The Axis-patella Index: A New Method for Measuring Patellar Height

Abstract

Background: The widely used methods for measuring patellar height are based on the anatomic landmarks of the patella and the proximal tibia. When the knee undergoes alterations in the articular surface or adjacent structures due to various diseases or operations, the accurate identification of tibial landmarks is challenging. The results of these conventional methods may be biased or even unavailable. The purpose of this study was to introduce and validate a new method, axis-patella index (API), for assessing patellar height. Methods: Patellar heights were respectively measured using the Insall–Salvati, modified Insall–Salvati, Blackburne–Peel, Caton–Deschamps, plateau-patellar angle, and axis-patella methods on the lateral radiographs of 649 knees without anatomic abnormalities. The inter- and intra-observer reliability of all methods were evaluated by calculating intraclass correlation coefficient and Bland-Altman plots. The associations between the API and the established methods were assessed using Pearson’s correlation coefficient. Results: The API was strongly correlated with the five aforementioned methods. Reliability analyses indicated high inter-observer reproducibility and intra-observer repeatability. The mean of API was approximately 1.5, hence the reference range was defined 1.2 - 1.9 according to the 95% confidence interval. Conclusions: The API, a new index of referencing the tibial shaft axis and the fibular head, providing a valid and accurate alternative for the assessment of patellar height. The reference range was 1.2 - 1.9 in radiographically normal knee.

An evaluation of a novel method for the MRI-based assessment of Caton-Deschamps index in the Knee

“…The traditional CD assessment of patellar height was performed on lateral X-rays (Fig. 1) [10,14]. The CD is obtained by measuring two different parameters; (1) the length of the articular surface of the patella and (2) the distance between the inferior aspect of articular surface of the patella and the anterior angle of the tibial plateau.…”