Abstract
Introduction
The femoral head/stem taper modular junction has several advantages; it also has the potential to result in fretting [1]. Stability of the taper junction is critical in reducing the risk associated with fretting. The purpose of this test was to measure the strength of various commercially available head-stem taper combinations under torsional loads to determine the effect of taper geometry and material on the strength of this taper junction.
Methods and Materials
CoCr femoral heads were tested with trunnions that were machined with both a large and small taper geometry, replicating commercially available stem taper designs, V40 (small) and C (large) (Table-1, Stryker Orthopaedics, NJ).
The femoral heads were assembled onto the trunnions with a 2 kN axial force. A multi-axis test frame (MTS Corp, MN) was used to test the head-trunnion combination by dynamically loading with a torque of ± 5Nm and a constant axial load of 2450N for 1000 cycles at 1.5 Hz (Figure 1). Samples were submerged in 25% diluted Alpha Calf Fraction Serum (Hyclone, UT). Upon completion of the dynamic test, a static torque to failure test was performed where the axial force of 2450N was maintained and the trunnion was rotated to 40° at a rate of 3°/sec.
The torque required to rotate the trunnion by 1° was determined for each specimen. Also, the torsional resistance, defined as change in torque/change in angle in the linear region of the torque-angular displacement data curve, was calculated for all the specimens. A limitation associated with the static test was that at 1° rotation it was difficult to differentiate between rotation of the trunnion inside the femoral head and physical twisting of the trunnion. Specimen groups were compared with a single-factor ANOVA test and a Tukey post hoc test at 95% confidence level.
Results
The dynamic test did not generate any rotation between the trunnion and the head. The difference in torque at 1° of rotation and torsional resistance was not statistically significant between any two specimen groups (Table 2, p > 0.05).
Discussion
The results of this study indicate that neither taper surface area nor material have a significant effect on the strength of the taper junctions under torsional loads, within the range of designs and material combinations tested. Alternatively, the results suggest that taper junctions are appropriately designed to eliminate the effects of taper surface area and material on their strength. Previously in a similar test setup, it was determined that the average torque generated with CoCr femoral heads articulating against a press-fit acetabular shell/polyethylene liner assembly is 3.86 Nm [3]. In the present study, the trunnion-head combinations were cyclically loaded with 5 Nm of torque for 1000 cycles and the strength of the taper junction upon completion of the static test was at minimum six-times greater than the torque generated at the articulating surfaces.
