Letter to the Editor regarding 'How symmetric are metal-on-metal hip resurfacing patients during gait? Insights for the rehabilitation'.
http://dx.doi.org/10.1016/j.jbiomech.2017.07.037 0021-9290/ 2017 Elsevier Ltd. All rights reserved. We read with great interest the article written by Resende and colleagues (Resende et al., 2017) entitled, ‘How symmetric are metal-on-metal hip resurfacing patients during gait? Insights for the rehabilitation’ Hip resurfacing arthroplasty (HRA) is a contentious subject given the high profile withdrawal of the DePuy Orthopaedic ASR metal-on-metal (MoM) implant (DePuy-ASR) due to high failure rates (de Steiger et al., 2011; Prosser et al., 2010). In their welldesigned kinematic study (Resende et al., 2017), the authors assessed whether a unilateral DePuy ASR HRA can result in symmetric gait. We applaud such a study, which aims to take assessment beyond functional scores, which have many well-documented limitations (Fitzpatrick et al., 1998). Despite analysing a withdrawn implant, the study demonstrates that all 12 ASR HRA patients having reported no pain. This is particularly reassuring as the mean time from surgery to gait assessment was 45 months, which puts this group of patients at the highest risk as most revisions occur in the early mid-term (Curtin et al., 2017; Laaksonen et al., 2017). Sadly, though, there was no mention of serum metal ions. It is well reported that raised serum metal ions are correlated to adverse local tissue reactions (ALTRs) (Laaksonen et al., 2017; Reito et al., 2013), which would negatively alter hip biomechanics. The prevalence of ALTRs in asymptomatic patients is also high (Chang et al., 2012; WynnJones et al., 2011). Moreover, the research looked at lower limb kinematic function and there is no evidence that these patients had any further imaging (ultrasound or MARS MRI) to ensure that there was not any surrounding soft tissue destruction. These omissions, which are now standard recommendations (MHRA, 2017), make it difficult to conclude as to whether these were well functioning or failing hips. Furthermore the authors present what appears to be a retrospective study as no preoperative data was presented. It is important to have baseline data in order to appreciate the effect of the intervention, which could be significantly worse or better. A single snapshot study, particularly with small numbers, could easily draw incorrect conclusions. Given this, the title of this paper would be better changed to ‘How symmetric are ASR hip resurfacing patients during gait’. Instead, the current title of the study would be better addressed by prospectively evaluating a HRA implant with a proven track record to answer the important question of whether HRA results in symmetric gait, and to give insight for rehabilitation. Conflict of interest