Osteochondritis dissecans (OCD) develops when a small area of bone just beneath the joint cartilage loses its blood supply, weakens, and begins to separate from the surrounding bone. The exact trigger varies from person to person, but the condition results from a combination of repetitive joint stress, disrupted blood flow, and individual vulnerability factors like genetics and possibly low vitamin D levels. It most commonly affects active children and young adults, with the knee, elbow, and ankle being the primary sites.
How the Lesion Forms
The process starts beneath the smooth cartilage surface of a joint, in a layer called subchondral bone. Blood vessels that feed this bone become disrupted, cutting off oxygen and nutrients to a small patch of tissue. Without adequate blood flow, the bone cells in that area begin to die. The cartilage sitting on top, which depends on healthy bone underneath for structural support, starts to soften, crack, and erode.
As the damage progresses, the weakened bone loses mineral density. Repeated force through the joint, particularly shearing and compressive loads, gradually loosens the affected fragment. In early stages, the piece stays in place but becomes soft and unstable. In advanced cases, the fragment detaches completely and floats freely inside the joint, where it can catch between moving surfaces and cause locking, swelling, or sharp pain.
Repetitive Stress and Microtrauma
The most widely recognized trigger for OCD is repetitive mechanical loading of a joint beyond what the bone can repair between sessions. This is why the condition disproportionately affects young athletes. Continual stress disrupts the small blood vessels feeding the subchondral bone and interferes with the bone’s normal rebuilding cycle, essentially creating damage faster than the body can fix it.
The specific mechanism depends on the joint involved. In the elbow, which is the second most common site, OCD of the capitellum (the rounded knob on the outer side of the lower arm bone) appears to result from repetitive compression between the forearm bone and the capitellum. In baseball pitchers, the rapid extension of the elbow during a throw creates a shearing motion across this surface that doesn’t occur during slower, passive movement. In the knee, repetitive weight-bearing activities like running and jumping deliver axial loads that concentrate force on the inner part of the thigh bone’s rounded end.
High training volume, dominant-leg overuse, repetitive jumping or kicking, and a history of falls or direct trauma to the joint all increase risk. Sports that combine high repetition with joint-specific stress, such as baseball, gymnastics, and soccer, show up frequently in studies of OCD patients.
Disrupted Blood Supply
Not all OCD cases can be explained by overuse alone. Some researchers believe certain individuals have vulnerable blood supply patterns in their subchondral bone to begin with. The blood vessels that feed bone near the growth plate (the area where bones lengthen during childhood) are still developing in young people, and their arrangement may leave certain zones with relatively fragile circulation. A single traumatic event or moderate repetitive stress could be enough to shut off flow in these areas, triggering the cascade of bone death and cartilage breakdown.
This vascular vulnerability helps explain why OCD is largely a condition of adolescence. The risk rises sharply after age 12 compared to children between 6 and 11, with the increase being 3 times higher in the knee, 7 times higher in the ankle, and 22 times higher in the elbow in the older age group. The condition is rare before age 10 or after age 50.
Genetics and Cartilage Structure
A small subset of patients develops OCD in multiple joints, a pattern that points to an inherited cause rather than a sports-related one. Familial osteochondritis dissecans has been linked to mutations in the ACAN gene, which provides instructions for making aggrecan, a protein that serves as a key structural component of cartilage. When the gene is mutated, the aggrecan protein can’t attach properly to the other building blocks of cartilage, resulting in weaker tissue throughout the body.
This form follows an autosomal dominant inheritance pattern, meaning a child needs only one copy of the altered gene (from one parent) to develop the condition. People with familial OCD typically have lesions in several joints, most often the knees, elbows, hips, and ankles, rather than a single site tied to a specific sport or activity.
Vitamin D Deficiency
A striking finding from research on juvenile OCD patients is the near-universal presence of low vitamin D levels. In one study of 80 young patients diagnosed with the condition, 97.5% had vitamin D levels below the recommended threshold of 30 ng/mL. The average level was just 10.1 ng/mL, and over a third of patients had levels below 10 ng/mL, which qualifies as severe deficiency. These patients had significantly lower vitamin D levels than a control group without OCD.
Vitamin D plays a central role in bone mineralization and repair. When levels are chronically low, bone is less able to rebuild itself after the normal microdamage that occurs with physical activity. This doesn’t mean low vitamin D alone causes OCD, but it may act as an important contributing factor, making subchondral bone more vulnerable to the repetitive stress that initiates the disease process.
Why Age Matters So Much
The distinction between juvenile and adult OCD is more than a label. It reflects a fundamental difference in the bone’s ability to heal. Children and adolescents whose growth plates are still open have actively growing, well-vascularized bone with strong regenerative potential. Many juvenile OCD lesions heal with activity restriction alone, without surgery, because the developing bone can restore blood flow and rebuild the damaged area.
Once the growth plates close at skeletal maturity, this healing advantage largely disappears. Adult OCD, which affects roughly 3.4 per 100,000 people per year, usually requires surgical intervention because the bone can no longer repair itself as effectively. This is one reason early detection in adolescents matters: a lesion caught while the growth plates are open has a much better chance of resolving on its own.
How Lesions Progress
OCD lesions are classified into four stages based on stability and cartilage integrity:
- Stage I: The bone underneath is softened, but the cartilage surface remains intact and the fragment is stable.
- Stage II: The cartilage shows partial cracking or discontinuity, but the fragment is still stable when examined.
- Stage III: The fragment has fully separated from the surrounding bone but hasn’t moved from its original position. It is unstable.
- Stage IV: The fragment has displaced or broken loose entirely, leaving an empty crater in the joint surface.
Understanding this progression matters because the earlier a lesion is caught, the more treatment options are available and the better the long-term outlook for the joint. Stage I and II lesions in young patients can often heal with rest and activity modification. By stage IV, the loose fragment can damage other cartilage surfaces inside the joint, and the risk of developing early osteoarthritis increases substantially.
Putting the Causes Together
OCD is best understood as a condition with multiple contributing factors rather than a single cause. The typical scenario involves a young, active person whose subchondral bone sustains repetitive stress during a period when the blood supply to that area is naturally vulnerable. If that person also has low vitamin D levels, a genetic predisposition to weaker cartilage, or biomechanical issues that concentrate force unevenly across the joint, the risk climbs further.
This is why prevention efforts focus on managing training loads in young athletes, teaching proper sport-specific mechanics (like throwing technique in baseball or landing form in gymnastics), and ensuring adequate nutrition for bone health. Reducing the mechanical insult to the joint and supporting the bone’s ability to repair itself addresses the two sides of the equation simultaneously.