Key Words: Bone, Cartilage, Mineralization, endochondral ossication
Yesterday I discussed the lameness workup on Classy, a 5 years old thoroughbred mare with OCD of the stifle. So what is OCD anyway?
Today Iwill describe the disease process, and some of the factors we believe predispose juvenile horses to this problem. Tomorrow I will go over some of the more common location for OCD lesions.
OCD stands for Osteochondrosis dissecans. Just to give you a hint at how complex this disease is, there is still debate over whether this even is an accurate name for the problem. To understand this disease at all, you must first understand how bones grow. A joint is a moving part, consisting of bones that slide along each other, separated and lubricated by joint fluid. The surface of the bone is covered by a layer of cartilage, which is softer and more compliant than bone, and therefore stands up better than more rigid bone to the forces exerted on joints during athletic activity.
Think about it - how do your foal's bones get longer and thicker as the foal grows? What happens is a process called endochondral ossification. The bones grow from the surface cartilage toward the underlying bone. The cartilage cells, called chondrocytes, divide and increase in number. As they mature, they become mineralized and eventually transform into bone. This is a rapidly ongoing process in the growing foal. If the transition from young chondrocyte to mineralized chondrocyte to bone doesn't occur correctly, there is a defect in bone maturation. Thus, osteochondrosis is a defect in endochondral ossification.
Try this image to help you imagine what happens. You are painting the jumps in your arena. You are getting tired, so instead of putting on several thin coats of paint, and allowing each to dry in between, you start globbing on thick layers of paint, not letting each layer dry. When you lay the paint on correctly, each layer adheres to the one beneath and you end up with a shiny surface of smooth paint that doesn't crack or peel. If you glob the paint incorrectly, you end up with bubbles underneath the surface and cracks and flakes on the surface soon after the paint dries. Exactly the same thing happens in foals with OCD. The cartilage to bone development is abnormal, and the resulting defects include cysts (just like the bubbles under your paint surface) and cartilage flaps that detach from the underlying bone just like your flaky paint, because the attachment to the underlying layers is not healthy.
Cysts lying just below a thin layer of unhealthy cartilage, flaky, cracked cartilage, and actual flaps of calcified cartilage that separate from the underlying bone are all manifestations of OCD. In all three cases, the smooth, gliding surface that is critical to pain free, athletic joint function, is lost. The result is swelling and pain during athletic activity. Many factors predispose horses to develop OCD. The particular combination of events in a given foal that result in OCD are complex and inter-related. Some of the major forces at play are: genetics, rapid growth and large body size, excess feeding of carbohydrate, abnormal stress and trauma, and mineral imbalance, specifically copper deficiency.
This is a very basic overview of the disease process called OCD. Tomorrow we will look at some of the most common sites in the body where OCD occurs, and the clinical significance of some of these sites.
