Dynamic photoelastic analysis of stress distribution in simulated canals using different heat-treated flat-side rotary instruments.

Abstract

Objectives: This study aimed to compare the stress produced by three heat-treated flat-side prototype rotary instruments and a non-flat side instrument on the internal walls of simulated canals with three different curvature degrees using the photoelastic technique.

Materials and methods: Thirty-six resin blocks with simulated canals, comprising three curvature types (45°, 60°, and double curvature), were used in the study, with 12 blocks allocated to each curvature type. These blocks were further divided into four experimental groups (n = 9 per group) based on the heat treatment of the instruments: gold, silver, blue, and a control group with non-flat-side gold instruments. The blocks were analyzed using a circular polariscope setup, with real-time birefringence patterns captured by a digital camera. Stress on canal walls was evaluated using a semi-quantitative scale. Supplementary tests (stereomicroscope, SEM, energy-dispersive X-ray spectroscopy, and DSC) were performed to interpret the results further. Data were analyzed using the Kruskal-Wallis test (α = 5%).

Results: Inter- and intra-observer agreements were 0.91 and 1, respectively. All instruments exhibited high stress patterns on canal walls. The highest stress was observed in the 45° and 60° blocks (middle third) and the double curvature blocks (apical third). The flat-side gold instrument recorded the highest stress in the coronal third, and the flat-side blue in the apical third (p < 0.05). All instruments displayed some degree of distortion after use.

Conclusions: Instrument type and canal curvature significantly influenced stress distribution across root canal thirds. All flat-side instruments exhibited high stress patterns, warranting caution in curved canals due to potential distortion and performance impact.

Clinical relevance: This study recommended caution when using newly designed flat-side instruments in curved canals due to potential stress on canal walls and greater distortion, which may affect performance and durability.