Following on from our most recent article on OCD and cystic lesions, this article will focus on lameness associated with degenerative conditions with particular reference to the knee. The knee of a thoroughbred is unfortunately a "weak point" and common site of lameness and career ending injuries.
Osteoarthritis, which is also known as Degenerative Joint Disease (DJD) is an irreversible and chronic joint disease that is typically the "end stage" of various joint insults. It may occur secondarily to "over-exercise" on an immature skeleton or be a consequence of acute injuries such as intra-articular fractures or joint infection. A horse's skeleton, just like its muscles, needs to adapt to the forces placed on it. Appropriate training or "conditioning" stimulates this process and there is a balance that needs to be achieved that stimulates adaption but not degeneration. This process starts at birth. The actual amount of exercise needed to achieve this balance will vary between horses and will depend on a number of factors, including conformation, genetics and track conditions. It is the skill of the breeder, trainer and their veterinary advisor that helps to determine the level of exercise needed to achieve this balance.
The early signs of osteoarthritis are important to recognise as changes to exercise/ training programs and pharmaceutical treatment can stop the formation or progression of this disease. These signs may include joint swelling, pain on joint flexion or subtle changes to the horse's action, such as a wider limb stance, changing of lead legs, "hanging", or a reluctance to work. If these signs are ignored more permanent changes can occur, resulting in radiographic changes of osteoarthritis and lameness. Chip fractures of the knee may occur as acute "overload" injuries but they more commonly occur in combination with osteoarthritis as this process causes a "weakening" of the bone. These chip fractures can occur as yearlings, although more commonly at 2 or 3 years of age. Although these chips can be removed, and many horses compete successfully, osteoarthritis cannot be reversed and ongoing modification to training programs and veterinary treatment are required to achieve racing success.
Studies of radiographic changes to the knee in sales yearlings have been performed. In one North American study, an incidence of 2.7% was reported and these horses were significantly less likely to start a race at 2 and 3 years compared to yearlings without lesions. This finding reflects the high clinical significance of problems in the knee.
Bone spur formation (arrows) in the knee observed on yearling radiographs