File Breakage in Conventional Versus Contracted Endodontic Cavities

OBJECTIVE: To compare rotations to failure and tip separation length of a nickel-titanium (Ni-Ti) rotary instrument within a simulated mesio-buccal canal of a mandibular molar with a conventional or contracted endodontic cavity. METHODS: Two identical lithium disilicate #30 crowns were milled. A conventional or contracted endodontic cavity was prepared. A custom glass tube was fabricated with taper and length replicating a mesio-buccal canal, including buccal and lingual curvature, and placed at the mesio-buccal orifice of each crown, held in a silicone mold. Instrumentation was simulated using 30/.04 Ni-Ti rotary files following manufacturer recommended 1.8 Nm torque and 500 RPM (n=20 per access type). Instrumentation was video recorded to determine time (seconds) and rotations to failure. The length of broken tips was measured. The experimental data were compared using a t-test (significance level 0.05). Stresses in the instruments were examined using finite element analysis. RESULTS: Number of rotations to failure (mean±standard deviation) was 599±126 for conventional and 465±65 and for contracted access; tip separation lengths (mean±standard deviation) were 3.99±0.29 for conventional and 4.90±1.02 mm for contracted access. Number of rotations to failure and tip separation lengths were significantly different between the two access openings (p<0.001). Finite element analysis confirmed higher file curvature and accompanying higher stress levels with contracted access and the maximum stress further from the tip. CONCLUSION: Within the limitations of this study, the contracted access caused earlier failure of the Ni-Ti instrument with longer tip separation lengths than the conventional access due to higher stresses towards the middle section of the instrument.