Air flow in air turbine handpieces – Leakage by design?

Air-turbine handpieces were first introduced in 1957 and although there have been some improvements in components and ergonomics, these devices do not appear to have been optimized and development appears to have been unsystematic. Furthermore, leakage of air from the head of current air-turbine handpieces is documented to present a risk of surgical emphysema if the oral epithelium is breached. Objectives: To develop a method of investigating the effects of design on performance and to conduct a preliminary investigation of those affecting air flow through the turbine and air leakage from the head casing. Methods: Test rigs were fitted with a turbine-bearing assembly (Topair 796, W&H, Burmoos, Austria). Air inlet and outlet diameters were varied: a) fixed outlet (3.2 mm) and a range of inlet diameters (0.9 – 2.1 mm), and b) fixed inlet (1.3 mm) and a range of outlet diameters (1.0 – 3.0 mm). Free-running speed and stall torque were measured (Darvell-Dyson handpiece-testing machine) for each test rig with a range of casing cover spacings. Results: Generally, increase in inlet diameter increased both speed and torque. However, increasing outlet diameter over the range 1.5 mm to 3.0 mm reduced speed. With smaller outlet diameters (< 2mm) increasing the spacing of the casing cover (in the range 0.05 – 0.20 mm) increased speed but did not appear to influence torque. Conclusion: Leakage of air from the handpiece head casing appeared to be necessary for optimal speed performance. Overall, the test rig was found to be effective for this type of work and might be used to investigate other factors such as rotor and casing design. Such work is essential for a better understanding and development of a theory of dental turbine behaviour. This would provide guidance on improved handpiece design.