by Julie Cary, DVM, MS
Diplomate, American College of Veterinary Surgeons
Associate, Cave Creek Equine Surgical Center
One form of lameness associated with suspensory ligaments is Degenerative Suspensory Ligament Disease (DSLD) and is a point of frustration within the veterinary community, both because of lack of great treatment options and because very little is understood today about what triggers the process, or how to avoid or slow the degeneration. Interestingly, some horses degenerate with very little associated pain, while others will experience such pain that euthanasia will be considered as a reasonable option.
The suspensory ligament runs along the back of the cannon bone attaching the top of the cannon bone to the sesamoid bones at the fetlock joint. Along with the distal sesamoidean ligaments, the suspensory ligament forms the sling that supports the fetlock. The name of the ligament reflects the “suspension” of the fetlock. The anatomically correct name for the suspensory ligament is the third interosseous muscle, which reflects the presence of muscle fibers within the ligament.
There are several types of injuries that can happen to the suspensory ligament. It is like any other tendon or ligament, which can get stretched beyond its capacity, causing tearing of the fibers, swelling within the structure, resulting in the classic “bow”. During times of overextension of the fetlock, small pieces of bone at the attachments of the suspensory ligament may be torn loose of the bone. These are known as avulsion fractures. There is also a class disease that results in the breakdown of the suspensory ligament without a traumatic or overuse origin including a condition known as degenerative suspensory ligament disease.
Degenerative suspensory ligament disease (DSLD) refers to cases that have seemingly spontaneous loss of the elastic property of the ligament. This loss of elasticity results in a gradual stretching of the ligament, which is seen as an increase in extension of the fetlock (it comes closer to the ground). The resulting conformation is a straight hock and fetlock that is at a more severe angle with times where the pastern bones are almost parallel with the ground. It is unknown if the conformation of the horse originally (ie, straight hocked) contributes to development of breakdown to the hindlimb suspensory. It is almost always seen in both hindlimbs, although one hindlimb may progress faster than the other.
There is some indication that this disease may follow family lines in certain breeds, such as Peruvian Pasos. Also, pregnant mares in the last part of their pregnancy seem to have an increased chance of having the painful form of this degeneration. It is suspected that this may be due to hormonal levels. These horses are often hospitalized and relatively heroic measures are taken in some cases to get the mare to term in order to save the foal.
Diagnosis of DSLD can be challenging when the process is in the initial stages. Horses are often positive to both upper and lower limb flexion with fairly exaggerated lameness occurring. Clinical judgment combined with the results of the flexions, diagnostic nerve blocks and imaging such as radiographs and ultrasound generally lead the veterinarian to the diagnosis of degenerative suspensory ligament disease. Blocking the lameness can be frustrating, as it often requires a couple of different blocks to relieve the lameness entirely. Radiographs can reveal displacement of the sesamoid bones. Ultrasound can be an indicator of the amount of damage to the ligament. Ultrasound of the suspensory ligament of the hindlimbs can be challenging in some cases because the splint bones can interfere with the image. Use of the MRI in the future will help further characterize the damage to the ligament.
Treatment considerations include shoeing changes to decrease pressure on the suspensory ligament. Pain control if required may include herbal remedies, phenylbutazone, acupuncture, or more potent medications such as epidural morphine, depending on the level of discomfort. It is important to avoid excess body weight in these cases, as that causes more pressure on the suspensory ligament. Currently, many practitioners are included the use of Omega fatty acids in their treatment regimes to decrease inflammatory effects. Exceedingly painful cases, such as occurs with pregnant brood mares may require more heroic efforts such as body sling support and epidural medications. The ultimate goal, at this time, is to provide comfort for the horse and to slow the progression of the degeneration. With time and further research, veterinarians may ultimately learn to more effectively avoid and treat the condition.