Anatomy

In the forelimbs the accessory ligament of the deep digital flexor tendon (ALDDFT) is a substantial structure, similar in size to the deep digital flexor tendon (DDFT). The ALDDFT continues the common palmar ligament of the carpus, originating principally from the palmar aspect of the third carpal bone.¹ Proximally the ALDDFT is broad and rectangular in cross-section; farther distally it becomes narrower and thicker and blends with the DDFT in the middle one third of the metacarpal region. Fibrous bundles run from the lateral border of the ALDDFT to the superficial digital flexor tendon (SDFT) in the proximal half of the metacarpal region, and these predispose the horse to develop adhesions between the ALDDFT and the SDFT in horses with severe superficial digital flexor tendonitis or severe desmitis of the ALDDFT. The ALDDFT forms the dorsal wall of the carpal canal, within which is the carpal synovial sheath, which is interspersed between the DDFT and its accessory ligament.

In the hindlimbs the size of the ALDDFT varies extremely, but it is generally smaller than in the forelimb and rarely more than half the thickness of the DDFT. The ligament is usually symmetrical in the left and right hindlimbs of an individual horse. A recent postmortem study showed that the ALDDFT was absent in 6% of 165 hindlimbs and was occasionally a bifid structure.²

A forelimb ALDDFT has a low modulus of elasticity and a moderate strength to rupture, whereas the DDFT has a high modulus of elasticity and a strength to rupture that is more than three times that of the forelimb ALDDFT. In the forelimb the ALDDFT is loaded during the late stance phase, during extension of the digital joints,¹ or when landing over a fence.³ The ALDDFT prevents overstretching of the DDFT by passively carrying the load during maximal extension of the distal interphalangeal and metacarpophalangeal joints. The ALDDFT also functions to facilitate carpal extension when the limb is loaded.¹ During flexion of the limb the ALDDFT is relaxed completely, and active muscle contraction results in the DDFT sliding proximally within the carpal sheath. The role of the ALDDFT in the hindlimb is less clear.

Desmitis of the ALDDFT usually occurs in forelimbs^4-9 but occasionally is recognized as a cause of hindlimb lameness.^4,10-13 In forelimbs desmitis may occur alone^4,7 as an acute injury or may develop secondarily to previous severe tendonitis of the SDFT. In the latter case the SDFT is enlarged substantially and wraps around the medial and lateral margins of the DDFT. Adhesions develop between adjacent structures. In horses with chronic, severe desmitis of the ALDDFT, additional injury may occur to the adjacent DDFT.^7,8 A flexural deformity of the metacarpophalangeal or metatarsophalangeal joint may develop after severe injury to the ALDDFT.^4,9 Occasionally a flexural deformity of the metatarsophalangeal joint develops in one or both hindlimbs without a history of lameness, but ultrasonography reveals chronic architectural changes in the ALDDFT.^11-13 The flexural deformity may be relieved by desmotomy of the ALDDFT, provided that contracture of periarticular soft tissues has not already occurred.

Pathogenesis

Degenerative aging changes take place in the ALDDFT of forelimbs, and these may be a substantial predisposing factor in the development of desmitis. The amount of fibrillar collagen and number of large collagen fibers in the ALDDFT decrease with increasing age.¹⁴ In a study of mechanical properties of the ALDDFT in relationship to age, failure forces of the ALDDFT of older horses were significantly lower than those of the ALDDFT in younger horses.¹⁵ Fibrillar ruptures developed in the ALDDFT of old horses at forces and strains that were approximately half those of the forces inducing total failure in young horses. It has been suggested that fibrillar rupture occurs at relatively low strains in older horses and that repetitive microtrauma may lead to clinical desmitis.^14,15

The incidence of desmitis of the ALDDFT is rather different from that of other tendonous and ligamentous injuries. Injuries occur more often in horses older than 8 years of age.^4-6 The incidence in Thoroughbreds is comparatively low (rare in the Standardbred), and therefore this is an unusual injury in event horses or racehorses, except in older steeplechasers. Desmitis is a relatively common injury in ponies (see Chapter 126) and crossbred horses, including pleasure horses.^4,5 The incidence in Warmblood horses is also high.⁶ Desmitis occurs in older show jumpers (see Chapter 115), especially Grand Prix–level horses,¹² in older dressage horses,¹² and also sometimes in young, extravagantly moving dressage horses (see Chapter 116). Desmitis is generally a unilateral forelimb injury (Figure 71-1), although occasionally it occurs bilaterally, and is a rare cause of hindlimb lameness.

Fig. 71-1 A, Transverse ultrasonographic image of the palmar metacarpal soft tissue structures, 11 cm distal to the accessory carpal bone, of an 8-year-old Grand Prix show jumper. Medial is to the left. The accessory ligament of the deep digital flexor tendon (ALDDFT) is considerably enlarged (arrows) and diffusely hypoechogenic, with focal anechoic areas dorsomedially. B, Longitudinal ultrasonographic image of the palmar metacarpal sort tissues in zone 2A of the same horse as in A. Proximal is to the left. There is a diffuse reduction in echogenicity of the ALDDFT and poor fiber pattern.

In hindlimbs, desmopathy of the ALDDFT has been seen most frequently in cob-type breeds^11-13 and Quarter Horses or Quarter Horse crosses,¹⁶ even in some horses or ponies of a comparatively young age, and does not appear to have been induced traumatically in all horses. In some of these horses the condition has been bilateral, developing simultaneously or sequentially in each hindlimb. Desmopathy frequently has been associated with a tendency to stand with the fetlock of the affected limb partially flexed (Figure 71-2