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

- There is a reliable ratio of patellar width to thickness - 1.8:1.
- Bristol index of patellar width:thickness can guide patellar resection and restoration.
- Mean patellar cartilage thickness is 4.1 mm.

BackgroundThe 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).Methods75 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.ResultsWe 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 4.1 mm (SD 1.3).ConclusionThe 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 evidenceLevel III