Occlusal Modeling

Many practitioners consider occlusal modeling to be a very complex procedure that often requires occlusal adjustments significant enough to distort the outcome of the restoration. Breaking down shape through visual observation simplifies the perception of anatomy, giving it new meaning. Everything becomes more understandable, and elements previously hidden in the overall image can be perceived as part of the overall shape.1 Modeling is an ordered and linked sequence of steps designed to achieve correct anatomy. Constructing the correct morphology requires a knowledge of anatomy and the ability to perceive the fine anatomical details of the occlusal surface (Fig 7-1). A simplified modeling technique helps practitioners replicate an esthetically pleasing and functionally effective occlusal anatomy.

FIG 7-1 Modeling is interpolating the residual anatomy. As used in this text, interpolate means taking information from the residual anatomy and projecting, joining, standardizing, and complementing it based on anatomical knowledge.

Modeling Versus Layering

Composite is applied to a tooth cavity with the aim of anatomical and functional restoration. Techniques used to re-create the occlusal surface of a restoration are known as modeling techniques. They concern form, anatomy, and function. Techniques used to fill the cavity up to the surface layer are known as layering techniques and depend on the nature of the material used, cavity size, and residual structural factors. This book, and particularly this chapter, does not discuss how many layers or which colors and materials to use, but rather focuses on the surface layer and how to shape the composite to achieve the correct anatomy and hence tooth function.

No single modeling technique is suitable for all clinical cases. Several modeling techniques are available, and clinical conditions will determine the choice. Modeling techniques can be divided into:

  • Stamping techniques
  • Subtractive techniques
  • Additive techniques (subdivided into cusp-by-cusp and simultaneous modeling techniques)

When constructing an occlusal surface, additive (Fig 7-2) and subtractive stages can coexist according to the sizes of the increments and the anatomical details being defined.

FIG 7-2 The first increment is placed in an additive technique.

Modeling Instruments

Accurate modeling is based on a knowledge of anatomy followed by the choice and application of the modeling technique most appropriate for the specific clinical case. Essential instruments for performing accurate modeling are condensers, sculpting instruments, and spatulas.

As can be seen in Fig 7-3, four tips can be fitted on two instruments to perform all the techniques necessary for modeling direct posterior restorations. The condenser packs the composite and models surface depressions. The sculpting tip sculpts grooves, fossae, and pits, moving the occlusal ends of ridges and removing excess composite. The spatulas adapt the composite and join it to the residual walls. Their different angles facilitate modeling, depending on the side of the tooth being reconstructed. In modeling procedures, a microbrush is also used for packing, compressing, pushing, and adapting (Fig 7-4).

FIG 7-3 (a and b) In posterior teeth, 90% of modeling can be carried out using this single instrument, which consists of a condenser and sculpting instrument. (c and d) Straight and angled spatulas are useful for all modeling procedures not performed with the single sculpting instrument.

FIG 7-4 The microbrush helps model and spread composite.

Handling composite

It is sometimes necessary to handle composite with gloves (eg, to form the increment into a ball). Handling composite using powdered gloves, even if uncontaminated by blood and saliva, detracts from the mechanical properties of the composite and must be avoided at all costs. Composite should only be handled when wearing powder-free gloves that have been washed with ethanol2,3 (Fig 7-5).

FIG 7-5 (a to d) Powder-free gloves should be washed with ethanol before handling composite with the fingers.

How to Build a Triangular Ridge

Additive modeling techniques always start by modeling a triangular ridge. An initial composite increment is formed into a ball and placed at the site from which the reconstruction procedure is to start. The composite is shaped, first mesially and then distally, with the flat face of a modeling spatula resting on the residual ridge. The ball of composite assumes the pyramid-like shape of a triangular ridge (Fig 7-6).

FIG 7-6 (a to d) Sequence for converting a ball of composite into a triangular ridge.

Once a pyramid-like ridge has been built, the morphology is improved, and excess composite is removed using a sculpting instrument (Fig 7-7). In addition to defining the pyramid shape, the spatula is rested on the residual ridge to follow its angle of inclination and transfer the correct gradient to the composite (Fig 7-8). Modeling carried out in this way reduces the risk of occlusal adjustments. This significantly enhances technique reliability, execution speed, and the predictability of the outcome.

FIG 7-7 (a to d) Removal of excess composite.

FIG 7-8 (a and b) Shaping the ball-shaped increments of composite into a triangular ridge.

How to Construct a Groove

Grooves can be constructed by buildup, subtraction, or both procedures in alternation. The buildup procedure involves placing an initial increment to form a ridge. When constructing the adjacent ridge, the composite increment is built up on the initial increment, which has already been cured. Buildup (Fig 7-9) can be carried out with a condenser or, more simply, by using a brush slightly moistened with resin modeling agent.4 The brush is touched gently against the composite to compress it and push it toward the first increment. As shown in Fig 7-10, the composite is adapted to a previously cured increment, creating a natural-looking groove.

FIG 7-9 (a and b) The ball-shaped increment is converted to a pyramid. (c to e) The increment is built up on the previously cured increment using a condenser or brush.

FIG 7-10 (a to d) The increments are projected toward the cured surfaces. (e and f) A natural groove is created.

A groove is sculpted by subtraction by running the pointed end of a modeling instrument along the groove under construction. It slides gently toward the outside, starting from the future central fossa before joining up with peripheral residual anatomy (Fig 7-11).

FIG 7-11 (a and b) Defining a groove using a subtractive technique. The process is centrifugal.

During groove construction, the path defined by the modeling instrument must always be guided from the center toward the periphery of the occlusal surface (Fig 7-12). If sculpting takes place from the periphery toward the center, composite is moved away from the marginal seal area, creating a gap at the tooth-restoration interface (Fig 7-13).

FIG 7-12 (a to c) Defining grooves using a subtractive technique.

FIG 7-13 If a groove is constructed by sliding from the periphery toward the center of the occlusal surface, there is a risk of moving the composite away from the marginal seal.

The Rule of Proportion

Proportion is the dimensional relationship established between the constituent parts of an object. Composite increments should be harmoniously projected, inclined, oriented, and extended from the residual perimeter ridge toward the center of the occlusal surface in a 3D composition that respects the rule of proportion. After adapting and condensing, each composite increment is modeled with the aim of projecting it (Fig 7-14) to the occlusal side of one of the ridges.

FIG 7-14 Diagram showing virtual projection of a ridge. The composite mixture must be projected toward the center of the occlusal surface by reading anatomical information from the residual perimeter ridge.

When the composite increment is projected toward the occlusal surface, it must respect the inclination (or gradient) of the ridge, which will vary according to the tooth’s anatomy and the patient’s chewing pattern (Fig 7-15). Projection will be supported by orientation: Which way should the top of the increment face? It is useful to think of the cusp tip as the center of a compass that can be used to correctly orient the increment (Fig 7-16), following the direction of the residual ridge but always in proportion to subsequent composite increments. The extension, ie, the length of the increment (Fig 7-17), will determine the volume occupying the center of the occlusal surface. If the increment is overextended, this error will trigger a domino effect, and the restoration will be an anatomical failure.

FIG 7-15 Inclination. (a) Each ridge has its own gradient running from the cusp tip to the base of the groove (arrow). (b) The projection of an increment must respect the gradient of the residual groove and follow it to the center of the occlusal surface (arrows).

FIG 7-16 Orientation. The cusp tip can be considered the center of a compass, and the ridge midline will follow a very specific orientation (dotted arrow).

FIG 7-17 Extension. As the increment is gradually projected, tilted, and oriented, it must also be extended to occupy the occlusal surface (arrows), respecting proportional ratios with other increments.

Occlusal Stamping Technique

This is the smartest way to produce an anatomically accurate and functionally effective model when the anatomical potential of the tooth to be treated is still excellent. This technique5,6 allows full reproduction of the original occlusal anatomical pattern (before cavity preparation), keeping the static and dynamic occlusal ratios almost unchanged while simplifying reconstruction and occlusal registration procedures. The first stage of the technique involves preparing a custom stamp. The cavity is then prepared and filled (after bonding procedures) with composite resin. The stamp, insulated by polytetrafluoroethylene (PTFE) tape, impresses the anatomy in the composite. The composite can be cured once the PTFE tape and excess composite have been removed. For a detailed description of the technique, see Fig 7-18.

FIG 7-18 (a) This tooth has an anatomically intact occlusal surface and is a good candidate for the occlusal stamping technique. (b) Insulation is spread on the occlusal surface for easy detachment of the materials that will be used to construct the occlusal stamp. (c) Note the generous amount of insulation applied to the occlusal surface. (d) An air syringe is used to blow away excess insulation to leave a uniformly smooth occlusal surface. (e) The best material for making an occlusal stamp is liquid dam, because it can take an accurate impression of the occlusal morphology and stay elastic enough for easy detachment from the tooth without breaking the stamp, even after curing. (f) Liquid dam is spread over the occlusal surface of the tooth to take an accurate impression of the entire grinding surface anatomy. The material must be spread to cover the cusp sides, which will provide a repositioning index during the modeling stage. (g) A brush is used as a carrier. The working part is coated with liquid dam and brought into contact with the liquid dam placed on the tooth. During placement, always orient it to ensure a favorable insertion axis during intraoral modeling. (h) The brush is placed in contact with liquid dam spread on the tooth’s occlusal surface, and everything is cured. (i) Once the material has been cured, the insulation allows the stamp to be easily removed from the tooth. The resulting stamp is a faithful anatomical negative of the occlusal surface. (j) After the occlusal stamp has been made, the cavity is prepared. (k and l) Once the cavity has been prepared, the tissues are hybridized. High-viscosity composite is used to reconstruct the cavity base by means of an incremental technique, taking care to leave 1 to 1.5 mm between the composite base and preparation margin. (m and n) A periodontal probe is used to measure the gap between the composite substrate and the most coronal extent of the preparation margin. The residual cavity is filled with high-viscosity composite, which is fitted to the cavity margins. The occlusal surface is then modeled. (o) PTFE tape is placed to isolate the composite from the occlusal stamp. (p) The stamp can be centered by using the cusp sides as a repositioning index. (q) Firm continuous pressure should be exerted on the mold with the aid of tweezers, repeating the procedure several times. Excess composite will overflow, leaving the shape of the stamp on the composite. (r) The transferred morphology can be seen on the PTFE. The color of the underlying composite can be discerned because the PTFE has become thinner and more translucent. (s) The PTFE tape is removed from the occlusal surface using forceps. There is no risk of accidentally pulling away the restorative composite. Only outlying excess composite will be lifted off, and this will not affect the marginal seal. (t) View of occlusally impressed shape and excess composite beyond the preparation limits. (u) Excess composite is gently removed using a pointed instrument. (v) A moistened brush is used to spread the composite toward the seal margin and ensure a tight fit. (w) Excess composite is removed, and the occlusal seal is improved to ensure accurate morphology. (x) A sculpting instrument is used to make the grooves more pronounced. (y to gg) The depth of the grooves is improved. (hh) When everything has been done to the clinician’s satisfaction, the composite can be cured to fix the final shape.

VIDEO: OCCLUSAL STAMP

Subtractive Technique

The subtractive technique, introduced by Dietschi and Spreafico,7 combines simplified layering with fast, effective subtractive modeling (Fig 7-19a). The first composite increments (dentin) are initially quickly modeled to define an anatomical blueprint of the dentin component of the occlusal surface (Fig 7-19b), then cured. Next, a continuous layer of enamel is applied to the occlusal surface (Fig 7-19c). The surface layer is sculpted with a pointed instrument to reflect the anatomical blueprint of the dentin component (Fig 7-19d) and thus replicate the final occlusal anatomy. The surface anatomy is gradually reproduced, excess composite resulting from the modeling is removed, and the material is spread toward the preparation margin to ensure a good fit to the marginal seal (Figs 7-19e and 7-19f).

FIG 7-19 (a) Contoured, hybridized cavity ready for restoration. (b) A blueprint of the occlusal anatomy is created in the base dentin layer. The anatomy is constructed starting from the central fossa, the anatomical center of the occlusal surface, before opening in a centrifugal direction toward the primary and secondary grooves. Starting from the central fossa, the occlusal anatomy is constructed by joining the primary and secondary grooves using anatomical information obtainable from the occlusal perimeter. Once the dentin morphology has been defined and the anatomical pattern is accurate, the composite is cured. (c) The dentin base is completely covered with enamel. The composite is compressed to penetrate the anatomical recesses sculpted in the dentin and spread to improve the marginal seal. (d) A pointed instrument is used to model the enamel layer by removing composite. The modeling follows the anatomical blueprint of the dentin layer, beginning from the central fossa. The anatomy is sculpted to join the grooves to the anatomy discernible on the occlusal perimeter. The enamel desaturates the chroma of the underlying dentin for a natural-colored restoration. (e) Once modeling is complete, the composite can be cured. Sculpting stages are always alternated with the use of a brush, which makes the grooves more shallow and natural-looking. (f) View of the restoration after applying a brown stain. Note how the 3D anatomy is enhanced.

For small cavities, a single-component variant of the subtractive technique can be used. This involves using a body composite (of intermediate translucency) that combines the optical properties of enamel with those of dentin. Once the cavity has been prepared (Figs 7-20a and 7-20b), tissue hybridization procedures are performed, and then the composite mixture is positioned and condensed (Figs 7-20c and 7-20d). After removing any excess (Fig 7-20e), the anatomy is defined with a sculpting instrument (Figs 7-20f to 7-20p). Grooves modeled by subtractive sculpting may be irregular and rough-surfaced. A brush slightly moistened with resin modeling agent smooths any roughness and spreads the composite toward the restorative margin. This ensures a long-lasting, stable marginal seal and ensures that the restoration blends in smoothly with the healthy tooth tissue (Figs 7-20q and 7-20r). Overly deep grooves would be difficult to polish. They could lead to the buildup of food residues and bacteria, allowing the formation of plaque that would be difficult to remove. The subtractive technique is summarized in Box 7-1.

FIG 7-20 (a and b) Prepared cavity and application of the bonding system. (c and d) A single increment of body composite is placed in the cavity using a spatula before it is flattened and compacted with the aid of a condenser. (e) The condenser is also used to remove excess composite. (f) A pointed instrument is used to fix the anatomical center of the occlusal design—the central fossa. The entire morphologic design to be constructed by composite subtraction will stem from the central fossa, attempting to blend in with anatomy of the residual tooth. (g to l) Starting from this anatomical center, a pointed instrument is used to define the grooves, following paths leading to residual anatomy on the occlusal perimeter. (m to p) Modeling follows a centrifugal pattern, defining the mesiobuccal, distobuccal, and working grooves until the protrusive and secondary grooves are completed. Segmenting the composite by subtraction reduces the negative effects that curing shrinkage could impose on a composite increment measuring over 2 mm. (q and r) The composite is spread toward the restorative margins prior to curing.

BOX 7-1 Subtractive technique

Pros

  • A blueprint of the occlusal morphology is created at the dentinal level. If mistakes are made, corrections can be made in the enamel application.
  • Rapid technique in a medium to small cavity.

Cons

  • A single continuous layer of enamel is spread over the entire occlusal surface; therefore, control of stress caused by shrinkage might be ineffective.
  • Managing shape using a single composite application is not a simple procedure, and its success is operator-dependent.
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May 25, 2021 | Posted by in General Dentistry | Comments Off on Occlusal Modeling
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