Equivalent stiffness after glycosaminoglycan depletion in tendon--an ultra-structural finite element model and corresponding experiments.

The glycosaminoglycan (GAG) side-chains of small leucine-rich proteoglycans have been postulated to mechanically cross-link adjacent collagen fibrils and contribute to tendon mechanics. Enzymatic depletion of tendon GAGs (chondroitin and dermatan sulfate) has emerged as a preferred method to experimentally assess this role. However, GAG removal is typically incomplete and the possibility remains that extant GAGs may remain mechanically functional. The current study specifically investigated the potential mechanical effect of the remaining GAGs after partial enzymatic digestion. A three-dimensional finite element model of tendon was created based upon the concept of proteoglycan mediated inter-fibril load sharing. Approximately 250 interacting, discontinuous collagen fibrils were modeled as having a length of 400 μm, being composed of rod elements of length 67 nm and E-modulus 1 GPa connected in series. Spatial distribution and diameters of these idealized fibrils were derived from a representative cross-sectional electron micrograph of tendon. Rod element lengths corresponded to the collagen fibril D-Period, widely accepted to act as a binding site for decorin and biglycan, the most abundant proteoglycans in tendon. Each element node was connected to nodes of any neighboring fibrils within a radius of 100 nm, the slack length of unstretched chondroitin sulfate. These GAG cross-links were the sole mechanism for lateral load sharing among the discontinuous fibrils, and were modeled as bilinear spring elements. Simulation of tensile testing of tendon with complete cross-linking closely reproduced corresponding experiments on rat tail tendons. Random reduction of 80% of GAG cross-links (matched to a conservative estimate of enzymatic depletion efficacy) predicted a drop of 14% in tendon modulus. Corresponding mechanical properties derived from experiments on rat tail tendons treated in buffer with and without chondroitinase ABC were apparently unaffected, regardless of GAG depletion. Further tests for equivalence, conservatively based on effect size limits predicted by the model, confirmed equivalent stiffness between enzymatically depleted tendons and their native controls. Although the model predicts that relatively small quantities of GAGs acting as primary collagen cross-linking elements could provide mechanical integrity to the tendon, partial enzymatic depletion of GAGs should result in mechanical changes that are not reflected in analogous experimental testing. We thus conclude that GAG side chains of small leucine-rich proteoglycans are not a primary determinant of tensile mechanical behavior in mature rat tail tendons.