Features July 2005 Issue

Hip Resurfacing: Not Yet Ready For Prime Time

A controversial alternative to hip replacement still has a hill to climb to prove its long-term effectiveness.

Though total hip replacement (THR) is a well-established and highly successful procedure that brings relief to some 200,000 Americans a year, it may soon have competition from another procedure expected to win approval in 2005 from the Food and Drug Administration—hip resurfacing.

Unlike THR, which removes the head and neck of the femur and replaces them with a metal ball and stem, resurfacing keeps the neck intact and resurfaces and reshapes the head, covering it with a metal cap. Both procedures require a cup to be placed in the hip socket (acetabulum).

Though hip resurfacing is not yet approved in the U.S.—resurfacing of the femoral head currently does have FDA approval, and one resurfacing device could receive FDA approval as early as next month—it has been in clinical use in Canada, Europe, and Australia for several years. Surgeons in those countries claim the procedure allows for greater range of motion than THR and often recommend it for younger, more active patients who still have healthy bone stock. Because both parts of the resurfacing implant (cap and cup) are made of metal (chromium-cobalt), physicians also believe that the device has the potential to last considerably longer than a traditional THR.

However, many U.S. surgeons remain skeptical of resurfacing, and are waiting for clinical studies to prove its long-term effectiveness.

“Resurfacing sounds good, and a lot of patients are asking about it, but we’re waiting for the long-term results to emerge,” says Mark I. Froimson, M.D., an orthopaedic surgeon at The Cleveland Clinic. “Until then, we aren’t ready to embrace a promising but unproven procedure.”

Size matters
The range of motion allowed by a hip implant is determined, in part, by the size of the femoral head used—the larger the head, the greater the range of motion permitted. Total hip replacements have traditionally used an all-metal ball that is about 60 percent the size of the original, approximately 28-32 millimeters in diameter. In resurfacing, the head can be restored to its original size, up to 52mm.

“Using a smaller head in THR is a biomechanical trade off,” says Wael Barsoum, M.D., an orthopaedic surgeon at The Cleveland Clinic. “You give up some range of motion in order to reduce friction.” 

Reducing friction
Friction, and the debris it creates, is the bane of hip implant longevity. The wear created by the metal head rotating against the hard plastic liner of the socket cup creates debris particles that can cause the THR to loosen or wear out prematurely. Using a smaller head creates less friction and helps slow the wear of the plastic liner, but these metal-on-plastic designs still often need to be replaced within 15 to 20 years, if not sooner.

Newer THR designs further reduce wear through the use of higher-tech materials. Instead of metal-on-plastic, some systems use ceramic-on-ceramic, others metal-on-metal. Both approaches, theoretically, could last much longer, perhaps 50 years or more, but these systems are so new that no meaningful long-term data exists.

Because the resurfacing implants are also metal-on-metal, they may prove to be just as durable as these new THR designs. But that doesn’t mean they are debris-free. “There is likely to be some release of metal ions into the blood stream, but of uncertain clinical consequence,” says Dr. Froimson. “It may be nothing, we just don’t know.”

Lower dislocation risk
Another advantage of resurfacing’s use of a larger femur head is a lower risk of dislocation. “The larger the head, the more stable the implant,” says Dr. Froimson. “With a smaller head, it’s easier for it to get out of place.”

While those receiving a THR have a dislocation risk of 1-3 percent over the life of the implant, a British study of 1,839 resurfacing patients found only one occurrence of dislocation—.05 percent—after following the group for eight years.

Higher fracture risk
What concerns U.S. surgeons most about the resurfacing procedure is fracture risk. “With resurfacing you face the risk of fracturing the femoral neck, which is left intact and is usually the weakest part of the natural hip joint,” says Dr. Froimson. “Such risk doesn’t exist with THR because the neck is replaced with metal.”

Approximately 1-2 percent of those who have undergone hip resurfacing will have such a fracture. This may occur because the remaining bone is not healthy enough; older patients, in particular, may not be suitable candidates for resurfacing because of poor bone quality. Or the femoral neck may fracture because the cap and cup are not fitted precisely enough to allow normal transfer of forces across the joint.

Hip resurfacing (left): The jury is still out on whether this new procedure is as good as, or better than, hip replacement (right).

Surgical challenge
Making sure the acetabular cup is properly positioned is one of the surgical challenges of resurfacing. “If the metal head doesn’t fit perfectly within the metal socket, you can create dangerous torque on the joint and fracture the femoral neck,” says Dr. Barsoum.

Properly placing the acetabular cup is easier in THR because the surgeon cuts away the femoral head in order to visualize the cup. But in resurfacing, the head is left intact, making it harder to see the cup.

“It makes the procedure more technically difficult, and more likely to lead to positioning errors,” says Dr. Froimson. Surgeons also make a larger incision in resurfacing than in THR, to help them see around the femoral head.

Burden of proof
The bottom line in evaluating any new medical procedure, beyond its safety, is whether it’s at least as good as what it is designed  to replace. And since the results from THR keep getting better, hip resurfacing has quite a hill to climb.

“THR is an extremely successful procedure, and it keeps improving in terms of durability and range of motion,” says Dr. Barsoum. “Ceramic-on-ceramic hip implants have been in use now for at least four years, and they give all appearances, at least in lab testing, of lasting well beyond the 20 years of metal-on-plastic systems.

“Why introduce the possible complication of femoral neck fracture when we have a familiar procedure and technology that can solve the problem for 99 percent of patients who ask about resurfacing?”

What resurfacing needs in order to gain physician acceptance is a long-term track record. As things currently stand, orthopaedic experts just don’t know if the promises of the new approach will hold up over time.

“We’re a conservative group by nature, “says Dr. Froimson. “We’ve seen lots of promising ideas come along that just didn’t pan out, so we’re slow to embrace anything new until it clearly proves itself.”