Form and Function of Joints
There is a definite correlation between joint angle, hoof flight pattern, and weight bearing. One might even wax a bit poetic and declare that as the joint angles, so flies and lands the hoof. That's where the poetry ends, however, because if there are improper joint and/or hoof angles during weight bearing, there can be lameness problems.
There are three types of joints--fibrous, cartilaginous, and synovial. While all three are highly important, the synovial joints are more apt to affect hoof flight if they are out of alignment.
Fibrous joints are pretty much immovable and are united by fibrous tissue. Those fibrous joints are found in the skull and between the shafts of some long bones.
Cartilaginous joints have a bit more movement, but their range is still limited. Included in these types of joints are the pelvic and spinal joints.
The synovial joints are the most active in the horse's body and, as mentioned above, are the ones that will normally have a strong bearing on hoof flight pattern. One can say that these joints serve as the horse's ball bearings. They consist of two bone ends covered by articular cartilage. This cartilage is smooth and resilient, which allows for frictionless movement of the joint when properly lubricated with synovial fluid.
Joint stability is maintained by a fibrous joint capsule, which attaches to both bones, and collateral ligaments. The collateral ligaments are located on either side of most joints. They are important in maintaining stability in joints such as the fetlock, knee, elbow, hock, and stifle. Other ligaments surrounding the joint capsule also play a supporting role--a prime example involves the distal sesamoidean ligaments and suspensory ligaments that, together with the sesamoid bones, make up the suspensory apparatus and hold the fetlock in position.
Other ligaments within the joint, such as the cruciate ligaments, also help with stability. These ligaments provide important support.
While ball bearings are designed to provide frictionless movement in vehicles, they can only continue to do so if they are lubricated. It is the same with the joints. The joint capsule contains an inner lining known as the synovial membrane. Its function is to secrete synovial fluid, which lubricates the joint.
It becomes immediately obvious that a sophisticated structure like the joint needs proper alignment if it is to remain healthy. A joint that is not aligned correctly bears inappropriate strain throughout the entire structure. Not only is proper hoof flight altered by misalignment, but the potential for injury and disease from improper weight bearing is magnified.
Before discussing just how joint angle affects locomotion and weight bearing, let's take a quick look at the various movements that occur when a horse travels. Our tutor is Hilary M. Clayton, BVMS, PhD, MRCVS, holder of the Mary Anne McPhail Chair in Equine Sports Medicine at Michigan State University. Although she has spoken and written at great length on this subject, we will capsulize her work here.
The swing phase, when the horse's hoof is not in contact with the ground, does not result in great stress on poorly aligned joints. In the swing phase, the limb is first pulled forward, then pulled backward in the final movement prior to ground contact. "The forces on tendons and ligaments of the joints during the swing phase are very small," Clayton says.
Any damage usually occurs from misalignment trauma during the stance or weight-bearing phase, which describes that period when the hoof is in contact with the ground. The phase is divided into ground contact, impact, loading, and breakover.
- Ground Contact--The first contact is either heel first, flat-footed, or toe first, depending on gait, speed, farriery, and/or lameness.
- Impact Phase--The impact phase occupies the first 50 milliseconds (one-twentieth of a second) after the hoof contacts the ground. During this time, the limb undergoes rapid deceleration that causes a shock wave to travel up the horse's limb. Important shock absorbers at this point are the joints. During the impact phase, the bones receive their maximum amount of shock, so this is the phase when most bone and joint injuries occur.
- Loading Phase--Loading and unloading occupy the period from the end of the impact phase until breakover. During this phase, forces are applied more gradually than during impact, and it is during this phase that ligaments and tendons are maximally loaded. Naturally, this is the phase when most soft tissue injuries occur.
- Breakover--The breakover phase begins when the heels leave the ground and begin to rotate around the toe of the hoof, which is still in contact with the ground.
Aligning the Front End
Let's start with the front end, because this is where the horse carries 60-65% of its weight. As a result of this "heavier" front end, a horse is more apt to have joint problems in the front legs than in the rear.
If the hoof flight is to be straight and true, all parts of the moving leg must be aligned, beginning with the shoulder. When we look at a horse from in front, we should see a straight line down the forearm, through the knee, and into the pastern and hoof. The distance between the legs at the chest should be the same as the distance between the feet on the ground. Few horses have perfect conformation, and the amount of deviation has a strong bearing on hoof flight abnormality and the potential for injury and joint disease.
Proper alignment in the front end begins in the shoulder. What this means is that the slope of the shoulder should match the slope of the pastern and hoof, with forearm and cannon bones forming a straight line through the knee until reaching the pastern. This alignment has all of the bones fitting neatly together with properly aligned joints.
One of the most troublesome abnormalities in front limb joint conformation is toeing out. These horses have pastern, knee, and/or shoulder joints out of alignment, with the toes pointing out instead of forward. There's not a big problem as long as the horse remains still, but when he moves, particularly at the trot, the abnormality causes serious hoof flight problems. The hooves will wing inward, sometimes striking the opposite leg during the flight phase. Injury, accompanied by lameness, is often the result.
The reverse problem of toeing out is toeing in. Here the toes point inward instead of straight ahead during the ground contact phase. As the horse travels, the hooves will swing outward during the flight phase--this is known among horsemen as paddling. While this abnormal hoof flight pattern puts undue strain on joints, there is at least a bit of good news involved--the horse doesn't strike itself.
Base-wide horses (those that stand with their hooves farther apart than the legs are at the chest) often have narrow chests, and often toe out as well. Base-narrow horses stand with their hooves closer together than the legs are at the chest, and usually have wide chests. They tend to toe in.
The pasterns are key shock-absorbing joints, and as a result must be strong, yet pliable. The pastern is made up of two bones between the fetlock joint and the hoof, and the joint between those two bones. Long pasterns tend to have these two bones longer than normal, and the pasterns tend to drop more horizontally especially during high-stress disciplines such as racing and jumping. This increased range of motion can put more stress on the suspensory apparatus.
Pasterns that are too short remain fairly upright even with high stress. This results in too much concussion traveling up the leg because these less flexible pasterns are less efficient shock absorbers.
Other joint alignment problems originate in the knee or carpal joint. One that was previously thought to be quite serious is bench knee. This means that the forearm and cannon bone are not properly aligned. The bench-kneed horse will have the forearm entering the knee on the inside or medial aspect, while the cannon bone below the knee joint will exit on the outside or lateral aspect. Though it has commonly been thought that bench knees put a horse at serious risk for injury especially in high-stress disciplines, recent research suggests that this might not be the case.
Two other conditions of the knee joint that can alter normal hoof flight and weight bearing and raise the injury potential are being over at the knee (with the forearm ahead of the cannon bone viewed from the side, also known as bucked knee), or being behind at the knee (the reverse of over at the knee, also called calf knee). Of the two, calf knee is the more serious because of the stress on tendons and ligaments that run down the back of the leg, and because of the predisposition to chip fractures from overextension of the joint.
Knees can be misaligned laterally or medially as well. These conditions are known as bow legs and knock knees, respectively. See the table on page 108 for more information on these.
Rear End Alignment
Deviations in joint angles of the rear limbs can also have a profound effect on hoof flight and weight bearing. As with the front end, we want to see proper alignment down through the bones and joints of the rear limbs.
When the joints of the hind limbs are not correctly aligned, the horse will often not be a good athlete because it is unable to make proper use of muscle power either in propelling itself forward or in putting on the brakes for a sliding stop (when it will have difficulty keeping the feet parallel).
One fairly common stance in horses is being base narrow; when the base-narrow horse is at rest, its rear hooves are closer together than its hocks and hip joints. Mild base-narrow conformation behind does not usually cause problems. If a base-narrow horse has a tendency to wing in, however, the horse might interfere and cut or bruise the opposite leg. The opposite condition, which is much less common, is being base-wide. This means that the hooves, when on the ground, are wider apart than the hocks and stifles.
In the hind limbs, toeing out is common and less of a problem than in the front end--in fact, a recent study found that roughly 70% of horses naturally toe out behind.
Joint abnormalities in rear legs can also influence the length of a horse's stride. Generally speaking, horses with more angulation to their joints can have a longer stride than horses with straighter joints, much like the actual length of a tightly coiled spring versus a loosely coiled one.
An example is the sickle-hocked horse, where the rear cannon bone is angled forward from a vertical line. Such a horse might be able to reach well under itself. Conversely, the horse that is straight behind--where there is very little angulation between the thigh bone (femur) and the tibia--will be unable to reach as far forward with the rear legs and show very little engagement of the hind limbs for collection.
Several hock misalignments can cause lameness problems as well; see the table on pages 110-111 for diagrams of, and stresses associated with, these problems.
Proper alignment in the lower limbs means that the slope of the pastern should be the same as or very similar to the slope of the foot so that the long pastern bone, short pastern bone, and coffin bone all fit smoothly and neatly together.
The problem involving improper joint alignment, with resultant alteration of hoof flight and weight bearing, is that very little can be done to correct the problem.
"By the time the horse is mature, the limb structures have adapted to the misalignment," Clayton says. "If the horse is still sound and performing well, alterations in foot balance should be made carefully if at all, because any change at this point may ruin the horse's adaptation to his problem."
Hoof trimming to encourage proper alignment can be done, but only during the very early formative stages in a foal, and must be approached with great skill and care.
Once the horse reaches maturity, what you see is what you get. By attempting corrective trimming at that stage, one is merely adding additional stress and pressure on the joint.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals