The crown resembles the natural dentition in form and function so the patient enjoys the use of the prosthesis without being reminded of the loss that necessitated restorative intervention.
To achieve this, one must be familiar with the composite form and nomenclature of the natural teeth, understand the determinants of occlusion and their effect on occlusal surface of the teeth, and be able to interpret the harmonious relationship of the axial and interproximal tooth surfaces with the adjacent teeth and architecture of the gingival tissue.
The lost wax casting technique
To convert wax pattern to cast metal
Surrounding the wax pattern with a heat resistant mold made of investment material, eliminating the wax by heating and then introducing molten metal into mold through a channel called sprue
Small variation in investing or casting can significantly affect the quality of the final restorations.
Successful castings depend on attention to detail and consistency of technique. So an understanding of the exact influence of each variable in the technique is important.
According to GPT “One of several esters of fatty acids with higher alcohols, usually monohydric alcohols”
“Dental waxes are combination of various types of waxes compounded to provide desired physical properties”.
“The contouring of a wax pattern” or wax base of a trial denture into desired form”.
“An agent applied to the die to provide space for the luting agent in finished castings”.
“The channel or hole through which plastic or metal is poured or cast into a gate or reservoir and then into a mold”.
“A wax form that is the positive likeness of an object to be fabricated”
“The process of covering or enveloping, wholly or in part, an object such as a denture, tooth, wax form, crown etc, with a suitable material before processing, soldering or casting”.
“Something that has been cast in a mold; an object formed by the solidification of a fluid that has been poured or injected into a mold”.
Dental waxes are organic polymers consisting of hydrocarbons and their derivatives (e.g. ether and alcohol).
Dental waxes are blend of natural and synthetic waxes, natural resins, oils, fats and gums.
Classification of Dental Waxes
Type I (hard)
Type II (soft)
Casting – Sheets, ready shapes, wax up
Boxing wax, Utility wax, sticky wax, carding wax, blockout wax, white wax
Corrective wax, bite registration wax
Casting wax: The wax used to make wax pattern for crowns, RPD framework.
- Post damming of complete maxillary denture.
- Producing wax bites of cusp tips for the articulation of stone cast.
Class A – 28 gauge pink 350C – max 10%
Class B – 30 gauge green 380C – min 60%
Class C – Readymade shapes blue
Composition : Paraffin – 40-60% – Main bulk
Dammer resin – to reduce flaking and to increase smoothness of casting
Carnauba, ceresin, candelila/ bees – to raise melting temperature and to decrease flow at mouth temperature
Dyes are added to increase color contrast.
According to American Dental Association (ADA) have categorized inlay wax into two types:-
Type-I: Medium – for direct wax pattern
Type-II: Soft – for indirect fabrication of wax pattern
- Should be uniform in molecular structure
- Color should be in contrast with die color
- No flakiness/ roughness
- Vaporization temperature – 5000C (9320F)
- Flow-wax begin to harden at 560C (1330F)
Max flow at 370C is 1% for Type-I
- Thermal property – 0.7% expansion at increase of 200C and contracts 0.35% when cooled from 370C – 250C. Glass transition temperature – 350C
- Wax distortion should be minimal (elastic memory).
Other Materials Used:
Resin modeling materials – urathane dimethacrylate oligomers, self cure resins
Peter Kieth Thomas (PKT) waxing instruments
No.1 – for adding larger increments of wax
No.2 – for adding smaller increments
No.3 – burnishing instrument
No.4 – Carving
No.5 – Carving
Wax is added by heating the shank, so that wax flows away from the hottest part of the instrument.
No.7 – Waxing spatula
- In addition to PKT, wax carvers No.2 ward and nos. ½ and 3 Hollenback carvers are used.
- Darby-Perry-Trimmer No.6 (DPT) wax burnisher
- Arch explorer No.23
Electric Waxing Units
These are preferred by some technicians because they allow precise control of the wax which is important for proper manipulation and carbon build up can also be prevented.
Micromatic waxing unit provides several units to aid in waxing.
- It is having wax pens with interchargeable tips
- Electric temperature indicator to maintain tip temperature
Combiladar CL-EW waxing unit has different size and style tips for waxing occlusion and margins of crowns. Temperature of the tip can be varied by sliding resistance bar.
Provision for Cement Space
It is space between internal surface of casting and prepared tooth surface.
Ideal : 20-40 mm
Factors increasing cement space
- Thermal and polymerization shrinkage of impression material
- Use of internal layer of soft wax
- Use of die spacers
- Increased expansion of investment mold
- Removal of metal from the fitting surfaces by grinding, air-particle abration, etching with aquiragia / electrochemical milling.
Factors decreasing cement space
- Use of resin/ electroplated dies
- Use of alloys with higher melting range
- Reduced expansion of investment
Die spacers are resins dissolved in volatile solvents, often with suspended pigments to identify the film thickness.
E.g. Adapt rite. Fit rite – for gypsum dies
Belle de st. Claire; Truf it – for epoxy resin dies
Sequence of Waxing
Dipping the die in melted wax
Flow wax onto the die with well heated instrument (only shank should be heated).
Wax pattern removal
Wax coping is removed to check the perfectly adapted reproduction of prepared tooth surface (magnifying / binocular 10x microscope used to check folds/ creases or internal surface of wax).
Over contouring should be avoided
Contact areas are build up
follow contour of adjacent tooth
flat or concave (tooth surface gingival to its height of contour immediately adjacent to soft tissues)
Functional occlusal wax up waxing to a functional core
Functional Occlusal Wax-up
To apply principles of articulation when waxing the occlusal surface or opposing occlusal surfaces of posterior teeth, the wax added technique for functional waxing was developed
Its advantages over carving technique are:-
- It provides organized step by step procedure to develop the occlusal anatomic form.
- This technique provides better understanding of the effects of articulation on individual elements of occlusal form, the shape and location of cusps, ridge and groove direction, and fossa depth.
The wax added technique
Step-I: Build up of cones
- The buccolingual position of cusps is determined by dividing the distance from lingual height of contour to tip of the buccal cusp in thirds.
- The opposing occlusion and dynamic function will influence the modification of cusp height and placement.
Step-II: Adding the triangular ridges
- A wax ridge is formed from the tip of the buccal cusp to the central developmental groove.
- There should be no interferences when the articulator is moved into eccentric excursions.
Step-III: Adding Axial contour ridges
- Buccal and lingual contour ridges are added to complete axial tooth form.
Step-IV: Adding mesial and distal cusp ridges
- These ridges will complete buccal and lingual outline from the teeth
- There should be no excursive contacts
Step-V: Adding marginal ridges
- This completes occlusal outline form of the tooth and form of occlusal interproximal embrasure.
Step-VI: Fill and refinement
- This technique is readily adaptable to any occlusal scheme. Both cusp-marginal ridge and cusp fossa scheme of occlusion can be established.
Cusp Height Location:
- By adding cones of wax, the position and height of cusp is determined by articulating with opposing teeth.
- Centric/ functional cusp should be positioned at buccolingual center of tooth.
- For each cusp triangular ridge is added. It runs towards the center of occlusal surface. Base of triangular ridge is at center of occlusal surface.
- It should be convex mesiodistally. This convex ridge should make pinpoint contact with opposing tooth.
- Each cusp has two secondary ridges adjacent to triangular ridge
- It is made convex with grooves where they meet the convexities of triangular ridges
- Most mesial and distal secondary ridges should be in continuous with marginal ridges.
To optimize the adaptation of wax pattern to the die, the margins must be reflowed and refinished immediately before investing.
- Minimise dissolution of luting agent
- Facilitates plaque control
In porcelain fused to metal restorations once the final contour of wax pattern has been completed, the pattern is cut back over an even thickness about 1mm to provide room for the porcelain.
Waxing to a functional Core
The technique consists of generating a wax occlusal path in the mouth and replicating the same path in stone. The generated path captures selected mandibular movements in wax. These mandibular movements are the replicated in stone cast.
Functionally generated path techniques will be discussed under five parameters:-
Type of guidance
Bilateral balanced occlusion – not used in FPD
Unilateral balanced occlusion
Anterior guided post occlusion
Placement of cusp tip contact
Relationship of occlusal tooth contacts to TMJ
Number of contact points
Functional core is a 3-dimensional representation of selected mandibular movements recorded in wax and replicated in stone. Whenever the wax pattern contacts functional core, the final restoration will contact mandibular teeth in that movement.
- The posterior teeth on the working side will disocclude the teeth on the non-working side.
- Functionally generated path is most accepted with this type of occlusion.
In unilateral balanced occlusion the working inclines (inner inclines of maxillary buccal cusp) contact the functional core, whereas the non-working inclines (inner inclines of maxillary lingual cusps) are relieved to eliminate balancing contacts.
The amount of contact desired on working inclines for 1st and 2nd molar region is less than that desired for cuspid-bicuspid region because mandible is a class-III lever system, second the teeth nearer to center of rotation will travel less distance than those farthest.
Anterior guided posterior occlusion:-
In this occlusion one or more of the anterior teeth disoccludes the posterior teeth in working and non-working movements. Both the working and balancing inclines are relieved and only cups tips should be in contact.
Occlusion schemes/ placement of cusp tip and cusp to marginal ridge and cusp to fossa
- Cusp to marginal ridge/ embrasure is more prevalent in natural dentition.
- Cusp to fossa relationship (Lundeen 1969) has more biomechanical advantages:
- Most stable tooth contact in ICP is established
- Occlusal forces can be most axially directed
- Wedging effect of a cusp articulated over the occlusal embrasure is avoided (Thomas, 1967).
Cusp-fossa relationship in anteriposterior cross section:
- Convex triangular ridges will cause “ball mill” effect.
- Wide centric principle “freedom in centric” concept compensates for irregular mandibular movements.
- Effect of increased cusp height
- Wear facets more commonly formed
- Can contribute to excessive lateral forces
- Relatively shallow, but effective cusp height
Science of physics says that “a force exerted at the point is inversely proportional to the square area therefore skillful arrangement of subtle concavities in the triangular and cusp ridges, a short stamp cusp can function as efficiently as a long cusp with a lesser stress potential.
Relationship of occlusal contact to the TMJ
In relation to TMJ teeth can be waxed in 3-positions:
Intercuspal position (ICP)
Centric relation – occlusion (CR-O)
Long centric (LC)
Establishing long centric position in functionally generated path is acceptable because functional core will not have interferences between ICP and CR-O.
Number of contact points:-
Point to point
3-point to 3-point (tripodization)
Point to surface
Tripodization is desired to establish condylar position at one point.
In long centric type of occlusion, the point to point or point to surface contact is acceptable.
Wax additive technique
The subtractive technique
Margin adaptation to die
The stone cast is trimmed on a model trimmer to within 2 mm of tooth preparation to prevent chipping of stone cast near margins.
- Saw cut is made through interproximal papillae.
- Cut wax pattern 2mm short of margins
- New wax is flown to cover margins with smooth internal adaptation.
- Excess wax is removed with Hollenback carver
- Wax is smoothen with silk or nylon cloth by gentle apical strokes.
Spruing Principles and Techniques
The first step in production of refractory mold is termed spruing the pattern. The wax pattern is attached to a conical base (crucible former) by a short extension of wax, plastic/metal (which is later removed) this connection is termed sprue former.
Basic requirements of spruing
- Sprue must allow the molten wax to escape from the mold.
- Must enable the molten metal to flow into the mold with little turbulence.
- The metal within it must remain molten slightly longer than the alloy that has filled the mold.
In general relatively large diameter sprue is recommended
A 2.5 mm (10-gauge) – molar and metal ceramic patterns
2.0 mm(12 gauge) – premolars and partial coverage restorations.
Other than centrifugal technique, like air pressure and in centripetal machines the narrow sprue is essential.
- Attached to bulkiest part of wax pattern, normally largest non-centric cusp is used.
- Should be placed at 45-degrees angle to the thickest portion.
- The attachment of sprue to wax pattern should be smoothened to minimize turbulance.
- It should not be restricted because necking increases casting porosity and reduces mold filling.
Small auxillary sprue / vents will improve casting of thin pattern. Their action help gases escape during casting or ensure that solidification begins in critical areas by acting as a heat sink.
Sprue is attached to crucial former, usually made of rubber. It constitutes base of the casting ring exact shape depends on type of casting machine used.
In modern machines, crucible former is tall to allow use of a short sprue and allow pattern to be positioned near the end of the casting ring.
Casting ring and Liner
It holds the investment in place during setting and restricts the expansion of the mold.
Normally a liner is placed inside the ring to allow for more expansion.
- Casting ring must be lined with asbestos or ceramic paper to permit greater freedom of expansion of investment.
- Wetting the liner increases hygroscopic expansion of mold. Studies have shown that absorbant dry liner removes water from the investment and makes a thicker mix, hence total expansion increases.
- Position of wax pattern should be exactly at the center of the casting ring. Te fixed prosthesis which are cast as one piece showed greater accuracy.
The wax pattern should be a precise mold for the final restorations. During fabrication of wax pattern the design principles for optimal biomechanical and hygienic function should be applied.
It is supposed to resemble the morphology of natural dentition.
Effect of die spacer on precementation space of complete coverage restorations. Int J Prostho 1997; 10:131
- One /two layer of die spacer demonstrated less precementation space than 3 or 4 layers.
- Bevel area no spacer should be applied.
Influence of finish line form and geometry on crown cementation. Int J Prostho 1992; 5: 137
Author concludes that finish line form and geometry did not affect fit of cast crown.
Effect of selected physical properties of waxes on investments and casting shrinkage. J Prosthet Dent 1996; 75: 211
- Casting shrinkage decreased when the flow of wax pattern is increased.
- Flow of pattern is increased in exothermic setting of investment.
- A larger casting ring is suggested for casting thick wax pattern, wax having high softening temperature and for low flow temperature wax.
Crown contours and gingival response: (Lee M. Jameson)
Author suggests –
- Overcontouring should be avoided in cervical 1/3rd and interproximal surfaces.
- Adequate tooth preparation should be done at gingival margins and interproximal surfaces.
- In exposed furcation areas fluting should be done to promote self-cleansing.
- Gingival trauma should be avoided during preparation.
Limiting wax pattern distortion caused by setting expansion: D.R.Davis. J Prosthet Dent 1987; 58:229
Author suggests the ring liner should be 3mm short of casting ring. The castings made with full length liner showed longer castings than normal.
Castings made with closed rings were more closely adapted to die than made with open rings.
Measurement of paint-on die spacers used for casting relief: by W.E. Compagni. J Prosthet Dent 1982; 47 : 606
A hobby shop point used as a spacer had the smallest film thickness and smallest standard deviation.
4 coats of point will produce double the relief than 2 coats and 6 coats will produce triple the double coats.
The effect of sprue attachment design on castability and porosity: R.G. Verrett. J Prosthet Dent 1989; 61:418
Straight and flared attachments produced better castability and less porosity, than either constricted or bend sprue attachments.
Study on the surface of resins that burnout without residues in the lost wax procedure: M.Krane et al. J Prosthet Dent 1998; 389
It is recommended that some resins replace waxes /wax-resin compositions.
Resin modeling materials – Light cured resins
A good wax pattern – key to nice tooth!