Abstract
Introduction
Because of concerns regarding excessive wear and short-term failures attributed to the metal-on-metal bearings, the use of metal-on-metal hip resurfacing arthroplasty (MOMHRA) has been greatly reduced since 2008, despite great mid-term results for well-designed implants and in certain patient populations. The true cause of excessive wear was then unknown. Therefore, identification of true risk factors for the procedure became paramount to refine indications and improve survivorship outcomes.
Methods
Over the last 10 years, a systematic search of the US national library of Medicine and National institutes of health with the key words “metal-on-metal” and hip resurfacing” was conducted and returned 2186 items. Of these items, 862 were deemed relevant to our research purposes and entered in our center's reference database from which this review was performed.
Results
Edge loading is the main culprit for high wear and high serum Co and Cr ion concentrations because it disrupts the fluid film lubrication of the device. Computation of the contact patch to rim distance (CPR), an estimate of the joint's functional coverage, is the best predictor of potential edge loading and excessive wear. Both in vivo and in vitro studies show that the wear of well-designed and well-positioned MOM bearings diminishes over time with continued use, an advantage only featured by MOM bearings. Systemic wear-related complications and hypersensitivity to metal once thought to be common are in fact rare occurrences. In addition, metal-related revisions only represent a small portion of the various modes of failure encountered with well-designed HRA. In our series of 1321 hips with only 0.5% lost to follow-up, 11 patients underwent revision surgery for excessive wear or adverse local tissue reaction. All but 2 had mal-positioned acetabular components (CPR distance <10mm). One of these 2 patients had serum cobalt and chromium levels of 13 and 9 µg/L respectively, despite a CPR distance of 18.3 mm, while the other showed a peri-prosthetic fluid collection estimated at 111cc on MRI. Component aseptic loosening (acetabular or femoral) remains, as is also the case for total hip arthroplasty (THA), the leading indication for revision surgery, even though substantial progress has been reported to reduce its incidence. Femoral neck fractures and loosening are associated with the surgeon's learning curve which can be avoided with proper training. The survivorship of the femoral component in our series after implementation of surgical technique changes is 99.2% at 10 years for an overall survivorship of 95.5%. A diagnosis of DDH is associated with a higher failure rate, particularly from socket loosening, while women without risk factors have a survivorship of 98.6% at 15 years.
Conclusion
HRA has often been, but should not be associated with the results of large head metal-on-metal THA which often present with a different set of failure mechanisms. Now that most of the risk factors for HRA have been identified, and solutions found, a balanced perspective of its results is needed because the long-term data available validate the low wear of MOM bearings which was predicted in the early hip simulator studies. Considering the tremendous progress made in surgical technique and the advanced tribological knowledge acquired with the last 20 years of investigations related to MOM HRA, all the conditions are present to make this procedure a success for lifetime durability as shown in our series where 44 patients (54 hips) have died without a revision at a mean follow-up time of 9.7 years (range, 1.8 to 19.7). For HRA, when devices with adequate coverage and clearance of the ball by the socket are used, severe developmental dysplasia and inadequate surgical technique are responsible for most failures at 10 to 15 years of follow-up. It is our opinion that the advantages of HRA over THA (such as absence of thigh pain, a low dislocation rate, no taper corrosion, and the preservation of proximal femoral bone mineral density and the ability to maintain high activity levels without penalty) now outweigh the risks of using a MOM bearing.
