Impacts of Contracted Endodontic Cavities on Instrumentation Efficacy and Biomechanical Responses in Maxillary Molars.

INTRODUCTION: Recently, we reported that in mandibular molars contracted endodontic cavities (CECs) improved fracture strength compared with traditional endodontic cavities (TECs) but compromised instrumentation efficacy in distal canals. This study assessed the impacts of CECs on instrumentation efficacy and axial strain responses in maxillary molars. METHODS: Eighteen extracted intact maxillary molars were imaged with micro-computed tomographic imaging (12-mum voxel), assigned to CEC or TEC groups (n = 9/group), and accessed accordingly. Canals were instrumented (V-Taper2H; SSWhite Dental, Lakewood, NJ) with 2.5% sodium hypochlorite irrigation, reimaged, and the proportion of the modified canal wall determined. Cavities were restored with bonded composite resin (TPH-Spectra-LV; Dentsply International, York, PA). Another 28 similar molars (n = 14/group) with linear strain gauges (Showa Measuring Instruments, Tokyo, Japan) attached to mesiobuccal and palatal roots were subjected to load cycles (50-150 N) in the Instron Universal Testing machine (Instron, Canton, MA), and the axial microstrain was recorded before access and after restoration. These 28 molars and additional 11 intact molars (control) were cyclically fatigued (1 million cycles, 5-50 N, 15 Hz) and subsequently loaded to failure. Data were analyzed by the Wilcoxon rank sum and Kruskal-Wallis tests (alpha = 0.05). RESULTS: The overall mean proportion of the modified canal wall did not differ significantly between CECs (49.7% +/- 12.0%) and TECs (44.7% +/- 9.0%). Relative changes in axial microstrain responses to load varied in both groups. The mean load at failure for CECs (1703 +/- 558 N) did not differ significantly from TECs (1384 +/- 377 N) and was significantly lower (P < .005) for both groups compared with intact molars (2457 +/- 941 N). CONCLUSIONS: In maxillary molars tested in vitro, CECs did not impact instrumentation efficacy and biomechanical responses compared with TECs.