Osteochondrosis is one of the most important and prevalent developmental orthopedic diseases of horses. Although its specific etiology is not known, it is considered to arise from a focal disturbance in endochondral ossification. The term osteochondrosis is currently used to describe the clinical manifestation of the disorder; however, the term dyschondroplasia is preferred when referring to early lesions because primary lesions are seen in cartilage.

Osteochondrosis has a multifactorial etiology that includes rapid growth, overnutrition, mineral imbalance, and biomechanics (ie, trauma to cartilage). Genetics has been implicated in some breeds (eg, Standardbred and Swedish Warmblood). The condition mainly affects articular growth cartilage, but the metaphysis may also be involved. If the physeal metaphyseal cartilage is affected, bone contours and longitudinal growth are disturbed (see Lameness in Horses: Physitis in Horses). Involvement of articular cartilage at the periphery of joint surfaces leads to regressive changes at the joint margins, dissecting lesions, and the formation of cartilage flaps or osteochondral fragments (osteochondrosis). Central articular lesions, because of weight-bearing effects, may lead to the development of subchondral cysts (see Arthropathies in Large Animals: Subchondral Cystic Lesions). Axial skeletal involvement includes vertebral articular facets, which may be associated with stenosis of the vertebral canal and, therefore, ataxia and proprioceptive deficits (ie, wobbler syndrome), but the relationship between these conditions is not clear.

Clinical Findings

The clinical signs of equine osteochondrosis are difficult to characterize specifically because of the wide range of lesions and sites involved. Many cases have no detectable clinical signs and are identified on presale radiographs. Furthermore, lesions of dyschondroplasia do not always progress to osteochondrosis and produce clinical signs. In severe cases, other signs of developmental orthopedic disease also may be apparent.

The most common presenting sign of osteochondrosis is a nonpainful distention of an affected joint (eg, gonitis, bog spavin). The exceptions to this are joints in which swelling is difficult to detect (eg, shoulder joint, medial femorotibial joint) and in which lameness is more often the first sign observed. 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 is usually absent or mild except for those sites mentioned above for which the earlier sign of joint swelling is difficult to detect. For example, lesions in the shoulder frequently result in moderate to severe lameness, muscle atrophy, and pain on joint flexion. In the stifle, some horses with subchondral bone cysts in the medial femoral condyle present with lameness severe enough that a fracture may be suspected and swelling may only be detected on careful examination. More severe signs are observed when osteochondral fragments come loose within the joint. This is often seen in yearlings or older horses that present with stiffness, flexion responses, and varying degrees of lameness. These signs are usually associated with the onset of training.

Diagnosis

Clinical diagnosis can often be made on the basis of signalment and signs. More definitive diagnosis requires the use of some specific clinical aids. Radiographic examination has been the traditional method of confirming diagnosis; however, early lesions involving cartilage without significant subchondral bone damage will not be visualized. In the distal limb, oblique views may be helpful; in the hock, because the most common site of a lesion is the distal intermediate ridge of the tibia, the best view is a plantarolateral/dorsomedial oblique. Ultrasonographic examination of the swollen joints can help delineate articular damage and synovial inflammation and determine if osteochondral fragments are intra- or extra-articular. The most accurate way to confirm diagnosis is by arthroscopy, and most of the predilection sites are accessible except for the cervical articulations.

Osteochondrosis

Scintigraphy has limitations in growing horses due to normal high activity in physes and sites of active endochondral ossification. It is a useful technique for detecting subchondral cysts and secondary degenerative changes in older horses. MRI is ideal for diagnosis of both early and late lesions but is not widely available. Also, sites that are most diagnostically challenging are generally in the proximal limb where access is difficult. Clinical pathology and the evaluation of synovial fluid is rarely helpful but can be used to eliminate inflammatory causes of swollen joints.

Treatment and Management

Management of osteochondrosis depends on the site and severity of signs. Mild cases recover spontaneously, and a conservative approach may be appropriate. In young animals (<12 mo old), this involves restricted exercise for some weeks combined with a reduction in feed intake to slow the growth rate. Particular care should be taken to ensure appropriate mineral supplementation (eg, suspected copper deficiency). It is controversial whether correcting the diet, once signs have developed, will actually assist resolution, but it may help limit or prevent further cases on stud farms. Intra-articular medication with hyaluronic acid may be beneficial, and injection of long-acting corticosteroids may help reduce swelling and improve any associated synovitis but is not recommended in young, growing horses.

Those cases considered for surgery are mainly treated arthroscopically. This technique has been successful in most affected sites, particularly the hock, stifle, and fetlock. Damaged cartilage, osteochondral fragments, and compromised subchondral bone are removed and the joint flushed extensively with sterile fluid. Prognosis following removal of discrete osteochondral fragments is good. In cases with more extensive osteochondral damage, the prognosis depends on the extent of the joint surface that must be removed. The prognosis is poor for cases with instability resulting from joint surface loss or in which secondary osteo-arthritis (degenerative joint disease) is advanced. This is often the case with shoulder osteochondrosis due to the difficulty in detecting early signs. Cases involving subchondral cysts have a guarded prognosis as they are often in important weightbearing areas of the joint and reconstruction of the joint surface is rarely possible.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVS, MRCVS; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD; Jean-Marie Denoix, DVM, PhD, Agregé; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS