Bristol index of patellar width to thickness (BIPWiT): a reproducible measure of patellar thickness from adult MRI.

Background: The restoration of an adequate patellar thickness is a key to the successful outcome of knee arthroplasty. This study investigated the relationship between the thickness of the native patellar and medial-lateral patellar width using magnetic resonance imaging (MRI).
Methods: 75 MRI scans of young adults, with an average age of 27 (range 16-40) were studied. Exclusion criteria included a diagnosis of degenerative joint disease, patello-femoral pathology or age under 16/over 40 (170 patients). The bony thickness of the patellar, the chondral thickness and patellar width were measured, as was the location of maximal patellar thickness. Inter/intraobserver variability was calculated and correlation analysis was performed.
Results: We found a strong correlation between patellar width and thickness (bone plus cartilage) (Pearson 0.75, P<0.001). The mean width to thickness ratio was 1.8:1 (standard deviation 0.1, 95% confidence interval 1.78-1.83). Without cartilage the ratio was 2.16:1 (SD 0.15, 95% CI 2.11-2.21), correlation was moderate (Pearson 0.59, P<0.001). The average maximal patellar cartilage thickness was 4.1mm (SD 1.3).
Conclusion: The strong correlation and narrow confidence intervals for the ratio of patellar width to thickness, suggest that patellar width might be used as a guide for accurate restoration of patellar thickness during total knee or patello-femoral replacement. After removing osteophytes we would recommend a ratio of 1.8:1. Further work is required to establish whether there is a relationship between anterior knee pain post total knee arthroplasty and an abnormal patellar width:thickness ratio.
Level of Evidence: Level III.
(Copyright © 2014 Elsevier B.V. All rights reserved.)