“Joint disease is very common. It is estimated that 60% of lameness is attributable to the joint,” says Professor C. Wayne McIlwraith, Director of the Orthopedic Research Center at the College of Veterinary Medicine and Biomedical Sciences, Colorado State University in Fort Collins, Colo. Indeed, whether you’re talking humans or horses, you can hardly go a day without hearing joint-related lingo: arthritis, capsulitis, DJD, glucosamine, cartilage, etc. But, what does it all really mean? What goes on in there, and what are we talking about when we discuss joint disease?

Joints are grouped into three categories classified on the basis of range of motion: synarthroses (immovable joints), amphiarthroses (slightly movable), and diarthroses or synovial joints (movable). But for the purposes of this article, “joint” will refer to the last category, the synovial joints. We will examine the joint as an organ — a group of tissues working together to perform a specific set of functions.

The synovial joint is not unlike a piece of machinery, and like a machine, in order to understand how it works and what can go wrong, we need to take it apart.

Anatomy — Structures and Function1

Loosely speaking, synovial joints are the hinges of the body — flexible points where two or more bones meet and interact. Looking at the joint, from the deepest part outward, we have:

• The bone ends: Covered by the articular cartilage.
• The articular cartilage: The Teflon coating over the ends of bone,” says McIlwraith.
• Ligaments: Fibrous bands…