooth leads to changes in the whole dentition: the teeth, which limit defect, displace, without having antagonists, they rise from the alveoli. (V.O.Popov, 1862, Godon, 1865).

     Displacement of the teeth does not develop in the intact dental arch and the healthy periodontium, there is a so-called “articulatory equilibrium”. The displacement of the teeth prevents the protection of each tooth on the side of its neighbor.

     The partial defects of the dentition lead to development of various forms of the maxillodental system pathology, which are manifested not only in a change of the position of separate teeth and dentitions, but also in the incorrect bite, modification of the alveolar processes and mucous membrane.

     The alveolar processes become atrophied at the place of the absent teeth and are hypertrophied on some part with the teeth, which have the antagonists. Changes in the height of occlusion occur as a result of the increased dental abrasion on the large area.

     The teeth, which do not have antagonists, may cause injury and inflammation of the mucous membrane of the alveolar processes of the opposite jaw. Complete or partial edentia is reflected both in the functional and the morphological changes in the temporal- mandibular joint. Atrophic- degenerate processes develop; there are changes in the gastrointestinal tract.

     The disturbances of the mastication result in changes in the functional activity of the salivary glands. Thus, prosthesis of the dentition defects is a preventive measure for preventing different changes of the local or general nature.

The figure schematically presents the construction of the soldered bridge prosthesis constructed from the stainless steel, on which three sections are isolated: crown 1, solder 2 and the cast intermediate part 3 of the dental prosthesis.

  The case, from which the crown 1 was constructed, was stretched and then swaged. The artificial teeth 3 were obtained by casting. The place of junction between the crown 1 and intermediate part 3 was filled with the solder 2; therefore the structure of all sections was not identical. The most unsatisfactory of three sections indicated is the solder.

     The solder testing as to the strength of junction of the soldered parts carried out in the Research Institute of nonferrous metallurgy as early as the 30s showed that the solder possessed satisfactory mechanical properties, although it was unstable to the effect of even weak acids.

     At the same time the literature data analysis of the results of experimental studies and clinical observations on the study of effect of different solder alloys on the biological medium, gives grounds to assert that the solder does not meet the requirements, to the dental prostheses, which are in the biological medium, as to physicomechanical properties.

     The main drawbacks of the solders are their low stability to the corrosive destruction in the oral cavity, electro-potential difference with the soldered metals, as well as change of the structure of the soldered metal during the process of soldering.

     This specifies a number of the positions, which have a negative nature.

     1. There is a formation of metal oxides in the oral cavity of people using soldered prostheses of chromium nickel steel. Depending on the nature of saliva, composition of other metalware, being in the oral cavity (prostheses, metallic fillings, inlays) as well as individual peculiarities of the organism, the formation of oxides may have the more or less expressed character.

     Blackening of the sites of soldering or presence of the sharply outlined spots on the steel prosthesis surface is evidence of the presence of such oxides. In this case a quantitative increase of the microcells in the saliva and the formation of salts of heavy metals are noted in the oral cavity of people using such prostheses, which adversely affects the secretory function of the stomach. The mechanism of effect of metal oxides on the organism during the electrolytic dissociation in the oral cavity is still studied insufficiently; however, their nonphysiological state is completely obvious and incompatible with the principles of preventive medicine.

     2. The pathologic state, which was called the phenomena of galvanism, develops in the oral cavity of those, who use soldered prostheses, which are associated with a potential difference and can appear both in presence of different metals or alloys and as a result of heterogeneity of one alloy.

Both factors, which specify a potential difference, can develop in presence of soldered bridge prostheses, since these prostheses consist of different alloys – chrome-nickel steel and solder, and the structure of each of these alloys acquires heterogeneous nature during the process of their manufacture.

While conducting the metallographic examination of the soldered bridge prostheses of chrome-nickel steel, it is revealed that even during thorough observance of technology along the contact line there are many nonsoldered sections. There are micropores in the layer of solder dividing them. Although the decrease of the thickness of the solder layer provides more durable junction of the soldered parts, however, it does not improve the alloy structure at the site of their junction, but in this case a quantity of micropores in the solder layer even increases. The two-phase structure of steel is observed both in the crown and in the intermediate part. There are sections of metal with the precipitation of chromium carbides on the margins of the metal grains.

It is impossible to improve the alloy structure by heat treatment (recrystallization), since the prosthesis should be heated to the temperature of 1, 000-1,100⁰C, and this will lead to deformation and separation of its parts.

But if we examine the clinical stages of construction of swaged-soldered prostheses, they included up to 5 visits of the patient to the dentist:

1 visit: Preparation of the teeth and taking of the impressions for constructing the swaged – soldered crowns.

2 visit: Fitting of the swaged crowns and their trimming as well as taking of the impressions for constructing soldering of the intermediate parts of the prosthesis (cast teeth).

3 visit: Fitting of the unpolished swaged – soldered constructions of the bridge prostheses and their trimming.

4 visit. Fitting of the polished swaged – soldered constructions of the bridge prostheses and constructions with their subsequent fixation.

5 visit. In spraying of the dental prostheses – the prosthesis construction takes from 20 to 30 days.

 

Thus, analyzing the aforesaid it is possible to make the following conclusions, namely:

1. At present the swaged crowns and soldered bridge prostheses do not meet the necessary modern requirements as the methods of swaging do not ensure the precise construction of the crown.

2. Presence of different alloys and metals in the soldered bridge prosthesis leads to the phenomenon of galvanism in the oral cavity.

3. Breakdown occurs very frequently at the site of soldering of the dental prosthesis elements.

4. The technical process of construction of different elements of the swaged- soldered dental prosthesis provides for the use of the strong acids.

 

EVALUATION OF THE WHOLE CAST PROSTHESES

– Construction of the wax reproduction of the component (wax pattern);

– Installation of sprues and creation of the casting block;

– Preparation of the mixture, utilized for formation of the covering layer of model;

– Coating of the wax reproduction of the component with a covering mass;

– Construction of the casting form;

– Melting of wax from the casting form with the subsequent drying and burning of the casting form;

– Melting of the dental alloy with the subsequent filling of the molten metal in the casting form;

– Cooling of the casting with its subsequent release from the molding mass and sprues;

– If necessary thermal processing of the cast components;

– Polishing, fitting, etc

     The sequential and thorough fulfillment of the construction stages of the dental prosthesis components enumerated above by the casting method of the cast models is the guarantee of a high quality of the cast component of the prosthesis, which can be achieved only by thorough fulfillment of the enumerated points in accordance with the existing procedures.

     Besides, the clinical stages of the whole cast construction of the dental prostheses in comparison with the clinical stages of the swaged – soldered construction of the dental prostheses requires only 3 visits, namely:

1 visit: Preparation of the teeth, taking of the impressions for constructing temporary protective constructions and fixation of the temporary protective constructions.

2 visit: Fitting of the polished whole cast constructions and fixation of the prosthesis.

3 visit. In spraying of the dental prostheses – the prosthesis construction takes from 3 to 5 days.

Thus, from the comparative analysis of the dental prosthesis construction by the method of swaging with the subsequent soldering and the casting method it is possible to make the following conclusions:

1. Obtaining dental prostheses by the casting method allows to get more uniform properties of the metal of the dental prosthesis, which allows to exclude electrochemical processes in the oral cavity;

2. The dental prostheses allow to compensate most fully the dentition defect, since cast crowns are more precise, they tightly cover the tooth neck and do not traumatize the gum tissue;

3. The dental prostheses are reliably protect by aesthetical coverings from the plastics or ceramic metal.

4. The mechanical strength and chemical stability of the dental prostheses increases, and, therefore, the period of their service.

5. Introduction of the technology of casting into dental practice allows to reduce the number of both clinical and laboratory stages in construction of whole cast bridge prostheses, which makes it allows to increase the quality of the dental prostheses.

6. Technology of construction of the whole cast bridge prostheses does not provide for the use of the strong chemical substances (acid, alkali, etc), which allows to improve the working conditions of the dental technicians.

7. In organization of the proper and qualitative training of the technician- caster, construction of the dental prostheses by the casting method by the cast models allows to increase several times productivity and effectiveness of work of the dental technicians.

 

 A bridge, also known as a fixed partial denture, is a dental restoration used to replace a missing tooth by joining permanently to adjacent teeth or dental implants.

Types of bridges may vary, depending upon how they are fabricated and the way they anchor to the adjacent teeth. Conventionally, bridges are made using the indirect method of restoration. However, bridges can be fabricated directly in the mouth using such materials as composite resin.

A bridge is fabricated by reducing the teeth on either side of the missing tooth or teeth by a preparation pattern determined by the location of the teeth and by the material from which the bridge is fabricated. In other words, the abutment teeth are reduced in size to accommodate the material to be used to restore the size and shape of the original teeth in a correct alignment and contact with the opposing teeth. The dimensions of the bridge are defined by Ante’s Law: “The root surface area of the abutment teeth has to equal or surpass that of the teeth being replaced with pontics”.^[1]

The materials used for the bridges include gold, porcelain fused to metal, or in the correct situation porcelain alone. The amount and type of reduction done to the abutment teeth varies slightly with the different materials used. The recipient of such a bridge must be careful to clean well under this prosthesis.

 

When restoring an edentulous space with a fixed partial denture that will crown the teeth adjacent to the space and bridge the gap with a pontic, or “dummy tooth”, the restoration is referred to as a bridge. Besides all of the preceding information that concerns single-unit crowns, bridges possess a few additional considerations when it comes to case selection and treatment planning, tooth preparation and restoration fabrication.

Case selection and treatment planning

When a single tooth requires a crown, the prosthetic crown will in most instances rest upon whatever tooth structure was originally supporting the crown of the natural tooth. However, when restoring an edentulous (without teeth) area with a bridge, the bridge is almost always restoring more teeth than there are root structures to support. For instance, in the photo at right, the 5-unit bridge will only be supported on three abutment teeth. To determine whether or not the abutment teeth can support a bridge without failure from lack of support from remaining root structures, the dentist employs Ante’s rule—which states that the roots of abutment teeth must have a combined surface area in three dimensions that is more than that of the missing root structures of the teeth replaced with a bridge. When the situation yields a poor prognosis for proper support, double abutments may be required to properly conform to Ante’s rule.

A semi-precision attachment between teeth #3 and #4, with the female on #4. Note the lingual buttons extending, in the photo, upward on #2 (on the left) and downward on #4. These are used to grasp the crowns with a hemostat and make them easier to handle. They can also be used to aid in removal of the crown in case there is an excessive amount of retention during the try-in. They are cut off prior to final cementation.

 

When a posterior tooth intended for an abutment tooth already possesses an intracoronal restoration, it might be better to make that bridge abutment into an inlay or an onlay, instead of a crown. However, this may concentrate the torque of the masticatory forces onto a less enveloping restoration, thus making the bridge more prone to failure.

The abument and pontic joined with GC pattern resin in a solder index and reinforced with an old bur (lying horizontally across the occlusal surface of the copings).

 

In some situations, a cantilever bridge may be constructed to restore an edentulous area that only has adequate teeth for abutments either mesially or distally. This must also conform to Ante’s rule but, because there are only abutments on one side, a modification to the rule must be applied, and these bridges possess double abutments in the majority of cases, and the occlusal surface area of the pontic is generally decreased by making the pontic smaller than the original tooth.

Tooth preparation

As with preparations for single-unit crowns, the preparations for multiple-unit bridges must also possess proper taper to facilitate the insertion of the prosthesis onto the teeth. However, there is an added dimension when it comes to bridges, because the bridge must be able to fit onto the abutment teeth simultaneously. Thus, the taper of the abutment teeth must match, to properly seat the bridge. This is known as requiring parallelism among the abutments.

When this is not possible, due to severe tipping of one of more of the abutments, for example, an attachment may be useful, as in the photo at right, so that one of the abutments may be cemented first, and the other abutment, attached to the pontic, can then be inserted, with an arm on the pontic slipping into a groove on the cemented crown to achieve a span across the edentulous area.

Restoration fabrication

Full dental bridge being machined using WorkNC Dental CAD/CAM software.

As with single-unit crowns, bridges may be fabricated using the lost-wax technique if the restoration is to be either a multiple-unit FGC or PFM. Another fabrication technique is to use CAD/CAM software to machine the bridge.^[2] As mentioned above, there are special considerations when preparing for a multiple-unit restoration in that the relationship between the two or more abutments must be maintained in the restoration. That is, there must be proper parallelism for the bridge to seat properly on the margins. Sometimes, the bridge does not seat, but the dentist is unsure whether or not it is only because the spatial relationship of the two or more abutments is incorrect, or whether the abutments do not actually fit the preparations. The only way to determine this is to section the bridge and try in each abutment by itself. If they all fit individually, it must have simply been that the spatial relationship was incorrect, and the abutment that was sectioned from the pontic must now be reattached to the pontic according to the newly confirmed spatial relationship. This is accomplished with a solder index.

The proximal surfaces of the sectioned units (that is, the adjacent surfaces of the metal at the cut) are roughened and the relationship is preserved with a material that will hold on to both sides, such as GC pattern resin. With the two bridge abutments individually seated on their prepared abutment teeth, the resin is applied to the location of the sectioning to reestablish a proper spatial relationship between the two pieces. This can then be sent to the lab where the two pieces will be soldered and returned for another try-in or final cementation.

 

Comparative characteristics of bridge dentures (stamp, slip-cast)

Teeth do not possess the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries, trauma, or wear, restora­tive materials must be used to reestablish form and function. Teeth require preparation to receive restora­tions, and these preparations must be based on fun­damental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment. Careful attention to every detail is im­perative during tooth preparation. A good prepara­tion will ensure that subsequent techniques (e.g., provisionalization, impression making, pouring of dies and casts, waxing) can be accomplished.

The principles of tooth preparation may be di­vided into three broad categories:

1.  Biologic considerations, which affect the
health of the oral tissues

2.            Mechanical considerations, which affect the
integrity and durability of the restoration

3.  Esthetic considerations, which affect the ap­pearance of the patient

Successful tooth preparation and subsequent restoration depend on simultaneous consideration of all these factors. Often improvement in one area will adversely affect another, and striving for perfection in one may lead to failure in another. For example, in the fabrication of a metal-ceramic crown, sufficient thickness of porcelain is necessary for a lifelike appearance. However, if too much tooth structure is removed to accommodate a greater thick­ness of porcelain for esthetic reasons, the pulpal tis­sue may be damaged (biologic consideration) and the tooth unduly weakened (mechanical considera­tion). An in-depth knowledge and understanding of the various criteria are prerequisites to the devel­opment of satisfactory tooth preparation skills. Predictable accomplishment of optimum tooth preparation (Fig. 2-1) often entails finding the best combination of compromises among the prevalent biologic, mechanical, and esthetic considerations.

BIOLOGIC CONSIDERATIONS

Surgical procedures involving living tissues must be carefully executed to avoid unnecessary damage. The adjacent teeth, soft tissues, and the pulp of the tooth being prepared are easily damaged in tooth preparation. If poor preparation leads to inadequate marginal fit or deficient crown contour, plaque con­trol around fixed restorations will become more dif­ficult. This will impede the long-term maintenance of dental health.

PREVENTION OF DAMAGE DURING TOOTH PREPARATION

Adjacent Teeth. latrogenic damage to an adja­cent tooth is a common error in dentistry. Even if a damaged proximal contact area is carefully re­shaped and polished, it will be more susceptible to dental caries than the original undamaged tooth surface. This is presumably because the original surface enamel contains higher fluoride concentra­tions and the interrupted layer is more prone to plaque retention. The technique of tooth prepara­tion must avoid and prevent damage to the adjacent tooth surfaces.

A metal matrix band around the adjacent tooth for protection may be helpful; however, the thin band can still be perforated and the underlying enamel damaged. The preferred method is to use the proximal enamel of the tooth being prepared for protection of the adjacent structures. Teeth are 1.5 to 2 mm wider at the contact area than at the cemento-enamel junction (CEJ), and a thin, tapered dia­mond can be passed through the interproximal con­tact area (Fig. 2-2) to leave a slight lip or fin of enamel without causing excessive tooth reduction or undesirable angulation of the rotary instrument.

Soft Tissues. Damage to the soft tissues of the tongue and cheeks can be prevented by careful re­traction with an aspirator tip, mouth mirror (Fig. 2-3), or flanged saliva ejector. Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared.

Pulp. Great care also is needed to prevent pulpal injuries during fixed prosthodontic procedures, Particular care is needed when preparing grooves or pinholes, because coolant cannot reach the cutting edge of the bur. To prevent heat buildup, these retention features should always be prepared at low rotational speed.

 

Chemical Action. The chemical action of cer­tain dental materials (bases, restorative resins, sol­vents, and luting agents) can cause pulpal damage, particularly when they are applied to freshly cut dentin. Cavity varnish or dentin bonding agents will form an effective barrier in most instances, but their effect on the retention of a cemented restora­tion is controversial.

Chemical agents are sometimes used for cleaning and degreasing tooth preparations. However, they have been shown to be pulpal irritants. Thus their use is generally contraindicated, particularly be­cause they do not improve the retention of ce­mented restorations.

 

Bacterial Action. Pulpal damage under restora­tions has been attributed, to bacteria that either were left behind or gained access to the dentin be­cause of microleakage. However, many dental mate­rials, including zinc phosphate cement, have an an­tibacterial effect; because vital dentin seems to resist infection,” the routine use of antimicrobials may not be advantageous. Many dentists now use an antimi­crobial   agent, such as Consepsis, after tooth preparation and before cementation, although the benefit has not been documented in clinical trials. 18 NOTE: All carious dentin should be removed be­fore placing a restoration that will serve as a foun­dation for a fixed prosthesis. An indirect pulp cap is not recommended, because its later failure is likely to jeopardize extensive prosthodontic treatment.

CONSERVATION OF TOOTH STRUCTURE

One of the basic tenets of restorative dentistry is to conserve as much tooth structure as possible consis­tent with the mechanical and esthetic principles of tooth preparation. This will reduce the harmful pulpal effects of the various procedures and materials used. The thickness of remaining dentin has been shown” to be inversely proportional to the pulpal response, and tooth preparations extending deeply toward the pulp should be avoided. Dowden20 has argued that any damage to the odontoblastic processes will adversely affect the cell nucleus at the dentin-pulp interface, no matter how far from the nucleus it occurs. For this reason, when assessing likely adverse pulpal response, the amount of dentin removed is important; particular care must be exercised when preparing vital teeth for com­plete-coverage restorations (Fig. 2–4).

Tooth structure is conserved by using the follow­ing guidelines:

1.              Use of partial-coverage rather than complete-coverage restorations (Fig. 2–5)

2.    Preparation of teeth with the minimum practical convergence angle (taper) between axial walls (Fig. 2–6)

3.    Preparation of the occlusal surface so reduction follows the anatomic planes to give uni­form thickness in the restoration (Fig. 2–7)

4.    Preparation of the axial surfaces so tooth
structure is removed evenly; if necessary,
teeth should be orthodontically repositioned.

5.    Selection of a conservative margin compati­ble with the other principles of tooth prepa­ration.

6.    Avoidance of unnecessary apical extension of
the preparation.

CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH

An improperly prepared tooth may have an adverse effect on long-term dental health. For example, in­sufficient axial reduction inevitably results in an overcontoured restoration that hampers plaque control. This may cause periodontal disease or dental caries. Alternatively, inadequate occlusal re­duction may result in occlusal dysfunction, and poor margin placement may lead to chipped enamel or cusp fracture.

Axial Reduction. Gingival inflammation is commonly associated with crowns and FPD abut­ments having excessive axial contours, probably be­cause it is more difficult for the patient to maintain plaque control around the gingival margin. A tooth preparation must provide sufficient space for the development of good axial contours. This will enable the junction between the restoration and the tooth to be smooth and free of any ledges or abrupt changes in direction.

Under most circumstances a crown should dupli­cate the contours and profile of the original tooth (unless the restoration is needed to correct a mal­formed or malpositioned tooth). If an error is made, a slightly undercontoured flat restoration is better because it is easier to keep free of plaque; however, increasing proximal contour on anterior crowns to maintain the interproximal papilla may be beneficial. Sufficient tooth structure must be removed to allow the development of correctly formed axial contours (Fig. 2–8), particularly in the interproximal and furcation areas of posterior teeth, where periodontal disease often begins.

Fig. 2–8.   A and B, Tooth preparations with adequate axial reduction allow the development of prop­erly contoured embrasures. Tissue is conserved by using partial coverage and supragingival margins where possible. C, Preparing furcation areas adequately is important; otherwise, the restoration will be excessively contoured, making plaque control difficult.

Margin Placement. Whenever possible, the margin of the preparation should be supragingival. Subgingival margins of cemented restorations have been identified as a major factor in periodontal disease, particularly where they encroach on the epithelial attachment. Supragingival margins are easier to prepare accurately without trauma to the soft tissues. They can usually also be situated on hard enamel, whereas subgingival mar­gins are often on dentin or cementum.

Other advantages of supragingival margins in­clude the following:

1.  They can be easily finished.

2.            They are more easily kept clean.

3.            Impressions are more easily made, with less
potential for soft tissue damage.

4.            Restorations can be easily evaluated at recall
appointments.

However, a subgingival margin (Fig. 2–9) is jus­tified if any of the following pertain:

1.  Dental caries, cervical erosion, or restorations
extend subgingivally, and a crown-lengthening procedure is not indicated.

2.           The proximal contact area extends to the gingival crest.

3.           Additional retention is needed.

4.           The margin of a metal-ceramic crown is to be
hidden behind the labiogingival crest.

5.  Root sensitivity cannot be controlled by more
conservative procedures, such as the applica­tion of dentin bonding agents.

6.  Modification of the axial contour is indicated.

Margin Adaptation. The junction between a ce­mented restoration and the tooth is always a poten­tial site for recurrent caries because of dissolution of the luting agent and inherent roughness. The more accurately the restoration is adapted to the tooth, the lesser the chance of recurrent caries or peri­odontal disease . 3° Although a precise figure for acceptable margin adaptation is not available, a skilled technician can make a casting that fits to within 10 u and a porcelain margin that fits to within 50 um, provided the tooth is properly pre­pared. A well-designed preparation has a smooth and even margin. Rough, irregular, or “stepped” junctions greatly increase the length of the margin and substantially reduce the adaptation of the restoration (Fig. 2-10). The importance of preparing smooth margins cannot be overemphasized. Time spent obtaining a smooth margin will make the sub­sequent steps of tissue displacement, impression making, die formation, waxing, and finishing much easier and will ultimately provide the patient with a longer-lasting restoration.

Margin Geometry. The cross-sectional configu­ration of the margin has been the subject of much analysis and debate .33 Different shapes have been described and advocated . For evaluation, the following guidelines for margin design should be considered:

1.  Ease of preparation without overextension or
unsupported enamel

2.  Ease of identification in the impression and on
the die

3.  A distinct boundary to which the wax pattern
can be finished

4.  Sufficient bulk of material (to enable the wax
pattern to be handled without distortion and to
give the restoration strength and, when porcelain is used, esthetics)

5.  Conservation of tooth structure (provided the
other criteria are met)

Although they are conservative of tooth struc­ture, featheredge or shoulderless crown prepara­tions should be avoided because they fail to provide adequate bulk at the margins. Over-contoured restorations often result from feath­eredge margins because the technician can handle the wax pattern without distortion only by increasing its bulk beyond the original contours. A varia­tion of the featheredge, the chisel edge margin, is formed when there is a larger angle be­tween the axial surfaces and the unprepared tooth structure. Unfortunately, this margin is frequently associated with an excessively tapered preparation or one in which the axial reduction is not correctly aligned with the long axis of the tooth. Under most circumstances, featheredges and chisel edges are unacceptable. Historically their main advantage was that they facilitated the mak­ing of impressions with rigid modeling compound in copper bands (a technique rarely used today), be­cause there was no ledge on which a band could catch. A chamfer margin  is particu­larly   suitable for cast   metal crowns and the room for adequate bulk of material, and can be placed with precision, although care is needed to avoid leaving a ledge of unsupported enamel.

 

Probably the most suitable instrument for mak­ing a chamfer margin is the tapered diamond with a rounded tip; the margin formed is the exact image of the instrument. Marginal accuracy de­pends on having a high-quality diamond and a true-running handpiece. The gingival margin is pre­pared with the diamond held precisely in the intended path of withdrawal of the restoration.

Tilting it away from the tooth will create an undercut, whereas angling it toward the tooth will lead to overreduction and loss of retention. The chamfer should never be prepared wider than half the tip of the diamond; otherwise, an unsupported lip of enamel could result . Some authorities have recommended the use of a diamond with a noncutting guide tip to aid accurate chamfer placement. However, the guide has been shown to damage tooth structure beyond the intended prepa­ration margin . Under some circumstances a beveled margin is more suitable for cast restorations, partic­ularly if a ledge or shoulder already exists, possibly from dental caries, cervical erosion, or a previous restoration. The objective in beveling is threefold: (1) to allow the cast metal margin to be bent or bur­nished against the prepared tooth structure; (2) to minimize the marginal discrepancy33 caused by a complete crown that fails to seat completely (how­ever, Pascoe has shown that when an oversized crown is considered, the discrepancy is increased rather than decreased); and (3) to protect the unprepared tooth structure from chipping (e.g., removing unsupported enamel). NOTE: When access for burnishing is limited, there is little adformed as the negative image of a vantage in beveling. This applies particularly to a gingival margin, where beveling would lead to sub­gingival extension of the preparation or placement of the margin on dentin rather than on enamel. Fa­cial margins of maxillary partial-coverage restora­tions should be beveled to protect the remaining tooth structure and to allow for burnishing.

Because a shoulder margin allows room for porcelain, it is recommended for the facial part of metal-ceramic crowns, especially when the porcelain margin technique is used. It should form a 90-degree angle with the unprepared tooth sur­face. An acute angle is likely to chip. In practice, dentists tend to underprepare the facial shoulder, leading to restorations with inferior es­thetics or poor axial contour.

Some authorities have recommended a heavy chamfer rather than a shoulder margin, and someind a chamfer easier to prepare with precision. Ear­lier work found less distortion of the metal framework during porcelain application, although with modern alloys, this doesn’t appear to be a problem.

A 120-degree sloped shoulder margin is used as an alternative to the 90-degree shoulder for the facial margin of a metal-ceramic crown. The sloped shoulder reduces the possibility of leaving unsupported enamel and yet leaves sufficient bulk to allow thinning of the metal framework to a knife-edge for acceptable esthetics.

A beveled shoulder margin is often recommended for the facial surface of a metal-ceramic restoration where a metal collar (as op­posed to a porcelain labial margin) is used. The beveling removes unsupported enamel and may allow some finishing of the metal. However, a shoul­der or sloped shoulder is preferred for biologic and esthetic reasons. This allows improved esthetics because the metal margin can be thinned to a knife edge and hidden in the sulcus without the need for positioning the margin closer to the epithelial attachment.

Occlusal Considerations. A satisfactory tooth preparation should allow sufficient space for devel­oping a functional occlusal scheme in the finished restoration. Sometimes a patient’s occlusion is dis­rupted by supraerupted or tilted teeth. When these teeth are prepared for restoration, the eventual occlusal plane must be carefully analyzed and the teeth reduced accordingly. Often consider­able reduction is needed to compensate for the supraeruption of abutment teeth.

Sometimes even endodontic treatment is neces­sary to make enough room. However, under these circumstances, violating the principle of conserva­tion of tooth structure is preferable to the potential harm from a traumatic occlusal scheme. Obviously, careful judgment is needed, and diagnostic tooth preparations and waxing procedures are essential to determining the exact amount of reduction required to develop an optimum occlusion.

Preventing Tooth Fracture. No tooth is un­breakable. If teeth are smashed together (as in an automobile accident, sport injury, or biting on a

hard object unexpectedly), a cusp may break. Cuspal fracture also can occur from parafunctional habits such as bruxism.

The likelihood that a restored tooth will fracture can be lessened if the tooth preparation is designed to minimize potentially destructive stresses. For example, an intracoronal cast restoration (inlay) has a greater potential for fracture because when occlusal forces are applied to the restoration, it tends to wedge opposing walls of the tooth apart. This wedging must be resisted by the remaining tooth structure; if the structure is thin (as with a wide preparation isthmus), the tooth may fracture during function. Providing a cuspal coverage restoration (onlay) rather than an inlay lessens the chance of such fracture. However, although not conservative of tooth structure, a complete crown is often a better solution, because it offers the greatest protection against tooth fracture, tending to “hold” the cusps of the tooth together.

 MECHANICAL CONSIDERATIONS

The design of tooth preparations for fixed prosthodontics must adhere to certain mechanical principles; otherwise, the restoration may become dislodged or may distort or fracture during service. These princi­ples have evolved from theoretical and clinical obser­vations and are supported by experimental studies. Mechanical considerations can be divided into three categories:

1.  Providing retention form

2.  Providing resistance form

3.     Preventing deformation of the restoration

RETENTION FORM

Certain forces (e.g., when the jaws are moved apart after biting on very sticky food) act on a cemented restoration in the same direction as the path of with­drawal. The quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal is known as retention. Only dental caries and porcelain failure outrank lack of retention as a cause of failure of crowns and fixed partial dentures .

The following factors must be considered when deciding whether retention is adequate for a given fixed restoration:

1.  Magnitude of the dislodging forces

2.              Geometry of the tooth preparation

3.  Roughness of the fitting surface of the
restoration

4.              Materials being cemented

5.  Film thickness of the luting agent

Magnitude of the Dislodging Forces. Forces that tend to remove a cemented restoration along its path of withdrawal are small compared to those that tend to seat or tilt it. A fixed partial denture or splint can be subjected to such forces by pulling with floss under the connectors; however, the

greatest removal forces generally arise when excep­tionally sticky food (e.g., caramel) is eaten. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and tex­ture of the restoration being pulled.

Geometry of the Tooth Preparation. Most fixed prostheses depend on the geometric form of the preparation rather than on adhesion for retention be­cause most of the traditional cements (e.g., zinc phosphate) are nonadhesive (i.e., they act by in­creasing the frictional resistance between tooth and restoration). The grains of cement prevent two sur­faces from sliding, although they do not prevent one surface from being lifted from another. This is anal­ogous to the effect of particles of sand or dust within machinery. They do not have a specific adhesion to metal, but they increase the friction between sliding metal parts. If sand or dust gets into an old-fashioned, mechanical camera or watch, the increase in friction can effectively jam the mechanism.

Cement is effective only if the restoration has a single path of withdrawal (i.e., the tooth is shaped to restrain the free movement of the restoration). The relationship between a nut and a bolt is an ex­ample of restrained movement. The nut is not free to move in any direction but can move only along the precisely determined helical path of the threads on the bolt.

The relationship between two bodies, one (in this case a tooth preparation) restraining movement of the other (a cemented restoration), has been studied mathematically and is known in analytical mechan­ics as a  closed lower pair of kinematic elements.   In fixed prosthodontics, a sliding pair is the only pair that has relevance. It is formed by two cylindrical* surfaces constrained to slide along one another. The elements are constrained if the curve that defines the cylinder is closed or shaped to prevent move­ment at right angles to the axis of the cylinder.

A tooth preparation will be cylindrical if the axial surfaces are prepared by a cylindrical bur held at a constant angle. The gingival margin of the prepara­tion becomes the fixed curve of the mathematical definition, and the occlusoaxial line angle of the tooth preparation should be a replica of the gingival margin geometry. The curve of a complete crown preparation is closed, whereas the grooves of a par­tial crown preparation prevent movement at right angles to the long axis of the cylinder. However, if one wall of the complete crown preparation is overtapered, it will no longer be cylindrical, and the ce­mented restoration will not be constrained by the preparation because the restoration then has multi­ple paths of withdrawal. Under these circum­stances, the cement particles will tend to lift away from rather than slide along the preparation, and the only retention will be a result of the cement’s limited adhesion.

Taper. Theoretically, maximum retention is ob­tained if a tooth preparation has parallel walls. However, it is impossible to prepare a tooth this way using current techniques and instrumentation; slight undercuts are created that prevent the restoration from seating.

An undercut is defined as a divergence between opposing axial walls, or wall segments, in a cervi-cal-occlusal direction. For instance, if the cervical diameter of a tooth preparation at the margin is narrower than at the occlusoaxial junction (reverse taper), it will be impossible to seat a com­plete cast crown of similar geometry. Undercuts can be present whenever two axial walls face in opposite directions. Thus the mesial wall of a complete cast crown preparation can be undercut relative to the distal wall; in addical wall can be undercut relative to the lingual wall; finally, in a partial veneer preparation, the lingual wall of a proximal groove can be under­cut relative to the lingual wall of the preparation.

A slight convergence, or taper, is necessary in the completed preparation. As long as this taper is small, the movement of the cemented restoration will be ef­fectively restrained by the preparation and will have

what is known as a limited path of withdrawal. As the

taper increases, however, so does the free movement of the restoration, and retention will be reduced.

The relationship between the degree of axial wall taper and the magnitude of retention was first demonstrated experimentally by Jorgensen in 1955. He cemented brass caps on Galalith cones of different tapers and measured retention with a ten­sile-testing machine. The relationship was found to be hyperbolic, with retention rapidly becoming less as taper increased, although the relationship was no longer hyperbolic when the internal surfaces of the caps were roughened. The retention of a cap with 10 degrees of taper* was approxi­mately half that of a cap with 5 degrees. Similar re­sults have been reported by other workers.

Selection of the appropriate degree of taper for tooth preparation involves compromise. Too small a taper may lead to unwanted undercuts; too large will no longer be retentive. The recommended con­vergence between opposing walls is 6 degrees, which has been shown to optimize retention for zinc phosphate cement.⁵⁵ Recognizing this angle is important, although there is no need to deliberately tilt a rotary cutting instrument to create a taper, since this will invariably lead to overprepa-ration. Rather, teeth are readily prepared with a ro­tary instrument of the desired taper held at a con­stant angulation. The rotary instrument should be moved through a cylindrical path as the tooth is prepared, and the taper of the instrument should produce the desired axial wall taper on the com­pleted preparation. In practice, many dentists expe rience difficulty consistently avoiding excessively tapered preparations, particularly when preparing posterior teeth with limited access .56 Some authori­ties recommend the routine use of grooves to reduce the incidence of restoration displacement. It is un­clear, however, whether accurate groove alignment is more easily achieved than axial wall convergence, and skillfully prepared axial walls at a minimal con­vergence are very conservative of tooth structure.

Surface Area. Provided the restoration has a limited path of withdrawal, its retention depends on the length of this path or, more precisely, on the surface area in sliding contact. Therefore, crowns with long axial walls are more retentive than those with short axial walls,-” and molar crowns are more retentive than premolar crowns of similar taper. Surfaces where the crown is essentially being pulled away from rather than sliding along the tooth, such as the occlusal surface, do not add much to total retention.

Stress Concentration. When a retentive fail­ure occurs, cement often adheres to both the tooth preparation and the fitting surface of the restora­tion. In these cases, cohesive failure occurs through the cement layer because the strength of the cement is less than the induced stresses. A computerized analysis of these stresses-‘‘ reveals that they are not uniform throughout the cement but are concen­trated around the junction of the axial and occlusal surfaces. Changes in the geometry of the prepara­tion (e.g., rounding the internal line angles) may re­duce stress concentrations and thus increase the re­tention of the restoration.

Type of Preparation. Different types of prepa­ration have different retentive values that corre­spond fairly closely to the surface area of the axial walls, as long as other factors (e.g., taper) are kept constant. Thus the retention of a complete crown is about double that of partial-coverage restorations 59 (Fig. 7-32).

Adding grooves or boxes to a preparation with a limited path of withdrawal does not markedly affect its retention because the surface area is not increased significantly. However, where the addition of a groove limits the paths of with­drawal, retention is increased .⁶⁰

Roughness of the Surfaces Being Cemented.

When the internal surface of a restoration is very smooth, retentive failure occurs not through the ce­ment but at the cement-restoration interface. Under these circumstances, retention will be increased if therestoration is roughened or grooved . The casting is most effectively prepared by air-abrading the fit­ting surface with 50 um of alumina. This should be done carefully to avoid abrading the polished sur­faces or margins. Airborne particle abrasion has been shown65 to increase in vitro retention by 64%.

Failure rarely occurs at the cement-tooth inter­face. Therefore, deliberately roughening the tooth preparation hardly influences retention and is not recommended, because roughness adds to the diffi­culty of impression making and waxing.

Materials Being Cemented. Retention is af­fected by both the casting alloy and the core or

buildup material. Laboratory testing results have yet to be confirmed by longer-term clinical studies, but it appears that the more reactive the alloy is, the more adhesion there will be with certain luting agents. Therefore, base metal alloys are better re­tained than less reactive high-gold content metals The effect of adhesion to different core materials also has been tested, with conflicting results. One laboratory study67 examining adhesion between ce­ments and core materials found that the cement ad­hered better to amalgam than to composite resin or cast gold. However, when crowns were tested for retention, higher values were found with the com­posite resin than with amalgam cores . The differ­ences may have been due to dimensional changes of the core materials, although the clinical implications of this finding are not clear.

Type of Luting Agent. The type of luting agent chosen affects the retention of a cemented restora­tion .69 However, the decision regarding which agent to use is also based on other factors. In gen­eral, the data suggest that adhesive resin cements are the most retentive , although long-term clinical evidence about the durability of the bond is not available.

Film Thickness of the Luting Agent. There is conflicting evidence about the effect of increased thickness of the cement film on retention of a

restoration. This may be important if a slightly over­sized casting is made (as when the die-spacer tech­nique is used).

The factors that influence the retention of a ce­mented restoration are summarized in Table 7-3.

RESISTANCE FORM

Certain features must be present in the preparation to prevent dislodgment of a cemented restoration. Mastication and parafunctional activity may subject a prosthesis to substantial horizontal or oblique forces. These forces are normally much greater than the ones overcome by retention, especially if the restoration is loaded during eccentric contact be­tween posterior teeth. Lateral forces tend to displace the restoration by causing rotation around the gin­gival margin. Rotation is prevented by any areas of the tooth preparation that are placed in compres­sion, called resistance areas (Fig. 7-35). Multiple resis­tance areas cumulatively make up the resistance form of a tooth preparation.

Adequate resistance depends on the following:

1.  Magnitude and direction of the dislodging
forces

2.  Geometry of the tooth preparation

3.  Physical properties of the luting agent

Magnitude and Direction of the Dislodging

Forces. Some patients can develop enormous bit­ing forces. Gibbs et al” discovered one individual (Fig. 7-36) who had a biting force of 4340 N (443 kg).* Although this is considered extraordinary, restorations should nevertheless be designed to withstand forces approaching such magnitude. In one laboratory study,-” a complete crown cemented on a nickel-chromium test die was found to be ca­pable of withstanding over 13,500 N (1400 kg)-a far greater force than would occur in the mouth-before becoming displaced (Fig. 7-37).

In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed. If a fixed prosthesis is carefully made with a properly de­signed occlusion, the load should be well distributed and favorably directed (see Chapter 4). However, if a patient has a biting habit such as pipe smoking or bruxing, it may be difficult to prevent fairly large oblique forces from being applied to a restoration. Consequently the completed tooth preparation and restoration must be able to withstand considerable oblique forces as well as the normal axial ones.

Geometry of the Tooth Preparation. As with re­tention, preparation geometry plays a key role in at­taining desirable resistance form. The tooth prepara­tion must be shaped so that particular areas of the axial wall will prevent rotation of the crown.

Hegdahl and Silness79 analyzed how these resisting areas alter as changes are made in the geometry of the tooth preparation. They demonstrated that increased preparation taper and rounding of axial angles tend to reduce resistance. Short tooth preparations with large diameters were found to have very little resis­tance form. In general, molar teeth require more par­allel preparation than premolar or anterior teeth to achieve adequate resistance form.”” The relationship between preparation height, or diameter, and resis­tance to displacement is approximately linear.,’

A partial-coverage restoration may have less re­sistance (Fig. 7-38) than a complete crown because it has no buccal resistance areas. Resistance must be provided by boxes or grooves (Fig. 7-39) and will be greatest if they have walls that are perpendicular to the direction of the applied force. Thus U-shaped grooves or flared boxes provide more resistance than V-shaped ones.’ -y The resistance form of an excessively tapered preparation can be improved by adding grooves or pinholes, because these interfere with rotational movement and in so doing subject additional areas of the luting agent to compression.

Physical Properties of the Luting Agent. Re­sistance to deformation is affected by physical prop­erties of the luting agent, such as compressive strength and modulus of elasticity. To satisfy AD A/ANSI specificatioo. 96 (ISO 9917), the compressive strength of zinc phosphate cement must exceed 70 MPa* at 24 hours (Fig. 7-40). Glass ionomer cements and most resins have higher com­pressive strength, whereas polycarboxylates have similar values.⁸²

Increasing temperature has a dramatic effect on the compressive strength of luting agents, particu­larly weakening reinforced zinc oxide-eugenol ce­ment (Fig. 7-41). An increase from room tempera­ture (23° C) to body temperature (37° C) halves the compressive strength of reinforced zinc oxide-eugenol cements, and a rise in temperature to 50c C (equivalent to hot food) reduces the compressive strength by over 80% .83 Equivalent testing of more modern cements has not been reported.

Zinc phosphate cements have a higher modulus of elasticity than do polycarboxylate cements, which exhibit relatively large plastic deformation. This may account for the observation that the reten­tive ability of polycarboxylate cement is more de­pendent on the taper of the preparation than is the retention with zinc phosphate cement.”-‘

The factors that affect the resistance to displace­ment of a cemented restoration are summarized in Table 7-4.

DEFORMATION

A restoration must have sufficient strength to pre­vent permanent deformation during function (Fig. 7-42). Otherwise, it will fail (typically at the restora­tion-cement, or the metal-porcelain, interface). This may be a result of inappropriate alloy selection, in­adequate tooth preparation, or poor metal-ceramic framework design.

Alloy Selection. Although Type I and Type II gold alloys (see Chapter 22) are satisfactory for in-tracoronal cast restorations, they are too soft for crowns and fixed partial dentures, for which Type III or Type IV gold alloys (or an appropriate low-gold alternative) are chosen. These are harder, and their strength and hardness can be increased by heat treatment.

High-noble metal content metal-ceramic alloys have a hardness equivalent to that of Type IV golds, whereas nickel-chromium alloys are considerably harder. These may be indicated when large forces are anticipated, such as with a long-span FPD, al­though their use presents certain problems (see Chapter 16). Adequate Tooth Reduction. Even the stronger alloys need sufficient bulk if they are to withstand occlusal forces. Largely based on empirical data, there should be a minimum alloy thickness of about 1.5 mm over centric cusps (buccal in the mandible, lingual in the maxillae). The less stressed noncentric cusps can be protected with less metal (1 mm is ad­equate in most circumstances) for a strong and long-lasting restoration. Occlusal reduction should be as uniform as possible, following the cuspal planes of the teeth; this will ensure that sufficient occlusal clearance is combined with preservation of as much tooth structure as possible. In addition, an anatomically prepared occlusal surface (Fig. 7-43) will give rigidity to the crown because of the “cor­rugated effect””6 of the planes.

When teeth are malaligned or overerupted, the occlusal surface needs to be prepared with the eventual restoration in mind. For example, a supraerupted tooth may need considerably more than 1.5 mm of reduction to result in adequate clear­ance to reestablish an ideal occlusal plane. Diagnostic tooth preparation and waxing are helpful in determining the correct tooth reduction.

Margin Design. Distortion of the restoration margin is prevented by designing the preparation outline to avoid occlusal contact in this area. Also, tooth reduction should provide sufficient room for bulk of metal at the margin to prevent distortion. As discussed earlier, one disadvantage of the feath-eredge preparation is that the resulting thin layer of gold is not as strong as the comparatively thicker restoration of a chamfer preparation.

The grooves and ledges incorporated in a partial-coverage restoration provide essential strengthen­ing for the casting, particularly an anterior pinledge retainer (Fig. 7-45).

ESTHETIC CONSIDERATIONS

The restorative dentist should develop skill in de­termining the esthetic expectations of the patient. Patients prefer their dental restorations to look as natural as possible. However, care must be taken that esthetic considerations are not pursued at the expense of a patient’s long-term oral health or func­tional efficiency.

At the initial examination it is important to make a full assessment of the appearance of each patient, noting which areas of which teeth show during smiling, talking, and laughing. The patient’s es­thetic requirements must be discussed and related to oral hygiene needs and the potential for disease. The final decision regarding an appropriate restora­tion can then be made with the full cooperation and informed consent of the patient.

METAL-CERAMIC RESTORATIONS

The poor appearance of some metal-ceramic restorations is often due to insufficient porcelain thickness. On the other hand, adequate porcelain thickness is sometimes obtained at the expense of proper axial contour (such overcontoured restora­tions almost invariably lead to periodontal disease). In addition, the labial margin of a metal-ceramic crown is not always accurately placed. To correct all these deficiencies, certain principles are recom­mended during tooth preparation that will ensure sufficient room for porcelain and accurate place­ment of the margins. Otherwise, good appearance would be achievable only at the expense of peri­odontal health.

Facial Tooth Reduction. If there is to be suffi­cient bulk of porcelain for appearance and metal for strength, adequate reduction of the facial surface is essential. The exact amount of reduction will de­pend to some extent on the physical properties of the alloy used for the substructure as well as on the manufacturer and the shade of the porcelain. A min­imum reduction of 1.5 mm typically is required for optimal appearance. Adequate thickness of porce­lain (Fig. 7-46) is needed to create a sense of color depth and translucency. Shade problems are fre­quently encountered in maxillary incisor crowns at the incisal and cervical thirds of the restoration, where direct light reflection from the opaque layer can make the restoration appear very noticeable. Be­cause opaque porcelains generally have a different shade from body porcelains, they ofteeed to be modified with special stains in these areas.”

With very thin teeth (e.g., mandibular incisors) it may be impossible to achieve adequate tooth reduc­tion without exposing the pulp or leaving a severely weakened tooth preparation. Under these circum­stances a less than ideal appearance may have to be accepted.

The labial surfaces of anterior teeth should be prepared for metal-ceramic restorations in two distinct planes (Fig. 7-47). If they are prepared in a single plane, insufficient reduction in either the cervical or the incisal area of the preparation will result.

Incisal    Reduction.    The incisal edge of a metal-ceramic restoration has no metal backing and can be made with a translucency similar to that of natural tooth structure. An incisal reduction of 2 mm is recommended for good esthetics. Excessive incisal reduction must be avoided because it re­duces the resistance and retention form of the preparation.

Proximal Reduction. The extent of proximal re­duction is contingent on exact predetermination of the location of the metal-ceramic junction in the completed restoration. The proximal surfaces of an­terior teeth will look most natural if they are re­stored as the incisal edges, without metal backing. This will allow some light to pass through the restoration in a manner similar to what occurs on a natural tooth (Fig. 7-48). Obviously, if the restora­tion is part of a fixed partial denture, the need for connectors will make this impossible.

Labial Margin Placement. Supragingival mar­gin placement has many biologic advantages. The restorations are easier to prepare properly and eas ier to keep clean. Nevertheless, subgingival margins may be indicated for esthetic reasons, particularly when the patient has a high lip line and the use of a metal collar labial margin is contemplated.

The patient’s smile is observed as part of the ini­tial examination (see Chapter 1). It is important to record which teeth and which parts of each tooth are exposed. Patients with a high lip line, which ex­poses considerable gingival tissue, present the greatest problem if complete crowns are needed. Where the root surface is not discolored, appear­ance can be restored with a metal-ceramic restora­tion having a supragingival porcelain labial margin-sometimes called a “collarless” design (see Chapter 24). If the patient has a low lip line, a metal supragingival collar may be placed because the metal is not seen during normal function. Metal margins generally have a more accurate fit than porcelain margins.

However, it cannot be assumed that the patient will be happy with a supragingival metal collar just because the metal is not visible during normal func­tion. Some patients have reservations about ex­posed metal, and the advantages of such supragin­gival margins must be carefully explained before treatment.

Metal collars can be hidden below the gingival crest, although there will be some discoloration if the gingival tissue is thin. Successful margin place­ment within the gingival sulcus requires care to en­sure that inflammation and/or recession, with re­sulting metal exposure, are avoided or minimized. The periodontium must be healthy before the tooth is prepared. If periodontal surgery is needed, the sulcular space should not be eliminated completely; rather, a postsurgical depth of about 2 mm should be the objective. Sufficient time should be allowed after surgery for the periodontal tissues to stabilize. Wise found that the gingival crest does not stabi­lize until 20 weeks after surgery. Margins should not be placed so far apically that they encroach on the attachment; extension to within 1.5 mm of the alveolar crest will lead to bone resorption. The margin should follow the contour of the free gingiva, being further apical in the mid­dle of the tooth and further incisal interproximally. A common error (Fig. 7-49) is to prepare the tooth so the margin lies almost in one plane, with exposure of the collar labially and irreversible loss of bone and papilla proximally.

PARTIAL-COVERAGE RESTORATIONS

Whenever possible, accomplishment of an estheti-cally acceptable result without the use of metal-ceramic crowns is preferred, not only because tooth structure is conserved but also because no restora­tive material can approach the appearance of intact tooth enamel. Esthetic partial-coverage restorations depend on accurate placement of the potentially visible facial and proximal margins. Understand­ably, many patients will not readily accept a visible display of metal. If a partial-coverage restoration is poorly prepared, the patient may demand that it be replaced by a metal-ceramic crown, and the result will be unnecessary loss of tooth structure and a greater potential for tissue damage.

Proximal Margin. Placement of the proximal margins (particularly the mesial, generally more visible, margin) is critical to the esthetic result of a partial-coverage restoration. The rule here is toplace the margin just buccal to the proximal contact area, where metal will be hidden by the distal line angle of the neighboring tooth. Tooth preparation angulation is critical and should normally follow the long axes of posterior teeth and the incisal two thirds of the facial surface of anteriors. If a buccal or lingual tilt is given to the tooth preparation, metal may be visible (Fig. 7-50).

The distal margin of posterior partial-coverage restorations is less visible than the mesial margin. Often in this area it is advantageous to extend the preparation farther beyond the contact point for easier preparation and finishing of the restoration and better access for oral hygiene.

Facial Margin. The facial margin of a maxillary partial-coverage restoration should be extended just
beyond the occlusofacial line angle. A short bevel is needed to prevent enamel chipping. A chamfer can be placed where appearance is less important (e.g., on molars) because this will provide greater bulk of metal for strength.

If the buccal margin of metal is correctly shaped (Fig. 7-51), it will not reflect light to an observer. As a result, the tooth will appear to be merely a little shorter thaormal and not as though its buccal cusp is outlined in metal. If the buccal margin is skillfully placed following the original cuspal con­tour, the final restoration will have an acceptable appearance.

When mandibular partial cast crowns are made, metal display is unavoidable because the occlusal surface of mandibular teeth can be seen during speech. A chamfer, rather than a bevel, is recom mended for the buccal margin because it provides a greater bulk of metal around the highly stressed centric cusp (Fig. 7-52). If the appearance of metal is unacceptable to the patient, a metal-ceramic restora­tion with porcelain coverage on the occlusal surface can be made.

Anterior partial-coverage restorations can be fab­ricated to show no metal (Fig. 7-53), but their prepa­ration requires considerable care. The facial margin is extended just beyond the highest contour of the incisal edge but not quite to the incisolabial line an­gle. Here the metal will protect the tooth from chip­ping but will not be visible.

PLANNING AND EVALUATING TOOTH PREPARATIONS

Tooth preparation is a technically complicated and irreversible procedure. Thus it is the practitioner’s responsibility to carry it out properly every time. Mistakes are often difficult, if not impossible, to correct.

DIAGNOSTIC TOOTH PREPARATIONS

Diagnostic tooth preparations are performed on ar­ticulated casts before the actual clinical preparation. They yield information with regard to the following: e Selecting the appropriate path of withdrawal for a fixed partial denture, particularly when the abutment teeth are tilted or have an atypi­cal coronal contour

Another advantage of diagnostic tooth prepara­tions is that the operator can practice each step of the intended restoration. Mistakes are not permanently destructive. Additionally, diagnostic preparations can be used in the prefabrication of provisional restorations, significantly reducing the appointment time at tooth preparation (the indirect/direct tech­nique is described in Chapter 15).

Diagnostic Waxing Procedures For all but the most straightforward prosthodontic treat­ment plans, a diagnostic waxing procedure should be performed. This is done on diagnostic tooth prepara­tions and establishes the optimum contour and oc­clusion of the eventual prosthesis. The procedure is of particular benefit if the patient’s occlusal scheme or anterior (incisal) guidance requires alteration. Evaluative Procedures during Tooth Prepara­tion. Each step of a tooth preparation should be carefully evaluated with direct vision or indirectly with a dental mirror. Alignment of multiple abut­ment teeth can be a special problem, and using the mirror helps to superimpose the image of adjacent abutment teeth. Complex preparations should be evaluated by making an alginate impression and pouring it in fast-setting stone. A dental surveyor (Fig. 7-57) can then be used to precisely measure the axial inclinations of the tooth preparation. The less experienced dentist may hesitate to make such an impression for fear of losing time. However, the in­formation obtained often saves time in subsequent В procedures by identifying problems that can then be addressed immediately. During tooth preparation, it is useful to learn to use the contraangle handpiece as both a measuring and a cutting instrument. This is done by concentrating on the top surface of the turbine head, which is perpendicular to the shank of the bur. If the top surface is kept parallel to the oc­clusal surface of the tooth being prepared, the bur will automatically be in the correct orientation (Fig. 7-58). To prevent undercuts or excessive conver­gence during axial reduction, the handpiece must be maintained at the same angulation. The correct taper is imparted by the diamond instrument. Keep­ing the turbine head at its correct angulation ini­tially is often most effectively done by supporting it with a finger of the opposite hand.

PATIENT AND OPERATOR POSITIONING

Learning the proper patient and operator positions is as beneficial as learning the proper preparation steps. Of particular importance are the advantages of obtaining a direct view of the preparation, which is always preferred to an indirect or mirror view. However, certain areas (e.g., the distal surfaces of maxillary molars) cannot be seen directly.

Inexperience, coupled with a hesitation to move the patient’s head into a more favorable position, can unnecessarily complicate tooth preparation. For instance, having the patient rotate the head to the left or right side can considerably improve the visi­bility of molar teeth that are being prepared. In most instances a direct view can be obtained by subtly changing the operator’s or the patient’s position. Having the patient open maximally does not neces­sarily provide the best view. If the jaw is partially open, the cheek may be retracted more easily (Fig. 7-59), and if the patient is encouraged to make a lat­eral excursion, the distobuccal line angle, together with the buccal third of the distal wall, may be seen directly. In practice, the mirror is essential only to visualizing a small portion of the distal surface. When preparing a complete crown, the parts of the tooth most easily seen should be prepared first, leaving the other areas for preparation with the help of the mirror as a final stage.

The principles of tooth preparation can be catego­rized into biologic, mechanical, and esthetic consid­erations. Often these principles conflict, and the prac­titioner must decide how the restoration should be designed. One area may be given too much empha­sis, and the long-term success of the procedure may be limited by a lack of consideration of other factors. Experience will help in determining whether preparations are “complete.” Each tooth prepara­tion must be measured by clearly defined criteria, which can be used to identify and correct problems. Diagnostic tooth preparations and evaluative im­pressions are often very helpful. The types of prepa­ration described in the following chapters are ex­plained in a step-by-step format. Understanding the pertinent theories underlying each step is crucial. Successful preparation can be obtained most easily by systematically following the steps. It is critical to refrain from “jumping ahead” before the previous step has been evaluated and, if necessary, corrected. If the clinician proceeds too rapidly, precious chair time will be lost, and the quality of the preparation will probably suffer.

After a patient has been examined and treatment planned for a crown or bridge procedure, a preliminary alginate impression for study models can be made. Subsequently, working casts can be created from the preliminary impression, which will be used to fabricate custom trays (Figure 1 and Figure 2). The philosophy behind making the final impression with custom trays is to provide a well-fitting impression tray that is comfortable for the patient, contains an adequate amount of impression material to obtain an impression that reproduces fine details, minimizes waste of expensive impression materials, and assures proper intraoral seating of the impression tray during the impression process.¹

Anesthetic Application

Applying topical anesthetic to the injection sites after drying the mucosa will provide patient comfort during the injections. For patients, the administration of the local anesthetic injection is probably the most nerve-racking part of the dental visit, no matter what procedure will be performed.

Shade Selection

Shade selection can be done while the dental team is waiting for the local anesthetic to take effect. Using a shade guide under natural lighting conditions, the dentist, dental assistant, and the patient should determine which shade matches the adjacent teeth best.²

Preparation and Evacuation

After the teeth to be prepared have been anesthetized, the dentist will use burs in a high-speed handpiece to reduce and shape the teeth. Simultaneously, the dental assistant will use the high-velocity evacuation system and the air/water syringe to keep the oral cavity clear of debris and maintain a clear path of vision for the dentist. When tooth preparation is complete, the dental assistant will examine the patient’s oral cavity to verify that it is clear of debris.

Tissue Management

There are various methods available for tissue management (Figure 3), the most common being the use of retraction cord. Retraction cords may be twisted or braided and are available in different sizes to accommodate various depths of sulcus.

 Using a Balshi packer or another of gingival cord-packing instruments available, gently push the end of the gingival retraction cord under the gingival margin. Continue advancing around the prepared tooth, packing the gingival retraction until arriving at the beginning end of the cord. Leave a piece of the cord sticking above the gingival margin for removal immediately proceeding placement of the wash or light- final impression material (Figure 4 and Figure 5). Some retraction cords are treated with a chemical agent to enhance the contraction of the tissue it contacts. Aluminum chloride, potassium aluminum sulfate, ferric sulfate, or zinc chloride cause the collagen fibers in the tissues around the capillaries to swell. The expansion of the collagen around the capillaries induces pressure on them, which causes them to constrict.^3,4 This contraction of the tissues allows the impression material to enter the sulcus to capture the margins of the tooth preparation. Astringedent^®, Astringedent^® X, ViscoStat^®, ViscoStat^®Wintermint, ViscoStat^® Clear (Ultradent Products, Inc, www.ultradent.com), Gingi-BRAID+ (DUX Dental, www.duxdental.com), and Hemodent™ Cord, (Premier Dental Products Co, www.premusa.com) are examples of tissue management products. Make sure you are familiar with the patient’s medical history before selecting a gingival retraction method. Astringedent X should be used on patients who are on the anticoagulant warfarin or aspirin therapy, or who are hemophiliacs to control bleeding.³ Also, gingival retraction cord should not be left in the sulcus for an extended period of time as this may cause gingival recession, especially if a periodontal condition is present.

Generally, chemically treated retraction cord can be left in place for 5 to 7 minutes and untreated cord for 10 to 15 minutes. Gingival retraction products containing epinephrine should be avoided in patients who have hypertension, heart disease, diabetes, or hyperthyroidism. In addition to the use of cords for gingival retraction, retraction caps (Roeko Comprecap, Coltène Whaledent Inc, www.coltenewhaldent.com; GingiCap Centrix, www.centrixdental.com), retraction gel (GelCord Tissue Management Gel, Pascal International, Inc, www.pascaldental.com), and gingival retraction paste (Expasyl™, Kerr Corporation, www.kerrdental.com; Traxodent Hemodent, Premier Dental Corporation, www.premusa.com) can be used. Roeko Comprecaps should be used in conjunction with an expanding polyvinyl siloxane material to press the gingival tissues away from the margins of the preparation. If the gingival tissues are too bulky and esthetics will not be compromised, the dentist can create a trough around the preparation using electrosurgery to open the sulcus.

Final Impression

After the tissues have been managed, the final impression materials can be prepared. The selection of the impression material depends on the dentist’s preference. A single-viscosity, monophase impression material is favored by many dental professionals because there is only one material to mix and it can reproduce fine details. A combination of medium-, heavy-, or putty along with a wash-type impression material can be used with a two-step technique (Figure 6 and Figure 7). When selecting an impression technique and materials, consider how many hands will be available for mixing the materials and loading them into a syringe and/or impression tray. An option is to use materials that are prepared using an automixer.

After the master impression is removed from the patient’s mouth, the impression should be disinfected and then examined by the dentist to ensure all necessary details were captured (Figure 8).⁷ A clear reproduction of the margins is needed because the dental laboratory technician must be able to read the margins on the dies produced from the impression to fabricate a well-fitting permanent prosthesis (if the necessary anatomic features were not reproduced, the impression would have to be repeated; this also may include reapplication of the gingival retraction method).

Immediately after the impression is completed, a bite registration should be recorded with a high-viscosity elastomeric impression material. The bite registration also needs to be disinfected before packaging to be sent to the laboratory.

Provisional Restoration Creation/Delivery

After the final impression has been completed, provisional coverage is provided.⁸ Options available for provisional coverage are: custom-made, preformed polymer, preformed polycarbonate, and aluminum. Stainless steel crowns also can be used as a form of provisional coverage in adults.⁹ Another possible option requires sending the working cast to the dental laboratory a week before the preparation visit. Provisional restorations can be fabricated by the dental laboratory technician from the pretreatment cast. These provisionals are reinforced with orthodontic wire or Kevlar fiber. Using laboratory-fabricated provisionals can save chair time, but adds an expense. This option is especially useful if the provisional restorations are expected to be worn for an extended time.

Provisional Restoration Cementation

Regardless of the type of provisional used, it will be cemented onto the prepared teeth using a temporary luting agent. A well-fitting provisional needs a minimal amount of temporary cement (Figure 9).

 An excessive amount of cement inside a crown or bridge may prevent proper seating onto the prepared tooth/teeth. After the cement has set, the gingival margins should be examined and any excess cement removed (Figure 10). A piece of knotted dental floss should be carefully passed interproximally to remove residual cement (Figure 11).⁹ If the provisional is a bridge, floss should be passed under the pontic areas as well.¹

Patient Instruction

After the provisional is in place, the patient should be instructed about the importance of plaque control and any modifications to his or her oral hygiene routine. Diet modifications while the provisional restoration is in place should be discussed. Equally important is informing the patient to notify the dental office immediately should the provisional restoration come off the preparation before the next scheduled appointment.⁸

Final Cementation Visit Steps

Removal of Provisional Restoration

The next visit is scheduled to permanently cement the prosthesis onto the prepared tooth/teeth. When the patient arrives, the dental assistant will remove the provisional restoration (Figure 12). The prepared tooth/teeth should be examined and any remaining temporary cement removed. The preparation is carefully rinsed and dried.

Restoration Cementation

The permanent restoration is tried in. If the dentist and patient are satisfied with the fit and appearance of the prosthesis, it can be cemented with the permanent cement designated by the dentist. After isolating the preparation, the dental assistant will need to mix the permanent cement according to the manufacturer’s directions and then coat the inner surface of the abutment teeth of the prosthesis with enough cement to ensure adequate adhesion of the prosthesis to the patient’s teeth. Again, excessive amounts of cement may prevent proper seating. The prosthesis is seated onto the preparation by the dentist and the patient is instructed to bite continuously until the cement has hardened. After the cement is set, the dental assistant should examine the area and remove any excess cement as was done with the provisional restorations.

Conclusion

Treatment sessions for fixed prosthodontic procedures are busy. An organized dental assistant will keep track of the multiple tasks involved in the multistep procedure and store instruments and materials so they are easily accessible and ready for use. By chunking out the multiple, coordinated procedures, the dental assistant can keep chair time to a minimum and help reduce stress for the patient and the dental team.