Factors affecting dental air-turbine handpiece bearing failure

Abstract

Objectives: To investigate the influence of various factors on air-turbine handpiece bearing failure through developing standard protocols for testing the bearing longevity. Methods: Groups of four air-turbine assemblies (Synea TA-98, W&H, Dentalwerk, Bü rmoos, Austria) were subjected repeatedly to a full binary combinatorial set of operating conditions: with and without lubrication, simulated clinical loading, and corrosion protection, all with autoclaving, to the point of failure. A control set was lubricated only. Lubrication (Assistina, W&H), autoclaving (ST-Im30b, Eschmann Bros & Walsh, West Sussex, England), simulated clinical loading (0.56 N at 45° to the turbine axis, after autoclaving), and corrosion protection during autoclaving (magnesium sacrificial anode) were used as required. Free-running speed (Hz) and bearing resistance (μNm) were determined (Darvell-Dyson testing machine) at baseline and after every 10 cycles until turbine failure. Three-way analysis of variance (lubrication × loading × corrosion protection) of log(cycles to failure), with α = 0.05, was used. Results: All autoclaved turbines had failed by 560 cycles, while the controls failed at 960-1000 cycles. All three main effects were significant: loading (p<10-6), lubrication (p<0.0002), and corrosion protection (p<0.02), as was the interaction lubrication × loading (p<10-6). No other interaction attained significance. Conclusions: Running under load was the most important factor affecting bearing longevity. While autoclaving clearly has a detrimental effect, lubrication effectively increases longevity. A sacrificial anode may be economically worthwhile to extend life further, but low-load usage patterns, as generally instructed, are confirmed as beneficial.