Is hip resurfacing a viable option for active patients with arthritis of the hip? With this question in mind, these authors offer a closer look at the literature and note that this procedure may have key advantages over total hip replacement.

Conventional total hip replacement has led to a remarkably improved quality of life for patients with hip arthritis. However, the unknown life span of conventional total hip replacement presents a significant quandary for young or active patients with hip arthritis. These patients are often caught in the delicate balance between quality of life and the possibility for future revision surgeries. Today, patients demand more from the hip prosthesis and are tipping the balance toward earlier surgery.
       Indeed, patients frequently request hip resurfacing over conventional hip arthroplasty. As a result, hip resurfacing has become the fastest growing orthopaedic surgery in the world.¹
       That said, hip resurfacing is similar to total hip arthroplasty in many ways. The surgical approaches are very similar and acetabulum preparation for both is identical. However, current hip resurfacing procedures use large diameter cups to accommodate large femoral heads. Surgeons place these cups directly to the acetabulum without bone cement, relying on bony ingrowth to the prosthesis.
       Hip resurfacing also differs with total hip arthroplasty in the preparation of the femur. Conventional hip replacement requires complete removal of the femoral head and cutting the neck of the femur. This allows one to place a large stem in the femoral shaft. Resurfacing attempts to preserve most of the femoral head and only replaces the surface of the hip joint. This technique not only preserves the femoral neck but also preserves the femoral canal, a very key point when it comes to revision surgery. All current hip resurfacing manufacturers use cement to fixate the head prosthesis to the femur.

How Hip Resurfacing Has Evolved Over The Years

To better understand the current innovations and technology of hip resurfacing, one must understand the history of the procedure. In fact, the concept was born early in the arthroplasty era, prior to the use of the standard stemmed femoral component which surgeons use in today’s total hip replacements.
       British hip surgeon Sir John Charnley, FRCS was the first to attempt to resurface the femoral head. Charnley used Teflon as an artificial joint surface in the early 1960s.² Rapid wear of the Teflon caused these devices to fail very quickly. Subsequent attempts at resurfacing in the 1970s also failed. Researchers mostly attributed the high failure rates to acetabular failure from osteolysis.³ They believed the osteolysis was caused by high polyethylene wear from equatorial bearing wear forces.
       In the 1980s, the Wagner resurfacing had a very thin (3 mm) polyethylene which articulated with the femoral head.⁴ However, the thin articulation was the cause of high failure rates. Revision rates with the Wagner resurfacing were as high as 60 percent at eight years post-op.⁵ Hip resurfacing in the United States was largely abandoned in the 1990s due to the failures of the last two decades and the success of standard total hip arthroplasty.
       Derek McMinn, FRCS, in the United Kingdom, and Harlan Amstutz, MD, in the United States, began introducing what we know today as the current generation of hip resurfacing.⁶ Modern resurfacing success results from designs of press fit acetabular componenets, large femoral heads and newer metal on metal articulation.

How Does Hip Resurfacing Compare To Conventional Total Hip Arthroplasty?

Granted, the tremendous success of current total hip replacements makes it difficult to make a case for change. However, hip resurfacing offers many unique and exciting advantages over total hip arthroplasty.
       One of the major advantages is increased range of motion. Studies have demonstrated increased hip abduction and extension. However, the clinical relevance of this increased range of motion has yet to be determined.
       Researchers have postulated that hip resurfacing may offer more normal hip kinematics and functionality than conventional hip arthroplasty. Mont et al., compared gait analysis in patients with normal hips, osteoarthritic hips, total hip arthroplasties and hip resurfacing.⁷ The study found that patients who underwent hip resurfacing walked faster than patients with total hip arthroplasty, and walked at a similar pace to patients with normal hips. Researchers also found that hip abductors and extensor movement were preserved in patients who had hip resurfacing as opposed to patients with standard hip replacements. Patients with resurfaced hips also exhibited better overall kinematics.⁷
       The large femoral head design of current hip resurfacing options leads to significantly decreased risk for hip dislocation in comparison to conventional hip arthroplasty. Traditional hip replacements have femoral head sizes of 22 to 32 mm whereas the large femoral heads used in resurfacing procedures have diameters that measure between 36 to 54 mm. Several recent studies demonstrate no dislocations with these large femoral heads.^8,9
       The most obvious advantage of hip resurfacing is the ability to preserve the proximal femur bone stock. Not violating the femoral canal allows surgeons many revision options that would not be available after a routine total hip replacement surgery.
       Conventional hip replacements can lead to femoral bone loss secondary to a phenomenon known as stress shielding. As bony ingrowth occurs between the stem and bone, there is less stress on the bone but this leads to a loss of bone stock due to the greater stiffness of metal in comparison to bone.
       Decreased bone stock leads to significant long-term challenges. Decreased bone stock can lead to fractures around the femoral stem. Also bear in mind that revision surgery becomes much more difficult in these cases.
       Since hip resurfacing does not involve the large stem common to conventional hip replacement, stress shielding and bone loss occur much less with hip resurfacing procedures. Harty et al., performed bone mineral density (BMD) tests on patients who had femoral neck fractures after undergoing resurfacing procedures with the Birmingham implant. They found the BMD did not decrease around the small neck stems.¹⁰ See the sidebar “A Quick Guide To Hip Resurfacing Implant Options.”
       Kishida et al., looked at hip resurfacing versus conventional total hip replacement in regard to bone stock. They concluded that resurfacing preserves bone stock more than total hip replacements by physiologically transferring the load to the proximal femur. Study investigators also concluded that hip resurfacing facilitated significantly better bone density than total hip arthroplasty.¹¹
       Another unique indication for hip resurfacing is significant deformity of the proximal femur. Given the small neck stem surgeons use in hip resurfacing procedures, they can completely avoid the femoral canal. Using a conventional hip stem would be impossible in this patient population. Also, bear in mind that another indication for hip resurfacing is the presence of retained hardware where a standard stem would be impossible.

A Quick Guide To Hip Resurfacing Implant Options

- * Birmingham Hip (Smith & Nephew). This device is used worldwide and has had FDA approval for several years. * Cormet Hip Resurfacing (Stryker Corporation). The FDA approved this hip resurfacing option in 2007. * Conserve Plus (Wright Medical Technology). While this device is utilized in Europe and Asia, it has not yet been approved by the FDA. * Durom (Zimmer). This device has not yet been approved by the FDA. * Articular Surface Replacement (DePuy). While this device has seen widespread use outside of the United States, it has not yet been approved by the FDA.

What You Should Know About Metal On Metal Articulations

Metal on metal articulation is one critical design feature that has led the success of modern resurfacing.
       However, it is not a new concept. Earlier metal on metal implants were attempted in the late 1960s and early 1970s. However, design flaws including poor manufacturing tolerances, early impingement and poor material selection led to increased failure rates.¹² Advances in metallurgy have led to improvements in wear rates and lower tolerance levels.
       Metal on metal articulations are significantly less volumetric with linear wear than metal on polyethylene articulations. This is evidenced in vivo and with hip stimulator testing.¹³ One particular analysis shows that metal on metal articulations have a 60 times lower volumetric wear rate than conventional polyethylene bearings.¹⁴ One critical component in the success of metal on metal bearings is the fluid film lubrication that exists between the cup and head. Polar bearings keep the ideal film lubrication between the cup and head.
       Overall, these changes have led to a very reliable bearing surface that contributes to the success of resurfacing today. Granted, no bearing surface is perfect. Metal on metal articulations do carry certain theoretical disadvantages that must be considered. Potential disadvantages include: carcinogenic concerns, metal hypersensitivity, and the existence of metal particles and metal ions.
       Resurfacing with metal on metal bearings and large heads bring an increased level of metal ions. One recent study looked at serum and urine cobalt and chromium levels over the course of two years following hip resurfacing.¹⁵ Initial serum levels peaked at six and twelve months, and slowly declined over the following year. Like other studies, the study found that metal ion levels decrease over time.¹⁵
       Some have postulated that metal ions may carry a nephrotoxic risk. While no long-term studies have addressed this risk, Back et al., did follow cobalt and chromium levels in the aforementioned study and found no adverse affect on renal function over two years following hip resurfacing.¹⁵ Another theoretical risk revolves around young women who may become pregnant. However, researchers have not yet shown an association between metal ion levels and teratogenicity. Brodner et al., analyzed maternal and placental levels of metal ions, and found that the ions did not cross the placental barrier.¹⁶ While this risk has never been proven, one should mention this possibility to any female of child-bearing age who is considering hip resurfacing.
       Lastly, carcinogenesis is another risk of metal ions that has yet to be proven. To date, there are no reports of increased tumors, leukemia or sarcomas associated with metal on metal prosthesis.

What About Contraindications And Potential Complications?

There are contraindications that one must consider when weighing the option of hip resurfacing.
       Any large defects of the femoral head or neck preclude good fixation of the stem, and can lead to early failure or fracture. In fact, femoral neck fractures make up the most significant complication unique to hip resurfacing.
       In a review of 3,497 patients who underwent hip resurfacing procedures, 1.46 percent incurred a femoral neck fracture.¹⁷ Researchers noted that women were twice as likely to suffer a femoral fracture following hip resurfacing than men. Researchers also found that the mean time to fracture was 15.4 weeks. This suggests that the majority of fractures typically occur early in the postoperative period.¹⁷
       Risk factors for femoral neck fractures reportedly include: intraoperative notching, varus replacement of the femoral stem, poor surgical exposure, surgeon experience, body mass index, female sex, femoral head cysts, patient age greater than 60, inflammatory arthritis and poor bone stock.¹⁸ Femoral neck fractures often displace the neck stem and require revision surgery, typically a revision to a conventional hip arthroplasty. However, Cossey et al., and other researchers have reported on conservative treatment of non-displaced femoral neck fractures.^19,20 They reported successful treatment with non-weightbearing and that the patients have had no functional deficits.
       Due to the large size of the acetabular cup, patients with a small acetabulum or an acetabulum with significant bone loss are not ideally suited for resurfacing.
       A low offset of the hip ball, varus neck shaft angle and limb length discrepancy of greater than 1 cm are all factors that one cannot easily compensate for when it comes to hip resurfacing. Accordingly, when these factors are present, it is better to opt for the conventional hip arthroplasty with modular components.
       Other potential complications include avascular necrosis, heterotopic ossification and infection. Infection rates among hip resurfacing cases are reportedly comparable to infection rates associated with conventional total hip arthroplasty. Treacy et al., recently reported on five-year infection rates with the current Birmingham resurfacing.²¹ There are currently no indications that resurfacing carries any increased risk.

Final Notes

An increasing percentage of patients in their 40s and 50s are undergoing total hip arthroplasty procedures. These patients have life expectancies greater than 30 years. We have also seen changing patient attitudes about hip arthroplasty. Today, patients seek to remain active rather than accept the limitations of hip arthritis. Hip resurfacing embraces functional capacity, bone conservation and increased revision options. Accordingly, this may be an appealing option for properly selected patients.

       Dr. Jinnah is a professor at Wake Forest University who specializes in hip surgery. He has received training at Cambridge University and the University of California at Los Angeles (UCLA). Dr. Jinnah’s previous faculty appointments include positions at Johns Hopkins University, the University of Maryland and the University of Oxford.

       Dr. DeCook is a senior resident in orthopedics at Wake Forest University.