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Technology Overview of Hip Resurfacing

Posted on: 05/20/2010
The American Academy of Orthopaedic Surgeons (AAOS or Academy) asked a panel of nine surgeons from around the United States to review the status of metal-on-metal hip resurfacing and give us a summary. The panel addressed four specific questions:

1) How revision rates compare between metal-on-metal hip resurfacing and total hip replacements.

2) Is it possible to tell which patients are going to have a successful resurfacing result?

3) Which one works better; hip resurfacing or replacement?

4) Are the results better with hip resurfacing when the procedure is done by experienced surgeons only on certain pre-selected patients?

Let's start by reviewing just what is a hip resurfacing procedure? Hip resurfacing arthroplasty is a type of hip replacement that replaces the arthritic surface of the joint but removes far less bone than the traditional total hip replacement.

The operation begins by making an incision in the side of the thigh. This allows the surgeon to see both the femoral head and the acetabulum (or socket). The femoral head is then dislocated out of the socket. Special powered instruments are used to shape the bone of the femoral head so that the new metal surface will fit snugly on top of the bone.

The cap is placed over the smoothed head like a tooth capped by the dentist. The cap is held in place with a small peg that fits down into the bone. The patient must have enough healthy bone to support the cap.

The hip socket may remain unchanged but more often it is replaced with a thin metal cup. A special tool called a reamer is used to remove the cartilage from the acetabulum and shape the socket to fit the acetabular component. Once the shape is correct, the acetabular component is pressed into place in the socket. Friction holds the metal liner in place until bone grows into the holes in the surface and attaches the metal to the bone.

Now, who should have hip resurfacing instead of a complete joint replacement? Because the hip resurfacing removes less bone, it may be preferable for younger patients who are expected to need a second, or revision, hip replacement surgery as they grow older and wear out the original artificial hip replacement.

Resurfacing can be used both for patients with osteoarthritic changes of the hip as well as for those who have dysplasia of the hip from birth. Dysplasia means the hip socket is too shallow to hold the femoral head in place. Partial or complete hip dislocation is often the result. Hip resurfacing may be more successful for hip osteoarthritis than for hip dysplasia but further study is needed before making any recommendations here.

Risk factors for resurfacing failure include small component size (used more often in women than men) and age (risk increases with age; older than 75 has the highest risk of failure). Younger age (less than 55) is more of a risk factor for total hip replacements.

Other factors have been studied such as smoking, body mass index (BMI), activity level, and menopausal status. None of these seemed to be significant in terms of success or failure of the resurfacing procedure. But the studies done so far are fairly limited in scope. The panel could not make any firm statements regarding the effect of these particular patient characteristics on outcomes or revision rates for hip joint resurfacing.

That brings us to the question of which is better: resurfacing or hip replacement? The answer to this remains clouded by the fact that the patients in the two groups are so different by age, weight, and activity level (younger, lighter, and more active in the resurfacing group).

The number and type of complications were also equal between hip resurfacing and total hip replacements. The three major complications associated with these two procedures are infection, loosening, fracture, and dislocation. Overall patient satisfaction was also equal between the two groups.

As for the final question about the difference in results for joint resurfacing based on surgeon expertise and type of patients selected --well, there was some evidence that surgeons experience and surgical technique made a difference. But the studies weren't high quality. The panel could not make any conclusions with confidence in this area.

Where does that leave us in comparing the outcomes and effectiveness of hip joint resurfacing compared with joint replacement? The panel points out that the quality of evidence is lacking. Study design, research methods, and quality of analysis just don't measure up in this area. More studies are needed in matched patient populations directly comparing these two treatment techniques. Long-term studies with follow-up over 10 years or longer are also needed.

This report as a summary of modern metal-on-metal hip resurfacing wasn't meant to provide an official position or recommendations from the Academy. The goal was to summarize the findings of studies to date and offer an educational tool for surgeons who are providing care and treatment for this particular group of patients.

Brian McGrory, MD, et al. Modern Metal-on_Metal Hip Resurfacing. In Journal of the American Academy of Orthopaedic Surgeons. May 2010. Vol. 18. No. 5. Pp. 306-314.

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