A bowed tendon is an injury to a tendon on the back of a horse’s lower leg, most commonly the superficial digital flexor tendon. The name comes from the visible outward curve, or “bow,” that appears along the cannon bone area when the tendon swells after tearing. It’s one of the most common and career-threatening soft tissue injuries in performance horses, particularly Thoroughbred racehorses and sport horses that work at high speeds.
Which Tendon Is Affected
The superficial digital flexor tendon runs down the back of the horse’s leg, connecting the muscle of the forearm to the second pastern bone. Its job is to flex the pastern and fetlock joints, absorbing enormous forces every time the horse lands on a front leg at speed. Because it sits just beneath the skin, it’s both vulnerable to direct trauma and easy to feel when something goes wrong.
When this tendon is overstressed, individual collagen fibers within it begin to tear. In most cases, acute injuries don’t happen all at once. They’re preceded by accumulated microdamage to the collagen fibers, meaning the tendon has been weakening over time before the moment it visibly fails. The resulting inflammation causes the tendon to swell outward, creating the characteristic bowed shape along the back of the cannon bone.
What Causes a Bowed Tendon
The underlying cause is mechanical overload. Any situation where the tendon absorbs more force than its fibers can handle puts a horse at risk. This happens most often during galloping, jumping, or working on deep or uneven footing. Fatigue plays a major role: as muscles tire during a workout or race, they lose their ability to absorb shock, and that load transfers directly to the tendons.
Poor conformation, such as long pasterns or a tendency to hyperextend the fetlock, increases the baseline strain on the tendon. Inadequate conditioning is another factor. A horse brought back to hard work too quickly after time off hasn’t given its tendons enough time to adapt to increasing loads. Bandaging that’s too tight or applied unevenly can also contribute by restricting blood flow or creating pressure points.
Signs and Symptoms
In an acute bow, the back of the cannon bone will feel hot, swollen, and painful to the touch. The horse will often show lameness ranging from mild to severe depending on how many fibers have torn. In some cases, the swelling is dramatic and obvious within hours. In milder injuries, you may notice only a slight thickening or firmness compared to the opposite leg.
The location of the swelling tells you something about the injury. A “high bow” sits near the knee, a “middle bow” is in the center of the cannon bone, and a “low bow” is closer to the fetlock. Middle bows are the most common. Chronic cases may show a permanent thickening of the tendon even after the initial heat and pain have resolved, because the original tendon fibers are replaced with scar tissue that is bulkier and less elastic than normal tendon.
How It’s Diagnosed
A veterinarian can often suspect a bowed tendon from a physical exam alone, but ultrasound is the standard tool for confirming the diagnosis and assessing severity. On an ultrasound image, a healthy tendon shows tightly organized, parallel fibers with a uniform brightness. A damaged tendon looks different: the cross-sectional area increases, the fibers appear disorganized, and dark (hypoechoic) regions show up where the tissue has torn or filled with fluid.
Measuring the cross-sectional area of the tendon is considered the most accurate way to detect and track an injury. Veterinarians also grade the fiber alignment on a scale from 0 to 3 based on how disrupted the pattern looks. They classify lesions as either “core” injuries, where the damage is concentrated in the center of the tendon, or “diffuse” injuries that spread across a wider area. This distinction matters because it influences treatment choices and how the tendon is expected to heal. Serial ultrasound exams over weeks and months allow veterinarians to track whether new fibers are organizing properly.
Immediate Treatment
The goals in the first days after a bow are straightforward: reduce inflammation, limit swelling, and minimize the amount of scar tissue that forms. Cold therapy is the first line of treatment. Ice or cold water is applied for 20 to 30 minutes, three to four times per day, and continued until the acute inflammation subsides.
After icing, the leg is wrapped in a firm, padded bandage. Bandaging increases pressure around the tendon, which helps prevent fluid from leaking out of blood vessels into the surrounding tissue. The bandage is typically kept on for about a month, with daily removal for short periods to prevent complications from constant pressure. Anti-inflammatory medications help manage pain and reduce the inflammatory response in the early stages. The horse is placed on strict stall rest to prevent further damage.
Rehabilitation Timeline
Recovering from a bowed tendon is slow. There are no shortcuts, and rushing the process is the fastest way to guarantee reinjury. The general framework starts with complete stall rest, then progresses to short periods of hand-walking. Over the following months, the intensity of controlled exercise is gradually and systematically increased until the tendon has healed enough to handle normal work.
A typical rehabilitation program spans 9 to 12 months or longer, depending on severity. The early weeks involve only stall rest and hand-walking for 10 to 15 minutes. Walking duration increases over several weeks before any trotting is introduced. Trot work begins in small amounts, often under saddle, and is slowly extended. Canter and faster work come last, and only after ultrasound confirms that the tendon is healing with acceptable fiber organization. Throughout this process, the veterinarian performs periodic ultrasound checks to make sure the tendon is responding well before each step up in intensity.
Regenerative Therapies
In the past two decades, biologic treatments have become widely used for tendon injuries in horses. The two most common are platelet-rich plasma (PRP), which concentrates the horse’s own growth factors and injects them into the lesion, and stem cell therapy, which introduces cells that can support tissue repair.
The evidence on these treatments is nuanced. A large systematic review and meta-analysis found that neither PRP nor stem cells significantly increased the likelihood that a horse would return to its previous level of performance compared to conventional treatment alone. However, the same analysis found that stem cells, and stem cells combined with PRP, did reduce the risk of reinjury. That’s a meaningful benefit for a condition notorious for recurring. In one study tracking horses for two years after stem cell treatment, the reinjury rate was 18%, compared to 44% in horses treated with conventional methods alone.
Prognosis and Reinjury Risk
The good news is that most horses with bowed tendons can return to work. In studies following horses after treatment, roughly 80 to 85% returned to their previous level of performance within 12 to 24 months. The bad news is that scar tissue never fully replicates the strength and elasticity of the original tendon. This is why reinjury rates are significant.
With conventional treatment alone, nearly half of horses reinjure the tendon within two years of returning to work. That number drops substantially with stem cell-based therapies, down to around 14 to 18% at the two-year mark. The risk is highest in horses returning to high-intensity disciplines like racing, eventing, or barrel racing. Horses moving into lighter work after recovery fare considerably better. For owners weighing their options, the severity of the initial injury, the quality of the rehabilitation program, and the demands of the horse’s intended use all factor into the long-term outlook.