Chapter 10 Posterior Direct Composites

Relevance of Posterior Composites to Esthetic Dentistry

When esthetic dentistry began its evolution, the posterior teeth were considered unimportant. As patient expectations have increased, more focus has been placed on the esthetic contribution of posterior teeth (Figure 10-1). With the mechanics of mandibular function, as humans speak, laugh, and exhibit the behaviors considered human, the incisal edges of the lower anterior teeth and the occlusal surfaces of the posterior teeth are critical (Figure 10-2).

FIGURE 10-1 A and B, Amalgams placed 20 years previously in the patient. The teeth were asymptomatic but were treated because of incomplete fractures and recurrent decay. C and D, Postoperative results at 3 days, occlusal (C) and lingual (D) views. Patient experienced no sensitivity afterward. E, High-magnification palatal view of the upper first molar.

FIGURE 10-2 The occlusal surface of the lower posteriors can have a dramatic impact on the esthetics of dynamic human faces.

Many patients inhibit behaviors and develop a lack of confidence from a lack of pride in the anterior teeth. The same problems occur with the patient’s quality of life with regard to the posterior dentition. Many practitioners have seen these behaviors in patients with unacceptable anterior teeth. It is a valid exercise to examine the psychology of what happens when posterior tooth esthetics are not ideal. These problems have an impact on both quality of life and self-esteem (Box 10-1). Interestingly, habits such as pursing of lips and raising the hand to cover the mouth are the same regardless of whether patients dislike the appearance of their anterior or their posterior teeth.

Box 10.1 Posterior Dentition’s Impact on Self-Image and Quality of Life

A senior partner at a large, powerful accounting firm commented that she would arrange the seating for business meetings so that her colleagues were seated directly in front of her rather than to the side. Why? So that they would not see her maxillary bicuspid, which had a small interproximal display of gold. The culprit was a gold onlay placed by her previous dentist 20 years earlier. Over time a dinginess often occurs with interproximal metal restorations (see Figure 10-1, A and B).

This conversation was initiated when the patient was shown buccal corridor photographs. A traditional anterior photograph or “lips” picture does not really show the side view. Photographs where people are looking either obliquely or laterally allow the dentist and patient to discuss problems seen only from these perspectives.

This patient referred herself to the practice because she distrusted self proclaimed “cosmetic dentists”who aggressively promote their procedures. Yet a routine use of intraoral photography allowed her to enter into a course of self-discovery and served as the basis for an honest exchange of ideas and concerns. Such a course can lead to esthetic dentistry procedures without the ethical dilemma of “pushing” esthetics on the patient.

Happily, this patient chose a comprehensive esthetic reconstruction (see Figure 10-1, C to E). She now smiles and laughs and conducts meetings without rearranging the chairs in the room.

Brief History of the Clinical Development and Evolution of the Posterior Composite Procedure

For anterior composites, North American dental schools rapidly integrated both materials and updates. Cavity preparations were adapted to the material very early on. In terms of longevity, anterior composites have been deemed acceptable by both practicing dentists and dental schools. It is important to talk about perception of success with teaching institutions and outcome studies because often they are unrelated. Two factors were key in the success of anterior composites and their early integration. First was the clear Mylar matrix—the simple, single-space filling technique. Second was an early recognition by dentists and dental schools that the cavity preparation benefits from significant changes in the preparations done for silicate restoration or gold foil restorations, which the composites replaced. Thirty years later, after constant evolution, the modern anterior cavity preparation that has little or no mechanical undercuts and long infinity edge margins bears little resemblance to the silicate and gold foil preparations that anterior composite has replaced.

Unlike with anterior composites, the integration of posterior composites has had a tumultuous journey because of the lack of a regimented cavity shape and filling technique. Posterior composites have a somewhat dysfunctional relationship with dental schools. Many dental schools have discouraged the placement of posterior composites, knowing full well that on graduation most dentists would place posterior composites on a wholesale basis. In the schools’ defense, the outcome studies referenced, combined with the instructors’ personal observations of the parade of failing, leaking composite fillings, validated schools’ resistance to embracing the use of posterior composite on a wide-scale basis. It should be noted, though, that the posterior composite can be the most minimally invasive, biomimetic, and esthetically pleasing of all posterior restorations.

The Five Cs

Cracks and Fractures

Cracks and fractures are now the third leading cause of tooth loss in industrialized nations. The original hope was that because composite restorations are adhesive, the composite would be able to repair or splint the tooth together after the cavity preparation has weakened the tooth. Unfortunately, this is not the case, as recent outcome studies have shown the same number of cuspal fractures in amalgam-restored and composite-restored posterior teeth. Some immediate strengthening of the tooth can occur after placement of a composite, but two important factors must be borne in mind. First, crack initiation and propagation in the human dentition take years or even decades to accomplish. Today dentists are seeing cracks and fractures in teeth with posterior composites because enough time has elapsed. Much of the immediate strengthening of the tooth lessens dramatically over time. Many teeth that had posterior composites placed 15, 20, or 30 years ago are now fracturing. Second, it is not possible to predictably rely on composite to keep the tooth splinted long term. That challenges the dental profession to rethink the entire posterior composite approach. Unless the cavity design is changed dramatically, clinicians should consider posterior composites to act as “white amalgam” in terms of tooth fracturing.

Dental practitioners also worry about cracks and fractures in the material itself. A full explanation of composite’s characteristics is beyond the scope of this text. The most important characteristics in preventive dentistry for cracks and fractures can be divided into two different camps: characteristics during placement and polymerization, and characteristics of the composite once it is in the tooth and functional. Radiopacity, flexural strength, modulus of elasticity, fracture toughness, and total fracture work all contribute, but the most important factor from a clinician standpoint is fracture toughness. Catastrophic problems occur with posterior composites lacking fracture toughness. The fracture toughness of composite materials in general is inferior to that of gold—one of the liabilities of composite. These concerns must be overcome.

Crack initiation must be carefully evaluated and managed during cavity preparation and placement of composite and finishing. This addresses both crack initiation in the tooth and crack initiation or propagation in the composite material.

The C-Factor

The C-factor is a concept that is bandied about by many in the dental community. Although the concept has never been proven scientifically, it is the best guide to the management of polymerization shrinkage in various cavity preparations. C-factor stands for configuration factor and expresses the ratio of internal walls versus external surfaces. A second way to describe C-factor is internal surface area versus external surface area. C-factor is a fundamental flaw in traditional cavity preparations because the parallel walls for resistance and retention work against the dentist during polymerization shrinkage (Figure 10-3).

FIGURE 10-3 As the light hits the composite, it will shrink toward the center. Most direct composites shrink 2% to 5%, determined linearly. The linear number is usually a higher number. The way the composite shrinks is very critical and is based on the shape of the cavity preparation, the number of opposing walls, how they oppose each other, and the angle at which they oppose each other. These variables are critical to the behavior of the shrinkage of composites.

As the curing light hits the composite, it will shrink toward the center (Figure 10-4). The shrinkage can be measured as either volume or linearly. On a linear basis, most direct composites shrink 2% to 5%. All composites shrink on polymerization at this point, but the way the composite shrinks is critical and is based on the C-factor. The shape of the cavity preparation, the number of opposing walls, how they oppose one another, and the angle at which they oppose one another are extremely critical to the behavior of composite shrinkage.