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Why Equine Bones Break and Tendons Rupture

Understanding anatomy and physics can help prevent severe injuries

May 9, 2016

Article last updated: March 29, 2021

The life of one of the most promising racehorses of our time was cut short in 2006 after a long struggle to recover from a shattered fetlock.

Barbaro’s injury at the Preakness Stakes and the heroic attempts to save him by the New Bolton Center at University of Pennsylvania School of Veterinary Medicine drew a lot of attention to the question of why so many young performance horses suffer from broken bones and ruptured tendons. Are these kinds of injuries inevitable? Should we give up trying to solve this problem?

During the 2016 International Hoof-Care Summit in Cincinnati, Ohio, veterinarian Renate Weller, professor of comparative imaging and biomechanics at the Royal Veterinary College, University of London, answered that question with a resounding “no.”

Research into equine biomechanics began in 1791 when the Royal Veterinary College was founded and began trying to figure out what made the most famous racehorse of the time, Eclipse, run so fast.

The very first biomechanics calculations ended up being incorrect, but eventually led to the development of the Royal Veterinary College’s Structure and Motion Laboratory. Studying a variety of animals — from dinosaurs to cockroaches — 55 researchers aim to understand locomotive biomechanics. The horse research group within the lab is most interested in what limits performance and causes injury. The conclusion is that biomechanics link performance and injury together, creating a triangle of problems.

Very Little Safety Factor

What do a racehorse, the Tower Bridge in London and a grand staircase all have in common? Each one of these can break, and the factor of safety predict when this will happen.

Factor of safety, used interchangeably with safety factor, is the ratio of the maximum stress that a structural part or other piece of material can withstand to the maximum stress estimated for it in the use for which it is designed.

“Safety factors tell you how much more load you can put on a structure before it breaks,” Weller told attendees of the Burney Chapman Memorial Lecture, presented by Life Data Labs. “They are determined by the load put upon the structure and the strength it has to withstand that load.”

Manmade structures typically can take more load than they are designed for. An elevator that is labeled as being able to handle eight passengers — a safety factor of 1.0 — can actually take much more load than that, perhaps a 1.8 or more.

Horses, however, have a safety factor as little as 1.2. Thoroughbred digital flexor tendons measured at the Structure and Motion Laboratory have shown safety factors as little as 1.1 or 1.2.

“In other words,” Weller says, “at any given point beyond their basic safety factor, they are close to breaking.”

How much load does a horse leg experience when in use? At the walk, the front limb of a horse experiences half the horse’s body weight, which is about a quarter of a ton. At the trot, the leg experiences half a ton. At a gallop, the leg experiences 2½ times the horse’s body weight.

“This is the equivalent of a car crashing down on the front leg of a horse every time it hits the ground,” Weller says. “This is a lot of force to cope with for an animal with such a small foot.”

Dealing With Load

So how do horses cope with these huge forces on the legs? They do it by distributing the load on different structures within the leg and using the leg like a pogo stick.

“When pressure is put on the leg, the fetlock joint creates a weird angle,” Weller says. “Why would an animal want an angle right in the middle of the leg when this makes the leg essentially unstable? It seems like it would make more sense for horses to have straight legs like humans or elephants, because then the ground direction force goes straight up through the leg and all structures are evenly loaded.”

In horses, the tendons on the back of the leg are very stiff and literally can’t be pulled apart. A lot of weight is put on them, so they are stretched by the horse’s body weight. This includes the suspensory ligament, which has material properties that are more elastic, like a tendon. Every time the horse’s leg hits ground, the fetlock joint tries to collapse and the tendons are stretched.

“It’s similar to what happens when you stretch a rubber band and then let go of one end: the rubber band flies away,” Weller says. “When you stretch the rubber band, you put energy into the system and when you let go, the energy is returned, causing the rubber band to fly away. This is a good system for the horse because at every step, the tendons return about 90 percent of the energy put into them. It’s a very cost-effective way to locomote.”

If the horse had springy legs that are less prone to breaking, they would be hard to control.

“There are local feedback loops that help keep the horse moving on a straight line,” Weller says. “The hyper extension of the fetlock joint is a key feature. This is why the fetlock suffers the most problems in racehorses; it is pushed to the limit of what it can tolerate. Associated with the fetlock joint is the suspensory apparatus, especially the proximal sesamoid bones. The sesamoid is the most commonly fractured bone in performance and racehorses.”

Like humans, horses move distinctly different from one another. The coffin joint angle of six horses was recorded at a trot. The above graph demonstrates the variability that occurs in each. This is helps explain why farrier work on one horse might not work on another.

The coffin joint, on the other hand, does not experience as much movement as the fetlock, but still is a high-action joint (Figure 1). At the trot, there is a big variation in coffin joint movement among horses. Six different horses will have six different coffin joint movements.

“Horses are individuals and move differently from one another,” Weller says, “which is why the farrier work on one horse may not work on another.”

During movement, the flexor tendon is stretched and puts pressure on the navicular bone. This is inherent in the design of the horse’s leg and is an area of weakness. Because the fetlock joint is unstable, it tries to collapse because the ground reaction force creates a lever arm, or what’s called in biomechanics, a “moment arm.”

Meanwhile, the tendons hold against the force that is trying to collapse the joint. On the flexor side on the back of the leg, there is a flexor “moment.” This is the force in the tendon and the moment arm of the tendon around the fetlock joint. The proximal sesamoid bones, the fetlock and the navicular bone at the coffin joint largely determine the moment arm of the tendon. While these don’t change, there is change in the force on the tendon. The heavier the horse or the faster the horse goes, the greater the ground reaction force.

When ground reaction forces (represented by the green lines) move toward the toe, moment arms (represented by the blue and orange arrows) get larger. In turn, greater force is placed on the tendon (represented by the red lines).

Farriers can influence where the ground reaction force enters the hoof. Horses already do this to an extent with conformation. Very upright horses have a close ground reaction force, which creates small forces in the tendon. Horses with a long sloping pastern have a ground reaction force vector that is longer, with larger forces in the tendon. With a longer toe, the ground reaction force moment arms get bigger, and force on the tendon grows (Figure 2). Calculations show that 1 centimeter of toe length in an average Thoroughbred comes to 50 kilos of force acting on the tendons.

“Racehorse trainers want long toes because they believe leverage is good for the horse,” Weller says. “However, horses with tendon issues don’t run very fast for very long. The limiting factor in performance in racehorses actually has to do with oxygen delivery to the muscles, not in the front end where you have the tendon mechanism.”

When horses have palmar foot pain, strain on the tendon gets worse.

“They try to get away from that and they start landing more toward the toe,” Weller says. “This increases the force on the tendon. So an area that is already a problem is made even worse. It’s important to get a horse out of the cycle.”

A study in the 1990s by Allan Wilson on lameness biomechanics proves this.

Conformation also plays a role. If a horse is standing flat on one foot and has a slightly counter rotated distal solar angle on another, the stretch in the digital flexor tendon is increased.

“Each degree of angle has a 4% effect on the deep digital flexor tendon,” Weller says. “When the deep digital flexor tendon is stretched, more pressure is put on the navicular bone.”

Clinical studies were conducted at the Structure and Motion Laboratory on the effect of conformation on the deep digital flexor tendon and navicular bone injuries. The results indicated that the steeper that angle, the better for the horse.

Strength Of Structure

“The horse is a weird animal that can run very fast to get away from a predator, but can also move a long time at a moderate speed,” Weller says. “They can do that because of their long legs with flexor tendons and the fetlock joint.”

To build a leg like this to accommodate those flexor tendons, the leg must be long and all one structure.

“If a cow leg hits the ground, it dissipates pressure because it splays,” Weller says. “Horses can’t do that. They have to take it how it comes.”

As the horse’s leg developed over time, it became thinner.

“It’s harder to run in thick boots, but not in light running shoes,” Weller says. “All the horse’s muscles are located proximally — all the muscles are above the knee.”

A consequence of this is that horse bones are fragile. The number one bone failure in racehorses is the long pastern bone. The cannon bone is also very fragile because it’s small.

Up to 50% of horses also suffer from tendonitis during racing season.

“We see a lot of tendon injuries,” Weller says. “The superficial digital flexor tendon gets stretched a lot. Other tendons experience similar strains, if not more. The suspensory ligament suffers a lot of strain in collected work in dressage horses. The superficial digital flexor tendon gets stretched up to 60% in collection.”

The optimal performance zone is where the risk of injury is low and almost meets optimal performance …

The superficial digital flexor tendon is large, and the reason is that it has to cope with a lot of forces.

“If you have a big rubber band, it’s trickier to stretch compared to a smaller rubber band,” Weller says. “That’s the trade off. We want the tendon to stretch, but also be strong enough to cope. Sometimes, the horse gets that wrong.”

The majority of orthopedic injuries are because of cyclical overloading, she says. It’s similar to the difference between breaking a matchstick and a paperclip.

“To break a paper clip, you have to fatigue the material,” Weller says. “That’s what’s happening in orthopedic injuries.”

Fortunately, the body can repair a microscopic crack, but the body has limits to what it can do. If a horse is pushed too hard, the body doesn’t have the opportunity to repair the cracks. Keep pushing it and bone will break.

“Thanks to research, we know how much a racehorse can do before there’s a risk of a bone breaking,” Weller says. “As little as 10,000 loading cycles will do it. There are 220 loading cycles per mile at a gallop. So they operate at a very narrow edge.”

The answer is not that horses should train less, she says, because structures need to be conditioned.

“Bone and muscle respond to training,” Weller says. “If you train, your bones try to be stronger and will put down more mineral. But there can be too much, and then bone becomes brittle.”

There’s a fine line between adaptation and pathological adaptation, and this is obvious in Thoroughbreds. The skill of the trainer is crucial to know how to balance training and rest. You need to train to get fit, and train for skill, so training is necessary.

“As you train more, your performance goes up quickly and then starts to plateau when you reach your potential,” Weller says. “The risk of injury goes the opposite way. When you start training, you haven’t accumulated any damage. Then the risk of injury goes up rapidly, until the whole system comes to a crash. You can’t perform if your tendon is ruptured.

As a horse trains, its performance increases. Unfortunately, the risk of injury also grows. Trainers must find the optimal performance zone, where the injury risk is low and almost meets optimal performance.

“A good trainer knows this. The optimal performance zone is where the risk of injury is low and almost meets optimal performance. In competitive sports, winner versus second place is not that big, so you push as much as you can without breaking it.” (Figure 3)

Reducing Risk

When it comes to bones, vibrations are good. They contribute to remodeling of the bone and the ability to absorb shock. When it comes to vibrations, the first 20 milliseconds when the leg hits ground is crucial.

“We know there’s a relationship between osteoarthritis of low motion joints, like the pastern and spavin joints, and vibrations,” Weller says. “Horses that work on a lot on roads get vibrations of certain frequencies and are more prone to osteoarthritis in those low motion joints than other horses.”

If you influence the foot you influence everything else …

The Structure and Motion Laboratory with the British Horse Society conducted a study of horses moving on bridle paths. When a horse goes out for a 40-minute hack in the U.K, it encounters 20 different surfaces, Weller says.

“When a person runs and goes from concrete to beach, he adjusts his leg stiffness,” she says. “He makes the leg stiffer in soft ground to modulate the impact he experiences. Have you ever missed a step in the dark and jarred your bones? It’s because you use your eyes to judge your leg stiffness.”

Unlike humans, horses have only one leg stiffness and have to cope with whatever ground they encounter. They dissipate shock with a measure of slide.

“If you put anything on the hoof that stops this slide, like studs or anything like that, you are taking out the shock absorbing ability,” Weller says. “Use the least that you can.”

Slide in plastic, steel and rubber shoes was compared at the Structure and Motion Laboratory. Rubber shoes were shown to completely stop slide, while the particular type of plastic tested gave the most slide.

The type of horseshoe — plastic, steel and rubber — can alter slide. In an experiment at the Structure and Motion Laboratory, rubber shoes completely stopped slide, while plastic offered the most.

“We thought we could give the horse more shock absorbance by using rubber shoes, but we didn’t take into consideration that the horse wouldn’t slide anymore,” Weller says (Figure 4).

Slide also depends on the surfaces the horse travels on because surfaces have a huge impact on shock absorption. Artificial surfaces versus turf shows a big difference (Figure 5).

Horses place their feet in a way to dissipate shock and load on the legs. Although textbooks say that horses should land a certain way, they don’t all land the same, Weller says. Within 12 strides, a horse can use three or four different ways of landing. Most horses land with lateral heel first, but a few land flat, and some land heel on.

“For this reason,” Weller says, “when watching a horse move, watch for more steps than just two or three.”

The other shock absorbing mechanism is heel defamation and the properties of the laminae. Farriers can have a huge effect on this.

“If you influence the foot,” Weller says, “you influence everything else.”

Bones and tendons aren’t the only reasons performance horses break down. Horses also go wrong because they have a problem with their motor.

“They have a very efficient muscle system, and a very high percentage of muscle, but they get inflammation of the muscle,” Weller says. “Racehorses are very prone to this.

The respiratory system is also a factor. At the gallop, a horse shifts 2,000 liters per minute.

“Every minute, a horse shifts the equivalent of 10 bathtubs in and out,” Weller says. “The horse is using its anatomy to do this. The gut of the horse goes forward, compresses the lungs and pushes the air out. That huge gut that should be a disadvantage is an advantage because it helps the horse breathe faster.”

But as with bones and tendons, overtraining can cause a problem for the horse.

“Push this system to the limit,” Weller says, “and you see bleeders.”

The anatomy of the equine leg enables the horse to obtain great speeds, but at the same time places it at risk for injury. Understanding the way bones and tendons function in racing and performance horses can help farriers take measures to keep hooves and legs healthy.

Farrier Takeaways

At a gallop, the equine front leg experiences the equivalent of a car crashing down on it each time it hits the ground.

For every centimeter of toe length in an average Thoroughbred results in 50 kilograms of force acting on the tendons.

The risk of a bone breaking increases after as little as 10,000 loading cycles. A horse has 220 loading cycles per mile at a gallop.

Anything that stops slide, such as studs, removes the ability to absorb shock.

Audrey Pavia

Audrey Pavia is a freelance writer and author of 23 books about animals. She lives with a host of animals at her Norco, Calif., home.