Influence of remaining coronal tooth structure location on the fracture resistance of restored endodontically treated anterior teeth.
STATEMENT OF PROBLEM: A restored endodontically treated tooth is less likely to fracture when there is axial tooth structure between the core base and preparation finish line. However, an accurate prognosis requires knowing whether fracture resistance depends on a complete circumferential distribution of tooth structure or tooth structure in a specific location related to the applied force. PURPOSE: This in vitro study investigated the fracture resistance of restored endodontically treated teeth when residual axial tooth structure was limited to one half the circumference of the crown preparation. MATERIAL AND METHODS: Fifty extracted maxillary anterior teeth were sectioned 18 mm from their apices, endodontically treated, and divided into 5 groups of 10 teeth each. Four groups were prepared with full shoulder crown preparations having axial wall heights of 2 mm around the preparation circumferences. In 3 of the groups with axial tooth structure, one half of the axial tooth structure was removed, palatally, labially, or proximally, and groups were identified according to the site of retained coronal tooth structure. For the fifth group, all axial tooth structure was removed to the level of the preparation shoulder. Thus, in 1 group the axial walls were circumferential, 360 degrees around the preparations (Complete group), in 3 groups the axial walls were continuous for 180 degrees (Palatal, Labial, and Proximal groups), and the last group had no retained coronal tooth structure incisal to the finish line (Level group). All 50 prepared teeth were then restored with quartz fiber posts (Bisco), composite resin (Bisco) cores, and metal crowns. A universal testing machine compressively loaded the tooth specimens from the palatal at a crosshead speed of 0.5 cm/min at an angle of 135 degrees to the long axis of teeth until failure occurred. A survival analysis was conducted using a log-rank test followed by Holm-Sidak pairwise tests (alpha=.05) to detect significant differences in median failure load between groups. The mode of failure was determined by visual inspection of all specimens. RESULTS: The median failure load (P<.001) was 607 N, 782 N, 358 N, 375 N, and 172 N for the Complete, Palatal, Labial, Proximal, and Level groups, respectively. The predominant mode of failure was an oblique palatal to facial root fracture for the groups with remaining coronal tooth structure. In the Level group, post debonding was the predominant mode of failure. CONCLUSION: For restored endodontically treated teeth that do not have complete circumferential tooth structure between the core and preparation finish line, the location of the remaining coronal tooth structure may affect their fracture resistance.