Explaining the Pain in Hip Replacement

Hip arthroplasty patients with unexplained pain may be due to tissue damage

(RxWiki News) Hip replacements are meant to give patients improved mobility and pain relief. But sometimes, hip replacement patients have post-surgery pain that doctors cannot explain. New research may offer an explanation.

In some cases, patients who have received a metal-on-metal hip implant have unexplained pain in their new hip joint. That pain is more likely due to tissue damage than wear on the implant, according to a study recently presented at a medical conference.

"Stay in contact with your surgeon after joint replacement."

For their study, Douglas E. Padgett, MD, chief of the Hip Service at the Hospital for Special Surgery (HSS) in New York, and colleagues wanted to find out what may be causing unexplained pain in patients with metal-on-metal hip implants.

The hip is a ball-and-socket joint. Over time or due to disease, this joint can become damaged and require replacement. There are a number of implant options for joint replacement, each one made of different materials. One of these options is the metal-on-metal implant, in which both the ball and socket are replaced with a metal prosthesis.

According to Dr. Padgett and colleagues, there is evidence that "failure of metal-on-metal hip devices due to unexplained pain is rising." Both harmful tissue reactions and wear on the implant could be to blame. Therefore, the researchers set out to assess rates of unexplained pain, implant wear and tissue damage in patients who went to the hospital for revision (second) surgery.

The study included 50 patients with hip replacements who went to HSS because of unexplained pain. These patients were compared to a control group of 48 other hip replacement patients who went to HSS because of a loose or out of place implant, infection or fracture.

To measure implant wear and tissue damage, the researchers used clinical exams and magnetic resonance imaging (MRI) in addition to looking at implants after removing them from patients. 

The researchers also looked at the degree of aseptic lymphocytic vasculitis-associated lesions (ALVAL), a sign of harmful tissue reactions to metal. ALVAL scores run the range of a 10 point scale in which higher scores are a sign of worse tissue reactions.

Results showed that 30 patients with unexplained pain (60 percent) had an ALVAL score of 5 or more, signaling a moderate to high harmful reaction to metal. Researchers found some buildup of metal deposits in the soft tissue of 12 percent of the patients.

Compared to the control group, the unexplained pain group had an average synovial thickness that was three times higher and a synovial fluid volume that was five times higher. Synovial membrane is a soft tissue found in certain joints, including the hip. Synovial fluid is also found in these joints. Increased synovial thickness and fluid volume can be signs of a harmful reaction.

High-grade tissue damage was found in 10 times as many patients in the unexplained pain group compared to the control group.

Results did not show any differences between the two groups that may have explained the difference in tissue damage. In other words, age, sex, body mass index (BMI), length of implantation and size or placement of the implants was more or less the same between groups. Additionally, both groups had similar signs of wear on their implants.

"We found that some patients had a significant amount of tissue damage but not a lot of wear, suggesting that factors other than wear are contributing to the problem regardless of whether the patients have pain," said study co-author Timothy Wright, PhD, Kirby Chair of Orthopedic Biomechanics at HSS, in a press statement.

"We have used the information from our study to develop guidelines for patients and surgeons," said Dr. Wright.

According to Dr. Padgett, "Hip implant patients with unexplained pain should be followed closely by their surgeon…Early identification of patients with unexplained pain is vital to avoid significant tissue damage."

This research was presented March 20 at the annual meeting of the American Academy of Orthopaedic Surgeons. As the study has yet to be published in a peer-reviewed, the findings should be interpreted with caution.

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Review Date: 
March 25, 2013