Last Updated on February 25, 2022 by Allison Price
Sesamoid injuries can be devastating and difficult to fix in horses. Here’s how to avoid it
The two tiny bones that sit at the back of the horse’s fetlock are both awe-inspiring and baffling to veterinarians. Sesamoids are the bones that anchor the suspensory system, which allows horses’ feet and fetlocks to move correctly. Sesamoid injuries can be severe due to their anatomy and location.
It is not surprising, given the location of the sesamoids, that high speeds can cause soft tissue injuries and fractures. For example, a racehorse’s fetlock may sometimes reach the ground and make contact with sesamoid bone. These bones can break if the pressure is too high.
Jeff Blea (DVM), a racetrack practitioner and former president of the American Association of Equine Practitioners, stated that horses have two proximal sesamoid bone on each limb. They, along with the long pastern and cannon bones, form the fetlock joint.
Blea explains how the sesamoids are protected by a complex system of ligaments. The suspensory ligament starts at the top of the cannon bones and runs down to the cannon bones, where it splits into two branches, one attaching to each sesamoid. Other ligaments link the sesamoids together, while the distal sesamoidean and pastern ligaments extend to the bottom of the bones. Blea says that it is a highly mechanical area from a physiological perspective. It is susceptible to increased tension and force as well as increased pressure.
Although the sesamoids might seem like an accident waiting for it to happen, Emma Adam BVetMed Dipl. ACVIM ACVS, PhD was a graduate of the University of Kentucky’s Gluck Equine Research Center. She is also a former assistant of Sir Michael Stoute, a champion racehorse trainer.
She says, “Our patella can be described as a sesamoid bony.” It is moving over the amazing structure of our knee. The sesamoids in horses provide a groove to these extremely strong flexor tendon, and mechanical support for the incredible unidirectional joint. They do both.
The size of the Sesamoid bone is about the same as a walnut and it’s somewhat pyramidal. This alone makes it difficult to fix a fracture by surgeons and the body. Adam also points out other difficulties.
She says that the Sesamoid bones are having a hard time because they lack blood supply. There is no musculature around them to lend blood supply and there is no periosteum (the soft protective tissue that covers bone).
Blood supply and periosteum are both important for bone healing. The sesamoids, therefore, are left to their own devices.
What goes wrong?
Sesamoids, like any other bone, can break if they are overstressed. Sesamoids can be injured due to the many ligaments that attach to them. The more elements there are, the worse the prognosis.
Blea states that sesamoids can fracture in one of three ways: apical (the top), mid-body (or basal) or both.
A veterinarian can usually remove an apical fracture arthroscopically, which is minimally invasive surgery that uses a fiberoptic camera. There are good prospects for a quick return to performance.
Blea says that the limiting factor in prognosis is based on whether or not there is a suspensory and how much suspensory branch attachment is involved. Your prognosis will drop if the suspensory is also damaged.
Blea says that mid-body or compound (breaks through skin) fractures often result in a poor prognosis for a horse’s return to performance. Sometimes, these horses can continue to a more rewarding second career.
Blea is the most pessimistic when it comes to basal fractures. He says that some people have succeeded with inserting screws into these fractures. “But, the problem is that you have the distal sesamoidean muscles pulling at the sesamoid. This creates even more tension.”
Fractures can also be fatal. Sesamoids can break down into too many pieces for removal or reassembling. These cases often end in euthanasia.
Blea says that horses in such a situation can be saved to breed or as companions through arthrodesis or fusing the joint. They won’t be athletically sound but can be pain-free.
Horses may also get sesamoiditis (bone inflammation). This can be caused by too much stress on the joints, but it can also happen in horses that are young and developing.
More research is required to determine if sesamoiditis correlates or not with a higher chance of future fractures. Sesamoiditis can also be caused by other factors, such as conformation, training regimes, and speed.
Both breed and age play a role. Adam states that Warmbloods are more susceptible to sesamoid injuries then Thoroughbreds. This is likely due to differences in body types and the fact that Warmbloods who are destined for eventing, jumping, dressage and dressage begin training later than racehorses.
Adams states that warmbloods aren’t subject to as many sesamoid injury. They can develop osteoarthritis-related changes. They may develop bony changes after the insertion of the suspensory and distal sesamoidean ligaments.
Adam states that a Warmblood will usually fracture a sesamoid if it happens. Adam says, “When you have a horse performing a canter-pirouette, it is easy to see the strain being put on the suspensory apparatus.” This maneuver requires a lot of effort.
Difficulties in Diagnosis
Broken bones can be caused by injuries. Fractures may not appear immediately on radiographs due to the time it takes for bone repair work to occur. These two factors can complicate the diagnosis of sesamoid fractures.
Blea says that sesamoids can fool people. Diagnostic anesthesia (blocking) may not be able to pinpoint the problem if a horse is lame. Blea says that many people mistakenly believe it is a foot problem. They’ll perform diagnostic anesthesia on the feet and the horse will be sound. They work on the foot and the horse has a sesamoid bone fracture a few weeks later.
Blea suggests that if diagnostic anesthesia narrows the search to the fetlock or a possible sesamoid injuries, but the radiograph shows nothing, Blea recommends resting the horse for 10-14 days, then radiographing the same area again.
Blea says that it is common to not diagnose problems with sesamoids until they have fractured. Before fracture, you may not notice any swelling, heat, inflammation or pain in the bone.
In such cases, the rehabilitation program typically involves keeping the horse in stalls for 30 days and hand-walking him for 60 days. Blea suggests that the horse is allowed to move on his own after the hand-walking period. To monitor healing, he takes additional radiographs for four months.
Prevention of Sesamoid Injuries to Horses
Sesamoid injuries to horses can be prevented by preventing them from happening. Blea and Adam emphasize the importance of building a solid training foundation before any horse can achieve top-performance.
Adam says that “Sesamoids may respond to training.” Training programs must take into consideration that bone, muscle, ligaments and muscles all get fit at different speeds. To avoid injury, a horse must also be healthy. Good shoeing habits and consistent, even footing are important to keep the horse’s fetlock area healthy.
Blea says, “It is important to have good medial-to-lateral (inner and outer) balance in your foot.”
Other management methods, such as providing healthy nutrition, are equally important.
The most important thing an owner or trainer can do for sesamoid injuries is to monitor them closely. Blea says that it is essential for trainers and vets to exercise diligence. Talk to the horse. Examine the legs. Talk to the rider.
The use of newer diagnostic techniques can help prevent future problems. These options include nuclear scintigraphy (which Blea says is “probably the most common method we use to diagnose sesamoid issues”, by visualizing bone remodeling), MRI or CT.
Sue Stover, DVM. PhD, Dipl. ACVS, Professor of Anatomy, Physiology, and Cell Biology at the J.D. John Peloso (DVM, MS), Dipl., are the Wheat Veterinary Orthopedic Research Laboratory and Wheat Veterinary Orthopedic Research Laboratory, Davis, California. ACVS is the owner, partner and surgeon at Equine Medical Center Ocala in Florida. Stover has found that more than half of all cases are due to catastrophic fetlock problems. This is based on the results of the California racetracks’ post-mortem program. She is currently investigating radiographic techniques to help her diagnose more accurately and sooner.
Peloso and others have discovered that standing MRI is extremely useful in diagnosing early sesamoid issues.
Peloso discovered two risk factors in his studies: an increase in the density of sesamoid bone that makes them more fragile and problems with the contralateral limb (the horse’s limb that compensates for the weaker limb).
He suggests that MRI could be used to assess bone density and early signs of injury in the contralateral limb. This could help catch any sesamoid damage prior to fractures.
Peloso cited a paper from Newmarket, England veterinarians that used standing MRI to identify cannon bone fracture pathology for 35.8% of racing Thoroughbreds. They could not confirm -radiographically – the diagnosis.
Peloso says that the clinical signs of these injuries can be subtle and hard to spot because they are located below the cartilage surface.
Take-Home Message
Sesamoid injuries can occur in horses when horses are driven at high speeds and have suspensory apparatus anatomy. Because changes can’t always be seen using radiographs and palpation, fracture diagnosis can be difficult. The best diagnostic tools for diagnosing problems are CT, MRI and nuclear scintigraphy. However, good management techniques and constant monitoring of the fetlock for signs of injury or lameness can help prevent sesamoid injuries in horses.
