Posterior Composite Resin Restorations: Keys to Long-Term Survivability

The continued clinical success of light-cured adhesive composite resin restorations depends greatly on attention to detail in each of the steps required to diagnose, prepare, and restore. Particularly in the posterior, the challenges of Class II carious lesions and replacement restorations demand accurate execution of technique.

This article provides a review of the critical factors in direct placement composite resin restorations in the posterior, including isolation, matrix systems, light-curing, and placement methods. Successful implementation of these key elements is essential for survivability of posterior composite restorations.

Techniques for posterior composite resin placement, especially for Class II restorations, have largely focused on minimizing composite resin shrinkage that causes stress within the body of the restoration during light-curing and volumetric shrinkage of the composite that may lead to microscopic gaps between the restorative material and the walls and margins of the restoration. To understand the concerns about polymerization shrinkage stress, clinicians should know the role of the cavity preparation in the development of these stresses as it relates to the C-factor (configuration factor).⁵ C-factor refers to the ratio of a tooth preparation's bonded to unbonded (free) surfaces (cavity walls). The higher the C-factor, the greater the potential for interference between the adhesion of cavity preparation walls and resin-based composite due to volumetric polymerization shrinkage and shrinkage stresses. This phenomenon may cause gaps between the restoration and tooth that could be responsible for postoperative sensitivity and/or recurrent caries and premature restoration failure.

Operator error has been suggested as a significant contributory factor in lack of longevity in posterior composite resin restorations.⁶ With this in mind, recommendations have been made for different placement techniques for Class II composite resins that focus on minimizing technical errors.^4,7-10 Some of the techniques that have been suggested for improved restoration longevity for posterior composite Class II restorations include: (1) incremental placement nanohybrid-hybrid composite; (2) incremental placement nanohybrid composite with first increment of a small amount of flowable in the proximal box; (3) bulk-fill composite resin only; (4) sonic placement of bulk-fill composite resin; (5) dual-cure bulk-fill composite resin; and (6) bulk-fill flowable composite with wear-resistant composite in stress-bearing/wear-prone areas.^11-16 The use of these techniques and advanced materials may overcome the challenges associated with restoration adaptation to cavity walls and margins through the minimization of shrinkage and gaps that occur due to restoration porosity induced by the trapping of air bubbles within high-viscosity composites.¹⁷

Successful light-curing of posterior composite restorations requires both selection of a light-curing unit (LCU) that will provide adequate energy to polymerize composite resin and sound clinical techniques to ensure that the light energy is delivered to the composite assuring adequate photopolymerizaton. When selecting and/or using a curing light, the clinician should have an understanding of the parameters of the LCU to achieve long-lasting restorations, as not all units are equivalent. Important features to consider when selecting a curing light include spectra wavelength, power density, timing for use, availability of accessories, configuration and diameters of curing probes/tips available for a device, and energy source to power the curing device (battery or plug-in), among others.^18,19

Because of variability among light-curing devices, it is important that clinicians are familiar with the unit they are using. A curing light should have a minimum irradiance value of 600 mW/cm² to 1000 mW/cm².¹⁹ While irradiance values are the most common benchmark used when comparing curing lights, they do not provide a complete picture of critical factors.^20,21 With the use of a laser beam analyzer, it recently became possible to perform site-specific measurements of irradiance and power-the beam profile-over the surface of the tips of curing lights.^20,22 The ideal beam profile should be an even distribution of irradiance and power over the entire surface of the light tip. For some lights the beam profile may reveal what appears to be hills and valleys with inconsistent and uneven radiant energy dispersion, ie, "hot" and "cold" spots.^20,22 The clinical implications of a beam profile are that if an overlay of the beam profile were to be placed on a tooth preparation it would reveal the regions of the preparation that are not receiving adequate radiant exposure to cure a dental resin.²³ Clinicians may request that the manufacturer provide the light-curing capacity of their LCU.

Isolation is another essential factor in the success of direct placement dental restorations. A controlled dry field free of saliva, debris, and other contaminants is key when performing operative procedures.²⁴ Available armamentarium includes absorbent cotton products (rolls, parotid shields, gauze), high- and low-volume evacuators (including a hygoformic), combined saliva ejectors and bite blocks, and rubber dam.²⁴

The rubber dam is considered the most effective mode of obtaining field isolation.²⁴ However, studies researching the impact of isolation of posterior restorations, particularly composites, do not conclusively indicate increased survivability associated with the use of this modality.^25,26 Evidence, however, does show that rubber dam isolation is consistent with improved enamel and dentin bonding and decreased microleakage.^27-29 Practitioners should always apply the principles of good isolation using the most appropriate methods to maximize the success of the restoration.

Though the routine placement of Class I composite resin restorations is not particularly difficult, placing a Class II and achieving proximal contact can be challenging. Unlike dental silver amalgam, composite resin is not packable and cannot move a matrix band to achieve an anatomic proximal contact. Composite resin by its chemistry is a viscous liquid that may be moved and displaced but cannot be made denser during placement.^30,31

To address this issue, dentists and manufacturers have designed specialized matrix systems that allow the clinician to achieve an anatomic proximal contact. Thin, dead-soft, stainless-steel matrices (0.001-in thickness) for use with a Tofflemire retainer and sectional matrices (0.001-in thickness) to be used with metal, spring-like rings provide advantages over thicker, more rigid stainless-steel matrices (0.002-in and 0.0015-in thickness) used for dental silver amalgam placement. These ring systems, which may feature enhanced silicone or composite wings, provide additional wedging of teeth to create separation to compensate for the reduced thickness of the matrix band to ensure good proximal contact. They also allow for improved contouring on the facial and lingual surfaces, especially when the preparation extends beyond the tooth line angles, and enable a more anatomic contour. These systems are especially useful for single proximal surface placement when compared to the use of a circumferential band.^24,31 The routine use of sectional matrices is generally accepted as a reliable approach to obtaining anatomically contoured Class II composite resin restorations.¹⁰

Most restorations placed in dental practice are direct composite resins to restore anterior and posterior teeth. It is estimated that 261 million direct composite resin restorations were placed worldwide in 2012.³² Posterior composites perform similar to amalgam.^32-34

For the purposes of decision-making, clinicians should know the problems associated with posterior composites. The most common failure modes reported for posterior composite restorations, especially Class IIs, include secondary caries and material fracture.^35-37 Also, larger composite resin restorations fail at higher rates than for amalgam.^33,38 Unlike amalgam, when posterior composite restorations fail, it happens in rapid progression. Therefore, periodic follow-up appointments are important for early detection and repair of these failures.³⁹ Restorations placed with rubber dam isolation showed significantly fewer material fractures that needed replacement compared with those placed without rubber dam isolation.^2,3 A growing body of evidence has demonstrated that the clinical survival of posterior composites may be >90% after 5 years and >80% after 10 years.^4,34,35,37

Though the use of adhesively placed posterior composite resin restorations has shifted focus to minimally invasive tooth preparation designs, it also has put an emphasis on increased skill among dentists in handling these materials.⁴⁰Best practices to achieve longevity of restorations include following the instructions for use from the manufacturer of the materials being placed, using isolation techniques that achieve a clean, dry field for restoration placement, and cavity preparation design consistent with the removal of caries and any previously existing defective restorations. Placement techniques previously described for composite resin will also contribute to improved clinical success.

Posterior composite resin restorations have demonstrated successful long-term clinical survivability. These restorations require a heightened attention to detail in the selection of devices, LCUs, and matrix systems. The composite materials chosen must be compatible with the curing light being used, and a reproducible technique for tooth isolation during restoration placement must be compatible with the selected material. While the use of these adhesively placed restorations demands considerable skill on the part of the dentist handling the materials, it allows for minimally invasive tooth preparation designs.

Nisha Ganesh, DDS
Assistant Professor, Department of General Dentistry
University of Maryland School of Dentistry
Baltimore, Maryland

Howard E. Strassler, DMD
Professor, Division of Operative Dentistry
Department of General Dentistry
University of Maryland School of Dentistry
Baltimore, Maryland