Class II composite restorations: importance of cervical enamel in vitro

Laboratory methods to simulate the mechanical degradation of resin composite restorations

OBJECTIVES

This study reviewed the literature to identify in vitro approaches that have been used to simulate the mechanical degradation and fatigue of resin composite restorations.

METHODS

A search for articles was carried out in 4 databases and included studies in which composite restorations were bonded to teeth and subject to cyclic loading. Articles were assessed for eligibility, and the following items were the extracted from the included studies: authors, country, year, materials tested, simulation device and details including load magnitude and frequency, number of cycles, type of antagonist, test medium, and temperature. Data were analyzed descriptively.

RESULTS

The 49 studies included showed a high level of heterogeneity in methods, devices, and test parameters. Nineteen different simulation devices were used, applying loads varying between 30 and 2900 N, and frequencies varying between 0.4 and 12 Hz. The load and frequency used most often were ~ 50 N (63.3%) and 1.5-1.7 Hz (32.7%). The number of cycles varied between 10 K and 2.4 M, 1.2 M was the most prevalent (40.8%). The majority of studies combined cyclic loading with at least one additional aging method: static liquid storage, thermo-mechanical cycling applied simultaneously, and thermal cycling as a discrete aging step were the three most frequent methods. The overall evidence indicated reporting problems, and suggested a lack of clinical validation of the research methods used.

SIGNIFICANCE

Validation studies, underlying clinical supporting data, and better reporting practices are needed for further improving research on the topic. Specific suggestions for future studies are provided.

Influence of adhesive-composite application modalities on their bonding to tooth structure and resistance of the performed restorations to failure

Background/purpose

The longevity of bonded composite restorations could be affected by the utilized adhesive-composite application techniques. This in vitro study aimed to evaluate the influence of adhesive-composite application modalities on their bonding values to tooth structure and on the failure resistance of the performed restorations on loading.

Materials and methods

Resin composite studs, 2 mm in diameter and 4 mm high, were bonded in 2 groups to flattened enamel and dentin surfaces of 80 extracted premolars using pre-cured (PC) and co-cured (CC) self-etch resin adhesive. Studs in each group were built-up in 4 subgroups using either multiple increments of nano-filled composite (IF, control) or single increment of preheated nano-filled (PH), bulk-fill (BF) and sonic-activated bulk-fill composites (SF). Another 80 premolars with standard class II cavities were also restored using the same adhesive-composite application modalities. All specimens were then stressed on a universal testing machine to assess the composite-tooth shear bond strength and the resistance of the performed restorations to failure. The modes of specimens’ failure were also assessed following each test.

Results

The PC adhesive provided higher bond strength to dentin (p < 0.05) and comparable bond strength to enamel in comparison to the CC one (p>0.05%). Both PH and BF composites showed lower bond strength to dentin in presence of PC adhesive (p < 0.05). Comparable bond strengths were noticed for PH, BF and SF composites to dentin in presence of CC adhesive (p > 0.05). PH and SF restorations presented the highest resistance to failure (p < 0.05).

Conclusion

Both incrementally and bulky-inserted composites offer clinically acceptable bond strength in presence of pre-cured resin adhesive. Both Preheated and sonic-activated composite restorations offer the highest resistance to failure on loading. The preheating procedure renders regular composite material suitable for bulk-fill applications.

Fracture strength of restorations in proximal cavities of primary molars

This study evaluated the fracture strength of various restorative materials for primary molars in dovetail and box-only class II cavity designs. Eighty extracted noncarious human primary molars were used. The teeth were randomly divided into two groups for either dovetail or box-only preparations. The teeth were then divided into four subgroups for each restorative material: glass ionomer cement (GIC), resin-modified glass ionomer cement (RMGIC), compomer, and composite. The restorations were tested for fracture strength. The loads at fracture and fracture mode were recorded and a scanning electron microscopy analysis was performed to observe the micromorphology of the borders between the teeth and the materials. The nonparametric Kruskal-Wallis and Mann-Whitney U-tests were used. Although there were significant differences between the restorative materials (p < 0.05), there were no differences between the fracture strength of the box-only and the dovetail cavity designs in any of the groups (p > 0.05) except the composite group. The fracture strength of the compomer and composite groups was significantly higher than that of the GIC and RMGIC groups (p < 0.05). A class II cavity could be selected as dovetail or box-only and compomer and composite are more resistant to fracture than GIC and RMGIC.