Ask the Doctors May 2004 Issue

Ask Dr. Marks: 05/04

I have had four hip replacements, and my hip still dislocates about every four to five months. What could be causing these failures? I really donít want to go through more surgery. Are there any alternatives?

The prevalence of total hip dislocation in the first year after surgery is between 1.9 and 4 percent. Although the greatest risk of dislocation occurs in the first year, the patient remains at risk for the life of the implant. The cumulative risks of first-time dislocation rises every year.

The Journal of Bone and Joint Surgery recently reported on a study investigating the long-term risks of dislocation after total joint arthroplasty. It was found that at 25 years the risk of dislocation is 7 percent. Patients at the highest risk are females with osteonecrosis (bone death) of the femoral head or a history of a fracture in the femur near the hip joint. Other factors that play lesser roles are inflammatory arthritis and age. Those being over the age of 70 are at greater risk. Of patients who dislocate, most will experience only one dislocation. Some go on to have recurrent dislocations due to a laxity of soft tissues around the joint or a worn-out implant.

Patients with frequent dislocations may need further surgery to regain stability of their artificial hip joint, and surgeons have a number of methods to achieving this stability. They can change the position of the components, tighten the soft tissue envelope by increasing the length of the neck of the femoral component, tightening the muscles around the hip by moving them to a new position further down the femur, or tightening the joint capsule. A larger prosthetic femoral head also increases the stability of the hip. In recent years there has been an acetabular component developed that firmly holds the hip in place, preventing further dislocations. This prosthesis is being used with increasing frequency.

Unfortunately, the nonsurgical alternatives for the repair of a dislocated artificial hip are limited. A brace that extends from above the waist to the knee or below the knee may be helpful to prevent dislocations.

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I understand there are laboratory tests a doctor can perform to see if my medications are working or if Iím having any side effects from my medications. What are they, and what exactly do they reveal?

The use of blood tests to monitor the efficacy of therapy and side effects is a common practice in medicine. For instance, if you are on anti-inflammatories for arthritis, the physician will monitor for side effects that could include abnormal renal function, anemia that could be a sign of bleeding, or liver function tests that might show effects on liver cells. If you are on aspirin in higher doses, a salicylate level test is often used to adjust dosage. In patients with inflammation of muscles who are treated with prednisone, the muscle enzymes CPK and aldolase are followed closely and allow doctors to judge the effect of therapy and adjust the prednisone dosage. In patients who have osteoporosis and are on the commonly used medications (Fosamax, Actonel, Evista), a urine test to assess bone turnover can be used to judge the effect of the therapy, long before a bone density test will show a change.

These are only a few of the hundreds of blood tests that doctors use every day to evaluate and monitor the drugs that patients receive.