Chapter 38The Carpus
Anatomy
The carpus has a dense joint capsule dorsally that blends with the overlying fascia and retinaculum. Synovium in young horses is often thickened or folded dorsally in the middle carpal joint and can interfere with visibility during arthroscopic surgery. This fold appears to smooth as horses age or as osteoarthritis develops. The antebrachial fascia blends with the retinaculum that functions to restrain extensor tendons. Retinaculum thickens and forms the medial and palmar borders of the carpal canal. The palmar retinaculum is sometimes severed in horses with carpal tenosynovitis and tendonitis (see Chapter 75). Anatomical considerations and flexor and extensor tendon injuries are discussed elsewhere (see Chapters 69 and 77). The sheathed extensor carpi radialis and common digital extensor tendons, located dorsally and dorsolaterally, respectively, limit carpal palpation and restrict access. Cul-de-sacs of distended antebrachiocarpal and middle carpal joint capsules can be palpated medial to the extensor carpi radialis tendon or between the extensor carpi radialis and common digital extensor tendons in a standing horse. Arthrocentesis and arthroscopic examination require careful placement of needles and instruments in these portals to avoid injury to tendons and sheaths. These portals can be easily felt as distinct depressions when the carpus is flexed. The sheathed lateral digital extensor tendon, located on the lateral aspect, should be avoided during arthrocentesis of the palmarolateral pouches. The sheathed extensor carpi obliquus tendon is small and passes obliquely over the antebrachiocarpal joint from lateral to medial to attach to the McII. This tendon can readily be seen medially during arthroscopic examination of the antebrachiocarpal joint. Extensor tenosynovitis must be differentiated from middle carpal and antebrachiocarpal joint effusion and hygroma. The antebrachiocarpal and middle carpal joints each have a palmarolateral and a palmaromedial outpouching through which arthrocentesis and arthroscopic evaluation can be performed. Unless greatly distended, the palmarolateral outpouchings are larger than the corresponding palmaromedial outpouchings. The palmarolateral outpouching of the antebrachiocarpal joint is in close proximity to the carpal sheath, and inadvertent penetration of the carpal sheath can occur during arthrocentesis or arthroscopic examination even when the palmarolateral outpouching is distended.
Knowledge of the communications and boundaries of the carpal joints is important in understanding the extent of disease processes and the results of diagnostic analgesia (see Chapter 10). The antebrachiocarpal joint is considered solitary, although in a single specimen in a cadaver study the joint communicated with the middle carpal and carpometacarpal joints.10 In some horses a communication appears between the antebrachiocarpal joint and the carpal sheath. The middle carpal and carpometacarpal joints always communicate (see Figure 10-8Figure 10-9Figure 10-10). Communication between the middle carpal and carpometacarpal joints and the carpal sheath is rare. The carpometacarpal joint has distinct distopalmar outpouchings located axial to the McII and the McIV that have secondary pouches interdigitating within the proximal aspect of the suspensory ligament (SL). These outpouchings explain inadvertent analgesia of the carpometacarpal and middle carpal joint while performing high palmar analgesia and possibly why lameness abates during middle carpal analgesia in horses with avulsion fractures of the proximopalmar aspect of the McIII or proximal suspensory desmitis.11
Clinical Characteristics and Diagnosis of Carpal Lameness
Imaging
Radiography
Normal radiological anatomy of the carpus is difficult because carpal bones overlap considerably, bones shift during flexion, and normal radiolucent defects and aberrant carpal bones can be difficult to interpret. In the skyline image of the distal row, the normal articulation between the third and fourth carpal bones can be superimposed on the lateral aspect of the third carpal bone and confused with a sagittal fracture (see Figure 38-1, B). On a DM-PaLO image the normal articulation between the second and third carpal bones should not be confused with a sagittal slab fracture, but this image is essential to diagnose sagittal fracture of the third carpal bone correctly, which runs parallel to this articulation. Radiolucent defects or osseous cystlike lesions are often seen in the ulnar carpal bone and are considered incidental findings, but when they appear in other bones, they can cause lameness regardless of whether communication with a joint exists. In LM and oblique images, the first and fifth carpal bones can be confused with osteochondral fragments. Radiolucent defects in the McII and the McIV often occur in the presence of the first (see Figure 37-7) and fifth (Figure 38-2) carpal bones but are normal. In a flexed LM image the radial carpal bone moves distally relative to the intermediate carpal bone. This normal finding is quite useful in determining the exact positioning of osteochondral fragments or other lesions on the proximal or distal surfaces of the radial and intermediate carpal bones. The flexed LM image is also highly useful for evaluation of the distal dorsal articular surface and subchondral bone of the radial carpal bone. Xeroradiography has largely been discontinued, but computed radiography and digital radiography are available at most institutions and many private practices and yield images superior to those obtained by conventional radiography, but positioning and exposure must still be optimized. Subtle radiological changes can be readily seen in most digital radiographic images, as can fragments, radiolucent defects, and other changes not previously visible on conventional images (see Figure 38-1). Care must be taken not to confuse normal articulations for fractures.
Scintigraphy
Scintigraphy is especially useful to diagnose early stress-related subchondral bone injury and differentiate carpal lesions from those of the proximal metacarpal region (see Figure 19-19). A common finding in young racehorses is carpal lameness localized by clinical signs and diagnostic analgesia with negative or equivocal radiological abnormalities. Focal areas of increased radiopharmaceutical uptake (IRU) are often found unilaterally or bilaterally, most commonly in the third carpal bone (see Figure 19-16). Scintigraphy can be used to verify or refute the importance of sclerosis in the third carpal bone. Scintigraphy is useful in diagnosing unusual fractures of the palmar aspect of the third carpal bone, corner fractures or table surface collapse of the third carpal bone or other carpal bones, and lesions that are not apparent or are located in obscure areas not depicted radiologically. Focal areas of IRU occur with many carpal injuries, and although sensitivity is high, the specificity of scintigraphic images is low and differentiation of specific types of injuries is difficult. Scintigraphy is most useful in localizing the site of injury, based on which additional radiographic images are obtained, or rest is recommended, followed by repeated radiographic examination.
Diagnostic Arthroscopy
In horses with scintigraphic evidence of IRU and those with OA but without radiological confirmation of osteochondral fragmentation, arthroscopic examination usually reveals cartilage damage, the extent of which can be graded. Prognosis is inversely related to degree of cartilage damage (see Figures 23-7 and 38-1). Occult osteochondral fragments, most commonly involving the third and radial carpal bones, and intercarpal ligament tearing are found frequently in these horses (see Figure 23-4).
Specific Conditions of the Carpus
Osteoarthritis
OA is the most common carpal problem, but clear differentiation of OA from osteochondral fragmentation is difficult, because both problems are intertwined. Horses with osteochondral fragments often develop OA, and horses with early OA, and some with chronic OA, develop osteochondral fragments. Pathogenesis of OA and osteochondral fragmentation appears similar if not identical in some horses, but OA of the equine carpus has two forms. The most common form is seen in racehorses or ex-racehorses that initially develop stress-related subchondral bone injury of the middle carpal and antebrachiocarpal joints that leads to, or accompanies, overlying cartilage damage and osteochondral fragmentation (see Figure 38-1). A second form of OA develops in nonracehorses and is less common. Horses are usually middle aged or older, but occasionally it occurs in younger horses. Typical clinical and radiological evidence of OA exists, but osteochondral fragments are unusual (see Chapters 61 and 84).
In many racehorses extensive OA and osteochondral fragmentation lead to retirement, but some are able to compete in other sporting events. Progressive OA can then develop later in life. In middle-aged to old nonracehorses, primary OA develops without stress-related subchondral bone injury, high-impact loading, or development of osteochondral fragmentation. This condition can be seen in Western performance horses, other sports horses, or even in horses and ponies used for pleasure riding. Often severe radiological evidence of OA is seen on initial examination when lameness is subtle (Figure 38-3). In fact, it has been proposed that the threshold of pain in riding horses with severe OA of the antebrachiocarpal joint may be higher than in those with similar conditions of the middle carpal joint, because observation of lameness by owners of these horses was a late event.30 Faulty conformation such as carpus valgus, back at the knee, or bench knee is seen in some horses, but in others neither mechanical nor training-related factors are present. OA in these horses can involve the antebrachiocarpal and middle carpal joints together or separately, but when disease involves the carpometacarpal joint, chronic and severe lameness develops (see Figure 3-2).