Abstract
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.
We aimed to compare patellar height measurements in patients with patellofemoral dysplasia.
In a retrospective analysis of the MRI scans of 33 knees in 29 patients with patellofemoral dysplasia we assessed 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.
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).
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.
The abstracts were prepared by Mr D J Bracey, Editorial Secretary. Correspondence should be addressed to him at Royal Cornwall Hospital, Treliske, Cornwall TR1 3LJ, England.
