Load transfer, damage, and failure in ligaments and tendons

Abstract

The function of ligaments and tendons is to support and transmit loads applied to the musculoskeletal system. These tissues are often able to perform their function for many decades; however, connective tissue disease and injury can compromise ligament and tendon integrity. A range of protein and non-protein constituents, combined in a complex structural hierarchy from the collagen molecule to the tissue and covering nanometer to centimeter length scales, govern tissue function, and impart characteristic non-linear material behavior. This review summarizes the structure of ligaments and tendons, the roles of their constituent components for load transfer across the hierarchy of structure, and the current understanding of how damage occurs in these tissues. Disease and injury can alter the constituent make-up and structural organization of ligaments and tendons, affecting tissue function, while also providing insight to the role and interactions of individual constituents. The studies and techniques presented here have helped to understand the relationship between tissue constituents and the physical mechanisms (e.g., stretching, sliding) that govern material behavior at and between length scales. In recent years, new techniques have been developed to probe ever smaller length scales and may help to elucidate mechanisms of load transfer and damage and the molecular constituents involved in the in the earliest stages of ligament and tendon damage. A detailed understanding of load transfer and damage from the molecular to the tissue level may elucidate targets for the treatment of connective tissue diseases and inform practice to prevent and rehabilitate ligament and tendon injuries. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3093-3104, 2018.

Keywords: damage; failure; injury; ligament; tendon.

Figure 1.

A) Collagen structural hierarchy in ligament and tendon from the molecule to the tissue. The collagen triple-helical molecule, or tropocollagen, is assembled of three alpha-chains and forms the fundamental component of ligament and tendon (reprinted with permission, from Handsfield et al.) B) Collagen molecules arrange longitudinally and laterally in a quarter-stagger pattern to form collagen micro-fibrils (top, length compressed 5 times for display). Transverse organization follows a quasihexagonal packing (bottom), with groups of 5 molecules forming the lattice-like structure (inset; adapted with permission, from Orgel). C) The quarter-stagger arrangement is responsible for the characteristic d-banding pattern of collagen fibrils, which is observed in electron microscopy (3D reconstruction from serial transverse SEM images – left; SEM backscatter image - right). Fibril-level SEM imaging demonstrates high level of alignment, high aspect ratio, and continuity of fibrils along the length of ligament and tendons (left adapted with permission, from Svensson et al.; right adapted with permission, from Provenzano et al.). D) Small leucine rich proteoglycans such as decorin proteoglycan interact with collagen at the fibril level, playing an important role in regulating fibril growth (reprinted from Kosho). E) Collagen fibrils assemble to form collagen fibers, which contain a characteristic crimp pattern that is observed by many optical techniques including brightfield microscopy (left, adapted with permission, from Legerlotz et al.), fluorescence microscopy (top, reproduced with permission, from Screen et al.) and second harmonic generation imaging (bottom). Tenocytes (top) and elastin (bottom, yellow) reside between and in register with collagen fibers. Groups of fibers are enclosed by a fascia-like layer of endotenon to form fascicles, with groups of fascicles comprising the ligament or tendon. Some tendons, such as the Achilles tendon, originate from multiple muscle bellies with a single bone insertion point; in these multi-muscle tendons, each muscle belly contributes a subtendon.

Figure 2.

(A-D) Representation of the elastin network in ligament tissue. (A) In an unloaded state, the crosslinked elastin network exists as an unorganized network of randomly coiled elastin fibers. (B) Under applied load, the elastin network elongates and (C) following selective digestion with elastase the network is disrupted and only fragments remain. (D) Representation of elastin residing along and between collagen fibers in ligament (A-D reprinted with permission, from Henninger et al.). (E-H) Multiphoton microscopy of collagen (E, green), elastin (F, cyan), and cells (G, red) in unloaded tendon/ligament. These images reveal the presence of elastin between collagen fibers and in register with fiber crimp. Scale bar, 30 µm (E-H adapted with permission, from Pang et al.).

Figure 3.

Force-displacement and stress-strain relationships at different levels of the ligament and tendon hierarchy. Each scale exhibits non-linear behavior and the behavior of higher level structures is due to a combination of the behavior of subscale features directly and their interactions. A) force-extension of a single tropocollagen molecule, stretched by optical tweezers. Since, the optical tweezer method is not capable of achieving failure loads for a collagen molecule, this curve only displays behavior of the molecule at low strains. The elongated toe region is a result of stretching the molecule to its contour length, estimated at 315 ± 44nm. While this toe region appears long with respect to the molecule force-extension curve, it is still short compared to the micron to millimeter scale toe regions observed for higher level structures (reprinted with permission, from Sun et al.). B) Stress-strain curves from human patellar tendon (HPT) and native rat tail tendon (N-RTT) fibrils, loaded in tension using an atomic force microscope (reprinted with permission, from Svensson et al.). C) Stress-strain curve of a single fascicle from rat tail tendon, loaded by an electro-mechanical test system. This shape of this curve holds strong similarity to that observed for collagen fibrils. The toe region at the fascicle level is likely the result of molecules stretching to their contour length and uncrimping at the fiber level.

Figure 4.

Mechanisms of load transfer at the fiber and fibril scale. (A) Deformation of photobleached lines at increasing tensile strain and along the length of a tendon fascicle during tensile testing of a notched specimen. The continuity and increased deflection of the photobleached lines reveal shear transfer across the strain localization caused by the notch (adapted from Szczesny et al.). (B-D) Elastin contribution to the tensile, shear, and transverse behavior of porcine MCL. While removal of elastin by elastase treatment reveals a contribution to tensile behavior, namely an extension in the toe-region, elastin is the primary determinant of shear and transverse mechanics suggesting it is a likely candidate mechanism for fiber-level shear load transfer (B adapted with permission, from Henninger et al.; C-D adapted with permission, from Henninger et al.). (E-G) Decorin does not contribute to the quasistatic or viscoelastic behavior of porcine MCL, demonstrated by unchanged material response following selective removal of the chondroitin and dermatan sulfate side chains. Thus, while proteoglycans have demonstrated an important role during tissue development, they do not contribute to the mechanics of mature tissues (E-F adapted with permission, from Lujan et al.; G adapted from Lujan et al.).

Figure 5.

Collagen molecular damage due to incrementally applied tensile strain. A) Average stress-strain curves for fascicles stretched to incremental levels of strain. B) Fluorescence images of rat tail tendon fascicles stretched to 5, 10.5, and 15 % strain, then stained with fluorescent collagen hybridizing peptide, which detects unfolded collagen α-chains. On the bottom, a brightfield image of a fascicle shows the orientation of the fascicles in the fluorescence images. Scale bars, 2 mm. C) Incremental molecular damage with increasing strain quantified by intensity of fluorescent-CHP staining (green) and trypsin digestion (blue). Correspondence between the two techniques confirms that CHP binding detects mechanically unfolded collagen. The orange dotted lines in (a) and (c) indicate the approximate transition strain from the linear region to the onset of damage as identified by deviation of the stress-strain curve from linearity, which correlates with the onset of fluorescent-CHP intensity (reprinted from Zitnay et al.).