Metal on metal (MoM) hip arthroplasty: An observational study on the effect of bearing contact surface area and contact pressure on metal ion levels

Abstract

Metal on metal wear has attracted significant media attention in recent years, owing to the issues associated with adverse reactions to metal ion debris, with subsequent revision arthroplasty procedures. To date, implant orientation has been the key. Studies on ceramic implants have utilised mathematical models in order to assess contact surface areas and pressures generated within the hip joint and their correlation to wear. To date, this has not been undertaken in metal on metal hips. We applied a previously published Hertzian ball in plane model to a cohort of metal on metal hip replacement patients from a single surgeon practice to evaluate the effect of contact pressures on metal ion levels. Statistical analysis was performed using SPSS v20. A total of 58 patients, with a male preponderance (65%), from a single surgeon practice were identified and recruited. Anaverage age of 52 (±9) years, mean body mass index 28.91 kg/m2 (±4.41) with no proportional difference between groups were noted ( p > 0.05). Mean ion levels were calculated for both cobalt 34.93(±21.90) nmol/L and chromium 40.10(±25.85) nmol/L. Implant orientation was recorded and, using the method described, contact pressures were calculated. Correlation testing revealed that abduction angle was significantly associated with cobalt ( r=0.344, p = 0.040) and chromium ( r = 0.336, p = 0.045) levels, but that the contact pressures were not associated with metal ion levels (Co, r = −0.033, p = 0.803, Cr, r = −0.095, p = 0.479).Our results reinforce that it is neither the body mass index nor the contact surface area or pressure generated that increases metal ion level, and that abduction angle remains essential in metal ion generation. The surgeon should be vigilant in ensuring correct implant orientation intra-operatively in order to reduce metal ion generation.