Proximal suspensory desmopathy in horses

Proximal suspensory desmopathy (PSD) of the hind limb is a commonly seen problem in many types of performance horses, especially those equine athletes that are required to heavily load their hind legs while performing in their respective disciplines. Dressage horses, cutting horses and jumpers are particularly at risk for this condition. In these horses, damage often occurs proximally in the tendon, and the diagnosis and treatment at this location can be frustrating for both practitioners and horse owners.

Performance horses are particularly at risk for this condition, due to the stress placed on the proximal suspensory ligament during exercise. (GETTY IMAGES/KOJI AOKI)

Many different treatment approaches have been tried—from complete rest to shock wave therapy to the use of regenerative medicine via intra-lesional injection (extracellular matrix powder, autogenous bone marrow, autogenous stem cells, platelet rich plasma and others). Recently another treatment approach, based on what is known about the anatomy and physiology of the hind limb proximal suspensory ligament (PSL), has been increasingly utilized. This procedure combines fasciotomy of the PSL and neurectomy of the deep branch of the lateral plantar nerve.

Though this procedure was first reported in 2006 and expanded upon in 2008, it has become more popular recently and is offering major relief to many affected horses. There are still some important issues with hind limb PSD diagnosis, patient selection and staging, and postsurgical rehabilitation, but it seems that fasciotomy-neurectomy can be a positive step and should be considered in the management of hind limb PSL damage.

Probable causes and pathology

Injury to the PSL occurs when the ligament is stretched beyond its normal physiological limit. Many factors contribute to this stress-induced injury, including straight, upright hind limb conformation, poor surfaces or uneven footing in the ring, fatigue and other underlying pathology.

(GETTY IMAGES/LYNN KOENIG)

It is currently thought that many cases of PSL injury are the result of accumulated, submaximal damaging events (athletic work trauma), rather than a single catastrophic incident. It is further believed that work trauma is superimposed on progressive degeneration of the ligament itself.

Sue Dyson, MA, PhD, a specialist in equine orthopedics at the Centre for Equine Studies at the Animal Health Trust in Newmarket, U.K., believes there should be a name change for this condition that better reflects both the actual ligamentous damage incurred and its probable cause.

"Proximal suspensory desmopathy, as opposed to desmitis, might be a more appropriate term in some horses in view of the degenerative histological characteristics of injured ligaments and the absence of signs of inflammation in the majority," she says.

According to Dyson, pain and the resulting lameness in these cases may originate from the suspensory ligament itself and from compression of associated nerves (i.e., plantar neuritis) and less from inflammation. It is the special anatomical architecture of the hind limb proximal suspensory ligament that is both the cause of problems in this area and the reason for success of the fasciotomy-neurectomy procedure.

The hind limb suspensory ligament is confined by the plantar aspect of the third metatarsal bone (MTIII), the second and fourth metatarsal bones (MTII and MTIV), and the deep fascia running between the plantar aspects of MTII and MTIV. Repetitive injury to this ligament results in increased tendon size due to initial intraligamental hemorrhage and edema, which is followed by removal of injured tissue and proliferation and migration of fibroblasts. When this normal repair process is not fully accomplished due to repeat stress trauma, there is resultant production of inelastic, cartilage-like tissue within the ligament. This chronically thickened, swollen ligament in its restricted "compartment" exerts significant pressure on the lateral plantar nerve, which is adjacent to this area, causing the pain and lameness that is often seen in these horses upon exercise.

Ferenc Tóth, DVM, PhD, a former researcher at the University of Tennessee's College of Veterinary Medicine, and his coworkers have summarized the clinical relevance of their observations on lameness caused by PSD, stating, "Horses lame because of PSD of the pelvic limb may remain lame after desmitis has been resolved because of compression of the deep branch of the lateral plantar nerve."

Tóth's final suggestion is that "excising a portion of this nerve may resolve lameness," foreshadowing future developments in treatment.1

Clinical signs and diagnosis

The clinical signs associated with hind limb PSL desmopathy can vary from extremely mild to moderate lameness. Horses may show classic, unilateral low-grade lameness and be unable to perform certain specific athletic movements (i.e., uneven or poorly performed canter pirouettes in the dressage ring, uneven stops or resistance and twisting when backing up in western events, uneven jumping and landing over fences and so on) or may exhibit very subtle performance problems that are only "felt" by upper-level riders as a resistance or hesitancy in specific movements. Even more subtle signs, such as altered head carriage, bolting or other behavioral changes, have been caused by PSL desmopathy.

Diagnosis of this particular injury can be difficult because of the number of structures present in the area of the PSL. Clinicians must try to rule out the distal limb and the hock joint and capsule as sources for the hind limb pain and lameness. Thermography can often confirm increased heat in the area of the PSL but cannot differentiate between ligamentous inflammation and neuritis at that location.

Proximal suspensory desmopathy cases often show no swelling or palpable tenderness. Regional anesthesia can be helpful in this diagnostic process, but closely associated structures in this area other than the PSL may also be affected so nerve blocks are not considered completely diagnostic. Ultrasound evaluation is the field diagnostic modality of choice, and abnormalities of an affected PSL include changes in size (bigger), shape (uneven), echogenicity (small- to large-core lesions) and fiber pattern (disrupted fiber development). But even ultrasound leaves something to be desired and can underrepresent the amount of damage to the PSL.

Robert Cole, DVM, DACVR, an assistant professor of radiology at Auburn University's College of Veterinary Medicine, cautions that a complete workup of a case of suspected PSL desmopathy should also include radiography. Primary bone pathology, distal intertarsal joint issues, avulsions and numerous other bony causes of lameness can mimic PSL desmopathy and should be properly investigated.

Cole also adds that magnetic resonance imaging (MRI) is rapidly becoming the gold-standard diagnostic tool for cases of PSL desmopathy, as it allows surgeons to know if the ligament is the primary issue or if nerve compression is the major cause of lameness. "Neurectomy is not a procedure done casually, and MRI really lets the surgeon be sure that it's the correct procedure for the condition," says Cole.

Treatment and outcomes

Initial cases of proximal suspensory desmopathy are treated routinely utilizing cold therapy, compression and rest. These injuries are monitored over the following eight weeks before any attempts at surgery are made.

"Case selection is extremely important for success when using a fasciotomy–neurectomy treatment approach for PSL desmopathy," explains Reid Hansen, DVM, DACVS, DACVECC, professor of equine surgery and lameness at Auburn University's College of Veterinary Medicine.

Some surgeons, such as John Peroni, DVM, MS, DACVS, an associate professor of large animal surgery at the University of Georgia's College of Veterinary Medicine, believe there are no contraindications for a fasciotomy at almost any point in the suspensory ligament injury timeline. "Relieving pressure can only help the underlying ligament heal, so performing this procedure—even in acute desmitis cases—is quite acceptable," states Peroni.

The neurectomy part of the procedure, however, is saved for a time much later in the disease process. Horses are given four to eight weeks of rest and then reevaluated. If significant healing is occurring, as evidenced by ultrasonographic changes in the thickness and character of the suspensory ligament, then the horse is given additional healing time along with appropriate slow rehabilitation.

Nathaniel White, DVM, MS, DACVS, a professor emeritus of equine surgery at the Marion DuPont Scott Equine Medical Center at Virginia-Maryland Regional College of Veterinary Medicine at Virginia Technical Institute, teaches that rest is the most common and often most effective treatment for proximal suspensory desmopathy.

"The greatest success with no relapses," says White, "includes absolute stall rest for four weeks prior to initiating controlled and gradual increase in walking concurrent with stall rest." Horses that do not respond to this initial healing phase are then candidates for fasciotomy and ligament splitting.

Excellent results are being reported for this approach to these performance injuries. White summarizes, "To date, 95 percent of horses with rear limb proximal suspensory desmitis, which did not heal with rest or rest and shock wave therapy, healed and returned to normal work after suspensory desmoplasty."

Horses that respond to time and treatment and eventually have well-healed suspensory ligaments, as shown with sequential ultrasound evaluations or preferably MRI, but that are still painful and lame are candidates for neurectomy. These athletes have strong enough ligaments to handle a return to work, and neurectomy can be beneficial in eliminating neuritis of the lateral plantar nerve and the associated lameness.

Horses that do not respond after longer periods of rest are labeled as chronic, and their thickened, fibrotic, hypoechoic suspensory ligaments are not likely to change. These horses are poorer candidates for a fasciotomy-neurectomy procedure. Once these horses undergo a neurectomy they are more likely to load the suspensory ligament, and there is no remaining pain response to keep them from excessive activity. A number of horses that have had neurectomies without well-healed ligaments have suffered catastrophic breakdowns of the hind limb suspensory apparatus following a return to exercise.

Fasciotomy and neurectomy of the proximal suspensory ligament and associated nerve is a procedure that offers hope to affected horses, but the selection of candidates must be done carefully. MRI should be utilized if at all possible to identify those horses best suited to these surgeries, and strict postoperative care and rehabilitation are required to achieve a positive outcome.

Dr. Kenneth Marcella is an equine practitioner in Canton, Ga.

Reference

1. Toth F, Schumacher J, Schramme M, et al. Compressive damage to the deep branch of the lateral plantar nerve associated with lameness caused by proximal suspensory desmitis. Vet Surg 2008;37:328-335.