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Rate of Hip Dislocation Higher After Conversion From Hemiarthroplasty to Total Hip Replacement

There are two main ways to replace a hip joint. The first is with a partial replacement or hemiarthroplasty. The head of the femur (thigh bone) is removed and replaced. The hip socket called an acetabulum isn't replaced. The second choice is a total hip replacement (THR). Both sides of the joint are removed and replaced.

Most of the time, hip replacements go smoothly with good results. But sometimes the THR doesn't work out, and the patient must have a revision surgery. The implant is removed and replaced with another prosthesis.

For patients with a hemiarthroplasty, conversion to a THR may be needed. The patient's native acetabulum is removed and replaced with a new socket. The femoral head implant must be replaced, too. A smaller head is needed to fit into the new socket.

Both the revision surgery and the conversion procedure come with a risk of hip dislocation. In this study, the rate of dislocation is compared between these two operations (revision versus conversion).

Patients who had either a revision operation or a conversion at one hospital by one surgeon were included in the study. All surgeries were done through a posterior (behind the hip) approach. The soft tissues were cut and repaired in the same way between the two procedures.

Comparing the two groups, there were no differences in ages at the time of the revision or conversion for those who did versus those who did not have a dislocation. Size of the implant parts, angles, and positions of the components were also compared.

The hemiarthroplasty conversion group had a much higher rate of dislocations. Some of the patients had multiple dislocations. Hip dislocations occurred most often in hips with smaller femoral heads. And even when the heads were the same size between the groups, the conversion group still had more dislocations.

In fact, the conversion group had more dislocations for every head size. A large head did not necessarily mean good hip stability. It is likely that cutting into the soft tissues around the hip during conversion is the main reason for later hip dislocation. This factor combined with the downsizing of the femoral head may have increased the risk of dislocation for the conversion group.

This study demonstrates the need to find ways to lower dislocation rates after both hip revision and conversion procedures. The surgeon must pay attention to soft-tissue tension on the femoral head and the greater risk of dislocation after conversion from a hemiarthroplasty to a THR.

References: Alexander P. Sah, M.D., and Daniel M. Estok II, M.D. Dislocation Rate After Conversion from Hip Hemiarthroplasty to Total Hip Arthroplasty. In Journal of Bone and Joint Surgery. March 2008. Vol. 90. No. 3. Pp. 506-516.

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