Shear load transfer in high and low stress tendons

Abstract

Background Tendon is an integral part of joint movement and stability, as it functions to transmit load from muscle to bone. It has an anisotropic, fibrous hierarchical structure that is generally loaded in the direction of its fibers/fascicles. Internal load distributions are altered when joint motion rotates an insertion site or when local damage disrupts fibers/fascicles, potentially causing inter-fiber (or inter-fascicular) shear. Tendons with different microstructure (helical versus linear) may redistribute loads differently. Method of Approach This study explored how shear redistributes axial loads in rat tail tendon (low stress tendons with linear microstructure) and porcine flexor tendon (high stress with helical microstructure) by creating lacerations on opposite sides of the tendon, ranging from about 20-60% of the tendon width, to create various magnitudes of shear. Differences in fascicular orientation were quantified using polarized light microscopy. Results and Conclusions Unexpectedly, both tendon types maintained about 20% of pre-laceration stress values after overlapping cuts of 60% of tendon width (no intact fibers end to end) suggesting that shear stress transfer can contribute more to overall tendon strength and stiffness than previously reported. All structural parameters for both tendon types decreased linearly with increasing laceration depth. The tail tendon had a more rapid decline in post-laceration elastic stress and modulus parameters as well as a more linear and less tightly packed fascicular structure, suggesting that positional tendons may be less well suited to redistribute loads via a shear mechanism.

Shear loads induce cellular damage in tendon fascicles

“…In the single laceration group a 25% decrease in peak load compared to the intact control was much smaller than the decrease seen by Kondratko et al in whole tendons (Kondratko et al, 2012), where a laceration of approximately 60% reduced the peak load to 55%. The double laceration group tested here also demonstrated a smaller drop in peak load than seen by Kondratko-Mittnacht et al using a similar testing protocol in whole tendon (55% compared to 80%) (Kondratko-Mittnacht et al, 2015). The variation in these laceration scenarios is likely a result of differing shear transfer mechanisms at the two hierarchical levels.…”

“…Mechanical compromise due to laceration is disproportional to laceration area (Kondratko et al, 2012), indicating that hierarchical levels of tendon do not behave as independent load bearing structures as depicted in simplified structural models. Instead, levels of shear-lag and load transfer above those seen during normal loading must occur between fibrils and fascicles, as suggested in other studies (Ahmadzadeh et al, 2013; Kondratko-Mittnacht, Duenwald-Kuehl, Lakes, and Vanderby Jr., 2015; Pensalfini, Duenwald-Kuehl, Kondratko-Mittnacht, Lakes, and Vanderby Jr., 2014; Szczesny and Elliott, 2014). Although some studies conclude that shear force transmission capability is negligible between isolated tendon fascicles (Haraldsson et al, 2008; Purslow, 2009), Kondratko et al report a larger shear load transfer potential between fascicles when observing whole tendon behavior (Kondratko-Mittnacht et al, 2015).…”

“…Authors used transversely overlapping cuts (each 60% of the tendon depth) such that no fascicles were contiguous grip-to-grip. In this configuration, tendons maintained 20% of their intact strength and stiffness (Kondratko-Mittnacht et al, 2015). …”

“…Beyond affecting mechanical function, abnormal levels of tendon loading mechanisms, such as shear, as utilized by Kondratko-Mittnacht et al in (Kondratko-Mittnacht et al, 2015) or in vivo by partial tears, tendinopathy, or local remodeling, would affect nearby cells. In response to altered mechanical load, tenocytes change tendon structure, composition, and mechanical properties by adjusting extracellular matrix (ECM) protein expression (Banes et al, 1999; Chiquet, 1999; M.…”

Shear lag sutures: Improved suture repair through the use of adhesives

“…As a test case for a clinically relevant suture repair scenario, the model was analyzed using realistic tendon and suture material properties and a variety of realistic suture lengths and adhesive properties for a typical flexor digitorum profundus clinical repair: L = 13 mm, r t = 2 mm, E t = 200 MPa, t a = 100 µ m, r s = 100 µ m, and E s = 2 GPa [38, 47, 48, 49, 50, 51, 52]. …”