Chapter 37The Metacarpal Region
This chapter discusses the examination and diagnosis of injuries to the metacarpal region of the horse. The detailed anatomy of the deep digital flexor tendon (DDFT) (see Chapter 70), the accessory ligament of the deep digital flexor tendon (ALDDFT) (see Chapter 71), the superficial digital flexor tendon (SDFT) (see Chapter 69), and the third interosseous muscle or suspensory ligament (SL) (see Chapter 72) is discussed elsewhere.
In the metacarpal region the interosseous ligaments attach the second (McII) and fourth (McIV) metacarpal bones to the third metacarpal bone (McIII). These ligaments ossify to a variable extent during skeletal maturation. Fibrous bands extend from the distal aspect of the McII and McIV to the medial and lateral proximal sesamoid bones (PSBs).
The proximal aspect of the third interosseous muscle or SL attaches to the proximal palmar aspect of the McIII and the palmar carpal ligament and lies between the McII and McIV. Large exostoses on the axial aspect of the McII or the McIV have the potential to impinge on the abaxial border of the SL. The carpal sheath (see Chapter 75) extends through the proximal third of the metacarpal region. The amount of fluid in the carpal sheath varies between horses but is usually bilaterally symmetrical.
The digital flexor tendon sheath (DFTS) envelops the SDFT and DDFT from the distal third of the metacarpal region to the middle of the middle phalanx. Palmar to the metacarpophalangeal joint, the sheath passes through the inelastic canal created by the palmar annular ligament (PAL), the palmar fibrocartilaginous surfaces of the PSBs, and the intersesamoidean ligament. Within the DFTS, proximal to the sesamoidean canal, the SDFT forms a ring (the manica flexoria) around the DDFT. In the palmar midsagittal plane, a synovial reflection, the vincula, attaches the SDFT to the DFTS wall. Proximal to the manica flexoria, the DDFT is attached medially and laterally by mesotendon to the DFTS wall. The lateral mesotendon is more substantial and extends farther distally.
The palmar and palmar metacarpal nerves innervate the palmar metacarpal region. The SL is innervated by the medial and lateral palmar metacarpal nerves, branches of the deep branch of the lateral palmar nerve. This nerve receives contributions from the ulnar and median nerves. The medial palmar metacarpal nerve has fibers only from the median nerve, whereas the lateral palmar metacarpal nerve has fibers from the ulnar and median nerves.1 The SDFT and DDFT are innervated by the palmar nerves.2
In the proximal metacarpal region, the palmar nerves lie beneath relatively thick fascia, whereas farther distally they are more superficial. In the proximal 5 cm of the metacarpal region, the palmar metacarpal nerves and the distal palmar outpouching of the carpometacarpal joint capsule are in close proximity.
The metacarpal region should be examined visually from all angles to identify any changes in contour caused by swelling. If the limb is hairy, subtle swellings may easily be missed, especially in heavier breeds of horses, and if injury to the metacarpal region is suspected, then clipping the hair to facilitate examination can be useful. Palpation of the metacarpal region should be performed systematically with the limb bearing weight and with the limb semiflexed. With the limb bearing weight, the contour of the dorsal and palmar aspects of the limb should be straight. Distention of the medial palmar vein may reflect local inflammation. The size of the SL and its branches is assessed by running both thumbs down the dorsal and palmar borders from proximally to distally, medially, and laterally; they should remain equidistant. With the limb lifted, the margins of each of the SDFT, DDFT, ALDDFT, and SL should be carefully palpated to detect rounding of the margins, enlargement, or change in texture. Each structure should be squeezed, starting proximally and working distally, gently at first and then with increasing pressure to determine whether pain can be elicited. Careful comparison should be made with the contralateral limb, bearing in mind that lesions may be present bilaterally. Assess the response in light of the horse’s temperament and the recent work history. Abnormal stiffness of a structure may reflect previous injury. It is necessary to roll away the digital flexor tendons and compress the SL against the palmar aspect of the McIII to assess the most proximal part of the SL.
The axial and palmar margins of the McII and McIV should be palpated to identify any new bone formation (a splint) and to determine whether applied pressure causes pain. It is also important to assess whether the axial margin of the splint is clearly demarcated from the SL. Firm pressure should be applied to the dorsal aspects of the McIII to identify pain.
When local analgesic techniques are performed in the metacarpal region, it is important to recognize the potential for local anesthetic solution to diffuse proximally from the site of injection and thereby desensitize structures farther proximally, partially, or completely. Minimum volumes of local anesthetic solution should be used (maximum 2 mL/site and less in ponies) to minimize the risks of misinterpretation. The horse should stand still after injection before reassessment of the gait and should be reevaluated no more than 10 minutes after injection. Even then, false-positive results may be seen.
The metacarpal region seems particularly prone to development of swelling at the site of injection, which potentially may be permanent. The limb should be thoroughly scrubbed with chlorhexidine before injection to minimize the risk of adverse reaction and because of the potential for inadvertent injection into a synovial cavity (the DFTS, carpal sheath, or carpometacarpal joint capsule). The use of nonirritating local anesthetic solution (mepivacaine) is strongly recommended. It is also suggested that a stable bandage be applied to the limb for about 18 hours after injection.
Perineural analgesia of the palmar nerves at the level of the base of the PSBs desensitizes the foot and pastern regions, but in some horses it may also alleviate pain from the metacarpophalangeal joint, the proximal phalanx, PAL, SL branches, and DFTS. In horses with pain arising from the metacarpal region, lameness often appears paradoxically worse after palmar (abaxial sesamoid) nerve blocks, especially with proximal suspensory desmitis. Complete desensitization of the distal third of the metacarpal region requires perineural analgesia of the palmar nerves at the junction of the proximal two thirds and distal one third of the metacarpal region and the palmar metacarpal nerves distal to the distal aspect of the McII and McIV, the so-called low palmar or four-point block. Care should be taken to avoid inadvertent injection into the DFTS, which can occur when performing the palmar block despite no synovial fluid appearing in the needle hub. Proximal diffusion may result in partial alleviation of pain from the body of the SL in association with ultrasonographic evidence of proximal suspensory desmitis. If a unilateral lesion is suspected, the block may be performed medially or laterally alone.
Desensitization of one specific PSB can be achieved by blocking the sesamoidean nerve by introducing a needle between the insertion of the SL and the dorsal aspect of the abaxial surface of the sesamoid bone. The needle is directed toward the apex of the bone, and 0.5 mL of local anesthetic solution is injected.3
Elimination of pain from the entire metacarpal region requires blocking the palmar nerves immediately distal to the carpus and the deep branch of the ulnar nerve (and thus the palmar metacarpal nerves). However, often it is useful to be more specific and desensitize the deep structures (the palmar aspect of the McIII and the SL) or the more superficial structures (SDFT, DDFT, and ALDDFT). Perineural analgesia of the deep branch of the lateral palmar nerve or of the palmar metacarpal nerves by subcarpal injection should theoretically not abolish pain from the fetlock or more distal aspect of the limb, but this does sometimes occur. It is therefore important to perform the low 4-point block first. Blocking the palmar metacarpal nerves immediately distal to the carpus runs the risk of inadvertent injection into the distopalmar outpouchings of the carpometacarpal joint or spread of the local anesthetic solution by diffusion. Theoretically the risk of failure to desensitize the most proximal aspect of the SL exists, but diffusion of local anesthetic solution usually removes pain; false-negative results sometimes occur. The response should be compared with that following intraarticular analgesia of the middle carpal joint. Occasionally subcarpal analgesia of the palmar metacarpal nerves relieves pain associated with a primary middle carpal joint lesion better than intraarticular analgesia. Intraarticular analgesia of the middle carpal joint may relieve pain associated with proximal suspensory desmitis or a palmar cortical fatigue fracture of the McIII.
Perineural analgesia of the lateral palmar nerve from the lateral4 or medial5 approach entails less risk of affecting the middle carpal joint, but it does not eliminate it totally. The potential to remove pain from the lateral aspect of the more distal part of the limb also exists. From the lateral approach there is also a substantial risk of desensitization of the carpal sheath. This block can be particularly useful in a difficult horse, which is easier to inject with the limb bearing weight rather than semiflexed.
Theoretically, blocking the ulnar nerve should not completely remove pain associated with the proximal aspect of the SL or the proximal palmar aspect of the McIII because of the contribution of fibers from the median nerve to the medial and lateral palmar metacarpal nerves. However, in practice it generally does. The ulnar nerve block can be particularly useful to differentiate proximal metacarpal region pain from carpal pain in horses with an equivocal response to subcarpal analgesia; however, occasionally carpal pain is reduced.
Perineural analgesia of the palmar nerves (2 mL/site) should not desensitize the deeper structures (the McIII, SL) but should alleviate pain from the more superficial structures (SDFT, DDFT, and ALDDFT). Local infiltration around a painful exostosis of the McII or the McIV seems to be the most effective way of determining whether pain from the exostosis is contributing to the lameness observed. However, occasionally perineural analgesia proximal to the painful splint exostosis is necessary to abolish pain.
Intrathecal analgesia of the DFTS usually results in improvement in lameness associated with pain from within the sheath, but a better response is frequently seen after perineural analgesia of the palmar and palmar metacarpal nerves. Injection into the DFTS is most easily performed on the palmar midline of the pastern region, distal to the proximal digital annular ligament. The likelihood of inducing iatrogenic hemorrhage at this site is small, and retrieval of synovial fluid usually is easier than from the DFTS proximal to the PAL. If distention of the DFTS is only mild, compression of the proximal part of the sheath by an assistant to increase distention in the palmar pouch in the pastern can be helpful. Injection into the carpal sheath is usually only indicated if the sheath is distended (see Chapter 75).
Radiographic examination of the metacarpal region is usually tailored to each particular horse concerning the images required and the exposure factors used. With localized periosteal new bone, often finely coned-down images and use of soft exposures (low kilovolts [peak]) are required to demonstrate the lesion best. Several similar images varying slightly in obliquity may be required rather than a full series of dorsopalmar, lateromedial, dorsolateral-palmaromedial oblique, and palmarolateral-dorsomedial oblique images. Therefore a flexible approach must be used, depending on the initial clinical signs. Little soft tissue covers the metacarpal region; therefore a grid is not required. A high-definition screen with slow-speed film provides the best detail, although this may not be necessary with digital or computed radiography. Diffuse swelling in the metacarpal region makes images appear flatter, lacking contrast. Exposure factors may need to be increased slightly.
Superimposition of the McII and McIV over the McIII can create confusing radiolucent Mach lines (see page 174). The nutrient canal in the McII and McIV varies in prominence6 and should not be confused as a fracture. Ossification varies between the McII and McIV and the McIII in normal horses.
Ultrasonographic examination of the metacarpal region is discussed in detail elsewhere (see Chapters 16 and 69 to 72). In large, cob-type horses, the skin and underlying subcutaneous tissues in the metacarpal region may be thick, making it difficult to obtain high-resolution images. Dense stubble may persist after fine clipping of the hair coat, making it difficult to maintain good contact. Deep skin folds may further complicate the issue. For these horses it may be necessary to amplify the power and gain controls of the ultrasound machine and increase the focal depth of the transducer. Image quality may be enhanced by application of copious amounts of the ultrasound coupling gel to the skin for at least 15 minutes before imaging.
Blood pool (soft tissue phase) and bone phase (delayed) images of the metacarpal region are particularly useful in horses in which pain has been localized to the metacarpal region but in which no clinically significant radiological or ultrasonographic abnormalities have been identified. Many exostoses involving the McII and the McIV, although clinically inactive, are associated with moderately increased radiopharmaceutical uptake (IRU). Clinically silent enostosis-like lesions also may have IRU.
Incomplete, Longitudinal Palmar Cortical Fatigue Fractures of the Third Metacarpal Bone and Stress Reactions
Incomplete palmar cortical fatigue fractures of the McIII are relatively common and almost invariably involve the medial aspect of the bone,11-14 may involve the metaphyseal and proximal diaphyseal region, and sometimes extend proximally to involve the carpometacarpal joint. They are believed to be fatigue or stress fractures because increased radiopacity may be present radiologically when lameness is first recognized, indicating previous bony reaction.
These fractures occur most commonly in young horses, but they also occur in skeletally mature horses. In some horses, a recent increase in work intensity can be identified, which may be a predisposing factor. These fractures have been identified in a variety of racehorses sports horses, including horses used for flat and harness racing, National Hunt racing, and point-to-point racing, dressage, eventing, and endurance riding.
Lameness usually is sudden in onset and may be unilateral or bilateral. In some horses with bilateral injury, lameness is insidious in onset, and the horse has loss of forelimb action. Lameness varies from moderate to severe and tends to be worst on hard ground, and often the horse appears to become lamer the farther it trots. After the horse turns at the walk, the lameness then improves and again deteriorates as the horse trots. If the lameness is bilateral and similar in degree in each limb, the horse moves with a short striding, stilted gait.
Lameness is substantially improved or alleviated by palmar metacarpal (subcarpal) nerve blocks or perineural analgesia of the deep branch of the lateral palmar nerve. Lameness may be improved in some horses by intraarticular analgesia of the middle carpal joint.
Dorsopalmar radiographic images of the proximal metacarpal region may reveal increased radiopacity of the proximal medial aspect of the McIII, with or without a longitudinal radiolucent line extending a variable distance proximodistally (Figure 37-1). If present, the radiolucent line is invariably located medial to the axis of the McIII. Several slightly oblique dorsopalmar images may facilitate identification of a fracture. Generally no radiological abnormality is detectable in other images, although very occasionally periosteal callus is seen on the palmar aspect of the McIII in a lateromedial image. In some horses, no detectable radiological abnormality exists at any stage.
Fig. 37-1 A, Dorsopalmar radiographic image of the right third metacarpal bone of a 6-year-old Thoroughbred hurdler with moderate right forelimb lameness recognized 7 days previously and alleviated by subcarpal analgesia of the palmar metacarpal nerves. Medial is to the left. The medial aspect of the metaphyseal and proximal diaphyseal regions of the third metacarpal bone has increased radiopacity. A poorly defined longitudinal radiolucent line (arrows) represents an incomplete longitudinal palmar cortical fatigue fracture. B, Dorsopalmar radiographic image of the right proximal metacarpal region of a 6-year-old Arabian endurance horse with a moderate right forelimb lameness of 3 weeks’ duration. Lameness was alleviated by perineural analgesia of the deep branch of the lateral palmar nerve. Well-defined increased radiopacity immediately proximal to the nutrient foramen surrounds a longitudinal radiolucent line in the third metacarpal bone (arrows). C, The same horse as in B after 4 weeks of box rest. The radiolucent line extends further proximally (arrows).
Diagnostic ultrasonography usually reveals no detectable abnormality. Rarely a defect in the palmar cortex of the McIII can be identified. Nuclear scintigraphic images usually demonstrate moderate to intense IRU in the proximal palmar aspect of the McIII in dorsal and lateral bone phase images (Figure 37-2). Early radiopharmaceutical uptake in the proximal palmar aspect of the McIII may also be evident in pool phase images. This pattern of uptake is indistinguishable from that associated with an avulsion fracture of the McIII at the attachment of the SL. It is not known whether horses without radiological change, but with scintigraphic evidence of an increased bone modeling in the proximal aspect of the McIII, would develop a radiologically evident incomplete palmar cortical fracture of the McIII if the horses were kept in work, or whether this is a different manifestation of the response of bone to exercise.
Fig. 37-2 Dorsal (A) and lateral (B) scintigraphic images of the carpal and proximal metacarpal regions of the forelimbs of an 8-year-old advanced event horse with acute-onset moderate left forelimb lameness of 3 weeks’ duration. The left forelimb is on the right in the dorsal image (A). Lameness was alleviated by subcarpal analgesia of the palmar metacarpal nerves. The character of the lameness was typical of a palmar cortical fatigue fracture, but no radiological or ultrasonographic abnormality was detected. Intense focal increased radiopharmaceutical uptake in the proximal, medial, palmar aspect of the third metacarpal bone is compatible with a stress or fatigue fracture.
MRI in a small number of Thoroughbred (TB) racehorses with no detectable radiological abnormality revealed diffuse areas of reduced signal intensity in the proximal palmar aspect of the McIII in T1 gradient echo (GRE) images and increased signal intensity in fat-suppressed and T2-weighted GRE images (surrounded by a hypointense rim in T2-weighted images, the result of a fluid-fat cancellation artefact), consistent with bone trauma.15 In some horses a cortical defect consistent with a fracture was identified, usually medially. The clinical characteristics of the lameness in these horses were not described and scintigraphy was not performed. It is not clear whether these injuries are stress-related bone injuries that reflect a continuum with palmar cortical fatigue fractures. Similar abnormalities have also been seen in endurance and other sports horses, in association with IRU, and similar lameness characteristics of those described for palmar cortical fatigue fractures.10 Generally the entire palmar aspect of the McIII is involved, sometimes accompanied by signal alterations in the McII or the McIV, unlike the palmar cortical stress fractures described above in which radiological abnormalities are usually restricted to the medial aspect of the McIII. However, the MR signal alterations were usually most extensive medially.
Most horses respond well to rest, with box rest for 1 month and then box rest and controlled walking exercise for another 2 months. Horses are then able to slowly and progressively resume normal work. The incidence of recurrent injury is small. Every attempt should be made to identify any problem with the previous training program that may have predisposed the horse to injury.
Transverse stress fractures of the distal metaphyseal region of the McIII are relatively uncommon.16 I have seen this fracture in horses of 4 to 7 years of age in the first season of racing over fences, in young polo ponies, and in pleasure horses that have galloped on the beach. These fractures are believed to be stress fractures because endosteal and periosteal callus has usually been identified radiologically at the first recognition of the lameness. Acute fractures have also been identified in young TB flat racehorses, sometimes only in light work.
In young immature TB racehorses there is usually acute-onset severe lameness unassociated with work. There is rapid development of swelling in the distal third of the metacarpal region on the palmar, palmaromedial, or palmarolateral aspects, which is exquisitely painful on palpation and may mimic cellulitis. Radiographs are usually negative initially, but within 7 to 10 days a tranverse radiopaque line becomes apparent across the distal metaphyseal region and ill-defined periosteal new bone proximally and distally, resulting in distortion of the distal aspect of the McII or the McIV. In some horses, a radiolucent fracture line may eventually be identified. Lameness takes several months to resolve, and there is slow remodeling of the periosteal callus.
In older horses, lameness is usually acute in onset after fast work, is moderate to severe, and may be unilateral or bilateral. Lameness may improve rapidly with the horse appearing sound within a few days. In horses with long-standing lameness, a change in contour of the distal dorsal aspect of the McIII is visible and is associated with periosteal callus formation. In these horses, pain may be elicited by firm pressure applied to the distal dorsal or palmar aspects of the McIII. Twisting the McIII may also induce pain. However, horses with acute lameness may have no localizing signs.