Tendon collagen fibrils ‐ Continuity and mechanical properties

Abstract

PurposeTendon force transmission depends on the length and mechanical behavior of collagen fibrils, two poorly understood parameters.MethodsCollagen fibrils from rat‐tail tendons (RTT) and human patellar tendons (HPT) were mechanically tested to failure using atomic force microcopy (AFM). The effect of cross‐link stabilization by NaBH4 in RTT was investigated. Continuity and structure of HPT fibrils was determined by electron microscopy serial sectioning (FIB‐SEM).ResultsFailure stress and high‐strain modulus were greater for the HPT (540±140 MPa, 4.3±1.4 GPa) than RTT fibrils (200±110 MPa, 1.4±0.7 GPa) (p<0.001) (see fig.). NaBH4 had no effect on RTT fibril mechanics. In contras, fascicles showed higher failure stress (58±18 MPa vs 35±11 MPa, p=0.02) and reduced failure strain (5.0±0.8% vs 9.7±4.5%, p=0.05). A total of 1800 HPT fibrils were traced through 25μm deep FIB‐SEM stacks (total length = 45 mm), and only one fibril end, 4 fibril fusions and 3 fibril loops were observed.ConclusionDifferences between RTT and HPT fibril properties are likely caused by greater cross‐link maturity in HPT, but seem unrelated to cross‐link lability since NaBH4 had no effect on RTT fibrils. That reduction did have an effect at the fascicle level indicates the presence of reducible interfibrillar bonds. HPT fibril length data suggests that fibrils rarely end, but rather fuse to form a functionally continuous structure.