Effect of collagen fiber orientation on mechanical properties of bone and myofascia in hindlimb unloading rats

It is well known that the bone loss which may lead to bone fracture risk occurs under microgravity and disuse. Besides bone mineral density (BMD), the role of collagen as the main component of bone in the mechanism has unknown. In addition, myofascia, which is also mainly composed of collagen, plays an important role in the force transmission in musculoskeletal system. In this study, the collagen alternations of hindlimb tibia and myofascia in tail-suspension rats, which hindlimb unloading as a model to simulate microgravity and/or disuse, were investigated and further the correlations with the mechanical properties were analyzed. The collagen fiber orientations were observed by second-harmonic generation (SHG) microscopy and quantitatively analyzed in MATLAB. Bone mechanical properties were measured by three-point-bending test and nanoindentation. Myofascial mechanical properties were explored by tensile test. The results showed: 1) the collagen fibers orientation both in bone and myofascia were dispersed, as standard deviation of collagen fiber orientation increased .2) the mechanical properties of tibia were decreased by unloading both at macro level (maximum load, breaking load and stiffness, etc.) and micro level (hardness). In myofascia, the yield strain and ultimate strain were significantly decreased. 3) correlation analysis showed bone collagen orientation was related to bone hardness/elastic modulus, and myofascial collagen orientation was related to myofascial elastic modulus and yield limit. Additionally, myofascial mechanical properties were related to bone mechanical properties. These suggested disordered collagen orientation may be related to the deterioration of mechanical properties in both bone and myofascia under simulated microgravity. The alternation of myofascial mechanical property may have a certain impact on the deterioration of bone mechanical properties through affecting the force transmission from muscle to bone.