Tendon injury of the superficial digital flexor tendon in Horses
The most common tendon injury in horses is damage to the superficial digital flexor tendon. This is the tendon closest to the skin on the back of the cannon bone. It can be palpated from the back of the knee down to the fetlock. The deep digital flexor tendon lies just under it, and under that, the suspensory ligament. In severe injuries the deeper structures may also be damaged. When the superficial digital flexor tendon, commonly referred to by veterinarians as the SDFT, is injured, it first begins to swell. This type of injury is known as tendinitis and it occurs in all sport horses.
Swelling of the Superficial Digital Flexor Tendon
Generally a bowed tendon is caused by a particularly hard strain, or from a blow to the leg, or an accidental striking by a rear hoof when the horse is tired. There is a general belief that bows usually occur from overwork in horses in training. It is thought that continual stretching with a little tearing each time
- without adequate time to heal
- will cause an eventual sudden tearing of a significant number of fibers in the tendon during an especially strenuous workout.
When the superficial digital flexor tendon, commonly referred to by veterinarians as the SDFT, is injured, it first begins to swell. This type of injury is known as tendinitis.
The use of diagnostic ultrasound has provided a method of determining just how much damage there is in that tendon. Those veterinarians who use ultrasound on a regular basis have learned that there is not much correlation between the external swelling and the real tissue tearing within the tendon. The more hopeful situation is when most of the swelling is from tissue fluid outside the tendon. Sometimes most of the swelling comes from a large blood pocket within the tendon. The important thing that ultrasound shows, however, is just how much tearing of the tendon has occurred and how large the core lesion is.
The majority of equine tendon and ligament injuries occur in the metacarpal (foreleg), metatarsal (hind leg) and pastern regions. The superficial digital flexor tendon (SDFT) and suspensory ligament (SL) are the most commonly affected structures. The SDFT is located just under the skin surface, followed by the deep digital flexor tendon (DDFT) , inferior check ligament and suspensory ligament. In the pastern region, the SDF and DDF tendons are also evaluated as well as the distal sesamoidean ligaments (straight and oblique) While this brochure focuses on these regions, the principles are similar for any musculoskeletal injury. Ultrasound can also be utilized to diagnose injuries in other areas such as the shoulder, stifle, fetlock, sacroiliac and pelvic region.
Normal tendons and ligaments demonstrate a homogeneously echogenic (evenly white) appearance on ultrasound when viewed on cross-section. Fiber pattern is evaluated by placing the transducer parallel to the tendon/ligament fibers. Normal tendons/ligaments demonstrate a long linear fiber pattern. Ultrasound machines are equipped with calculation packages so that we can measure the size of the tendon or ligament.Injured tendons and ligaments demonstrate an increased cross-sectional area (size), decreased echogenicity (a black or gray appearance) and a disrupted fiber pattern. The above images demonstrate the appearance of an acute mild, moderate and severe tear of the superficial digital flexor tendon.
Mild Tear of the Superficial Digital Flexor Tendon
Moderate Tear of the Superficial Digital Flexor Tendon
Severe Tear of the Superficial Digital Flexor Tendon
Most tendon and ligament injuries require 9-12 months for optimal healing. Key to the success of returning your horse to work is regular ultrasound exams to check the progress of healing throughout his or her rehabilitation. Injured tendons/ligaments should demonstrate an improved size, echogenicity and fiber pattern at each recheck exam, as seen in the two images below. This horse sustained a severe tear of his superficial digital flexor tendon as seen on the left image. The image on the right shows significant evidence of healing with an improved echogenicity and fiber pattern.
Severe Acute Superficial Digital Flexor Tendon Injury
5 Months Later: Marked Improvement of the Injury
However, some injuries are slower to demonstrate evidence of healing on ultrasound. This is often the case with suspensory ligament and deep digital flexor tendon injuries. The images below illustrate a horse with a persistent lesion of his suspensory ligament after four months of layup. In this situation, a horse may be allowed to progress in his rehabilitation as long as there is no evidence of new injury and the horse is not demonstrating any signs of lameness.
Severe Acute Suspensory Ligament Injury
4 Months Later: Lesion is Still Easily Visible
- Warm skin, hot, heat
- Forelimb pain, front leg
- Hindlimb pain, hind leg
- Forelimb swelling, mass in fore leg joint and/or non-joint area
- Hindlimb swelling, mass in hind leg joint and/or non-joint area
- Forelimb lameness, stiffness, limping fore leg
- Hindlimb lameness, stiffness, limping hind leg
- Generalized lameness or stiffness, limping
Early handling of a tendon injury still remains about the same as it has for years. Cold application helps to reduce the initial swelling, and should be applied as soon as the injury is discovered, if it is fresh. Within 48 hours, heat application and other means of increasing local blood circulation to the injury is helpful in reducing swelling. The application of DMSO is common, and seems to offer significant help in most cases. Rest is as important in this early stage as it always has been. Continual stall rest should be provided until a veterinarian has assessed the damage and outlined an appropriate course of treatment and care.
There are several types of surgery that may be recommended by a veterinarian to help the horse with a quicker, more complete healing. Seldom, however, is there ever a return to full function for the leg which has sustained a bow large enough to see at a distance.
These tendons glide within a sheath, with normal lubrication to allow for easy movement. Tendinitis will often cause adhesions to form between the tendon and its sheath. One of the primary concerns early on is to prevent these adhesions as much as possible. Some veterinarians like to inject drugs in the sheath or in the tendon itself. Hyaluronic acid is expensive but popular. Sometimes corticosteroids are injected, and sometimes beta aminoproprionitrile fumarate (BAPN) is used.
Superior Check Desmotomy