Osteochondroma



Osteochondroma


G. Petur Nielsen, MD

Andrew E. Rosenberg, MD










An osteochondroma is shown arising from the distal femur, which is capped by mineralized cartilage and grows in a direction away from the joint. There is a linear fracture through the base of the stalk.






A typical osteochondroma that has a mushroom-shaped configuration and well-formed hyaline cartilage cap is seen. The cartilage cap overlies newly formed cancellous bone, which contains fatty marrow.


TERMINOLOGY


Abbreviations



  • Osteochondroma (OCE)


Synonyms



  • Exostosis


  • Osteocartilaginous exostosis


Definitions



  • Benign cartilage-capped tumor that originates in metaphysis or region of apophysis


  • Arises on surface of bones and enlarges through process of endochondral ossification


  • Osteochondroma occurs in 2 clinical settings



    • As solitary lesion


    • Multiple lesions (multiple hereditary osteochondromatosis)


ETIOLOGY/PATHOGENESIS


Neoplastic Process



  • May occur sporadically, but when a manifestation of hereditary multiple osteochondromas (HMO), an autosomal dominant genetic disorder, they tend to be polyostotic and bilaterally symmetrical


  • Previously, osteochondromas were believed to develop from lateral displacement or redirection of growth plate or defects in surrounding periosteal cuff of bone



    • Based on the observation that osteochondromas never arise on surface of existing cortex and were induced experimentally in animals by manipulating the growth plate


  • Recent studies have shown that chondrocytes in cartilage cap have inactivating mutations involving genes EXT1 and EXT2 on chromosomes 8q24 and 11p11-12



    • Indicates that chondrocytes are derived from monoclonal cellular proliferation and strongly suggests that neoplastic component of osteochondromas resides in cartilage cap


  • Involvement of EXT3 on chromosome 19p also seen in some cases


  • Genetic abnormalities of osteochondroma have not been identified in dysplasia epiphysealis hemimelica or metachondromatosis


  • Small minority of osteochondromas are associated with previous radiation



    • Most patients are young



      • 25% of patients ≤ 5 years who receive total body radiation develop osteochondroma


  • Patients with metachondromatosis have both osteochondromas and enchondromas


CLINICAL ISSUES


Epidemiology



  • Incidence



    • Most common primary bone tumor


    • Accounts for 36-50% of benign bone tumors and 8.5-15% of all primary neoplasms of bone


    • Approximately 80-85% of osteochondromas are solitary


  • Age



    • Most patients are in their 2nd decade of life at time of diagnosis



      • Incidence is 35/1,000,000 individuals 0-18 years of age


      • Age range is 8-77 years with average of 21 years


  • Gender



    • Male preponderance; M:F = 1.5-2:1


Site



  • Typically arise in appendicular skeleton



    • In metaphysis of bones derived from endochondral ossification



      • Distal femur, proximal tibia, proximal humerus


  • Rarely arise in epiphysis




    • Known as Trevor disease or dysplasia epiphysealis hemimelica in this location


  • Can involve flat bones, such as ilium and scapula


  • Rare in small bones of hands and feet and vertebrae, and never arise in craniofacial skeleton


Presentation



  • Many are asymptomatic



    • Incidental finding on imaging studies


  • Some are 1st detected as slowly enlarging firm mass present for many years


  • May be painful



    • Impingement upon adjacent neurovascular structures


    • Inflamed overlying bursa: Inflammatory bursitis


    • Fracture of stalk


  • Interferes with range of motion


Natural History



  • Slow growth that usually ceases at puberty



    • Osteochondromas exhibit greatest amount of growth while physeal plates are open


  • Malignant transformation should be considered if osteochondroma grows rapidly


Treatment



  • Small and asymptomatic osteochondromas do not require intervention



    • May sustain various complications that cause them to become symptomatic and justify removal


  • Excision usually adequate therapy



    • Cortex is transected near base of lesion, which is well defined in pedunculated lesions


    • Overlying bursa should also be removed with cartilage cap


    • If portion of perichondrium or cartilage cap is left behind, then lesion may recur locally


Prognosis



  • Malignant degeneration of solitary osteochondroma is very uncommon (0.4-2%)


  • Development of dedifferentiated component, such as high-grade osteosarcoma or pleomorphic spindle cell sarcoma, is exceedingly rare


IMAGE FINDINGS


Radiographic Findings



  • Arises from surface of bone


  • Cortices of lesion and underlying bone and their marrow cavities are in direct continuity


  • In long tubular bones, lesion typically points away from nearest joint in reaction to forces exerted by muscles and tendons



    • Direction of osteochondroma is not diagnostic feature


  • Cartilaginous surface is lobulated and may contain calcifications


  • Base can be narrow or broad (sessile lesion)


  • Lesion can sometimes be heavily mineralized


MR Findings



  • T1-weighted image shows similar signal intensity of medullary cavity of lesion and underlying bone


  • T2-weighted image shows high signal intensity cartilaginous cap



    • Low signal intensity surrounding cap represents perichondrium


  • Helpful in evaluating thickness of cartilaginous cap


CT Findings



  • Nicely shows continuity of cortices and medullary cavity of osteochondroma and underlying bone


  • Useful in demonstrating thickness of cartilaginous cap


  • Helpful in showing bursa formation


Bone Scan



  • Hot in growing individuals


  • In adults, osteochondromas are often not hot on bone scan



    • Exceptions: Fracture, bursitis, malignant transformation



MACROSCOPIC FEATURES


General Features



  • Mushroom shaped


  • Outer layer consists of thin sheath of fibrous tissue that overlies pearly gray-white cartilaginous cap



    • In older individuals, cartilaginous cap may be very attenuated or gone


    • May contain gritty areas of calcification or cystification


  • Cartilage cap of variable thickness; ranges from < 1 inch to several inches in depth



    • Surface of cap is smooth and lobulated


    • Base has sharp but undulating margin with underlying cancellous bone


  • Cartilage cap may demonstrate variable degrees of mineralization



    • In form of irregular stipples or ring-like calcifications



      • Due to partial or complete “framing” of cartilage lobules by ossification or amorphous mineralization


  • Base of cartilage cap undergoes enchondral ossification and merges with areas that have appearance of cancellous bone


Size



  • 1 cm to > 20 cm in greatest dimension



    • Average: 3-6 cm


MICROSCOPIC PATHOLOGY


Histologic Features



  • Outer layer is perichondrium



    • Composed of dense fibrous tissue that overlies hyaline cartilage cap


    • Overall architecture recapitulates that of disorganized growth plate


  • Peripheral portion of cap is least cellular with individual chondrocytes surrounded by abundant hyaline cartilage matrix


  • Cellularity of cartilage increases in deeper layers


  • Chondrocytes become arranged in vague columns


  • In deeper layers, chondrocytes enlarge with more abundant cytoplasm


  • Chondrocytes exhibit minimal cytologic atypia and no mitotic activity



    • Scattered binucleate cells may be present


  • Matrix calcifies at base of cap


  • Chondrocytes undergo necrosis



    • Portions of mineralized cartilage are resorbed by osteoclasts at base of cap


    • Residual struts of cartilage matrix that are left behind act as scaffolding for bone deposition


  • Newly formed trabeculae mimic primary spongiosa at base of normal growth plate


DIFFERENTIAL DIAGNOSIS


Bizarre Parosteal Osteochondromatous Proliferation (BPOP)



  • Contains reactive fibrous tissue that overlies cellular cap of cartilage, which undergoes enchondral ossification


  • Underlying cortex is intact


Periosteal Myositis Ossificans

Jul 6, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Osteochondroma
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