By Garrett Metcalf, DVM
The suspensory ligament is a very important structure to the locomotion, soundness and support of the lower limb of a horse. It is a common tissue that can become injured acutely or sustain chronic injury over time that eventually causes lameness. Horse owners are typically aware of the suspensory ligament but don’t always understand the importance of the structure and what it does biomechanically for the horse. This article will cover anatomy, biomechanical function of the suspensory, types of injuries and treatment options.
Anatomy and Biomechanics of the Suspensory Ligament
The suspensory ligament is truly named as a muscle called the interosseous muscle because it often contains muscle fibers, but it has evolved to act more like a ligament than a muscle. The suspensory ligament can be divided into three sections. The proximal suspensory is at the very top or the origin where the suspensory begins by attaching to the back of the cannon bone. The body or the middle section of the suspensory is between the proximal part and the branches of the suspensory ligament. The last section of the suspensory is the branches. The suspensory at this level splits into two branches that attach to each sesamoid bone behind the ankle or fetlock joint. This section is where the suspensory ends or inserts onto the sesamoid bones.
The suspensory has its own nerve innervation that provides sensation to it by a small branch off of a larger nerve just above were the suspensory begins. This is important as you read later in part of the treatment options when dealing with injury to this structure.
The suspensory can be simply thought as a shock absorber or springs on a cars suspension system. It is made to absorb force and load that is applied to the limb with each step. It is able to do this with thousands of collagen fibers that act like a rubber band that is able to stretch and then return to its original size once again ready to absorb more load just like a shock absorber on a car. The suspensory and sesamoid bones act as a pulley/cable system to keep the fetlock from extending excessively and supporting the lower limb. These fibers when looked at with an ultrasound look like a section of rope or cable made up of many small strands of fibers to create the entire structure of the suspensory. This can be used to illustrate what the injuries look like in the horse. Imagine when a section or bundle of these fibers break it weakens the entire structure of the suspensory just like strands in a rope breaking and fraying causing the rope to not be as strong as it was before. The breaking of the fibers causes significant pain and inflammation leading to lameness in the horse.
Diagnosing Suspensory Injuries
Multiple breed of horses and disciplines can be subject to suspensory desmitis. Suspensory injuries are common among sport horses, races horses and western performance horses. Injuries to the suspensory ligament can be acute sudden injuries that leave the horse quite lame initially with sometimes notable swelling, heat and pain with palpation of the leg or chronic multiple injuries overtime that cause enlargement of the suspensory ligament. Acute injuries may only need to be diagnosed with an ultrasound examination of the suspensory ligament.
Chronic injuries usually require a lameness examination, localization of the pain with diagnostic anesthesia and then ultrasound imaging of the ligament. A common history of a chronic hind limb suspensory issues is that the horse has had repeated hock injections that were working for a period of time but the injections stop working or are only lasting for a short period of time. This is because the hind proximal suspensory is closely located near the lower hock joints and the medication decreases the inflammation and pain around the proximal suspensory for a period of time.
Read more in the June 2019 issue of Oklahoma Farm & Ranch.