Abstract
INTRODUCTION
Component positioning of an artificial hip joint plays a key role in durability of implant. Despite the fact that a number of numerical, experimental and clinical studies have been carried out to investigate the effects of cup inclination on polyethylene wear, steep inclination has been reported to be associated with both high and low volumetric wear. Moreover, how cup anteversion affects wear and its interaction with inclination are still unclear. To address these knowledge gaps, in this study wear and contact mechanics of a hip joint under various cup positioning has been investigated by using FEA (Finite Element Analysis).
METHOD
A Pinnacle® Marathon neutral liner 36×56mm was chosen to model the wear and creep over 3 million cycles (mc) based on the Archard's law and modified time hardening model in ANSYS, respectively. Central composite design of response surface method was used to generate 9 FEA runs, where the operative inclination angles varied from 35º, 45º to 55º and operative anteversion angles differed amongst 0º, 15º and 30º. The range of cup angles were chosen to be similar to the Lewinnek “golden” safe zone for dislocation. The gait cycle as specified in ISO 14242-1 was applied to the femoral head.
RESULT
Edge contact takes place with steep inclination and anteversion, resulting in a decreased contact area (wear contour) which intersects with cup rim, consequently increasing the contact pressure and wear penetration depth. According to the FEA result, maximum wear rate, contact pressure, wear depth and Mises stress were found in the model with highest inclination and anteversion angles. However, within the range of positioning angles studied, the predicted volumetric wear rates only vary from 13.68 mm3/mc to 13.92 mm3/mc, which are comparable to the corresponding wear rates measured in vitro, i.e. 12.9±3.8 mm3/mc (inclination = 35º, anteversion = 0°) and 15.4±5.0 mm3/mc (inclination = 55º, anteversion = 0°).
DISCUSSION
Increased wear rate, contact pressure, wear depth and stress were predicted with a combination of steep cup inclination and anteversion, resulting from the edge contact condition. While the increase in stress and wear was modest within the range of positions tested, combinations of higher inclination and version are not uncommon and would be expected to result in more significant increases. Hence in a total hip replacement surgery it is desirable to ensure that the contact area lies well within the bearing area by avoiding excessive inclination and anteversion angles.
