Abstract
Introduction
Clinically relevant attributes of an orthopedic bearing material include its strength, oxidative stability, and wear resistance. Recent reductions in bearing wear and oxidation have been realized by crosslinking (HXLPE), and through the incorporation of α-tocopherol (VE). VE infusion has improved the oxidative stability of HXLPE in vitro [1] and in vivo (as evidenced by retrievals) [2]. However, concern has been raised that adding VE may increase the frictional torque of bearings, potentially increasing the risk of trunnionosis [3]. This study compares the 3D frictional torques of HXLPE vs Vitamin-E HXPLE, against 28 mm and 36 mm metal and ceramic heads.
Methods
Samples were made from prints for commercially available Ringloc liners (Biomet, IN). The HXLPE group was made from ArCom XL, and the VE-HXLPE was made from E1 HXLPE (Biomet, IN). Femoral heads were of cobalt chrome (ASTM F1537), or ceramic (Biolox Delta™). Testing was performed at EndoLab®, Germany. An ISO 14242–1 compliant six-station simulator and 3D gait cycle was utilized. During the cycle, the abduction/adduction range was −4°/+7°, the flexion/extension was +25°/-18°, and the external/internal rotation was −10°/+2°. Testing was performed at 37±2 °C, at 1 Hz, and with a maximum dynamic load of 3.0 kN. Lubrication medium was calf serum, EDTA, and antibiotics diluted in DI water (30 g/l of protein). Measurements were averaged across 5 cycles after 120 completed cycles of motion, and after 200 cycles. Analysis was performed using Minitab with multiple 2-way ANOVAs, with a p=0.05 significance threshold.
Results
There was a statistically significant effect from head size (p≤0.0005). 36 mm heads exhibited higher torque (4.25±0.31 Nm, 200 cycles) than the 28 mm heads (2.90±0.08 Nm). There was no statistically significant effect (p≥0.409, all outputs) from head type (metal vs ceramic). In the 36 mm groups, there was no statistically significant effect (p≥0.300, all outputs) from the liner material (HXLPE vs VE-HXLPE). Within the metal head groups, the inclusion of VE statistically significantly decreased (p≤0.018, all outputs) the average mean frictional torques by 8.07±4.6%.
Conclusion
A larger head size increased the frictional torque. In the metal head groups the infusion of VE decreased the frictional torque, and VE did not statistically significantly change the torque in the ceramic groups. A previous report found that VE increased torque [3], but evidence herein does not support that conclusion. Limitations of the previous study include; use of a simplified rotation torque method, non-physiologic lubrication, lack of statistical analysis, unknown implant design, and unknown VE concentration and processing methods. Based on these results, no adverse clinical effects are anticipated due to VE-associated changes in frictional torque. Short-to-mid-term clinical results have not shown any adverse effects [4].
