Modular implant design affects metal ion release following metal-on-metal hip arthroplasty: a retrospective study on 75 cases

Metal-on-Metal (MoM) total hip arthroplasty (THA) has been associated to wear and metal-ions release, controversially related to a variety of clinical complications. Little is known about the relevant design-dependent parameters involved in this process. The present study investigated the correlation between metal ion release in blood and revision rate as a function of: (i) specific MoM implant modular design parameters, (i.e. acetabular cup and femoral head diameters, taper adapter material and size, femoral neck material and modularity and stem size); (ii) MoM bilaterality. Co and Cr ions concentration levels in blood of 75 patients were retrospectively-evaluated with a mean follow-up of 4.8 years (range: 1.8-6.3). Patients were divided in a unilateral and a bilateral group. Statistical analysis was performed to find any significant difference related to acetabular cup diameter, femoral head diameter, taper adapter material/size, neck material/size and stem size. The bilateral MoM group had 4-times higher metal ion levels in blood than the unilateral one (p=0.017 only Cr), related to a higher revision rate (30% vs 20%): differences were 10-times higher particularly with a 48 mm femoral head diameter (p=0.012) and a Ti-alloy neck (p=0.041). Within the monolateral group using a shorter taper adapter and a shorter neutrally-oriented neck demonstrated higher ion levels (p=0.038 only Cr and p=0.008 only Co, respectively). The aforementioned design-features and MoM bilaterality are important risk-factors for metal-ion release in modular MoM THA.