Sesamoid Bones: They Take A Lot Of Pressure And Raise A Lot Of Questions For Researchers

As racing continues its quest to reduce injury rates, one key area of interest for many experts is the proximal sesamoid bones.

Most racing fans who have heard of sesamoid bones know about the two small, triangular bones held inside the suspensory ligament that form the back of the equine ankle, but horses (and humans) actually have other sets of sesamoids in the body. The two that form each ankle are called the proximal sesamoid bones. The human kneecap or patella is present in the horse as a component of the stifle and is also considered a type of sesamoid bone. The navicular bone in the internal structures of the hoof is also a type of sesamoid. Sesamoids exist because they reduce friction on joints by gliding over the joint’s surface, helping to pull the limb back and forth.

The proximal sesamoid bones are part of the ankle or fetlock, which drops down toward the racing surface to absorb the horse’s weight during a footfall. The joint flexes farther down the harder the foot falls. The elastic tendons and ligaments (particularly the suspensory ligament) are crucial during this shock absorption procedure, and the proximal sesamoids are hard at work in this moment too – which may mean it’s not surprising that they’re a common source of injury.

Existing research suggests that sesamoid fractures or suspensory apparatus failures are associated with 30 to 50 percent of fatal injuries in Thoroughbred racehorses. At a recent virtual session of the University of Kentucky’s annual Equine Showcase, researchers said that makes them a crucial area of study – but we have to start from the beginning.

Scientists would like to know how the structure of the proximal sesamoids changes in response to intense exercise like racing. We know bones change their shape and structure in proportion to the amount and types of forces placed on them through exercise in a process called bone remodeling. (You can learn more about bone development and remodeling here.) It would be helpful to know if somewhere in that process, sesamoid bones undergo abnormal changes that could signal or predispose an upcoming fracture.

According to Dr. James MacLeod, researcher and faculty member at the Gluck Equine Research Center, scientists first need the answers to more basic questions about proximal sesamoids. In order for researchers to know what is considered an abnormal structural change, they have to know what’s normal for these particular bones – what size, shape, and internal structure is typical? How do they develop? When do they develop? How much variation is there in size, shape and structure between individuals, between breeds and between sports?

Unfortunately, MacLeod said, existing science is somewhat light on the answers to these questions.

“It turns out that in the horse, very little information was published about proximal sesamoid bone development and maturation in a normal sense,” he said.

When trying to answer the basic question of when these bones develop, MacLeod and his colleagues dug up two publications in textbooks suggesting that these particular bones don’t begin to form in a developing equine fetus until very late in gestation, between Day 290 and 330 in what’s typically a 340-day gestation. The end of ossification (hardening) for the bones was, according to these textbooks, complete at around month three or four of the foal’s life.

“We had evidence right away that there was much more to know about the development of proximal sesamoid bones,” he said.

Soon after the research team began their inquiries, Dr. Emma Adam, assistant professor at Gluck, used advanced imaging to discover that the very beginnings of cartilage (which would eventually transition to bone) were beginning to form in what would become the fetlock at Day 46 of gestation. At that point the fetus was only three centimeters long, with a tiny forming limb only three millimeters long.

Currently, MacLeod and his colleagues are in the process of learning more about the variability of the bones in adults, assembling lots of samples from horses who have died for reasons independent of development or injury to the sesamoid bones. Researchers want to study them grossly (recording observations detectable without a microscope) as well as at a microscopic level. They’re looking at elements like bone volume, which refers to the amount of a bone that is minerals. Researchers already know that sesamoid bone volume increases with age as an animal matures and the bone itself grows. Next, MacLeod said, we need to learn how bone volume may change when the horse grows old enough to begin exercise.

Another element that could be important in microscopic bone changes is the trabeculae, which are the bands or thin rods of tissue that together make up the hard structural elements of the bone. MacLeod hopes researchers will learn more about the orientation of these little beam-like supports – are they isotropic, meaning their orientation creates a look of sameness throughout a sample, or are they anisotropic, meaning many of them lie in a single, similar orientation? This matters because it impacts how easy a substance is to break. If you think about chopping an anisotropic piece of wood, he points out, it’s easy to do with the grain because all the strands of the block’s interior structure are pointing more or less the same direction. If you chop against that grain, it suddenly becomes tougher. With an isotropic substance like metal, its components are oriented in all different directions at a cellular level, making it equally difficult to cut or split no matter how you approach it — there’s no area or angle of weakness on a microscopic level.

The initial step to understanding these elements of the bone’s structure is to get as many samples as possible from a wide cross section of ages and breeds. Those breed differences could be really important, too — it won’t help racehorses if the team develop their sense of normal sesamoid bones from Shetland ponies.

“You’d certainly expect [to see differences],” he said. “The skeletal system in general matures differently between different breeds. Small horses and ponies actually mature faster than larger horses.”

There could also be important differences in what’s “normal” between male and female animals, as well as large, heavy-bodied and fine-boned horses within the same breed.

For now, MacLeod said his team has more questions than answers, but he is hopeful that soon – maybe even by next year’s annual equine research showcase – he can provide some.

“I think as we ask the questions, as we generate quality data sets, as we advance imaging technologies, I think we will be able to answer those questions,” he said.

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