Regional depth-specific subchondral bone density measures in osteoarthritic and normal patellae: in vivo precision and preliminary comparisons

Computed tomography-based depth-specific image processing is able to precisely identify regional differences between healthy patellae and patellae with osteoarthritis. This study aims to assess the precision errors and potential differences in regional, depth-specific subchondral bone mineral density (BMD) in normal and osteoarthritic (OA) human patellae in vivo using CT-based density analyses. Fourteen participants (2 men and 12 women; mean age, 51.4; SD, 11.8 years) were scanned using clinical quantitative CT (QCT) three times over 2 days. Participants were categorized as either normal (n = 7) or exhibiting radiographic OA (n = 7). Average subchondral BMD was assessed at three depths relative to the subchondral surface. Regional BMD analysis included: total lateral facet BMD, total medial facet BMD, and superior/middle/inferior BMD of lateral and medial facets at normalized depths of 0-2.5, 2.5-5, and 5-7.5 mm from the subchondral surface. We assessed precision using root mean square coefficients of variation (CV%). We evaluated differences between OA and normal BMD by (1) calculating percentage differences between the groups (in relation to normal BMD) (2) relating percentage differences to respective CV% errors and (3) determining effect sizes using Cohen's d. Root mean square CV% precision errors ranged from 1.1 to 5.9 %. Percentage differences between OA and normal BMD varied from -1.6 to -30.1 % (BMD lower in OA patellae). In relation to precision errors, percentage differences were, on average, 5.5x greater than CV% errors. Cohen's d effect sizes ranged from -1.7 to -0.1. Largest differences were noted at depths of 2.5-5 and 5-7.5 mm from the subchondral surface. Patellar subchondral BMD measures were precise (average CV%, a parts per thousand currency sign3 %). This region- and depth-specific CT-based imaging tool characterized regional standardized BMD differences between normal and OA patellae in vivo.