When you call the veterinarian to help you pinpoint the intermittent lameness in your horse, the last two words you want to hear are “suspensory injury.”

“Like tendons, suspensory ligaments can take a very long time to heal,” said Lisa Fortier, DVM, PhD, ACVS, associate professor of large animal surgery at Cornell University (N.Y.). “When the suspensory ligament does heal, re-injury is very common. Most people, when they hear their horse has a suspensory injury, have the same gasping response because there’s no quick fix.”

According to a 1999 study performed by the University of California, Davis, Center for Equine Health, suspensory ligament injuries were the most common injuries or conditions affecting horses, second only to colic. Despite the commonality of the diagnosis, researchers and veterinarians haven’t been able to speed up or perfect the healing process.

But a better understanding of the causes behind the injury and evolving stem cell research is giving what mayhave once been a career-ending injury a chance for a full recovery.

Primary Function Of The Suspensory Ligament

The suspensory ligament consists of a strong band of fibrous tissue that begins at the top and back of the cannon bone between the splint bones. Approximately two thirds of the way down the cannon bone, the suspensory ligament splits into two branches (medial and lateral), each of which inserts into one

of the paired proximal sesamoid bones at the back of the fetlock. Smaller extensor branches run to the front of the limb to join the major extensor tendon of the limb.

The suspensory ligament’s function is crucial to a horse’s movement since it prevents excessive extension of the fetlock joint during the weight bearing or stance phase of the stride.

Unnecessary stress can occur to the suspensory ligament when a horse travels at fast speeds, lands wrong after a jump or applies too much force to the area. Typically, the damage is cumulative over a period of time and may be considered a repetitive strain injury.

The suspensory ligament is much like the structure of cable on a bridge. Small bundles of wire are grouped together in small units before being combined with others to form larger and larger parallel bundle groups. The suspensory ligament is composed in a similar fashion but instead of bundles of wire, it’s bundles of fibers that are aligned longitudinally. It’s the parallel alignment of the fibers that allows for maximum strength and elasticity.

Injury to the suspensory ligament occurs when the load placed on it exceeds the collective strength of the entire fiber bundle groups, causing tearing, fraying or rupturing of the individual fibers or fiber bundle groups.

“The suspensory supports most of the back of the fetlock so it’s more prone to injury because it holds so much strain,” explained Fortier. “Injuries to the suspensory branches seem to take longer to heal and can often be career-ending because they are smaller in diameter than the main body of the suspensory ligament, meaning that there are fewer fibers holding up the back of the fetlock.”

Identifying A Suspensory Injury

“It’s extremely easy to misdiagnose a suspensory injury,” said Tim Ober, DVM, the show jumping veterinarian for the U.S. Equestrian Team. “It’s easy to over-diagnose, and it’s easy to miss one because generally we think of suspensory injuries as things that get worse with exercise. So, if you take a horse on a brief exam, you may miss entirely even seeing the lameness because you don’t ask the horse to do enough to stress the suspensory and make him lame.”

The clinical symptoms of a suspensory injury vary considerably, depending on the site of the injury and the severity of the damage. Overloading the suspensory ligament can cause fiber damage in one of three places: at the origin, in the body or in one or both of the branches.

Acute or more recent injuries tend to be characterized by heat, swelling and pain on palpation of the affected area. In acute injuries lameness may range from mild to severe, sometimes lasting only a few days and appearing somewhat transient.

With proximal suspensory desmitis, the origin of the suspensory ligament is damaged. There is normally no heat or swelling, although some pain just below the back of the knee or hock when the leg is in a flexed position may be present. The lameness may be subtle, developing gradually and often undiagnosed for a substantial period of time.

Suspensory body desmitis (injury to the body of the suspensory) is arguably the most common type of injury to the suspensory ligament. Many horses are able to function in spite of the injury during its early stages, often causing the injury to become chronic. As the ligament thickens, it can put pressure on the splint bones, resulting in splint bone fractures.

With injury to the suspensory branches there is usually a thickening of the affected region. A varying degree of lameness is present, but heat and tenderness may or may not be evident. If the ligament is ruptured (this rarely occurs), then the fetlock will sink either fully or partially toward the ground, depending on the comprehensiveness of the rupture.

 Another important component of the suspensory appara-tus includes the distal sesamoidean ligaments in the pastern region. The suspensory branches are proximal to the sesamoid bones and the distal sesamoidean ligaments are distal (or below). The distal sesamoidean ligaments lay deep within the soft tissue of the pastern, thus prohibiting easy visualization of inflam-mation or swelling. Once injured these ligaments can become problematic or chronically affected. Additionally, the bone insertion on either the sesamoid bones or the pastern (first phalanx) can be affected as well.

“It’s common for an owner to call and say that his horse is lame and then rest it for two to three days before the vet gets there to see it. During that time the suspensory has quieted down enough that if you don’t take the horse out of the stall and make him work enough to demonstrate that lameness and look for the trend of getting worse, then you can easily miss a suspensory injury that way,” said Ober.

Ultrasound machines are used to identify disrupted fiber patterns.

“The quality of ultrasound machines has improved tremendously,” said Ober. “We can see more subtle variations in suspensory ligaments and have learned as a result that suspensories have a high concentration of fibers within the ligaments that are easily confused with injury because of the disruption of the fiber patterns.

“Suspensory injuries are often misdiagnosed because there are a lot of subtle abnormalities on totally normal horses. So, the vet needs to have done all the work, blocking carefully, and know exactly where the horse hurts before they jump to a diagnosis on an ultrasound,” Ober cautioned.

Underlying Issues

“I think we’re seeing fewer origin injuries on average and maybe a few more branch injuries in recent years,” commented Ober. “I think that may have to do with changes in the footing. Synthetic or synthetic-sand mixes have more consistency, and I feel we see fewer soft tissue injuries in general due to the footing. However, we are seeing more front end jamming in general because the horse’s foot doesn’t slide as much on the synthetic footing when it lands.”

And while suspensory injuries are a direct result of excessive strain on the ligament during strenuous exercise, underlying front foot pain can cause the horse to stress the suspensory ligament in an effort to protect himself from pain.

“Many horses that have begun to experience relatively acute foot pain will palpate with a lot of reactive activity in their

suspensory ligaments,” said Ober. “These are horses that are recently lame. If we were to ultrasound them, they would ultrasound normal. Block their feet and take their lameness away, and then treat their feet and their suspensories quiet down.

“That is the pattern over and over again. So, with that in mind, when you see a horse that is acutely lame with an obvious suspensory injury that doesn’t block until you get to the suspensory ligament you better take an extra look at the feet because at a given moment we can only see what’s at the top of the plate. We can’t see clearly sometimes what’s underneath the surface. The follow-up care for that horse needs the flag for careful assessment of the feet. Make sure that the feet are comfortable, and the suspensory heals properly,” said Ober.

When the suspensory damage is in the hind limbs, Ober has observed that by treating lower back pain, he’s able to alleviate some of the pressure on the hind limbs, thus improving the healing process.

“I have had so many horses that stalled in their recovery because of suspensory origin injuries, and that stalling was improved by treating their lower backs, which is something that I have learned by observation,” Ober said.

“Can I explain biomechanically? Only retroactively. Horses with lower back pain carry themselves further out behind, and that’s equivalent to raising the heel, which stresses the suspensory area below the hock,” he added. “That particular horse’s center of balance would not be underneath himself, causing him to stress the suspensory origin.”

The Healing Process

As is the case with many injuries, early diagnosis of a suspensory injury is immensely important because the earlier it’s detected, the better chance at confining damage to only heat and inflammation.

If suspensory injuries are left untreated for an extended period of time, a progressive thickening of the ligament occurs, which if severe, can put pressure on surrounding areas, causing more structural damage.

“The earlier the detection of a suspensory injury, the better likelihood there is for long-term recovery,” said Greg Ferraro, DVM, a veterinary professor and director of the U.C. Davis’ Center for Equine Health.

Rarely will the repaired structure be as strong as before the injury because the structural integrity cannot be duplicated. There-fore, months of stall rest and a slow return to work are essential components in rehabbing a horse with a suspensory injury.

“In cases of suspensory injuries we’re not talking in terms of days or weeks,” insisted Ferraro. “We’re talking months or longer depending on the degree of the injury.”

 Tendon and ligament healing is more difficult than the healing of other bodily tissues because whereas the body has the ability to produce new connective tissue for repair, the tissue of tendons and ligaments doesn’t reproduce the tissue into the original structure of longitudinal bundles of fiber. That being the case, the repair seldom recreates a structure that can match its original strength or function.

Instead, the body produces an abundance of connective tissue that wraps arbitrarily around the damaged area, forming a dense scar in an attempt to “glue” the damaged fibers together.

“Suspensory ligaments heal with scar tissue that’s less elastic than the ligament itself, so the healed tissue is much stiffer than the surrounding normal suspensory ligament. When a horse returns to work, the junction of the stiff scar tissue and the healthy normal tissue is usually where re-injury occurs,” Fortier explained.

Multiple products and techniques, including shock wave, therapeutic ultrasound, hyaluronic acid, bone marrow, electromagnetic stimulation, magnetic therapy, platelet rich plasma and harvested stem cells from bone marrow or fat, all purportedly speed up or improve the healing of tendon or ligament injuries. So far no long-term studies are available to prove these claims.

For most veterinarians, stem cell therapy is the gold standard of tendon and ligament repair. Several veterinary universities, including U.C. Davis, Cornell, Colorado State, Guelph (Ont.), and Prince Edward Island, are currently studying mesenchymal stem cells (adult stem cells found in bone marrow, blood, brain, and fat that can be induced to form nerves, cartilage, bone, tendon and ligaments) in hopes of improving healing and recovery time. There needs to be more research done in regards to the origin of the stem cells (i.e., bone marrow, fat, embryonic) and the results of healing.

Stem cells are known to produce more than 30 different types of growth factors and tissue chemicals that stimulate healing. In addition, stem cells help recruit other local and systemic stem cells to help focus on repairing damaged tissue, as well as promote or suppress T-cell function.

“We know that a horse can heal faster with stem cell therapy,” said Ferraro, who believes that the therapy offers the most hope for tendon and ligament repair science has ever come across. “But as of right now the healing that’s taking place is still with scar tissue, not tendon and ligament tissue. We hope that in a couple of years we will be able to heal the damaged areas with tendon and ligament tissue.

“Right now vets are waiting until they have identifiable lesions before injecting the stem cells into the lesions,” he added. “We want to manipulate the cells so that they can be given intravenously, which means we could intervene a lot earlier by a process called regional perfusion [see “Regional Limb Perfusion: Zeroing In On Infection, June 18, p. 50]. This would allow us to inject stem cells directly into the blood supply of the tendon or ligament. Injecting a needle into the site of an injury can create further injury to the affected area. But, if we can get the intravascularly administered cells to go to the site of the injury then we can intervene really early and stop the scar tissue healing before it really gets going.”

Injecting stem cells directly into the site of an injury as opposed to into the blood stream can actually interfere with the healing process since some of the injected cells die and must be removed during the healing process, which is why

the theory of healing with mesenchymal stem cells offers so much hope.