An Inside Look at Joints
Exploring Healthy and Resilient Joint Function in our Horses
By Alexa Linton, Equine Sports Therapist
The relationship between our horses and their joints can feel scary, conjuring up words like “arthritis” and “lameness.” Our focus on joint function and integrity, including how this plays a role in a healthy hoof mechanism and legs, is designed to create a knowledgeable foundation to support your horse for years to come. Hopefully, by the end of this article you will have a better understanding of joints – most specifically in the legs – how they function, how to help them last longer (preferably for the entire life of your horse), how to recognize when there is an issue, and how to address that issue in the best way. Put your geek hat on – we are diving in!
Synovial joints are the most common and most movable, accounting for the majority of the joints in the lower extremities. They are also the most prone to disease and injury. Photo: Dreamstime/Svetlana Golubenko
When it comes to horses, we are looking at three main types of joints:
- Synovial joints – the most common and most movable, making up the majority of the joints in the lower extremities;
- Fibrous joints – immobile joints, such as those between the cranial bones; and
- Cartilaginous joints – slightly movable joints occurring, for example, between the vertebrae.
For this article we’ll focus on synovial joints, because they are the most common joint in the lower extremities and the most prone to injury and disease due to how mobile they are.
A synovial joint is a bit of a wonder. These joints are formed when two bones come into relationship and are designed, in six different formations - pivot, hinge, condyloid, saddle, plane, and ball-and-socket - to create mobility and absorb compression. In order to perform those functions, a stabilizing joint capsule is developed by the body, made up of a fibrous capsule at each bony end, collateral ligaments (strong and stabilizing fibrous tissue), and at certain joints (specifically the stifle) cruciate ligaments, as well as other stabilizing ligaments and tendons. In the inner layer of the joint, there is a synovial membrane lining the sides of the capsule and responsible for secreting hydrating synovial fluid to lubricate, nourish, and cleanse the joint. The synovial fluid contains highly lubricating hyaluronic acid, a component you have probably heard of as it is often used in joint supplementation. Disease in the joint appears to commonly be connected to a depletion of hyaluronic acid, leaving the joint more susceptible to breakdown. Flexible hyaline cartilage covers the ends of the bone. It’s important to note that there is fluid stored within this cartilage, which is distributed over the surface of the cartilage during weight-bearing, and then reabsorbed when weight bearing stops.
This information becomes applicable to our horses as we create our joint-happy warm-up routine. Contrary to what many of us have been taught, the best warm-up is one that promotes this distribution of fluid through healthy joint action. This means starting slow and allowing the horse to move at his own pace with a focus on the lubrication of the joints, especially those of the lower extremity, imagining the release of fluid with each step. I now listen to my 20-year-old mare Diva and her pace, working together to preserve the integrity of her joints as much as possible as she ages, by easing into the transitions and being careful not to ask her to “move out” before her body is ready and her shock-absorbing joints are working at their prime.
The Synovial Joints
To begin our exploration of synovial joints, let’s start with a look at the front limbs, bearing 60 to 65 percent of the horse’s weight. The glenohumeral (shoulder), elbow, knee, fetlock, pastern, and coffin joints experience more force and concussion than the hind limbs, particularly for horses that are used for jumping and racing, which means these joints are more susceptible to injury and disease. The elbow and shoulder joints, both hinge joints, tend to be protected from these concussive forces by the lower joints, although recent studies show that the elbow joint, made up of the humerus, radius, and ulna, can be susceptible to cartilage breakdown which is often related to hoof balance and to being ridden or driven, leading to a horse that feels “just a little off” or tentative moving downhill. It is thought that up to 90 percent of injuries in the front limbs happen from the fetlock joint down, with about 10 percent of injuries happening at the knee joint, which matches up to our wrist joint. Just like our wrist joint, the knee is extremely complex - quite literally full of bones, eight in all. It includes three main joints, one between the radius and the first row of four carpal bones, one between the two rows of carpal bones, and one between the lower row of carpal bones and the cannon bone, plus a number of articulations between the carpal bones themselves. Just like us, horses can experience tendon inflammation in the carpal canal, especially in horses that are over at the knee or experiencing incorrect hoof balance.
Lower down, the cannon bone connects with the first phalanx to create the fetlock joint, along with the medial and lateral sesamoid bones, supported by the suspensory ligament.
Adventuring further down, we come to the less mobile and less shock-absorbing pastern joint, made up of the first phalanx and the second phalanx. Into the hoof, we find the more mobile and hopefully, highly shock-absorbing, coffin joint, which connects the second phalanx and third phalanx (the coffin bone). The coffin bone, nestled within the hoof mechanism, can be negatively impacted by mechanical or metabolic laminitis, with rotation potential in more serious cases. A misalignment in the coffin bone and joint, accompanied by pain and inflammation in the sensitive lamina attaching the hoof wall to the capsule, can create trouble up the leg and beyond, and needs to be addressed immediately by a trusted veterinarian and farrier. Another article topic all by itself is the accompanying navicular bone and its close friends the deep digital flexor tendon, which supports the navicular bone, and the lubricating and friction-reducing navicular bursa.
On to the hind limbs, made up of the coxofemoral joint, the stifle joint, the hock joint, as well as the fetlock, pastern and coffin joints, mirroring the front limb. Hind limbs bear less weight than the front limbs, and primarily function in creating impulsion rather than shock absorption. Arguably, the most complex joint of the rear limb is the stifle, the largest joint in the horse, which is the equivalent to the human knee joint and essential to movement, as well as supporting the horse during sleep via the stay apparatus attached to the patella bone. The stifle consists of the femoropatellar joint between the femur bone and the patella, the medial and lateral femorotibial joints side-by-side between the femur and tibia, two menisci (handy structures designed to keep the femur in place as it glides over the tibia, the medial and lateral collateral ligaments), as well as three main stabilizing ligaments attaching the patella to the tibia. This joint has many components that need to work together for healthy function, so as you can imagine, stifle issues are common. This joint is frequently impacted by cruciate ligament rupture, patellar luxation, and osteochondritis dissecans (OCD), all great reasons to have a trusted veterinarian and bodyworker on your wellness team.
The hock joint, similar to the human ankle, is made up of ten bones, a bevy of stabilizing ligaments and four joints. These are the tarsocrural, which is a high-motion (90 percent of the movement in the hock happens here) ball-in-socket; and the shock-absorbing, lower-motion, and more injury-prone proximal intertarsal, distal intertarsal, and tarsometatarsal joints (collectively known as the tarsal joints). The tarsal joints of the hock are also prone to OCD, a developmental disease caused when the cartilage at the joint doesn’t form properly, and osteoarthritis (OA), both of which are progressive inflammatory conditions and all the more reason to start horses in focused training or riding only after these joints are fully formed. These conditions can also be influenced by nutrition, genetics, rapid growth, and hormonal balance. If one of these painful conditions is suspected in any of the joints we have discussed, consult your veterinarian for a full lameness assessment.
With my osteopathy brain, I can’t help but look at how these many parts function as a whole. As you can imagine, conformation plays a role, as the way the horse is weight-bearing and his relationship with gravity can’t help but have an impact on the integrity of his joints. As everything is connected, what is happening in the limbs naturally impacts what sits above them, namely the spine, shoulder girdle, and pelvis function, and vice versa. I always recommend regular visits with a knowledgeable equine body worker as a part of your strategy to preserve joint integrity and to catch compensation and stress points before they become a bigger issue. And to swim in somewhat controversial waters, correct hoof balance goes a long way to sustain the joints and soft tissues of the lower extremities. Take a moment to observe the impact of hoof balance on the horse’s legs - a long toe, underslung heel, upright hoof, or side-to-side imbalance can do a number on the joints above it. My go-to resource for clients and students is The Essential Hoof Book by Susan Kauffman and Christina Cline, and remember, it is never worth sticking with a farrier who is not a fit for your horse.
It is also important to recognize the impact of the rider’s weight on the integrity of the joints. If your horse is young or just starting out under saddle, take your time introducing rider weight, interspersing with short sessions of biomechanically correct and gently progressive groundwork (I love the Academic Art of Riding by Bent Branderup) to gradually build strength and resiliency in tendons, ligaments, and joint capsules. Keep top of mind that the majority of joints aren’t fully formed until the horse is more than four or five years old. Remember that there is no rush, and you are investing time to support a horse to have a long and healthy future. With my mare turning 20 this year and feeling fabulous, I see firsthand the benefits of starting out slow with lots of restorative time, short focused rides (about 15 minutes of concentrated strenuous work only), room to move, play and graze, easily absorbed vitamins, minerals and fatty acids, and regular bodywork. Glucosamine, MSM, hyaluronic acid, and chondroitin can prevent and restore joints from degeneration.
Realistically, just like humans, any horse can sustain a joint injury, no matter how careful we are. In this case, a trusted team including a veterinarian, bodyworker, and farrier, and a plan with your horse in mind are critical. More to come on the rehabilitation process in future articles.
This article was originally published in the Summer 2019 issue of Canadian Horse Journal.
Main article photo: iStock/GlobalP