Clinical and laboratory stages of production whole casted crown

Clinical and laboratory stages of production whole ncasted crown.

Errors and complications in prosthetic by artificial crowns.

Background

 

A ndental crown is a cap-like restoration used to cover a damaged tooth. Crowns ncan give support to misshapen or badly broken teeth and permanently replace nmissing teeth to complete a smile or improve a bite pattern. They may be molded nfrom metal, ceramic, plastics, or combinations of all three. They are cemented nin place and coated to make them more natural looking. Historically, a variety nof materials have been used as tooth replacements. The ancient Egyptians used nanimal teeth and pieces of bone as primitive replacement materials. More nrecently, artificial teeth have been fabricated from substances such as ivory, nporcelain, and even platinum. With modern technology, high quality tooth nreplacements can be made from synthetic plastic resins, ceramic composites, and nlightweight metal alloys.

Design

 

There nare several key factors to consider in the design of dental crowns. First, nappropriate raw materials with which to make the crown must be identified. nThese materials must be suitable for use in the oral cavity, which means they nmust be acceptable for long term contact with oral tissues and fluids. Crowcomponents must have a good safety profile and must be non-allergenic and nnon-carcinogenic. The American Dental Association/ANSI specification #41 n(Biological Evaluation of Dental Materials) lists materials which have been deemed nsafe for use. In addition to safety considerations, these materials must be nable to withstand the conditions of high moisture and mechanical pressure, nwhich are found in the mouth. They must be resistant to shrinkage and cracking, nparticularly in the presence of water. Metal is preferred for strength but nacrylic resins and porcelain have a more natural appearance. Therefore the nselection of crown material is, in part, dependent on the location of the tooth nbeing covered. Acrylic and porcelain are preferred for front teeth, which have nhigher visibility. Gold and metal amalgams are most often used for back teeth nwhere strength and durability are required for chewing but appearance is less ncritical.

The nsecond factor to consider when designing a crown is the shape of the patient’s nmouth. Dental restorations must be designed to mimic the bite properties of the noriginal tooth surface so the wearer does not feel discomfort. Since every nindividual’s mouth is different each crown must be custom designed to fit perfectly. nSuccessful crown design involves preparation of an accurate mold of the oral ncavity.

Raw Materials

 

There nare four main types of materials used in crown construction: The plasters used nto create the mold, the materials from which the crown itself is made (e.g., nmetal, ceramic, plastic), the adhesives used to cement the crown in place, and nthe coatings used to cover the crown and make it more aesthetically appealing.

Molding plasters

 

Plaster nmolds are made from a mixture of water and gypsum powder. Used for dental napplications since the 1700s, gypsum is finely divided calcium sulfate ndihydate. Different types of plasters are used depending on application: nimpression plaster is used to

Aimpression of the tooth to be crowned is taken to record its shape. The nimpression plaster is mixed and then placed in a tray that is fitted over the nteeth. The tray is held still in place until the plaster hardens. When the tray nis removed from the mouth, it retains a three dimensional impression of the ntooth that is to be covered. This impression is a negative, or reverse, image nof the tooth.

 record nthe shape of the teeth, model plaster is used to make durable models of the noral cavity, and investment plaster is used to make molds for shaping metal, nceramics and plastics. Waxes are also sometimes used in this regard.

Crowconstruction materials

Metals nare frequently used in crown construction because they have good hardness, nstrength, stiffness, durability, corrosion resistance, and bio-compatibility. nMetals formulated as mixtures of mercury have been historically used. In fact, none source notes that metal amalgam was used as a dental restorative as early nas 1528. Common alloys used in crowns are based on mixtures of mercury with nsilver, chromium, titanium, and gold. These mixtures form a blend than can be neasily shaped and molded, but which hardens in a few minutes.

 

Ceramics nare well suited for use in crowns because they have good tissue compatibility, nstrength, durability and inertness. They can also be made to mimic the nappearance of real teeth fairly closely. However, the tensile strength of nceramic is low enough to make it susceptible to stress cracking, especially ithe presence of water. For this reason, ceramic is most often used as a coating nfor metal-structured crowns. The two primary types of ceramics used in crowns nare made from potassium feldspar and glass-ceramic.

 

The nfirst resin used in denture materials was vulcanized rubber in 1839. Since nthen, a number of other resins have been developed which are more suitable for ndental applications. Today, acrylic polymer resins are commonly used identures and crowns. Specifically, polymethyl methacrylate is most often used. nThis type of resin is made by mixing together chemical entities known as nmonomers with activating chemicals which cause the monomers to react and link ntogether to form long chains called polymers. Some of these resins harden at nroom temperature as this reaction progresses. Others require heat or nultraviolet light to catalyze the change.

 

Special ndental adhesives, or dental cements, are used to hold the crown in place. These ncan be classified as either aqueous or nonaqueous. The aqueous type include nzinc phosphates, polycarboxylate cements, glassionomer cements, and calcium nphosphate cements. The nonaqueous type include zinc oxide-eugenol, calcium nchelates, and acrylic resins such as polymethyl methyacrylate.

 

Coatings nare used to make the crown appear more natural. Porcelain is used in this nregard, but it is difficult to work with and hard to match to the tooth’s nnatural color. Resins similar to the ones used in tooth construction are also nused to create tooth-colored veneers on crowns. These resins have an advantage nover other veneers in that they are inexpensive, easy to fabricate, and can be nmatched to the color of tooth structure. However, acrylic coatings may not nadhere to the crown’s surface as well as porcelain or other materials. nTherefore, the prosethedontist may design the crown’s surface with mechanical nundercuts to give the coating a better grip. Resin coatings also have nrelatively low mechanical strength and color stability and poor abrasion and nstain resistance as compared to porcelain veneers.

The Manufacturing nProcess

Creating the mold

1 nBefore beginning, the dentist may need to prepare the area where the crown is nto be Two types of crown preparation: anterior and posterior.

 installed. nThis may require the removal of 2-3 millimeters of tooth structure from the nfour sides and the biting edge. Then, an impression of the tooth is taken to nrecord its shape. This step uses impression plaster which is the softest and nfastest setting type of dental plaster. The impression plaster is mixed with a nsmall amount of water until it is fluid. This slurry is placed in a tray that nis fitted over the teeth. The tray is held still in place until the plaster nhardens. When the tray is removed from the mouth, it retains a three ndimensional impression of the tooth that is to be covered. This impression is a nnegative, or reverse, image of the tooth.

2 The nnext step is to prepare another type of plaster, known as model plaster. This ntype of plaster is harder than the impression plaster. Once again the plaster nis mixed with the appropriate quantity of water. Then the slurry is poured into nthe impression mold. In this way a positive model of the tooth can be made. nThis positive model made from the negative impression mold is called a cast. nThe cast is used by the dentist for study purposes.

3 The nimpression is also used to make a mold, called an investment, which is capable nof withstanding high temperatures. This is an important consideration because nsome metals and ceramics require temperatures higher than 2,372° F (1300° C) nfor molding. These investments are made from calcium phosphate mixed with silica nand other modifying agents.

Fabrication

4 nFabrication of the crown is done by filling the investment with the appropriate nmaterial. In the case of metals, this is done at a high temperature so the nmetal is molten. For ceramics and plastics, the mixture is initially fluid but nmay require the addition of heat to cause the materials to cure and harden. A nvertical vise may be used to help pack the casting investment tightly. The nprocess also requires the mold first be treated with a release agent to ensure nthe crown can be easily removed after it has hardened. Some acrylic resins must nbe heated for up to eight hours to make sure they are fully cured. After the nprocessing is done and the investment has cooled, the mold is broken apart and nthe crown is removed.

Installation

5 After nthe crown has been successfully completed, it is ready for installation. The nprosethedontist applies cement to the inside of the crown surface and then fits nit into place over the tooth. Because of the number of processing steps there nmay be a slight discrepancy in the fit and the crown may require minor grinding nand smoothing of its surface to ensure it fits correctly.

Finishing

6 The ncrown may require a finishing coat to seal it and improve its natural nappearance. Such coatings are typically acrylic polymers. The polymer can be npainted on as a thin film, which hardens to a durable finish. Some polymers nrequire a dose of ultraviolet light to properly cure.

Quality Control

 

Good nquality control is critical to ensure the crown fits and looks natural in the npatient’s mouth. Every crown is unique because every person’s mouth is ndifferent and every crown is custom molded to fit. To ensure appropriate fit nand feel, fine details can be added to the crown by hand after the molding nprocess is completed. Even with minor adjustments, quality problems and nfailures in crowns are likely to occur. Key quality control issues include nfailures due to biological factors (such as caries, recurrent decay, nsensitivity problems, and periodontal diseases), mechanical reasons (including nfracture of the crown surface, and poor cementation), aesthetic problems n(discoloration of the surface), and damage due to traumatic accidents. In such nsituations it may become necessary to reposition or remove a crown to allow for neither replacement or other dental operations. There are special crown and nbridge removal systems that have been developed for easy removal of these nprosthedontics. This is accomplished by placing a precision vertical channel ithe surface of the crown, then threading the surface until the cement layer has nbeen broken. The crown can then easily be lifted from the underlying tooth nwithout force.

Casting Procedures

Casting nis the process by which a wax pattern of a restoration is converted to a nreplicate in dental alloy. The casting process is used to make dental nrestorations such as inlays, onlays, crowns, bridges, and removable partial ndentures. Because castings must meet stringent dimensional requirements, the casting nprocess is extremely demanding. In dentistry, virtually all casting is done nusing some form or adaptation of the lost-wax technique. The lost-wax technique nhas been used for centuries, but its use in dentistry was not common until n1907, when W.H. Taggart introduced his technique with the casting machine.

 Casting ncan be defined as the act of forming an object in a mold .The object thus nformed is also called as a casting .

 Objectives nof casting

 1) nTo heat the alloy as quickly as possible to a completely molten condition.

 2) nTo prevent oxidation by heating the metal with awell adjusted torch .

 3) nTo produce a casting with sharp details by having adequate pressure to the well nmelted metal to force into the mold.

 STEPS IN nMAKING A CAST RESTORATION

 1 n. TOOTH PREPARATION .

 2 n. IMPRESSION .

 3 n. DIE PREPARATION .

 4 n.WAX PATTERN FABRICATION .

 – nThere are 4 methods for making wax patterns for a cast restoration .

 5. nSPRUING .

 a) nSprue Former . (sprue pin ).

 -provides nchannel for the molten metal .

 -made nof wax , plastic or metal .

 -reservoir nis attached to the sprue .

 -ideally nlength of sprue is 3/8 th” to ½”

 Lost nWax Process

 The nlost wax casting process is widely used as it offers asymmetrical casting nwithnvery fine details to be manufactured relatively inexpensively. The process ninvolves producing a metal casting using a refractory mould made from a wax nreplica pattern.

 The nsteps involved in the process or the lost wax casting are:

 1 n.Create a wax pattern of the missing tooth / rim

 2 n.Sprue the wax pattern

 3 n.Invest the wax pattern

 4. nEliminate the wax pattern by burning it (inside the furnace or in hot water). nThis will create a mould.

 5 n. Force molten metal into the mould – casting.

 6 n.Clean the cast.

 7 n.Remove sprue from the cast

 8 n. Finish and polish the casting on the die .

 The nlost-wax technique is so named because a wax pattern of a restoration is ninvested in a ceramic material, then the pattern is burned out n(“lost”) to create a space into which molten metal is placed or cast. nThe entire lost-wax casting process . A wax pattern is first formed on a die of nthe tooth to berestored or, occasionally, directly on the tooth. All aspects of nthe final restoration are incorporatedinto the wax pattern, including the nocclusion, proximal contacts, and marginal fit. Once the wax pattern is ncompleted, a sprue is attached, which serves as a channel for the molten metal nto pass from the crucible into the restoration. Next, the pattern and sprue are ninvested in a ceramic material, and the invested pattern is heated until all nremnants of the wax are burned away. After burnout, molten metal is cast into nthe void created by the wax pattern and sprue. Once the investment is brokeaway, the rough casting ispickled to removed oxides. Finally, the sprue is nremoved and the casting is polished and deliveredto the patient. If all steps nhave been done well, the final restoration will require minimal modificatioduring cementation into the patient’s mouth.

 Dimensional nChanges in the Lost-Wax Technique

 If nmaterials used during the casting process didn’t shrink or expand, the size of nthe final cast restoration would be the same as the original wax pattern. nHowever, dimensional changes occur in most of the steps and, in practice, the nfinal restoration may not be exactly the same size as the pattern. The nmanagement of these dimensional changes is complex, but can be summarized by nthe equation:

 wax nshrinkage + metal shrinkage = wax expansion + setting expansion + hygroscopic nexpansion + thermal expansion .This equation balances the shrinkage (left nsideof equation) against the expansion (right side ofequation) that occurs nduring the casting process. If the final restoration is to fit the die, the nshrinkage and expansion during the casting process bmust be equal. Shrinkage nforces in the casting process come from two sources: wax and metal. Although nthe die restricts the wax from shrinking to a large degree while the pattern is non the die, residual stresses may be incorporated into the pattern and released nduring investing, when the pattern isremoved from the die. Furthermore, if the ninvestingis done at a temperature lower than that atwhich the wax pattern was nformed, the wax willshrink significantly because of the high coefficientof nthermal expansion of waxes. Metal shrinkage occurs when the moltenmetal nsolidifies, but this shrinkage is compensated by introducing more metal as the ncasting solidifies. However, once the entire casting has reached the solidus ntemperature of the alloy, shrinkage will occur as the casting cools to room ntemperature. As for wax, the metallic shrinkage that occurs below the solidus nis caused by the coefficient of thermal expansion for the alloy. Cooling nshrinkage may reach 2.5% for an alloy that cools from a high solidus ntemperature (1300″ to 1400′ C), depending on the coefficient of thermal nexpansion of the alloy. A typical shrinkage range for most alloys is 1.25% to n2.5%. Furthermore, because the casting is solid at this point, the only npossible compensation mechanismis to start with a void space that is 1.25% to2. n5% too large. Thus, shrinkage of wax and metalmust be compensated with nexpansion in the investment if the casting is to have the appropriate ndimensions.

Accuracy of the nLost-Wax Technique

 A ncasting should be as accurate as possible, although a tolerance of rt0.05% for nan inlay casting is acceptable. If the linear dimension of an average dental ninlay casting is assumed to be 4 mm, +0.05% of this value is equal to only n+2ym, which suggests that if two castings made for the same tooth have a nvariation of 4 ym, the difference may not be noticeable. To visualize this ndimension, recall that the thickness of an average human hair is about 40 ym. nTherefore the tolerance limits of a dental casting are approximately one-tenth nthe thickness of a human hair. To obtain castings with such small ntolerancelimits, rigid requirements must be placed not only on the investment nmaterial but also on theimpression materials, waxes, and die materials. nNaturally, technical procedures and the proper handling of these materials are nequally important. The values for the setting, hygroscopic, and thermal nexpansions of investment materials may vary from one product to another, and nslightly different techniques may be used with different investments. In each ncase, the values obtained for any one property should be reproducible from one nbatch to another and from one casting to another.

 The nSprue :

 Definition:

 Its na channel through which molten alloy can reach the mold in an invested ring nafter the wax has been eliminated. Role of a Sprue: Create a channel to allow nthe molten wax to escape from the mold. Enable the molten alloy to flow into nthe mold which was previously occupied by the wax pattern.

 

 FUNCTIONS nOF SPRUE

 1 n. Forms a mount for the wax pattern .

 2 n. Creates a channel for elimination of wax .

 3 n.Forms a channel for entry of molten metal

 4 n. Provides a reservoir of molten metal to compensate for the alloy shrinkage .

 SELECTION nOF SPRUE

 1 n. DIAMETER :

 It nshould be approximately the same size of the thickest portion of the wax npattern .

 Too nsmall sprue diameter suck back porosity results .

 2 n. SPRUE FORMER ATTACHMENT :

 Sprue nshould be attached to the thickest portion of the wax pattern .

 It nshould be Flared for high density alloys & Restricted for low density nalloys .

 3 n. SPRUE FORMER POSITION

 Based non the

 1 n.Individual judgement .

 2 n.Shape & form of the wax pattern .

 Patterns nmay be sprued directly or indirectly ..

 Indirect nmethod is commonly used

 Reservoir nprevents localised shrinkage porosity .

 Reservoir nAnd Its Location

 Reservoir nportion of a Spruing system is a round ball or a bar located 1mm away from the nwax pattern. Reservoir should be positioned in the heat centre of the ring . nThis permits the reservoir to remain molten for longer and enables it to nfurnish alloy to the pattern until they complete solidification process . Round nball reservoir & a bar reservoir also called connector

 Significance nof Reservoirs:

 Reservoir nis the largest mass of any part of the Sprue system & it is present in the nheat centre of the ring, it is the last part to solidify. These properties nallow continuous feeding of the molten alloy to compensate for Solidificatioshrinkage & avoid Shrinkage porosity

 Spruing nTechnique:

 Direct nSpruing:

 The nflow of the molten metal is straight(direct) from the casting crucible to npattern area in the ring. Even with the ball reservoir, the Spruing method is nstill direct. A basic weakness of direct Spruing is the potential for suck-back nporosity at the junction of restoration and the Sprue.

 Indirect nSpruing:

 Moltealloy does not flow directly from the casting crucible into the pattern area, ninstead the alloy takes a circuitous (indirect) route. The connector (or nrunner) bar is often used to which the wax pattern Sprue formers area attached. nIndirect Spruing offers advantages such as greater reliability & npredictability in casting plus enhanced control of solidification shrinkage n.The Connector bar is often referred to as a “reservoir

 Armamentarium n:

 1. nSprue .

 2 n. Sticky wax .

 3 n. Rubber crucible former .

 4 n. Casting ring .

 5 n. Pattern cleaner .

 6 n. Scalpel blade & Forceps .

 7 n. Bunsen burner .

 I n. Procedure for single casting :

 A n2.5 mm sprue former is recommended

 for nmolar crowns 2.0 mm for premolars & partial coverage crowns .

 II n. Procedure for multiple casting :

 Each nunit is joined to a runner bar .

 A nsingle sprue feeds the runner bar

 4 n. SPRUE FORMER DIRECTION

 Sprue nShould be directed away from the delicate parts of the pattern

 It nshould not be at right angles to a flat surface .(leads to turbulance à porosity .)

 Ideal nangulation is 45 degrees .

 5 n. SPRUE FORMER LENGTH

 Depends non the length of casting ring .. Length of the Sprue former should be such that nit keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue nformer should be no longer than 2 cm. The pattern should be placed as close to nthe centre of the ring as possible.

 Significance

 Short nSprue Length:

 The ngases cannot be adequately vented to permit the molten alloy to fill the ring ncompletelyleading to Back Pressure Porosity.

 Long nSprue Length:

 Fracture nof investment, as mold will not withstand the impact force of the entering nmolten alloy.

 Top nof wax should be adjusted for :

 6 nmm for gypsum bonded investments .

 3 n-4 mm for phosphate bonded investments .

 TYPES OF nSPRUES

 I n. – Wax . II . Solid

 – nPlastic . Hollow

 – nMetal .

 VENTING

 Small nauxilliary sprues or vents improve casting of thin patterns .

 Acts nas a HEAT SINK .

 WAX nPATTERN REMOVAL

 Pattershould be removed in line with its path of removal

 WETTABILITY

 To nminimise the irregularities on the investment & the casting a wetting agent ncan be used .

FUNCTIONS nOF A WETTING AGENT

 1 n. Reduce contact angle between liquid & wax surface .

 2 n.Remove any oily film left on wax pattern .

 DISTORTION nOF THE PATTERN

 Distortiois dependant on temperature &time interval before investing .

 To navoid any distortion ,

 Invest nthe pattern as soon as possible .

 Proper nhandling of the pattern .

 PREREQUISITES

 Wax npattern should be evaluated for smoothness , finish & contour .

 Patteris inspected under magnification & residual flash is removed .

 CRUCIBLE nFORMER

 It nserves as a base for the casting ring during investing .Usually convex ishape.

 May nbe metal , plastic or rubber .

 Shape ndepends on casting machine used .

 Modermachines use tall crucible to enable the pattern to be positioned near the end nof the casting machine .

 Casting nring

 CASTING nRING LINERS

 Most ncommon way to provide investment expansion is by using a liner in the casting nring .Traditionally asbestose was used .

 Noasbestose ring liner used are :

 1) nAluminosilicate ceramic liner .

 2) nCellulose paper liner .

 The naim of using a resilient liner is to

 -. nallow different types of investmentbexpansion (act as a cushion)

 _. nfacilitate venting during casting procedure.

 _. nfacilitate the removal of the investment block after casting.&. prevent the ndistortion by permitting the outward expansion of the mold.

 The ncasting ring holds the investment in place during setting and restricts the nexpansion of the mold. Normally a resilient liner is placed inside the ring nleaving about 2-3 mm from both ends to allow for supporting contact of the ninvestment with the casting ring.

 Purpose nof Casting Ring Liner

 Ringer nliner is he most commonly used technique to provide investment expansion. To nensure uniform expansion , liner is cut to fit the inside diameter of the ncasting ring with no overlap. Thickness of the liner should not be less thaapproximately 1mm. Place the liner somewhat short of the ends of the ring, 3mm, ntends to produce a more uniform expansion, therefore less chance for distortioof the wax pattern & mold .

 Traditional nmaterial for lining casting rings until it was learned that it posed a npotential health risk to dental laboratory technicians . Asbestos fiber bundles nwere found to produce hazardous-size respirable particles capable of causing nlung disease.

 Non-asbestos nRing Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose ncombination Safety of the ceramic ring liners remains uncertain, because naluminum silicate also appears capable of producing hazardous-size respirable nparticles

 RINGLESS nINVESTMENT TECHNIQUE

 Used nfor phosphate bonded investments .

 This nmethod uses paper or plastic casting ring .

 It nis designed to allow urestricted expansion .

 Useful nfor high melting alloys .

 Investing nTechnique

 Investing nis the process by which the sprued wax pattern is embedded in a material called nan investment. The investment must be able to withstand the heat and forces of ncasting, yet must conform to the pattern in a way such that the size and nsurface detail are exactly reproduced. In dentistry, gypsum- and nphosphate-bonded investment materials are the two types of materials used for nthis purpose . After spruing, the pattern a casting ring is added to contaithe investment while the investment material is poured carefully around the npattern. For the setting and hygroscopic expansion of an investment to take nplace more uniformly, some allowance must be made for the lateral expansion of nthe investment. Solid rings do notpermit the investment to expand laterally nduringthe setting and hygroscopic expansions of themold.

 To novercome this lateral restriction, a ceramic paper liner is placed inside the nring.The ceramic paper liner is cut to fit the inside ofthe metal ring and is nheld in place with the finger.The ring containing the liner is then dipped nintowater until the liner is completely wet and wateris dripping from it. The nring is shaken gently toremove the excess water. After the liner has nbeensoaked, it should not be touched or adaptedfurther with a finger because nthis reduces itscushioning effect, which is needed for the lateral expansion of nthe investment. A liner that is about3 mm short at each end of the ring is npreferred.When the liner is equally short at each end of thering, the investment nis locked into the ring, and uniform expansion of the cavity form occurs.

 During ninvesting, the water-based gypsummaterial must flow around the pattern and ncaptureevery surface detail. However, the wax sur-faces generally are not neasily wetted by water.The surface of a wax pattern that is not ncompletelywetted with investment results in surface irregularities in the ncasting that destroy its accuracy.These irregularities can be minimized nbyapplying a surface-active wetting agent on thewax. The function of the nwetting agent is toreduce the contact angle of a liquid with the waxsurface. nWetting agents also remove any oily filmthat is left on the wax pattern from nthe separatingmedium. Thecontact angles are 98′ for the plain wax surfaceand n61″ for the treated wax surface. The lowercontact angle indicates that the ntreated wax surfacehas an affinity for water, which results in theinvestment nbeing able to spread more easily overthe wax. Because the surface-active agents narequite soluble, rinsing the wax pattern with waterafter the applicatiodefeats the purpose of theiruse.

 The ndistortion of the wax pattern after itsremoval from the die is a function of nthe temperatureand time interval before investing. Thenearer the room ntemperature approaches the softening point of the wax, the more readilyinternal nstresses are released. Also, the longer apattern is allowed to stand before ninvesting, thegreater the deformation that may occur, even atroom temperature. nA pattern should therefore beinvested as soon as possible after it is nremovedfrom the die, and it should not be subjected to awarm environment during nthis interval. In anycase, a pattern should not stand for more than20 to 30 nminutes before being invested. Once itis properly invested and the investment nhas set,there is no danger of further pattern distortion,even if it remains for nsome hours before the finalstages of wax elimination (burnout) and casting

 Investment nTechniques

 During ninvestingof the pattern, the correct water powder ratioof the investment mix, a nrequired number ofspatulation turns, and a proper investing techniqueare nessential to obtain acceptable castingresults. There are two methods of ninvesting thewax pattern: hand investing and vacuum investing.In both cases, nthe proper amount of investmentpowder and water should be used, followingthe nmanufacturer’s instructions exactly. Thewater is added first, followed by the nslow additionof the powder to encourage the removal ofair from the powder. The npowder and liquid aremixed briefly with a plaster spatula until all thepowder nis wetted.

 Ihand investing, the cover of the bowl containingthe investment mix is placed nover thebowl . The cover contains a mechanicalmixer, and the mixing is done by nhand,usually for 100 turns of the spatulator. The settingrate of an investment ndepends on the number ofspatulation turns, which also affects the nhygroscopicexpansion. The investment, after beingspatulated, is placed on the nvibrator to eliminatesome of the air bubbles from the mix and tocollect all of nthe mix from the sides of the rubber bowl into the center. Thefilled ring is nthen set aside for the investment toset completely, which usually requires 45 nto60 minutes. When a phosphate-bonded investmentis used, the ring is slightly noverfilled, the topof the ring is not leveled off, and the investmentis allowed nto set. After the investment has set, the excess investment is ground off using na modeltrimmer. This procedure is necessary because anonporous, glassy surface nresults, which must beground off to improve the permeability of the

 investment nand allow for gases to readily escape from the mold during casting.

 Ivacuum investing, special equipment is used to facilitate the investing noperation. With this equipment, the powder and water (or special liquid) are nmixed under vacuum and the mixed investment is permitted to flow into the ring nand around the wax pattern with the vacuum present. Although vacuum investing ndoes not remove all the air from the investment and the ring, the amount of air nis usually reduced enough to obtain a smooth adaptation of the investment to nthe pattern. Vacuum investing often yields castings with improved surfaces whecompared with castings produced from hand-invested patterns. The degree of ndifference between the two procedures depends largely on the care used in hand ninvesting. Whether hand- or vacuum-investing procedures are used in filling the ncasting ring, the investment should be allowed to harden in air before burnout nof the wax.

 Single step ninvesting technique:

 The ninvesting procedure is carried out in one step either by brush technique or by nvacuum technique.

 a). nBrush technique:

 The naccurate water-powder ratio is mixedunder vacuum. A brush is then used to npaintthe wax pattern with mix then the casting ringis applied over the crucible nfromer and thering is filled under vibration until it iscompletely filled.

 

 b). nvacuum technique:

 • nThe mix in first hand spatulated, and then withthe crucible former and patteris place, then ring is attached to the mixing bowl.

 • nThe vacuum hose is then attached to theassembly. The bowel is inverted and the nring isfilled under vacuum and vibration

 Two-step ninvesting technigue:

 The ninvesting procedure is carried out in twosteps:

 • nFirst, the wax pattern is painted with a thick mix andis left till complete nsetting, the set investment block(first cost) is immersed in water for about ntenminutes . the casting ring is then applied over the crucible former and nfilled with the properly mixedinvestment (second coat) till the ring is ncompletely filled and the mix is left to set.The two-step investing technique nis recommendedwhenever greater amount of expansion is required. Thistechnique nalso minimizes the distortion of the waxpattern and provides castings with nsmoother surfaces.

 • nThe investment is allowed to set for the recommendedtime (usually one-hour) nthen the crucible former isremoved. If a metal sprue former is used, it is nremovedby heating over a flame to loosen it from the waxpattern. Any loose nparticles of investment should beblown off with compressed air should be placed nin ahumidor if stored overnight.

 Wax nelimination (burnout):

 Wax nelimination or burnout consists of heating the investment in a thermostatically ncontrolled furnace until all traces of the wax are vaporized in order to obtaian empty mold ready to receive the molten alloy during procedure.

 

 • nThe ring is placed in the furnace with the sprue hole facing down to allow for nthe escape of the molten wax out freely by the effect of gravity .

 • nThe temperature reached by the investment determines thethermal expansion. The nburnout temperature is slowly increased in order to eliminate the wax and water nwithout cracking the investment.

 •For ngypsum bonded investment, the mold is heated to650 -6870 c )to cast precious nand semiprecious

 precious nalloys.

 • nWhereas for phosphate-bonded investment, the mold is heated up to 8340 c to ncast nonprecious alloys at high fusing temperature.

 The nring should be maintained long enough at the maximum temperature (“heat soak”) nto minimize a sudden drop in temperature upon removal from the oven. Such a ndrop could result in an incomplete casting because of excessively rapid nsolidification of thealloy as it enters the mold.

 • nWhen transferring the casting ring to casting, a quick visual check of the sprue nin shaded light is helpful to see whether it is properly heated. It should be a ncherry-red color .

 CASTING

 Melting n& Casting Technique Melting & Casting requires Heat source to melt the nalloy Casting force, to drive the alloy into the mould

 

 Casting nTorch Selection Two type of torch tips: Multi-orifice Single-orifice nMulti-orifice tip is widely used for metal ceramic alloys. Main advantage is ndistribution of heat over wide area for uniform heating of the alloy. nSingle-orifice tip concentrate more heat in one area.Three fuel sources are nused for Casting Torch; Acetylene ,Natural Gas ,Propane

 CASTING nCRUCIBLES

 Four ntypes are available ;

 1) nClay .

 2) nCarbon .

 3) nQuartz .

 4) nZirconia –Alumina .

 Casting nMachines

 It nis a device which uses heat source to melt the alloy casting force .

 Heat nsources can be :

 1) nReducing flame of a torch .

 ( nconventional alloys & metal ceramic alloys )

 2) nElectricity .(Base metal alloys )

 Advantages nof electric heating :

 -heating nis evenly controlled .

 -minimal nundesirable changes in the alloy composition .

 – nAppropriate for large labs .

 Disadv n:

 Expensive n.

 Casting nmachines use :

 1) nAir pressure .

 2) nCentrifugal force .

 3) nEvacuation technique .

 Alloys ncan be melted by :

 1) nAlloy is melted in a separate crucible by a torch flame & is cast into the nmold by centrifugal force .(centrifugal C M )

 2) nAlloy is melted by resistance heating or by induction furnace & then cast ncentrifugally by motor or spring action (springwound CM electrical resistance )

 3) nAlloy is melted by induction heating cast into mold centrifugally by motor or nspring action .(Induction CM )

 4) nAlloy is vacum melted by an argon atmosphere

 Torch nmelting / Centrifugal casting machine

 Electrical nresistance /Heated casting machine

 Melting nof the alloy should be done in a graphite or ceramic crucible .

 Adv n:

 -Oxidatioof metal ceramic restorations on

 overheating nis prevented .

 -Help nin solidification from tip of the casting to the button surface .

 Inductiocasting machine

 Commonly nused for melting base metal alloys.

 Adv n:

 – nHighly efficient .

 – nCompact machine withlow power consumption

 -No npre heating needed ,

 – nsafe & reliable.

 Direct ncurrent arc melting machine

 A ndirect current arc is produced between two electrodes :

 The nalloy & the water cooled tungsten electrode .Temp used is 4000 degrees .

 Disadv n:

 High nrisk of overheating the alloy .

 Vacuum nor pressure assisted casting machine

 Moltealloy is drawn into the evacuated mold by gravity or vacuum & subjected to naditional pressure

 For nTitanium & its alloys vacuum heated argon pressure casting machines are nused .

 Accelerated ncasting method

 This nmethod reduces the time of both bench set of the investment & burnout .

 Uses nphosphate bonded investments which uses 15 mnts for bench set & 15mnts for nburnout by placing in a pre – heated furnace to 815 degrees .

 Effect nof burnout on gypsum bonded investments

 Rate nof heating has influence on smoothness & on overall dimensions of the ninvestment

 Rapid nheating causes cracking & flaking which can cause fins or spines .

 Avoid nheating gypsum bonded investment above 700 degrees .Complete the wax nelimination below that temp .

 Effect nof burnout on phosphate bonded investments

 Usual nburnout temp is 750 -1030 degrees.

 Although nthey are strong they are brittle too .

 Since nthe entire process takes a long time two stage burnout & plastic ring cabe used .CLEANING AND PICKLING ALLOYS

 The nsurface oxidation or other contamination of dental alloys is a troublesome noccurrence. The oxidation of base metals in most alloys can be kept to a nminimum or avoided by using a properly adjusted method of heating the alloy and na suitable amount of flux when melting the alloy . Despite these precautions, nas the hot metal enters the mold, certain alloys tend to become contaminated othe surface by combining with the hot mold gases, reacting with investment ningredients, or physically including mold particles in the metal surface. The nsurface of most cast, soldered, or otherwise heated metal dental appliances is ncleaned by warming the structure in suitable solutions, mechanical polishing, nor other treatment of the alloy to restore the normal surface condition.

 Surface ntarnish or oxidation can be removed by the process of pickling. Castings of nnoble or high-noble metal may be cleaned in this manner by warming them in a n50% sulfuric acid and water solution . . After casting, the alloy (with sprue nattached) is placed into the warmed pickling solution for a few seconds. The npickling solution will reduce oxides that have formed during casting. However, npickling will not eliminate a dark color caused by carbon deposition The effect nof the solution can be seen by comparing the submerged surfaces to those nthat have still not contacted the solution. the ordinary inorganic acid nsolutions and do not release poisonous gases on boiling (as sulfuric acid ndoes). In either case, the casting to be cleaned is placed in a suitable nporcelain beaker with the pickling solution and warmed gently, but short of the nboiling point. After a few moments of heating, the alloy surface normally nbecomes bright as the oxides are reduced. When the heating is completed, the nacid may be poured from the beaker into the original storage container and the ncasting is thoroughly rinsed with water. Periodically, the pickling solutioshould be replaced with fresh solution to avoid excessive contamination.

 Precautions nto be taken while pickling

 With nthe diversity of compositions of casting alloys available today, it is prudent nto follow the manufacturer’s instructions for pickling precisely, as all npickling solutions may not be compatible with all alloys. Furthermore, the npractice of dropping a red-hot casting into the pickling solution should nbeavoided. This practice may alter the phase structure of the alloy or warp nthin castings, and splashing acid may be dangerous to the operator. Finally, nsteel or stainless steel tweezers should not be used to remove castings nfrom the pickling solutions. The pickling solution may dissolve the tweezers nand plate the component metals onto the casting. Rubber-coated or Teflotweezers are recommended for this purpose.

 FLUXING

 To nprevent oxidation of gold alloys during melting always use a reducing flux .

 Boric nacid & borax are used .

 Casting nof glass or ceramic

 A ncastable ceramic is prepared in a similar manner as metal cast preparation .

 Glass nis heated to 1360 degrees & then cast.

 Phosphate nbonded investments are used for this purpose .

 CASTING nDEFECTS

 Classificatio(combe ):

 1) nDistortion.

 2) nSurface roughness .

 3) nPorosity .

 4)Incomplete ncasting .

 5) nOxidation .

 6) nSulfur contamination .

 Distortion

 It nis usually due to the distortion of wax pattern.

 To navoid this :

 Manipulatioof the wax at its softening temp

 Invest nthe pattern at the earliest .

 If nstorage is necessary store it in a refrigerator .

 Surface nroughness

 May nbe due to :

 Air nbubbles on the wax pattern .

 Cracks ndue to rapid heating of the investment .

 High nW/P ratio .

 Prolonged nheating of the mold cavity .

 Overheating nof the gold alloy .

 Too nhigh or too low casting pressure .

 Compositioof the investment .

 Foreigbody inclusion.

 

 POROSITY

 May nbe internal or external .

 External nporosity causes discolouration .

 Internal nporosity weakens the restoration .

 Classificatioof porosity .

 I n.Those caused by solidification shrinkage :

 a) nLocalised shrinkage porosity .

 b) nSuck back porosity .

 c) nMicroporosity .

 They nare usually irregular in shape .

 II n) Those caused by gas :

 a) nPin hole porosity .

 b) nGas inclusions .

 c) nSubsurface porosity .

 Usually nthey are spherical in shape .

 III n) Those caused by air trapped in the mold :

 Back npressure porosity .

 Localised nshrinkage porosity

 Large nirregular voids found near sprue casting junction.

 Occurs nwhen cooling sequence is incorrect .

 If nthe sprue solidifies before the rest of the casting , no more molten metal is nsupplied from the sprue which can cause voids or pits

 (shrink npot porosity )

 This ncan be avoided by –

 – nusing asprue of correct thickness .

 – nAttach the sprue to the thickest portion of the pattern .

 -Flaring nof the sprue at the point of atttachment .

 -Placing na reservoir close to the pattern .

 Suck nback porosity

 It nis an external void seen in the inside of a crown opposite the sprue .

 Hot nspot is created which freezes last .

 It nis avoided by :

 Reducing nthe temp difference between the mold & molten alloy .

 Microporosity n:

 Fine nirregular voids within the casting .

 Occurs nwhen casting freezes rapidly .

 Also nwhen mold or casting temp is too low .

 Pihole porosity :

 Uposolidification the dissolved gases are expelled from the metal causing tiny nvoids .

 Pt n& Pd absorb Hydrogen .

 Cu n& Ag absorb oxygen .

 Gas ninclusion porosities

 Larger nthan pin hole porosities .

 May nbe due to dissolved gases or due to gases Carried in or trapped by molten metal n.

 Apoorly nadjusted blow torech can also occlude gases .

 Back npressure porosity

 This nis caused by inadequate venting of the mold .The sprue pattern length should be nadjusted so that there is not more than ¼” thickness of the ninvestmentbetween the bottom of the casting .

 This ncan be prevented by :

 – nusing adequate casting force .

 -use ninvestment of adequate porosity .

 -place nthe patterot more than 6-8 mm away from tne end of the casting .

 Casting nwith gas blow holes

 This nis due to any wax residue in the mold .

 To neliminate this the burnout should be done with the sprue hol facing downwards nfor the wax pattern to run down.

 Incomplete ncasting

 This nis due to :

 – ninsufficient alloy .

 -Alloy nnot able to enter thin parts of the mold .

 -Whethe mold is not heated to the casting temp .

 -Premature nsolidification of the alloy .

 -sprues nblocked with foreign bodies .

 -Back npressure of gases .

 -low ncasting pressure .

 -Alloy nnot sufficiently molten .

 

 Too nbright & shiny casting with short & rounded margins :

 occurs nwhen wax is eliminated completely ,it combines with oxygen or air to form ncarbon monoxide .

 Small ncasting :

 occurs nwhen proper expansion is not obtained & due to the shrinkage of the nimpression .

 Contaminatioof the casting

 1) nDue to overheating there is oxidation of metal .

 2) nUse of oxidising zone of the flame .

 3) nFailure to use a flux .

 4) nDue to formation sulfur compounds .

 Black ncasting

 It nis due to :

 1) nOverheating of the investment .

 2) nIncomplete elimination of the wax .

Preparing nthe Tooth
nIf you need a crown, you nmay also need endodontic or root canal ntreatment on the tooth. Such treatment may be recommended if you have a lot of ndecay in the tooth. This increases the risk of infection or injury to the ntooth’s pulp. nNot everyone who needs a crown will also need a root ncanal.

Before placing a crown, your dentist may need to build up a nfoundation to support it. A foundation is needed if large areas of the tooth nare decayed, damaged or missing. If you are receiving the crown after root ncanal treatment, your dentist may insert a post-and-core foundation.

To place a crown, your dentist will file down the tooth to nmake room for the crown. If you are receiving an all-metal crown, less of the ntooth needs to be removed because these crowns can be made thinner than PFM or nceramic ones.

After filing down the tooth, there are two ways to make a npermanent crown. Most crowns require two visits to the dentist. You receive a temporary ncrown at the first visit and wear it while your permanent crown is nmade. Some dentists have a machine that makes a crown in one visit.

Tooth 12 is ncovered with temporary crown

With the traditional approach to making a crown, your dentist nfirst will use a piece of thread or cord to push the gum away from the tooth. nThen the dentist will make an impression n(copy) of the tooth using a rubber-like material. The impression material sets nin about five minutes. Then it is removed.

Your dentist will also take an impression of the teeth above nor below the tooth that will receive the crown. The purpose is to make sure the ncrown will fit into your normal bite.

The impressions are sent to the lab, where the crown is made. nDuring that time, you will have a temporary crown placed. These crowns are nusually made of plastic. They can be made in advance by the laboratory or made nby the dentist during your preparation visit. Then the dentist fits the ntemporary crown to your tooth.

These crowns are not meant to last for a long time. In some ncases, however, a temporary crown can stay in place for a year or longer. If it nneeds to last longer, a lab-made plastic crown is best. It is stronger and will nlast longer than a temporary plastic crown that is made by the dentist.

Temporary cement is used to keep this crown in place. It is nspecial cement that is designed to be weak. This allows your dentist to easily nremove the temporary crown so your permanent crown can be placed.

At a second visit, your dentist will remove the temporary ncrown and test the permanent one. Sometimes crowns need more polishing or nglazing or some other adjustment before they are placed. Once the crown is nready, it is permanently cemented on your tooth.

If your dentist owns a Cerac or CAD-CAM unit, a crown can be nmade in one visit. First, a 3D image of the prepared tooth is fed into the nunit. Then a computer makes the crown from a block of porcelain. The dentist nplaces the crown during the same visit.

 

Indications

1. The nonly option remaining.

2. nUsually reserved for molar and lower premolar teeth.

3. nFlame-shaped diamond

4. nExcavator

 

Procedure

If the nproblem is such that a partial crown will not function, the only option is the nfull crown restoration, and it is the treatment court of last appeal. Full ncrowns are most frequently placed on molar teeth and occasionally on lower nbicuspids.

The nsituations that require a complete crown include teeth that have exten­sive but nquestionable restorations or extensive carious involvement. A significant ncontributor to this difficulty is a large defective Class V amalgam nrestoration, and the only way a casting will perform is to make a complete ncrown. Another situation requiring this type of crown is presented by teeth nthat are very short and do not provide resistance unless a full crown is nutilized.

Many ntimes the locations of the gingival margins are predetermined by the existing nrestoration or caries and decalcification that must be enclosed. This oftedictates that these margins, with emphasis on the interproximal and facial, will nbe in the gingival sulcus. If adequate resistance for the restoration and effec­tive ncoverage can be met without placing the margin in the sulcus, that is the rec­ommendation. nMany times the lingual surface will be intact and it is easy to ter­minate the nmargin above the level of the gingiva, which eases the maintenance problems of nthe marginal gingiva.

The ninitial procedure is the occlusal reduction. The reduction may be done using a nwheel diamond or a tapered round-end diamond. The same amount of reduction is nrequired as for other posterior teeth, with a minimum of 1 mm wherever functiooccurs. The occlusal reduction should show correlation with occlusal morphology nof the tooth and not be an arbitrary flattening of the occlusal surface.

Followin;g nthe occlusal preparation, proceed to the buccal and lingual reduction. A ntapered round-end diamond is useful for this purpose. The gingival finish line nwill have been predetermined by the needs of retention, defective restorations, nor caries. The reduction should reflect a chamfer effect at the gingi­val ncavosurface. The buccal and lingual morphology is observed, and the reduc­tiois done to produce an even thickness of metal in the completed restoratioconsistent with the shape of the tooth.

The nbuccal and lingual walls are structured to provide good resistance to ndisplacement. There is a danger of over-angulating the walls and losing the npotential resistance for the casting. This reduction is carried toward the nadjacent teeth. To avoid overcutting at the interproximal, a thin flame-shaped ndiamond is selected.

The nfinish line must be located gingival to the contact and be on healthy enamel. nIf an amalgam core or restoration  is part of the prepa­ration, it is nexpected that the finish line for the casting will terminate gingival to the nrestoration. The proximal walls must relate to each other in the same man­ner nas to the buccal and lingual to provide optimal resistance. They will be nslightly inclined toward each other.

This nshould complete most of the reduction, and now with a fine grit fin­ishing ndiamond or a No. 1170 bur the preparation is made smooth. All sharp corners or nangles are slightly rounded. For some of the smoothing it is best that the bur nrun at a reduced speed to avoid excess cutting.

The nocclusal clearance is checked in the same manner as discussed for pre­vious nposterior cast preprations.

Consideratiomust be given to clearance when the patient goes through eccentric movements, nand the preparation must have adequate reduction to allow the restoration to nhave a acceptable function in all movements. This information must be placed ninto a suitable articulator to allow for a reliable wax-up and casting.

The nfinal segment of the preparation is the placement of a groove on the buccal nsurface, and usually this will be in the anatomical buccal groove. This groove nis of value, as it helps to provide positive orientation during the placement nof the casting. It will tend to limit the potential for slight rotation of the ncrown during seating, as the groove will guide or key it into the specific nplacement. At times it will prove of value by providing a degree of nsupplemental retention, which may be helpful to a restoration with marginal nretention. On occasion when the buccal segment of the tooth is badly destroyed nthis groove may be placed on the lingual portion of the preparation. If the npreparation  requires a pin-supported amalgam or resin buildup, the groove nmay be placed m the amalgam or resin.

This ngroove may be formed by using a thin, tapered round-end diamond, or a No. 171 nbur may be used effectively for this purpose. If the bur is used, it is advised nto use it at reduced speed to avoid overcutting. This groove extends just short nof the gingival finish line.

Resume of Partial and Full Crown Preparation

1. Gingival nfinish line when possible should be located occlusal to the gingiva.

2. nGrooves with adequate length are preferred over proximal boxes for resistance nform.

3. Do nnot terminate a finish line through a wear facet.

4. nOcclusal reduction should occur first using wheel or football-shaped diamond. nWith a wax bite check for a minimum clearance of 1mm.

5. nLingual reduction with bullet-nose diamond 2D-T.

6. nInterproximal reduction with thin or flamed-shaped diamond to avoid cutting nadjacent tooth.

7. nProximal grooves are located for maximum length and will line up toward the nbuccal cusp tip.

8. nPlace buccal bevel toward facial to complement esthetics and function.

9. A nV-shaped connector may be used between grooves.

10. With na seven-eighths crown one groove is located on the facial surface.

11. For na full crown preparation the facial reduction is done with the same instrument nas for the lingual. A definite chamfer is needed for the buccal finish line.

12. nPlace a groove on the facial surface of the preparation for positive seating nand use of tapered diamond or a No. 171 bur.

 

 

Dental nMetalcasting

The Setup

 For nthe level of detail and precisioecessary for dental metalwork, we use the nlost wax method. Once a crown pattern has been waxed, we sprue it with a length nof wax cut from a spool. The attachment is done by carefully heating the sprue nand sticking it to the pattern. Obviously, insufficient heat will produce a nweak bond, and excessive heat will warp the wax pattern. Often, the pattern end nof the sprue is pinched down to a slightly smaller diameter. This tapering nincreases the velocity of the metal flowing into the pattern. I cannot explaithe physics of this, or tell you why the increased velocity helps the final nproduct; I only know that it does. The other end of the sprue is attached to nthe base of the ring mold. In contrast, this attachment should be tapered out nslightly to allow for a smooth entrance for the metal. If venting is necessary, na length of spruing wax is attached at the relevant location on the pattern, nand ran back to the base former end. With a centrifugal casting machine and ndental investment, venting is almost never necessary (the stuff is expensive nfor a reason). For large crowns or bridges, a reservoir (a plastic or wax ball n”interrupting” the sprue, and placed close to the pattern) is nsometimes added to eliminate shrinkage and porosity. This is usually not nneeded, as the base former creates a large cavity in the investment ring, and nthus a natural reservoir. Incidentally, the metal left in this cavity is called na button.

The Molding nProcess

 

 The nmold material we use is called investment. I tried to find some information othe chemical make up of dental investment, but I was unable to find anything. nThe investment we use comes from Microstar Corporation. As I have already nstated, its not cheap, but it is excellent material if precision and fine ndetail are high priorities.

 The ninvestment is mixed with water and a special liquid that is sold separately. The nratio of water and this special liquid depends on how “tight” or n”lose” you want your investment. This sacred knowledge belongs to the nwaxers, which I am not, so if you want to uncover those secrets, I’m afraid you nmust either experiment yourself, or find a waxer in a dental lab and appease ntheir wishes for them to part with the ways of their ancient art. Once mixed, nthe investment is vaccuum mixed to remove air bubbles. While the investment is nmixing, we spray a small amount of debubblizer on the patterns, and blow off nthe excess. If too much debubblizer is left on the pattern, the investment will nbe diluted in that area, and create a weak spot. Debubblizer is a liquid that nreduces the angle of contact when flowing a thick liquid into a mold, or onto a npattern. This makes the investment slide across the pattern, instead of rolling nacross it and trapping air pockets between the pattern and the investment nmaterial. I do not know the chemical content of debubblizer, but it feels and nacts like slightly soapy water. You then carefully pour the investment into the nformer. Patience, practice, and keeping your eyes on what you are doing help to nget a good pour.

Burnout of the Wax

 Now nwe set a timer to tell us when the investment is near the hottest stage of its setup nprocess. This information is different with each type of investment, and is ngiven on the package. Basically, this is after the investment is hard, but well nbefore being fully cured. When this point is reached, the ring is removed from nthe former. The non-gated end is scraped well, as although the investment is ndesigned to be finely porous, any surfaces that have been formed against a nsmooth surface will also be smooth, and thus nearly air-tight. Scraping removes nthis smooth surface layer, allowing gases to escape freely once we are casting. nThe ring is then placed in an oven and left until the wax has burned completely nout, and the ring has reached the proper temperature for the type of metal that nwill be cast. It is important to act quickly, for the ring should still be as nhot as possible when placed in the oven to avoid temperature shock. This is nonly necessary when speed is a factor, and as in all places of work, time is nmoney. If speed is not a factor, you can place a cold ring in a cold oven and heat nit slowly.

 The ndevice we use for casting is called a broken arm centifugal casting machine. nPlease excuse the over simplified imagary, but I couldn’t get my hands on a ncamera, and I was already taking too long getting this article together, so I nslapped these basic diagrams together:

Casting the Crown

 

 Once nthe burnout is complete, and the ring is properly heated, we wind the casting nmachine and set the retaining pin. We then “break” the caster’s arm. nThe caster arm has a pivoting joint just before the crucible and cradle. nRotating the “wrist” as I call it, in the opposite direction from how nthe arm will spin, adds extra force to the initial swing, and this makes all nthe difference in the world. The ingot(s) are then placed in the crucible ready nto melt. We use a different crucible for each type of metal, so that left over ntraces of metal left in the crucible from previous castings do not contaminate nour end product. We do keep our crucibles clean, but having a crucible for nevery type of metal is prudent, and prudence throughout our process is what nkeeps our dentists happy, and our lab in business!

 We nput the torch to the metal, careful to get an even melt. Most metals will begito pool naturally, but some non-precious metals we use actually maintain their nshape when heated to their melting temperature. For these metals, we tap the ncrucible occasionally, watching for the “shell” to break, and the nmetal to pool. Once the metal pools, we back the torch off a little to avoid noverheating, as we do not wish to evaporate important elements from the metal. nThe ring is then taken out of the oven and placed in the cradle. The crucible nis slid forward to meet the ring, the pin is dropped, the torch is taken away, nand… woosh! Now for the most part, all the metal flows into the ring. But nsometimes, a little metal does not make it into the ring because of the n”jerk” of the arm straitening out. This is not a big problem, but whecasting gold we follow the arm with our hand for a split second before complete nrelease in order to dampen the sudden jerk. Over time, a drop of gold here and nthere add up, and since gold casts very easily, we lose nothing by slowing the nvery beginning of the acceleration.

Complications of na Tooth Crown

Significance of Crown Discomfort

Sensitivity naround a crown is common and to be expected following a new procedure. Most npatients report a degree of hot and cold sensitivity following new crowplacement. Temporary crowns are more prone to thermal sensitivity due to the nfact that they fit less tightly and completely than permanent crowns do. A npatient may often wait a few weeks to find that sensitivity has all but ndisappeared. In severe or prolonged cases, however, a dentist should be ncontacted with the concern.

Common Problems nWith Food

A npotential complication occurs when a bit of food, such as candy or a popcorkernel, becomes jammed between a dental crown and existing teeth. Soft tissues ncan become damaged in these instances, causing further complications. nToothpicks or dental floss may be used to attempt to free objects trapped nagainst a crown. Over-the-counter numbing agents such as Anbesol may help to nrelieve pain. Once again, it is imperative that if pain persists, a qualified ndentist be contacted for an opinion.

 

Use nTime to Heal Gum Discomfort

The nedge of most dental crowns ends right at the gum line or a tiny bit below. nHence, sensitivity in the gums will usually occur just following the fitting of na dental crown. Swollen gums can be treated with aspirin, and rinsing the mouth nwith saltwater several times daily can assist greatly in alleviating paiassociated with a new crown. The crown making process has a tendency to cause ngums to swell, so time and care can often solve the problem naturally.

Emergency nSituations

Loose ncrowns occur relatively frequently in temporary crowns and it is also commofor permanent crowns to become loose over time. The affected area should be ncleaned as well as possible with toothpaste or floss, and a temporary fixative nmay be applied to keep the crown in place until an office visit. Due to the nextreme pain and discomfort you might feel, this issue should be dealt with by na dentist as soon as possible.

Considerations

If, nafter time, any type of pain in the dental crown persists, a dentist should be ncontacted to discuss options with a patient. Analgesics, like ibuprofen or naspirin, are often recommended to remedy temporary discomfort. Ointments and nfixatives are made especially for people experiencing complications from new ndental crowns, and these can be bought relatively inexpensively over the ncounter. You should gauge for yourself whether the pain is too much to endure nbeyond a brief period of time. A crown becomes buffered and finished as you nbite down on things and use your teeth, so oftentimes allowing the mouth to get nused to the crown is necessary before a dentist can make further adjustments.

Tell if tooth ncrown is not fitted properly

1.    nCheck to see if your crown is loose. When fitted properly, your crowshould be cemented, or bonded, over your natural tooth. This is to prevent the nweak tooth from further damage. However, wheot properly fitted, you will nnotice that the crown feels loose. Or it may even fall out. This can occur if ncrown doesn’t have enough support from your natural tooth to maintain a proper nfit after it’s been bonded. A loose-fitted dental crown can be the result of nimproper construction at the dental laboratory or inadequate preparation of your ntooth. Another reason can be because of additional decay after the crown was nconstructed.

2.    nCheck your bite. If your dental crown does not properly fit, you may notice nthat the crown’s shape is incorrect. This is known as a dental fracture. For ninstance, when you bite down your dental crown may be higher than the rest of nyour teeth. Or when you chew, your crowned tooth makes contact with the food nbefore your natural teeth. Thus, your dental crown’s bite may seem off or out nof alignment with your other teeth. Your dentist may have to buff or reshape nyour dental crown so the shape is more in sync with your natural bite. This nproblem is caused if your dentist misjudged your bite before your crown’s nspecifications were sent to the laboratory or by improper construction such a ndental laboratory processing error.

3.    nCheck for space. Feel the edge of your dental crown. It should fit properly nwith no gaps or space. However, if not properly fitted, you will feel an opespace between your dental crown and the tooth. In addition to an improper fit, na space or gap between the edge of your dental crown and your tooth is a prime narea for cavities or further tooth damage.

You caalso have a problem with proper fit if your dental crown is too bulk or slim. nSo you may know that the crown is improperly fitted to your natural tooth if it ntraps food between teeth or irritates the inside of your cheek.

I) nPermanent dental crowns: Common problems, pains, sensitivities.

 

Listed nbelow are some of the types of complications that a person might experience nwith a tooth that has had a permanent crown placed. If you do notice any of nthese or any other symptoms, you should let your dentist know sooner rather nthan later, so they can evaluate them and make a treatment recommendation in a ntimely fashion.

A) nProblems that cause tooth pain.

 

The nsame events and circumstances that can lead to the need for a dental crow(such as tooth fracture, tooth decay or a lost filling) can also have a ndetrimental effect on a tooth in other ways. And for this reason, even a tooth nthat previously seemed just fine, either before the crowning process was beguor even many years after, may begin to display some type of symptoms.

 

While nthe timing of the symptoms may seem suspicious, they’re most likely aindication that all was not well with the tooth before the crowning process was nbegun, and, unfortunately, the full extent of these problems could not be ndefinitively identified beforehand.

Commoproblems.

 

For nexample, sometimes after the crowning process has been begun, or even long nafter it’s been completed, a problem with a tooth’s nerve becomes apparent, nthus creating the need for root canal therapy. (We describe the relationship nbetween dental crowns and the need for root canal treatment  in greater ndetail here.)

 

These nteeth may just be a little tender to biting pressure or, at the other extreme, ninvolve severe pain that lasts for hours. The discomfort may be triggered by a nstimulus (hot things tend to set these teeth off, and the pain typically nlingers), or else it occurs spontaneously. Often it has a throbbing, sometimes na radiating, nature.

 

With nother cases, it’s possible that a fractured tooth has cracked seriously enough nthat even a crown cannot securely splint its broken parts together. Teeth nhaving this complication will continue to be painful in response to biting npressure, even after having a crown placed.

 

Neither nof these situations are ones that your dentist can diagnose or anticipate with ncertainty. If you have symptoms develop or persist, all you can do is report nthem to your dentist so they can evaluate your status and make a treatment nrecommendation.

B) nCrowned teeth whose bite doesn’t seem right.

 

Your ndentist will evaluate the way your new crown comes together with its opposing nteeth before they cement it in place. Even so, you may find (especially after nyour numbness has worn off) that your crown’s shape is still not quite right.

 

You may nfind that your crowned tooth touches first when you bring your teeth together. nOr maybe as you slide your teeth from side to side, you can feel some aspect of nthe crown seems too prominent (is too “high”).

 

This type nof problem is usually an easy fix for your dentist. They simply need to buff nyour crown down, so its shape is more in harmony with your bite. Don’t expect nthis type of problem to take care of itself, however, because it won’t. Ifact, if this condition is not remedied in a timely fashion, it can lead to nserious consequences, such as a need for root canal treatment.

 

C) nCrowned teeth that are sensitive to hot and cold.

 

After a ndental crown has been cemented into place, a person may notice that their tooth nis sensitive to hot and cold foods and beverages. The location of this sensitivity nis usually right at the edge of the crown, down by the gum line.

 

As aexplanation, it might be that the crown doesn’t cover over the tooth quite as nfar as it ideally should (possibly on a scale so small you can’t even visualize nit). It’s this “exposed” surface that responds vigorously to hot and ncold stimuli.

 

This ntype of problem may have an easy fix. The cure might be as simple as using ntoothpaste that’s marketed as a treatment for “sensitive teeth” (the nactive ingredient in these products is usually potassium nitrate). Or, the ndentist may apply a solution to the tooth that helps to protect (and therefore ndesensitize) the exposed surface.

Castable nmetal alloys used in dentistry

Their nhistory, the types, their uses and their toxicities

Metal nCastings

Metal Castings are made by fabricating a hollow mold, pouring a nmolten metal into it, allowing the metal to solidify and separating the now nsolid metal casting from the mold.  Ultimately, all metallic nobjects originate from castings.  In dentistry, metal castings are used to restore teeth, nreplace teeth, and as frameworks for removable partial dentures.  Today, nmetal castings are also used as metal frameworks to support porcelain crowns or nfixed partial dentures in order to produce strong, and yet very nesthetic restorations.

The history of the lost nwax technique in dentistry

The lost wax technique was probably invented in ancient China nor Egypt.  The technique consists of  carving a wax replica of aitem that is to be duplicated in gold.  The wax is invested (imbedded) iplaster or clay and burned out leaving an image (hole) where the wax used to nbe.  Then the image is filled with molten gold through a small hole in the ninvestment.  This technique works quite nicely for fairly large castings, nbut gravity alone is not sufficient to draw gold into the very fine detail nnecessary to fabricate a tiny filling for a tooth.

Prior to 1855, dentistry consisted mostly of nextracting decayed and abscessed teeth and replacing them with some sort of removable ndenture.  Silver amalgam, made from shaved silver coins mixed with nmercury, was invented in France in 1819, but was an unreliable filling nmaterial  due to the haphazard way it was formulated.   While nitinerant entrepreneurs traveled the countryside plugging amalgam into decayed nteeth, most reputable dentists refused to use it.  Gold leaf was first nused to fill teeth in about 1483 by Giovanni d’Arcoli, but the technique was nextremely tedious and expensive and only the most wealthy and determined patients ncould afford and withstand having their decayed teeth repaired this nway.  

The cohesive gold foil technique was perfected nand codified in 1855.   It was much less tedious and less expensive nthan using gold leaf and made restoration of decayed teeth a real option for a nwider swath of consumers.  The gold foil technique consists of a rather nlaborious and still fairly expensive process involving hammering tiny pieces of npure (cohesive) gold foil into an equally laboriously prepared cavity preparation.  nOnly reasonably affluent people could afford this sort of dentistry, but it was nreliable, and gold foil became the industry standard for repairing damaged nteeth.  In 1895, G.V. Black standardized a reliable and safe formula for dental amalgam .  nThis made it possible for the average (non wealthy) person to save a decayed ntooth rather than having to extract it.  Unfortunately, not all dentists nwere on board with the mercury, so many remained wedded to the gold foil ntechnique. 

In 1907, William H. Taggart invented a ncentrifugal casting machine for use with the lost wax technique.  With ncentrifugal force replacing gravity as a method of filling the casting image ninside an investment, it became possible to cast small, highly detailed nobjects.  He worked up the procedures for the technique and patented nit,  however he eventually lost the patent when it was discovered that a nDr. Philbrook of Denison, Iowa had published a paper on the subject twenty five nyears before.   Taggart’s procedure involved carving a wax patterdirectly inside of a patient’s open mouth.  Today, a dentist takes aimpression and sends it to a dental laboratory.  The lost wax technique is nexplained below:

The Lost Wax Technique

A dentist laboriously drills out the decay from a ntooth and refines the shape of the preparation, being careful that there are no nundercuts which might interfere with an unrestricted path of withdrawal. 

The dentist then takes an impression of the prepared ntooth.  This impression is then sent to the lab for fabrication of the nrestoration.  The images presented here show how a gold crown is nfabricated in the laboratory. 

After pouring the impression with a fine plaster ncalled dental stone, the die (the plaster model of the prepared tooth) is thecovered with wax and then carved into the appropriate tooth shape.

Then, a sprue (simply a small wax rod) is attached nto the wax replica.  In the image below, the sprue is the green extensiofrom the crown down toward the casting ring cap at the bottom.  The bulb nin the sprue serves as a reservoir for the gold to help equalize the pressure nof the liquid gold so it flows evenly into each wax pattern

The casting ring cap is then fitted over the ncasting ring.  The casting ring serves as a container to hold the plaster n(actually in improved investing plaster) which is then flowed around the wax npatterns.

Once the investment has set, the casting ring cap nis removed leaving the sprues sticking up out of the now hard investment.  nThe cylinder, with its invested wax, is placed in a very hot oven.  Whethe wax burns away, the plaster in the ring then contains a space in the shape nof the original wax filling (actually a hollow three dimensional image of the filling with attached sprue).

The image is then filled with molten gold using a ncentrifugal casting machine. 

By immersing the still hot plaster with its gold ninnards in water, the plaster would shatter away leaving behind the casting nwhich includes the gold filling and the attached sprue.

After removing the sprue, the gold casting was npolished up and cemented into the original cavity preparation in the tooth.

This technique works equally well for fillings iteeth as well as full gold crowns.  When a casting does not replace the ncusp of a tooth, it is called an inlay:

When a casting replaces one or more cusps on a ntooth it is called an onlay:

Taggart’s centrifugal casting machine made it possible nto apply enough “gravity” to force the molten gold into the tiny ninvested image of a filling.  The picture on the left shows a moderversion of Taggart’s invention.  The white piece with the hole in it is a nsmall crucible used to melt the gold alloy with a gas nand forced air torch.  The burned out image (originally invested in a metal ncylinder, called a casting nring), is placed behind the hole in the crucible.  The orange nstand contains a spring which has been wound several times in preparation for the ncasting operation.  Once the image is in place and the alloy has beemelted, the technician allows the locking pin that sticks up on the left side nof the base to drop.  This releases the armature, and when the technicialets the armature loose, the armature, along with the crucible and its attached ncasting ring spins at considerable speed.   The crucible apparatus nswings out so that it is facing the counterweights on the opposite side of the narmature.  Centrifugal force forces the melted alloy through the hole ithe crucible and molten metal proceeds to fill the image in the casting ring nbehind it.

Unfortunately, Taggart’s technique did not nproduce the accuracy that many dentists demanded for these small restorations, nso most dentists still resisted the introduction of cast metal restorations ifavor of gold foil or the newly improved silver amalgam, both of which always nproduced the tightest restorations possible.  Still, even in the 1910’s, nwealthy people wanted high class dentistry and were willing to pay more for the nprivilege of not having to sit around suffering while the dentist hammered gold ninto the cavity preparation.  Thus, cast gold restorations began to ncompete successfully with gold foil almost immediately, in spite of the fact nthat the castings did not fit the preparation perfectly.  Since gold was nthe metal used to make the crowns worn by kings, it suited the mentality of the nday to think of being able to afford the services of an expensive dentist as nsomething that brought a royal distinction to the patient.  Thus the term n”gold crown” was something like an advertising slogan.  The term n”crown” was used to denote any gold restoration applied to a single ntooth, including gold foil restorations, inlays and onlays.  Today, the nterm “crown” is reserved for any full coverage restoration, whether ngold or porcelain, and the terms “inlay”, “onlay” and n”filling” are used to denote restorations that cover only a part of a ntooth’s clinical crown.

In 1929, Coleman and Weinstein invented cristobalite investment to replace the plaster of Paris, neliminating most of the shrinkage and distortion problems which had plagued the nproduction of gold castings up to that point.  (Cristobalite is one of the nthree crystalline configurations of silica.  It has unique thermal nexpansion qualities which makes it especially suitable as an investment nmaterial for metal casting.)  Even cristobalite investment did not produce nperfect castings, and it was not until the 1940’s that cristobalite investment nmaterials were formulated that compensated for all of the distortions nencountered in the original lost wax technique.

Complications of a dental crown are uncommon, but any dental nprocedure involves risks and the potential complications. Complications may nbecome serious in some cases. Complications can develop during the procedure or nyour recovery.

Risks and potential ncomplications of a dental crown include:

·    nAllergic reaction to the crown 

·    nAnesthetic complications, such as nallergic reaction and nerve or blood vessel injury

·    nChipped or loosened crown

·    nInfection in the tooth

·    nInjury to the mouth

·    nLoss of crown if it falls out

·    nSensitivity to heat and cold in the naffected tooth

Reducing your risk of ncomplications

You can reduce the risk of ncertain complications by:

·    nFollowing activity, dietary and nlifestyle restrictions and recommendations before your crown procedure and nduring recovery

·    nInforming your dentist if you are nnursing or if there is any possibility that you may be pregnant

·    nNotifying your dentist immediately nof any concerns after the procedure such as pain, fever, and difficulty nchewing 

·    nTaking your medications exactly as ndirected 

·    nTelling all members of your care nteam if you have any allergies, especially metal allergies if your dentist is nusing metal crown material