Does spontaneous cartilage thickening occur in osteoarthritic knees? Data from IMI-APPROACH and the OAI

Determine whether spontaneous cartilage thickening (i.e. without specific intervention) occurs in patients with or at high risk of knee osteoarthritis (OA) and compare characteristics with knees exhibiting cartilage thinning.

1447 knees from IMI-APPROACH and OAI with radiographic and MRI data at baseline, one year, and two years were included. Cartilage thickening was defined as an increase in both radiographic average whole-joint minimum joint space width (mJSW) and MRI mean cartilage thickness over the total femorotibial subchondral bone area (ThCtAB) according to per-patient fitted linear regression lines over the two-year period. Knees with decreases in both were categorized as thinning. Characteristics in structural and clinical parameters between both groups were compared.

Cartilage thickening was observed in 203 knees (14%). Patients with thickening were younger (median 59 vs 64 years, p<0.001), more often female (69% vs 59%, p=0.011), and had lower KL grades (32% KL grade 0 or 1 vs 21%, p<0.001). Over two years, thickening knees showed an increase in mJSW (+0.11mm/year) and ThCtAB (+0.03mm/year) while thinning knees showed a decrease (-0.15mm/year and -0.05 mm/year, respectively). No significant differences were found in BMI or weight change. Fewer hyaluronic acid injections were observed during follow-up in the thickening group (p=0.045). Sensitivity analyses, using alternative definitions of thickening based on JSW and MRI thresholds, confirmed these findings.

Spontaneous cartilage thickening occurs in a relevant proportion of knees with or at risk of knee OA. This intrinsic thickening warrants further investigation into the mechanisms and clinical relevance.