Ilio-femoral impingement angle: a new radiographic measurement.

Background: Hip dislocations after total hip arthroplasty (THA) are a debilitating complication, often occurring in deep flexion. Current 3D modelling allows for simulating the flexion angle at which bone-on-bone impingement occurs, but it is resource-intensive and not widely available for routine clinical use. Therefore, we propose the ilio-femoral impingement angle (IFIa) as a novel, simple radiographic measure to help identify patients at higher risk for dislocation using standard radiographs.

Methods: A radiographic study was performed on 117 consecutive patients who underwent preoperative planning for THA with 3D remodelling from 2021 to 2022 at a single institution. The flexion angle was recorded at which the femur impingement on the pelvis was identified from the 3D model. This was compared to the angle subtended between a vertical line, the centre of the femoral head and a line rotated upwards to the first point of contact with the ipsilateral ilium. This angle was named the ilio-femoral impingement angle (IFIa).

Results: The mean IFIa was 142.29° ± 11.49°, and it demonstrated a strong correlation with the 3D impingement angle (r = 0.69,  0.05). The inter-rater reliability of the IFIa measurement was high, with an ICC of 0.87, confirming the consistency of this novel radiographic measure.

Conclusions: This study demonstrates that the novel radiographic measurement of the IFIa represented the deep flexion impingement angle of the femur on the ilium. The IFIa may be used as simple and cost-effective alternative measurement to model impingement during flexion.