Factors of patellar instability: an anatomic radiographic study.

We analyzed the radiographs and computed tomography (CT) scans of 143 knees operated on for symptomatic patellar instability and 67 contralateral asymptomatic knees, together with 190 control knee radiographs and 27 control knee scans, to determine the factors affecting patellar instability. Four factors were relevant in knees with symptomatic patellar instability: (1) Trochlear dysplasia (85%), as defined by the crossing sign (96%) and quantitatively expressed by the trochlear bump, pathological above 3 mm or more (66%), and the trochlear depth, pathologic at 4 mm or less. (2) Quadriceps dysplasia (83%), defined a present when the patellar tilt in extension is more than 20% on the CT scans. (3) Patella alta (Caton-Deschamps) index greater than or equal to 1.2 (24%). (4) Tibial tuberosity-trochlear groove, pathological when greater than or equal to 20 mm (56%). The factors appeared in only 3%-6.5% of the control knees. The etiology of patellar instability is multifactorial. Determination of the factors permits an effective elective therapeutic plan which aims at correcting the anomalies present.