Osteochondritis dissecans of the knee (OCD) is a condition in which cracks form in the articular cartilage (joint lining) because of a problem with the underlying bone. We don’t understand exactly why but the blood supply to the bone near the joint (subchondral bone) is interrupted. This loss of blood flow causes the subchondral bone to die in a process called avascular necrosis. The bone is then reabsorbed by the body leaving an empty space under the articular cartilage. The articular cartilage is not directly affected by the lack of blood supply because it gains it’s nutrition from the synovial fluid in the joint. Unfortunately the loss of structural support for the articular cartilage allows it to crack and fragment. This results in loss of the smooth joint lining surface and sometimes movement of these osteochondral (bone and cartilage) fragments within the joint.
There are two main types of OCD: the adult form, which occurs after the physis closes (growing art of the bone); and the juvenile form, which occurs in patients with an open epiphyseal plate (some people believe that the adult form is undiagnosed persistent juvenile OCD).
OCD predominantly affects adolescent and young adult patients. Many of these patients are involved in athletic persuits and this condition can have a dramatic impact on them. OCD can lead to pain, swelling, mechanical symptoms and inability to continue to play sports and is the most common cause of a loose body in the knee in adolescents. Clinical findings are often subtle so diagnosis requires a high index of suspicion and limited range of motion may be the only clinical sign. Most patients provide a history of trauma as their reason for their presentation but this is usually minor and probably not relevant. In some patients the affected femoral condyle is tender on palpation and the patient may walk with the leg externally rotated in order to avoid impingement of the lesion on the medial femoral condyle.
The diagnosis is made on xray but MRI has a key role in determining the stability of the lesion. Conservative management is the mainstay of treatment for stable lesions. The majority of patients respond to conservative treatment but those with unstable lesions require arthroscopic management. Unfortunately the affected knee may progress to degenerative arthritis while the patient is still young.
The etiology of OCD remains unknown, although several possibilities—including family history, repetitive micro-trauma, growth disorders, and ischaemia have been proposed. The true incidence is unknown but is higher in males than females (29 per 100,000 in males and 18 per 100,000 in females). OCD can involve other joints including the shoulder, elbow, hip, and ankle, but the knee is the most commonly affected. The natural history of OCD of the knee remains unclear and distinguishing between those lesions that may go on to heal and those that will not heal remains a challenge. High quality diagnostic, prognostic, and therapeutic studies that reported data separately for adults and children are rare. Many of the publications dealing with OCD of the knee are level IV evidence (case series).
The primary goals of treatment are to relieve pain, improve knee function, and prevent progression of the degenerative joint process. As with all surgical procedures the risks include infection, bleeding, dvt and persistent pain. The newer arthroscopic approaches have a relatively low risk compared to more invasive approaches but the more invasive treatments to salvage and/or reconstruct the cartilage and/or bone are still sometimes required. Nonsurgical treatment also presents challenges because it is difficult to predict which stable juvenile OCD lesions will heal.
Imaging
OCD is a radiologic diagnosis. If OCD of the knee is suspected, AP, lateral, notch-view (knee in flexion) and skyline patella xrays should be ordered. AP films alone may miss a lesion on the posterior aspect of the medial femoral condyle. If a lesion is seen the contralateral knee should also be xrayed. Plain films will detect a circumscribed area of necrosis but are a poor method of assessing articular cartilage and cannot be used to determine stability. If the xrays are normal the diagnosis is almost certainly not OCD.All OCD lesions seen on xray should be staged for stability with MRI. MRI has a 97 percent sensitivity for detecting unstable lesions. Other than arthroscopy, MRI is the most accurate method for staging lesions with Stages I and II being stable. Stages III and IV are unstable lesions with the cartilage breached and synovial fluid entering between the fragment and underlying bone.
On MRI, the presence of high-signal changes on T2 images signifies the presence of fluid between the fragment and intact bone. The overlying articular cartilage can still be intact in an unstable fragment. Distinguishing between stages II and III is important in planning for surgery. If the MRI demonstrates an unstable lesion (stage III or IV) then arthroscopy should be used to check the cartilage surface.
Conservative treatment of stable lesions is the general rule but there are no prospective randomized clinical trials proving that this is the right thing to do. The existing literature often groups studies of the adult and juvenile forms of OCD, as well as the variety of joints affected, making evidence-based conclusions difficult. Prognosis worsens with age and physis closure. Therefore, the goal of management of juvenile OCD is to promote resolution of the lesion before physis closure and unstable lesions are managed surgically. In the adult form, therapy is aimed at preserving function and preventing the development of early osteoarthritis.Factors such as location of the lesion, relationship to weight-bearing surface, stability, physis closure and clinical presentation should be considered. Approximately 50% of lesions resolve over a period of 10 to 18 months with conservative measures. Girls younger than 11 years of age and boys younger than 13 have an excellent chance of complete resolution. Patients over 20 years of age tend to have poorer outcomes and the likelihood of requiring surgery is increased. Unstable lesions (stages III and IV) in patients with a closed physis have a particularly poor prognosis and more aggressive intervention is indicated in older symptomatic patients.
Running and jumping sports should be avoided for six to eight weeks with the goal of activity modification being to allow symptom-free activities of daily living. Conditioning exercises and quadriceps strengthening may help and if the patient remains symptomatic a period of non weight bearing on crutches may be indicated. Immobilisation is no longer used as prolonged splinting leads to quadriceps atrophy and stiffness which may complicate the condition. Persistent symptoms in a compliant patient (or complaints of joint catching or grinding suggesting detachment and a loose body) are an indication for arthroscopy.
Depending on surgical findings, a loose body may be removed, a fragment excised, cartilage debrided or a lesion drilled to promote revascularization. Small fragments (<5mm) or multiple OCD defects are typically removed and the base of lesion drilled to create bleeding to encourage fibrocartilage formation. Larger fragments (>5mm) in weight bearing areas are reduced and fixed where possible. This is usually done with resorbable Kirschner wires or pins. MACI (cartilage) grafting is indicated for very large symptomatic lesions (2x2cm) and while it works very well to relieve day to day symptoms it rarely gets the patient back to competitive running sports. Following surgery, range-of-motion exercises should be initiated early. Quadriceps strengthening helps promote overall knee stability and wellbeing. Weight bearing is usually restricted for 6 weeks to allow healing of the bone. Patients should be examined at three-month intervals until symptoms resolve and imaging studies are indicated for evaluation of clinical deterioration only.
OCD is a relatively rare disorder but is an important cause of joint pain in active adolescents. Xray is a simple inexpensive test which provides the diagnosis and should be performed in any adolescent complaining of problems with their knee. MRI scanning and a surgical opinion should be sought for all patients with OCD lesions even though most will be treated conservatively.
KEY POINTS
Osteochondritis dissecans of the knee predominantly affects adolescent and young adult patients.Clinical findings are often subtle so diagnosis requires a high index of suspicion and limited range of motion may be the only clinical sign.
The diagnosis is made on xray but MRI has a key role in determining the stability of the lesion.
The majority of patients respond to conservative treatment but those with unstable lesions require arthroscopic management.
A surgical opinion should be sought for all patients with OCD lesions even though most will be treated conservatively.