The Hip “Windshield Wiper Sign”: An Osteochondral Defect of the Anterolateral Femoral Head, Predicts Instability and Allows Planning of Combined Arthroscopic Cartilage Therapy and Periacetabular Osteotomy

The new radiographic "Windshield Wiper Sign" detects hip instability and an instability-related osteochondral defect of the anterolateral femoral head. It is visible on plain radiographs, as well as MRI and during arthroscopy, in otherwise considered normal or borderline hips based on the lateral center-edge angle (LCEA). In the last two decades, femoral acetabular impingement syndrome (FAIS) with labral lesions and cartilage lesions were treated with excellent results. Recently the focus shifted to complex cases, revision surgery, and capsular management preventing postoperative hip instability. (Micro-)instability of the hip causes poor outcomes, but preoperatively there has been no Gold Standard for radiographic assessment of hip instability. On plain radiographs we use the LCEA and the anterior center-edge angle (ACEA), we can also use the Femoro-Epiphyseal Acetabular Roof (FEAR) Index and The Gothic Arch Angle (GAA). These signs help detect in which cases arthroscopic surgery alone would provide poor results and a concurrent osteotomy, for example a periacetabular osteotomy (PAO), is needed. We now understand that hip instability in hips otherwise considered normal hips or borderline dysplastic can be detected by the Windshield Wiper Sign. This is important knowledge for concomitant arthroscopic combined with osteotomy, as the need for cartilage therapy can be determined preoperatively.