(Radiographics. 2000;20:1407-1434.)
© RSNA, 2000
Imaging of Osteochondroma: Variants and Complications with Radiologic-Pathologic Correlation1
Mark D. Murphey, MD ,
James J. Choi, MD , 2,
Mark J. Kransdorf, MD,
Donald J. Flemming, CDR, MC, USN and
Frances H. Gannon, MD
1 From the Departments of Radiologic Pathology (M.D.M., J.J.C.) and Orthopedic Pathology (F.H.G.), Armed Forces Institute of Pathology, 6825 16th St NW, Bldg 54, Rm M-133A, Washington, DC 20306; the Departments of Radiology and Nuclear Medicine, Uniformed Services University of Health Sciences, Bethesda, Md (M.D.M.); the Department of Radiology, University of Maryland School of Medicine, Baltimore (M.D.M.); the Department of Radiology, National Naval Medical Center, Bethesda, Md (D.J.F.); and the Department of Radiology, Mayo Clinic, Jacksonville, Fla (M.J.K.). Received April 12, 2000; revision requested April 27 and received May 19; accepted May 19. Address correspondence to M.D.M. (e-mail: murphey@afip.osd.mil).
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Abstract
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Osteochondroma represents the most common bone tumor and is a developmental lesion rather than a true neoplasm. It constitutes 20%–50% of all benign bone tumors and 10%–15% of all bone tumors. Its radiologic features are often pathognomonic and identically reflect its pathologic appearance. Osteochondromas are composed of cortical and medullary bone with an overlying hyaline cartilage cap and must demonstrate continuity with the underlying parent bone cortex and medullary canal. Osteochondromas may be solitary or multiple, the latter being associated with the autosomal dominant syndrome, hereditary multiple exostoses (HME). Complications associated with osteochondromas are more frequent with HME and include deformity (cosmetic and osseous), fracture, vascular compromise, neurologic sequelae, overlying bursa formation, and malignant transformation. Malignant transformation is seen in 1% of solitary osteochondromas and in 3%–5% of patients with HME. Continued lesion growth and a hyaline cartilage cap greater than 1.5 cm in thickness, after skeletal maturity, suggest malignant transformation. Variants of osteochondroma include subungual exostosis, dysplasia epiphysealis hemimelica, turret and traction exostoses, bizarre parosteal osteochondromatous proliferation, and florid reactive periostitis. Recognition of the radiologic spectrum of appearances of osteochondroma and its variants usually allows prospective diagnosis and differentiation of the numerous potential complications, thus helping guide therapy and improving patient management.
Index Terms: Osteochondroma, 40.3113, 40.1542 • Osteochondromatosis, 40.365, 40.782 • Bone neoplasms, 40.3113, 40.1542
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LEARNING OBJECTIVES FOR TEST 5
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After reading this article and taking the test, the reader will be able to:
- Describe the radiologic spectrum of osteochondroma, its variants, and complications, as well as features that allow distinction of malignant transformation.
- Understand the pathologic basis of the radiologic features of osteochondroma, its variants, and complications.
- Recognize the clinical and radiologic manifestations of hereditary multiple exostoses.
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Introduction
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Osteochondroma is the most common benign tumor or tumorlike lesion of bone. The radiographic appearance of this tumor is often diagnostic and reflects its pathologic characteristics, that is, a lesion composed of cortical and medullary bone with an overlying hyaline cartilage cap. However, it is the continuity of this lesion with the underlying native bone cortex and medullary canal that is pathognomonic of osteochondroma. These lesions may be solitary or multiple, with the latter associated with the syndrome hereditary multiple exostoses (HME). Complications are commonly associated with these exophytic masses and include cosmetic and osseous deformity, fracture, vascular compromise, neurologic sequelae, overlying bursa formation, and malignant transformation. Variants of osteochondroma include subungual exostosis, dysplasia epiphysealis hemimelica, turret exostosis, traction exostosis, bizarre parosteal osteochondromatous proliferation, and florid reactive periostitis. Although radiography is often diagnostic, additional imaging modalities including bone scintigraphy, ultrasonography (US), computed tomography (CT), and magnetic resonance (MR) imaging are frequently employed in evaluation of these lesions, particularly when they are symptomatic or in unusual locations. The purpose of this pictorial review is to illustrate the many varied imaging appearances of both uncomplicated and symptomatic osteochondromas and their variants, with an emphasis on the pathologic cause of these manifestations.
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Pathologic Characteristics
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Osteochondromas are developmental lesions rather than true neoplasms and are often referred to as an osteocartilaginous exostosis (or simply exostosis). These lesions result from the separation of a fragment of epiphyseal growth plate cartilage, which subsequently herniates through the periosteal bone cuff that normally surrounds the growth plate (encoche of Ranvier) (1–4) (Fig 1). The mechanism for this separation is not entirely clear, although it likely results from the "cut-back" remodeling during growth of the long bone. Persistent growth of this cartilaginous fragment and its subsequent enchondral ossification (maturation) 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 (1–4) (Fig 2). Osteochondromas enlarge from growth at the cartilage cap, identical to a normal physeal plate. After adolescence and skeletal maturity, osteochondromas usually exhibit no further growth.
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Figure 1. Osteochondroma development. Diagram illustrates the normal long bone with the epiphysis (tan), epiphyseal plate (blue), and "cut back" zone of remodeling (yellow). Periosteal bone cuff or encoche of Ranvier (large arrowhead) is shown with herniation of a small amount of physeal tissue (small arrowhead). This tissue migrates into the metaphysis from patient growth (arrows). The intracortical location of this physeal tissue and subsequent growth results in osteochondroma formation.
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Figure 2. Photograph of a coronally sectioned specimen of an osteochondroma of the rib shows marrow (*) and cortical (arrowheads) continuity with the underlying parent bone. The lobulated hyaline cartilage cap (open arrow) is seen and is similar in appearance to the costal cartilage (solid arrow).
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Several previous studies lend support to this concept of development of osteochondromas. D'Ambrosia and colleagues (5) experimentally produced osteochondroma by transplanting epiphyseal plate tissue into cortex. Cases of traumatically and surgically induced osteochondromas (secondary osteochondroma) have also been reported and are usually related to Salter-Harris injuries that cause in vivo transplantation of physeal tissue (4–8).
Irradiation is also associated with development of osteochondroma, related to its damaging effect on the epiphyseal plate, which causes undifferentiated cartilage tissue to migrate into the metaphysis as described by Langenskiöld and Edgren (9) and others (10–15). Subsequent growth of intracortical cartilage foci leads to the formation of osteochondromas that are identical pathologically and radiographically in all respects to other exostoses. The prevalence of development of osteochondroma in various studies of irradiated patients ranges from 6% to 24% (10–15), with the largest series reporting the lower frequency (15). Osteochondromas represent the most common benign radiation-induced tumor, and such lesions were first reported in 1950 (16). Radiation-induced osteochondroma occurs most frequently in patients who undergo radiation therapy for neuroblastoma or Wilms tumor between the ages of 8 months and 11 years (11–15). In these cases, radiation dose is usually 1,500–5,500 cGy, although Neuhauser and coworkers (17) reported a case that developed after a single dose of 125 cGy (12–17). The latent period before discovery of the osteochondroma following radiation treatment is 3–17 years (mean, 5–12 years) (11–15). Lesions can occur anywhere in the radiation field and may have a higher prevalence with total body irradiation as opposed to localized irradiation (14). Other skeletal manifestations of irradiation may be apparent (scoliosis, atrophy, radiation necrosis), and in rare cases, malignant transformation of a radiation-induced osteochondroma to chondrosarcoma has been reported (2,18).
At gross examination, the cartilage cap of an osteochondroma has a bosselated, shiny, and glistening, bluish-gray surface, reminiscent of cauliflower (Fig 3a). Osteochondromas vary dramatically in size but are usually between 1 and 10 cm. The thickness of the cartilage cap is typically 1–3 cm in young patients, whereas in adults it is often only a few millimeters thick or entirely absent, leaving a surface composed of eburnated bone (1,2,19–23). The hyaline cartilage cap is also usually undulating and lobulated, with areas that invaginate into the underlying medullary component of the lesions (Fig 3a, 3b). On cut sections, the cartilage cap often appears with opaque yellow-to-gray areas due to calcification within the cartilage matrix.
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Figure 3a. Resected solitary benign osteochondroma. (a, b) Photographs of coronally sectioned gross specimen (a) and coronally sectioned whole mount specimen (hematoxylin-eosin [H-E] stain) (b) show yellow marrow in the lesion (*) and blue hyaline cartilage cap (arrowheads). There is undulation of the hyaline cartilage that invaginates in several areas into the medullary component (straight arrows) of the osteochondroma. (c) Photomicrograph (original magnification, x250; H-E stain) reveals yellow marrow (*) and trabeculae (straight arrows) within the osteochondroma, the hyaline cartilage cap with columns of cartilage cells (arrowheads), and fibrous capsule or perichondrium (curved arrows in b and c).
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Figure 3b. Resected solitary benign osteochondroma. (a, b) Photographs of coronally sectioned gross specimen (a) and coronally sectioned whole mount specimen (hematoxylin-eosin [H-E] stain) (b) show yellow marrow in the lesion (*) and blue hyaline cartilage cap (arrowheads). There is undulation of the hyaline cartilage that invaginates in several areas into the medullary component (straight arrows) of the osteochondroma. (c) Photomicrograph (original magnification, x250; H-E stain) reveals yellow marrow (*) and trabeculae (straight arrows) within the osteochondroma, the hyaline cartilage cap with columns of cartilage cells (arrowheads), and fibrous capsule or perichondrium (curved arrows in b and c).
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Figure 3c. Resected solitary benign osteochondroma. (a, b) Photographs of coronally sectioned gross specimen (a) and coronally sectioned whole mount specimen (hematoxylin-eosin [H-E] stain) (b) show yellow marrow in the lesion (*) and blue hyaline cartilage cap (arrowheads). There is undulation of the hyaline cartilage that invaginates in several areas into the medullary component (straight arrows) of the osteochondroma. (c) Photomicrograph (original magnification, x250; H-E stain) reveals yellow marrow (*) and trabeculae (straight arrows) within the osteochondroma, the hyaline cartilage cap with columns of cartilage cells (arrowheads), and fibrous capsule or perichondrium (curved arrows in b and c).
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At microscopic examination, the cartilage cap appears to merge with the underlying bone and is covered with a thin layer of fibrous capsule that functions as a perichondrium (1–3,24) (Fig 3c). The cartilage cap resembles a growth plate with columns or clusters of chondrocytes evenly distributed and maturing in an enchondral process (Fig 3c). These chondrocytes can be mildly increased in number with binucleate forms (caused by mechanical or traumatic forces) that, in certain instances, may be mistaken for a more histologically aggressive process (1,2,24). The process of enchondral ossification leads to medullary bone, typically with yellow rather than hematopoietic marrow (Fig 3), and large calcified areas or calcific debris. Examination by electron microscopy reveals proliferating and degenerating chondrocytes that are indistinguishable from normal chondrocytes.
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Solitary Osteochondroma
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Solitary osteochondroma is a frequent lesion estimated to occur in 1%–2% of individuals undergoing extensive radiographic evaluation (1–4,25,26). These lesions constitute 20%–50% of benign bone tumors and 10%–15% of all bone tumors (2,25,26). The vast majority of solitary osteochondromas are asymptomatic and, if detected, are found incidentally (25,26). Symptomatic lesions usually occur in younger patients, with 75%–80% of such cases being discovered before the age of 20 years (1–4,25,26). Solitary osteochondroma has a male predilection, ranging from 1.6–3.4 to 1 (25).
The most common symptom related to osteochondroma is a nontender, painless cosmetic deformity related to the slowly enlarging exophytic mass. Additional complications that cause symptoms include osseous deformity, fracture, vascular compromise, neurologic sequelae, overlying bursa formation, and malignant transformation (27).
Any bone that develops from preformed cartilage (enchondral ossification) may develop an osteochondroma. The long bones of the lower extremity are most frequently affected (50% of cases) and are more commonly involved than those of the upper extremity by a ratio of 2 to 1 (1–4,25,26). As with other bone tumors, osteochondromas occur most often about the knee (40% of cases) (2,22,25). The femur is the single most frequently affected bone (30% of cases), with distal involvement being three times more common than proximal involvement (25). Tibial osteochondromas account for 15%–20% of cases and most commonly occur in a proximal location (2,22,25). The humerus is also a frequent site of osteochondromas (10%–20% of cases) (2,25,26). Other more unusual locations of osteochondroma include small bones of the hands and feet (10% of cases), scapula (4%), pelvis (5%), and spine (2%) (2,22,25,26). Long bone lesions frequently affect the metaphysis, with the diaphysis being a rare location.
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 (Figs 4, 5). The areas of osseous continuity between parent bone and osteochondroma may be broad, involving a large portion of the bone circumference (dimension of the lesion base exceeding its length—sessile osteochondroma), or narrow, with a bulbous tip (pedunculated osteochondroma). Identifying the characteristic cortical and medullary continuity between lesion and parent bone on radiographs is dependent on lesion type (sessile or pedunculated), location, and image projection. This relationship is usually well delineated in lesions of long bones, particularly pedunculated osteochondromas, with standard radiographic projections (although often only on one view) (Fig 6). However, in osteochondromas of flat bones with complex anatomy (ie, pelvis, spine, scapula) and sessile lesions, the continuity and thus the diagnosis may not be apparent on radiographs alone (Fig 7). Pedunculated lesions usually point away from the nearest joint owing to the forces of the overlying tendons and ligaments (although not typically attached to the osteochondroma).
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Figure 4a. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4b. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4c. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4d. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4e. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4f. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 4g. Benign solitary osteochondroma of the tibia in a 15-year-old boy with lesion growth. (a) Initial lateral radiograph of the ankle shows pathognomonic features of osteochondroma with a lesion composed of cortical and medullary bone protruding from the underlying tibia (open arrow). The cortical (solid arrows) and medullary (*) continuity with the tibia was seen on radiographs only in the lateral projection. The cartilage cap is not mineralized and cannot be seen. (b-d) Axial MR images (repetition time msec/echo time msec = 600/16) obtained before (b) and after (c) intravenous administration of gadolinium-based contrast material also reveal the cortical (arrowheads) and marrow (*) continuity with the underlying bone and yellow marrow in the lesion. The hyaline cartilage cap is 3 cm thick (curved arrows), shows mild peripheral and septal contrast material enhancement (straight arrows), and becomes very high signal intensity on the sagittal short-inversion-time inversion recovery (3,000/17, 90 msec inversion time) MR image (d). (e) Lateral radiograph obtained 2 years later shows lesion growth and mineralization that simulate malignant transformation but represent only growth in the skeletally immature patient. (f) Bone scan demonstrates marked increased uptake of radionuclide. (g) Photograph of the sagittally sectioned specimen correlates with the imaging appearance, revealing yellow marrow (*) and the thick hyaline cartilage cap (arrows). Foci of mineralization (x) are also seen, as noted previously in the MR images (b-d).
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Figure 5a. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 5b. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 5c. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 5d. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 5e. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 5f. Benign solitary sessile osteochondroma of the fibula in a 19-year-old man. (a-e) Radiograph (a), axial CT scans (bone [b] and soft-tissue [c] windows), and axial T1-weighted (500/20) (d) and T2-weighted (2,000/90) (e) MR images show the marrow and cortical continuity of an osteochondroma and underlying fibula (large arrowheads). The various degrees of maturity and calcification in the areas of enchondral mineralization cause a heterogeneous appearance on CT and MR images and can be seen in the photograph of the axially sectioned gross specimen (f). Mature areas of bone formation contain yellow marrow architecture (*). Areas of calcified cartilage show high attenuation on CT scans (solid arrows in b and c), are white in the gross specimen (curved arrows), but are very heterogeneous on T2-weighted MR images (low signal intensity with all pulse sequences in densely calcified areas [small arrowheads in d and e] versus higher signal intensity in areas with more prominent components of nonmineralized cartilage [solid arrows in e]). Nonmineralized hyaline cartilage cap (open arrows) is thin (1 cm) and has more fluid characteristic at CT and MR imaging, reflecting its high water content, and has a blue color on the gross specimen. It is more difficult to detect on the soft-tissue window CT scan (c) than with MR imaging.
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Figure 6a. Solitary benign pedunculated osteochondroma of the femur in a 22-year-old man with an associated fracture. (a) Radiograph of the knee reveals a pedunculated osteochondroma with marrow and cortical continuity to the underlying femur (arrowheads). The lesion points away from the knee joint and a lucent area at the base represents a fracture (arrow). (b) Sonogram shows posterior acoustic shadowing from the ossified lesion component and small thin hypoechoic hyaline cartilage cap (*), both of which are easily distinguished from the more superficial hyperechoic fat and muscle. (c) Photograph of the coronally sectioned whole-mount specimen (H-E stain) shows the cortex (open arrows), yellow marrow space (solid arrows), cartilage cap (*), and a portion of the healing fracture (arrowheads).
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Figure 6b. Solitary benign pedunculated osteochondroma of the femur in a 22-year-old man with an associated fracture. (a) Radiograph of the knee reveals a pedunculated osteochondroma with marrow and cortical continuity to the underlying femur (arrowheads). The lesion points away from the knee joint and a lucent area at the base represents a fracture (arrow). (b) Sonogram shows posterior acoustic shadowing from the ossified lesion component and small thin hypoechoic hyaline cartilage cap (*), both of which are easily distinguished from the more superficial hyperechoic fat and muscle. (c) Photograph of the coronally sectioned whole-mount specimen (H-E stain) shows the cortex (open arrows), yellow marrow space (solid arrows), cartilage cap (*), and a portion of the healing fracture (arrowheads).
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Figure 6c. Solitary benign pedunculated osteochondroma of the femur in a 22-year-old man with an associated fracture. (a) Radiograph of the knee reveals a pedunculated osteochondroma with marrow and cortical continuity to the underlying femur (arrowheads). The lesion points away from the knee joint and a lucent area at the base represents a fracture (arrow). (b) Sonogram shows posterior acoustic shadowing from the ossified lesion component and small thin hypoechoic hyaline cartilage cap (*), both of which are easily distinguished from the more superficial hyperechoic fat and muscle. (c) Photograph of the coronally sectioned whole-mount specimen (H-E stain) shows the cortex (open arrows), yellow marrow space (solid arrows), cartilage cap (*), and a portion of the healing fracture (arrowheads).
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Figure 7a. Solitary benign osteochondroma of the pelvis in a 70-year-old man with vague hip pain. (a) Pelvic radiograph shows nonspecific sclerosis over the right supraacetabular region (arrow). Bone scan (not shown) revealed only mild radionuclide uptake. (b) CT scan reveals an osteochondroma with characteristic marrow (*) and cortical (arrowheads) continuity. The overlying soft-tissue attenuation could simulate a thick nonmineralized cartilage cap (arrows) and the possibility of malignant transformation. However, this finding represents iliacus muscle draped over the osteochondroma rather than a cartilage cap. (c) Photograph of a coronally sectioned whole-mount specimen reveals the cortical (solid arrows) and medullary (*) continuity with the underlying parent bone and lack of significant cartilage cap (open arrow).
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Figure 7b. Solitary benign osteochondroma of the pelvis in a 70-year-old man with vague hip pain. (a) Pelvic radiograph shows nonspecific sclerosis over the right supraacetabular region (arrow). Bone scan (not shown) revealed only mild radionuclide uptake. (b) CT scan reveals an osteochondroma with characteristic marrow (*) and cortical (arrowheads) continuity. The overlying soft-tissue attenuation could simulate a thick nonmineralized cartilage cap (arrows) and the possibility of malignant transformation. However, this finding represents iliacus muscle draped over the osteochondroma rather than a cartilage cap. (c) Photograph of a coronally sectioned whole-mount specimen reveals the cortical (solid arrows) and medullary (*) continuity with the underlying parent bone and lack of significant cartilage cap (open arrow).
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Figure 7c. Solitary benign osteochondroma of the pelvis in a 70-year-old man with vague hip pain. (a) Pelvic radiograph shows nonspecific sclerosis over the right supraacetabular region (arrow). Bone scan (not shown) revealed only mild radionuclide uptake. (b) CT scan reveals an osteochondroma with characteristic marrow (*) and cortical (arrowheads) continuity. The overlying soft-tissue attenuation could simulate a thick nonmineralized cartilage cap (arrows) and the possibility of malignant transformation. However, this finding represents iliacus muscle draped over the osteochondroma rather than a cartilage cap. (c) Photograph of a coronally sectioned whole-mount specimen reveals the cortical (solid arrows) and medullary (*) continuity with the underlying parent bone and lack of significant cartilage cap (open arrow).
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The hyaline cartilage cap is quite variable in its appearance on radiographs. Frequently, the chondroid nature of this region is suggested by the identification of arcs and rings or flocculent calcification. The thickness of the cartilage cap is usually not well evaluated with radiography unless there is extensive mineralization (Figs 4, 5).
Bone scintigraphy of osteochondromas varies and is directly correlated with the degree of enchondral bone formation (23,26,28–30). As such, there is generally more prominent radionuclide uptake in osteochondromas in younger patients, although this increased activity often persists well beyond the time of skeletal maturity (Fig 4f). Osteochondromas without increased radionuclide uptake (quiescent lesions) are more frequent in older patients (23).
The three-dimensional imaging capability of CT often allows optimal depiction of the pathognomonic cortical and marrow continuity of the lesion and parent bone in osteochondromas (21,23,31–34). This is particularly true for lesions in complex areas of anatomy, such as the
pelvis or spine, and for those with a broad stalk of attachment. In fact, in our opinion, very thin sections available with CT are often superior to MR imaging in these cases. Measurement of hyaline cartilage cap thickness with CT has met with more variable success in the literature (19,21,31). Mineralization in the cartilage cap allows very accurate measurement with CT. However, it can be very difficult to accurately measure the thickness of an entirely nonmineralized cartilage cap because it cannot be easily differentiated from surrounding muscle or bursa (Figs 5, 7). In fact, Hudson and coworkers (31) believed CT did not reliably depict cartilage caps less than 2.5 cm in maximum thickness. In our opinion, CT is usually accurate in measuring cartilage cap thickness, and we suggest that previous studies were relatively early in experience with musculoskeletal CT. The nonmineralized areas of cartilage cap are often lower in attenuation than muscle, corresponding to its high water content (75%–80%) (Fig 5). The cartilage cap in benign osteochondromas in studies by Kenney et al (21) and Hudson et al (31) averaged 6 mm and 8 mm,
with a maximum of 2.5 cm. It is vital to emphasize that the hyaline cartilage cap thickness is significantly dependent on the degree of skeletal maturity. 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 (Fig 4).
US also enables accurate measurement of the thickness of the hyaline cartilage cap thickness (19,31,35). In a study by Malghem and coworkers (19), US revealed a mean measurement error of less than 2 mm, with hyaline cartilage cap thickness less than 2 cm. In this study, US was more accurate than CT and similar to MR imaging in evaluation of cartilage cap thickness. US allows surrounding fat (hyperechoic) and muscle to be easily distinguished from the nonmineralized cartilage cap, which appears as a hypoechoic layer (Fig 6b). Areas of mineralization in the cartilage cap and the underlying osseous component show posterior acoustic shadowing. Disadvantages of US include operator dependence, inability to evaluate deep lesions inaccessible to the probe, and lack of evaluation of the osseous components of the lesion.
MR imaging also demonstrates cortical and medullary continuity between the osteochondroma and parent bone (Figs 4, 5), and this distinctive feature is often better seen with this modality as opposed to radiography in complex areas of anatomy. Cortical bone is devoid of mobile protons and remains low signal intensity with all pulse sequences, whereas the medullary component has an appearance of yellow marrow. MR imaging is the best radiologic modality for visualizing the effect of the lesion on surrounding structures and evaluating the hyaline cartilage cap. Mineralized areas in the cartilage cap remain low signal intensity with all MR pulse sequences, although as enchondral ossification proceeds, yellow marrow signal is ultimately apparent. The high water content in nonmineralized portions of the cartilage cap had intermediate to low signal intensity on T1-weighted images and very high signal intensity on T2-weighted MR images (19,36–38) (Figs 4, 5). These features typically allow accurate measurement of the cartilage cap thickness and distinction from overlying muscle on MR images. However, in young patients with active growth and maturation from normal enchondral ossification in the cartilage cap, there may be marked heterogeneity with all MR pulse sequences from the mixture of these tissues (nonmineralized cartilage, calcified cartilage, and ossification with yellow marrow). Low signal intensity at the periphery of the lesion represents the perichondrium (36). Enhancement after the intravenous administration of gadolinium-based contrast material reveals septal and peripheral enhancement in the cartilage cap (37–40) (Fig 4c).
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Hereditary Multiple Exostoses
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HME, also known as familial osteochondromatosis or diaphyseal aclasis, is characterized by the development of multiple osteochondromas (1,2). HME was differentiated from the solitary form of osteochondroma in 1786, with the earliest known description of an afflicted family reported by Boyer in 1814 (41). The estimated prevalence of HME is 1:50,000 to 1:100,000 in Western populations (42–45) and may be as high as 1:1,000 in the Chamorros, the indigenous people of Guam and the Mariana Islands (42,46).
The number of exostoses, the degree and type of angular deformity, and even the rate of malignant transformation vary significantly in HME, even within families. The disease shows an autosomal dominant inheritance pattern, with incomplete penetrance in females (47,48). Approximately two-thirds of affected individuals have a positive family history (49). The specific genetic abnormalities have recently been detected, with three distinct loci: one each on chromosomes 8, 11, and 19 (49–52).
The genetic basis for HME stems from the identification of similar osseous changes in Langer-Giedion syndrome, a contiguous gene syndrome that maps to chromosome band 8q24.1 (53,54). This observation suggests that the gene involved in HME also maps to this region and is responsible for the skeletal changes in Langer-Giedion syndrome (42,53,54). Genetic linkage analysis of this region confirms the presence of a locus for HME as being EXT1 on chromosome 8, and additional loci on chromosomes 11 and 19, designated as EXT2 and EXT3 (42,52–55).
Sequence analysis of the EXT loci in patients with HME has suggested that the EXT genes on chromosomes 8, 11, and 19 (49,52–55) function as tumor suppressor sites. These observations have also suggested a model in which inactivation of EXT genes leads to formation of an exostosis, with subsequent inactivation of a second EXT gene (or possibly another gene) causing malignant transformation (48,49,51–55). Recent genetic studies have suggested that three phenotypic patterns exist for HME and correlate with the three genotypes (49). Males are affected approximately 1.5 times more commonly than females (48). The observation of unequal sex ratios, with a significant male preponderance, supports the concept of incomplete penetrance in females (48). This incomplete penetrance may be the result of hormonal factors or perhaps an X-linked modifying gene (48).
The multiplicity of lesions and associated deformity, particularly with severe involvement, lead to early radiologic evaluation and diagnosis of HME, in contrast to the relatively later diagnosis of solitary osteochondromas (49). In fact, the diagnosis of HME is made at birth in some cases (48,49). Most patients are diagnosed by age 5 years, and virtually all are diagnosed by age 12 years (2,49). In families with the genetic predisposition, members who do not demonstrate lesions by age 12 years will not manifest the disease (48,49).
The skeletal distribution of lesions varies, with some authors noting that the typical distribution is bilateral and symmetric, whereas others report a strong unilateral predominance (2,49). This difference may relate to the genetic type of HME, as Carroll and coworkers (49) have noted asymmetric involvement in two of the three genotypes. Nearly every bone, with the exception of the calvaria (above the skull base), has been described as being involved in HME. Specific sites of involvement include the scapula and ribs (40% of cases), humerus (50%–98%), elbow (35%–40%), wrist (30%–60%), hands (20%–30%), pelvis (5%–15%), hips (30%–90%), knees (70%–98%), ankles (25%–54%), and feet (10%–25%) (2,25,26,56) (Fig 8).
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Figure 8a. Manifestations of HME in three patients. (a) Photograph of a 30-year-old patient with HME demonstrates marked forearm abnormalities and milder knee and ankle valgus deformities that result in a mild short stature. (b-d) Pelvic (b), lower extremity (c), and hand (d) radiographs of an 8-year-old patient show extensive undertubulation with widened metaphyses (Erlenmeyer flask deformity). These growth disturbances are much more apparent than the pelvic osteochondromas. Coxa and genu valgus, lateral ankle tilt (arrowhead), extrinsic erosion of the left fibula by the tibial osteochondroma (straight arrow), and osseous bowing of the radius with pseudo-Madelung (curved arrow) deformities are also seen. (e) Photograph of a coronally sectioned whole-mount specimen (H-E stain) from a 9-year-old patient reveals multiple osteochondromas involving the femur. The largest lesion medially shows characteristic medullary and cortical continuity (solid arrows) with the underlying parent bone as well as the overlying hyaline cartilage cap and perichondrium (open arrow). Yellow marrow within the lesion (M), undertubulation of bone with Erlenmeyer flask deformity (*), and multiple smaller lesions (arrowheads) are also seen.
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Figure 8b. Manifestations of HME in three patients. (a) Photograph of a 30-year-old patient with HME demonstrates marked forearm abnormalities and milder knee and ankle valgus deformities that result in a mild short stature. (b-d) Pelvic (b), lower extremity (c), and hand (d) radiographs of an 8-year-old patient show extensive undertubulation with widened metaphyses (Erlenmeyer flask deformity). These growth disturbances are much more apparent than the pelvic osteochondromas. Coxa and genu valgus, lateral ankle tilt (arrowhead), extrinsic erosion of the left fibula by the tibial osteochondroma (straight arrow), and osseous bowing of the radius with pseudo-Madelung (curved arrow) deformities are also seen. (e) Photograph of a coronally sectioned whole-mount specimen (H-E stain) from a 9-year-old patient reveals multiple osteochondromas involving the femur. The largest lesion medially shows characteristic medullary and cortical continuity (solid arrows) with the underlying parent bone as well as the overlying hyaline cartilage cap and perichondrium (open arrow). Yellow marrow within the lesion (M), undertubulation of bone with Erlenmeyer flask deformity (*), and multiple smaller lesions (arrowheads) are also seen.
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Figure 8c. Manifestations of HME in three patients. (a) Photograph of a 30-year-old patient with HME demonstrates marked forearm abnormalities and milder knee and ankle valgus deformities that result in a mild short stature. (b-d) Pelvic (b), lower extremity (c), and hand (d) radiographs of an 8-year-old patient show extensive undertubulation with widened metaphyses (Erlenmeyer flask deformity). These growth disturbances are much more apparent than the pelvic osteochondromas. Coxa and genu valgus, lateral ankle tilt (arrowhead), extrinsic erosion of the left fibula by the tibial osteochondroma (straight arrow), and osseous bowing of the radius with pseudo-Madelung (curved arrow) deformities are also seen. (e) Photograph of a coronally sectioned whole-mount specimen (H-E stain) from a 9-year-old patient reveals multiple osteochondromas involving the femur. The largest lesion medially shows characteristic medullary and cortical continuity (solid arrows) with the underlying parent bone as well as the overlying hyaline cartilage cap and perichondrium (open arrow). Yellow marrow within the lesion (M), undertubulation of bone with Erlenmeyer flask deformity (*), and multiple smaller lesions (arrowheads) are also seen.
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Figure 8d. Manifestations of HME in three patients. (a) Photograph of a 30-year-old patient with HME demonstrates marked forearm abnormalities and milder knee and ankle valgus deformities that result in a mild short stature. (b-d) Pelvic (b), lower extremity (c), and hand (d) radiographs of an 8-year-old patient show extensive undertubulation with widened metaphyses (Erlenmeyer flask deformity). These growth disturbances are much more apparent than the pelvic osteochondromas. Coxa and genu valgus, lateral ankle tilt (arrowhead), extrinsic erosion of the left fibula by the tibial osteochondroma (straight arrow), and osseous bowing of the radius with pseudo-Madelung (curved arrow) deformities are also seen. (e) Photograph of a coronally sectioned whole-mount specimen (H-E stain) from a 9-year-old patient reveals multiple osteochondromas involving the femur. The largest lesion medially shows characteristic medullary and cortical continuity (solid arrows) with the underlying parent bone as well as the overlying hyaline cartilage cap and perichondrium (open arrow). Yellow marrow within the lesion (M), undertubulation of bone with Erlenmeyer flask deformity (*), and multiple smaller lesions (arrowheads) are also seen.
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Figure 8e. Manifestations of HME in three patients. (a) Photograph of a 30-year-old patient with HME demonstrates marked forearm abnormalities and milder knee and ankle valgus deformities that result in a mild short stature. (b-d) Pelvic (b), lower extremity (c), and hand (d) radiographs of an 8-year-old patient show extensive undertubulation with widened metaphyses (Erlenmeyer flask deformity). These growth disturbances are much more apparent than the pelvic osteochondromas. Coxa and genu valgus, lateral ankle tilt (arrowhead), extrinsic erosion of the left fibula by the tibial osteochondroma (straight arrow), and osseous bowing of the radius with pseudo-Madelung (curved arrow) deformities are also seen. (e) Photograph of a coronally sectioned whole-mount specimen (H-E stain) from a 9-year-old patient reveals multiple osteochondromas involving the femur. The largest lesion medially shows characteristic medullary and cortical continuity (solid arrows) with the underlying parent bone as well as the overlying hyaline cartilage cap and perichondrium (open arrow). Yellow marrow within the lesion (M), undertubulation of bone with Erlenmeyer flask deformity (*), and multiple smaller lesions (arrowheads) are also seen.
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Imaging of individual osteochondromas in HME is identical to that of solitary lesions and involves both pedunculated and sessile types of lesions. Carroll et al (49) noted that the amount of involvement and deformity of the forearm and distal leg were a measure of the overall disease extent. Increased number of exostoses, prominent angular deformity, and shortening in these areas indicated more severe, generalized skeletal involvement (49) (Fig 8). In addi
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Abstract
LEARNING OBJECTIVES FOR TEST...
