Osteochondrosis is a joint disease that can affect all horses, but is found to be most common in Warmblood horses. It is a disease that can cause lameness and create long term problems if left untreated, but it is also expensive to get the necessary treatment. Because it is a joint disease it is not really an issue that can be ignored as joint damage can increase of fragments are left in the joint space as well as being painful for the horse. It is important to know what OC is and therefore get closer to understanding why it occurs in some horses and not others.
The joints most commonly studied in OC incidences are;
1. femoropatellar (stifle)
2. tarsocrural (hock) and
3. metatarsophalangeal (fetlock)
As the growth rate in horses is greater in the hind limb than forelimb during early stages of development. OC occurs when there is a disturbance of the physiologic process of endochondral ossification, leading to locally thickened cartilage plugs. At the same time cartilage canals disappear during chondrification. This means that the articular cartilage depends exclusively on the synovial fluid for diffusion of nutrients. Therefore, excessive thickening of cartilage can lead to certain areas lacking nutrition and hence to focal necrosis and weakening of the tissue.
Osteochondritis dissecans (OCD), as well as subchondral bone cysts, are commonly considered to be a manifestation of osteochondrosis. The epiphyseal ossification centre advances out until ossification is completed, leaving a layer of cartilage. This layer of cartilage becomes the articular cartilage. If there is a disturbance in endochondral ossification, an area of retained cartilage can be formed with a consequent defect in the bone. Cracking can then proceed in this retained cartilage to give a flap or fragment of cartilage that may contain bone. These flaps and fragments on the surface of the joint result in osteochondritis dissecans. Also, because of joint incongruity, this may lead to degenerative joint disease (DJD). Unlike bone, articular cartilage shows very limited capability of remodelling and repair after maturation. This indicates that the cartilage layer needs to develop fully and correctly during this phase. Osteochondrotic lesions may disappear due to remodelling in the juvenile horse, but lesions that are still present when the cartilage is fully matured will not repair and become a serious factor form the animal welfare point of view.
Physitis of the joint also occurs and can be described as a clinical symptom especially if swelling of the growth plates is associated with lameness; radiographs are taken to ensure that there is not a significant problem within the growth plate. Clinical signs may be divided broadly into two categories; those seen in foals <6 mo old and those seen in older animals. Often the first sign noted in foals is a tendency to spend more time lying down. This is accompanied frequently by joint swelling, stiffness, and difficulty keeping up with other animals in the paddock. An accompanying sign may be the development of upright conformation of the limbs, presumably as a result of rapid growth. Fetlock osteochondrosis is particularly seen in younger foals (<6 mo old). Lameness in older horses is not always apparent, but more than likely a horse will show signs of stiffness, reduced flexion and even muscular atrophy where movement had be compromised.
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