Treating wounds of the equine distal limbs

Wounds of the lower limbs of the horse can be challenging to treat successfully, especially those that may involve the deep structures, such as tendons, ligaments and synovial (joint) spaces.

Photo 1: A wound more than eight hours old. The laceration is on the cranial and mid-portion of the radius, severing the extensor muscles, and includes the periosteum of the radius. (PHOTO: COURTESY OF DR. JAMES A. ORSINI, NEW BOLTON CENTER, UNIVERSITY OF PENNSYLVANIA)

"Rapid and accurate recognition of damage to deep structures is mandatory for appropriate case management and a favorable prognosis," says Henry W. Jann, DVM, MS, Dipl. ACVS, at Oklahoma State University's Center for Veterinary Health Sciences.

"Our knowledge of how tendons and ligaments heal is constantly expanding and controversial," Jann says. "We're not really certain of the physio-logical mechanisms that turn tendon healing on and off, but we're learning more every day."

Veterinarians know that the healing process is controlled to a large extent by growth factors. To assist healing, practitioners have some new therapies in their armamentarium, including stem-cell and shock-wave therapy.

"But, in terms of the precise biomechanical pathways and how to turn them on and off, we've still got a lot to learn," Jann says.

Photo 2: Second intention wound healing in the mid- to distal part of the front limb of a horse. Notice the healthy granulation tissue and an area at the level of the fetlock and pastern that needs surgical debridement. This wound would benefit from a skin graft. (PHOTO: COURTESY OF DR. JAMES A. ORSINI, NEW BOLTON CENTER, UNIVERSITY OF PENNSYLVANIA)

While it is known that training and exercise help strengthen bones and muscles, just how tendons respond to exercise and the best way to train horses to prevent tendon injury are not well understood. Tendons do get stronger to a certain extent in young horses, but in the mature horse it's hard to get them healed well enough to attain the physical strength required of an equine athlete. With any tendon laceration, the potential for severe hemorrhage is always present because of the proximity of major arteries to the flexor tendons and their relatively superficial location.

"Ligament healing follows essentially the same pattern as tendon healing," Jann notes, "although the intrinsic and extrinsic patterns have not been as clearly defined."

Unlike tendons, ligaments do respond positively to exercise.

Diagnosis

When examining a wound in the lower extremities that has the potential of penetrating or compromising deep-tissue structures, the horse's posture and ambulation will provide clues to potential tendon, ligament or joint damage.

Photo 3: Fresh wound in the mid-cannon bone area as a result of a wire cut. These wounds can be problematic due to secondary problems with compromised blood supply and bone injury. (PHOTO: COURTESY OF DR. JAMES A. ORSINI, NEW BOLTON CENTER, UNIVERSITY OF PENNSYLVANIA)

"When tendons are compromised, there is an alteration in limb conformation during ambulation or upon weight- bearing," Jann explains. Damage to each of the various tendons, ligaments and joints — superficial deep flexor tendon (SDFT), deep digital flexor tendon (DDFT), metatarsophylangeal or metacarpophylangeal joint (MP), fetlock joint, distal interphalangeal joint (DIP) and coffin joint — has its own specific presentation/appearance. "These changes in conformation are consistent and can be relied upon for categorization of compromise to a specific tendon or tendon group," Jann says.

Treatment

"The treatment of tendon lacerations is arguably one of the most difficult endeavors a surgeon is confronted with," Jann suggests.

The veterinary surgeon can sew the ends together, do tenorrhaphy, but returning the animal to full function is another issue.

"Just getting them to recover from the anesthesia, getting the wound to heal, getting a functional animal back, are real challenges," says Jann. "If an infection gets going, it's a challenge to resolve. It's similar to human surgery. If you cut your hand badly, when a tendon is affected it's pretty hard to regain full function."

During the repair process, tendons sometimes heal too well, producing scar tissue that is inelastic. Normal tendon tissue is elastic. It is what enables kangaroos or deer to jump or a human athlete to dunk a basketball.

It's not so much a function of strength, but rather the spring that the tendons provide. When they tear, whether lacerated or cut, they tend to fill in with scar tissue that doesn't stretch like normal tendon.

Immobilization of the horse's limb is imperative to prevent further lower-leg damage. Various methods (e.g., the Kimzey Leg Saver or a self-constructed splint of plastic PVC pipe) can be used to keep the limb from further movement and injury.

Once the limb is immobilized, the next concern is hemorrhage, common with tendon lacerations. A pressure bandage will reduce active bleeding, and systemic antibiotics are warranted due to usually excessive contamination.

"Most of the wounds that we see in horses are pretty contaminated," Jann says. "Horses usually find the most non-sterile thing in the environment (to cause) a laceration."

Wound debridement is of critical importance. For tendon injuries "it cannot be overemphasized," Jann says, adding, "It would be appropriate to say that debridement is the sine qua non for successful repair of traumatic tendon lacerations."

But proper debridement of all infected, contaminated tissue can be a problem.

In the early stage of treatment, the surgeon should focus, not only on the tendon, but on the entire wound, Jann says. The tissue that most often seems to be most contaminated is the paratenon, which must be meticulously debrided.

Even then microscopic foreign debris remains, and irrigation is essential to removing these remaining sources of infection. Large volumes (2 to 3 liters) of irrigation solution (0.1 percent providone-iodine and lactated Ringer's) are recommended, along with active suction.

Suturing tendons usually is indicated.

"The three goals of tendon repair are minimizing gap formation, minimizing adhesion formation and creating a minimal interference to the intrinsic vasculature of the tendon," Jann explains.

After the deep structures are repaired "every effort should be made to close the wound primarily," particularly the paratenon, he says. This tissue layer should be closed over the tenorrhaphy site.

"This step is important because the paratenon provides the cells from which the tendon scar is formed," says Jann. And it prevents tendon tissue from adhering to the subcutaneous tissue and skin. These upper layers are closed separately.

Primary wound healing helps prevent formation of granulation tissue and maximizes overall limb function.

Sheathed flexor zones

Tendon lacerations that occur in the sheathed zones (i.e., digital synovial sheath of the fetlock joint) are more problematic because of the potential for developing septic tenosynovitis. "This is at best a career-threatening and potentially a life-threatening sequel to any traumatic wound involving a tendon sheath," Jann says.

In horses, any tendon laceration in the palmar and/or plantar aspect of the fetlock or pastern area automatically invades the tendon sheath, because the tendons are contained within sheaths in these anatomic zones. In performance horses, these areas are the ones traumatized most often.

"One recent report lists the digital sheath as the most commonly contaminated and infected synovial structure," Jann says.

Wounds in the joint space

"Any significant traumatic wound of the distal limb that is in close proximity to a joint should be treated as if the joint were invaded until proven otherwise," Jann cautions.

It is important to determine if the wound is in a joint space, and there are techniques to do this.

The joint can be palpated, radiographed or saline may be injected into the joint to see if it comes out the wound.

"If we're not sure, I would rather be on the safe side," Jann states. Use antibiotics, clean the wound and use regional perfusion to make sure the wound is kept free of infection.

If septic arthritis sets in, it is unlikely the horse will be functional again.

"Septic arthritis is a condition that should be avoided at all costs," Jann says.

"The old adage about an ounce of prevention being worth a pound of cure applies to the correlation of wounds of the distal limb to the potential for septic arthritis in one of the diarthrodial joints, particularly the fetlock, tarsus and carpus," he says.

The severity of joint contamination and/or infection are highly influenced by rapid and aggressive treatment.

Stepwise treatment for joint-space wounds includes inspection, antimicrobial therapy and lavage.

After wound margins are thoroughly cleansed and prepped to prevent contamination of the deeper aspects, especially if joint spaces are suspected of being involved, the wound should be inspected digitally.

The easiest way to determine if the synovial space has been compromised, even minimally penetrated, is to assess its permeability.

Sterile saline injected intra-articularly with sufficient pressure to distend the joint fully, with leakage noticed in or around the wound location, will indicate joint capsule involvement.

If that is the case, the serious nature of the wound (i.e., from a minor laceration to a career- or life-threatening event) should be conveyed to the client. With the joint space compromised, it is of critical importance to treat the wound within 24 hours to ensure a good outcome — 85 percent survival, 50 percent for return to function.

Suspicion of joint involvement also makes it critical to use a broad-spectrum antibiotic immediately. Based on serial culture and sensitivity, the proper drug should be selected and then continued for three to six weeks, or for two weeks after clinical signs have been resolved.

Once joint involvement has been determined, lavage is important. This is best done under general anesthesia, via arthroscopy in a surgical setting, though it may be done in the sedated standing animal under field conditions.

Use copious fluids (i.e., providone iodine-lactated Ringer's), with the wound carefully debrided.

Prognosis

"Most of the wounds we see in horses' extremities are so infected, quite frankly if a person had a wound like that and went to a surgeon, he would probably just amputate," Jann says.

But, unlike people and dogs, who can function on less than their full complement of limbs, horses need all four legs to function.

Overall prognosis for flexor-tendon lacerations is favorable. Wounds that involve tendon sheaths can be successfully treated with a favorable prognosis.

So too is the prognosis for extensor-tendon lacerations in the non-sheathed and sheathed zones.

For joint-space wounds, aggressive therapy, initiated within hours of the injury, is essential for a favorable prognosis.

Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.