Orthopedics

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Review Article 

Skeletal Osteochondromas Revisited

Andreas F. Mavrogenis, MD; Panayiotis J. Papagelopoulos, MD, DSc; Panayotis N. Soucacos, MD, FACS

  • Orthopedics. 2008;31(10)
  • Posted October 1, 2008

Abstract

Skeletal osteochondromas or osteocartilaginous exostoses represent the most common of all benign bone tumors and 10% to 15% of all bone tumors.1-3 Osteochondromas are solitary or multiple, pedunculated or sessile exophytic outgrowths from the bone surface that are composed of cortical and medullary bone with an overlying hyaline cartilage cap. Marrow and cortical continuity with the underlying parent bone defines the lesion.2,4,5

Osteochondromas usually occur in children or adolescents between 10 and 15 years, and increase in size throughout childhood, ranging from 1 to 10 cm. After adolescence and skeletal maturity, osteochondromas usually exhibit no further growth.2,6 In adults, growth or imaging alterations of an osteochondroma suggest the rare diagnosis of malignant transformation; however, extensive growth of osteochondromas without histological evidence of malignancy has been reported.7,8

Skeletal osteochondromas are considered by some authors as true tumors and by others as growth disturbances or developmental lesions that result from the separation of a fragment of epiphyseal growth plate, which subsequently herniates through the periosteal bone cuff that normally surrounds the growth plate (encoche of Ranvier).4 Therefore, an osteochondroma can arise in any bone that develops from enchondral ossification.2,9

Osteochondromas enlarge from growth at the cartilage cap, identical to a normal physeal plate. D’Ambrosia and Ferguson4 experimentally produced osteochondroma by transplanting epiphyseal plate tissue into bone cortex. Persistent growth of this cartilaginous fragment and its subsequent enchondral ossification result in a subperiosteal osseous excrescence with a cartilage cap that projects from the bone surface. The stalk of the osseous protuberance must be in direct continuity with the underlying cortex and medullary canal to be considered a true osteochondroma.2,4,5,9

Solitary skeletal osteochondromas are usually located in the metaphyses of the long bones, extending away from the adjacent joint, forming the typical pattern of stalactites and stalagmites. The long bones of the lower extremities are most frequently involved2,9-13; the knee (40%)9,12,14 followed by the humerus (10%-20%)9,12,13 are more frequently involved. Other more unusual locations include the small bones of the hands and feet (10%), scapula (4%), pelvis (5%), and cranial base and jaw9,12-15; the spine is affected in 1.3% to 4.1% of cases. The cervical spine is most frequently affected (50% of lesions, most commonly the C2 vertebra), followed by the thoracic spine (most commonly the T8, followed by the T4 vertebrae), and the lumbar spine (Figure 1).16,17

The vast majority of solitary osteochondromas are asymptomatic and diagnosed incidentally.12,13 Symptomatic lesions usually occur in younger patients; 75% to 80% of such cases are discovered before age 20 years.9,12,13 A male to female ratio of 1.6-3.4 to 1 of solitary osteochondromas has been reported.12

Clinical symptoms may be related to mechanical effects, cosmetic deformity, neurovascular impingement, pseudoaneurysm formation, fracture, overlying bursa formation, or malignant transformation. Painless swelling and cosmetic deformities related to the slowly enlarging mass are the most common complaints.18,19 Mechanical irritation of soft tissue can result in bursa formation that presents as a soft-tissue mass suspicious for a malignant degeneration.1,20 Neurological symptoms secondary to osteochondromas of the spine are not common since most lesions do not protrude into the spinal canal or neural foramens.

Spontaneous regression of skeletal osteochondromas or “evanescent exostosis” has been reported.21-26 The exact mechanisms of spontaneous regression of osteochondromas remain unknown. This phenomenon has a predilection for very young male patients. Initially, it has been attributed to the early cessation of lesion growth and progressive incorporation to the adjacent cortical bone.21 Later, spontaneous regression of osteochondromas has been attributed to “metachondromatosis” that is characterized by both multiple enchondromatosis and multiple osteochondromatosis.27 Copeland et al22 suggested that spontaneous regression of skeletal osteochondromas may be due to cessation of cap growth…

Skeletal osteochondromas or osteocartilaginous exostoses represent the most common of all benign bone tumors and 10% to 15% of all bone tumors.1-3 Osteochondromas are solitary or multiple, pedunculated or sessile exophytic outgrowths from the bone surface that are composed of cortical and medullary bone with an overlying hyaline cartilage cap. Marrow and cortical continuity with the underlying parent bone defines the lesion.2,4,5

Osteochondromas usually occur in children or adolescents between 10 and 15 years, and increase in size throughout childhood, ranging from 1 to 10 cm. After adolescence and skeletal maturity, osteochondromas usually exhibit no further growth.2,6 In adults, growth or imaging alterations of an osteochondroma suggest the rare diagnosis of malignant transformation; however, extensive growth of osteochondromas without histological evidence of malignancy has been reported.7,8

Pathogenesis

Skeletal osteochondromas are considered by some authors as true tumors and by others as growth disturbances or developmental lesions that result from the separation of a fragment of epiphyseal growth plate, which subsequently herniates through the periosteal bone cuff that normally surrounds the growth plate (encoche of Ranvier).4 Therefore, an osteochondroma can arise in any bone that develops from enchondral ossification.2,9

Osteochondromas enlarge from growth at the cartilage cap, identical to a normal physeal plate. D’Ambrosia and Ferguson4 experimentally produced osteochondroma by transplanting epiphyseal plate tissue into bone cortex. Persistent growth of this cartilaginous fragment and its subsequent enchondral ossification result in a subperiosteal osseous excrescence with a cartilage cap that projects from the bone surface. The stalk of the osseous protuberance must be in direct continuity with the underlying cortex and medullary canal to be considered a true osteochondroma.2,4,5,9

Location

Solitary skeletal osteochondromas are usually located in the metaphyses of the long bones, extending away from the adjacent joint, forming the typical pattern of stalactites and stalagmites. The long bones of the lower extremities are most frequently involved2,9-13; the knee (40%)9,12,14 followed by the humerus (10%-20%)9,12,13 are more frequently involved. Other more unusual locations include the small bones of the hands and feet (10%), scapula (4%), pelvis (5%), and cranial base and jaw9,12-15; the spine is affected in 1.3% to 4.1% of cases. The cervical spine is most frequently affected (50% of lesions, most commonly the C2 vertebra), followed by the thoracic spine (most commonly the T8, followed by the T4 vertebrae), and the lumbar spine (Figure 1).16,17

Clinical Presentation

The vast majority of solitary osteochondromas are asymptomatic and diagnosed incidentally.12,13 Symptomatic lesions usually occur in younger patients; 75% to 80% of such cases are discovered before age 20 years.9,12,13 A male to female ratio of 1.6-3.4 to 1 of solitary osteochondromas has been reported.12

Figure 1: Epidemiology of skeletal osteochondromas
Figure 1: Epidemiology of skeletal osteochondromas.

Clinical symptoms may be related to mechanical effects, cosmetic deformity, neurovascular impingement, pseudoaneurysm formation, fracture, overlying bursa formation, or malignant transformation. Painless swelling and cosmetic deformities related to the slowly enlarging mass are the most common complaints.18,19 Mechanical irritation of soft tissue can result in bursa formation that presents as a soft-tissue mass suspicious for a malignant degeneration.1,20 Neurological symptoms secondary to osteochondromas of the spine are not common since most lesions do not protrude into the spinal canal or neural foramens.

Spontaneous regression of skeletal osteochondromas or “evanescent exostosis” has been reported.21-26 The exact mechanisms of spontaneous regression of osteochondromas remain unknown. This phenomenon has a predilection for very young male patients. Initially, it has been attributed to the early cessation of lesion growth and progressive incorporation to the adjacent cortical bone.21 Later, spontaneous regression of osteochondromas has been attributed to “metachondromatosis” that is characterized by both multiple enchondromatosis and multiple osteochondromatosis.27 Copeland et al22 suggested that spontaneous regression of skeletal osteochondromas may be due to cessation of cap growth followed by active resorption. The authors argued that a traumatic lesion such as a fracture may stop the growth of the osteochondroma by injury to the cartilaginous cap, vascular supply disruption, or stimulus of the reparative process in the adjacent bone.22 Reston et al26 suggested that spontaneous regression of osteochondromas is unpredictable and can occur after closure of the growth plates. Morphologically, sessile osteochondromas regress spontaneously without any intercurrent circumstance, while pedunculated osteochondromas suffer a traumatic episode before regression. The spontaneous resolution of a broad-based osteochondroma may be more frequent since the interaction with the adjacent bone is more extensive than in the pedunculated case.26

Secondary skeletal osteochondromas have been reported after radiation therapy for nonosseous childhood malignancies such as neuroblastoma or Wilms’ tumor, and after trauma, usually Salter-Harris injuries that cause in vivo transplantation of growth plate tissue into the cortex.4,5,28-34 Radiation has been associated with the development of osteochondromas by its damaging effect on the growth plate that causes undifferentiated cartilage tissue to migrate into the metaphysis.30-34 Radiation-induced osteochondromas are identical pathologically and radiographically to primary osteochondromas. The prevalence of radiation-induced osteochondromas ranges from 6% to 24%, with the largest series reporting the lower frequency.30-34 The prevalence is higher with total body irradiation as opposed to localized irradiation.34 Malignant transformation of a radiation-induced osteochondroma to chondrosarcoma has also been reported.9,35

Imaging

The radiographic appearance of solitary osteochondroma, particularly in long bones, is frequently pathognomonic. The lesion is composed of cortical and medullary bone protruding from and continuous with the underlying bone. The areas of osseous continuity between parent bone and osteochondroma may be broad (sessile osteochondroma) or narrow (pedunculated osteochondroma). Pedunculated lesions usually point away from the nearest joint owing to the forces of the overlying tendons and ligaments forming the stalactites and stalagmites. In young patients, the undertubulation of bone in the metaphyses about the hip and knee may be the first radiographic manifestation of disease prior to mineralization and visualization of individual osteochondromas. A widened metadiaphyseal junction and a distal femoral Erlenmeyer flask deformity may be apparent.18

The hyaline cartilage cap has a variable radiographic appearance, including arcs and rings, flocculent calcifications, and variable mineralization.2,3 Unless extensively mineralized, determining the thickness of the cartilaginous cap is often difficult or impossible on radiographs due to difficulty in differentiating from surrounding muscle or bursa formation.

Bone scintigraphy findings of osteochondroma vary and are directly correlated with the degree of enchondral bone formation and metabolic activity.13,36,37 In younger patients, a more prominent radionuclide uptake in osteochondromas is usual, while osteochondromas without increased radionuclide uptake (quiescent lesions) are more frequent in older patients. However, increased activity often persists well beyond the time of skeletal maturity. Positive bone scintigraphy does not allow differentiation from lesions with malignant transformation.36

Ultrasonography enables accurate measurement of the thickness of the hyaline cartilage cap, as it allows surrounding fat (hyperechoic) and muscle to be easily distinguished from the nonmineralized cartilage cap (hypoechoic layer).38,39 In a study by Malghem et al,39 ultrasonography revealed a mean measurement error of <2 mm for osteochondromas with hyaline cartilage cap thickness <2 cm. Disadvantages of ultrasonography include operator dependence, inability to evaluate deep lesions inaccessible to the probe, and lack of evaluation of the osseous components of the lesion.

Computed tomography (CT) scanning using three-dimensional imaging reformation allows optimal depiction of the pathognomonic cortical and marrow continuity of the lesion and parent bone, especially for osteochondromas in complex areas of the anatomy, such as the pelvis or spine, and for those with a broad stalk of attachment.36-38 Computed tomography is also useful for accurate measurement of the mineralized hyaline cartilage cap thickness. However, it can be difficult to accurately measure the thickness of an entirely nonmineralized cartilage cap because this cannot be easily differentiated from surrounding muscle or bursa.2,38,39 In addition, CT scans may be less accurate in evaluating the thickness of 1- to 2.5-cm-thick cartilaginous caps.38

Magnetic resonance imaging (MRI) is the best imaging modality for evaluating cortical and medullary continuity between osteochondroma and parent bone; identification of complications such as fractures, osseous deformity, neurovascular compromise, and bursa formation; and delineation of the mass to the surrounding muscular and neurovascular structures for surgical planning. However, in young patients with active growth and maturation from normal enchondral ossification in the cartilage cap, there may be marked heterogeneity with all magnetic resonance pulse sequences from the mixture of these tissues (nonmineralized cartilage, calcified cartilage, and ossification with yellow marrow).5,40-42

Magnetic resonance imaging may be the optimal method for evaluating the cartilaginous cap and identify lesions suspicious for malignant transformation.40,42 The cartilaginous cap shows high signal intensity on T2-weighted images due to the high water content of cartilaginous tissue and is easily distinguished from adjacent muscle and the ossific stalk. The overlying perichondrium attached to the cartilaginous caps appears as a low signal intensity band on T2-weighted images. Mineralized areas in the cartilage cap show low signal intensity in all magnetic resonance pulse sequences. Enhancement after intravenous administration of gadolinium-based contrast material reveals septal and peripheral enhancement in the cartilage cap.2,3,5,39-41

Histology

On gross examination, the cartilage cap of an osteochondroma has a bosselated, shiny, and glistening bluish-gray surface, reminiscent of cauliflower. The cartilage cap lies at the periphery of the lesion where growth occurs. It often shows opaque yellow-to-gray areas due to calcification within the cartilage matrix, and appears to merge with the underlying bone, covered with a thin layer of fibrous capsule that functions as a perichondrium.9,39,43 The thickness of the cartilage cap is typically 1 to 3 cm in young patients, whereas in adults it is often only a few millimeters thick or absent, leaving a surface composed of eburnated bone.9,14,39 Increased thickness of the cartilage cap is a recognized feature in young patients in response to continued active growth and should not be viewed as a finding of malignant transformation in skeletally immature patients.2,38,43,44

On cut sections, the medullary cavity of the osteochondroma is continuous with the underlying bone, as is the cortex and perichondrium covering the tumor. Microscopic features of enchondral ossification are observed at the junction of the cartilaginous cap and underlying cancellous bone. The cartilage cap resembles a growth plate with columns or clusters of proliferating and degenerating chondrocytes that are indistinguishable from normal chondrocytes and are evenly distributed and maturing in an enchondral process.2,9,14,39,43 These chondrocytes can be mildly increased in number with binucleate forms that occasionally may be mistaken for a histologically more aggressive process.9,43 Enchondral ossification leads to medullary bone, typically with yellow rather than hematopoietic marrow and large calcified areas or calcific debris.

Variants of Skeletal Osteochondromas

Variants of skeletal osteochondromas include hereditary multiple exostoses or Bessel-Hagen disease, subungual exostosis or Dupuytren’s exostosis, dysplasia epiphysealis hemimelica or Trevor disease, turret or traction exostoses, bizarre parosteal osteochondromatous proliferation or Nora’s lesion, florid reactive periostitis, and metachondromatosis.2,5,27

Hereditary Multiple Exostoses

Hereditary multiple exostoses, also known as familial osteochondromatosis, diaphyseal aclasis, or Bessel-Hagen disease, is characterized by multiple skeletal osteochondromas and variable skeletal deformities depending on the size and number of lesions. The incidence is at least 1 in 50,000, which makes it one of the most common inherited musculoskeletal conditions. Most patients are diagnosed by the age 5 years and virtually all by 12 years.2,6,45,46 The disease shows an autosomal-dominant inheritance pattern, with incomplete penetrance in women.47 Men are affected approximately 1.5 times more than women; 66% to 90% of patients with hereditary multiple exostoses have a family history of the condition.48 Mutations at 3 gene loci of chromosomes 8, 11, and 19 and mutations in 1 of the 2 EXT genes causing defective enchondral ossification by defective heparan sulfate biosynthesis maintain the proliferative capacity of chondrocytes and promote phenotypic modification to bone-forming cells.49,50

Multiple exostoses may occur bilaterally or may predominate on one side of the body. The size and number of lesions vary considerably. The bones about the knee followed by the proximal femur and humerus are most commonly affected. The most frequent locations in the flat bones include the pelvis and the scapula. The spine is affected in 9% of cases.2,10,11 Shortening and bowing of the long tubular bones, including a pseudo-Madelung deformity of the forearm and short broad femoral neck, are common.51

Taniguchi52 reviewed 41 children with hereditary multiple exostoses and devised a practical classification system of 3 groups based on the extent of forearm involvement: group I, with no involvement of the distal forearm; group II, with involvement of the distal forearm but without shortening of the radius or ulna; and group III, with involvement of the distal forearm and shortening of the distal radius or ulna.52 Children in group I are mildly affected and present at an older age, whereas those in group III are severely affected, present at a much earlier age, and are more likely to experience malignant transformation.52 The lesions tend to enlarge until skeletal maturity at a growth rate proportionate to the overall growth of the patient. Patients with smaller and fewer lesions may never become symptomatic.11 Short stature is a frequent clinical feature (40% of cases) that may be the result of the development of exostoses during childhood and early puberty, at the expense of longitudinal bone growth53; childhood height is correlated with severity of involvement.52

Imaging of individual osteochondromas in hereditary multiple exostoses is identical to that of solitary lesions and involves both pedunculated and sessile lesions. However, in hereditary multiple exostoses, sessile osteochondromas are the rule rather than the exception. Small sessile lesions in the pelvis may create an undulating cortical contour on CT scans (wavy pelvis sign).2

Complications such as pain, restriction of motion, deformities and shortening of the long bones, fracture, neurovascular compression, mechanical symptoms, and bursa formation are more common compared to solitary lesions. The most severe complication of the disease is sarcomatous transformation. Fortunately, many of the sarcomas in this setting are low-grade chondrosarcomas and can be treated successfully with wide excision. Malignant degeneration seldom occurs in the first decade of life or after. The high range of reported incidence of malignant degeneration (0.5%-25%) can be attributed to the inability to detect all hereditary multiple exostoses patients without malignant degeneration.44,54,55 However, recent studies estimate the lifetime risk of hereditary multiple exostoses malignant degeneration to be approximately 2% to 5%.2,10,11,48,56,57

Patients with hereditary multiple exostoses require continued surveillance both clinically and radiologically to evaluate progression of deformities and development of complications. Skeletal surveys and bone scintigraphy in the first and second decades of life to establish a baseline and to evaluate metabolically active lesions are advocated. Continuous growth of a lesion, especially when painful or with a cartilaginous cap of >2 cm after skeletal maturity, has been associated with an increased risk of sarcomatous transformation.16 The imaging features of malignant transformation of multiple osteochondromas are the same as solitary osteochondromas.

Treatment of patients with hereditary multiple exostoses is more problematic and complex than that of patients with solitary osteochondromas. Surgical treatment is often directed to correct the associated deformities rather than restricted to the exostoses alone. Multiple surgical procedures are often performed in these patients.11,57

Subungual Exostosis

Subungual or Dupuytren’s exostosis is a common lesion that shares some radiographic features with osteochondroma but pathologically represents a distinct entity. Chronic and repetitive trauma and infection have been implicated in the pathogenesis of subungual exostosis. Initially, the lesion is characterized by proliferating fibroblasts presumably induced by trauma. Cartilage metaplasia then develops, which progresses to mature ossification. In contrast to osteochondroma, there is typically no continuity with the underlying cortex and medullary canal, and the cartilage cap consists of fibrous cartilage rather than hyaline cartilage.58

Subungual exostosis classically arises from the dorsal or dorsomedial aspect of the distal phalanx, with a variable relationship to the nail bed. The vast majority of cases (86%-90%) involve the toes, with a predilection for the great toe (77%-80%). The lesion may be painful and show secondary overlying skin ulceration.58 Subungual exostoses may demonstrate an alarming growth rate that may lead to concern for chondrosarcoma; however, malignant degeneration of subungual exostoses has not been reported. Treatment consists of complete surgical excision. The recurrence rate varies from 11% to 53%.58

Dysplasia Epiphysealis Hemimelica

Dysplasia epiphysealis hemimelica, also called tarsomegaly, tarsoepiphyseal aclasis, or Trevor disease, is a rare skeletal developmental disorder characterized by osteochondromas involving the epiphyses of the long bones. The reported incidence is 1 in 1,000,000, with no hereditary evidence.5,59 The condition is 3 times more frequent in men than women. The typical radiographic findings are irregular ossifications of the epiphysis; the medial aspect of the epiphyses is involved nearly twice as often as the lateral aspect.60 As the bone matures, the ossification becomes confluent and forms a mass extending from the epiphysis.60

Epiphyseal involvement is hemimelic; the most common sites are the talus, distal femur, and proximal and distal tibia. Based on extent and distribution, 3 forms of dysplasia epiphysealis hemimelica have been described: a localized form (monostotic involvement), a classic form (>1 area of osseous involvement in a single extremity), and a generalized or severe form (entire single extremity involvement). The localized form usually affects the bones of the hindfoot or ankle. The classic form shows characteristic hemimelic distribution and accounts for more than two-thirds of cases; it typically involves >1 epiphysis within a single lower extremity, particularly about the knee and ankle.5

Similar to hereditary multiple exostoses, dysplasia epiphysealis hemimelica is diagnosed at a young age because of an antalgic gait, palpable mass, varus or valgus deformity, or limb length discrepancy. Surveillance until skeletal maturity is indicated to evaluate progression of the disease and not to assess malignant degeneration, which has not been reported.5 Surgical intervention is more frequently required compared to solitary osteochondromas because of epiphyseal involvement.61,62 Surgery is directed at improving joint congruity and preventing secondary osteoarthritis. Corrective osteotomies may be necessary to treat residual deformities.61,63

Turret Exostosis

A turret or traction exostosis, as originally described by Wissinger in 1966,64 is a smooth, dome-shaped, extracortical mass arising from the dorsum of a proximal or middle phalanx of the hand.The mechanism of injury is related to deep laceration to the digital extensor mechanism resulting in the disruption of the periosteum and formation of a subperiosteal hematoma. As the hematoma matures, it becomes ossified and forms a bony excrescence on the dorsal surface of the phalanx that often diminishes the excursion of the extensor tendon of the finger.

Bizarre Parosteal Osteochondromatous Proliferation and Florid Reactive Periostitis

Bizarre parosteal osteochondromatous proliferation, or Nora’s lesion, and florid reactive periostitis are osteochondroma-like reactive lesions.65-67 The small bones of the hands and feet are more frequently involved.9,65-67 Radiographically, they appear as laminated or mature periosteal reaction and soft tissue calcification that, when it matures, resembles an osteochondroma. However, the cortical surface of the affected bone remains normal in florid reactive periostitis.68 Treatment for these lesions is local excision.64 The overall rate of recurrence for turret exostosis of the hand is 20%, compared with 55% for bizarre parosteal osteochondromatous; resection of a mature as opposed to an immature lesion markedly reduces the recurrence rate.64,65,67

Metachondromatosis

Metachondromatosis was described by Maroteaux in 1971 as an autosomal-dominant disorder characterized by osteochondromas of the hands and feet, paraosseous calcifications or ossifications, usually juxtaarticular, and metaphyseal enchondromas in the long tubular bones and iliac crest with a very capricious progression, since they can grow, stay, or even disappear.27,69,70

Complications of Skeletal Osteochondromas

Mechanical complications such as restricted joint motion, snapping tendons, tenosynovitis, and premature osteoarthritis; deformities; fractures; vascular compromise such as pseudoaneurysm formation, compression, displacement, and stenosis or complete vascular occlusion (Figures 2A-C); neurological sequelae such as peripheral nerve entrapment neuropathy, spinal nerve roots compression, cranial nerve deficits, radiculopathy, spinal stenosis, cauda equina syndrome, and myelopathy; overlying bursa formation; and malignant transformation have been reported for skeletal osteochondromas.2,3,18,71-103 Additional, rarely reported complications include osteomyelitis, infarction of the cartilage cap or osseous component, hemarthrosis, saphenous neuritis, posterior tibial compartment syndrome, pleural effusion or hemothorax, and hematuria.2,3,9,18,75 Complications and their imaging appearances associated with hereditary multiple exostoses and solitary osteochondroma are identical. However, the prevalence and severity of these manifestations in hereditary multiple exostoses are greater owing to the multiplicity of lesions.2

Figure 2A: Figure 2B: Figure 2C: Figure 2D:
Figure 2F:Figure 2G: Figure 2E:
Figure 2: AP (A) and lateral (B) radiographs of the knee of a 17-year-old adolescent with a skeletal osteochondroma of the left fibula. The patient presented with clinical symptoms of left leg and foot arterial compromise. Magnetic resonance angiography showed compromise of the left popliteal and anterior tibial artery (C). Through a posterolateral upper leg approach and exposure of the popliteal artery and tibial nerve (arrow) (D), surgical excision of the osteochondroma was performed (E). AP (F) and lateral (G) radiographs after excision of the tumor.

Mechanical irritation and cosmetic deformity by an underlying exostosis is the most common clinical presentation of osteochondromas and frequently leads to surgical resection.9,18 Fracture of an osteochondroma is an unusual complication resulting from localized trauma and typically involves the base of a pedunculated lesion about the knee.18 No significant incidence of nonunion has been reported. Interestingly, regression or resorption of a solitary osteochondroma occurring both spontaneously and following a fracture has been reported.21,23,76

Pseudoaneurysm formation is the most common vascular complication of skeletal osteochondromas81,82; the popliteal artery is most frequently involved.77-79 Pseudoaneurysm formation may be attributed to the formation of bone spicules, ossification of the cartilaginous cap, or direct trauma of the blood vessel by the exostosis or friction of the artery in proximity with the osteochondroma.72,82,84 The predominance of popliteal artery involvement is related to the frequency of osteochondromas in this location as well as to the lack of mobility of the popliteal artery proximally at the adductor canal and distally by its branches.72,77,79,82,84

Vascular complications can be evaluated using angiography (venous or arterial), Doppler ultrasonography, contrast material-enhanced CT or MRI, and angiography.72,77 Treatment of vascular complications of osteochondromas depends on the location and extension of the tumor, and includes tumor excision, primary closure or repair of the vascular defect, embolization, and anticoagulation.82,84-86

Neurological sequelae can be associated with both peripheral and central (vertebral or skull base) osteochondromas. Peripheral lesions may compress nerves, leading to entrapment neuropathy, more frequently of the peroneal nerve.73,74,87-91 Myelopathic symptoms have been reported in 34% and 77% of patients with solitary and multiple spinal osteochondromas, respectively. Smaller lesions extending into the spinal canal that are more likely to be symptomatic are difficult to detect on standard radiographs.73,74,91

First described by Orlow in 1891, who coined the term “exostosis bursata,” a thickened bursa formation over the cartilaginous cap of skeletal osteochondromas usually due to friction with the underlying structures is a rare (1.5%) complication.16 Most of these bursae are seen in association with osteochondromas of the ventral surface of the scapula,1,19,20,92 the hip (lesser trochanter), and the ribs.16,93

Osteochondroma-associated bursae are lined by synovium and may become large, inflamed, infected, or hemorrhagic. The thickened bursa is attached to the perichondrium of the cartilaginous cap and can mimic chondrosarcomatous transformation.1,20 In addition, the bursa may contain chondral or fibrin bodies; chondrometaplasia can occur within the synovial lining, leading to secondary synovial chondromatosis.71,94

Preoperative diagnosis to differentiate bursa formation of an osteochondroma from malignant transformation of the cartilaginous cap is important.20,39,92,93,95 On radiographs, a bursa appears as a soft-tissue mass overlying the osteochondromas, containing areas of chondroid mineralization that represent intrabursal fragments.20 Ultrasonography is particularly helpful for distinguishing the anechoic bursal collection with posterior acoustic enhancement from the solid hypoechoic tissue of the underlying hyaline cartilage cap.39,92 The bursa appears as a fluid-filled mass showing low attenuation on CT scans, and homogeneous low signal intensity on T1-weighted and high signal intensity on T2-weighted MRIs; calcified or noncalcified chondral filling defects may be apparent. Magnetic resonance pulse sequences that allow differentiation of free water (bursa) from bound water (cartilage cap), such as magnetization transfer and fat-suppressed three-dimensional spoiled gradient-recalled sequences, have been successfully used to distinguish these 2 structures.93,95

Malignant transformation is the most feared sequelae of osteochondroma. The exact incidence of malignant transformation of solitary osteochondromas is unknown, since a number of these are asymptomatic and never diagnosed.2,14,96-98 The reported incidence is 0.4% to 2.2% in patients with solitary osteochondroma and up to 27.3% in patients with hereditary multiple exostoses.2,44,97,99-105 However, as the follow-up period gets longer, the incidence of sarcomatous transformation for solitary osteochondromas may increase as high as 7.6%.102

The most frequent malignancy is chondrosarcoma arising from the cap. Secondary or peripheral chondrosarcoma arising in a skeletal osteochondroma accounts for 8% of all chondrosarcomas and is usually solitary and low histological grade. Multifocal (with hereditary multiple exostoses) and dedifferentiated chondrosarcomas and cases of malignant fibrous histiocytoma and osteosarcoma (arising from the lesion base) have also been reported.5,14,44,96,97,106,107 The loci on chromosomes 8 and 11 have been associated with malignant transformation when loss of heterozygosity occurs.104,105

Malignant transformation before age 20 is unusual. Centrally located osteochondromas about the pelvis, hips, and shoulders are particularly more prone to malignant transformation.2,8,10,98,103 Clinical and radiographic suspicion of sarcomatous transformation is indicated by the presence of new onset of pain, continuous growth of the tumor after epiphyseal plate closure, thickness of the cartilaginous cap >1.5 cm, scattered or irregular extensive calcifications, irregular or indistinct lesion surface, focal regions of radiolucency in the interior of the lesion, erosion or destruction of the adjacent bones, and a significant soft-tissue mass, particularly containing scattered or irregular calcification.38,99,100,101,108

The thickness of the hyaline cartilage cap is an important criterion in determining malignant transformation.2,38,39,48,97,102,103 In a study by Hudson et al,38 the thickness of the cartilage cap in benign osteochondromas averaged 9 mm, with a maximum of 2.5 cm. For lesions with malignant transformation, mean measurements of 3.9 cm (range, 0.5-15 cm) and 4.6 cm (range, 1-15 cm) were reported by Ahmed et al102 and Wuisman et al,97 respectively. In general, a cap thinner than 1 cm usually indicates a benign condition, a cap between 1 and 2 cm is questionable, and a cap thicker than 2 cm generally corresponds to malignant transformation.2,38,39,43,44,97,102

Wide excision and limb salvage is the treatment of choice for secondary chondrosarcoma. As with primary chondrosarcomas, radiation therapy and chemotherapy are not used, except in cases of dedifferentiation.44,97,102,109,110

If treated with adequate surgery, the outcome of patients with secondary chondrosarcoma is favorable, with a long-term survival of 70% to 90%. The local recurrence rate varies with the adequacy of the tumor margins, from 0% to 15% in cases with wide resection to 57% to 78% in cases with marginal or intralesional resection.44,97 Most recurrences appear within the first 5 years. Metastases are unusual, occurring in approximately 3% to 7% of patients, and most commonly affect the lungs.2,44,97 Prognosis is worse for patients with grade 2 tumors, tumors in the trunk, multiple exostoses, local recurrences, metastases, and dedifferentiation of the secondary tumor.14,97,109

Treatment of Skeletal Osteochondromas

Treatment of skeletal osteochondromas is individualized. Patients with small asymptomatic or minimally symptomatic lesions, typical imaging findings, and no functional or mechanical impairment or progressive deformity should be observed regularly for the possibility of spontaneous regression or malignant transformation.103,110-115 However, treatment should aim also at prevention of deformities.103,115

Surgical excision is a successful form of treatment for symptomatic osteochondromas with a low morbidity.114 Ideally, operative intervention should be delayed until skeletal maturity, but, in symptomatic patients, partial excision preserving the physis may be necessary for the relief of symptoms and the prevention of progressive joint deformity. However, partially excised skeletal osteochondromas in skeletally immature patients may become larger and cause plastic deformation of the bones and deformities of the adjacent joints. In addition, partial excision is associated with a high rate of recurrence, so a close follow-up is required.116 Following surgical excision, remodeling of the bones usually occurs gradually, mostly in the youngest patients.117,118

Large symptomatic lesions may be resected at their base, where there is continuity to underlying bone (Figure 2).110,111 Giant symptomatic skeletal osteochondromas should be treated with giant bone resection and reconstruction of the defect using giant cadaveric bone allograft and rigid internal fixation. Extensive bone resection is necessary for giant lesions for the possibility of recurrence or malignant transformation; reconstruction of the defect using bone allograft and rigid internal fixation ensure mechanical stability and healing of the bone defect (Figure 3).

Surgical excision of osteochondromas occurring at the lateral aspect of the distal tibia is hampered by the difficult access to this area.117 Current techniques use an anterior approach with or without osteotomy of the fibula, but this makes access to the posterior aspect of the tibia difficult.117,118 Alternatively to the anterior approach, complete excision of a distal tibial skeletal osteochondroma with a satisfactory outcome can be obtained by removal and subsequent replacement of the distal fibula after excision of the tumor and fixation with a semitubular plate.117 Distal fibula resection,119 arthrodesis of the distal tibiofibular joint in adults,120 and fibular rotational osteotomy via a transfibular approach121 have been reported for surgical excision of distal tibia skeletal osteochondromas.

Asymptomatic osteochondromas of the spine can be followed without surgical intervention, whereas symptomatic lesions are treated surgically through decompression laminectomy or hemilaminectomy; total vertebrectomy through an anterior approach has also been reported.112,113 Computed tomography is indispensable for defining the size of the spinal lesion and its relationships with surrounding structures, for planning surgical treatment, and for following up its evolution. Neurological recovery is usually the rule after excision of symptomatic spinal osteochondromas.122

Figure 3A: AP radiograph of the knee of a 32-year-old man with a skeletal osteochondroma of the right femur Figure 3B: Lateral radiograph of the knee of a 32-year-old man with a skeletal osteochondroma of the right femur Figure 3C: Wide excision of the tumor (left) and a large bursa (right) was done through an anterolateral approach to the thigh

Figure 3D: Reconstruction of the defect was done using fresh frozen cadaveric femoral head as a bone void filler and rigid internal fixation with a titanium low contact plate and screws
Figure 3E: AP radiograph after excision of the tumorFigure 3F: Lateral radiograph after excision of the tumor
Figure 3: AP (A) and lateral (B) radiographs of the knee of a 32-year-old man with a skeletal osteochondroma of the right femur. Diagnosis of the tumor was obtained incidentally at childhood. The patient presented with new onset of pain and growth of the tumor over the past year. Wide excision of the tumor (left) and a large bursa (right) was done through an anterolateral approach to the thigh (C). Reconstruction of the defect was done using fresh frozen cadaveric femoral head as a bone void filler and rigid internal fixation with a titanium low contact plate and screws (D). AP (E) and lateral (F) radiographs after excision of the tumor.

The risk of complications related to surgical resection of benign osteochondromas is up to 13%; these include neurapraxia, arterial laceration, compartment syndrome, intraoperative or postoperative fracture,111,114 recurrences that may be symptomatic, neuroma formation, wound infection, and growth arrest because of growth plate injury.116

The overall recurrence rate after resection of skeletal osteochondromas has been estimated at 2%.110,111 It is important to completely resect the overlying perichondrium since inadequate excision of this tissue significantly increases the risk of recurrence. Leakage of the often myxomatous cartilage tissue into the surgery bed can also result in soft-tissue recurrence, although this is rare.111,116

Conclusion

Skeletal osteochondromas represent the most common bone tumors. The cortical and medullary continuity of the lesion with the underlying parent bone is pathognomonic. Variants of osteochondromas include hereditary multiple exostoses, subungual exostosis, dysplasia epiphysealis hemimelica, turret or traction exostosis, bizarre parosteal osteochondromatous proliferation, florid reactive periostitis, and metachondromatosis.

Numerous complications are associated with osteochondromas, including mechanical effects and deformity, fracture, vascular compromise, neurologic sequelae and overlying bursa formation. Malignant transformation, although rare, is one of its severe complications. These complications are more common in patients with multiple exostoses as opposed to solitary lesions. Treatment of solitary lesions should be individualized; small asymptomatic lesions should be treated with observation alone. Larger or giant symptomatic osteochondromas should be treated with wide bone resection and reconstruction of the defect using bone allografts and rigid internal fixation.

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Authors

Drs Mavrogenis, Papagelopoulos, and Soucacos are from the First Department of Orthopedics and Attikon General University Hospital, Athens University Medical School, Athens, Greece.

Drs Mavrogenis, Papagelopoulos, and Soucacos have no relevant financial relationships to disclose. Dr Morgan, CME Editor, has disclosed the following relevant financial relationships: Stryker, speakers bureau; Smith & Nephew, speakers bureau, research grant recipient; AO International, speakers bureau, research grant recipient; Synthes, institutional support. Dr D’Ambrosia, Editor-in-Chief, has no relevant financial relationships to disclose. The staff of ORTHOPEDICS have no relevant financial relationships to disclose.

The material presented at or in any Vindico Medical Education continuing education activity does not necessarily reflect the views and opinions of Vindico Medical Education or ORTHOPEDICS. Neither Vindico Medical Education or ORTHOPEDICS, nor the faculty endorse or recommend any techniques, commercial products, or manufacturers. The faculty/authors may discuss the use of materials and/or products that have not yet been approved by the US Food and Drug Administration. All readers and continuing education participants should verify all information before treating patients or utilizing any product.

Correspondence should be addressed to: Panayiotis J. Papagelopoulos, MD, DSc, Athens University Medical School, 4 Christovassili St, 15451, Neo Psychikon, Athens, Greece.

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