Anatomy

In the forelimb the deep digital flexor tendon (DDFT) has three heads: the humeral head, the largest, and the smaller radial and ulnar heads.¹ The tendon of the humeral head develops 8 to 10 cm proximal to the antebrachiocarpal joint, but muscular tissue persists to the level of the antebrachiocarpal joint, where the ulnar and radial heads join. The merged tendon is triangular in cross-section within the carpal canal but becomes more rounded in the metacarpal region. The accessory ligament of the DDFT (ALDDFT) merges with the DDFT in the middle third of the metacarpal region.

In the hindlimb the DDFT is formed by a large lateral digital flexor tendon and a smaller medial digital flexor tendon. The lateral digital flexor tendon incorporates the caudal tibialis tendon and passes over the sustentaculum tali within the tarsal sheath. The medial digital flexor tendon passes over the proximal tubercle of the talus, on the medial aspect of the talus, in its own synovial sheath. These two tendons fuse in the proximal metatarsal region. In the proximal metatarsal region the DDFT is a large oval structure that becomes smaller farther distally. The ALDDFT in the hindlimb varies in size and is generally comparatively smaller than in the forelimb and may be absent; in some horses it is a bifid structure.

At the fetlock region the DDFT becomes wider, elliptical, and fibrocartilaginous and is enclosed within the digital flexor tendon sheath (DFTS). In the pastern region the tendon becomes bilobed. At the level of the proximal part of the middle phalanx the dorsal part of the tendon becomes a fibrocartilaginous pad. Distally the DDFT is molded to the palmar or plantar aspect of the navicular bone. The DDFT is broad, has a terminal fanlike expansion containing cartilage, and inserts on the facies flexoria of the distal phalanx, delineated dorsally by the semilunar line and the adjacent surface of the cartilage of the foot.

The DDFT has a high modulus of elasticity (1585 MPa) and a considerable strength to rupture (approximately 1700 daN).¹ The ALDDFT has a low modulus of elasticity (490 MPa) and a moderate strength to rupture (approximately 490 daN). The DDFT limits carpal and fetlock extension under high loads. In the fetlock region the DDFT is under tension and compression. It is therefore fibrocartilaginous in this region and in the pastern, where the tendon is under pressure from the tuberositas flexoria, a transverse prominence on the proximal palmar aspect of the middle phalanx. In the digit the DDFT facilitates flexion of the proximal interphalangeal joint during weight bearing and stabilizes the distal interphalangeal joint.

The position of the DDFT and the navicular bone varies considerably during the stance phase. In the full weight-bearing position the DDFT is in close contact with only the distal border of the navicular bone, but during propulsion it comes into full contact with the palmar aspect of the bone. The tendon is stretched maximally as active contraction of the muscle bellies and the elasticity in the tendon result in elevation of the fetlock and extension of the distal interphalangeal joint. During the swing phase of the stride the DDFT relaxes. The DFTS facilitates displacement of the digital flexor tendons during flexion and extension.

DDFT injuries occur most commonly in the fetlock or pastern regions within the DFTS or within the hoof capsule. Injuries in the metacarpal or metatarsal regions and the carpus or tarsus are less common.^2-22

Deep Digital Flexor Tendonitis Associated With Recurrent Desmitis of the Accessory Ligament of the Deep Digital Flexor Tendon

Injuries of the DDFT in the carpal or metacarpal region, proximal to the DFTS, are rare except in association with chronic desmitis of the ALDDFT (see Chapter 71). Recurrent desmitis may be accompanied by pathological lesions of the DDFT.² Because of the close proximity of the DDFT and its accessory ligament, it is difficult to assess each structure accurately by palpation, especially with chronic enlargement of the ALDDFT, which may wrap around the borders of the DDFT. Ultrasonographic examination may reveal slight enlargement of the DDFT. The dorsal border may be less well defined, and diffuse hypoechogenic regions may occur within the DDFT, extending a variable distance proximodistally. These injuries usually result in recurrent lameness.

It has also been noted that in association with substantial enlargement of the superficial digital flexor tendon (SDFT) because of chronic tendonitis, the DDFT becomes smaller in cross-sectional area.³ With chronic enlargement of the ALDDFT, the DDFT also may reduce in size.

Primary deep digital flexor tendonitis in the proximal metacarpal region is rare. A single case was recorded by Genovese and Rantanen⁴ in an 8-year-old Quarter Horse used for English pleasure riding. Lesions have been identified using magnetic resonance imaging in a small number of horses.^5,6 Occasionally, traumatic injuries of the DDFT have been seen within the carpal sheath (see Chapter 75).

Deep Digital Flexor Tendonitis in the Carpal Sheath Secondary to Solitary Osteochondroma or a Distal Radial Physeal Exostosis

Lesions of the DDFT within the carpal sheath are an unusual cause of lameness except secondary to irritation by a solitary osteochondroma or a distal radial physeal exostosis.⁷ An osteochondroma is an exostosis continuous with the cortex of the bone and is covered by cartilage. The osteochondroma develops immediately proximal to the distal radial physis, often medial to the midline. Lameness is sudden in onset and usually is accentuated by carpal flexion. There is usually distention of the carpal sheath, but some horses with physeal exostoses have severe, sporadic lameness with no localizing clinical signs. An osteochondroma is readily identifiable radiologically, but some physeal exostoses are more readily identified using ultrasonography. Ultrasonographic examination from the distal medial aspect of the antebrachium also reveals the abnormal bone contour, an abnormal amount of fluid within the carpal sheath, and an irregular dorsal contour of the DDFT. Treatment is by tenoscopic surgical removal of the osteochondroma or exostosis and debridement of any torn fibers of the DDFT. The prognosis for return to athletic function is excellent.

Deep Digital Flexor Tendonitis within the Digital Flexor Tendon Sheath in the Fetlock Region

Some enlargement of the DFTS is common in hindlimbs, often unassociated with lameness, but occurs less frequently in forelimbs. Sudden-onset lameness associated with distention of a DFTS in a forelimb or a hindlimb may be caused by a variety of different lesions, but deep digital flexor tendonitis always should be considered^5,6 (see Chapter 74). It is rare to identify lesions of the DDFT within a DFTS that is not distended. Some horses develop deep digital flexor tendonitis after long-term chronic enlargement of the DFTS.

Lameness associated with DDFT lesions within the DFTS occurs more frequently in hindlimbs than in forelimbs and in horses from a variety of disciplines. The condition usually occurs unilaterally, although it has been seen bilaterally in the hindlimbs of several Warmblood dressage horses³ and in show jumpers.

Lameness varies from mild to moderately severe. Distention and thickening of the DFTS may make accurate palpation of the DDFT difficult. In some horses pain can be elicited by palpation of the margins of the tendon or by firm pressure applied to its palmar (plantar) aspect. The tendon should be assessed throughout its length, proximal and distal to the fetlock. In the acute stage there may be localized heat. Passive flexion of the lower limb may induce pain. If forelimb lameness is only mild in straight lines, it may be exaggerated on the lunge on a soft surface, especially in medium and extended trot. Distal limb flexion often accentuates the lameness. Occasionally in hindlimbs lesions of the DDFT within the DFTS have been the cause of sporadic lameness. The intermittent nature of the lameness makes definitive identification of the cause a diagnostic challenge. Such a history seen in conjunction with distention of the DFTS should prompt ultrasonographic examination (Figure 70-1).¹⁶

Fig. 70-1 Transverse (A) and longitudinal (B) ultrasonographic images of the left hindlimb of a horse with sporadic left hindlimb lameness. There was chronic moderate distention of the digital flexor tendon sheath of both hindlimbs. There is diffuse reduction in echogenicity in the dorsal half of the deep digital flexor tendon (A, arrows) and loss of fiber pattern (B, arrows).

Intrathecal analgesia of the DFTS usually results in substantial improvement but rarely alleviates lameness. Better improvement is seen after perineural analgesia of the palmar or plantar nerves and palmar metacarpal (plantar metatarsal) nerves (a so-called low 4-point block) proximal to the distended DFTS. In horses in which the metacarpophalangeal (metatarsophalangeal) joint capsule also is distended, performing intraarticular analgesia may be necessary to be sure that distention is not contributing to pain.

Definitive diagnosis requires ultrasonographic examination. Four types of lesions involving the DDFT have been identified: enlargement and change in shape of the tendon, focal hypoechoic lesions within the tendon or on its border, mineralization within the DDFT, and marginal tears.^8-14 The first three are readily diagnosed using diagnostic ultrasonography, but the marginal tears are much more difficult to identify. Surgical exploration may be required for definitive diagnosis.⁸ Acute-onset focal hypoechogenic areas generally are not seen with preexisting adhesion formation, although any of the other lesions may be.

The normal DDFT changes in its shape and cross-sectional area from proximally to distally, but it is usually bilaterally symmetrical. A normal DDFT is uniform in its echogenicity, and its margins are clearly defined. At the site at which the ALDDFT merges with the DDFT there may be a relatively hypoechogenic region, especially in the hindlimbs. This is a normal variant. Hypoechoic artifacts are induced readily in the distal fetlock and pastern regions if the ultrasound transducer is not perpendicular to the tendon, and in these regions evaluating the SDFT and DDFT simultaneously is difficult. Echogenic synovial plicae (mesotendon) extend medially and laterally from the DDFT to the DFTS wall in the proximal recess of the DFTS (Figure 70-2, A). These are seen much more obviously when the tendon sheath is distended and should not be mistaken for marginal tears or adhesions. With chronic tenosynovitis, these plicae may become thickened. Distal to the fetlock is an echogenic palmar (plantar) synovial fold that should not be confused with an adhesion (see Figure 70-2, B