Elizabeth Greene, Ph.D.
Extension Equine Specialist
University of Vermont
[reproduced from Summer 2001 Caution:Horses, Vol. 6, No. 2]
Dr. Diagnosall, the expert psuedoveterinarian.
The yearling colt "Ripandsnort" was presented to Dr. Diagnosall with right hind leg lameness. "Ripandsnort" is a Quarter Horse/Thoroughbred/Standardbred/Appaloosa/(insert your favorite breed) that is going to begin training under saddle soon. He has been on a high performance/growth diet and has adequate pasture and some supplemental alfalfa hay available.
Dr. Diaganosall did a general lameness exam, trotted the horse on a hard, level surface, used hoof testers, and performed flexion tests.
no obvious signs of injury
no painful reaction to the hoof testers
no obvious tendon sheath swelling
But, flexion tests on the hocks caused the horse to move off slightly more lame after flexion of the right hock.
With the permission of the owners, Dr. Diagnosall took four different radiographs: a front-to-back view, a side-to-side view and two different oblique (i.e. two different angle) shots of "Ripandsnort's" right hock.
What did Dr. Diagnosall conclude?
From the tests, Dr. Diagnosall determined that "Ripandsnort" has Osteochondritis Dessicans (OCD), which is a form of Developmental Orthopedic Disease (DOD). The terminology used by veterinarians to describe DOD can be very confusing to the layman. Developmental Orthopedic Disease encompasses a broad category of musculoskeletal problems that are sometimes referred to as metabolic bone disease, metabolic cartilage disease, or developmental skeletal disease. Examples of DOD include Angular Limb Deformities, Contracted Tendons, Club Feet, and Osteochondrosis. Osteochondrosis refers to problems related to bone and cartilage development, and one type of this is called Osteochondritis Dessicans.
In order to understand "Ripandsnort's" problems, we need to understand how cartilage growth in the joint occurs and what might affect the process. As the horse grows, there are several stages of normal cartilage and bone formation. Normal endochondral ossification (the conversion of cartilage to bone near the joint) involves the following steps: chondrocyte (cartilage cell) proliferation, maturation and growth, followed by chondrocyte degeneration, death and calcification, vascular invasion, and finally, bone formation and remodeling. As these steps occur, nutrients are provided by two sources, diffusion through the cartilage from the synovial fluid (fluid in the joint) and from the capillaries that are located in the existing bone.
In Osteochondrosis for some reason, the cartilage cells do not go through the normal degeneration process. This leads to a thickening of the cartilage layer, which can cause some of the cartilage cells to be cut off from the blood supply, interfering with nutrient availability. Osteochondrosis may show up on a radiograph as an unevenness, dent or pit in the articulating surface and subchondral bone (bone immediately underlying the articular surface).
Osteochondritis Dessicans can occur when the affected area becomes necrotic (dies). This may cause the formation of a flap (OCD), or cause a piece of the cartilage to come completely loose and float around in the joint, which may ossify with time and is commonly referred to as a joint mouse. The Osteochondritis portion of OCD refers to an inflammatory reaction in the bone marrow and joint. The breakdown of the cartilage can trigger a local inflammatory response by the body. The signs of inflammation are pain, swelling, heat, redness and loss of function. The lameness and pain that is observed by the owner is a result of synovitis (inflammation of the lining of the joint capsule). The Dessicans refers to a flap of cartilage that has come loose from the base as a result of the necrosis. When examining the radiographs, Dr. Diagnosall looked for joint mice or flattened areas on the articulating surface. If present joint mice would show up as radiopague (white) fragments within the joint space.
The clinical signs of OCD are lameness, swelling, and distension of the joint capsule. Some factors that have been associated with development of OCD and other Developmental Orthopedic Diseases include, genetic predisposition, conformation, mechanical stress and traumatic injury, lack of exercise, rapid growth spurts, endocrine imbalances and nutritional factors. The endocrine factor is usually tied to an imbalance of thyroid hormone, which influences the maturation of chondrocytes. Several nutritional problems have been tied to increased occurrence of OCD as well. Young, growing horses that are fed excess levels of protein and/or energy in their diet may be potential candidates for OCD. Also, deficiencies or imbalances in ratios of calcium to phosphorus can predispose the horse to OCD problems, since those particular minerals are critical components of bone. The Nutrient Requirements of Horses (1989) recommended ratios of calcium to phosphorus can range from 1:1 to 5:1 without being detrimental as long as phosphorus levels in the diet are adequate. Copper and zinc ratios have also been implicated in OCD problems since an excess zinc level will inhibit copper absorption. Copper is an essential component of lysyl oxidase. A deficiency of lysyl oxidase will lead to reduced strength and stability of bone.
What are the care and treatment options for "Ripandsnort?"
If Dr. Diagnosall does not find evidence of joint mice or cartilage defects on the radiographs, he may opt for conservative treatment. This could consist of stall rest, nutritional program evaluation and adjustment, nonsteroidal anti-inflammatory drugs and radiographic reevaluation after 6-12 months. If there continues to be problem after several months, he may choose to try interarticular joint therapy (joint lavage-flushing the joint, or injection of joints with Hyaluronic Acid).
However, if there is evidence of obvious pathological cartilage damage or mice, arthoscopic surgery is the treatment of choice. The flaps, mice and necrotic cartilage can be removed with minimal damage to the surrounding tissues, but not necessarily minimal expense. In addition, Dr. Diagnosall may want to take radiographs of the other hock, since bilateral occurrence of OCD is common.
Bohanon, T.C. 1995. The Horse: Diseases and Clinical Management. W.B. Saunders Co. Edited by C.N. Kobluk, T.R. Ames, and R.J. Geor.
National Research Council. 1989. Nutrient Requirements of Horses. National Academy Press, Washington, D.C.
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