Influence of different load application devices on fracture resistance of restored premolars

Marginal adaptation of different monolithic zirconia crowns with horizontal and vertical finish lines: A comparative in vitro study

Background

This study evaluated the influence of different tooth preparation techniques and zirconia materials on marginal adaptation.

Methods

Forty-eight healthy human maxillary first premolars were divided into two primary groups based on preparation design: group A (chamfer) and group B (vertical). Within each main group, there were three subgroups, comprising eight teeth each, distinguished by the type of zirconia material employed (Zircad LT, MT, and Prime by Ivoclar Vivadent). All the samples were prepared by the same operator using a dental surveyor. Intraoral scanning was performed on the prepared teeth. SironaInLab CAD 20.0 software was used to design crowns, which were subsequently generated using a 5-axis milling machine. The crowns were cemented to their respective teeth with self-adhesive resin cement. Marginal gap measurements were taken in micrometers (μm) before and after cementation at 16 sites per sample using a digital microscope at×230 magnification. The collected data were evaluated using statistical analysis using the independent t-test, paired t-test, and ANOVA at an 0.05 significance level.

Results

The vertical preparation group exhibited the smallest marginal gap, while the chamfer group displayed the largest. This disparity was statistically significant (P<0.05) for pre- and post-cementation measurements across all materials. There were no significant differences between the different monolithic zirconia crowns.

Conclusion

The vertical preparation design illustrated significantly better marginal adaptation than the chamfer preparation design. Comparisons between materials showed comparable marginal gaps. The mean values of the marginal gaps in all groups increased significantly after cementation.

On the fracture behavior of molar teeth with MOD cavity preparation

Mesial-occlusal-distal (MOD) cavity preparations are commonly used to restore damaged teeth. While numerous in vitro cavity designs have been devised and tested, no analytical frameworks for assessing their resistance to fracture seem to exist. This concern is addressed here by resorting to a 2D slice specimen cut from restored molar teeth with rectangular-base MOD cavity. The evolution of damage due to axial cylindrical indentation is followed in situ. The failure begins with a rapid debonding along the tooth/filler interface and continues with unstable cracking from the cavity corner. The debonding load q_d is fairly fixed while the failure load q_f is insensitive to the presence of filler, increasing with cavity wall thickness h and reducing with cavity depth D. The growth of the corner crack is studied using a 2D fracture analysis in conjunction with the FEM technique. The ratio h = h/D emerges as a viable system parameter. A simple expression for q_f given in terms of h and dentin toughness K_C is developed that predicts well the test data. In vitro studies on full-fledged molar teeth with MOD cavity preparation show that the fracture resistance of filled cavities often exceeds by a large margin that of unfilled ones. Indications are that this may reflect load sharing with the filler. Thus, the fracture resistance of the unfilled cavity provides a lower bound to a compromised MOD filling after long-term aging in the mouth. This bound is well predicted by the slice model. Finally, it is recommended that MOD cavities be prepared, if applicable, such that h > D regardless of the tooth size.

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.

Effects of different cusp coverage restorations on the fracture resistance of endodontically treated maxillary premolars

STATEMENT OF PROBLEM

Cusp coverage restorations for the restoration of endodontically treated teeth represent a conservative approach in terms of function and esthetics. However, limited scientific data are available regarding the optimum reduction design and thicknesses.

PURPOSE

The purpose of this in vitro study was to evaluate the fracture resistance and fracture patterns of cusp coverage restorations with different cusp reduction designs and reduction thicknesses on endodontically treated maxillary premolars (ETMPs) with mesio-occluso-distal (MOD) cavities.

MATERIAL AND METHODS

One hundred sixty-five extracted intact human maxillary premolars were divided into 11 groups: G1-10 (test groups) and G11 (intact group). In the test groups, all of the teeth were restored with composite resin after canal treatments with MOD cavities. However, the cusps of the G1-9 teeth were reduced with combinations of different thicknesses (1.5, 2.5, and 3.5 mm) and designs (beveled, horizontal, and anatomic). The specimens were subjected to 10(5) cycles of 50 N mechanical loading. Next, the specimens were subjected to a compressive load at a crosshead speed of 0.5 mm/min until fracture. The fractured specimens were analyzed to determine the fracture pattern. Two-way ANOVA followed by the Fisher least significant difference (LSD) test was used to analyze the interaction between groups.

RESULTS

The fracture resistance values increased with increases in the cusp reductions. The fracture resistance values of the G6 (2.5 mm, anatomic) and G9 (3.5 mm, anatomic) groups were significantly greater than that of the MOD group. However, the G6, G8 (3.5 mm, horizontal), and G9 groups were comparable with G11. The highest restorable fracture rates were observed in G6 and G9.

CONCLUSIONS

Cusp reduction design and thickness influenced the fracture resistance and fracture patterns of cusp coverage restorations of ETMPs with MOD cavities. The teeth restored with anatomic cusp reduction designs with reduction thicknesses of at least 2.5 mm exhibited greater fracture resistance and more frequent restorable fractures.

Influence of crosshead speed on failure load and failure mode of restored maxillary premolars

We analyzed the effect of the crosshead speed of an applied load on failure load and failure mode of restored human premolars. Fifty intact, noncarious human premolars were selected. Class II mesio-occlusodistal preparations were made with a water-cooled high-speed preparation machine, and the teeth were restored with composite resin. The specimens were divided into five groups (n = 10 each) and tested individually in a mechanical testing machine, in which a 6.0-mm-diameter steel cylinder was mounted to vary the crosshead speed: v0.5: 0.5 mm/min; v1: 1.0 mm/min; v2.5: 2.5 mm/min; v5: 5.0 mm/min; and v10: 10.0 mm/min. The cylinder contacted the facial and lingual ridges beyond the margins of the restorations. Peak load to fracture was measured for each specimen (N). The means were calculated and analyzed with one-way analysis of variance followed by Tukey's test (a = 0.05). The mean load at failure values were (N) as follows: v0.5, 769.4 ± 174.8; v1, 645.2 ± 115.7; v5, 614.3 ± 126.0; v2.5, 609.2 ± 208.1; and v10, 432.5 ± 136.9. The fracture modes were recorded on the basis of the degree of the tooth structural and restorative damage: (I) fracture of the restoration involving a small portion of the tooth; (II) fractures involving the coronal portion of the tooth with cohesive failure of the composite resin; (III) oblique tooth and restoration fracture with periodontal involvement; and (IV) vertical root and coronal fracture. Varying crosshead speeds of 0.5-5.0 mm/min did not influence the failure load of restored maxillary premolars; however, increasing the crosshead speed to 10 mm/min decreased the failure load values and the degree of tooth structural damage.