Defects in ProTaper S1 instruments after clinical use: fractographic examination

Area of Science:

• Endodontics
• Materials Science
• Dental Instrument Failure Analysis

Background:

• Nickel-titanium rotary instruments are widely used in endodontic procedures.
• Instrument separation during clinical use is a significant concern for dentists.
• Understanding failure modes is essential for improving instrument design and clinical protocols.

Purpose of the Study:

• To investigate the failure modes of ProTaper S1 nickel-titanium instruments separated during clinical use.
• To differentiate between torsional, flexural, shear, and fatigue failure mechanisms.
• To correlate fracture surface characteristics with failure modes.

Main Methods:

• Analysis of 27 fractured ProTaper S1 instruments using scanning electron microscopy (SEM).
• Classification of fractures based on lateral view (torsional/flexural) and fracture surface characteristics (shear/fatigue).
• Statistical comparison of fractured segment lengths between shear and fatigue failure groups using Student's t-test.

Main Results:

• Lateral examination classified most fractures as flexural (25/27).
• High-magnification SEM revealed fatigue striations in 18 instruments, indicating fatigue failure.
• Shear fracture characteristics were observed in 9 instruments; fatigue was absent.
• Mean fractured segment length was significantly greater in fatigue failures (4.3 mm) than shear failures (2.5 mm).

Conclusions:

• Macroscopic or lateral examination is insufficient to determine the true failure mechanism of nickel-titanium instruments.
• High-magnification SEM analysis of the fracture surface is essential for accurate failure mode identification.
• Fatigue appears to be a significant factor contributing to the separation of rotary endodontic instruments during clinical use.