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Equine Joints:

Introduction to the types of joints in horses.

Equine Health Books

Equine Joints: An Overview

It is much the same with Even with perfect conformation (which is rare), the performance horse and its joints are still subjected to daily wear and tear that places stress on joints. The horse's joints are designed to efficiently absorb shock, permit frictionless movement, and effectively bear the weight of a body that can weigh 1,200-1,500 pounds (550 - 650 kg) or more.

Veterinary science has provided medical and surgical tools for helping repair equine joints, but there will always be the limitation of having to work with what's there.

In this we will examine the types of equine joints so that we understand the terminology and anatomy. Then we will look at the forces generated by various forms of competition and diagnostics. And, finally, we'll look at treatment.

Types of Joint

There are three different types or classifications of joints - fibrous, cartilaginous, and synovial.

Fibrous joints are the least likely to be afflicted with disease because they are more or less immobile. They include joints in the skull and those between the shafts of some long bones.

Cartilaginous joints don't have a high propensity for disease because they, too, have limited movement. These are the joints of the pelvis and vertebrae as well as growth plates, which extend a bone's length during the horse's growing years.

That brings us to synovial joints which are most likely to suffer disease and injury because they are the most active joints in the horse's body. They consist of two bone ends covered by articular cartilage. It is this cartilage within the joint that is so smooth and resilient that, when properly lubricated, enables frictionless movement of the joint.

Of course, in addition to the two bones covered with smooth, resilient material, you will need something to hold the whole thing together and lubricate it. The joint's stability is maintained by a fibrous joint capsule which is attached to both bones and, also, to the collateral ligaments which are located on either side of most joints. They are key components of the fetlock, knee, elbow, hock, and stifle joints.

Other ligaments within joints, e.g. the cruciate ligaments in the stifle, help stabilize some joints.

Ligaments external to the joint capsule provide additional support. Good examples are the distal sesamoidean ligaments and suspensory ligaments that, together with the sesamoid bones, make up the suspensory apparatus and hold the fetlock in its correct position.

An prime enemy of joint health is friction. To protect against friction we need lubrication. And where does this lubrication within a joint come from? The joint capsule contains an inner lining called the synovial membrane. This lining secretes the synovial fluid that lubricates the joint. A key ingredient in this fluid is hyaluronic acid, also known as sodium hyaluronate or hyaluronan, which lubricates the synovial membrane. Another substance in synovial fluid, a protein called lubricin, is the primary lubricant of the cartilage. In some cases of joint disease there is a depletion of this vital fluid.

Synovial Joints

For a quick look at the synovial joints, we will start with the forelimbs which bear 60-65% of the horse's weight and are thus subjected to greater concussive effects than the rear legs when a horse is moving at speed. Of course, with competition horses such as cutting and reining horses the heavy stress shifts to the rear limbs.

The knee or carpal joint is composed of three main joints comprising seven individual bones arranged in two rows and numerous ligaments that keep everything in its proper place - when all is working well.

The knee rests on top of the cannon bone or third metacarpal, which is flanked on either side by splint bones - the second and fourth metacarpals. Resting on top of the knee structure is the radius. Correct conformation is necessary for this joint to function at its peak and remain sound.

Just think of the stress on this complicated structure if the horse is over or back at the knee. Inappropriate conformation puts undue stress on the structures even at the walk. The stress is greatly magnified if the horse with poor conformation is traveling at speed, performing athletic maneuvers, or going over jumps.

The lower end of the cannon bone connects with the long pastern bone (also known as the first phalanx or P1) at the fetlock joint. One of the key jobs of the fetlock joint is to absorb shock. Next is the joint where the long pastern bone connects with the short pastern bone (second phalanx or P2). This is the pastern joint and it, too, is a shock absorber.

Then there is the coffin joint, which is composed of the second and third phalanges (P3 is also known as the coffin bone) and the navicular bone. There is a great deal of elasticity and motion in this joint, as well as shock absorbing capability.

Other integral parts of the front limbs are the shoulder and elbow joints. However, by the time concussion reaches them, its effects have been dissipated very effectively by the other joints, so the shoulder and elbow joints aren't as prone to concussive injury and disease.

The two key synovial joints in the rear limbs that are different from those in the front limbs are the hock and stifle joints. The hock or tarsal joint joins the tibia with the metatarsal bones. The horse's hock joint is a bit like its knee in that it is composed of a number of bones. There are four separate joints in the hock, with only the top one providing significant motion. Like the knee joint, the hock joint is held together by a complex set of ligaments.

The other joints from the hock down - pastern, fetlock, and coffin - function similar to their counterparts in the front leg. Moving upward from the hock, we come to the stifle joint, which is the horse's largest synovial joint and is analogous to the human knee. However, unlike the human knee, there are three separate joint compartments.

Above the stifle joint is the hip joint, which is of ball and socket construction and is stabilized by strong bands of ligaments. Here the upper end of the femur fits into a socket on the pelvic bone.

Each joint is stabilized by a complex network of tendons, ligaments, and muscles. When all is well, this complex network enables a joint to function in a smooth, synchronized fashion. However, when any part of the network malfunctions because of injury or disease, repair via medical treatment might be necessary.

© Les Sellnow
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