Nanostructure of collagen fibrils in human nucleus pulposus and its correlation with macroscale tissue mechanics

Collagen fibrils are the main structural components of the nucleus pulposus tissue in the intervertebral discs. The structure-property relationship of the nucleus pulposus (NP) tissues is still unclear. We investigated the structure of individual collagen fibrils of the NP and evaluated its correlation with the bulk mechanical properties of the tissue. Collagen fibrils were extracted from the NP of discs retrieved from adolescents during scoliosis correction surgery, and the extracts were confirmed by SDS-PAGE. The diameters of the individual collagen fibrils were measured through atomic force microscopy, and the compressive mechanical properties of the tissues were evaluated by confined compression. The correlations between the nanoscale morphology of the collagen fibrils and the macroscale mechanical properties of the tissues were evaluated by linear regression. The SDS-PAGE results showed that the fibril extracts were largely composed of type II collagen. The mean diameter of the collagen fibrils was 92.1 +/- 26.54 nm; the mean swelling pressure and compressive modulus of the tissues were 6.15 +/- 4.3 kPa and 1.23 +/- 0.7 MPa, respectively. The mean fibril diameter had no linear correlation (R(2) = 0.30) with the swelling pressure of the tissues. However, it had a mild linear correlation with the compressive modulus (p = 0.023, R(2) = 0.68). This is the first study, to our knowledge, to evaluate the nanostructure of the individual collagen fibrils of the nucleus pulposus and its relationship with macroscale mechanical properties of the NP tissues.