Mechanical properties of the equine superficial digital flexor tendon relate to specific collagen cross-link levels.
BACKGROUND
Damage to the flexor tendons, particularly the superficial digital flexor tendon (SDFT), is one of the most common musculoskeletal injuries sustained by horses competing in all disciplines. Our previous work has shown that SDFTs from different individuals show a wide variation in mechanical strengths; this is important clinically as it may relate to predisposition to injury. The high mechanical strength of tendon relies on the correct orientation of collagen molecules within fibrils and stabilisation by the formation of chemical cross-links between collagen molecules. It is not known whether the variation in SDFT mechanical properties between individuals relates to differences in collagen cross-link levels.


OBJECTIVE
Enzyme-derived, intermolecular cross-linking of tendon collagen correlates with mechanical properties of the SDFT.


METHODS
SDFTs were collected from 38 horses and mechanically tested to failure. Structural and material properties were calculated from the load/deformation plot and cross-sectional area for each tendon. Following mechanical testing, pyrrolic cross-link levels were measured using a spectrophotometric assay for Ehrlich's reactivity and pyridinoline levels were quantified by HPLC. Cross-link levels were correlated with mechanical properties and statistical significance tested using a Pearson's correlation test.


RESULTS
Pyrrole cross-link levels showed a significant positive correlation with ultimate stress (P = 0.004), yield stress (P = 0.003) and elastic modulus (P = 0.018) of the tendons, despite being a minor cross-link in these tendons. There was no significant correlation of mechanical properties with either hydroxylysyl- or lysyl-pyridinoline levels.


CONCLUSIONS
Given the low absolute levels of pyrrole, we suggest that the correlation with high mechanical strength is through an indirect mechanism. Understanding the nature of the relationships between pyrrole cross-links, other matrix characteristics and tendon material properties may allow development of strategies to identify horses at risk from tendon injury and be of value in informing training practices.
Division of Surgery and Interventional Science, Institute of Orthopaedics and Musculoskeletal Science, RNOHT, University College London, UK.
C T
CT
Thorpe
R J F
RJ
Stark
A E
AE
Goodship
H L
HL
Birch
D000818:Animals; D002102:Cadaver; D003094:Collagen / Q000737:chemistry / Q000502:physiology*; D006736:Horses / Q000502:physiology*; D013053:Spectrophotometry; D013314:Stress, Mechanical; D013710:Tendons / Q000502:physiology*
D003094:Collagen