In Japan, osteoarthritis (OA) of the hip secondary
to acetabular dysplasia is very common, and there are few data concerning
the pathogeneses and incidence of femoroacetabular impingement (FAI).
We have attempted to clarify the radiological prevalence of painful
FAI in a cohort of Japanese patients and to investigate the radiological findings.
We identified 176 symptomatic patients (202 hips) with Tönnis grade
0 or 1 osteoarthritis, whom we prospectively studied between August
2011 and July 2012. There were 61 men (65 hips) and 115 women (137
hips) with a mean age of 51.8 years (11 to 83). Radiological analyses
included the α-angle, centre–edge angle, cross-over sign, pistol
grip deformity and femoral head neck ratio. Of the 202 hips, 79
(39.1%) had acetabular dysplasia, while 80 hips (39.6%) had no known
aetiology. We found evidence of FAI in 60 hips (29.7%). Radiological
FAI findings associated with cam deformity were the most common.
There was a significant relationship between the pistol grip deformity
and both the α-angle (p <
0.001) and femoral head–neck ratio
(p = 0.024). Radiological evidence of symptomatic FAI was not uncommon
in these Japanese patients. Cite this article:
The radiological evaluation of the anterolateral femoral head is an essential tool for the assessment of the cam type of femoroacetabular impingement. CT, MRI and frog lateral plain radiographs have all been suggested as imaging options for this type of lesion. The alpha angle is accepted as a reliable indicator of the cam type of impingement and may also be used as an assessment for the successful operative correction of the cam lesion. We studied the alpha angles of 32 consecutive patients with femoroacetabular impingement. The angle measured on frog lateral radiographs using templating tools was compared with that measured on CT scans in order to assess the reliability of the frog lateral view in analysing the alpha angle in cam impingement. A high interobserver reliability was noted for the assessment of the alpha angle on the frog lateral view with an intraclass correlation coefficient of 0.83. The mean alpha angle measured on the frog lateral view was 58.71° (32° to 83.3°) and that by CT was 65.11° (30° to 102°). A poor intraclass correlation coefficient (0.08) was noted between the measurements using the two systems. The frog lateral plain radiograph is not reliable for measuring the alpha angle. Various factors may be responsible for this such as the projection of the radiograph, the positioning of the patient and the quality of the image. CT may be necessary for accurate measurement of the alpha angle.
