The Windshield Wiper Sign, An Instability-Related Osteochondral Defect of the Anterolateral Femoral Head.

PURPOSE: To investigate a radiographic sign believed to be 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 for patients who underwent periacetabular osteotomy (PAO) with the senior author between March 2021 and September 2023. 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 in the native hip. 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. These patients were then compared with a control group of arthroscopically treated hips without hip instability.

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. Of the 50 control group hips reviewed, the WSW sign was not identified.

CONCLUSIONS: The WSW sign is an uncommon radiographic finding in patients with hip instability. When identified, it can be predictive of substantial instability, especially in cases which are otherwise considered borderline dysplasia or normal based on LCEA.

LEVEL OF EVIDENCE: III, case-control study.