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Surface Replacement Hip Arthroplasty
Currently, younger patients make up the fastest growing group of total hip arthroplasty (THA) patients; however, American Academy of Hip and Knee Surgeons (AAHKS) surveys have demonstrated that younger patients report decreased satisfaction and less frequent return to high-level activities than older THA patients. Surface replacement arthroplasty (SRA) is a potential treatment option for this group of younger, high-demand, hip arthroplasty patients.
Hip resurfacing is not a new concept. Early designs for SRA, beginning in the 1950s through 1970s, often used polyethylene components that produced unacceptable wear with excess acetabular bone loss and implant failure. Retrieval data from the Wagner and Amstutz implants demonstrated especially high rates of wear and osteolysis, leading to decreased interest in SRA by the early 1980s. The technique remained appealing, however, especially in younger patients, because SRA allows greater preservation of bone stock (on the femoral side) and more anatomic restoration of the hip joint compared to THA.
Metal-on-metal THA implants were used in Europe for many years but were not commonly accepted in the United States. First-generation European metal-on-metal components, including the McKee-Farrar and Ring designs, demonstrated low wear rates on retrieval analysis. Using metallurgy and technology from the successful metal-on-metal THAs, McMinn and Treacy introduced the modern SRA, the Birmingham Hip Resurfacing (BHR) System (Smith and Nephew Memphis, TN) ( Fig. 4.1 ) in 1997. Early SRA issues due to thin polyethylene components and acetabular bone loss were eliminated by solid metal ingrowth acetabular components. Manufacturing techniques were developed for the BHR to mimic the high carbide content, low tolerances, and high implant stiffness that were successful in the McKee-Farrar and Ring THA designs.
Birmingham Hip Resurfacing System (Smith & Nephew, Memphis TN).
With over 20 years of use outside the United States, SRA use received Food and Drug Administration (FDA) approval in 2006. Although many modern SRA designs have been used, the BHR remains the only SRA currently approved and available for use in the United States. Additionally, the BHR is one of two SRA implants to receive a 10A or better rating from the Orthopaedic Device Evaluation Panel (ODEP) and remains the only SRA implant to receive the maximum 10A∗ ODEP rating.
Results
SRA remains a technically challenging procedure with increased implant costs. Thus, compared to THA, SRA must demonstrate similar revision and complication rates along with clinical advantages to warrant its continued use.
According to Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) data, the overall revision rate for SRA is greater than that of THA at 5 and 8 years; however, certain patient factors equalize revision rates: primary diagnosis, age, gender, and component size. Osteoarthritis has the lowest early revision rate, with other diagnoses such as osteonecrosis, dysplasia, and inflammatory arthritis showing significantly higher revision rates. For men younger than 55 years old with a primary diagnosis of osteoarthritis, SRA has equivalent or superior survivorship to THA at 8 years. The BHR has shown superior survivorship to other SRA designs in the registry.
Surgeons from Oxford, England, have demonstrated greater than 98% BHR survivorship at 12 to 15 years in appropriately selected patients. In a more recent report from a single American institution, 10-year survivorship was 98% for men younger than 60 years old with a diagnosis of osteoarthritis and femoral head sizes of at least 48 mm. Another North American multi-center study demonstrated equivalent complication and revision rates at 2-year follow-up compared to published THA results, and a recent systematic review found similar revision and complication rates.
There also is evidence to suggest that SRA patients have superior activity scores and faster return to sport than THA patients. Studies have demonstrated that SRA more accurately restores leg lengths and femoral offset; 100% restoration of femoral neck bone density also is achieved by 6 months after surgery, which may play a part in the reported decrease in thigh pain for SRA patients compared to THA patients.
Cobalt and chromium ion debris causing adverse local soft-tissue reactions or systemic toxicity remains a major concern with SRA. Current AAHKS guidelines recommend cobalt and chromium blood level checks on any SRA patient demonstrating symptoms of adverse tissue reaction or metal toxicity. For patients with bilateral BHR, a maximal threshold value of 5.5 μg/L for both cobalt and chromium is sensitive for identifying patients at risk for metallosis. An asymptomatic increase in metal ion levels also has been reported between early and mid-term follow-up in patients with unilateral SRA. If elevated blood cobalt and chromium levels are found, advanced imaging with either metal-subtraction sequence MRI or ultrasound is recommended to evaluate for local soft-tissue reaction. Certain SRA implants have demonstrated substantial issues with metallosis and soft-tissue reactions, but the BHR has demonstrated a less than 1% rate of adverse soft-tissue reaction secondary to metallosis in appropriately selected patients. Appropriate component positioning plays an important role in decreasing the potential for edge wear and the development of metallosis.
Indications and Patient Selection
Currently, SRA is most often indicated for young (<65 years old), highly active males with femoral head sizes greater than or equal to 48 mm who have a diagnosis of osteoarthritis. Near-perfect proximal femoral anatomy is required; patients with large femoral head cysts or osteonecrosis are not considered SRA candidates. Preoperative leg-length discrepancy is a relative contraindication to SRA. Patients with pre-existing renal disease also are not considered candidates for SRA because of the potential for metallosis. At our institution, we also no longer consider patients with significant acetabular dysplasia to be arthroplasty candidates.
Preoperative Radiographic Evaluation and Templating
As with all arthroplasty surgery, careful preoperative templating, radiographic evaluation, and planning are critical to success ( Fig. 4.2 ). The femoral head and neck bone quality should be normal. Significant cystic change within the femoral head is a contraindication for SRA. If the femoral neck is enlarged by remodeling, there may not be a clear delineation between the head and neck, with the head being larger than the neck. If the neck and head are of the same width, especially along the superior neck as seen on an anteroposterior radiograph, then removing bone from the head will risk notching the femoral neck and thus risk neck fracture.
The first step in templating is to measure the size of the femoral component. A template is laid over a radiograph of the proximal femur. The width of the opening of the femoral component should be wider than the femoral neck by 2 to 4 mm total. If not, the next larger template should be used. Then, the center post of the implant is aligned over the center of the femoral neck on radiograph. The line from the top of the greater trochanter to where the line on the template intersects the lateral cortex is measured and documented ( Fig. 4.2A ). This distance will be used when measuring the valgus angle of the implant intraoperatively ( Fig. 4.2B ).
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