Effect of Preparation Design on Fracture Resistance and Failure Mode of Root Canal Treated Maxillary Premolars Restored with Onlay Restorations

Purpose: Cusp fractures are among the major reasons for the extractions of root-treated teeth. With the intention of increasing information about the mechanical behavior of minimally invasive cusp coverage restorations, the aim of current study was to evaluate the fracture resistance and failure modes of endodontically treated maxillary premolars restored with onlays employing various cusp reduction designs (anatomic, horizontal and beveled) to identify the optimum design and to assess whether these designs influence the restorable fracture rate.
Materials and methods: Fifty defect-free maxillary premolar teeth were divided into 5 groups: onlay with anatomic occlusal reduction design (OA), onlay with horizontal occlusal reduction design (OH), onlay with beveled occlusal reduction design (OB), full-coverage crown (CR) and non-restored sound teeth (NR). In OA, OH, OB and CR groups, the access cavities were sealed using bonded composite after endodontic treatment. For OA, OH and OB groups, cusps were prepared with anatomic, horizontal and beveled designs. Except for NR group, all teeth were restored with resin-infiltrated ceramic restorations. After thermal aging for 10000 cycles between 5°C and 50 °C, all specimens were submitted to compressive axial load at a crosshead speed of 1 mm min-1 till failure. The data were analyzed using ANOVA test.
Results: The highest load to fracture values (N) was recorded in control non-restored group (1657±167.7N), followed by CR, OA, OB and OH groups where the mean values were 887.5±40.3, 789.4±54.8, 722.2±46.2 and 634.8±74.2N respectively. Significant differences were found among fracture values of test groups (p<.05). The intact teeth predominantly fractured with favorable fracture patterns followed by CR group; while both OB and OH groups showed the heights non-favorable pattern of fracture. Group OA represented moderate percentage of favorable fracture.
Conclusions: Root treated teeth restored with anatomic cusp reduction design displayed greater fracture resistance and greater rates of restorable fractures than root treated teeth submitted to horizontal and beveled reduction deigns.