Tendon and Ligament Injuries
Tendon and ligament injuries in horses cause both economic and personal hardships for horse owners and industry professionals. A prolonged period of layup and rehabilitation is necessary, but whether the horse will be sound at the end of the rehabilitation period is uncertain. Lameness due to tendon and ligament injury is common in performance horses, affecting up to 25% of racehorses over their careers.
Tendons connect muscle to bone, providing elasticity, gait efficiency, and support to the lower limb alignment. Ligaments connect bone to bone, giving structural support for joints and maintaining suspension of the fetlock joint as part of the suspensory apparatus in the horse. Tendon and ligament injuries can be classified into three types: traumatic laceration or rupture; acute inflammation with swelling and pain (tendinitis); and a more subtle degenerative injury resulting from a failure to heal because of repetitive damage.
The connective tissues that make up tendons and ligaments are closely related. The highly organized structure of a tendon enables it to be both strong and elastic. The cells within a tendon produce the extracellular matrix that is organized into the fibers responsible for a tendon’s unique mechanical properties. The tendon fibers are made of the protein collagen. The collagen forms long interlaced fibers in lengthwise alignment with the tendon, but the fibers also have a pleated pattern that, like a spring, gives elasticity to the tendon.
When a tendon is injured, inflammation ruptures or degrades tendon fibers. A healed tendon rarely has the exact structure of a normal tendon. Abnormal orientation, size, and organization of the collagen fibers that replace the original structure have less strength and elasticity. This is thought to increase the risk of re-injury once the healing process is complete.
Because tendons and ligaments are primarily made up of tissue matrix, they have a relatively small number of blood vessels and cells that can produce new normal tendons. In a damaged tendon, an efficient inflammatory process degrades and removes the injured fibers and matrix so that the tendon can heal with minimal scar tissue. It is thought that poor healing in tendons results from a prolonged and inefficient inflammation process. Therefore, tendon tissue requires as many as nine to 12 months for complete healing. Even with a careful rehabilitation program, re-injury is common.
The failure to heal completely might be due to the inability of tendons to remodel or because of the repetitive forces these structures experience regularly during exercise. Then, at a critical point during exercise or overexertion, the low-grade injury can no longer hold up to normal use or, perhaps, to an overload, and an acute lesion with heat, swelling and lameness forms. This injury typically starts in the center or core of the tendon (called a core lesion), where blood and serum form a clot that replaces the tendon fibers and creates more inflammation. This inflammation results in more damage over the following days or weeks.
While veterinary ability to diagnose tendon and ligament injuries has improved dramatically in recent years and new treatment modalities are available and in use, a long layup period and the risk of recurrence are still factors to consider for recovery. Newer treatments such as injection with stem cells or platelet-rich plasma are promising, but their benefit has not been fully characterized and they might not decrease the time required for healing and remodeling.
Further research into the detection, causes, and best treatments for tendon and ligament injuries is needed.