Para-articular chondromas, also known as intracapsular chondromas, are one of the three variants of extraskeletal chondromas, the other two being synovial chondromatosis and soft tissue chondroma. These tumors arise from chondroid metaplasia of the joint capsule 1.
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Epidemiology
These tumors are rare and about thirty cases have been reported in the English literature 1. They occur at a wide age range of 12 to 75 years. No sex predilection exists. The knee is the most commonly affected joint, at which the tumor is typically found inferior and medial to the patella 1-3.
Diagnosis
The diagnosis of these tumors is suspected based on clinical and imaging findings. It is typically confirmed through pathologic findings after surgical excision.
Clinical presentation
Patients present with non-specific symptoms including pain, limited range of motion, and joint swelling. Physical examination may be normal or show a palpable mass with warmth and erythema of the adjacent joint 4.
Pathology
The pathogenesis of these tumors is a matter of controversy. They most likely arise from cartilaginous metaplasia of the joint capsule or the connective tissue in its vicinity 1.
Microscopic appearance
They are comprised of various-sized hypercellular lobules of mature hyaline chondrocytes. Variable degrees of calcification or ossification may be present, hence the name intracapsular osteochondroma. Cellular pleomorphism, if noted indicates malignancy. If the specimen is covered with the synovial cells it is diagnosed as synovial chondromatosis 4.
Radiographic features
They are seen as soft tissue masses with different degrees of calcification/ossification.
Plain radiograph
May be normal or show curvilinear and ring-like calcification or variable amounts of ossification in the region of the tumor. Remodeling could be present at adjacent bony surfaces 2,4.
CT
Reveals an iso to hyperdense soft tissue mass with no evidence of invasion to the underlying bone. CT has more sensitivity in detecting possible calcification and ossification of the tumor 4.
MRI
Is the modality of choice for characterizing possible para-articular chondroma, where a well-circumscribed lobulated mass is seen near the joint capsule. Evidence of pressure erosion could be found in the adjacent bones 4.
Signal characteristics
T1: hypointense or isointense to muscle
T2: hyperintense with areas of calcification/ossification showing signal loss
T1 C: peripheral and septal pattern of enhancement is seen in most cases
Treatment and prognosis
Surgical excision is the treatment of choice. A recurrence rate of 15-20% is reported after surgery 1,4.
Differential diagnosis
synovial chondromatosis: appear as multiple intra-articular nodules in contrast to the solitary lesion of para-articular chondroma 2
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