One of the most common lower leg injury active, athletic horses experience is commonly known as a bowed tendon. The two major tendons located behind the cannon bone between the knee and the fetlock are the superficial digital flexor tendon and the deep digital flexor tendon. These tendons connect the large muscles above the knee or hock to the lower parts of the leg allowing flexion. When a horse is performing at speed, these structures come under intense and severe pressure.
The superficial digital flexor is the most injured structure of the two tendons. It is a flattened strip of white tissue made up primarily of a protein called collagen. When the tendon is injured, these collagen fibers are torn resulting in pain and its associated lameness, heat and swelling. The superficial digital flexor tendon is most often injured in the middle third of its length. This is termed “a middle bow”. There are also low bows near the fetlock and high bows near the back of the knee. The name “bow” springs from the bowed appearance of the back of the leg when viewed from the side. The middle third of the superficial digital flexor tendon (SDFT) is narrower than the top or the bottom and receives less blood supply making that area more susceptible to injury.
Overextension of the fetlock joint experienced during galloping, rapid turns, or stopping causes excessive strain on the wide flattened area of the SDFT. This strain is transferred up the tendon to its weaker middle part. Predisposition to the injury may also be brought about by poor conformation, improper conditioning, poor footing, inappropriate trimming, or direct trauma. It is also possible for improper bandage application to cause stress pressure and injury to the tendon.
In the past the diagnosis of a bowed tendon was made by physical exam. Other technologies including thermography (a way to image heat in the affected part) were also commonly used as diagnostic tools. With the introduction of ultrasound imaging in equine medicine, the diagnosis of the injury and the evaluation of its severity became greatly improved. The ultrasound image more accurately measures the percentage of the tendon affected and the amount fluid or edema associated with the injury. Ultrasound remains the diagnostic technique of choice by veterinarians due to the technical difficulties and cost of Magnetic Resonance Imaging (MRI).
Basic treatment of superficial digital flexor tendonitis, which is the medical term for a bowed tendon, is relatively simple and like that of a person with a sprained ankle. First, non-steroidal anti-inflammatory drugs (Banamine, Phenylbutazone, Ketoprofen and Fibrocoxib or Equioxx) are given either by injection or orally. Cold hydrotherapy, rest, and bandage support are essential.
Tendon injury is one area where new technologies have significantly reduced tendon healing time and scarring. Therapeutic laser treatment is available at reasonable cost. Laser therapy increases blood flow, helps bring in new materials to aid in healing, and helps eliminate molecules which would slow healing. A second therapy is the use platelet rich plasma, a component of the horse’s own blood. This product can be processed for injection by the veterinarian and injected back into the lesion. The components of this injection call on the horse’s own healing process to speed up the formation of new tissue. Finally, stem cell technology is available to help heal injured tendons. A tissue sample is taken from either the horse’s bone marrow or fat. Cells from these tissues are grown and allowed to develop into new cells which when injected into the tendon will greatly reduce healing time and decrease the likelihood of re-injury.
It is interesting that the horse is one of the first species to benefit from this technology, and modern veterinary research is leading the way to such new therapies. If you are an active rider, you will eventually encounter a horse with a tendon injury. Today with rapid intervention, modern diagnostics, and therapies the prognosis for a return to active life is better than ever.
Provided by Lee Delaney, D.V.M.
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