CYTOKINE DYNAMICS AND MRI-BASED CARTILAGE CHANGES OVER 5 YEARS AFTER ACL INJURY

Anterior cruciate ligament (ACL) injury triggers an acute intra-articular inflammatory response, characterized by elevated synovial fluid levels of pro‑ and anti‑inflammatory cytokines that decline over time. These cytokines collectively influence the balance between cartilage breakdown and repair.

In this exploratory study, we examined whether cytokine concentrations at different time points after ACL injury were associated with subsequent changes in cartilage thickness, and identified time periods with the strongest associations.

The KANON cohort includes patients with an acute ACL tear in a previously uninjured knee (n=121; mean age 26 years at injury; 74% men). Synovial fluids were collected at baseline (0 to 6 weeks after injury) and at follow-ups at 16, 30, 52, 104, and 260 weeks. Cytokines IL-6, IL-8, IL-10, and TNF were quantified using multiplex immunoassays, and IL-1α and LAP TGFβ-1 by proximity extension assay by Olink. MRI-based cartilage thickness was measured in 16 femorotibial cartilage subregions within each knee at baseline, 2 and 5 years using quality-controlled manual segmentation of 3D SPGR images (1.5 T Philips Gyroscan). A location-independent subregional cartilage thickness change score (ChangeScore) was calculated as the sum of all absolute changes without taking the sign (±) into account. Associations between cytokine concentrations at each visit and ChangeScores over two periods (baseline to 2 years, and 2 to 5 years) were examined using linear regression models adjusted for age and BMI at baseline, and sex. Variables were standardized (z-scores) to allow comparison of effect sizes. No adjustments for multiplicity were made.

Early (0-2 years) Despite markedly elevated cytokine concentrations in the acute post-injury phase (Figure 1A), baseline cytokine levels were not significantly associated with cartilage ChangeScore 0 to 2 years (Figure 1B). However, cytokine concentrations at 16, 30, and 52 weeks showed partial statistically significant associations with cartilage ChangeScore during the first 2 years, suggesting that subacute inflammation may be related to early cartilage matrix loss and gain. Late (2-5 years) Cytokine levels measured during the first year were not associated with cartilage changes between 2 and 5 years, whereas cytokine concentrations at the 2- and 5-year visits showed modest but statistically significant associations with ChangeScore 2 to 5 years, indicating a link between persistent low-grade inflammation and ongoing change. Among the six cytokines, IL-6 and IL-10 showed the largest effect sizes per standard deviation change in concentration and the most persistent associations with cartilage thickness ChangeScores across both time periods, whereas IL-1α displayed the weakest and most transient association (Figure 1B).

Cytokines in synovial fluid after ACL injury show time-dependent associations with longitudinal MRI-based cartilage changes. The strongest associations occur during the subacute phase rather than immediately after injury, suggesting a potential therapeutic window for interventions targeting inflammation to modify cartilage change.