Chapter 36 The Metacarpophalangeal Joint
The metacarpophalangeal (fetlock) joint is an intensely loaded, high-motion joint that is frequently injured in athletic horses. Fetlock region lameness can occur in horses of any occupation, but the joint is at particularly high risk in horses performing at maximal speed. Countering the high load experienced by the joint is the elastic suspensory apparatus that constrains the range of extension. Both the superficial and deep digital flexor muscle tendon units additionally serve to actively support the fetlock joint because of their position on the palmar aspect of the joint. Loss of this active support by the digital flexor tendons may lead to overload of the suspensory apparatus support and some degree of hyperextension. Extreme extension of the metacarpophalangeal joint results in impingement of the proximal rim of the proximal phalanx against the dorsal aspect of the third metacarpal bone (McIII), high compressive forces on the distal, palmar aspect of the McIII opposing the proximal sesamoid bones (PSBs), and both tensile and bending forces on the PSBs intercalated within the suspensory apparatus.1 Complex forces of torsion on the loaded fetlock joint result in a myriad of injuries.
The metacarpophalangeal joint is anatomically composed of the distal aspect of the McIII, the proximal phalanx, and both PSBs. A single synovial space extends on its palmar aspect at least 3 cm proximal to the apex of the PSBs. Dorsally, a bilobed synovial “pad” is located on the proximal dorsal articular rim of the distal aspect of the McIII; this pad presumably functions to help cushion the impingement of the McIII and proximal phalanx. The ligamentous elements of the fetlock joint are complex and important. Well-developed medial and lateral collateral metacarpophalangeal and metacarposesamoidean ligaments constrain the almost purely sagittal motion of this joint, but both abduction and rotation occur.2 The suspensory branches insert on the proximal, palmar abaxial margins of the PSBs and functionally continue through the distal sesamoidean ligaments to attach the distal portion of the PSBs to the proximal phalanx (cruciate or deep and oblique or middle) and the middle phalanx (straight or superficial). The intersesamoidean ligament attaches the axial aspects of the PSBs.
The digital flexor tendon sheath and its contents are described elsewhere (see Chapter 74). The common digital extensor tendon is unsheathed as it passes over the dorsal aspect of the joint. Branches of the medial and lateral palmar and metacarpal nerves primarily supply innervation to the fetlock joint,3 but small subcutaneous branches of the ulnar nerve supply a minor amount of dorsal sensory innervation.
Careful physical examination to detect heat, swelling (effusion with or without periarticular fibrosis), and pain with palpation or manipulation is critical for diagnosis of lameness of the metacarpophalangeal joint. However, physical findings may be subtle or seemingly nonexistent, whereas diagnostic analgesia often pinpoints the joint or region as the source of pain. Interpretation of fetlock flexion test results should be made with caution because many horses have false-positive responses to forced flexion of this joint, especially if the toe is used to increase leverage. Pain originating in the limb distal to the fetlock joint often is exacerbated by fetlock or lower limb flexion tests. False-negative responses to fetlock joint flexion also are common, especially in horses with subchondral bone injury and chip fractures. Flexion tests are discussed further in Chapter 8. Increased fluid in the fetlock joint can also be a false localizing sign. The presence of an effusion in the absence of heat and pain may indicate some derangement in synovial function but not necessarily point to the source of a clinically relevant lameness. Lameness characteristically occurs with weight bearing and usually, but not always, is worse with the limb on the inside of a circle. If clinical signs do not adequately localize lameness, perineural or intrasynovial analgesia can specify the region relatively easily. In the absence of localizing clinical signs, the foot and pastern regions should first be eliminated as sources of pain by a midpastern digital nerve block with a dorsal subcutaneous ring. Ideally this is followed by intraarticular analgesia of the metacarpophalangeal joint. If the lameness does not improve within 15 to 20 minutes of intraarticular analgesia, the clinician should perform a low palmar block or palmar and palmar metacarpal nerve blocks. In most horses with intraarticular chip fractures, synovitis, capsulitis, and osteoarthritis (OA), lameness improves after intraarticular analgesia. Horses with major fractures, nonarticular fractures, subchondral bone injuries, and tendon and tendon sheath lesions usually are only markedly improved with perineural analgesia. Perineural techniques are superior to definitively abolish pain associated with the fetlock joint. Pain in horses that do not respond to intraarticular analgesia should not necessarily be presumed to originate elsewhere. False-positive results to intraarticular analgesia can also be obtained; lameness associated with a suspensory branch injury or injury of the proximal aspect of the oblique, cruciate, or straight sesamoidean ligaments may be substantially improved or abolished. Subchondral bone pain will not always, or completely, be eliminated with intraarticular analgesia.
Several techniques are used for intrasynovial analgesia of the fetlock joint. One author (DWR) prefers to inject the local anesthetic solution with the horse’s limb on the ground. The dorsal aspect of the fetlock is readily accessible when the horse is bearing weight. As the clinician faces forward and presses the back of his or her arm against the horse’s carpus, a 22-gauge, 2.5-cm needle attached to a 75-cm extension set is inserted horizontally into the dorsal aspect of the joint just proximal to the margin of the proximal phalanx and deep to the common digital extensor tendon. One of us (DWR) uses 10 to 15 mL of 2% mepivacaine, but many practitioners use no more than 6 to 10 mL. One author (SJD) prefers to inject into the palmar pouch through the metacarposesamoidean ligament, with the limb flexed, using a 21-gauge, 3.8-cm needle, injecting 6 to 10 mL of 2% mepivacaine depending on the size of the horse. Other techniques are described in Chapter 10.
The fetlock joint is easy to image accurately because of the size and accessibility. Numerous flexed and oblique radiographic images to specifically silhouette or separate structures help to identify subtle lesions. The same advantages apply to scintigraphic and ultrasonographic imaging. For example, flexed lateral scintigraphic images can help determine whether a lesion in the palmar aspect of the fetlock involves the base of a PSB or the palmar surface of the McIII. A minimum set of radiographs should include dorsopalmar, lateromedial, flexed lateromedial, and both oblique images. The size of the region and absence of overlying soft tissues results in excellently detailed radiographs, even with portable x-ray machines, if suitable screen–film combinations are used or computed or digital radiography is available. A number of specific radiographic images should be obtained if certain lesions are suspected. For example, if a subchondral injury involving the distal palmar aspect of the McIII is suspected, the oblique images should be taken in a more proximal to distal direction than usual. If a lateral condylar fracture is identified, a partially flexed dorsopalmar image should be used to help identify comminution along the distal palmar aspect of the fracture line.4,5 Osteochondrosis of the sagittal ridge of the McIII and incomplete sagittal fractures of the proximal phalanx are lesions that can be difficult to identify in overexposed lateromedial images or underexposed dorsopalmar images, but digital radiography has largely eliminated the need to obtain numerous exposures with different techniques. Reference to the location of soft tissue attachments in the fetlock region is necessary for accurate interpretation.6
Nuclear scintigraphy has proven to be an exceptionally valuable tool to evaluate the fetlocks of active racehorses because so many injuries involve those joints, especially the distal metacarpal/metatarsal condyles. Flexed lateral images and dorsopalmar/plantarodorsal images allow excellent anatomic discrimination of which specific structures in the fetlock region have increased radiopharmaceutical uptake (IRU).7 In particular, flexed lateral and dorsal scintigraphic images can be very helpful to separate palmar metacarpal and PSB uptake. Scintigraphy is also useful for detection of subchondral and trabecular bone injuries in sports horses.8,9
Ultrasonographic evaluation of the fetlock can be valuable for evaluation of tendon and ligament injuries around the fetlock, especially the digital flexor tendons, the palmar annular ligament (see Chapter 74), and the dorsal synovial pad/plica, as well as the bone margins.10,11 Ultrasonography (96%) was more accurate than radiology (44%) in predicting the number and location of osseous fragments on the dorsal aspect of the metacarpophalangeal and metatarsophalangeal joints identified using arthroscopy.12 Three-dimensional imaging modalities such as computed tomography (CT)13-15 (see Chapter 20) or magnetic resonance imaging (MRI) (see Chapter 21) can also be very valuable in the fetlock, especially in areas like the distal palmar McIII condyle, the axial aspect of the PSBs, and the periarticular soft tissues.16-19
Conditions specific to the metatarsophalangeal joint and a detailed description of sagittal fractures of the proximal phalanx are discussed elsewhere (see Chapters 35 and 42). Injuries of the digital flexor tendon sheath, superficial and deep digital flexor tendons, the palmar annular ligament, and the proximal digital annular ligament are discussed in Chapter 74. Suspensory ligament (SL) branch injuries are discussed in Chapter 72. Injuries of the distal sesamoidean ligaments are discussed in Chapter 82. Traumatic disruption of the suspensory apparatus is discussed in Chapter 104.
Virtually every young racehorse has one or more episodes of metacarpophalangeal capsulitis or synovitis that is characterized by heat, effusion, and pain with flexion. Overt lameness usually is mild and often not evident after the horse warms up. Signs typically manifest as the horse increases the speed and distance of its exercise regimen. If the typical localizing signs are missed, the major indication may be a decrease in performance. In horses with chronic disease, visible thickening of the periarticular tissues usually is noticeable, with decreased range of motion of the joint. In mature sports horses, capsulitis and/or synovitis is less common as a primary condition but is more likely to be associated with sudden-onset moderate to severe lameness.
A diagnosis of primary capsulitis or synovitis usually is made by clinical observations and the absence of radiological or scintigraphic bony abnormalities. Synovitis and capsulitis may also accompany other causes of fetlock joint lameness, such as chip fractures. Synovial fluid analysis has not been particularly useful in diagnosis or prognosis for most fetlock joint injuries except to help identify infection.
In most young horses the condition resolves with nonsteroidal antiinflammatory drugs (NSAIDs), adjustments in training, and increased fitness. Intraarticular hyaluronan (20 mg) is helpful in horses with acute, mild to moderate synovitis, but a combination of hyaluronan with a low dose of a corticosteroid (e.g., 3 to 5 mg of triamcinolone acetonide or 20 to 30 mg of methylprednisolone acetate) is more consistently effective in resolving the clinical signs of inflammation. Injection should be followed by a decrease in exercise intensity for at least 1 to 2 weeks. Postexercise icing of the involved fetlock joints can be helpful. Generic OA treatments, including oral glucosamine with or without chondroitin sulfate, intramuscular polysulfated glycosaminoglycans (PSGAGs), and intravenous hyaluronan merit consideration, but they do not usually yield the consistent response seen with intraarticular therapy. Topical application of 1% diclofenac sodium (Surpass, IDEXX Pharmaceuticals, Inc., Greensboro, North Carolina, United States) appears to reduce inflammation.
Chronic capsulitis/synovitis is caused by repetitive injury of the dorsal aspect of the fetlock joint. The lesion is defined by a thickening of the normally dorsally located bilobed synovial pad that hangs down on either side of the sagittal ridge of the McIII. With extreme extension of the fetlock joint, the dorsal rim of the proximal phalanx impinges on the synovial pad, and repetitive trauma results in its inflammation and subsequent fibrosis. The tissue can become so thick that the dorsal profile of the joint is visibly disfigured. The characteristic swelling is asymmetrical on the midproximodorsal aspect of the fetlock (Figure 36-1) rather than simply spherical in outline as in a typical osteoarthritic fetlock joint (this is called an osselet). Exercise inflames the tissue further, and clinical signs of lameness or diminished performance can result.
The diagnosis of proliferative synovitis is based on physical examination, radiography, ultrasonography, or a combination of these modalities. The most common radiological sign of the lesion is a crescent-shaped, radiolucent “cut-out” on the dorsal aspect of the McIII at the level of the joint capsule attachment (Figure 36-2). The proliferative lesion may undergo dystrophic mineralization and be radiologically visible. Radiographic contrast studies can be used, but ultrasonography is simpler and more reliable (Figure 36-3). Normal thickness on ultrasonographic examination has been described as less than 2 mm,10,11 but mere identification of a slightly thicker than average structure is certainly not an indication for surgical excision. Many older racehorses have a substantially thicker synovial pad without any associated discomfort. Horses with severe OA have proliferative synovitis in the palmar pouch, and a large concave outline of the distal palmar aspect of the McIII is seen proximal to the PSBs (Figure 36-4 and Figure 34-17), which is associated with a poor prognosis.
Fig. 36-2 Chronic proliferative (villonodular) synovitis of the metacarpophalangeal joint with a typical erosive lesion (arrows) along the proximal dorsal margin of the sagittal ridge of the third metacarpal bone seen in a lateromedial radiographic image. The fibrotic tissue may have small fragments of bone and cartilage or dystrophic mineralization.
Fig. 36-3 Transverse (left) and longitudinal (right) ultrasonographic images of the distal dorsal metacarpal region. Ultrasonographic examination can confirm a thickened synovial pad, but the diagnosis usually is made based on physical examination and radiological findings. McIII, Third metacarpal bone; SP, synovial pad.
Fig. 36-4 Flexed lateromedial radiographic image of a Thoroughbred racehorse with severe chronic synovitis and osteoarthritis that led to the development of a large erosive lesion of the distal palmar aspect of the third metacarpal (McIII) bone. There is a large concave outline of the palmar cortex of the McIII just proximal to the proximal sesamoid bones.
Treatment for proliferative synovitis usually consists of aggressive intraarticular therapy (e.g., hyaluronan and corticosteroids), rest, and alterations in training. Many horses require frequent medication until the joint becomes stiff enough to prevent dorsal impingement. Horses appear to be able to work through this problem as they develop increasing strength and fitness. If the horse does not respond to medical treatment, surgical excision using an arthroscopic technique is recommended.20,21 Surgical debridement must be followed with continued medical treatment and careful attention to the training regimen. It is important to recognize that the thickening of the proximal dorsal synovial pad may be just one part of a chronic, osteoarthritic joint. In such horses, any therapy such as surgery directed solely at this lesion probably will fail.
Well-rounded osteochondral fragments in the synovial pad on the dorsoproximal aspect of the fetlock have been described in Warmblood horses in both forelimbs and hindlimbs, either as an incidental finding in presale radiographs obtained at 2 and 3 years of age (n = 102) or in association with fetlock joint pain in mature horses (n = 2).22 Radiological differentiation between these fragments and those that are a manifestation of osteochondrosis may not be possible. However, ultrasonographic examination has confirmed their presence within the synovial pad, and this has been verified at arthroscopic examination and by histology. Histological examination revealed that these fragments consistently comprised an osseous center covered with cartilage on one side and surrounded by fibrous tissue. In a study of 127 joints, multivariable analysis of variance showed a significant association between the presence of a fragment more than 10 mm in length and severe synovial proliferation, wear lines on the dorsal aspect of the McIII, and cartilage erosions. The long-term clinical significance of such lesions has not been determined. The etiology of these fragments has not been established.
OA of the metacarpophalangeal joint is common, especially in racehorses, endurance horses, polo ponies, event horses, and show jumpers. There is usually a progression of disease from medial to lateral, reflecting the biomechanical loading of the joint.23 The degree of lameness varies from mild to severe depending on the stage of the disease and recent work history. There is often associated synovitis and/or capsulitis, with palpable distention and/or thickening of the joint capsule. However, the absence of joint effusion does not preclude the presence of OA. There is often resentment of passive flexion of the joint and exacerbation of lameness by flexion; however, the absence of these clinical signs also does not preclude the presence of OA. The range of passive joint motion may be reduced because of increased stiffness of the soft tissues on the dorsal aspect of the joint.
Lameness is usually abolished by perineural analgesia using a “low four-point block,” but the response to intraarticular analgesia is variable, depending on the degree of subchondral bone pain. Increased intraarticular pressure may be a significant contributor to pain and lameness.24 Radiography usually reveals periarticular modeling on the dorsoproximal aspects of the proximal phalanx, medially more than laterally, with or without modeling of the distal dorsal aspects of the McIII. There may be thickening of the proximal subchondral bone plate of the proximal phalanx. Subchondral radiolucent areas are rare. With advanced OA there may be supracondylar lysis of the distal palmar aspect of the McIII and modeling of the proximal and distal articular margins of the PSBs. In some horses the only radiological change is narrowing of the joint space, reflecting advanced articular cartilage loss and a poor prognosis. It is important not to overlook this and to appreciate its clinical significance.
Intraarticular medication with corticosteroids (triamcinolone acetonide) and hyaluronan or PSGAGs is usually effective in early OA, combined with correction of any foot imbalance. Modification of the training program and selection of work surfaces may be important. Local therapy (e.g., whirlpool boots, spa therapy) can be of benefit. The value of oral nutraceuticals is difficult to quantify but may have a management role. Autologous conditioned serum (interleukin–1 receptor antagonist) is being used with increasing frequency with anecdotally favorable results. If the clinical signs are inconsistent with the radiological abnormalities or there is a poor response to treatment, further investigation by exploratory arthroscopy (in the presence of joint effusion) or MRI (in the absence of joint effusion) may be warranted.
Trauma to the fetlock, the result of hitting a jump, getting a show jump pole tangled between the forelimbs, falling, or stumbling, may result in sudden onset of lameness associated with effusion of the metacarpophalangeal joint.8,25 Lameness is abolished by intraarticular analgesia. At the time of acute onset of lameness there may be no detectable radiological abnormality, and follow-up radiographs may also have no detectable lesion. Although there may be a transient response to intraarticular medication, effusion and lameness usually persist. Exploratory arthroscopy may reveal a focal area of cartilage loss usually on the dorsal distal aspect of the condyle of the McIII, with exposure of subchondral bone, and some softening of the immediately surrounding articular cartilage or peeling off of a large piece of cartilage—a “delamination” injury. Debridement of such lesions invariably results in clinical improvement. If the lesion is located dorsal to the region of maximum weight bearing, the horse may become sound, but lesions on the major weight-bearing aspect result in a guarded prognosis for return to full athletic function.
Alternatively, follow-up radiographs may reveal a radiolucent area in the subchondral bone of the distal aspect of the McIII, seen best in a dorsal 25° proximal-palmarodistal oblique image. Such osseous cystlike lesions are usually associated with an overlying area of full-thickness cartilage loss. Like the cartilage lesions described above, lesions on the weight-bearing part of the McIII usually result in persistent lameness, but some horses have returned to full athletic function following debridement of more dorsally located lesions. However, the presence of extensive wear lines in the articular cartilage reflecting concurrent OA warrants a guarded prognosis. Trauma through an accident while jumping has also resulted in osseous fragmentation of the dorsoproximal medial or lateral aspects of the proximal phalanx. High-detail radiographs may be necessary to differentiate this fragmentation from periarticular osteophyte formation. However, the immediate recognition of such radiological abnormalities soon after an accident is diagnostic of traumatic joint injury rather than OA. Such fragmentation may be associated with a traumatically induced deep groove in the opposing articular cartilage of the distal aspect of the McIII; therefore, although arthroscopic removal of the fragments is likely to result in clinical improvement, long-term repeated intraarticular medication of the joint is likely to be required to maintain soundness. Secondary OA may ensue.
Subchondral bone injury is an extremely important cause of lameness involving the metacarpophalangeal joint, especially in racehorses.26,27 Similar lesions are being recognized with increasing frequency in elite endurance horses and also in other sports horses. There are no specific localizing clinical signs in many horses other than variable lameness and an observable diminution in performance. Signs such as heat, swelling, or response to flexion can be absent or extremely subtle, even in horses with overt lameness. Although subchondral bone injury is far more common in racehorses, it appears that single-step overload injuries that focally damage a portion of the McIII or the proximal phalanx can occur in nonracehorses. Such injuries may result in radiologically visible increased radiopacity or radiolucency. The severity of the lameness in such horses can be surprising, and infection may be a differential diagnosis.