Effects of collagen unwinding and cleavage on the mechanical integrity of the collagen network in bone

The objective of this study was to investigate how molecular level changes in the collagen network affect its mechanical integrity. Our hypothesis is that the cleavage and unwinding of triple helices of collagen molecules significantly reduce the mechanical integrity of the collagen network in bone, whereas collagen crosslinks play a major role in sustaining the structural integrity of the collagen network. To test this hypothesis, the collagen molecular structure was altered in demineralized human cadaveric bone samples in the following two ways: heat induced unwinding and pancreas elastase induced cleavage of collagen molecules. Along with control specimens, the treated specimens were mechanically tested in tension to determine their strength, elastic modulus, toughness, and strain to failure. Also, the percentage of denatured collagen molecules and amounts of two major collagen crosslinks (hydroxylysylpyridinoline and lysylpyridinoline) were determined using high-performance liquid chromatography techniques. It was found that unwinding of collagen molecules may cause more reduction in stiffness (E) but less strain to failure (ε_f) than cleavage. Both collagen denaturation types cause similar changes in the strength (σ_s) and work to fracture (W_f) of the collagen network with no significant changes in hydroxylysylpyridinoline and lysylpyridinoline crosslinks. The results of this study indicate that the integrity of collagen molecules significantly affect the mechanical properties of the collagen network in bone, and that collagen crosslinks may play an important role in maintaining the mechanical integrity of the collagen network after collagen denaturation occurs.