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The horse’s tendons are prone to strain injuries, especially when participating in disciplines, such as racing, jumping and eventing.
Tendons are fibrous connective tissues that attach muscle to bone and allow the limbs to move. Tendons serve as a mechanical buffer for muscles, absorbing and storing energy from concussive forces. They also act like springs, swiftly releasing stored energy to power muscles and propel the leg forward. This mechanism helps to prevent wear-and-tear on the muscles. [1]
Tendons are resistant to tearing, but they are prone to injury because they lack stretching ability. Any forceful strain or awkward movement that pushes the tendon beyond its stretching limit can result in injury. [3]
Injured tendons are slow to heal because of a lack of blood supply to the area. While the body can regenerate connective tissues, they usually do not regain their original strength after an injury.
Fortunately, several treatments are available to help horses recover from a tendon injury, including non-invasive, newly developed treatments and surgical approaches. If your horse is affected by a tendon injury, consult your veterinarian to determine an individualized treatment plan.
Tendons of the Horse’s Legs
Tendons are critical for the horse’s ability to walk, run, jump, and perform various athletic activities. They play an important role in the horse’s limbs, transmitting forces generated by muscles to produce movement and provide stability to the skeletal system.
The tendons in a horse’s leg include the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), and extensor tendons.
The superficial digital flexor tendon is located below the hock, or knee, and extends down the cannon to the pastern bone. It is responsible for flexing the digit and experiences high loading stress when the horse is in movement. This makes the SDFT susceptible to overstrain injuries. [3]
The deep digital flexor tendon runs along the back of the cannon bone in the leg, connecting the short pastern to the coffin bone in the hoof. The DDFT is very strong when compared to the SDFT and has high propulsion power. [4]
The extensor tendons run down the front of the cannon and pastern bones. They allow the horse to extend the fetlock in a forward movement. The extensor is not weight-supporting and is less likely to be injured than the flexor tendons. [5]
Signs of Injury
Most tendon injuries present with similar clinical signs, range from mild to severe depending on the extent and location of the injury.
Typical signs of a tendon injury include:
- Swelling and heat in the back of the leg
- Lameness (mild to severe)
- Bulging or distension in the back of the leg
- Sinking of the fetlock
Risk Factors for Tendon Injuries
Any horse can develop a tendon injury. However, certain management and training practices can put horses at higher risk of injury.
Some factors that can increase the risk of a tendon injury include: [3]
- Poor foot balance (low heels, long toe)
- Exercising on hard track surfaces
- Muscle fatigue
- Lameness in the opposite limb
- Introducing heavy work in unconditioned horses
- Excess weight (obesity, carrying heavy loads, etc.)
Types of Tendon Injuries
The tendons in a horse’s legs are susceptible to different types of injuries. One common form is overstrain injuries, which occur due to repetitive micro-damage that gradually weakens the tendon over time.
In horses undergoing consistent training, tendons can lose their elasticity, strength, and ability to withstand high stress. Repetitive stress placed on a tendon can lead to disruption of collagen fibers and injury. [3]
Tendons can also suffer from acute injuries resulting from traumatic events. A significant impact or forceful event such as a fall or collision with a jumping pole can result in bruising or tearing of a tendon. Moreover, sharp objects can cause penetrating wounds that may sever the tendon and lead to infection.
Superficial Digital Flexor Tendon Injuries
The SDFT is highly prone to injury and one of the leading causes of lameness in racehorses. The SDFT in horses is the functional equivalent of the Achilles tendon in humans, and injuries to this tendon can take a year or longer to heal.
Up to 80% of horses with an SDFT injury experience re-injury, primarily due to the weaker scar tissue that forms to replace the damaged tendon. Between 19-70% of performance horses with an injured SDFT are retired from sports due to their injury. [6][7][8]
Injuries to the SDFT are more common in high-impact sports, including racing, eventing, jumping and polo. The most common injury to this tendon is tendonitis, otherwise known as a bowed tendon.
SDFT strains may result in core lesions that can be seen via ultrasound and excessive swelling. In acute injuries, this lesion is often accompanied by edema (fluid retention). [9][10]
Deep Digital Flexor Tendon Injuries
Injuries to the DDFT commonly occur in the pastern region rather than around the cannon bone. They can be recognized by the presence of fluid distension and swelling in the pastern area.
Overstrain injuries to the DDFT can cause persistent, low-grade lameness in horses. Deep digital flexor tendon damage is commonly seen in dressage and show-jumping horses. [10]
Treatment for DDFT injuries can involve 4-12 months of stall rest for the horse, along with physical rehabilitation. [11]
Extensor Tendon Injuries
Injuries to the extensor tendon in horses are infrequent due to its location on the body. Unlike the weight-bearing flexor tendons, the extensor tendon has less of an impact on performance and typically heals well without invasive treatment. [9]
Injury to the extensor tendon rarely affects the horse’s gait, although stumbling during walking may occur until the leg is healed. This can be mitigated through corrective hoof shoeing or trimming.
If a horse falls or strikes its lower leg, the extensor tendon can suffer lacerations. This trauma can lead to infection in the tendon sheath. [5]
Tendon Sheath Injury (Tenosynovitis)
Tenosynovitis is a condition that involves inflammation of the tendon sheath, usually occurring in response to trauma. It is common in working and performance horses and can cause varying degrees of swelling, lameness and pain. [12]
Tendons are surrounded by a protective layer called the tendon sheath, consisting of connective tissues that contain lubricating synovial fluid. The sheath protects the tendon from abrasion and wear, while facilitating smooth movement without friction or resistance. [2]
Tenosynovitis is characterized by swelling and distension of the tendon sheath in the lower leg. This results from the buildup of synovial fluid in the affected area, known as synovial effusion.
Tenosynovitis in horses can be classified into three primary types: acute, chronic, and septic. Acute and chronic cases typically stem from injury or trauma, while septic tenosynovitis occurs when the tendon is penetrated, leading to infection. [13]
Typical clinical signs of advanced septic tenosynovitis in horses include the following:
- Reluctance to bear weight on the affected limb
- Swollen tissues around the wound
- Elevated temperature or fever
- Depression
Septic tenosynovitis can result in significant swelling and lameness in the lower leg, requiring immediate medical intervention to address the infection. Early identification and treatment, ideally within 24 hours of symptom onset, significantly improve the prognosis for the horse. [14]
Diagnosis
Tendon injuries caused by strain are highly prevalent in equine sports, making this a relatively straightforward condition for your veterinarian to diagnose.
Typically, diagnosis relies on the horse’s training and health records, alongside identifying inflammation in the affected area. [9]
Physical Examination
Your veterinarian will perform a physical examination to assess the damage to the horse’s tendon. Horses with tendon injuries may not be lame, because lameness is more closely related to the degree of inflammation rather than the severity of the injury. As a result, lameness exams may not provide useful diagnostic information. [9]
Before palpating the injured area, your veterinarian will look for visual signs of swelling in the affected area. Mild tendon injuries with limited swelling may only be visible when the horse is clipped.
Palpating the tendon between the thumb and forefinger allows your veterinarian to feel for any subtle thickening indicative of an overstrain injury. Your veterinarian will assess the leg for abnormalities both while it is bearing weight and when it is not, to identify any potential issues.
Diagnostic Ultrasound
A common technique for diagnosing tendon injuries is ultrasonography. This cost-effective and non-invasive imaging technique employs high-frequency soun