Abstract
INTRODUCTION
Metal-on-metal hip resurfacing arthroplasty (MoMHRA) is a surgical option in the treatment of end-stage hip disease. The measurement of systemic levels of metal ions gives an insight into the wear occurring and is advocated by regulatory bodies as routine practice in the assessment of resurfaced hips. However, the acceptable upper levels of Chromium (Cr) and Cobalt (Co) ions concentration with clinical significance still have to be established. The aim of this study is to address this issue in unilateral and bilateral resurfaced hips.
METHODS
453 patients with unilateral MoMHRA and 139 patients with bilateral MoMHRA at >12 months postoperative were retrospectively identified from an independent hip specialist's database. Routine metal ion levels were measured at last follow-up (ICPMS protocol). Radiological assessment included measurement of acetabular component orientation using EBRA, calculation of contact patch to rim (CPR) distance, and evaluation for any adverse X-ray findings. The cohort was divided into the well functioning group (Group A) and the non-well functioning group (Group B). A well functioning resurfacing gad to fulfil all of the following criteria (bilateral patients had to fulfil criteria for both hips): no patient reported hip complaints, no surgeon detected clinical findings, HHS> 95, CPR distance> 10mm, no abnormal radiological findings and no further operation scheduled. Upper levels (acceptable limits) of Cr/Co were considered to be represented by the top margin of the box-whisker plot [upper limit = 75th quartile value + (1.5 x interquartile range)] in Group A.
RESULTS
251 unilateral MoMHRAs patients (55%) and 58 patients with bilateral MoMHRAs (42%) comprised Group A. The majority of males were in Group A compared to the majority of females who belonged in Group B (p<0.001); subsequently Group A patients had bigger size components (p<0.001). Unilateral Group A [Cr: 2.0 µg/l (SD: 1.5)/ Co: 1.8µg/l (SD: 1.2)] patients had significantly lower ions than Group B [Cr: 7.3µg/l (SD: 17.3)/ Co: 6.6µg/l (SD: 18.1)] patients (p <0.001). Similarly, Group A bilateral patients [Cr: 3.8µg/l (SD: 2.7)/ Co: 2.8µg/l (SD: 1.9)] had significantly lower ions that Group B [Cr: 10.7µg/l (SD: 16)/ Co: 8.5µg/l (SD: 15.8)].
The upper levels (safe were: Cr: 4.6µg/l / Co: 4.0µg/l for unilateral MoMHRAs and Cr: 7.4µg/l / Co: 5.0µg/l for bilateral MoMHRAs. Unilateral MoMHRAs had significantly higher ion levels compared to bilateral patients (p <0.001). Sensitivity and specificity of these upper levels in predicting poor function were respectively 25% and 95% for Cr and 22% and 96% for Co.
DISCUSSION
The findings of this study suggest that both unilaterally and bilaterally resurfaced patients with well functioning implants have low metal ion levels with upper levels of Cr: 4.6µg/l / Co: 4.0µg/l for unilateral MoMHRAs and Cr: 7.4µg/l / Co: 5.0µg/l for bilateral MoMHRAs These results indicate that the upper acceptable limit of metal ion levels in resurfaced hips is lower than the previously MHRA recommended threshold, however the study was conducted with very low tolerance for what was considered a clinically problematic hip. Well-functioning bilateral resurfacing have higher ion levels compared to well-functioning unilateral resurfacing.
