CHAPTER 126 Collateral Desmitis of the Coffin Joint
Collateral desmitis of the coffin joint has long been ignored as a potential cause of foot lameness in horses because of practitioners’ inability to image the collateral ligament with radiography and the absence of telling clinical signs. First ultrasonography—and later magnetic resonance imaging (MRI)—have focused attention on this important structure and its appearance in health and disease. The frequency with which collateral desmitis has been diagnosed has therefore increased dramatically in recent years. In one study it was considered to be the second most important single injury responsible for foot lameness in horses without radiographic abnormalities (behind digital deep digital flexor tendonitis), whereas in another study it was found to be the most common injury in Warmblood sport horses.
The collateral ligaments of the coffin joint originate from depressions on the distal aspect of the medial and lateral epicondyles of the middle phalanx. In the standing horse, the collateral ligaments course in a vertical direction, perpendicular to the ground, toward their insertion on small depressions on the dorsoproximomedial and dorsoproximolateral aspects of the distal phalanx. These depressions are adjacent to the lateral and medial margins of the coffin joints, and they lie immediately dorsal to the medial and lateral ungular cartilages. The function of the ligaments is to support the coffin joint during movement in the sagittal, frontal, and transverse planes. It has been determined that asymmetric foot placement, with the hoof quarters at different heights, results in rotation and sliding of the distal and middle phalanges in both the transverse and frontal planes. This type of collateral motion is encountered mainly during the stance phase of asymmetric propulsion, such as when the horse is moving in circles, and places particular stress on the collateral ligaments of the coffin joint.
Collateral ligament injury can affect the collateral ligament’s origin on the distal epicondyles of the middle phalanx or its insertion on the distal phalanx. This injury arises predominantly at the bone-ligament interface and is referred to as enthesopathy. Enthesopathy can result in an osteolytic or an osteoproliferative bone response at the interface and occasionally a combination of both. When injury mainly affects the midportion of the ligaments, the term desmitis is used. Injury may consist of mild to moderate fiber damage (sprain) or severe fiber damage, with elongation or separation of a substantial portion of the lateral ligament, resulting in joint instability that can lead to partial or complete rupture with subluxation or luxation. There are no reports detailing the exact nature of desmitis and enthesopathy of the collateral ligament, but clinical experience indicates that instability of the coffin joint as a consequence of severe ligament injury is rare. Osteoarthritis as a sequel of collateral desmitis appears to be equally uncommon.
Horses of all breeds or disciplines can be affected, but horses used for jumping appear to be at increased risk. Knowledge of an acute injury is rarely reported in the history. In one report, however, nearly 20% of horses with a diagnosis of collateral desmitis had a history of being treated for a foot abscess before referral for diagnosis.
Most horses with collateral desmitis or enthesopathy do not have localizing signs. Distension of the dorsal pouch of the coffin joint may be detected but is not a characteristic finding. A discrete, palpable swelling may occasionally be found at the level of the origin of one of the ligaments, immediately proximal to the dorsomedial or dorsolateral aspect of the coronary band (Figure 126-1). The medial collateral ligament is injured nearly twice as often as the lateral. In a small proportion of horses, both lateral and medial ligaments are affected.
Figure 126-1 Right front foot of a 6-year-old Warmblood mare with intermittent right forelimb lameness of 3 months’ duration. Notice the firm enlargement over the dorsolateral aspect of the pastern immediately proximal to the coronary band (solid arrow) caused by desmitis with enlargement of the lateral collateral ligament of the coffin joint. Open arrow indicates the normal concave profile of the skin over the medial collateral ligament.
Some investigators report that a pain response cannot be induced by passive manipulation of the digit in horses with collateral desmitis of the coffin joint. In the opinion of the second author of this chapter, however, horses with collateral desmitis are likely to react painfully to digit flexion performed with the examiner pulling on the toe of the hoof capsule while facing caudally. Horses with collateral enthesopathy are less likely to resent digit flexion. The second author further believes that a positive flexion test is a favorable prognostic sign in horses with collateral ligament injury because it reflects minimal or no involvement of the attachment sites of the ligament. Monitoring the pain response to digit flexion is also useful in deciding when a horse with collateral desmitis may return to training.
Horses with collateral desmitis or enthesopathy typically have mild to moderate foot lameness of more than 2 months’ duration, with all the hallmarks of palmar foot pain. Affected horses have been described as having navicular syndrome without radiologic abnormalities. A horse may occasionally have acute-onset, moderate to severe lameness, but this is an atypical presentation. Lameness is usually mild when the horse is trotted in a straight line, but it invariably worsens when the horse is trotting in a circle.
About 33% of all horses with collateral desmitis are lame in only one limb. Another third have obvious signs of bilateral lameness, and yet another third of horses only has evidence of mild contralateral lameness when regional anesthesia is performed on the lame or more severely lame limb.
In a study in which the first author was involved, only 24% of 21 horses with MRI evidence of collateral ligament disease had significant improvement in lameness after intra-articular anesthesia of the coffin joint. However, other practitioners who relied on ultrasonography for diagnosis of desmitis have suggested that 80% of affected horses became sound after anesthesia of the coffin joint. It is useful to know that anesthesia of the navicular bursa does not result in improvement in lameness caused by collateral desmitis. A palmar digital nerve block, however, improved lameness significantly in 72% of 21 horses with MRI evidence of collateral ligament disease. In that study, all horses with collateral ligament disease became sound after an abaxial sesamoid nerve block. Lameness will also improve significantly in most cases if a palmar digital nerve block or abaxial sesamoid nerve block is only performed uniaxially on the side of the affected ligament.
Not all horses suspected of having soft tissue injury in the foot have the benefit of undergoing MRI examination. In these instances, the response to different anesthetic techniques in the foot can help to distinguish between the two most common soft tissue injuries, collateral desmitis and digital tendonitis of the deep digital flexor tendon (Table 126-1).
Because even normal collateral ligaments can produce variable signal characteristics on MRI images, the synergistic use of different imaging modalities along with MRI considerably strengthens the final diagnosis.