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Update on Hip Resurfacing Using Metal-on-Metal Implants

Posted on: 08/21/2008
It's been 10 years since the first total hip resurfacing was developed as an alternative treatment to total hip replacement. 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.

It is used most often in England, Australia, and Western Europe. Several implants have received FDA approval for use in the U.S. We may expect to see results of studies from the U.S. but for now, most data comes from these other countries.

And thanks to researchers and scientists who presented the results of their studies at the First Annual Course on Total Hip Resurfacing, we have some important knowledge about this procedure. In this report, the status of cementless acetabular metal-on-metal implants is presented.

The acetabulum refers to the hip socket where a thin-shelled metal cup is inserted. The femoral component used during hip resurfacing is placed on the outside of the femoral head. The femoral shaft is preserved.

The greatest success appears to depend on choosing the right patient for the procedure. Good bone stock and bone density is essential. Femoral neck fractures are the number one cause of resurfacing failure.

If the femoral neck isn't strong enough with some give (bending strength), there is a great risk of fracture during the first six months. Women with osteopenia or osteoporosis and men with small bones are most susceptible to this problem. These two groups of patients may not be eligible for joint resurfacing. Older adults (65 years of age and older) and anyone who is obese (body mass index greater than 35) are also excluded.

After 10 years of study, it looks like younger men (between the ages of 40 and 60) are the best candidates. But as the implants, technology, and techniques improve, this may change. Patients especially interested in this bone-sparing treatment alternative are athletes and sports participants. Early intervention for painful, limiting arthritis with joint resurfacing may result in continued sports activity.

Once the proper patients have been selected, surgeons must use careful placement of the implant. Just the right amount of femoral neck-shaft angle is important. Too much varus (inward angulation) can place excess strain on the bone (especially at the neck where fractures tend to occur).

Another important surgical technique is cement distribution. Too much cement or uneven distribution can lead to problems. Damage to the bone can result in fracture. Newer implants are cementless. In the case of the acetabular component, the liner is press-fit into the space without any potentially damaging adhesives (glues or cements). X-rays are used to confirm the position and fit of the acetabular implant.

Much of the research has focused on the effects of metal (cobalt-chromium) alloys. Wear patterns and exposure to tiny metal ions that flake off and enter the blood or urine have been studied in cadavers (joints preserved for study) and in live patients. It appears that acetabular cup angle makes a difference in the amount of metal measured in blood levels.

The resurfacing procedure isn't perfect yet. The number of femoral fractures is down but sometimes revision procedures are needed. Defects in the bone, excess wear, or loosening of the implant may lead to a second operation. Usually, the joint is converted to a total hip replacement.

For now, the resurfacing technique is considered an alternative (not a replacement) for the total joint procedure. It may save bone and make it possible not only to remain active but also to convert to a total hip replacement years later (if needed).

As surgeons continue to refine the technique and manufacturers improve the implants, hip resurfacing will have improved outcomes. There will be fewer complications. More patients will qualify for this type of surgery.

Technology will also aid in improving the results of this procedure. Advanced CT navigation methods will allow surgeons to reduce errors and improve accuracy of implant placement. And future materials using ceramics and polymers may be developed to reduce internal toxicity to patients.

Many more studies are needed to analyze the effect of resurfacing on motion, activity level, and function. Future studies comparing resurfacing to standard joint replacement has some merit. But the authors suggest one approach may not be superior to the other. They each offer a treatment option to specific (different) patient groups.

Michael A. Mont, MD, and Thomas P. Schmalzried, MD. Modern Metal-On-Metal Hip Resurfacing: Important Observations From the First Ten Years. In The Journal of Bone and Joint Surgery. Supplement 2008. Vol. 90-A. No. 3. Pp. 3-11.

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