Duncan F. Peters
Diagnosis and Treatment of Suspensory Ligament Injuries
Suspensory ligament injury in the performance horse is a frustrating condition for both veterinarians and owners, but the development and use of better diagnostic equipment are leading to an improved understanding of this condition. Equine sport disciplines differ dramatically, and the demands on the horse make it imperative that the practitioner has a good understanding of the elements of the discipline in which a horse under his or her care is engaged. The pathophysiology of an injury may be similar among horses used in different sport disciplines, but the ability of the horse to return to competition after injury may be dependent on the stresses related to its specific discipline. For these reasons, suspensory ligament injuries of similar severity may carry different prognoses, depending on the performance discipline. Suspensory ligament desmitis or desmopathy is a common injury in the sport horse. Injury occurs in three general regions of the ligament, largely dictated by the discipline in which the horse competes: the proximal aspect or origin of the suspensory ligament, the body, and the branches.
Proximal Suspensory Desmopathy
Proximal suspensory desmitis or desmopathy (PSD) is seen commonly in both the forelimbs and hind limbs, but the prognosis is generally poorer when the latter are involved. Lameness caused by PSD in the forelimb can be of variable severity but manifests as a decreased cranial phase of the stride, which is more evident when the affected limb is on the outside of the circle. In horses with hind limb PSD, the gait is notably different: there is delayed takeoff of the limb from the stance phase, with the limb appearing to hyperextend slightly behind the body. The cranial phase of the stride is reduced because the horse does not want to fully bear weight on that limb during the stance phase of the stride. Additionally, the entire hindquarters on the affected side appear to hesitate in their cranial movement and to dip or roll slightly toward the affected side just as the foot leaves the ground. This can sometimes be misleading and misinterpreted as intermittent or partial upward fixation of the patella. With both forelimb and hind limb desmopathy, the lameness is generally worsened when the horse works in deeper or heavier arena footing. This is because of the decreased recoil energy available in the soft tissue structures of the limbs in deeper footing. With forelimb PSD, the canter and lope can be helpful gaits for diagnosis of the problem. Horses with left forelimb PSD tend to have a very different right-lead canter, compared with the left-lead canter. The stride will be shorter, less fluid, and more upright in nature as a result of the horse’s reluctance to bear weight on the left front limb during the diagonal stance phase. When circling to the right, horses with left forelimb PSD are often reluctant to maintain forward impulsion and tend to want to break to the trot. Riders describe a “pogo” effect in the affected limb. Careful palpation of the proximal part of the suspensory ligament with pressure directed dorsally against the palmar surface of the third metacarpal bone (MC3) or plantar surface of the third metatarsal bone (MT3) can usually elicit sensitivity of varying degrees. Most commonly, pain is more evident on the medial aspect of proximal MC3 or MT3. Comparable palpation of the contralateral limb, in the same region with similar pressure exerted, should always be done to determine the validity of the sensitivity. In addition, unilateral palpable enlargement of the ligament in the proximal palmar MC3 or plantar MT3 region can frequently be appreciated. Passive flexion of the lower part of the limb will generally not elicit a dramatic painful withdraw reflex response, but the lower limb dynamic flexion test will usually exacerbate the lameness. In the hind limb, the upper limb dynamic flexion test often worsens the lameness to a greater degree than the lower limb dynamic flexion test. This can be similar to flexion test reactions seen with tarsal or stifle joint issues, and care must be taken to fully isolate the source of the pain to the proximal suspensory ligament region.
Diagnostic Anesthesia
Local analgesia can be helpful in isolating the source of pain arising from PSD. Generally, local infiltration of 2% mepivacaine infiltrated around either the lateral palmar nerve (3 to 4 mL) or the medial and lateral palmar metacarpal nerves (2 to 3 mL each) will greatly (>90%) reduce the degree of lameness and dramatically improve the character of the gait in the front limb. Additionally, intraarticular anesthesia of the intercarpal joint can alleviate pain in the proximal suspensory ligament in some horses. The examiner should note that horses with PSD commonly show some graduated improvement in response to regional nerve blocks in the lower part of the limb (i.e., the proximal digital [abaxial] and low palmar [low four-point] blocks), before the lameness is essentially alleviated with the more proximal anesthesia. This is likely because nerve supply to stretch receptors within the ligament is not specifically localized to one region. Injury to other local structures also must be considered when the lameness responds to anesthesia of the proximal metacarpal region. Pain arising from damage to the second or fourth metacarpal bones, their interosseous ligaments, the proximal aspect of the third metacarpal bone, the distal aspect of the palmar joint capsule of the carpus, and the dorsal aspect of the distal check ligament is likely reduced with blocking of this area. Migration of the local anesthetic proximally also may desensitize carpal joint structures.
Anesthesia of the deep ramus of the lateral plantar nerve or tarsometatarsal joint or local infusion of anesthetic into the proximal plantar MT3 region often alleviates the pain associated with ligament injury in horses with hind limb proximal suspensory syndrome. The use of magnetic resonance imaging (MRI) and computed tomography (CT) has enabled distinguishing a variety of conditions that have previously been attributed to and grouped with PSD of the hind limb. Abnormal structural and metabolic findings associated with the intertarsal ligaments, individual tarsal bones, the proximal aspect of MT3 and MT4, the plantar ligament of the tarsus, and the proximal aspect of the suspensory ligament now provide better understanding of the causes of pain originating in this region. These findings can aid in more targeted treatment and allow a more accurate prognosis.
Imaging
Imaging of proximal suspensory ligament injuries can be challenging, especially in the hind limb. Ultrasonographic examination is generally accepted as the initial tool in identifying defects. It should be acknowledged that commonly the extent of structural deviation from normal may not correlate well with the severity of clinical lameness: some horses have minimal ultrasonographically evident changes but marked lameness. Enlargement of the ligament and poor distinction of its linear margins may be the most useful signs of abnormality. The unaffected limb should always be used for comparison. Occasionally, a focal anechoic core lesion can be appreciated and makes the diagnosis more definite. A focus of irregular bony remodeling seen in the proximal palmar aspect of MC3 or plantar MT3 is indicative of entheseopathy, which may be accompanied by bony avulsion or chronic ligament or bone attachment irritation. Sequential ultrasonographic examinations should be used to follow the progress of any abnormalities, and in the case of acute injury, the changes seen on ultrasonography may become more severe 2 to 3 weeks after the initial injury. Effectiveness of treatment and rehabilitation programs can be monitored with good clinical evaluation and regular ultrasonographic examination.
Radiology, nuclear scintigraphy, MRI, and CT all have their place in delineating different aspects of PSD. Digital radiology is useful for examination of avulsion fractures or sclerotic changes (indicating chronicity) on the palmar aspect of MC3 or plantar MT3 and any bony changes associated with the carpal or tarsal bones. Radiopharmaceutical uptake during scintigraphic examination indicates inflammatory metabolic activity of bone, soft tissue, or both. This can be a useful indicator of the site of injury and useful for determining the progress of rehabilitation. Magnetic resonance imaging may reveal subtle changes of inflammation in bone and ligament, as well as structural changes and lesions, such as adhesions and exostoses, that indicate chronicity of disease. This information aids with forming a prognosis. Use of MRI also has identified distal tarsal bone inflammation that commonly accompanies hind limb PSD. Computed tomography can be used to differentiate some of the bone–ligament interface lesions that occur with proximal suspensory desmitis or desmopathy.
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