Femoro-acetabular impingement may not always cause hip osteoarthritis, study suggests

Researchers found that impingement frequently occurs during common Japanese activities requiring a large range of hip motion.

Although femoro-acetabular impingement frequently can occur during common activities, such as sitting in certain positions, it may not always lead to hip osteoarthritis, a study by researchers in Japan suggests.

Femoro-acetabular impingement that results from a hip defect has been proposed as a cause of primary hip osteoarthritis (OA). However, the prevalence of primary hip OA is lower among Japanese and other Asian populations, despite cultural activities commonly requiring a larger range of hip motion compared to Western lifestyles, according to a press release announcing the study findings.

Because of this, Mitsuyoshi Yamamura, MD, and colleagues at Kyowakai Hospital in Osaka, investigated whether impingement actually occurs during some traditional Japanese hip positions. The researchers used open-configuration MRI to examine the hips of five healthy Japanese women aged 18 to 26 years during five sitting postures, including: sitting straight, bowing while sitting straight, sitting cross-legged, squatting and W-sitting, which involves bending the legs behind the person.

Investigators detected points of impingement by multiple plane reconstructed (MPR) views and with the acetabular rim depicted circumferentially, according to the study, published online ahead of print on the Web site for the Journal of Orthopaedic Research.

"Impingement was considered to have occurred when, on MRI, the anterior femoral head-neck junction approached the acetabular rim and the femoral head was seen to float from the bottom of the acetabulum with the acetabular rim acting as a fulcrum," the authors wrote.

"No subjects complained of hip pain while maintaining any of the positions, even though the MR imaging process took from 10 to 14 minutes," they noted.

The researchers found that impingement occurred in all hips during the W-sitting position and in two of the five subjects during squatting. Also, the largest hip internal rotation angle was seen in the W-sitting position, according to the release.

"These findings show that impingement occurs frequently during daily Japanese activities. Thus, depending on race, femoro-acetabular impingement might not always cause primary osteoarthritis of the hip," the authors wrote.

The low incidence of primary OA among Asian populations may be related to soft tissue laxity around the hip, the authors speculated, citing reports that joint laxity or range of motion differs by race. Also, impingement did not appear to be associated with pathology in the current study as well as another study involving shoulder areas, according to the release.

However, the authors acknowledged that the study was small and thus the results cannot necessarily be generalized to all Asian populations. Also, it is not known whether study participants will eventually develop hip OA, the release said.

For more information:
Yamamura M, Miki H, Nakamura N, Murai M, et al. Open-configuration MRI study of femoro-acetabular impingement. J Orthop Res. 2007. Published online ahead of print June 28, 2007.