The Pathogenesis of Soft Tissue Injuries in Horses
Understanding the injury cascade and how to help horses on the road to recovery
As an equine veterinarian who practices a fair bit of performance horse medicine, I often hear from clients, “Well, thank goodness it’s just a bowed tendon,” as opposed to arthritic changes. I am uneasy with that mindset and try to express to clients that soft tissue injuries are not ideal scenarios by any means.
There is no such thing as just a strained sesamoidean ligament, just a torn intercarpal ligament, just a ruptured collateral ligament. Rather, these injuries can be very serious, and the prognosis for return to function is far from guaranteed.
The Injury Cascade
Soft tissue injuries and healing occur in three phases, as described below.
The inflammatory phase
Whether the horse has a tendon strain, ligament rupture, or muscle belly hematoma, the inflammatory phase of injury occurs immediately and is characterized by redness, heat, pain, and swelling.
“At the cellular level this corresponds to constriction of blood vessels for immediate hemostasis (arrest of bleeding) and formation of a blood clot, which serves as a preliminary scaffold for invading white blood cells that act to minimize infection and digest necrotic (dying) tissue by phagocytosis,” says Kelly Giunta, VMD, Dipl. ACVSMR, an associate at Blue Ridge Equine, in Earlysville, Virginia. “Release of histamine and serotonin by mast cells leads to increased blood flow to the site of injury.”
This increased blood flow allows cells, growth factors, and oxygen to the site of injury so the second phase of healing can begin.
The proliferative phase
The second stage, known as the proliferative or reparative phase, typically begins a few days after the injury occurs and continues for a couple of months.
“Cytokines produced by the white blood cells signal fibroblasts (cells that play important roles in tissue repair) to migrate to the site of injury and proliferate,” Giunta explains. “Fibroblasts synthesize matrix proteins, including collagen, elastin, and proteoglycans. The initial matrix is composed of type III collagen, which has different structural properties than the normal type I collagen found in tendons and ligaments.”
This means the healing matrix is weaker than the healthy tendon or ligament matrix and, thus, prone to reinjury.
The remodeling phase
The final remodeling phase usually begins a few months after injury and might continue for years.
“During the remodeling phase, tenocytes (tendon cells) and collagen fibers become aligned in the direction of stress,” says Giunta. “Type III collagen is replaced by type I collagen, and there is a decrease in the amount of water and proteoglycans in the matrix. Fibrous tissue gradually changes to scarlike tendon tissue. The repaired tissue never completely regains the biomechanical properties it had prior to injury, and the biochemical and ultrastructural characteristics remain abnormal even at 12 months.”
Imaging in 2022
Gone are the days of diagnosing a tendon injury on feel alone. If you want an accurate diagnosis and a proper treatment protocol, images of the injury are a necessity. Myra Barrett, DVM, Dipl. ACVR, ACVR-Equine Diagnostic Imaging, is an associate professor at the Colorado State University College of Veterinary Medicine & Biomedical Sciences and a partner at Inside Information Radiology, both in Fort Collins. As a board-certified radiologist, Barrett has made a career of imaging the equine patient. Here are the imaging options currently available for diagnosing soft tissue injuries.
Radiographs (X rays) are the mainstay for equine lameness diagnosis. They are essential tools in the practitioner’s kit, being portable; however, X rays have their limitations.
While radiographs are excellent for diagnosing fractures, osteoarthritis, and other skeletal disease, they present limitations for defining soft tissue structures. “Basically, the X ray gives us a general outline of soft tissues,” says Barrett. “They all look very gray. It’s like seeing someone’s shadow. You know they’re there, you just can’t tell any detail about them.”
Radiographs are helpful in a few select instances for soft tissue injuries. Soft tissue structures attach, or insert, at specific locations along the bone, each called an enthesis. “When there is an injury, the tendon or ligament next to that bone will be affected, called an enthesopathy,” Barrett explains. Where enthesopathies occur, the body often lays down mineral in a process called mineralization, which X rays can reveal.
Ultrasound is an everyday tool for equine practitioners, especially those who do large amounts of lameness or repro work. They are portable, lightweight, and have a plethora of uses. Plus, they’re the best modality for imaging soft tissue structures on the farm. “It’s a great first-line screening tool for soft tissue injuries,” says Barrett.
As with radiography, the convenience of ultrasound has its drawbacks. “They’re best used in areas where we can get to more easily,” she says. “Ultrasound can’t get through bone and can’t get through the hoof capsule.”
For example, veterinarians can use ultrasound to see the collateral ligament of the coffin joint. However, it only provides a look at the proximal third of the ligament. A healthy-looking proximal third collateral ligament does not rule out a deeper lesion enveloped in the hoof capsule below. Furthermore, the quality of the machine can make a difference with the image. “Think of it as a cheap flip phone camera versus the newest smartphone camera; one clearly will have better detail,” Barrett says.
Finally, proficiency at ultrasound takes practice, time, and experience. Advanced ultrasound for certain cases might be beyond some general practitioners’ scope. (Learn about ultrasonography’s role in lameness cases at TheHorse.com/185541.)
Nuclear scintigraphy (bone scan), computed tomography (CT), and magnetic resonance imaging (MRI) are necessary for some lameness cases. These advanced modalities are commonly found at referral and teaching hospitals.
Veterinarians perform bone scans on the standing horse, using trace amounts of a radioactive substance to identify changes in the skeletal metabolism. The horse receives a Technetium injection the day of the procedure. The bone scan then picks up any increased areas of bony turnover. These areas correspond to lesions such as inflammation, fractures, and other pathologies. Practitioners typically recommend bone scans when they can’t pinpoint a lameness to a specific location.
While practitioners generally use CT for distal (lower) limb lameness, some clinics with CT can image an equine head and neck.
“CT is just an X ray machine on steroids,” says Barrett. “CT is still X ray technology, just coming at you from all angles and is reconstructed in all sorts of different planes.” CT is usually faster and cheaper than MRI. It’s also generally a better modality for visualizing bone than soft tissue.
“MRI looks at protons from the water in your body,” Barrett says. “It gives you a lot more detail about bone and soft tissue. You get this really great global picture of bone and soft tissue.” MRI is becoming more available and popular for equine patients. The advent of standing MRIs allows for a distal limb image without general anesthesia. “It is the gold standard for musculoskeletal injuries,” she adds.
The Road to Recovery
Soft tissue injuries are frustrating, mostly due to the long, unpredictable healing process. “Bone has the ability to regenerate, and in most cases the healed bone obtains 100% of the biomechanical properties of uninjured bone,” says Giunta, unlike healed soft tissue injuries.
The simplest explanation why is blood flow. Bone has better blood supply—delivering those healing cells, growth factors, and oxygen mentioned earlier—than tendon, ligament, or cartilage and, therefore, can heal faster, she says.
Indeed, any injured tissue healing depends on blood flow. For example, meniscal injuries in the stifle are very slow to heal because they are virtually avascular (lack blood vessels), explains Giunta.
The horse determines much of its own success. “The sheer size and weight of a horse adds challenges to healing soft tissue injuries by increasing the stress and load on the damaged structure,” she says. “For certain horses, keeping them quiet and resting may be very difficult due to energetic and explosive behavior.”
Making a Plan
Previously, the age-old approach to soft tissue injuries was prolonged stall rest and hope. Thankfully, equine medicine has evolved. Once the veterinarian makes a diagnosis, he or she must formulate a plan that considers several factors.
“The most basic way to evaluate the injury is to take the approach of signalment. This includes breed, age, sex, and size of the horse, and then I can usually alter therapy based on these findings,” says Billy Hodge, DVM, Dipl. ISELP, based at Meadow Lane Equine Clinic, in Surrey, British Columbia, Canada. “This would seem vague but coupled with the information of the injury can make a huge difference with therapy. Of course, things such as the injury specifics, where the injury is, how lame the horse is or isn’t, as well as duration of the injury also matter.
Also consider the horse’s temperament and willingness to tolerate rehabilitation.
Finances must be part of the conversation. Many of the treatment protocols available offer great success, but they are not cheap. Biologic products, for example, are expensive. While stem cell harvest and injection might be gold standard for a particular injury, the treatment could simply be cost-prohibitive for some owners.
Finally, determine your end goal as the owner. Like any injury, return to athletic soundness is far from guaranteed—doubly so for soft tissue pathologies. I strive to build a protocol that fits the owner’s budget and goals, while being realistic about the injury’s prognosis.
A variety of options are available to treat soft tissue injuries—almost too many to list. We will highlight the mainstays of soft tissue therapeutic intervention.
Corticosteroid injections have been the hallmark of performance horse medicine since its inception. While they offer excellent anti-inflammatory powers and work wonderfully to treat synovitis (joint lining inflammation), they offer relatively few direct benefits for soft tissue injuries compared to other options.
Biologics are gaining momentum in the equine community. Since I graduated veterinary school, more and more options for biologic devices are coming available. No longer is platelet-rich plasma (PRP) a novel therapy!
Biologics are therapies derived from the patients themselves. The practitioners process the horse’s blood in a specific way to isolate the animal’s healing properties and use them for injection. Interleukin-1 receptor antagonist protein, PRP, autologous protein solution, alpha-2-macroglobulin, and stem cells all have different mechanisms of action for treating orthopedic injuries. Most often, veterinarians inject them into a joint or lesion directly. Many are portable and readily available to even ambulatory practitioners.
Shock wave and laser
These two noninvasive treatment options work in different ways to help increase blood flow to the lesions, which can allow for a faster recovery time. Extracorporeal shock wave therapy (ESWT) involves applying a series of low-energy acoustic waves directly over the injury. They work by initiating microtrauma at the application site, which results in a healing cascade. Light amplification by stimulated emissions of radiation, or laser, emits light photons at the injury site. They penetrate deeply to work at a cellular level, reducing inflammation by increasing blood circulation and delivery of healing mediators to that site.
In some cases surgical intervention is the gold standard. Veterinarians can use arthroscopy to thoroughly examine and treat cartilaginous or meniscal injuries. For example, a meniscal injury diagnosis on ultrasound often shows only part of the picture. A stifle arthroscopy allows a complete view of the joint and meniscus, while permitting the surgeons to remove any meniscal lesions.
The age-old question: To confine a healing horse or not? Often, this decision is not black and white. When dealing with a lameness case, the first thing that crosses my mind is how the horse will deal with prolonged rest. To be separated from other horses is difficult. If stall rest poses a problem, I often suggest keeping the horse in with a buddy. If that doesn’t work, I lean on safe sedatives to keep the horse calm during recovery.
The equine community appears to be getting away from strict stall rest. “I prefer to never stall rest and to allow stall and small turnout with walking, unless the horse is severely lame at the walk,” says Hodge. “I use movement as the basis for horses to get moving and get better as soon as possible.
“Each horse will heal differently,” he continues. “The maturation of the healing to be able to load the injury is never easy. This takes slow increased loading, and then the tissue gets stronger and more resistant to reinjury. Manipulating the injury to load more and then not reinjure but remodel to be stronger—that is the key.”
Soft tissue injuries can occur in any horse, whether it’s a competitive hunter, a racehorse, or an old pony on pasture. Diagnosis and management are key, as is setting your expectations early. “Anything can heal, but can it heal and return to its previous function?” says Hodge. “That is the end goal, and in horses that is dependent on factors like age, sex, breed, injury timeline, and the financial requirement to fix the horse.”
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