T2 and T1ρ Mapping Reveals Time-Dependent Cartilage Response to In-Scanner Cyclic Compression After ACL Reconstruction

Anterior cruciate ligament injury is a major risk factor for the development of post-traumatic osteoarthritis, with cartilage degeneration frequently occurring despite successful reconstruction surgery. There is a growing need for sensitive, non-invasive imaging techniques to detect early biochemical changes in cartilage before irreversible structural damage occurs. This study aimed to evaluate the response of tibiofemoral cartilage to controlled biomechanical loading in healthy individuals, and in patients six- and twelve-months post anterior cruciate ligament reconstruction using quantitative magnetic resonance imaging. We employed quantitative relaxometry (T2 and T1ρ mapping) in conjunction with a custom-built pneumatic loading device capable of applying functional, in-scanner mechanical loading (0.5 Hz, 50% body weight) to the knee joint. The scans were conducted before and after loading in 12 healthy controls (6 males, 6 females, 27.3±5.7 years old) and 27 post-operative patients (12 males, 15 females, 25.4±5.8 years old). The surgical cohort was further stratified into symptomatic and asymptomatic subgroups based on clinical outcomes. Biomechanical loading led to different quantitative relaxometry outputs between the six- and twelve-month post-operative time points. Across all patients, T2 values increased after biomechanical loading by 8% and 7% at six- and twelve-month time points, respectively. However, our current loading scheme did not lead to MR relaxometry differences between healthy and post-surgery cohorts. Meanwhile, for ligament reconstruction patients, we found increased relaxometry values due to biomechanical load at both time points post-surgery. Biomechanical loading alters the structure and relaxivity of the knee cartilage, and T1ρ may be a more meaningful quantitative metric for post-ligament reconstruction evaluation.