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Osteochondroma / Osteocartilaginous Exostosis


- Discussion:
    - a developmental dysplasia of peripheral growth plate which forms a cartilage capped projection of bone found near metaphyses of long bones;
          - peripheral chondroblast grows outward from the metaphysis, acting as an ectopic growth plate, which ceases growth at skeletal maturation;
          - hence, there is an excrescene of trabecular bone capped by a thin zone of proliferating cartilage;
    - it is the most common benign bone tumor;
    - usually occurs in long bones, but may occur any bone that is preformed in cartilage;
    - diff dx:
          - multiple cartilaginous exostoses;
                  - patients have polyostotic tumors
                  - look for short stature, clubbing of radius, & angular deformity of the lower limbs;
                  - these patients have an increased risk for secondary chondrosarcoma after the age of 30 years;
          - parosteal osteosarcoma
                  - may present as a symptomatic "exostosis" that increases in size in adults;
    - tumor growth:
          - lesion growths by enchondral ossification of proliferating cartilage cells in its cap;
          - tumor will continue to enlarge during skeletal growth, but will become latent at skeletal maturity;
                    - however, the lesion may continue to grow into the 3rd decade;
                    - occcassionally a lesion grows more rapidly than expected;
          - most common locations are proximal or distal femur, proximal humerus, proximal tibia, pelvis, and scapula;
                    - in areas other than the knee, more likely to undergoe malignant degeneration;
                    - may occur in the spine and cause neurologic damage;
    - malignant transformation:
          - risk of sarcomatous transformation in solitary exostosis is about 1%, but in MHE, risk approaches 10%;
          - evidence for transformation: (to chondrosarcoma)
                    - cartilaginous cap thicker than 1 cm in an adult (in child may be 2-3 cm thick) as seen by MRI;
                    - sudden or marked increase in uptake on bone scan in an adult (inconsistent w/ normal latency seen w/ skeletal maturity);
                    - confirmation by CT or MRI imaging of a soft tissue mass or displacement of a major neurovascular bundle;

- Clinical Presentation:
    - look for a firm, nontender, immovable mass arising near end of the long bone;
    - a symptomatic lesion, may be caused by irritation of overlying soft tissues which may go on to form a fluid filled bursa;
    - bursal fluid may be mistaken for a soft tissue mass;

- Diagnostic studies:
    - x-ray appearance of an exostosis is either flat, sessile lesion or a peduculated (stalk like) process;
    - peduncultaed osteochondromas are oriented in proximal direction;
    - x-ray hallmark is blending of tumor into underlying metaphysis;
    - look for a well defined metaphyseal excrescence of bone w/ a mottled density;
    - calcification:
           - cartilaginous cap displays irregular areas of calcification;
           - amount of calcification and bone formation increase w/ age;

- Microscopic Exam:
    - on microscopic exam, cartilaginous cap is seen to have same pattern as normal growth plate but it will be less organized;
    - underlying trabeculae form by endochondral ossification of cap and contain central cores of calcified cartilage.
    - may uniform but expanded cartilage cells w/ small round or elongated nuclei which may be positioned in rows similar to a physis;
    - polymorphy and hyperchomasy of cartilage cells is an expected finding in young children;
    - note that the cartilagenous cap may be upto 1 cm in width in adolescence and that a cap greater than 3 cm is consistent w/ low grade chondrosarcoma;

- Treatment:
    - no treatment is required if the diagnosis is not in doubt and if the patient is relatively asymptomatic;
    - surgical resection is indicated for persistant irritation (from bursitis) or for neurovascular comprimise;
    - surgical resection is also indicated for continued osteochondroma growth after skeletal maturity (in which case malignancy is suspected);
    - definitive treatment includes marginal excision of an active exostosis,  including the cartilaginous cap & overlying perichondrium;
           - deep bony base has minimal activity and may be removed piecemeal.
           - the cartilaginous cap should not be traumatized during its removal;
    - prognosis for a solitary exostosis is excellent (< 5% recurrence following marginal excision)

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Correlative radiographic, scintigraphic, and histological evaluation of exostoses.

Secondary chondrosarcoma in osteochondroma: report of 107 patients.