Restorative Dentistry: Developing Posterior Composite Restorations-A Biomechanical Concept

Differing physical and mechanical characteristics of composite resin and metallic restorations require a distinctive adhesive preparation design divergent from that of the classic amalgam preparation. Composite resin has a greater potential for bonding to tooth structure than does amalgam and, as such, minimal mechanical retention is required. Accordingly, clinicians confined to the mechanical principles associated with amalgam must reexamine operative procedures for adhesive restorations and institute a new, “non-mechanical ideology.” ^9,10

The restorative procedure requires the removal of the carious lesion and/or defective restoration, development of the proper cavity form consistent with enamel rod orientation, and placement of the restorative material. The composite restoration not only provides strength to unsupported or weakened tooth structure but, because of a low thermal conductivity, also eliminates most postoperative discomfort.^{11,12, 13}

A good adhesive preparation design requires maximum preservation of remaining tooth structure, with no extension for prevention. Since composite requires less volume to resist clinical fracture than amalgam, the preparation is limited to access to the lesion or defect.^14,15 The width of the preparation should be as narrow as possible because the wear resistance of the restoration is directly proportional to dimension.¹⁶ As a result, increased buccolingual width of the preparation can trespass into the centric holding areas.¹⁶ To allow for better resin adaptation, all internal line angles should be rounded and the cavity walls should be smooth.¹⁷ When there is enamel present to increase the potential for bonding, beveling should be restricted to the gingival and proximal margins. Restricted beveling increases fracture resistance by enlarging the bulk of the restoration, expands the bonding surface area and decreases microleakage by exposing the enamel rods for etching.¹⁸ The occlusal cavosurface margin should not be beveled because increased width of the preparation may infringe upon the centric holding area, accelerating the wear of the restoration.¹⁶ These new principles of design for adhesive restorations replace the traditional mechanistic approach to the restoration of teeth with applications of biomechanical concepts.

Application of the aforementioned adhesive design principles also requires a comprehensive understanding of the complex interplay between polymerization shrinkage and adhesion. The cross-linking of resin monomers into polymers is responsible for an unconstrained volumetric shrinkage of 2% to 5%.¹⁹ The uncompensated forces may exceed the bond strength of the tooth-restoration interface, resulting in a gap formation from a loss of adhesion.²⁰ Bacterial and fluid penetration through the marginal gaps may occur, causing colonization of microorganisms,²¹ recurrent caries, and postoperative sensitivity with possible subsequent irritation of the pulp,²² all of which effectuate clinical failure.^23,24

However, even if the adhesion process is effective, shrinkage forces generated by a high modulus material or a high volumetric shrinkage can result in stresses being transferred in a pulpal direction, resulting in dentinal tubular fluid movement that stimulates the odontoblastic process. This pressure change may be responsible for postoperative sensitivity upon mastication. Almost all stimuli known to procedure dentinal pain in a physiologically sound tooth are related to fluid movement in the dentinal tubules. This liquid movement stimulates the odontoblast process, a baroreceptor (i.e., a nerve receptor sensitive to pressure) that leads to neural discharge (i.e., depolarization). This neural pulpal activation is interpreted as pain.^25,26,27 Also, these residual stresses can result in cuspal flexure or enamel fracture.²⁴

Factors that influence polymerization shrinkage include type of resin, filler content of the composite, elastic modulus of the material, curing characteristics, water sorption, cavity configuration, and the intensity of the light used to polymerize the composite²⁸ (Figure 1). Managing and combating the undesirable effects of polymerization shrinkage can be accomplished through a variety of stress-reduction techniques²⁹ that include: selective bonding in appropriate cavity configurations; the application of liners and bases that act as shock absorbers in deeper cavity preparations; reducing light intensity from curing units; and utilizing a combination of selective bonding and layering of small increments of a low-shrinking composite resin.²⁸

In certain situations, there are no free surface areas present within the cavity. Thus, the ratio between the free and bonded restoration surfaces (i.e., C-factor)³⁰ is high, creating shrinkage stresses that are higher than the bond strength³¹ (Figure 2). This can result in pbodyial delamination from the tooth structure interface, generating marginal gaps and/or enamel fractures.³² The process of selective bonding creates free surfaces within the cavity, thereby reducing the configuration factor of the restoration. The adhesive layer seals the dentin, yet does not adhere to the restoration; therefore, the gap formation is confined to the internal aspect of the cavity, creating a free surface within the cavity and reducing the C-factor. This enables greater flow during polymerization, resulting in a more stress-resistant marginal adaptation.¹²

Using low-intensity curing light sequences to reduce shrinkage stress controls the plasticity (i.e., flow capacity) of the restoration during polymerization, while the final mechanical stability of the restoration remains unaffected.^28,33 When composites are polymerized with high curing light intensities, larger gaps between the cavity walls and the restorative material are created than are found with the use of low-intensity lights. Considerable stress reduction occurs during the first 20 seconds of polymerization. Employing a lower-intensity light power during the first 20 seconds extends the visco-elastic stage of setting, an interval during which stress can be pbodyly relieved by flow and elastic strain.²⁹ The correlation between the rate of conversion and the rate of shrinkage stress development requires a slower stiffness development, which may result in overall stress reduction by allowing more yielding of the free surface of the restoration to the underlying contracting bulk.²⁸ A reduced initial conversion rate of the resin material promotes a greater level of marginal adaptation at the interface of the cavity and restoration.³³ This tends to cause less damage at this interface.^28,33,34

Incremental layering has been advocated for use in large composite restorations to avoid the limitation of depth-of-cure, reduce the effects of polymerization shrinkage and enhance esthetic results from the multi-layering of color.^35,36 However, it is the anatomy of the tooth that should guide the clinician in developing the correct interpretation of form and color. Incremental placement of successive layers of dentin and enamel composites creates high diffusion layers that allow an optimal light transmission within the restoration, providing a more realistic depth of color and natural surface and optical characteristics. A polychromatic effect is achieved by stratifying various shades and opacities of the restorative composite. Due to the variations in natural teeth, the combination of different composite shades must be applied in relation to the natural tooth anatomy and specifically adapted to individual clinical situations. The following technique utilizes both the incremental layering of composite and the stratification of color to create a natural chromatic integration.³⁷

Preoperative Protocol
Prior to initiating the restorative procedure, an occlusal analysis of the anatomical morphology of the tooth is performed and transferred to a hand-drawn diagram. This diagram serves as a restorative road map for the clinician and can include such information as dentin and enamel intercolor contrasts, translucency patterns, crazing, hypocalcification spots, incisal and gingival blending, and occlusal stain patterns. Also, a preoperative selection of composite resins, tints and modifiers, along with their shade and orientation, is recorded. Shade selection should be accomplished prior to rubber dam placement to prevent improper color matching as a result of dehydration and elevated values (Figure 3). When teeth dehydrate, the air replaces the water between the enamel rods, changing the refractive index and making the enamel appear opaque and white.³⁸

In addition, the preoperative occlusal stops and excursive guiding planes are recorded with bodyiculation paper. These can be transferred to a hand-drawn occlusal diagram, recorded via an intraoral or digital camera or indicated and reviewed on a stone model. This initial registration is valuable in preparation design when determining placement of centric stops beyond or within the confines of the restoration, determining proper restorative material thickness of the bodyificial enamel and dentin, and minimizing finishing procedures.³⁹

Restorative Stage
A 43-year-old woman presented with defective amalgam restorations on the mandibular left first and second molars. The existing restorations exhibited open margins with recurrent decay (Figure 4). After thorough examination and assessment, the patient expressed interest in replacing the existing restorations with tooth-colored alternatives. Once anesthesia was administered, the treatment site was isolated with a rubber dam to achieve adequate field control and protect against contamination.^40,41 Upon removal of the existing amalgam restorations, a caries disclosing solution (Seek® & Sable™, Ultradent Products, Inc., South Jordan, UT) aided in the detection and identification of the irreversibly infected carious tissue and guided its removal.^{3, 42} The carious dentin was removed with a slow-speed carbide round bur #6 (Midwest Burs, DENTSPLY Professional, Des Plaines, IL) and spoon excavators. The occlusal outline extended to include carious enamel, provide access to the carious dentin, remove any residual amalgam staining, and provide access for the application of restorative materials. Healthy tooth structure should only be removed when the occlusal outline requires extension to a point beyond or within the previously indicated functional stops. The preparation was completed with a finishing diamond and cleaned with a 2% chlorhexidine solution (Consepsis® Scrub, Ultradent Products, Inc.), rinsed and lightly air-dried. The cavosurface enamel margins were etched for 30 seconds with 37.5% phosphoric acid (Figure 5), rinsed for 5 seconds and gently air-dried for 5 seconds. A self-etching primer (One Coat Bond, Coltène/Whaledent Inc., Cuyahoga Falls, OH) was applied to the dentin with a disposable applicator tip for 20 seconds with a continuous motion and air-dried lightly for 2 seconds (Figure 6). A bonding agent (One Coat Bond) was applied to the enamel and dentin surfaces with a disposable applicator tip for 20 seconds, lightly air-dried and light-cured for 20 seconds (Figure 7).

The cavity preparation was filled incrementally utilizing an A-2/B-2 duo-shaded hybrid composite (SYNERGY®, Coltène/Whaledent) based on the preoperative shade mapping diagram. Each increment was gently condensed with a clean, non-sticking ball-tipped composite condenser to ensure complete adaptation to the underlying resin and tooth structure. Each increment was light-cured for 10 seconds using the boost mode (Optilux™ 501, Kerr/Sybron Dental Specialties Inc., Orange, CA) (Figures 8a through 8c).

To reduce the possibility of cuspal flexure, a composite hybrid with a low volumetric polymerization shrinkage should be selected.²⁴ Also, this problem can be reduced by a diagonally layering the hybrid in increments of 1 mm to 2 mm^43,44 and feathering the material up the cavity wall, following the anatomical morphological contours, which will minimize the wall-to-wall shrinkage and reduce intercuspal stress. Continue to condense and shape the composite resin to correspond to cusp development and dentin replacement (Figures 9a and 9b).

Once the final bodyificial dentin layer was developed, the internal characteristics (i.e., creation of pits and fissures, staining of grooves, or the creation of internal color within the restoration) were applied using a #00 sable brush. An ochre-tinted resin was applied in the previously formed invagination (Figure 10). If the chroma is too high, it can be diluted using an untinted resin and a small brush or removed with a clean applicator tip. The tint and the final dentin layer were polymerized for 10 seconds in the boost mode.

An infinitesimal amount of brown tint was applied according to the shade diagram. Also, a diluted white wash was applied to correspond to the adjacent second bicuspid and the shade diagram (Figure 11). These tints should be polymerized before placing additional stratification materials to stabilize the characterization and prevent color mixing. This color variation allows the development of a 3-dimensional appearance within the restoration.

It is important to anticipate the final result and not trespass into the bodyificial enamel zone, preserving space for this anatomical envelope of composite. An A-1/D-2 duo-shaded hybrid composite (SYNERGY) was sculpted with a curved metal instrument and smoothed with a sable brush to correspond to the functional and anatomical occlusal morphology (Figures 12a and 12b). As previously discussed, a thorough preoperative occlusal registration and careful shaping of the composite resin to those confines prior to curing facilitates the establishment of anatomic morphology and minimizes the finishing protocol. At least 1 study revealed that a reduction in finishing results in less damage to the composite and improved wear and clinical performance.⁴⁴ After placing the last layer of composite and prior to final curing, an oxygen inhibitor, glycerin (Insure, Cosmedent, Inc., Chicago, IL; or DeOx®, Ultradent Products, Inc.), was applied in a thin layer with a brush to the surface of the restoration and light-cured for a 2-minute post-cure; the longer the composite is subjected to the curing light, the more effective the polymerization.¹⁴

To replicate the natural anatomical form and texture, the initial contouring was performed with a series of finishing burs. The occlusal refinement was achieved with #30 fluted egg-shaped finishing burs (BluWhite diamonds and carbides #7406 and #9406, Kerr/Sybron Dental Specialties Inc.), closely observing the tooth-resin interface and using a dry protocol. After the initial finishing procedure, the margins and surface defects were sealed. All accessible margins were etched with a 37.5% phosphoric acid semi-gel, rinsed, and dried. A composite surface sealant (OptiGuard®, Kerr/Sybron Dental Specialties Inc.) was applied and cured to seal any cracks or microscopic porosities that may have formed during the finishing procedures. The use of a surface sealant has been shown to reduce the wear rate of posterior composite resin restorations.⁴⁵

The rubber dam was removed, and the patient was asked to perform closure without force and then centric, protrusive, and lateral excursions. Any necessary occlusal equilibration was accomplished with a #30 egg-shaped finishing bur, and the final polish was repeated. The final polish was initiated with silicone rubber points or cups, which are composed of aluminum oxide pbodyicles and silicone that permit surface defects to be effectively eliminated (Figure 13). The definitive polish was accomplished with a brush with impregnated bristles (Jiffy®, Ultradent Products, Inc.) at a low speed and light pressure under water irrigation and air spray. The contact was tested with unwaxed floss to ensure the absence of sealant in the contact zone, and the margins were inspected.

The surface quality of the composite is not only influenced by the polishing instruments and polishing pastes, but also by the composition and the filler characteristics of the material. Newer formulations of composites with smaller pbodyicle size, shape, and orientation provide a level of polishability that compares to porcelain and enamel. Although clinical evidence of polishability with these new small pbodyicle hybrids appears promising, the long-term durability of the polish requires evaluation in future clinical trials. The postoperative result achieved through the use of this small-pbodyicle hybrid composite resin utilizing an adhesive design concept reflects a harmonious integration of anatomical form and internal depth of color (Figures 14a and 14b).

Recent developments in adhesive technologies, composite resin materials, and placement techniques have revolutionized the delivery of minimally invasive direct restorations. Rather than employing mechanical principles of preparation design, clinicians can opt to use biomechanical concepts when selecting contemporary restorative materials and placement techniques. Although the practice philosophy for the modern restorative dentist has remained the same, the mindset of the clinician must be transformed to continue to explore and develop ideas, techniques, and protocol. These modern concepts enable not only the creation of an esthetic posterior resin restoration, but also the preservation and reinforcement of sound tooth structure and the achievement of long-term success.

Douglas A. Terry, DDS
Assistant Professor
Depbodyment of Restorative Dentistry and Biomaterials
University of Texas Health Science Center, Dental Branch
Houston, Texas

Adjunct Faculty
University of California
Los Angeles, California

Center for Esthetic Dentistry
Private Practice-Esthetic and Restorative Dentistry
Houston, Texas

Karl F. Leinfelder, DDS, MS
Adjunct Professor
University of North Carolina
Professor Emeritus
University of Alabama
Tuscaloosa, Alabama