by: Les Sellnow
The mechanical engineering involved in the structuring of equine joints is both complex and masterful. Not only do healthy joints allow the horse to move freely, but they also help to effectively absorb concussion, especially when the horse is traveling at speed. In this article and one to follow next month, we want to take an in-depth look at joints and some of the problems that afflict them. Before we can discuss joint diseases, fractures, and other problems, we first must understand how joints are constructed and how they function. That will be the thrust of this article.
The information that follows has been gleaned from experts in the field, such as C. Wayne McIlwraith, BVSc, PhD, Diplomate ACVS, of Colorado State University; the late and legendary O. R. Adams, DVM, MS, also of Colorado State, who authored the book, Lameness In Horses; as well as Robert A. Kainer, DVM, MSD, and Thomas O. McCracken, MS, both of Colorado State.
To begin, there are three different types or classifications of joints--fibrous, cartilaginous, and synovial. Simply put, the purpose of a joint is to allow the back to flex and the limbs to bend.
Fibrous joints are basically immovable and united by fibrous tissue that ossifies with age. Included in this category are most joints of the skull (suture) and those between the shafts of some long bones (syndesmosis).
Cartilaginous joints have limited movement, such as the pelvis and vertebrae, and the growth plates, which extend a bone's length (physis) during the animal's early years.
The most active joints and the ones most apt to sustain injury or be attacked by disease are the synovial joints. Synovial joints, which are the horse's ball bearings, consist of two bone ends covered by articular cartilage. The cartilage within the joint is smooth and resilient, which allows for frictionless movement. 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.
There are other ligaments within the joint itself, such as the cruciate ligaments, that also help to stabilize some joints, such as the stifle joint.
Other ligaments outside the joint cavity also lend support. 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 its correct position.
In addition to the fibrous joint capsule, the joint capsule itself also contains an inner lining layer called the synovial membrane. It secretes the synovial fluid that provides lubrication within the joint.
As we will see later, there are various disease processes that affect the nature of the synovial fluid and which can produce a number of soundness problems for the horse. While there is much that can go wrong within the joint, nature's overall plan for their development is masterful.