Osteochondritis Dissecans of the Knee
Brian M. Ralston, MD; James S. Williams, MD; Bernard R. Bach, Jr, MD; Charles A. Bush-Joseph, MD; William D. Knopp, MDTHE PHYSICIAN AND SPORTSMEDICINE - VOL 24 - NO. 6 - JUNE 96
In Brief: Osteochondritis dissecans is a disorder in which a fragment of cartilage and subchondral bone separates from an articular surface. The etiology is uncertain, although trauma and ischemia have been implicated. The knee is most commonly affected, but the elbow and ankle may also be involved. Patients typically present during their adolescent or early adult years with nonspecific knee pain and swelling that worsens with activity. The diagnosis is confirmed by radiographic findings. Management decisions are based on the patient's age and the stability, location, and size of the lesion.
Osteochondritis dissecans (OCD)—a painful fragmentation of an articular surface—most commonly affects the knee joint; other less common sites include the capitellum of the elbow and the talar dome of the ankle (1). Although lesions in the shoulder, hand, wrist, and hip joints have also been reported, such lesions are rare (1,2). Within the knee, OCD lesions occur at the medial femoral condyle (80% to 85% of cases), the lateral femoral condyle (10% to 15% of cases), and the patella (5% of cases)(figure 1) (1). Within the medial femoral condyle, the lesion is most commonly observed on the lateral, non-weight-bearing surface.
The reported prevalence of OCD is 30 to 60 cases per 100,000 people (1). Patients usually present in their teenage years (those who have OCD of the patella usually present in their 20s and 30s), but the disorder may manifest later in life. It has been estimated that 4% of all cases of osteoarthritis of the knee diagnosed in men were the direct result of OCD (3). However, the lesion is not always symptomatic and is sometimes an incidental radiographic finding. Bilateral disease is present in 30% to 40% of patients (1,4,5,6). Males are affected three times more often than females (5,6).
A Question of Stability
The OCD fragment consists of intact articular cartilage attached to a piece of subchondral bone of variable thickness. The fragment may be in its normal anatomic location with a smooth articular surface (stable), or the lesion may become loosely attached to its bony base or may detach to become a loose body within the joint space (unstable).
The cartilage remains viable even if the fragment is a loose body, since cartilage is nourished by synovial fluid rather than direct blood supply. However, repetitive trauma and the loss of mechanical support may cause the cartilage to undergo softening and degenerative changes (5).
The bony part of the fragment is largely avascular. It is usually covered with a fibrocartilaginous scar, similar to that seen in a fracture nonunion, which reflects the chronic nature of the problem (2,6). By contrast, the bone of the underlying femoral base from which the fragment separates has normal vascularity. This is a distinguishing characteristic between OCD and osteonecrosis, in which the underlying bone is avascular (3).
There is no histologic evidence that OCD is an inflammatory process, thus "osteochondritis" is a misnomer. The process is more typical of degeneration, and might be more appropriately termed "osteochondrosis."
Although OCD has been well described, the cause of the disorder remains uncertain. Many etiologies have been proposed, including trauma, ischemia, and additional factors that might predispose patients to the disorder (1-4,6-9).
A traumatic origin is supported by multiple studies (1,3,5,6,9). Approximately 40% of patients presenting with OCD of the knee have a history of major or repetitive knee trauma, and 60% of patients presenting with OCD participate in a high level of athletic activity (3).
Certain aspects of normal knee anatomy may be a causal factor in OCD. The medial femoral condyle lies close to the medial tibial spine, and bears a broad attachment of the posterior cruciate ligament. Repetitive shear stresses from the tibial spine during activity and traction from the posterior cruciate ligament may account for the frequency of lesions over the lateral aspect of the medial femoral condyle (1,2,4,6,9).
Trauma may also be exacerbated by underlying knee abnormalities, including biomechanical malalignment and internal derangement. For example, OCD has been associated with genu valgum, genu varum, and meniscus tears (3,4,8,9). Discoid menisci have been noted in 20% of patients with OCD of the lateral femoral condyle. Many patients who have OCD of the patella have a history of patella subluxation, suggesting an association with ligament laxity and trauma (4).
While trauma may be the starting point for the development of OCD, it is likely that vascular insufficiency ultimately leads to fragment separation. In rapidly growing bone, the blood supply to the epiphysis and secondary centers of ossification can be tenuous (1,6), and a single traumatic event or repetitive microtrauma may interrupt the vascular supply.
The high incidence of multiple and bilateral lesions is difficult to explain by trauma and ischemia alone, and suggests that there are additional factors that predispose some persons to develop OCD (1,5,9). Various theories have been proposed, including genetic or endocrine factors, generalized ligamentous laxity, and abnormalities of secondary ossification centers (1,2,3,9). It is likely that the etiology of OCD is multifactorial.
Patients with OCD of the knee typically present with poorly localized, aching knee pain and swelling. Symptoms of knee locking or giving way may develop as the disease progresses. The pain is exacerbated by strenuous activity and twisting motions, especially internal rotation of the tibia, which causes the medial tibial spine to strike the lateral aspect of the medial femoral condyle (the site of most OCD lesions). As a result, patients may walk with the affected leg externally rotated (3). Many patients with lesions of the lateral femoral condyle feel a painful "clunk" with knee flexion and extension (7).
On physical exam, the affected knee typically maintains full range of motion, unless a loose body causes mechanical locking. Thigh circumference may be diminished on the affected side because of disuse atrophy. A joint effusion may be noted, especially if the patient has been active (6). With the knee flexed, diffuse tenderness may be elicited over the involved femoral condyle. However, this finding is nonspecific and often seen in patients with patellofemoral disease.
Wilson (10) described a useful diagnostic test for OCD of the medial femoral condyle (figure 2). When the knee is extended, the medial tibial spine rubs against the medial femoral condyle. Patients who have OCD experience pain at approximately 30° of flexion. External rotation of the tibia relieves the discomfort.
Patients who have OCD of the patella usually present with retropatellar pain and crepitus (3). Physical examination findings are nonspecific and may be similar to the findings in other causes of anterior knee pain, such as patellofemoral syndrome. Fragments generally remain stable, and they rarely become loose bodies (3). Although OCD of the patella is uncommon, it should be considered in young patients who have refractory patellofemoral symptoms.
While OCD can be suspected clinically, it is confirmed by radiographic findings. Plain radiographs are usually adequate to visualize the lesion and should include anteroposterior (AP), lateral, axial (either sunrise or Merchant), and tunnel views (figure 3: not shown). The tunnel view is particularly useful in visualizing lesions that may not be easily seen on a standard AP radiograph (7). The classic finding is a radiolucent semilunar line outlining an oval shaped fragment of bone. Sclerosis of the underlying femoral base correlates with an unstable fragment and a poor prognosis for spontaneous healing (6).
Plain radiographs reveal differences between medial and lateral femoral condyle lesions that correlate with symptoms and prognosis. The tunnel view demonstrates that typical medial femoral condyle lesions are anterior and face obliquely away from the tibiofemoral articular surface; therefore, they are less likely than lateral lesions to cause degenerative changes. They may involve the patellofemoral joint and cause symptoms such as pain and crepitus. On lateral radiographs, medial femoral condyle lesions are usually located within an area bounded by a Blumensaat's line (figure 3c: not shown) (6).
By contrast, lesions of the lateral femoral condyle (figure 4: not shown) are usually larger, more posterior, and involve more of the weight-bearing surface of the tibiofemoral joint (6). Many patients feel a painful "clunk" with knee flexion and extension (7). On lateral radiographs, lateral femoral condyle lesions are usually located posterior to a line extending from the posterior femoral cortex (figure 4: not shown) (7).
The diagnosis of OCD of the patella is also made radiographically. The typical lesion is located at the inferior aspect of the underside of the patella (figure 1c).
Other radiographic techniques that have been used to diagnose and characterize OCD include arthrography, computed tomography (CT), magnetic resonance imaging (MRI), and bone scan. Arthrograms have been proposed as a means of demonstrating loss of continuity of the articular cartilage, but in the absence of fragment separation, they are of little value (6). CT scans can provide excellent bony detail, but cannot be used to assess fragment separation (3). Thus, arthrography or CT alone provide insufficient information for the diagnosis and management of OCD.
MRI is useful in OCD management decisions because it provides excellent imaging of the articular surface, helps determine the size and viability of the subchondral bone fragment, and reveals information about fragment stability (6,11). In T2-weighted images, a bright signal between the fragment and the subchondral bone is consistent with synovial fluid. This finding indicates that the fragment is unstable and will require surgical treatment (11). MRI is also useful for evaluating the progress of fragment healing and revascularization (1,11).
Bone scan has been used as a prognostic indicator (12). There is a close correlation between radionucleotide uptake and healing potential. Increased uptake reflects increased blood flow to the area and is associated with a better prognosis. By contrast, a normal scan in the presence of abnormal radiographs predicts diminished healing potential. If serial bone scans demonstrate stable or decreasing radionucleotide uptake in the presence of persistently abnormal roentgenograms, the lesion is unlikely to heal with conservative management. Bone scans are particularly useful in determining the need for operative intervention if fragments are large and nondisplaced (12).
Once the diagnosis of OCD has been made, the goals of treatment are to reduce pain, restore the continuity of the articular surface, and decrease the likelihood of future degenerative joint disease (1,4,6,7). Patients with lesions of the lateral femoral condyle should be referred to an orthopedic surgeon because these lesions are less stable, are difficult to manage, and often require surgical treatment. Management options for lesions of the medial femoral condyle depend on the patient's skeletal maturity and the stability, location, and size of the lesion.
Except in the case of patellar OCD, for which prognosis is not clearly related to skeletal maturity (6), younger patients have a better prognosis than older patients. In 120211, Pappas (2) developed a classification scheme for OCD that was based on outcome. Category 1 includes children up to the age of early adolescence (girls 11 and younger and boys 13 and younger). Patients in this group have open physes and have an excellent prognosis. Category 2 comprises patients near skeletal maturity (females 12 to 20 and males 14 to 20). This group has an intermediate prognosis. Category 3 includes all patients over age 20. These patients have the poorest prognosis and a tendency toward fragment instability, loose body formation, and degenerative joint disease (1,2,3,6,8).
To simplify management decisions, patients may be divided into two treatment groups. "Juvenile/young adult" patients include Pappas categories 1 and 2 (20 years or younger); "adult" patients represent Pappas category 3 (older than 20 years). An algorithm for the diagnosis and management of OCD is presented in figure 5 (not shown).
Conservative treatment. Nonsurgical management is indicated for a juvenile/young adult patient with a lesion in the medial femoral condyle and no evidence of fragment instability on plain radiographs. The basis of conservative treatment is modification of activity to promote bone healing at the site of fragment separation. An initial 1 to 2 week period of immobilization and minimal weight-bearing is helpful to control pain and initiate healing. However, prolonged immobilization and casting should be avoided because joint motion is important for articular cartilage nutrition and healing (3,6).
Activities should be modified for 6 to 12 weeks (1,3). Younger patients generally require a shorter period of activity modification than older patients. In general, rapid or strenuous movement of the lower extremities should be avoided, especially high-impact activities such as running, cutting, and jumping. Activities of daily living and upper body exercise are permitted.
After the period of activity modification, decisions regarding a patient's return to normal physical activity should be made on an individual basis, based on symptoms. Low-impact exercises such as bicycling or swimming are usually well tolerated and may be recommended initially. This is followed by a gradual return to full activity.
Full activity may be permitted once the following criteria are met:
Plain radiographs usually reveal evidence of healing between 3 and 6 months after treatment (2,5,6). Thus, if the patient is improving clinically, persistently abnormal plain radiographs during the first 12 weeks of nonsurgical management do not necessitate further diagnostic imaging or referral to an orthopedic surgeon. Although MRI or bone scan may reveal characteristics associated with instability and poor healing potential, it is not necessary to perform them initially. Rather, MRI or bone scan may be reserved to follow the progress of healing if the patient has no improvement of symptoms over the first 4 to 6 weeks of conservative therapy or if plain radiographs are persistently abnormal after 12 weeks.
"Silent" OCD noted incidentally on radiographs of an asymptomatic juvenile or young adult patient usually needs no further evaluation or treatment and can be managed by observation (7). However, surgical treatment for silent lesions in adults is often recommended to prevent future sloughing and loose body formation (2,7).
Surgical treatment. Surgery may be indicated for juvenile/young adult patients with stable lesions of the medial femoral condyle if conservative therapy fails to relieve symptoms or if there is no radiographic evidence of healing after 12 weeks (1). Surgical repair is recommended for adult patients regardless of the stability of the lesion (1,7). Unstable fragments require surgery regardless of the patient's age (4,6,13).
Many types of surgical intervention have been proposed for OCD treatment, including transarticular drilling, fixation of the fragment, bone grafting, and fragment excision (4,6,13). A detailed description of these procedures is beyond the scope of this discussion. In general, surgery is performed to promote healing at the junction of the fragment's subchondral bone and the underlying bony base, and to restore a normal, smooth articular surface.
Arthroscopy is generally used for the initial surgical management: It can be a diagnostic procedure as well as a therapeutic tool (6). For example, an osteochondritic lesion that appears stable on radiographs may be discovered by arthroscopic probing to be only loosely adherent. Such a finding would affect the choice of surgical management. For juvenile/young adult patients with stable lesions who fail to heal with conservative management, arthroscopic drilling through the fragment into the underlying bone may promote healing through vascular ingrowth (1,3,6).
Unstable lesions and most lesions of adult patients require a more aggressive approach that involves fixation of the fragment and possibly bone grafting (1-3). Partly attached fragments are generally managed using arthroscopy, while large loose bodies may require an open procedure (1,13). A common approach is to remove fibrous scar from the underside of the fragment; curettage the bony base to the point of bleeding; drill holes; and fix the fragment in place with pins, wires, bone pegs, or screws (1,2,6,13-15). If the articular surface is depressed, a bone autograft or allograft can be used to build up the femoral base before fixation (1,2).
Occasionally, replacement of the osteochondritic fragment is unnecessary or impossible. Small lesions over non-weight-bearing areas can be excised, followed by abrasion arthroplasty, which promotes the ingrowth of fibrocartilage to fill in the defect (1-3). Fragment excision is also indicated for lesions that are unsalvageable due to comminution or a subchondral bone shell that is too thin. If a large articular crater remains, an osteochondral autograft or allograft may be used to fill the defect (2,3).
For OCD lesions of the patella, early referral to an orthopedic surgeon is indicated because conservative management is often unsuccessful. Surgical treatment for patella lesions typically involves fragment excision, followed by curettage and drilling of the bony patellar crater to promote fibrocartilage ingrowth into the defect (3,6). The prognosis depends on the size of the lesion (6).
The goal of returning patients to activity is most easily achieved for children and adolescents and for those who have stable lesions of the medial femoral condyle. For these patients, brief immobilization, followed by activity limitations will usually bring about gradual healing. However, adult patients, or those who have persistent symptoms or unstable lesions, will likely require surgical intervention. In addition, patients who have OCD of the lateral femoral condyle or patella should be referred to an orthopedic surgeon.
Dr Ralston is a family physician and practices at the Kildare Clinic in Chicago. He is a faculty member in the Department of Family Practice at MacNeal Hospital in Berwyn, Illinois. Dr Williams is a fellow, Dr Bach is associate professor and director of sports medicine, and Dr Bush-Joseph is assistant professor, all in the Section of Sports Medicine in the Department of Orthopedic Surgery at Rush Medical College, Rush-Presbyterian-St Luke's Medical Center, in Chicago. Dr Knopp is director of the sports medicine fellowship in the Department of Family Practice at MacNeal Hospital. Dr Bach is an editorial board member of the The Physician and Sportsmedicine. Address correspondence to Brian M. Ralston, MD, Kildare Clinic, 5101 Kildare Ave S, Chicago, IL 60632; e-mail to: [email protected]