Surface roughness of retrieved femoral heads in CoCr-Polyethylene Hip Bearings – A retrieval assessment with 11-17 years follow-up

Abstract

Metal-on-polyethylene (MPE) bearings have been the gold standard in total hip arthroplasty (THA) for over 40 years. However, even for the improved polyethylenes steadily gathering a reputation of very high wear resistance, it also clear that there are additional issues to consider (Fig. 1). It is well documented that impingement of metal-on-polyethylene (MPE) is a serious risk that can damage the polyethylene liner, produce additional wear debris and also lead to 3rd-body abrasion and roughening of the femoral head. 1-3 Adding to these risks is the contemporary use of larger femoral heads with the consequence of adjusting to thinner polyethylene liners once believed to be a contraindication in cup design. 4 The superior surface finish of CoCr implants mated with polyethylene bearings is believed to be one factor in the longevity of metal-on-polyethylene bearings (MPE). However, it may be that under long-term conditions in vivo, these 3rd body-wear particles eventually degrade the MPE surfaces. 2,5-7 The two-body and three-body wear mechanisms associated with MPE is an unavoidable risk given the nature of the hip mechanics. Studies of cup impingement have documented an incidence ranging 45-68%. However in MOM studies, impingement evidence has climbed to 96%. 8 We therefore sought evidence of roughening damage to the femoral heads as a result of 3rd-body wear. A study of MPE retrievals with average 12-years follow-up (N = 35 cases) 9 described CoCr roughness averaging 62nm (Ra= 41-80nm). A subsequent retrieval study (N=43: 6-years) with half the follow-up time described much lower roughness of 10nm (Ra), with roughness for non-worn surfaces reported as low as 3nm. 10 The 1st goal of this study was determine if femoral heads became rougher or smoother in MPE bearings with time (10-20 years). The 2nd goal was to determine if CoCr roughness simulated in laboratory tests was an adequate representation of that occurring in-vivo.