Abstract
Introduction
Dislocation is one of the major factors for revision surgery. Current literature states that the usage of larger bearing couples (> 36 mm) have the potential of reducing the risk of dislocation. Smaller ceramic-on-ceramic bearing couples (< 36 mm) have demonstrated very low wear rates. But does the wear behaviour change with increasing diameter? Therefore, the aim of this study was to compare wear rates of larger ceramic-on-ceramic bearing couples for total hip arthroplasty.
Materials and Methods
Wear tests according to ISO 14242 with 36, 40 and 44 mm zirconia platelet toughened alumina (ZPTA) bearings were performed in a servo-hydraulic hip simulator. In total, the specimens were loaded up to 5 million cycles. Wear was measured gravimetrically every million cycles. For each diameter three different combinations regarding clearance and roundness were chosen. One combination represented in tolerance parts (70 μm clearance, < 5 μm roundness). The other two combinations represented parts at the lower end and at twice the upper end of the tolerance band regarding clearance and out of specification parts regarding the roundness.
Results
In general, ball heads showed higher wear than liners. 44 mm bearings showed highest and 36 mm bearings showed lowest run-in wear rates (Table 1). Except for the 36 mm bearing negligible influence on wear rates caused by the different clearance and roundness was found. The highest linear wear rate was found for the 36 mm bearing having a clearance of 20 μm and a roundness of 15 μm. The smallest linear wear rate was also found for the 36 mm bearing. Here, in tolerance parts were tested.
Discussion and Conclusion
Generally, the current study shows extremely low wear rates for ZPTA/ZPTA even for larger diameter bearings. Except for the 36 mm bearing different machining tolerances in the investigated range seem to have only minor effects on the wear rates of larger diameter bearings under ISO 14242 test conditions. The wear rates determined in the current study might be lower in reality due to metal transfer from the tapers and the cups to the ball heads and liners occurring while assembling and disassembling procedures throughout testing.
Larger diameter bearings increase range of motion, stability and reduce the risk of impingement. Thus, larger diameter ZPTA bearings have the potential to minimize the dislocation risk while at the same time having maintained the extremely low wear rates of small diameter ceramic articulations.
