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Understanding Equine Proximal Suspensory Desmitis
Keri Camp, DVM
Suspensory ligament injuries are one of the most common and frustrating injuries that we deal with in our equine athletes. Inflammation or injury to the suspensory ligament or its branches is termed “suspensory ligament desmitis” and can occur in both the forelimbs and hind limbs. In this article, we will discuss the location and function of the suspensory ligament, how injuries occur, and the various treatment options available.
Ligaments are fibrous connective tissues that join bones to other bones. They are stabilizing structures that prevent over extension, over flexion, or over rotation. Ligaments are composed of parallel bundles of collagen and are very strong, but also non-compliant. These bundles of collagen are arranged longitudinally in the direction of the pull on the entire structure. This arrangement allows for maximal strength and longitudinal stretch with minimal cross sectional size. Doctors Ferraro, Stover, and Whitcomb at the Center for Equine Health at UC Davis equate this structure to a bridge cable in which small units are grouped to form larger and larger bundles.
In the horse, the suspensory ligament is located on the back of the cannon bone and plays an important role in the suspensory apparatus. The suspensory ligament originates near the top of the cannon bone, and fills the space between the splint bones. It lays underneath the deep digital flexor and superficial digital flexor tendons. Approximately two-thirds of the way down the cannon bone, the ligament splits into two branches, one on the inside (medial) aspect, and one on the outside (lateral) aspect. These branches each have attachments on the paired proximal sesamoid bones. From these attachments, fibers of the suspensory ligament continue to the front of the leg and join the common digital extensor tendon.
The primary purpose of both the suspensory ligament and the suspensory apparatus is to prevent over extension of the fetlock joint during weight bearing. The suspensory apparatus consists of the proximal sesamoid bones, the distal sesamoidean ligaments, and of course, the suspensory ligament and its branches. The ligamentous portions of the apparatus sustain tension with fetlock extension while the sesamoid bones sustain compression at the back of the fetlock. The overall effect of this system is to prevent the fetlock from dropping to the ground during standing or weight bearing.
Ligaments become injured when the load placed on them exceeds the strength of the fiber bundle groups. Overextension of the fetlock joint during weight bearing is the most common cause of damage to the equine suspensory ligament, resulting in tearing of either the individual fibers or bundles of fibers. The degree of injury depends on the number of fibers torn. The suspensory ligament can be injured at any point along the body or branches, however, this article will focus on injuries to the origin and body, termed proximal suspensory desmitis.
Signs of a suspensory ligament injury can be as noticeable as heat, swelling, and a sudden onset of lameness; or can be very mild such as a barely noticeable intermittent lameness in chronic cases. The variety of signs exhibited and sometimes variable blocking patterns can make diagnosis challenging. In addition, the body of the suspensory lies underneath the superficial and deep digital flexor tendons, making swelling and pain with palpation difficult to appreciate. Acute, or recent, injuries may show heat, swelling, and pain with palpation of the suspensory ligament. More chronic or long term, injuries may show transient or mild lameness or thickening of the ligament. Your veterinarian will use a thorough physical exam to palpate all soft tissue and ligamentous structures and a lameness exam, often including peripheral nerve blocks, to localize the limb and structure that is causing the lameness.
Ultrasonography is the gold standard for diagnosing tendon or ligament injuries and for monitoring healing of injuries. Injuries to the suspensory ligament often show increased diameter of the ligament; decreased echogenicity, or darker, black to gray areas; and disruptions in the fiber pattern. Your veterinarian will want to monitor your horse’s healing with serial ultrasounds during the rehabilitation period. Signs of healing may include a decrease in the diameter of the suspensory ligament; increased echogenicity, or more “whiteness”; and a return to a more normal fiber pattern. Other diagnostics your veterinarian may recommend include nuclear scintigraphy, often called a bone scan, or an MRI.
In addition to damage to the suspensory ligament fibers, horses can sustain damage to the back of the cannon bone. During the event that damages the ligament, small chip-type fractures can be sustained to the back of the cannon bone. These pieces are generally not loose or moving, as they are held in place by the suspensory ligament and other over lying structures. They will generally heal with time, and do not carry a worse prognosis for return to athletic activity than a suspensory injury without bone involvement.
Ligament healing is time consuming and often frustrating. Although the body can produce new connective tissue, new tissue is not organized in the same fashion as the original ligament, and the resulting repaired ligament is not as strong as the original. The most important factor in returning your horse to its athletic career is to minimize the amount of damage done in the first place; making a prompt diagnosis is essential. Your veterinarian will also institute anti-inflammatory therapy to help quiet down the body’s inflammatory response to the injury. Healing times for ligament injuries will often take months, as a result of the decreased blood flow to these structures.
Rehabilitation times and protocols will vary depending on the location and severity of your horse’s injury. Your veterinarian will use ultrasonographic evidence of healing to adjust and increase your horse’s exercise level. The most important factor will be following your veterinarian’s instructions, as excessive loading of the newly repaired tissue can cause further damage.
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