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.