Operational Stability of Three Tooth Margin Preparation Instruments

Objective: Achieving precise tooth margins is critical for restoration longevity and relies heavily on the clinician's tactile control of the instrument. However, objective, quantitative comparisons of operational stability among modern preparation instruments are lacking. This study aimed to quantitatively evaluate and compare the operational stability of three tooth margin preparation instruments by analyzing vertical force ( Fz ) characteristics under varying settings. Methods and Materials: The ultrasonic preparation device was tested across 10 power levels (groups S1-S10: 2-20 Watts, in 2-Watt increments), and the electric high-speed handpiece was evaluated at five rotational speeds (E1: 10,000 rpm, E2: 50,000 rpm, E3: 100,000 rpm, E4: 150,000 rpm, E5: 200,000 rpm). A turbine handpiece served as the control (group T). Each instrument was rigidly mounted to a multifunctional friction and wear tester via custom clamping collars and performed ten 10-second no-load test cycles. Vertical force ( Fz ) data from the stable operational phase (2-8 seconds) were analyzed. Welch's ANOVA and Dunnett's T3 post hoc tests were used for statistical comparisons (α = 0.05), with the standard deviation of Fz serving as the metric for operational stability. Results: The turbine handpiece (control) generated occlusally directed forces, whereas the electric and ultrasonic preparation devices produced gingivally directed forces. The maximum-power ultrasonic group (S10) showed the highest absolute Fz value (0.1573 ± 0.0290 N), while the maximum-speed electric handpiece (E5) showed the lowest (0.0201 ± 0.0120 N). Welch's ANOVA revealed significant differences between the control and the ultrasonic groups ( p < 0.0001), as well as between the control and the electric groups ( p = 0.0007). The turbine handpiece and high-power ultrasonic groups (S7-S10) exhibited significantly greater force variability (SD: 0.0237-0.0424 N) than the electric handpieces (SD: 0.0053-0.0120 N), indicating lower operational stability. Conclusions: The three tooth margin preparation instruments differed in vertical force direction and magnitude across different settings. The maximum-power ultrasonic preparation device generated the highest force, and the maximum-speed electric handpieces generated the lowest. Higher-power ultrasonic preparation devices and turbine handpieces showed poorer force stability, whereas electric high-speed handpieces maintained superior stability across all speeds.