Reliability of a new hip lateral view to quantify alpha angle in femoroacetabular impingement

Radiographic measurement of the alpha angle (AA) in femoroacetabular impingement (FAI) is not well codified and invasive techniques such as MR- or CT-arthrography remain the gold standard. Excessive acetabular coverage described in pincer-type FAI can be seen on plain radiographs but has never been quantified and anterior center edge (ACE) angle, described on the false-profile view (FP) to measure anterior acetabular coverage has never been evaluated in FAI.

In this study we wanted to determine if a plain radiograph could efficiently measure AA compared to CT-arthrography and if ACE could quantify the acetabular coverage in FAI.

We developed a hip view combining a lateral view and a FP, called profile view in impingement position (PIP). Twenty-six patients operated for FAI had CT-arthrography, PIP and FP. Nineteen control subjects had the PIP. AA were measured twice by three raters and ACE once. We compared AA measured on patients between CT and PIP, on PIP between patients and controls, ACE measured on patients between PIP and FP, and did a reproducibility analysis. Means were compared by paired or unpaired t-tests; reproducibility was measured by intraclass correlation coefficient (ICC).

Mean AA was 65.8° (range, 48-85°) on CT-arthrography and 63.9° (range, 50-87°) on PIP (P>0.05). ICC for PIP measures were 0.8-0.9 for intra-rater and 0.6-0.9 for inter-rater reliability. Mean AA on PIP in patients was 63.3° (range, 52-87°) and 44.9° (range, 34-67°) in controls (P<0.001). Mean ACE was 26.8° (range, 14-41°) on PIP and 32.8° (range, 18-56°) on the FP (P=0.015).

The PIP is a reliable view to measure the AA in FAI as measures on PIP and CT-arthrography were not significantly different with a good reproducibility. All of the painful hips and 2 controls had an AA>50°. PIP was not efficient to measure ACE.

Level III, case-control study.