OP3.4 The Windshield Wiper Sign: A Radiographic Finding of Hip Instability

Purpose: The purpose of this study was to introduce a radiographic sign indicative of hip instability and acetabular suction seal disruption in the native hip, coined the “windshield wiper” (WSW) sign.

Materials and Methods: A retrospective review was performed of 250 patients who underwent periacetabular osteotomy (PAO) with the senior author between March 2021 and September 2023. All patients with the radiographic WSW sign in a native hip were identified and included in the analysis. A WSW sign was identified on plain films as a concave or flat osteochondral defect on the anterolateral femoral head extending medial to the head-neck junction with resultant loss of femoral head sphericity. Every patient underwent a standardized series of radiographs, as well as computed tomography and magnetic resonance imaging. All patients underwent arthroscopy prior to PAO to address intra-articular pathology and other indicated procedures. The osteochondral defect and resultant suction seal disruption were verified during arthroscopy.

Results: Of 250 patients reviewed, a total of 19 hips in 17 patients (prevalence of 7.6%) demonstrated radiographic evidence of the WSW sign. All patients with a WSW sign presented with symptomatic clinical hip instability requiring a PAO. The mean patient age was 31.2 years, with a mean lateral center edge angle (LCEA) of 14.3 degrees. There were 13 hips (68.4%) with dysplasia, 4 (21.1%) with borderline dysplasia, and 2 (10.5%) with a normal LCEA. All patients with a WSW sign and LCEA ≥20° displayed significant femoral antetorsion abnormalities. All arthroscopic videos and images demonstrated a compromised suction seal.

Conclusions: The WSW sign is an uncommon radiographic finding in patients with hip instability. When identified, it can be predictive of substantial instability, which may be valuable for patient prognosis and counseling, as well as preoperative evaluation, diagnosis, and surgical planning, especially in cases which are otherwise considered borderline dysplasia or normal based on LCEA.