Repeated subrupture overload causes progression of nanoscaled discrete plasticity damage in tendon collagen fibrils

Abstract

A critical feature of tendons and ligaments is their ability to resist rupture when overloaded, resulting in strains or sprains instead of ruptures. To treat these injuries more effectively, it is necessary to understand how overload affects the primary load-bearing elements of these tissues: collagen fibrils. We have investigated how repeated subrupture overload alters the collagen of tendons at the nanoscale. Using scanning electron microscopy to examine fibril morphology and hydrothermal isometric tension testing to look at molecular stability, we demonstrated that tendon collagen undergoes a progressive cascade of discrete plasticity damage when repeatedly overloaded. With successive overload cycles, fibrils develop an increasing number of kinks along their length. These kinks-discrete zones of plastic deformation known to contain denatured collagen molecules-are accompanied by a progressive and eventual total loss of D-banding along the surface of fibrils, indicating a loss of native molecular packing and further molecular denaturation. Thermal analysis of molecular stability showed that the destabilization of collagen molecules within fibrils is strongly related to the amount of strain energy dissipated by the tendon after yielding during tensile overload. These novel findings raise new questions about load transmission within tendons and their fibrils and about the interplay between crosslinking, strain-energy dissipation ability, and molecular denaturation within these structures. Keywords: collagen fibril; mechanical overload; discrete plasticity; tendon strain; molecular denaturation A critical mechanical feature of tendons and ligaments is their resistance to catastrophic failure when overloaded, resulting in a strain or sprain rather than a rupture. Strains and sprains are common. In 2006 and 2010, nearly 40% of all occupational injuries in the United States resulting in time off work were strains or sprains.1,2 The resulting economic impact of these injuries is sizable. In 2009, workplace injuries cost $170 billion U.S. dollars, half of which was attributed to lost wages and productivity.3 With such a high rate of incidence, strains and sprains not only accounted for more lost days of work than any other occupational injury, but more than the next three leading causes combined.

Quantitative phase measurements of tendon collagen fibres

“…For the fibres extracted from overloaded tendons, the observed decrease in refractive index contrast can be similarly explained. Previous work has shown that the repeated overload regime used here causes collagen fibrils to enlarge as their periphery undergoes molecular denaturation [11]. In addition to changes in water content, collagen denaturation -uncoiling of the collagen molecule's native triple-helix -may have also contributed to the changes in refractive index contrast observed in the present study with heat treatment and mechanical overload.…”

“…The other tendon reached a maximum stress of 51.0 MPa at a strain of 18.9%. As expected from previous experiments [11,13], progressively smaller maximum stresses, and larger maximum strains were recorded during the subsequent four overload cycles. Tensile overload did not affect fibre diameter, but significantly reduced refractive index contrast, causing a mean decrease of 21% relative to the control samples (Figure 4).…”

“…Each tendon was then mounted in the grips of a servo-hydraulic materials testing system using an intergrip gauge length of 15 mm. At a strain rate of 0.5%/s, each tendon was subjected to five cycles of repeated tensile overload, as described previously [11,13]. Briefly, using custom written software programmed under LabVIEW, each tendon was stretched until the slope of its loaddeformation response reached zero.…”

Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury

“…Although the specific structural mechanisms leading to failure were not investigated in the current study, Veres and colleagues have suggested that repeated subrupture loading results in fibril denaturation events that form progressive fibril kinks and the accumulation of damage in bovine tail tendons (Veres et al, 2013; Veres and Lee, 2012). Such changes have been shown to primarily occur early during repeated loading (Veres et al, 2013), which was also observed in the current study. In addition, our laboratory has recently developed a novel method to investigate crimp during mechanical testing (Buckley et al, 2013a), which could be another imaged-based metric for subrupture damage accumulation.…”

“…Several studies have used fatigue loading to induce damage in tissue (Fung et al, 2010; Fung et al, 2009; Sereysky et al, 2012; Wren et al, 2003), investigate theoretical endurance limits in tendon (Schechtman and Bader, 1997, 2002; Wang et al, 1995), quantify subsequent biological changes (Andarawis-Puri et al, 2012a; Andarawis-Puri et al, 2012b; Legerlotz et al, 2011), and identify potential causes of failure (Veres et al, 2013; Veres and Lee, 2012). Although the specific structural mechanisms leading to failure were not investigated in the current study, Veres and colleagues have suggested that repeated subrupture loading results in fibril denaturation events that form progressive fibril kinks and the accumulation of damage in bovine tail tendons (Veres et al, 2013; Veres and Lee, 2012).…”