Page 354 - The Veterinary Care of the Horse

Causes

It may develop following:
VetBooks.ir • sprains

• repeated low-grade trauma due to poor conformation and normal work

• unsuitable training programmes

• obesity

• a fracture
• infection

• osteochondritis dissecans (see page 242)

The incidence of DJD increases with the age of the horse and is a reflection of wear and tear.

The course of the disease

The thickness of articular cartilage is limited as it has no blood supply and relies on diffusion

of nutrients from the synovial fluid. Thus it can only withstand a certain amount of shock
absorption during athletic exercise. Consequently, forces are absorbed by the soft tissues

surrounding the joint and the underlying subchondral bone. This can cause microfractures in
the bone immediately under the articular cartilage. Initially the repair of these improves the
strength of the bone and its ability to absorb the shock. However, continued stress results in

increased density of the bone with subsequent loss of deformability and the risk of
mechanical damage to the articular cartilage is increased.

The chondrocytes are responsible for the maintenance of the cartilage matrix. Under
normal circumstances they maintain a balance between breakdown and repair. Once the

cartilage has been damaged by mechanical trauma, they release destructive enzymes which
further break down the cartilage. Cells within the synovial membrane also release

degradative enzymes; these include prostaglandins, cytokines and matrix metalloproteinases.
Together these contribute to destruction of the articular cartilage as the rate of destruction

exceeds the rate of repair. As a result the cartilage becomes thinner and less able to withstand
normal forces. Small fissures develop and the surface of the cartilage becomes fibrillated.

Eventually fissures develop through the full thickness of the articular cartilage and pieces of
cartilage and the exposed subchondral bone are released into the joint space. As the cartilage
is lost, secondary changes occur in the surrounding bone and soft tissues. This is seen on

radiographs (Figures 8.6a and b) as:
• loss of joint space