Shear loads induce cellular damage in tendon fascicles

Abstract

Tendon is vital to musculoskeletal function, transferring loads from muscle to bone for joint motion and stability. It is an anisotropic, highly organized, fibrous structure containing primarily type I collagen in addition to tenocytes and other extracellular matrix components contributing to maintenance and function. Tendon is generally loaded via normal stress in a longitudinal direction. However, certain situations, including fiber breakage, enzymatic remodeling, or tendon pathology may introduce various degrees of other loading modalities, such as shear-lag at the fiber level, potentially affecting cellular response and subsequent function. Fascicles from rat tail tendon were dissected and placed in one of three paired groups: intact, single laceration, or double laceration. Each pair had a mechanically tested and control specimen. Single laceration fascicles contained one transverse laceration to mimic a partial tear. Double laceration fascicles had overlapping, longitudinally separated lacerations on opposite sides to cause intra-fascicular shear transfer to be the primary mechanism of loading. Elastic properties of the fascicle, e.g. peak load, steady state load, and stiffness, decreased from intact to single laceration to double laceration groups. Surprisingly, 45% of the intact strength was maintained when shear was the primary internal load transfer mechanism. Cellular viability decreased after mechanical testing in both laceration groups; cell death appeared primarily in a longitudinal plane where high shear load transfer occurred. This cell death extended far from the injury site and may further compromise an already damaged tendon via enzymatic factors and subsequent remodeling associated with cell necrosis.

Keywords: Cellular viability; Mechanics; Shear; Tendon; Viscoelasticity.

Figure 1

Specimen preparation and mechanical testing setup. Pair 1 (double laceration) was prepared with 2 overlapping lacerations on opposite sides of the fascicle, one specimen was mechanically loaded, one was not. Pair 2 (single laceration) was prepared with a single, mid-substance, transverse laceration, one specimen was loaded, one was not. Both ends of the fascicles were gripped in soft-tissue grips with rough, interlocking plates on both sides.

Figure 2

Mechanical response of experimental groups (intact, single laceration, and double laceration). The addition of a laceration decreased the mechanical properties in peak load (a), steady state load (b), and stiffness (c). Load decay ratio demonstrated larger value in the double laceration group compared to the other two groups (d). An asterisk (*) indicates significance (p≤0.05).

Figure 3

Cellular response in each experimental sub-group. Average cellular viability ratio values resulted in differences between the single and double laceration mechanically tested sub-groups and all other sub-groups, excluding each other (not all differences shown on the graph). An asterisk (*) indicates significance (p≤0.05).

Figure 4

Confocal images of a representative intact pair. (a) Shows the control, not tested specimen and (b) shows the intact, mechanically tested specimen.

Figure 5

Confocal images of a representative single laceration pair. (a) Shows the control, not tested specimen and (b) shows the lacerated, mechanically tested specimen. Horizontal dark band corresponds to region of non-viable cells.

Figure 6

Confocal images of a representative double laceration pair. (a) Shows the control, not tested specimen and (b) shows the lacerated, mechanically tested specimen. Horizontal dark band corresponds to region of non-viable cells.

Figure 7

Load decay ratio, normalized to the average intact value, for intact rat tail tendons and fascicles and those tested after introduction of overlapping lacerations. The lacerated fascicles demonstrated an increase in load decay ratio compared to the intact fascicle, while the lacerated tendon had a smaller load decay ratio than the intact tendon (though not significant; p=0.22). Data for whole tendon were measured by Kondratko-Mittnacht as a part of (Kondratko-Mittnacht et al., 2015). An asterisk (*) indicates significance (p≤0.05).