PROBLEM:

Radiographs and a CT tomogram are shown from five different patients with the same disease. What is the most likely diagnosis?

DISCUSSION:

Osteochondritis dissecans represents a localized injury of subchondral bone and adjacent cartilage at an articular surface. In this disorder, it is commonly believed that the bone undergoes avascular necrosis, with the overlying cartilage exhibiting degenerative changes. This can lead to the separation of the dead bone and damaged cartilage from its adjacent healthy tissue, resulting in the formation of an osteochondral body.1 As seen in the preceding radiographs, the knee, ankle, and elbow are the most common sites of osteochondritis dissecans. It also occurs less commonly at the hip, shoulder, and other joints.

This disease affects two distinct groups of patients. The juvenile form of the disease involves patients between the ages of five and fifteen years of age in whom the physes remain open. The adult form of the disease affects older adolescents and adults whose physes are closed.

The exact etiology of osteochondritis dissecans remains controversial. Theories implicating trauma, ischemia, abnormal ossification, and genetic predisposition have been proposed, but none of these has been universally accepted.1,2,3 The precipitating cause is most likely multifactorial, though some mechanisms may have stronger associations with certain joints. Accordingly, treatment of this disease is also not uniformly agreed upon, remaining dependent on the skeletal age of the patient, the specific joint involved, and the exact location within the joint. Clinical symptoms, etiology, radiographic findings, and treatment options for the knee, ankle, and elbow joints are discussed below.

Osteochondritis dissecans of the knee afflicts men twice as often as women, and it is seen most frequently in adolescents. This disease is relatively rare in those who are younger than 10 years or older than 50 years of age. Early symptoms are non-specific, usually consisting of complaints of a dull ache in the knee. Catching, clicking, popping, locking, or the feeling of a loose body in the joint can all manifest as late symptoms when the defect has undergone a complete or partial separation.1 A history of trauma to the knee is common, but not universal. The medial condyle of the femur is affected approximately 85% of the time. On routine anteroposterior and lateral radiographs the bony lesion can be seen as a well defined area of sclerotic subchondral bone that is separated from the normal areas of the femur by a radiolucent zone (figure 1).2 A third view, the notch or tunnel view, can best delineate the lesion in the non-weight-bearing portion, or inner aspect, of the medial femoral condyle (figure 2).1 Purely chondral lesions, however, call for arthrography, CT arthrography, MRI, or arthroscopy for proper diagnosis. These latter studies can also better characterize loose fragments, degree of displacement, and orientation of major fragments.3 Those patients who are skeletally immature should also have the opposite knee examined radiographically.

Although there is no standard classification system, Cahill and Berg have described a system for use with AP and lateral radiographs.4 In their technique, the AP radiograph is divided into five segments: the medial compartment is divided into segments 1 and 2; the notch is segment 3; and the lateral compartment is divided into segments 4 and 5. The lateral radiograph is separated into three sections: section A is anterior to the Blumensaat line; section B lies between the Blumensaat line and a median extended from the posterior femoral cortex; and section C describes the area posterior to a line extended along the posterior femoral cortex.

For osteochondritis dissecans of the knee, the major differential diagnosis to consider is spontaneous osteonecrosis of the knee. This entity generally is seen in older patients who present with a sudden onset of symptoms. This defect usually involves the weight-bearing portion of the medial femoral condyle and may be a true osteonecrosis, though its etiology also remains uncertain.2

Treatment varies markedly for the juvenile and adult forms of osteochondritis dissecans of the knee. Patients with the juvenile form (with open physes) usually can be successfully managed non-operatively with protected weight-bearing and limitation of activity.3 A knee immobilizer or splint may be used, and careful follow-up with serial radiographs is needed to assess the patient's progress.

Indications for operative intervention in this population include presence of loose bodies, persistence of symptoms for 6 to 12 months, lack of evidence of healing, or approach of skeletal maturity.1 Adult forms of the disease do not fare well with non-operative repair techniques and, therefore, surgical treatment is preferable for optimal results in adult patients. Such techniques include retrograde drilling of the intact articular cartilage; curettage and drilling of the defect; fixation of fragments with compression screws, bone pegs, or pins; and bone grafting to restore articular surface congruity.3 Strict postoperative instructions for non-weight bearing are essential to optimize the chances of healing, although early range-of-motion exercises are allowed. Patients who are older than 60 years of age with a large osteochondral defect may be candidates for knee arthroplasty. Younger patients with large defects, however, may experience significant trouble with advanced degenerative joint disease, although recent work with osteochondral allografts has shown promising results.1

Patients with osteochondritis dissecans of the ankle have lesions in either a posteromedial or anterolateral location on the talus. This occurs more often in young adults, and men are affected more often than women. Patients with posteromedial lesions may present with no symptoms and no history of trauma; the lesions frequently heal spontaneously, and the patients usually do not develop extensive arthritis.1 Those patients with anterolateral lesions, however, can present with pain, swelling, catching with walking, and a history of trauma. These lesions are strongly associated with trauma, they rarely heal without intervention, and patients often develop early arthritis.1

The traumatic etiology of osteochondritis of the ankle is clearer than that for the knee. Lateral lesions most likely result from impaction of the talus against the fibular articular surface by inversion and dorsiflexion, whereas medial lesions may arise from inversion and plantar flexion with lateral rotation against the tibia.3 These lesions may, in fact, be osteochondral fractures. Similar to the knee, radiographs reveal a well defined area of subchondral bone separated from the normal bone by a radiolucent line (figure 3).2 Berndt and Harty have developed a radiographic classification system divided into four stages: stage 1 is a small area of compression of subchondral bone; stage 2 describes a partially detached osteochondral fragment; stage 3 is a completely detached osteochondral fragment remaining in the underlying crater; and stage 4 describes an osteochondral fragment that is displaced.5 CT can be helpful for accurate determination of the size and orientation of the lesion before surgery (figure 4). MRI also has been shown to correlate well with findings from arthroscopy.2

Berndt and Harty's stages are used as treatment guidelines.1,3 Stage 1 and 2 lesions may best be treated non-operatively. Medial stage 3 lesions also fare better when treated non-operatively with immobilization of the limb and non-weight-bearing for up to 6 months. However, stage 3 lateral lesions and all stage 4 lesions are best treated operatively. This involves excision of the fragment, and curettage and drilling of the crater. If the fragment is in good condition, it may be pinned after débridement and preparation of the crater. Lateral lesions usually can be approached anteriorly because the lateral malleolus is posterior to the tibia. Medial lesions have been approached without the use of osteotomy of the medial malleolus by "grooving" the anteromedial surface of the distal tibia to allow improved exposure.1 These techniques are utilized to prevent or slow degenerative changes in the ankle, and they have generally produced good results.

Osteochondritis dissecans of the elbow usually occurs as a result of overuse injuries. Symptoms may include swelling, pain, and limited motion, with intermittent locking occurring if loose bodies are present. Such patients are usually adolescents and young adults who are involved in throwing activities. Repetitive microtrauma from throwing results from the increased valgus stress placed on the elbow, with compressive forces placed on the radial head and capitellum.3 Plain radiographs will show a radiolucency of the lateral or central portion of the capitellum (figure 5). MRI can be useful for evaluating the presence and stage of the lesion.

The major differential diagnosis to consider is Panner disease ("little leaguer's elbow"). This entity involves rarefaction and fragmentation of the entire capitellum, but is seen in children between four and eight years of age, is self-limiting, and improves with rest. Panner disease should be suspected if, on radiography, the entire capitellum is found to be involved.2

Again, younger patients have better results with non-operative treatment. Operative treatment includes débridement of loose bodies, as well as curettage and drilling to stimulate vascular growth.3 Unlike osteochondritis dissecans of the knee and ankle, results of treatment for osteochondritis dissecans of the elbow have not been as favorable. Degenerative changes and limitation of motion are expected results.

In summary, the radiographic findings of osteochondritis dissecans of various joints are similar. However, each different joint involved is unique, with different etiologies proposed for the lesion, as well as different treatment strategies. It may be best to consider osteochondritis dissecans of the knee, ankle, and elbow as three distinct entities sharing the same name in order to find the best method of treatment.

References

1. Crenshaw AH (ed): Campbell's Operative Orthopaedics, ed 8. St. Louis, Mosby, 1992.

2. Resnick D (ed): Diagnosis of Bone and Joint Disorders, ed 3. Philadelphia, WB Saunders, 1994.

3. Schenck RC Jr., Goodnight JM: Osteochondritis dissecans. J Bone and Joint Surg 78-A (3):439-456, 1996.

4. Cahill BR: Osteochondritis dissecans of the knee: Treatment of juvenile and adult forms. J Am Acad Orthop Surg 3:237-247, 1995.

5. Berndt AL, Harty M: Transcondylar fractures (osteochondritis dissecans) of the talus. J Bone Joint Surg 41-A(9):988-1020, 1959.

This series of diagnostic challenges is prepared by David J. Sartoris, MD,Professor, Department of Radiology, University of California School of Medicine,

San Diego, CA.