Assessing the wear of the modular taper interface in retrieved metal-on-metal hip replacements

Measuring the amount of wear in the case of revised hip replacements is considered to be a prerequisite of understanding and assessing true in vivo performance of the implant. Whilst most case studies have been focused on developing methods for measuring wear on the bearing surface an increased revision rate of LHMOM implants compared to resurfacing suggests that all interfaces have to assessed to evaluate overall wear. This paper describes the main characteristics of a method developed by the authors for quantifying taper wear as well as general taper interface parameters.
Previous studies have mostly relied on visual inspection to assess the wear of the taper interface. The method aims at characterizing any surface and form changes caused by the wear process through the use of a roundness measurement machine. The measurement process enables a full 360 degree scan on the taper surface using a diamond tip stylus. Wear is determined by analyzing the data through a custom developed Matlab program. Along with assessing the volumetric wear linear wear and contact parameters can also be determined.
The method was applied to 100 retrieved LHMOM femoral head components in order to quantify the wear at the taper junction. Initial results show that the taper interface is a contributing factor to overall wear with a wear volumes ranging from 0.2 to 25 mm3. The wear on the femoral heads can mostly be characterized as a localized area covering the whole contact length between the stem and the femoral head. Maximum linear depth is mostly present in the distal end of the femoral head. In some cases wear is only present on one half of the contact length suggesting unilateral loading. The results suggest the geometric form errors between components are one of the contributing factors of wear at the taper interface.