It’s a diagnosis no horse owner wants to hear. Bowed tendons are a serious cause of horse lameness and can be career ending for equine athletes.
Although most often a condition of racehorses, it can occur in other horses as well.
A bowed tendon is an injury to the superficial digital flexor tendon, which runs along the back of the leg, directly behind the cannon bone. Swelling from these injuries creates the characteristic bump on the back of the leg.
Horses can injure many other tendons and ligaments, but they are located deeper below tissue so there is no obvious swelling or bow in those. Most bowed tendons occur in the front legs.
The dense collagen that makes up tendons connects muscles to bone. In horses, tendons are a key anatomical adaptation to high speed running. They efficiently transfer muscle energy to mechanical movement, allowing horses to run for long distances at rapid speeds while saving energy.
Among domestic animals, tendon injury seems to be mainly an issue with horses.
Cows have similar lower leg structures, but tendon injuries are rare because we don’t ask our cattle to run races, jump fences or carry out other athletic tasks.
But Achilles tendon injuries in human athletes share many similarities to horses, leading some researchers to suggest that horses are a useful model to study this injury in people.
Bowed tendons most often occur after prolonged micro injuries to the tendon that eventually overwhelm the body’s ability to heal and eventually lead to lameness.
The accumulation of these micro injuries is the reason why bowed tendons are more common in older horses. Less frequently, tendons rupture with limited previous injury.
Diagnosis is based on appearance of a bump (bow) at the back of the leg accompanied by heat, pain, lameness and swelling.
Ultrasound of the affected tendon is useful to isolate the precise location of the damage, measure the size of the defect and monitor the healing process. Most vets have portable machines they can use to scan horses right on the farm. Researchers and high-end equine clinics may use magnetic resonance imaging (MRI) but this test is expensive and not readily available.
Tendons are made of collagen, a tissue with few cells, limited blood supply and poor ability to regenerate.
These characteristics make tendon injuries notoriously difficult to treat. The mainstay of treatment is strict rest with gradual increases in controlled activity. Other supportive care including cold therapy, anti-inflammatory medications, hyaluronic acid and shock wave therapy may help healing.
New treatments, including stem cells and platelet-rich plasma, may hold promise but have little scientific evidence to show they are better than traditional treatments.
Depending on the nature of the injury, horses with bowed tendons may be pasture sound, OK for pleasure riding or even return to high performance. But horses with tendon injuries are at high risk of re-injury because the healed site is filled with scar tissue that is never as strong as the original. Return to work must be gradual to build up endurance and lost muscle.
Preventing tendon injuries is a challenge since most are the result of wear and tear. Proper conditioning and gradually increasing fitness levels is important since most tendon injuries occur when horses are tired. Most tendon injuries in racehorses occur at the end of the race.
On the other hand, time off for rest and recovery is also essential to allow for healing after exercise. Some veterinarians will even ultrasound tendons of non-lame horses to check for signs of injury.
Bowed tendons are serious, but prompt diagnosis and treatment are important to maximize the chances of a successful recovery.
Dr. Jamie Rothenburger is a veterinarian who practices pathology and a PhD student at the Ontario Veterinary College. Twitter: @JRothenburger