Reliability and interobserver variability in radiological patellar height ratios

This study evaluated the reliability and interobserver variability of five patellar height ratios as measured by two examiners on standard radiographs : Insall-Salvati (IS), modified Insall-Salvati (MIS), Blackburne-Peel (BP), Calon-Deschamps (CD), and Labelle-Laurin (LL). Plain lateral radiographs w ith a knee flexion angle of 20 degrees for IS, MIS, BP, and CD ratios and 9 0 degrees for the LL method of 22 knees of 21 patients with varying patholo gical knee conditions were analyzed. Statistical results revealed a low int erobserver variability with high correlation coefficients (0.86 for IS, 0.8 2 for MIS, 0.86 for BP, 0.92 for CD, and 0.81 for LL; P > 0.3) and low mean interobserver errors. However, regarding the reliability of the radiograph ic results of the different methods for patella alta, baja, or norma we fou nd varying results in 68% of the patients. In two patients the patellar hei ght was classified as alta, norma, or baja depending on the ratio used. Reg arding the definitions of patellar height used by the authors of these meth ods, we found the lowest number of normal patellae with the IS ratio and no patella alta for the CD ratio. The LL method revealed the highest number o f patella alta. The BP ratio showed intermediate results for both patella a lta and baja, being the most moderate method. This study showed that there was a good interobserver reliability for the evaluation of patellar height according to the common radiological ratios. However, the high frequency of differing results between the different radiographic ratios showed that pa tellar height classification as "alta," "norma," or "baja" depends heavily on the chosen index. The differing results were due mainly to the normative patellar height data and to anatomical differences. Based on these finding s we recommend a ratio using the articular surface of the patella in relati on to the joint line. We recommend the BP method because it revealed the lo west interobserver variability and discriminated best among the groups alta , norma, and baja.