Defining the reference range of the Plateau–patella angle in Taiwanese patients with osteoarthritis

BACKGROUND: The plateau-patella angle (PPA) is a relatively new method for assessing patellar height. The currently accepted normal range (21°-29°) was established in Western populations, but ethnic differences in knee morphology may affect the PPA. No data are available for the Taiwanese population. This study aimed to determine the reference range in Taiwanese patients with osteoarthritis. METHODS: Preoperative lateral radiographs of 334 knees undergoing total knee arthroplasty were reviewed. Patellar height was evaluated using the Insall-Salvati index (ISI), Caton-Deschamps index (CDI), Blackburne-Peel index (BPI) indices and the PPA. Correlations between different measurement methods, intraobserver and interobserver reliabilities of the PPA were calculated. Two PPA ranges-21°-29° and 16°-24° (±1 SD of the mean value in this study)-were compared with conventional indices. RESULTS: 196 knees were included. The PPA value was normally distributed. It was moderately correlated to the ISI and highly correlated to the CDI and BPI. The 21°-29° PPA range identified more knees having an abnormal patella position (64% of the knees) than the results of the ISI, CDI, and BPI. In contrast, the 16°-24° range showed comparable results to those of the other indices. CONCLUSION: The PPA should be interpreted carefully when using a single normal range to all ethnic populations. Our cohort suggests that a range of 16°-24° may be more appropriate for the Taiwan osteoarthritic patients.