Objectives: To examine whether the quantity of cartilage or semiquantitative scores actually differ in knees with mild radiographic osteoarthritis (OA) compared with knees without OA.
Methods: Framingham OA Study participants had knee tibiofemoral MRI-based measurements of cartilage. Using 3D FLASH-water excitation sequences, cartilage volume (VC), thickness (ThCtAB) and subregional ThCtAB were measured and cartilage scored semiquantitatively (using WORMS). Using weight bearing radiographs, we defined mild OA as K/L=2 and nonOA as K/L=0. Differences between OA and nonOA knees in median cartilage measurements were tested using the Wilcoxon rank sum test.
Results: Among 948 participants (one knee each), neither VC nor regional thickness (ThCtAB) were different in mild versus nonOA knees. In mild OA, cartilage erosions in focal areas were missed when cartilage was quantified over large regions like the medial tibia. For some but not all subregions of cartilage, especially among men, ThCtAB was lower (p<.05) in mild OA than nonOA knees. Because semiquantitative scores captured focal erosions, median WORMS scores were higher in mild OA than nonOA (all p<.05). In moderate/severe OA (K/L grades 3 or 4), OA knees had much lower ThCtAB and higher WORMS scores than knees with nonOA.
Conclusions: In mild OA, the focal loss of cartilage is missed by quantitative measures of cartilage volume or thickness over broad areas. Regional cartilage volume and thickness (e.g. medial tibia) are not different in mild OA versus nonOA. Subregional thickness may be decreased in mild OA. Semiquantitative scoring which assesses focal cartilage damage differentiates mild OA from nonOA.