A number of measurements of patella height exist all of which use a position on the tibia as a reference. The Patellotrochlear Index has recently been proposed as a more accurate reflection of the functional height of the patella and described in normal knees.

Aim: A comparison of patellar height measurements in patients with patellofemoral dysplasia.

Method: A retrospective analysis of the MRI scans of 33 knees in 29 patients with patellofemoral dysplasia to assess the inter- and intraobserver reliability of four patellar height measurements: the Patellotrochlear Index (PTI), Insall-Salvati (IS), Blackburne-Peel (BP) and Caton-Deschamps (CD) ratios. We also assessed the correlation between the different measurements in predicting patella alta. Three blinded observers on two separate occasions performed the measurements.

Results: There were 21 females and 8 males with a mean age of 21.4 years (13–33).

Statistical analysis revealed good inter-observer reliability for all measurements (0.78 for PTI, 0.78 for IS, 0.73 for BP and 0.77 for CD). Intra-observer reliability was also good (0.80, 0.83, 0.75, 0.78 respectively). When comparing the different measurements for patella alta there was a weak correlation between the PTI and the others. There was a strong correlation between the CD and BP ratios (0.96) and a moderate correlation between IS and CD and IS and BP ratios (0.594 and 0.539 respectively).

Conclusion: All measurements are reproducible. The PTI however suggests patella alta exists in different patients to that suggested by the established measures. We propose the PTI as a more clinically relevant measure.