Acute modification of biomechanical properties of the bone-ligament insertion to rat limb unweighting.

We investigated the acute adaptation of the rat femur-medial collateral ligament-tibia (FMT) complex to 7 days of limb unweighting by means of a hind-limb suspension protocol. Male, young adult, Harlan Sprague-Dawley rats were randomly assigned to either control or suspended groups. Rats deprived of hind limb-to-ground contact forces had a 42% decrease in soleus muscle mass compared with the control group. Medial collateral ligament (MCL) length and cross-sectional area were measured, and each FMT complex was tension tested to failure. All failed at their tibia-MCL insertion. The ultimate load in the FMT and the peak Kirchhoff stress in the MCL (occurring immediately before insertion site failure) were significantly reduced in the suspended group. The suspended MCLs were 9.7% larger in area and 5.7% shorter in length than the controls under the same preload (0.25 N). We found no significant differences between the control and suspended MCLs in Green strain, stretch, or deformation immediately before insertion site failure, nor did we find a significant difference in the MCL tangent modulus. This study indicates that even acute periods of limb unweighting can structurally compromise bone-ligament insertions. Further, this study implies that the adaptations responsible for this structural compromise must involve acute changes in the intrinsic zone (or zones) of the bone-ligament insertion.