Knee Malposition During Radiographs Can Produce Clinically Relevant Errors in Measurements of Patellar Height.

PURPOSE: To quantify the amount of error in commonly used radiographic measurements of patella alta with progressive malposition of the knee. METHODS: Three-dimensional models were derived from computed tomography scans of patients with unilateral patellar instability. Models with knee flexion angles between 20° and 30° were projected onto 2-dimensional radiographs to create a perfect lateral radiograph. Progressive internal/external rotation and ad/abduction of the knee were performed to simulate malpositioning of the knee. For each 5° increment of aberrant positioning, corresponding measurements of posterior and distal condylar overlap were measured, and measurements of patella alta were compared, including Insall-Salvati (IS), modified IS, Blackburne-Peel Index, Caton-Deschamps Index, and patellotrochlear index, with a 0.05 change considered to be clinically significant. RESULTS: A total of 40 knees from 20 patients were included in this study. On radiographic views, for every 5° of aberrant rotation or ab/adduction, the overlap between the posterior condyles increased by approximately 4 mm (P < .001). IS measurements were the least sensitive to aberrant motion (range -0.023 to 0.012). Caton-Deschamps Index and Blackburne-Peel Index showed clinically and statistically relevant deviations at larger degrees of external rotation, whereas modified IS showed statistically relevant deviations in both abduction and adduction (0.054 at 10°). CONCLUSIONS: Malposition of the knee on 2-dimensional radiographs produced clinically relevant errors in the measurements of patellar height that varied by the method of measurement. The extent of condylar overlap can be used as a surrogate to estimate the extent of malpositioning and the potential for error in the measurement of patella alta on radiographic images. LEVEL OF EVIDENCE: Level III, case-control study.