Friction in orthopaedic zirconia taper assemblies

The torque resistance of zirconia ceramic heads/titanium taper trunnion jun ctions was tested in accordance with ISO 7206-9:1994(E); using twelve modif ied heads of 32 mm diameter under representative physiological conditions. Test parameters studied included assembly force, vertical load during test (test load) and head length. Mean torque resistances measured were 8.9 N m for a 1 kN test load and 15 N m at 4 kN test load. Coefficients of friction calculated for the torsional stability ranged from 0.06 to greater than 1. 0. Multiple regression analysis confirmed that the failure torques measured we re significantly dependent on test load (beta = 0.77; P < 0.001) whereas as sembly force and head length played a lesser, insignificant, part in the va riation. Data from push-on/pull-off tests were used to calculate coefficien ts of friction under axial loading, which were significantly correlated wit h taper angle and material. Torque testing shows greater variability than p ush-on/pull-off tests for similar combinations, and for zirconia heads on o ther tapers. The coefficients of friction measured (0.16-0.31) are signific antly different from values typically used in stress analyses.