Perspectives on metal-on-metal hip resurfacing

Abstract

The overall aim of this thesis was to study patients with metal-on-metal hip resurfacing (MoM-HR). More specifically, we investigated if there are any advantages in patient-reported outcome measures (PROM) after MoM-HR compared to conventional total hip arthroplasty (THA), if there are any factors predicting a failing hip resurfacing prosthesis, and potential socioeconomic differences between patients operated with MoM-HR or uncemented THA.

In Study I, the aim was to investigate potential self-reported outcome differences between MoM-HR patients and those operated with conventional hip arthroplasty. It was a nationwide register-based matched cohort study of 726 patients (363 MoM-HR vs. 363 conventional THA). The data was retrieved from the Swedish Hip Arthroplasty Register (SHAR). The outcome measures of interest were postoperative Hip Disability and Osteoarthritis Outcome Score (HOOS), EQ-5D, VAS Pain, and VAS Satisfaction. At a mean 7-year follow-up, MoM-HR patients reported better postoperative adjusted estimates in HOOS ADL (estimate 4.3, 95% CI 1.8–6.9) and HOOS Sport/Rec (estimate 7.8, 95% CI 3.8–12). There were no differences in the other studied functional outcome measures. I presented this study at the 2018 annual meeting of the Orthopaedic Research Society.

Study II was a single-institution cohort study of 288 patients. The aim was to investigate the possibility of eliminating “unsafe” serum metal ions after MoM-HR by careful patient selection and optimal implant positioning. Potential risk factors of elevated serum metal ions were also studied. The main findings were that even if the risk of elevated serum metals could be reduced when operating patients with “optimal” properties and implanting the MoM-HR optimally, the risk was not eliminated. Moreover, insufficient cup anteversion was the strongest risk factor of elevated serum metals >5 µg/l. I presented this study at the 2021 EFFORT Congress.

Study III was a cohort study based on the same cohort as study II. The aim was to study factors associated with prosthesis failure and the necessity of serial follow-up including serum metal analysis and hip X-rays. Incorrect cup anteversion on the postoperative X-ray, serum metal concentrations >5 µg/l at initial follow-up, and femoral head component size <50mm were found to be the main risk factors of revision surgery. The relative risk of revision was 11 times greater in patients with ≥ 1 risk factors compared to those without any risk factors. Although only a few patients had a notable change in either the position of the femoral component or a significant increase in serum metal concentrations, they had an increased risk of revision. I presented this study at the 2024 American Association of Orthopaedic Surgeons annual meeting.

In Study IV, the aim was to investigate preoperative socioeconomic differences between patients who underwent a MoM-HR and a matched group of patients who underwent uncemented THA in a 1:1 ratio. It was a nationwide register-based case-control study of THA-operated patients in Sweden between 1999 and 2014. The study was based on data from the Swedish Hip Arthroplasty Register, the National Patient Register, and Statistics Sweden. In total, 15,871 patients were included in the study. The matched cohort consisted of 1,481 MoM-HR patients and 1,481 uncemented THA patients. In the matched cohort, the proportion of patients with low education and low-income levels was larger in the uncemented THA group compared to the MoM-HR group. Another finding was that patients with low education levels were associated with a lower probability of undergoing a MoM-HR surgery compared to patients with the highest education level.

In conclusion, the potential benefits of MoM-HR do not outweigh its risks, and socioeconomic disparities appear to influence the choice of innovative hip arthroplasty. I hope this thesis provides clarity on the possible advantages and disadvantages of MoM-HR, offering beneficial information to orthopaedic surgeons and patients with hip osteoarthritis in need of hip arthroplasty.