A review of the equine suspensory ligament: Injury prone yet understudied

Guest, Deborah J; Birch, Helen L; Thorpe, Chavaunne T

Abstract

The suspensory ligament (SL) is a key component of the elaborate and highly adapted suspensory apparatus in the horse. In addition to contributing to stabilisation of the metacarpophalangeal joint, the SL has a spring like function to reduce the energetic cost of locomotion. Although the SL is highly prone to injury in horses of all ages and competing in a wide range of disciplines, knowledge regarding fundamental structure-function relationships in the SL is lacking, particularly compared with other injury-prone tendinous structures such as the superficial digital flexor tendon. In this review, we discuss current knowledge of SL composition, structure and mechanical properties and describe the epidemiology , aetiology and pathophysiology of injuries. We evaluate different diagnostic approaches and treatment modalities and identify key areas for future research. K E Y W O R D S equine, function, injury, structure, Suspensory ligament 1 | SUSPENSORY LIGAMENT COMPOSITION AND STRUCTURE The specialised function of the suspensory ligament (SL) is achieved by the highly adapted composition and structural arrangement of the tissue comprising the ligament. The SL originates on the proximo-palmar/ plantar aspect of the third metacarpus/tarsus and continues distally, bifurcating and inserting onto the abaxial surface of the proximal sesa-moid bones (Figure 1). The extensor branches extend medially and laterally around the metacarpophalangeal joint, fusing with the common digital extensor tendon. 3,4 The SL is often divided into three regions, the proximal portion, the body, and the branches (Figure 1B,C). The SL is the anatomical equivalent of the interosseus muscle in other species, and may be referred to by that name in the horse. However, the equine SL has become highly adapted during evolution, with an almost complete loss of muscle fibres except in the proximal region, and an increase in collagen content. 4 These evolutionary adaptations have resulted in the development of a unique structure, with characteristics reminiscent of tendon, ligament and muscle. 5 The SL has several functions during locomotion. It is a key component of suspensory apparatus, preventing hyperextension of the metacarpophalangeal joint during stance phase. 3,4 Indeed, at maximal metacarpophalangeal joint extension the SL and accessory ligaments of the superficial and deep digital flexor tendons are predicted to contribute over half of the total support moment around the joint. 6 The SL also functions to store and return elastic energy in a similar manner to the forelimb superficial digital flexor tendon (SDFT), reducing the energetic cost of locomotion. 7 At rest, the SL contributes to the passive stay apparatus, allowing horses to remain standing for long periods of time with little muscular effort. 8,9 The predominant extracellular matrix (ECM) protein that comprises the SL is type I collagen, with values varying from 34% to 65% dry weight (Table 1). 5,11 The collagen content varies along the length of the ligament, with a greater collagen content distally, as the proportion of muscular tissue diminishes. 5 These values are lower than those in the SDFT which is 75% collagen by dry weight (Table 1). Collagen molecules are stabilised by intermolecular crosslinks, including hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP). While HP concentrations are significantly lower in

Citation

Guest, D. J., Birch, H. L., & Thorpe, C. T. (2024). A review of the equine suspensory ligament: Injury prone yet understudied. Equine Veterinary Journal, 1-16. https://doi.org/10.1111/evj.14447

• Journal Article Type: Review
• Acceptance Date: Nov 8, 2024
• Online Publication Date: Dec 27, 2024
• Publication Date: 2024
• Deposit Date: Nov 28, 2024
• Publicly Available Date: Dec 3, 2024
• Journal: Equine Veterinary Journal
• Print ISSN: 0425-1644
• Electronic ISSN: 2042-3306
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Pages: 1-16
• DOI: https://doi.org/10.1111/evj.14447