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.
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.
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).
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).
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).
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.
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.
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).
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).
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.
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.
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.
VIDEO: OCCLUSAL STAMP
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).
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.
- 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.
- 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.