What is Osteochondrosis?
Osteochondrosis (OC) is a common developmental bone abnormality that can result in lameness in horses. In a growing horse, the end of the long bones start as cartilage and then converts into bone as the horse grows. OCs occurs when there is a disruption in the cartilage converting into bone, (endochondral ossification) most commonly near a joint surface, see figure 1. The cause of OC is thought to be due to lack of nutrition to the developing bone. Initially, nutrients are brought to the developing cartilage through small vessels, however, as the cartilage begins to turn to born, vessels diminish and the joint synovial fluid becomes the sole source of nutrition to the cartilage. This can lead to areas of retained cartilage which then die and split off. The term osteochondritis dissecans (OCD) is used when a flap of cartilage or a joint mouse is present. Alternatively, Osteochondrosis(OC) is used when there is a defect in the cartilage, but there is no free edge. The most common joints affected are in the stifles, hocks, and the fetlocks.
Figure 1: Disruption in endochondral ossification due to poor blood supply leads to defects in the subchondral bone and retention of cartilage. Adams Lameness 2011, pg 2633.
What factors contribute?
Many factors contribute to the cause of OC. Although, the exact inciting factor is unknown; genetics, rapid growth, conformation, trauma and dietary imbalances have all been implicated. Studies have shown animals fed a high energy diet and diets low in copper or high in phosphorus may predispose horses to development of OC. Thoroughbreds and Quarter Horses are overrepresented in breed prevalence; however, a variety of breeds can be affected. Horses tend to be around one year of age when clinical signs manifest, but OCs can manifest later in slow growing breeds such as Warmbloods.
Diagnosis is mostly based on presenting clinical signs, signalment and radiographic evidence. Presenting clinical signs often include acute onset of joint swelling that may correlate with a recent increase in level of exercise. Horses tend to have a very stiff gait which is exacerbated by flexion tests. Diagnosis can be confounded in that horses may show both radiographic and clinical signs, clinical signs without radiographic abnormalities (but arthroscopic abnormalities), and radiographic abnormalities with no clinical signs. Radiographic signs include areas in the bone that appear dark or borders that are irregular.
In mild cases, such as small lesions in very young horses, a more conservative approach can be taken. A reduction in feed intake is recommended to slow growth rate along with rest. Intra-articular corticosteroids and hyaluronic acid may be helpful as well in reducing swelling and associated synovitis. More often, surgical intervention via arthroscopy (see figure 3) is needed when lameness is a factor and the horse is over 8 months of age. The arthroscope is inserted into the joint and the cartilage flap or the bone fragment is removed. The bone underlying the defect is smoothed out to reveal healthy bone to ensure optimal healing.
Overall, the prognosis for a return athletic function is good post surgery, with more severe lesions and those in the shoulder joint having a worse prognosis. Success rates have ranged from 75 to 90% depending on the affected site.