Novel method of measuring patellar height ratio using a distal femoral reference point

Purpose: Patellar height is an important factor in patellar tracking and alters the force of the patellofemoral joint reaction. Several methods for measuring patellar height ratio have been described, with no single method recognised as a gold standard. This study developed a new measurement method using a distal femoral reference, where the normal values of measurement are unaffected by varying angles of knee flexion. Methods: Eighty-five volunteers had radiographs taken of their knees at 0°, 15°, 30°, 45° and 60° of flexion in a supine position. Patellar height was assessed by five different measurement methods: Insall-Salvati (IS), modified Insall-Salvati (MIS), Caton-Deschamps (CD), Blackburne-Peel (BP) and our new method. Three independent examiners sequentially conducted all measurements under identical conditions. Results: The normal value obtained with the method reported here was 1 ± 0.1standard deviation (SD) (IS 1.1 ± 0.2, MIS 1.8 ± 0.25, CD 1.08 ± 0.2, BP 0.9 ± 0.2). IS and the new method tended to have better reliability in the face of varying knee-flexion angles than did CD, BP or MIS. All methods had good to excellent levels of interobserver variation (IS 0.969, new method 0.811, MIS 0.768, CD 0.735, BP 0.708). Conclusion: The new patellar height ratio measurement method proved to be accurate and reproducible for evaluating a normal population. This method offered the benefit of using the distal femur as a reference landmark, and, thus, the resulting measurements were not altered by varying degrees of knee flexion.