Courtesy of UC Davis Veterinary Medicine
Tendons and ligaments are composed of fiber-like connective tissue elements that are carefully aligned in longitudinal bundles that run in the direction of force or pull on the entire structure. These bundles of fibers are grouped together, beginning in small units, then combined with others to form larger and larger parallel fiber bundle groups—much like the structure of a cable on a bridge.
The alignment of fibers in the long axis of this “biological cable” is integral to the tendon or ligament’s ability to stretch under load while maintaining its strength and integrity. The parallel alignment of the fibers allows for maximum strength and longitudinal elasticity with minimal total cross-sectional area (size).
The tendon or ligament becomes injured when the load placed on it exceeds the combined strength of the entire fiber bundle groups (i.e., cable strength). The injury is similar to stretching a piece of elastic too far so that it does not return to its original size and cannot sustain the load it could before being overstretched.
For the equine suspensory ligament, this most often occurs through overextension of the fetlock during the maximal weight-bearing that occurs at the middle of the stance phase of the stride.
Damage often involves tearing or rupturing individual fibers or fiber bundle groups. The fibers fray, tear, and lose their integrity perpendicular to the long axis (the direction of pulling force) of the tendon or ligament. The degree of damage depends on the number of fibers torn.
Signs of Injury
The clinical signs of a tendon or ligament injury can be quite varied. Acute (recent) injuries are often characterized by heat, swelling and pain on palpation of the affected area.
Lameness can range from mild to severe and may be somewhat transient, sometimes lasting only a few days. Chronic injuries often result in persistent thickening of the tendon or ligament and an intermittent or persistent lameness.
Diagnosis of Tendon or Ligament Injuries
The gold standard for diagnosis of injury to the tendons and ligaments in horses is by ultrasound examination. Normal tendons and ligaments show a homogenously echogenic (evenly white) appearance on ultrasound when viewed on cross-section. Normal tendons and ligaments demonstrate a long linear fiber pattern. Injuries show up as increased cross-sectional areas (size) with decreased echogenicity (black or gray appearance) and a disrupted fiber pattern.
The ultrasound image show a severe chronic injury to the top of the suspensory ligament. The left-hand image (transverse view) shows severe enlargement (arrows) with a mottled, heterogenous appearance (in the circle). The right-hand image (longitudinal view) shows enlargement (arrows), absence of normal fiber pattern, and short, croppy fibers. This can be seen by comparing the patterns of fibers of the superficial digital flexor tendon (SDFT) and deep digital flexor tendon (DDFT) above the suspensory ligament (SL).
Healing of the Suspensory Ligament
Key to the success of returning your horse to work regardless of the medical therapy employed is regular ultrasound evaluations to check the progress of healing throughout the rehabilitation. Injured tendons and ligaments should show a progression toward a more normal appearance in size, echogenicity and fiber pattern at each recheck exam.
Some injuries are slower to demonstrate evidence of healing on ultrasound. This is often the case with suspensory ligament injuries.
Healing of tendons and ligaments is more difficult than healing of tissue in other parts of the body. While the body has the ability to produce new connective tissue for repair, with tendons and ligaments it does not organize the tissue into the original structure of longitudinal bundles of fiber. Therefore, the repair rarely recreates a structure that can match its original strength or function.
To use the bridge-cable analogy once again, while the wires of the cable are reproduced, they are not interwoven into the loose ends of the cable and thus do not usually form the integrated bundles found in healthy tendon or ligament. Rather, the body forms an abundance of connective tissue but merely wraps it haphazardly around the area of damage to form a dense scar in an attempt to glue or weld the damaged ends of the biological cable back together.
While this scar tissue response may give the appearance of re-establishing the look and feel of the normal ligament, the repaired structure will rarely be as strong as before because the structural integrity cannot be duplicated.
Consequently, the single most important factor to the recovery of athletic performance following tendon or ligament injury is to minimize the amount of damage to the structure to ensure that the fewest number of fibers within the ligament are torn. To do this, an early diagnosis of the damage is essential.
The second most important factor to recovery is to start effective anti-inflammatory therapy immediately. Injury to a horse’s suspensory ligament is quickly followed by a pronounced inflammatory response characterized by increased blood flow and swelling within the ligament. While this initial response is designed to set the stage for eventual healing, if unchecked it can result in further damage to fiber bundle units adjacent to the damaged area and create an even larger loss of structural integrity.
Finally, the healing of tendons and ligaments occurs very slowly, over a long period of time. These structures have minimal numbers of blood vessels within them by nature of their tight configuration of fiber bundles. Without a large blood flow, the tissues are not able to clean away the debris of damage and institute repair processes rapidly. As such, convalescent periods for horses with substantial suspensory ligament injury are generally measured in months rather than days or weeks.
Initially, stall rest with handwalking is required. Your horse should not have access to unrestricted exercise such as pasture or arena turnout during the first several months. The injured tendon or ligament cannot withstand sudden heavy loading
during this time and is highly susceptible to injury. Your veterinarian can recommend a controlled exercise program—complementary to the horse’s medical treatment—that allows gradual loading of the tendon/ligament in increasing amounts so that it can heal to the best of its ability.
Recheck ultrasound exams are generally performed every 60 days to assess healing and to prevent injury. Ultrasound can detect evidence of tendon or ligament damage before a new injury occurs.
Perhaps the most important factor in a horse’s full recovery from a tendon or ligament injury is patient and owner compliance. Some horses and owners tolerate confinement better than others. A rehabilitation program requires patience
and commitment. Because it can be difficult to work with a fit horse that is suddenly not able to exercise, consult your veterinarian to develop
a recovery plan that works for you and your situation. In the end, this plan will give you the best chance to have your horse return to his preinjury level of function.