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OSTEOCHONDRITIS DISSECANS (OCD)
Overview
Osteochondritis dissecans (OCD) is a relatively common developmental disease that affects horses of all types. Although the exact cause of OCD and the steps leading up to it are not definitively known, there is a reasonable consensus about the general cause. In order to understand how OCD occurs, one must first understand how joints form. Differentiation of bones and joints occurs through a process known as endochondral ossification. Endochondral ossification is responsible for the formation of normal bones as well as normal articular cartilage, the smooth covering at the ends of bones which allows joints to glide smoothly. Normal pain-free joint function depends on a smooth cartilage surface of consistent thickness with a strong supporting plate of bone underneath, known as the subchondral bone plate. Failure of the developmental process leads to a disturbance in the formation of joint cartilage and the bone supporting it (Figures 1a-d). The resultant cartilage and subchondral bone is irregular in thickness and weaker than in normally developing areas. There may be a loss of blood supply and necrosis of the abnormal tissue. Forces placed on these abnormal areas when the horse ambulates may cause further damage to the abnormal tissue. This can lead to several problems which are recognized as OCD, including local detachment and fracture of cartilage and bone. These detached areas may remain partially attached to the surrounding tissue, which then exist as loose flaps of tissue, or may become completely detached, resulting in free-floating islands which can travel throughout the joint. These loose flaps and islands of abnormal cartilage and bone can cause inflammation in the joint, and over time may cause damage to the normal cartilage.
Figure 1a). Normal development results in a bone (shown in blue) with a subchondral bone plate to which a smooth cartilage cap is attached (shown in yellow).
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Figure 1b). Abnormal development due to a defect in endochondral ossification results in an area of thickened and/or weakened abnormal bone and cartilage (shown in green).
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Figure 1c). Trauma or exercise can further damage the abnormal area as the horse flexes and extends the joint.
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Figure 1d). Separation of the abnormal bone and cartilage from the underlying and surrounding tissue results in an OCD fragment, which can form a flap or can detach and float as a free body within the joint.
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Causes and Risk Factors
Several causes of OCD are known, although the disease is generally considered to be multifactorial. As a result, the disease is not usually caused by any one factor, but rather a combination of several factors acting together. These known factors include:
Rapid growth and large body size: An unusually rapid phase of growth and/or growth to a large size can be associated with OCD formation.
Nutrition: Diets that are very high in energy or have an imbalance in trace minerals, low-copper diets in particular, can lead to OCD formation.
Genetics: Risk of OCD may also be partially inherited, although the mode of inheritance is not well defined and other factors are often required before an OCD fragment forms. Genetics can also be responsible for the horse’s response to the other factors presented here, as well as rate of body growth.
Hormonal imbalances: Imbalance in certain hormones during development, including insulin and thyroid hormones, can encourage OCD formation.
Trauma and exercise: Trauma to a joint, including routine exercise, is often involved in formation and loosening of the OCD flap or fragment.
Incidence and Prevalence
Since all these factors are involved in a complex series of interactions it is not possible to predict which horses will develop OCD, and is therefore difficult to prevent the formation of OCD in individual animals. Clinical prevalence of OCD is usually between 5 and 25% in a given horse population, but radiographic signs of abnormal development can be as high as 60% in certain groups (Wittwer et al. J Vet Med A Physiol Pathol Clin Med 2006).
Signs and Symptoms
OCD may be detected as early as 5 months of age, or as late as skeletal maturity (approximately 3 years of age). However, in young horses, it is recommended that the final diagnosis should be made when the horse is about 1 year of age. The most common sign of OCD is effusion (swelling) in the joint of a young horse (Figure 2). Often, the joint swelling is first noticed shortly after the horse begins a formal training and exercise program. Depending on the location and severity of the OCD, the horse may be noticeably lame on the leg, may only be lame during high-speed work, or may not have detectable lameness at all. OCD can occur in virtually all joints, however there are several joints which are affected much more commonly. The most commonly involved joints are the hock, the stifle, and the fetlock. The shoulder can also be involved, although this is seen with less frequency in horses.
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Figure 2. Both hocks in this picture have severe effusion, as shown by the red arrows. Notice how the joint bulges outward. Most horses with OCD will have a milder degree of swelling.
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When to Seek Veterinary Surgical Advice
Any horse with a swollen or effusive joint should be examined by a veterinarian in order to properly diagnose OCD and to rule out other causes of effusion. Lameness, if present, is usually mild. Horses with severe lameness and joint swelling probably have a more serious problem and should be examined on an emergency basis. Diagnosis of OCD involves careful examination of the joints, may involve a lameness examination, and radiographs. Since OCD can occur in many places within the joint several different radiographic views are usually taken from different angles. Since OCD is often bilateral, radiographs of the opposite joint may be indicated, even if there is little or no swelling in that joint. OCD fragments will usually show up on the radiograph as a piece of bone that seems to “float” within a defect in the main bone (Figure 3). Occasionally an OCD fragment is made entirely of cartilage and cannot be seen on the radiograph; only the defect in the main bone may be visible in these cases. Free-floating fragments of bone may also be seen on occasion. It is possible for older horses to be diagnosed with OCD incidentally without apparent clinical signs. These fragments are usually found when radiographs of the joint are taken for another reason, such as a prepurchase examination. In these cases a veterinary surgeon can make recommendations about whether treatment is indicated. Find an ACVS Veterinary Surgeon.
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Figure 3. This is an example of how an OCD fragment typically looks on a radiograph. The OCD fragment is circled in red. Notice that the fragment seems to “float” within a defect in the main bone.
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Prevention
Prevention of OCD can be difficult due to the multifactorial nature of the disease. Owners should try to develop a balanced feeding program with a goal of providing proper trace mineral amounts and preventing excessive growth rates. Consultation with a veterinarian and laboratory feed evaluation may be necessary. Although understanding of the role of genetics in the development of OCD is still incomplete, more information concerning this factor will likely be forthcoming in the coming years.
Treatment
OCD is best treated by surgical removal of the abnormal bone and cartilage in most cases. A veterinary surgeon can make specific recommendations about treatment. The most common technique used for surgical removal of OCD fragments is arthroscopy (Figure 4). Arthroscopic removal is usually preferable to arthrotomy (an open joint approach), since skin incisions are small, less surgical trauma is caused, post-surgical recovery time may be reduced, and a more thorough exploration of the joint can be performed at the time of surgery. Arthroscopic OCD fragment removal usually requires general anesthesia. The surgeon makes two or more small incisions into the joint through which an arthroscope is placed to visualize the joint and instruments are introduced to remove the OCD fragments.
Figure 4. This is an arthroscopic view of an OCD fragment in a joint. A blunt probe is being used to palpate the fragment. Notice how the fragment is slightly raised above the surrounding cartilage and there is a clear line of separation between the fragment and the surrounding tissue.
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Aftercare
Surgical aftercare recommendations depend on the location and severity of the OCD but typically involve a period of rest followed by progressive exercise. Full return to training may require several months. Postoperative bandaging will be required for some OCD locations. Medication may be prescribed, including anti-inflammatory medications. A recheck examination and suture removal may also be required. It is important to understand the aftercare requirements before committing to surgery. Specific recommendations are made by the surgeon for each case.
Prognosis
Following proper treatment, prognosis for athletic function is good to excellent for many types of OCD. Some OCD locations, such as the shoulder, may have a reduced prognosis. It is important to discuss the expected outcome, including appearance of the operated joint, with the surgeon during treatment selection.
—Christopher R. Byron, DVM
Diplomate ACVS
Posted 4/02/2005
Updated 6/17/2008 by Dr. Byron
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