Equine joint disorders
Osteochondrosis dissecans (OCD) refers to joint lesions that occur when the cartilage becomes separated from the underlying bone. Further separation continues until pieces of cartilage or cartilage+bone are floating free in the joint. Common sites of OCD in the horse include the lateral trochlear ridge of the distal femur, cranial intermediate ridge of the distal tibia (DIRT lesions), lateral trochlea of the talus, and caudal aspect of the humeral head.
Subchondral bone cysts are another form of osteochondrosis. However, instead of the overlying articular cartilage becoming detached from the subchondral bone, the cartilage becomes infolded. Cysts may also occur secondary to trauma. Common sites for developmental cysts include the medial femoral condyle, glenoid fossa, and distal cannon bone.
Histologically, osteochondrosis is characterized by abnormal chondrocyte differentiation and formation of defective intracellular matrix. The process of ossification is interrupted and abnormally thick cartilage develops. Associated poor vascular supply to the thickened cartilage results in inadequate chondrocyte nutrition. The abnormal cartilage is an area of focal weakness and can separate or infold. This is referred to as abnormal endochondral ossification.
Several factors have been proposed as the inciting factor for osteochondrosis. These include genetic predisposition, rapid growth, nutritional mismanagement, and trauma. None of these factors can be identified as most important.
Osteochondrosis often affects rapidly growing animals; however, it appears to be more closely related to a high energy diet than to rapid growth. Controlled diet studies strongly implicate excess digestible energy (often excessive carbohydrates) as a factor in the development of osteochondrosis. Excess protein did not seem to make a difference. Diets high in calcium and phosphorus may induce osteochondrosis, as can abnormal copper (low) and zinc (high) levels. In another study, mares with wobbler syndrome (probably cervical OCD) were bred to similarly affected stallions. Offspring were not wobblers but had a high incidence of developmental orthopedic disease. In Standardbreds, tarsocrural OCD has a heritability rate of 0.52 (high). Finally, trauma may precipitate disease when biomechanical forces act on structurally abnormal cartilage. This correlates with the identification of clinical signs in horses just starting training.
Older horses can develop osteochondral cysts and fragments due to wear and tear on joints and related changes in bone and cartilage structure.
Osteochondrosis is most commonly diagnosed in young horses being put into work. This timing is more likely related to closer examination of the horse than to development at this stage of life. Joint effusion is the most common presenting complaint. Lameness may or may not be apparent but is more typically seen with cysts due to the weight bearing locations. Diagnosis is confirmed by radiographs. As this is a developmental disorder, both joints (eg both stifles or both hocks) should be radiographed as should any other joints (particularly fetlocks) with effusion. Radiographs may not show lesions in mild cases; ultrasound and/or arthroscopy may be needed to fully evaluate the joints.
Nonsurgical treatment includes rest, controlled exercise, and minimizing joint inflammation. It may be successful depending upon the site of the lesion and the intended use of the horse. As fragments can lead to osteoarthritis due to synovial damage, surgical removal is recommended. Surgical treatment has become the treatment of choice and involves removing loose pieces of cartilage and debriding defective subchondral bone. Cysts may be injected with steroids using ultrasonographic guidance. Prognosis is favorable for hock and stifle joints, depending upon the amount of cartilage damage at the time of surgery (up to 70-80% success rates). Prognosis is less favorable for shoulder, pastern, and fetlock joint lesions. These areas tend to develop progressive osteoarthritis.
Osteochondrosis is abnormal cartilage development (failure of endochondral ossification) leading to osteochondral fragments (joint mice) or cysts. Fragments occur in non-weight bearing areas but lead to local inflammation and joint changes. Cysts occur in weight bearing areas. The primary presenting complaint is often synovial effusion (fluid in the joint). Lameness is more likely to be associated with cysts; many horse with osteochondral fragments have joint effusion but are not lame. Lesions are frequently bilateral.
Pathogenesis of osteochondrosis dissecans: How does this translate to management of the clinical case? Equine vet. Educ. (2016) 28 (3) 155-166
Workshop Report. Third International Workshop on Equine Osteochondrosis, Stockholm, 29–30th May 2008, Equine vet. J. (2009) 41 (5) 504-507