laboratory steps in fixed partial denture
- 1. LAB PROCEDURES IN FIXED PARTIAL DENTURE DR. RAJ SALVI
- 2. Introduction • The fabrication of a fixed partial denture is an indirect laboratory procedure as direct fabrication in mouth is not possible. • It involves making a working (master) cast with removable dies on which the wax pattern is fabricated. • The pattern is invested and cast using the lost wax process. • The metal restoration is then veneered with resin or most commonly ceramic as required.
- 3. The lab procedures involved in fabricating a fixed partial denture involve: • Pouring working casts and dies • Fabricating wax patterns • Spruing • Investing • Casting • Veneering • Soldering
- 4. Working cast and dies • Working cast is a master cast that reproduces the entire arch including the prepared and unprepared teeth, relevant soft tissues and edentulous areas. • The die is a positive reproduction of only the prepared teeth (abutments). • Wax patterns are fabricated and the critical margins are finished on a die. • This is then placed on the articulated working cast to check the occlusion, axial contours and interproximal contacts. • Dies may be removable but are an integral part of the same working cast or they can be also separate from the cast.
- 5. Requirements Working casts • The prepared and unprepared tooth surfaces must be accurately reproduced. • The cast must be free of voids in critical areas like prepared teeth and adjacent teeth. • Should allow proper articulation with the opposing casts. • The soft tissues including the residual ridges should be recorded properly.
- 6. Dies • They must be an accurate reproduction of the prepared tooth, both in dimension and surface details, without voids. • Unprepared tooth structure cervical to finish line should also be accurately reproduced to aid in development of proper cervical contour. • The margins must be clearly visible and easily accessible. • Its form should aid easy handling during waxing and other procedures.
- 7. Materials for working casts and dies Requirements • Should be accurate. • Should be strong and resistant to abrasion. • Should be easy to section and trim. • Must be in a contrasting colour to that of the wax so that the margins can be easily visualized. • Must be compatible with the impression material. • Must be wettable by wax and should also be compatible with separating agents so that the prepared wax pattern does not stick to it.
- 8. Gypsum products The most commonly used die material is type IV gypsum also called die stone. Type V gypsum can also be used if higher expansion is required. Advantages • Easy to use and are cost effective. • Good compatibility with all impression materials. • Adequate surface detail reproduction and dimensional accuracy. Disadvantage • Poor abrasion resistance. Several methods have been used to increase abrasion resistance: • Gypsum hardeners such as aqueous colloidal silica or soluble resin solutions can be used instead of water. • The surface of the die stone can be treated with a resin such as epoxy, acrylic or cyanoacrylate (such a treatment can form an excessively thick surface layer).
- 9. Resins Epoxy resin and polyurethanes are used. Advantages • High strength and abrasion resistance. • Good detail reproduction. Disadvantages • Expensive. • Polymerization shrinkage (less of a problem with newer materials and polyurethanes). • Not compatible with polysulphides and hydrocolloids.
- 10. Electroformed dies This is the process of coating the impression by placing the impression in an electroplating bath. Pure silver or copper is deposited on the impression and a cast is then poured with type IV gypsum or resin. Advantage • Good abrasion resistance and detail reproduction. Disadvantages • Distortion may occur if not handled properly. • Time-consuming procedure • Special equipment is required. • Incompatible with polyethers • Silver electroplating uses cyanide solution which is extremely toxic.
- 11. Divestment • Used both as a die and investment material. • To make the working cast and die, it is mixed with colloidal silica. • It cannot be used with high-fusing alloys (base metal and metal ceramic) and can be used only for conventional gold alloys. Type IV gypsum is the most popular and commonly used die material for fixed prosthodontics.
- 12. Fabrication There are two methods of fabrication: 1. Working cast with separate dies 2. Working cast with removable dies
- 13. Working cast with separate dies The impression is poured twice. First, only the prepared teeth are poured with type IV gypsum for making a die and the partial cast is removed from the impression. Then a full arch cast is poured for the working cast, which is then articulated. The wax pattern is first fabricated on the die and then transferred to the working cast for checking contour, contact and occlusion. It is finally returned to the die to refine the margins.
- 14. Advantages • Simple method. • Working cast requires only minimal trimming. • Intact gingival tissues around the prepared teeth in the working cast guide proper contouring of cervical aspect. Disadvantages • May be difficult to transfer and seat delicate patterns from dies and working casts. • Distortion is a concern.
- 15. Technique • Moisture is removed from the elastomeric impressions by using a surfactant and from hydrocolloid impressions by blowing with air. • The required amount of die stone is mixed as per the manufacturer’s instructions and the prepared and adjacent sections of the cast are poured. • The stone is built up to a height of approximately 1.0 inch (2.5 cm) over the preparation. This allows bulk for an adequate handle on the die. • It is allowed to set for 1 h and the sectional cast is removed from the impression.
- 16. •The handle of the die should be octagonal in cross-section, slightly larger in diameter than the preparation and parallel to the long axis of the tooth. The ideal length of the handle should be about 1 inch. • An acrylic bur is used to trim the die below the margin and the contour near the margin can be finished using a scalpel blade. • The contour of the die apical to the finish line should be similar to natural tooth root to allow good axial contours of the finished restoration. • A separate working cast is then poured of the entire arch with base and articulated.
- 17. Handle should be octagonal and one inch in length. Die trimming influences axial contours
- 18. Working cast with removable dies In these, the die is an integral part of the working cast and can be removed to facilitate fabrication and finishing of the restoration. Advantage The wax pattern need not be removed from the die while transferring it to the working cast. This reduces the chances of distortion of the wax pattern. Disadvantage Gingival portion of the abutment is destroyed while making the die and hence it becomes difficult to verify the emergence profile of the restoration. Removable dies can be made using the following: 1. Dowel pins 2. Di-Lok trays
- 19. 1. Dowel pins Dowel pins are most commonly used to make removable dies. These are also called ‘die pins’. Definition: A metal pin used in stone casts to remove die sections and replace them accurately in the original position (GPT8). Types There are various types of commercially available dowel pins all only differing in their mechanism to prevent rotation of the removable die. They are also available with a metal or plastic sleeve to facilitate accurate repositioning on cast
- 20. (A) Flat-sided single dowel, (B) curved dowel, (C) straight double dowel with common head, (D) two separate parallel dowels, (E) dowel with plastic sleeve.
- 21. The following methods are employed to position the dowel pin in the working cast: 1. Prepour technique: The pins are oriented before the impression is poured. 2. Postpour technique: The pins are placed after the cast has been poured. Prepour technique • Any type of dowel pin can be used. • A bobby pin is most commonly used to hold and position the dowel pin. Other materials that can be used for this purpose are anaesthetic needles, paper clips and paper matches.
- 22. • A dowel pin is placed between the arms of a bobby pin. • The bobby pin is then positioned buccolingually across the impression such that the dowel pin will be centred directly over the preparation, without touching the impression, and along its long axis. • The bobby pin is attached to the impression with sticky wax. • Die stone is mixed and poured into the impression covering the knurled end of the dowel pin and at least 3–4 mm beyond the gingival margins of the teeth. • Paper clips are placed in the stone before it sets in other areas of the cast to aid retention to the base.
- 23. • After the stone sets, the bobby pins are removed from the impression and a small ball of soft utility wax 5 mm in diameter is placed on the tip of each dowel to aid in removing the dowel pin after setting. • The open lingual/palatal space of the impression is covered with baseplate wax or a paper towel to enable pouring of a complete base. • The cast is lubricated around each dowel with Vaseline or separating medium to permit easy separation of the dies from the working cast.
- 24. • The base of the cast is poured with dental stone. • After the stone has set, the cast is removed from the impression and the excess is trimmed till the wax balls are exposed. The wax is removed to expose the tips of the dowel pins. • A die cutting saw with thin blades is used to section the dies. Cuts are made vertically and interproximally on either side of the prepared teeth, such that it converges towards the base. • The exposed end of the dowel is gently tapped with an instrument handle to loosen and remove the die. • Advantage: Simple method without any elaborate equipment. • Disadvantage: Pins can be displaced while the cast is poured.
- 25. FIGURE 40.4 Dowel pins positioned with bobby pins attached to impression with pins or sticky wax. FIGURE 40.7 Die sectioned with saw.
- 26. ii. Postpour technique • In the postpour technique, a full arch cast is first poured to cover the entire impression. • After setting, the cast is removed from the impression and base is trimmed such that at least 5 mm of base is available beyond the gingival margins of the teeth. • Holes are drilled with fissure bur on the base of the cast corresponding to the centre of the prepared teeth and other areas of cast so that all the holes are parallel. • Dowel pins are cemented with cyanoacrylate or luting cement such as zinc phosphate on the prepared holes.
- 27. • Wax ball and grooves are placed as described previously. • Separating media is applied around the dowels on the prepared teeth. • The cast is boxed and the base is poured with dental stone. • Die is sectioned and removed. • Advantages: ○ Final impression can be poured immediately without any obstructing pins. ○ Removable section is planned with master cast rather than impressions. • Disadvantage: 1. Fracture of first pour of cast if made too thin 2. The pins may get displaced while pouring the cast
- 30. Di-lok tray system • This technique uses a special tray to pour the cast. This special tray has orientation grooves on the inner aspect. • Actually the tray is made of multiple components, which can be assembled or dismantled as required. • Impression is poured using a two-pour technique. • The first pour is poured upto the level of the impression and the second or base pour is poured after positioning the rim of the di-lok tray over the impression.
- 31. • Before the second pour is set, the base of the di-lok tray is assembled and the cast is allowed to set. • Later, the di-lok tray is dismantled and the grooves on the base of the cast formed by the di-lok tray is used as a guide to do die sectioning. Advantages • Simple and easy to prepare • The cast can be mounted in an articulator.
- 33. PINDEX SYSTEM This system was introduced by Coltene Whaledent, Switzerland 1. Die pins are placed into the cast after it is poured (Postpour technique). 2. This system consists of a drilling machine with a platform over which the cast can be placed. 3. A laser pointer above the cast can be used to position the cast at the exact location where the die pin holes should be drilled. 4. Once the position is confirmed the drill is activated by pressing on a foot pedal. 5. The drill extends upwards and drills holes into the cast exactly below the laser point into which the die pins are inserted.
- 34. Procedure 1. The master impression should be poured with die stone taking care to add enough stone to provide a base 8-10 mm beyond the free gingival margin. 2. The base of the model should be trimmed parallel to the occlusal plane. 3. The palatal portion of the cast should be removed using a palatal trimmer. 4. The trimmed model should be placed on the Pindex machine platform and oriented to the correct position using the laser beam.
- 35. 5. The drill is activated to make die pin holes in the cast. 6. Pindex system uses dual pins to prevent rotation. (Each die pin has two arms—one long and one short). 7. The die pins should be coated with cyanoacrylate and fully seated into the pin hole. 8. Once the die pins are attached the cast should be painted with a separating medium. 9. Base formers should be used to make the base for the model.
- 36. 10. Once the base is set, the model should be removed from the base by gently tapping with a hammer /mallet. 11. The model should be reseated into the base and die sectioning should be done using precision saws. Advantage • Allows accurate placement of the die pins. Disadvantage • Special equipment is required.
- 38. Die spacer Definition: An agent applied to a die to provide space for the luting agent in the finished casting (GPT 10). • It is applied prior to commencing the fabrication of wax patterns. • The die spacer is a material that is applied to the surface of a die to create space between the tooth and the restoration. • This space is required for the cementing material. • The margins should not be coated with the spacer.
- 39. •A band of about 1 mm adjacent to the finish line is not coated. •This allows for adequate marginal adaptation. • The thickness of the die spacer depends on the number of coats applied. A thickness of about 20–40 μm is adequate. • The solvent of the die spacer has a tendency to evaporate, hence a thinner is supplied along with the spacer.
- 40. Wax patterns A wax pattern of the restoration is the next step in the fabrication of a fixed partial denture. For any restoration involving castings (lost wax process), a wax pattern is essential. It is used for making the metal portion of all metal and metal veneered with ceramic or resins. A wax pattern can be fabricated directly or indirectly. • In the direct technique the pattern is waxed on the prepared tooth in the patient’s mouth. It is made with type I inlay wax and can be adopted for small restorations like inlays and posts. It is practically impossible to make extracoronal restorations directly as it is cumbersome, technique sensitive and will take away a lot of chairside time.
- 41. • The indirect technique is the most commonly used method for fabricating fixed partial dentures. The pattern is fabricated on a die made from an impression of the prepared tooth. It is made using type II wax. It offers the following advantages- 1. Allows better visualization of the restoration. 2. Margins can be accessed and finished better. 3. Chairside time of clinician is reduced. 4. Better standardization in technique as it is fabricated in a laboratory.
- 42. The type II wax (inlay wax) should fulfil the following ideal requirements: 1. It should have a contrasting color so that the margins of the pattern can be distinguished from the cast. 2. It should have a slightly lower melting point compared to type I wax. 3. It must not flow at room temperatures. It must flow readily when heated without losing its properties. 4. It should be rigid at low temperatures. 5. It should not flake or chip during carving. 6. It should burn out without any residue. 7. It should be rigid and dimensionally stable.
- 43. PROCEDURE • The indirect wax pattern is fabricated by preparing a wax coping followed by adding layers of wax to build-up the axial and occlusal contours. • Finally, the margins of the patterns are finished before casting. • Coping preparation, surface contouring and marginal finish will be done is a single step for all metal or DICOR ceramic retainers • whereas in case of metal ceramic retainers, the coping is alone prepared and cast using a metal ceramic alloy the contours and margins are directly prepared using ceramic and fired.
- 44. • For metal ceramic restorations with ceramic facings, all areas where metal is to occupy are contoured to a greater detail. • The area where the facing is to occupy is left to the level of the coping and a sharp cut back design should be provided in the wax pattern to mark the metal-ceramic junction.
- 45. PREPARATION OF DIE • Prior to coping fabrication, it is necessary to prepare the die to obtain a precise restoration. Margin preparation • It refers to trimming the area apical to the finish line and is also known as 'Ditching’. • Ditching is done to reveal the preparation margins. • It should be done with a pear shaped bur and must be smoothed with the discoid end of a tanner carver.
- 46. Application of die hardener- • Die hardeners are applied on the surface of the die to increase the surface hardness. • Die hardeners are also applied to the finish line of the die to prevent abrasion by waxing instruments. • The most commonly used die hardeners include Acrylic resin lacquer and Cyanoacrylates.
- 47. Application of die spacer • A die spacer is an agent applied to the die, to provide space for the luting agent in the finished casting. • The most commonly used die spacers include resins, oil paints, nail polish and thermoplastic resinsdissolved in volatile solvents. • Thinners are also available to decrease the viscosity of the die spacer. • Thinners should be applied 0.5 -1 mm short of margin. • The optimal thickness is 25 microns but the thickness may vary from 20-140 microns.
- 48. Application of lubricant • These materials are used to prevent wax from adhering to the die and to facilitate easy removal of the coping Oils, liquid soap, and detergents are used as lubricants. • The lubricant should always be applied via a fine brush
- 49. COPING FABRICATION • Coping fabrication starts with the addition of margin wax at the occlusal line angles first. • This is done to improve the thickness of the coping in these sites as it is prone to become weak in these regions. • The coping can be fabricated either using wax or heated sheet of resin. Usually wax is preferred. • Wax can be coated on the tooth either using a wax spatula or by dipping the die into hot wax. • The excess dipping wax should be removed using a carver.
- 50. • The coping wax may distort in the marginal region due to uneven shrinkage. • Thus, 1-2 mm of coping wax is removed in the marginal region (Cut Back) and is replaced with marginal wax. • The cervical wax is adapted on the cervical region. • Cervical wax is a tension free wax that undergoes extremely less shrinkage after modeling and is ideal for this region as it demands extremely high precision. • While adding a new layer of wax, the previous layer should be melted in order to avoid formation of voids or flow lines on the inner surface of the restoration. • The finished coping must be cleaned and checked for clearance in the articulator.
- 52. Preparing the axial contours Axial contours include the contour of the buccal, lingual and proximal surfaces. They should be prepared based on the following principles- • Establishing the proximal contact is the most important component of axial contouring. The proximal contacts determine the health of the gingiva (interdental papilla). • Usually posterior proximal contacts occur at the occlusal third when viewed from the side. Exceptions include maxillary molars where it occurs in the middle third. • The proximal tooth contact should be facially located when viewed occlusally. Exceptions include the maxillary molars where it is located in the center of the proximal surface when viewed occlusally.
- 54. • The contact area should be optimum to prevent food wedging between the teeth (narrow space) or injury to the gingiva (wide space). • The axial surface below the contact should be flat or concave never convex. A convex axial surface will lead to food engorgement into the interdental gingival sulcus • The heights of contour on the lingual surface of maxillary posteriors occur in the cervical third. The same in the mandibular posteriors occur in the middle third.
- 55. • The lingual prominence is greater for mandibular posteriors compared to maxillary posteriors. • The subgingival axial surface should be flat to promote efficient cleaning. The part of the axial surface that extends sub-gingivally forms a part of the emergence profile. • The emergence profile is a straight plane extending up to the height of contour. • Overcontouring the axial surface can lead to food entrapment and gingival inflammation. • Any depressions in the wax pattern should be removed by filling with wax.
- 56. Preparing the occlusal contours or developing the occlusal morphology • The occlusal surface of the retainer should be built after completing the axial contours. • The occlusal morphology of a retainer should be in harmony with the adjacent/opposite teeth • Special instruments like PK Thomas carvers are used to carve the occlusal surface of the restoration.
- 57. Procedure • The tail end of the PKT carver should be used to build cones of wax over the areas where cusps are to be formed. The cones should be placed exactly on the cusp location. • The marginal ridges and the cusp ridges are built using the same instrument • Additional wax is added near the peripheral ridges and contoured in harmony with the axial surface.
- 58. • Next, the partially waxed occlusal surface is occluded against the opposing cast. At this stage, the occlusal surface is said to resemble a 'fish’s mouth’. • Next the internal inclines of the cusp are contoured. The ridges should be convex to obtain a single point of contact instead of an area of contact with the opposing cusp.
- 59. • The wax pattern is transferred from the cast to the refractory die. • The marginal regions of the patterns should be re-melted and additional wax should be added in this area to produce good marginal fit. • The margins are finished to a fine edge using the burnisher. • A cotton pellet soaked in die lubricant is held with a cotton plier. It is used to finish the occlusal grooves. • The axial surface is finished with a cotton roll. One end of the roll is dipped in die lubricant and the wet cotton is applied over the axial surface. • The other end (dry end) of the roll is used to buff the wet wax until a smooth finish is obtained. Finishing the margins of the pattern
- 60. The various steps in a casting procedure are: • Preparing the wax pattern for casting • Spruing the wax pattern • Attaching the sprue to the crucible former • Investing the pattern in a casting ring • Burnout of the wax pattern • Casting • Recovery • Finishing and polishing. CASTING
- 61. Spruing the Wax Pattern The wax pattern should be sprued before investing. This sprue acts as a channel for metal flow during casting. Sprue former and sprues A sprue is defined as, "The channel or hote Through which plastic or metal is poured or cast into a gate or reservoir and then into a mold". A sprue button is defined as, 'The material remaining in the reservoir of the mold after a dental casting' —GPT. Materials for making sprue 1. Wax 2. Plastic 3. Metal
- 62. Sprue diameter • It should be larger than the thickest portion of the pattern. • Reservoir should be provided to prevent localized shrinkage porosity. Recommended sprue diameters • 2.5 mm (10 gauge) sprues for molar metal ceramic restorations. • 2.0 mm (12 gauge) sprues are preferred for premolar partial veneer restoration. • Narrow sprues are sufficient for casting to be done on centrifugal machines.
- 63. Positioning the sprues • A sprue should be placed at 45° angulation near the bulk of the pattern. The axial walls should have an obtuse angle (135°) in relation to the sprue. • This type of placement helps to prevent air entrapment during investing and the occurrence of suck back porosities after casting. • The point of attachment should be flared to prevent turbulence during metal flow • Venting: Small auxiliary sprues or vents should be placed in order to improve the casting as they form pathways for the gases to escape.
- 65. Attaching the Sprue to the Crucible Former • The sprue is attached to the crucible former made usually of rubber. The crucible former is a conical structure with a base placed at one end of the ring. • A crucible former can be defined as,"The base to which a sprue former is attached while the wax pattern is being invested in refractory investment; • a convex rubber, plastic or metal base that forms a concave depression or crucible in the refractory investment"— GPT.
- 66. • The free end of the sprue should be attached to the tip of the crucible former. The crucible former is removed once the investment sets. • The sprue should be adjusted such that the terminal end of the wax pattern is only about 6 mm away from the free end of the investment. • This distance should be maintained because the air in the mould space should escape out through the pores in the investment during casting.
- 67. Investing the Wax Pattern The wax pattern is invested in a casting ring. A casting ring is defined as, "The inferior portion of the refractory flask that provides a negative likeness or dimple into which a metal is cast in the refractory investment"—GPT. The investment is mixed according to the manufacturer’s instructions and used to invest the wax pattern positioned inside the casting ring. The wax pattern should be coated with a surface tension reducing agent or surfactant to improve the wetting of the investment. The investment material is chosen based on the type of alloy being used. • HIGH NOBLE ALLOYS- gypsum-bonded investments • BASE METAL ALLOYS- phosphate bonded investments
- 68. Ring liners like asbestos (not used now due to health hazards) Cellulose should be used to aid in uniform expansion of the mold. Wetting the liner increases the hygroscopic expansion of the investment. The ring liner should be coated on the walls of the casting ring and kept moist before investing the ring. After investing, the investment should be left to bench set for a minimum of one hour
- 69. Burnout or Wax Elimination • Burnout is defined as, "The removal of wax from the mould usually by heat" —GPT. • Remember that during burn out, the wax does not melt but it directly sublimes into volatile gases, which escape through the pores of the investment. • After bench setting the investment, it is placed in a furnace heated around 200°C/400°F for 30 minutes. To burn out the remaining traces of wax, the investment is heated further to a final burnout temperature (650°C/1200°F) for 45 minutes. • Heating affects the expansion of the investment; hence, the temperatures should be standardized.
- 70. The following steps should be followed in any casting technique: • Setting up the casting machine • Pre-heating the crucible • Weighing the alloy • Casting the alloy
- 71. SETTING UP THE CASTING MACHINE- • The centrifugal casting machine should be given three clockwise turns and locked in position using a pin • The crucible for heating the alloy is positioned on the casting machine. • The flame torch is adjusted. Oxy acetylene gas is used for ordinary alloys and oxygen gas is used for casting metal ceramic alloys.
- 72. PREHEATING THE CRUCIBLE • If the crucible is not pre-heated, it may lead to freezing of the alloy within the crucible leading to an incomplete casting. • A casting crucible is porcelain receptacle where the alloy can be placed and melted. • The posterior end of the crucible will have an opening through which the metal flows out during centrifugation. • The opening of the crucible should be aligned to the opening of the sprue in the invested ring.
- 73. Casting the alloy • The alloy is placed in the crucible and heated using an open flame from a torch. • The reducing part of the flame should be used in order to prevent oxidation of the alloy. • A little flux should be added to metal ceramic alloys and heated further till they ball up (like mercury) and obtain a mirror like surface that appears to spin. • In case of Nickel Chromium alloys, casting can be done once the sharp edges get rounded. • Once the alloy is ready to be cast, the locking pin of the machine is released and casting is completed.
- 74. RECOVERY • Recovery of casting involves the removal of residual investment adherent to the cast surface. • Gypsum-bonded investments quickly disintegrate when quenched in water. However, residual investment should be removed using a toothbrush or an ultrasonic cleanser. • Phosphate-bonded instruments do not disintegrate easily and should be removed using a blunt instrument or by sandblasting using aluminum oxide.
- 75. CERAMIC VENEERING The ceramic or acrylic veneers in metal resin or metal ceramic restorations are usually added after soldering. These veneers are added onto the cutback area provided in the cast framework. 1. Metal Preparation • The casting should be cleanly recovered from residual investment. Recovery includes sandblasting and ultrasonic cleaning. • The gingival surface of the pontic should be reduced. • Care should be taken to prevent over reduction. • Remember the gingival surface of the pontic should always be covered with porcelain.
- 76. 2. Porcelain Application- • After preparing the metal, an opaque layer of porcelain should be applied over the metal surface. • Next the gingival surface of porcelain is coated with cervical porcelain. • Layers of cervical, body and incisal porcelains should be used to build- up the facial surface. • When two or more adjacent units are built –up together, the porcelain in the interdental area should be sectioned to demarcate junction of the two units.
- 77. • A separating liquid is applied over the edentulous ridge prior to building up the gingival surface of the pontic. • Next the porcelain is fired as per the manufacturer ‘s instructions. • After firing the core porcelain, glaze porcelain is added and fired as usual
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