Objectives: To examine whether the quantity of cartilage or semiquantitative scores actually differ in knees with mild radiographic osteoarthritis compared with knees without osteoarthritis.
Methods: Framingham Osteoarthritis Study participants had knee tibiofemoral magnetic resonance imaging-based measurements of cartilage. Using three-dimensional FLASH-water excitation sequences, cartilage volume, thickness and subregional cartilage thickness were measured and cartilage scored semiquantitatively (using the whole-organ magnetic resonance imaging score; WORMS). Using weight-bearing radiographs, mild osteoarthritis was defined as Kellgren/Lawrence (K/L) grade 2 and non-osteoarthritis as K/L grade 0. Differences between osteoarthritis and non-osteoarthritis knees in median cartilage measurements were tested using the Wilcoxon rank sum test.
Results: Among 948 participants (one knee each), neither cartilage volume nor regional thickness were different in mild versus non-osteoarthritis knees. In mild osteoarthritis, cartilage erosions in focal areas were missed when cartilage was quantified over large regions such as the medial tibia. For some but not all subregions of cartilage, especially among men, cartilage thickness was lower (p<0.05) in mild osteoarthritis than non-osteoarthritis knees. Because semiquantitative scores captured focal erosions, median WORMS scores were higher in mild osteoarthritis than non-osteoarthritis (all p<0.05). In moderate/severe osteoarthritis (K/L grades 3 or 4), osteoarthritis knees had much lower cartilage thickness and higher WORMS scores than knees without osteoarthritis.
Conclusions: In mild osteoarthritis, the focal loss of cartilage is missed by quantitative measures of cartilage volume or thickness over broad areas. Regional cartilage volume and thickness (eg, medial tibia) are not different in mild osteoarthritis versus non-osteoarthritis. Subregional thickness may be decreased in mild osteoarthritis. Semiquantitative scoring that assesses focal cartilage damage differentiates mild osteoarthritis from non-osteoarthritis.
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▸ Additional supplementary files 1–3 are published online only at http://ard.bmj.com/content/vol69/issue1
Funding This study was supported by National Institutes of Health (NIH) AR47785, NIH AG18393 and by the National Heart, Lung, and Blood Institute’s Framingham Heart Study N01-HC-25195. SR is the recipient of an educational grant by the Swiss Society of Rheumatology.
Competing interests FE is CEO of Chondrometrics GmbH, a company providing magnetic resonance image analysis services. He provides consulting services to Pfizer, MerckSerono, AstraZeneca, and Wyeth. MH has a part-time appointment with Chondrometrics GmbH. AG is president of Boston Imaging Core Lab, LLC (BICL), a company providing radiological image assessment services. He is a shareholder of Synarc, Inc. FR is a shareholder of BICL. Other authors declare no conflict of interest.
Ethics approval Approval for the study was obtained from the Boston University Medical Center Institutional Review Board.
Patient consent Obtained.