March 2011

Can hip IR predict FAI?

In the moment: Sports medicine

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Physical exam could be used to screen

By Jordana Bieze Foster

Physical exam findings including hip range of motion can be used to help screen athletes for femoroacteabular impingement even before symptoms occur, according to research presented in February at the annual meeting of the American Academy of Orthopaedic Surgeons.

Two separate studies reported that asymptomatic athletes with limited hip ROM were more likely to demonstrate radiographic signs of FAI. However, the relative effectiveness of hip ROM as a screening tool may depend on how those radiographic findings are defined, which in and of itself is a topic of considerable debate.

Researchers from the University of Utah analyzed 67 male collegiate football players, 62 of whom were asymptomatic, and found at least one positive radiographic finding of FAI in 95% of 134 hips. More than three quarters of hips had radiographic evidence of a cam abnormality (suggesting femoral head/neck asphericity) and two-thirds had a pincer abnormality (acetabular overcoverage).

Hip ROM was measured using the impingement (flexion, adduction, internal rotation) test, FABER (flexion, abduction, external rotation) test, supine flexion exam, supine internal/external rotation exam, sitting internal/external rotation test, and prone internal/external rotation test.

All four ROM tests correlated significantly with alpha angle as measured on frog-lateral radiographs, but only supine internal rotation was mildly correlated with alpha angle on anteroposterior radiographs. (Alpha angle, a measure of femoral sphericity and an indicator of cam FAI, is the angle between the longitudinal axis of the femoral neck and a line connecting the femoral head center to the point where the head deviates from a best-fit sphere. Abnormal alpha angle was defined as greater than 50° on either AP or frog-lateral radiographs.) No ROM tests correlated significantly with any of the radiographic measures of pincer FAI.

The researchers calculated that predicting cam FAI with 80% sensitivity would require internal rotation cutoff values of 21°-26° for supine IR, 37°-41° for prone IR, and 38°-41° for sitting IR. However, the specificities associated with those cutoff values were 50% or less, according to Ashley L. Kapron, a graduate student in biomedical engineering at the university who presented the group’s findings at the AAOS meeting.

The Utah findings are consistent with those of a study from the University of Rochester presented at the same meeting, in which physical exam findings suggestive of FAI were compared to radiographs and magnetic resonance images in asymptomatic high school athletes.

Of 226 athletes between 12 and 18 years old who underwent sports pre-participation physical exams, 19 were identified as having less than 10° of hip internal rotation in 90° of flexion; six of those also had a positive anterior impingement test. Radiographic and MRI findings for 11 athletes from that group were compared to imaging findings for 10 age-matched control athletes with normal hip ROM.

Mean alpha angle as measured on MRI was 58.1° in the group with limited hip ROM, compared to 45.1° in the control group; that difference was statistically significant. In addition, more than two-thirds of hips from the limited ROM group had cam lesions on plain radiographs.

Such findings are encouraging for those working to develop FAI screening protocols for asymptomatic athletes. However, important pieces of the puzzle are still missing.

“It is unknown which of these patients will eventually develop symptomatic femoracetabular impingement,” Kapron noted.

There is also the question of how an abnormal alpha angle is defined and what the threshold should be for clinical significance. Cutoff values ranging from 42° to 68 have been reported in the FAI literature, and a Thomas Jefferson University study of 420 asymptomatic nonathletes presented at the AAOS meeting found mean alpha angles of 59° in men and 46° in women.

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