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Developmental Orthopedic Disease in HorsesBy Kentucky Equine Research Staff · July 15, 2013

The term developmental orthopedic disease (DOD) encompasses all general growth disturbances and orthopedic problems seen in the growing foal as well as some found in older horses. This nonspecific term is an improvement over previous descriptions of the various limb anomalies of young horses because earlier terms implied that they all had a common cause and pathogenesis (mechanism of disease development). It is still to be determined how closely related the various forms of DOD may be, but it is important that the term not be used synonymously with osteochondrosis. It is now considered inappropriate for all these conditions to be presumed to be manifestations of osteochondrosis, a condition that is defined below.

The conditions currently classified as DOD were previously designated as metabolic bone disease. However, it is felt that this term is misleading because it refers specifically to bone, whereas many of these problems are seen essentially as joint and growth plate problems. Severe forms of DOD are seen sometimes when there is little or no aberration in bone histomorphometry, implying questionable change in bone metabolism. DOD has become the generally accepted term. When the term was first coined, it was categorized to include a number of conditions and problems including osteochondrosis, acquired angular limb deformities, physitis, subchondral cystic lesions, flexural deformities (may be secondary to osteochondrosis or physitis), cuboidal bone abnormalities, and juvenile osteoarthritis. Among these, individual instances may or may not be associated with osteochondrosis.

Osteochondrosis is a defect in endochondral ossification that can result in a number of different manifestations, depending on the site of the endochondral ossification defect. These manifestations include osteochondritis dissecans (OCD), which is a disease process of the articular surface of joints that results in bone chips. As mentioned previously, the epiphyseal ossification center advances out until ossification ceases, 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 OCD.

Another manifestation of osteochondrosis may include some subchondral cystic lesions. These are radiolucent cyst-like structures that occur in epiphyseal bone. Not all subchondral cystic lesions or bone cysts are necessarily the result of osteochondrosis. Recently in a project funded by the American Quarter Horse Association, it was shown that subchondral cystic lesions can develop from defects in the joint surface and in mature animals. However, there is no question that some subchondral bone cysts occur as a result of defective endochondral ossification. Osteochondrosis can also result in lesions in the physis or metaphyseal growth plate. Horses with cervical vertebral malformations (wobblers) are also included. The exact role of osteochondrosis and the pathogenesis of cervical vertebral malformation are still uncertain. In some instances, angular limb deformities will be associated with retained cartilage associated with the physis and this would be evident on radiographs. Such cases are a small minority of angular limb deformities.

There has been a problem or a point of confusion between reports of clinical instances of DOD and radiographic surveys of horses not necessarily showing clinical signs. This has led to some questionable conclusions, not only regarding the causative factors of DOD but also the effect of treatment or the significance of various problems. There are many instances where radiographs on a prepurchase examination will show OCD, for instance, but that OCD may not be causing clinical problems because the fragment has not separated. That is why it is important to distinguish between the two. Most of the major radiographic studies have involved Standardbred horses. In one study in Quebec, each horse’s racing performance at two years of age was related to the radiographic lesions diagnosed at approximately 17 months of age and before training (there were two generations of 41 and 32 yearlings, respectively; five stallions and 46 mares were also radiographed). No complete clinical examinations or lameness diagnoses were made. Radiographic lesions were found in 31 (25%) of the horses (8 adults and 23 yearlings), of which 60% had a single problem, and 40% had between two and four radiographic problems. Subchondral bone cysts were detected in 14 (11.3%) horses (6 carpus, 5 fetlock, 4 pastern, 2 hock, and 2 stifle).

Juvenile osteoarthritis lesions were diagnosed 78 times in 35 (47.9%) of the yearlings (40 pastern, 13 fetlock, 11 carpus, 8 coffin, 6 hock) based primarily on the basis of osteophytes. Sesamoiditis was also diagnosed in yearlings. The average winnings and number of starts were compared between radiographically normal horses, the OCD or subchondral bone cyst horses, and the juvenile OA horses; no significant differences were found. Although the radiographic lesions did not seem to be associated with poor racing performance, the authors of the study noted the lack of clinical data and the relatively small numbers. Improved studies are needed in which clinical signs are correlated with  radiographs and, possibly more important, all horses are radiographed and then followed to document how many of those horses with x-ray changes develop clinical problems. This would solve many of the current issues at the yearling sales.

The high incidence of clinically apparent physitis and flexural deformities was emphasized in another study in Canada. Mild to moderate physitis and flexural deformities (concurrent with physitis in most cases) occurred in 88% of 42 weanlings between weeks 6 and 8 of a study looking at the effect of dietary energy and phosphorus on blood chemistry and development of growing horses. In these instances, the clinical signs largely resolved on their own by five weeks. Dietary treatment did not influence the incidence, nor was it related to daily weight gain. Another study defined incidence of DOD in Thoroughbred horses in Ireland over a period of 18 months. It was found that angular limb deformities and physitis together constituted 72.9% of the cases treated. The peak incidence of DOD problems occurred between weaning and the end of December. In a retrospective study, 193 of 1,711 (11.3%) were treated for DOD (21 had more than one type) and are detailed as follows: 92 angular limb deformities, 64 physitis, 18 flexural deformities, 7 wobblers, and 28 osteochondritis dissecans, juvenile arthritis or other joint problems.. More than half the animals treated (53.9%) recovered completely, achieving their expected sale value as yearlings; 27.5% showed incomplete recovery and mild to moderate loss of sale value; and 18.7% were either euthanized or lost much of their sale value. It was also noted that 67.7% of the animals showed some evidence of DOD, but only 11.3% were deemed to need treatment. These studies show that when DOD occurs, it commonly involves angular limb deformities and physitis problems that spontaneously self-correct. It is the ones that often do not self-correct, such as subchondral bone cysts, where continuing research is needed to reveal ways to avoid or treat these problems in bone development.