Chapter 77Extensor Tendon Injury
The extensor tendons of the carpus and digit and the respective synovial sheaths are vulnerable to injury because they lie relatively unprotected in the subcutaneous fascia on the dorsal surface of the limb. This chapter discusses lameness caused by injuries and disorders of the extensor tendons of the carpus and digit. Injuries of the extensor tendons of the hock (gastrocnemius and calcaneal tendons) are discussed elsewhere (see Chapter 80).
The extensor carpi radialis (ECR) muscle is the largest extensor muscle of the forelimb and has a prominent muscle belly on the cranial aspect of the antebrachium. The ECR muscle extends the carpus, smoothes carpal joint movement by dampening oscillations as the hoof strikes the ground, and flexes the elbow.1,2 The extensor carpi radialis tendon (ECRT) extends through most of the muscle belly and appears on the surface of the muscle in the middle of the antebrachium. The tendon passes through the middle groove at the distal end of the radius, over the dorsal aspect of the carpus, and inserts on the metacarpal tuberosity on the dorsal proximal aspect of the third metacarpal bone.1 As the tendon passes over the carpus, it is bound by the extensor retinaculum and enveloped by a synovial sheath, which extends from 8 to 10 cm proximal to the carpus, to the level of the middle carpal joint. Distal to the sheath the ECRT is attached to the carpometacarpal joint capsule. There is usually a small bursa beneath the tendon at the level of the third carpal bone.1,3
The common digital extensor (CDE) muscle is a compound muscle with three heads (humeral, radial, and ulnar) that lies lateral to the ECR muscle on the craniolateral aspect of the radius and acts to extend the digit and carpal joints and to flex the elbow. The main common digital extensor tendon (CDET) appears on the surface of the muscle in the middle of the antebrachium and passes distally through the lateral groove on the cranial distal aspect of the radius and over the capsule of the carpal joints. As the tendon passes distally over the dorsal aspect of the metacarpal region, it courses medially and reaches the dorsal midline just proximal to the fetlock. At the level of the distal aspect of the proximal phalanx the tendon becomes wider as it is joined by extensor branches of the interosseous tendon (suspensory ligament). The CDET inserts on the extensor process of the distal phalanx and the dorsal surface of the proximal extremities of the proximal and middle phalanges.
The CDET is enveloped by a synovial sheath as it passes over the carpus. The sheath extends from about 8 cm proximal to the carpus to the proximal end of the metacarpal region.1,3 A bursa is present between the tendon and the dorsal pouch of the fetlock joint capsule.1,3,4
A small tendon also arises from the smaller head (the radial head) of the CDE muscle. This tendon runs through the synovial sheath of the principal tendon and then passes laterally to fuse with the lateral digital extensor tendon (LDET) or may continue separately between the CDET and LDET to the fetlock. A small tendon may also arise from the ulnar head of the CDE muscle, which either fuses with the principal tendon or inserts on the joint capsule dorsal to the fetlock joint.
The lateral digital extensor (LDE) muscle is smaller than the other extensor muscles and is situated caudal to the CDE muscle. The action of the LDE muscle is to extend the digit and carpus. The LDET arises at the level of the distal third of the antebrachium and passes distally through the groove on the lateral styloid process of the radius and over the lateral aspect of the carpus. The tendon becomes larger and flattens distal to the carpus as it joins the tendon of the radial head of the CDE muscle and a strong band from the accessory carpal bone. The LDET runs lateral to the CDET and gradually inclines toward the dorsal aspect of the metacarpal region and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx.1
A synovial sheath envelops the LDET as it passes over the carpus. It begins 6 to 8 cm proximal to the carpus and extends to the proximal end of the metacarpal region.1,3 At the fetlock a small bursa lies between the tendon and the joint capsule.1
The extensor carpi obliquus is a small muscle that extends the carpus. The tendon of the extensor carpi obliquus arises at the distal end of the radius and courses distally, cranially, and medially over the ECRT and then passes through the oblique groove at the distal end of the radius to insert on the base (head) of the second metacarpal bone. The tendon is enveloped by a synovial sheath.1
In the hindlimb, the long digital extensor (LoDE) muscle is situated superficially on the craniolateral aspect of the limb and acts to extend the digit and flex the hock. The tendon of the muscle begins in the middle of the muscle belly and passes distally over the dorsal aspect of the hock, where the tendon is bound by the extensor retinacula and enveloped by a synovial sheath. The sheath extends from slightly proximal to the level of the lateral malleolus of the tibia to the proximal third of the metatarsal region. The long digital extensor tendon (LoDET) is joined by the LDET about 10 cm distal to the tarsus. In the angle of this union the extensor digitorum brevis also joins the principal tendon of the LoDE.1 Distal to this point the arrangement of tendons is the same as in the forelimb.
The LDE muscle lies on the lateral surface of the crus caudal to the LoDE muscle. The LDET runs through the entire length of the muscle belly and emerges at the level of the distal third of the tibia. The tendon descends through the groove on the lateral malleolus of the tibia, where it is bound by the extensor retinacula, and usually blends with the LoDET. In some horses the tendon does not insert on the LoDET but passes separately lateral to the LoDET and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx, like the corresponding tendon in the forelimb.1 The tendon is surrounded by a synovial sheath that extends from 4 to 6 cm above the lateral malleolus of the tibia to the proximal third of the metatarsal region.1,3
Many disorders of the extensor tendons, particularly acute injuries, may be diagnosed by careful clinical examination, gait analysis, and palpation. Diagnostic perineural or intrathecal analgesia may be useful in horses with chronic injuries to the extensor tendons to determine the importance of clinical findings. Synoviocentesis of distended tendon sheaths or bursae also may be helpful to distinguish between tenosynovitis caused by injury or infection.
Plain radiography is of little value in identifying soft tissue injuries of the extensor tendons unless radiodense foreign material or bone fragments are present. Radiology may be useful to evaluate mineralization within the tendons or the synovial sheaths, enthesopathy, and associated osseous abnormalities. Contrast radiography may demonstrate a penetrating tract, intrathecal adhesions, or synovial fistulae between the extensor tendon sheaths and carpal joints.5,6
Ultrasonography is a safe, noninvasive method of evaluating the extensor tendons, the synovial sheaths, and the bursae. A 7.5- or 10-MHz transducer and standoff pad are used to image the extensor tendons and the respective tendon sheaths. Careful evaluation of transverse and longitudinal images allows the clinician to determine the extent of tendon damage, the presence of foreign bodies within the tendon or tendon sheaths, synovial membrane hyperplasia, and intrathecal adhesions.7
Conditions Affecting the Extensor Tendons
Laceration of the Digital Extensor Tendons
Lacerations of the extensor tendons commonly occur in the metacarpal or metatarsal regions of the limbs because of their superficial location at these sites (see Chapter 81). Extensor tendon lacerations are more common in the hindlimb8-11 and are frequently associated with considerable soft tissue damage and opening of the extensor tendon sheaths.
Transection of an extensor tendon below the carpus or tarsus leads to reduced ability to extend the digit, which results in an exaggerated, rapid flip of the hoof at the end of the swing phase of the stride, intermittent knuckling over at the fetlock, or tripping at the walk. The horse, however, will bear weight in a normal posture if the foot is placed flat on the ground. This gait abnormality is more obvious in horses with hindlimb injuries and when the laceration is near the fetlock because remaining peritendonous fascial attachments provide some support to the distal part of the tendon in more proximal injuries.
Lacerations with transection of the extensor tendons proximal to the carpus, and at or just proximal to the tarsus, are also common. Transection of the CDET and ECRT commonly occurs proximal to the carpus and often results in pain on flexion of the carpus. Transection of the LoDET proximal to or at the tarsus results in greater tarsal flexion during the swing phase of the stride and intermittent knuckling of the digit. Within a few days of transection of an extensor tendon the horse learns to adapt its gait, and tripping and knuckling of the fetlock become less frequent.
The diagnosis of extensor tendon laceration is often apparent from the gait deficit. If necessary, the diagnosis may be confirmed by exploration of the wound with a sterile, gloved finger after aseptic preparation and wound lavage. Plain and contrast radiography should be used to evaluate concomitant joint or bone damage and to help to identify foreign bodies.
After aseptic preparation, wounds over the extensor tendons should be debrided and lavaged. It is important to debride any exposed or obviously devitalized bone and, where possible, to cover bone with skin to reduce the risk of sequestrum formation. Primary apposition of the lacerated tendon is unnecessary even if a large gap has formed between the distracted tendon ends. Progressive ultrasonographic evaluations show that fibrous tissue develops between the transected tendon ends; this tissue gradually becomes more organized and regains the linear arrangement of collagen along the line of the original tendon. This fibrous tissue provides an adequate mechanical link between the tendon ends, allowing extensor function of the digit to return.12
Although lacerations of extensor tendons may heal without external coaptation, wound healing is facilitated and exuberant granulation tissue formation is controlled if limb immobilization is used during the first 3 to 6 weeks after injury. Immobilization may be achieved using a polyvinylchloride (PVC) splint or cast. Full-limb PVC splints may be applied to the dorsal or palmar aspect of the forelimb to prevent knuckling of the fetlock and carpal flexion. In the hindlimb, a distal limb splint applied to the dorsal or plantar aspect of the limb can be used. Application of a shoe with a toe extension may help facilitate flat foot placement and prevent tripping or knuckling over of the fetlock in the early postinjury period. Casts provide an inexpensive and efficient means of immobilizing the limb and tendon ends and are especially useful when extensor tendon lacerations occur with extensive, contaminated wounds, in which primary wound closure cannot be achieved.
After removal of the cast or splint, the horse should remain confined to a box stall for another 4 to 6 weeks. During this period the toe should be trimmed short so that it does not catch to cause knuckling and disruption of the organizing fibrous tissue. A controlled exercise program of in-hand walking exercise may be initiated at this time to strengthen the tendon and improve gliding function. Passive range of movement exercises may also be beneficial. If no signs of knuckling are present after 10 to 12 weeks of controlled exercise, a gradual return to athletic use may begin.
The prognosis for return to athletic function after extensor laceration is good.8-11