Relationship between magnetization transfer ratio and axial compressive strain in tibiofemoral articular cartilage

The objective of this study was to determine the relationship between magnetization transfer ratio (MTR) and strain in ex vivo bovine cartilage using 9.4T magnetic resonance imaging (MRI) and to compare this relationship to the relationship between T2 and strain.

A previously designed custom electropneumatic loading device was used to compress together osteochondral blocks of bovine femoral and tibial cartilage and bone within a 9.4T Bruker MRI scanner. Stepwise loads were applied to compress cartilage to targets of 10%, 20% and 30% strain. Images were acquired for unloaded cartilage and after each load had been held constant for 20 min to minimize creep effects. A custom algorithm was used to quantify T2 and MTR (average, by depth, and column wise) in the region of contact, and to calculate axial cartilage strain. Repeated measures correlation was used to investigate potential correlations between MTR and strain, and T2 and strain.

Mean MTR vs strain correlations were r_rm = -0.90 (CI = -0.96 to -0.75) in the tibia and r_rm = -0.70 (CI = -0.87 to -0.35) in the femur. Mean T2 vs strain correlations were r_rm = -0.84 (CI = -0.94 to -0.59) in the tibia and r_rm = -0.11 (CI = -0.57 to 0.41) in the femur. Column-wise analyses produced negligible or weak correlations (-0.07 to -0.34 for T2 and MTR).

MTR increases with strain in the region of contact, and MTR is more strongly correlated to strain than T2. MTR shows promise as a surrogate measure of strain, especially when averaged over the region of contact.