Osteoarthritis is the most common joint disorder worldwide, yet the cause of osteoarthritis of the hip is still unknown. One condition that may play a role is femoro-acetabular impingement (FAI). Osteoarthritis is the most common joint disorder worldwide, yet the cause of osteoarthritis of the hip is still unknown. One condition that may play a role is femoro-acetabular impingement (FAI), in which the femoral head of the thighbone causes damage by rubbing abnormally on the hip socket (acetabulum). FAI caused by an abnormality in the hip socket can lead to osteoarthritis, but it is not known if FAI that is not caused by a defect can also lead to the condition. Recognizing that the Asian lifestyle requires a larger range of hip motion than the Western lifestyle, a new study examined FAI in Japanese patients with normal hips. The study will publish online in the Journal of Orthopaedic Research, the official journal of the Orthopaedic Research Society.

Led by Mitsuyoshi Yamamura of Kyowakai Hospital in Osaka, Japan, researchers conducted a study on five healthy female volunteers between the ages of 18 and 26. They defined impingement using an open-configuration MRI, which allows imaging of the hip joint throughout the entire range of motion, by imaging subjects in the W-sitting position (in which the legs are bent behind the person) in two variations with the legs flexed to different degrees. Images were then obtained for five sitting positions, including sitting straight, bowing while sitting straight, sitting cross-legged, W-sitting, and squatting. Most of these positions are used in eating, socializing and in religious or traditional ceremonies and squatting is the position usually used for defecation in Asia and the Middle East.

The results showed that impingement occurred in all subjects in the W-sitting position and was also seen in two subjects in the squatting position. The largest hip internal rotation angle was seen in the W-sitting position. “No subjects complained of hip pain while maintaining any of the positions, even though the MR imaging process took from ten to 14 minutes,” the authors note.

Populations in the Middle East and Asia have a low incidence of osteoarthritis in those with normal hips even though they regularly adopt positions that induce FAI, which suggests that FAI might not cause degenerative change in the hips. The researchers speculate that this may be related to soft tissue laxity around the hip, citing reports that joint laxity or range of motion differ by race. In addition, they note that impingement did not appear to be associated with pathology both in the present study and another study involving the shoulder area. Another reason FAI may not cause hip damage is that the positions in the study were static, as opposed to repetitive trauma, which the study did not evaluate.

The authors acknowledge that since the study was so small, the findings cannot necessarily be generalized to all Asian populations. Also, it is not known whether the subject in the study will develop osteoarthritis in the future. However, they note it is remarkable that FAI was seen in all five subjects. “This suggests that, depending on race, femoro-acetabular impingement might not always be a cause of osteoarthritis of the hip,” they conclude. “Further work in this area, including healthy males and patients with abnormalities, will confirm this conclusion.”