Part One
Equine Joint: Structure and Function
Overview
A joint is defined as an anatomic union or junction between two or more bones.
Synovial joints are the most common type of joint in the horse’s body. The main anatomic feature that distinguishes a synovial joint from a fibrous (skull fissures) or cartilaginous (verterbral) joint is the presence of a joint capsule surrounding the joint and the existence of synovial fluid that lubricates the joint. Examples of synovial joints are: Stifles, Hocks, Fetlocks, and Coffin joints.
Synovial Joint Function
The two primary functions of a synovial joint are to permit movement and transfer load between bones. In a normally functioning joint, both of these tasks are achieved in an efficient and pain-free manner. The secret of how this frictionless, painless movement occurs lies primarily in the structure and function of the articular cartilage lining the ends of opposing bones.
Articular Cartilage
At the ends of the bones in a synovial joint exists a thin layer of articular cartilage.
This tissue is extremely specialized to withstand the very high loads that occur during physical activity and especially performance events. Articular cartilage is composed largely of water (70-80% of articular cartilage is water), type II collagen fibers, proteoglycan molecules such as aggrecan, chondrocytes (cartilage cells), and a variety of miscellaneous (yet important) molecules. The most common proteoglycan in articular cartilage is aggrecan—a very large proteoglycan that plays a pivotal role in the function of articular cartilage.
During weight bearing, the aggrecan molecules, which are already very tightly packed together, become even further compressed. During this compression, water molecules (that were attracted to the negatively charged aggrecan molecules) are forced from the extracellular matrix of the cartilage and all of the negatively charged branches of the aggrecan molecule repel each other like similar ends of a magnet. As a result, the ends of the bones are protected by this layer of shock-absorbing articular cartilage and the load is transmitted between the opposing bones.
When Things Go Wrong: Osteoarthritis
In situations where the chondrocytes are unable to maintain a healthy, fully functional extracellular matrix, problems ensue. Osteoarthritis, defined as the erosion of articular cartilage, is one such example of what can go wrong in a joint. In horses with osteoarthritis, the balance between the degradation and synthesis of the extracellular matrix is disrupted favoring the destruction of the tissue. Osteoarthritis is a major cause of lameness in athletic horses and can be a painful debilitating condition that might be career-ending or even life-threatening.
Fast Facts
■ Synovial joints are highly movable joints that function to permit movement and transfer load between bones.
■ Synovial joints are made of two or more bones covered with a layer of articular cartilage. A joint capsule connects the bones and creates a cavity, which is filled with synovial fluid.
■ Movement and load transfer is achieved in a frictionless and painfree manner in a normally functioning joint. To achieve this, the articular cartilage located at the ends of the connecting bones must function normally.
■ In joints with inflammation—such as an infected joint or a joint with osteoarthritis—the articular cartilage is damaged and movement is no longer as efficient or pain-free as it should be.
Robin Peterson Illustrations
Equine Stifle Joint
Mandi Holland DVM
CSR Equine Sports Medicine Team
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