You are in: eMedicine Specialties > Radiology > MUSCULOSKELETAL Bone IslandArticle Last Updated: May 8, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Sidney P Regalado, MD, Assistant Professor of Radiology, Department of Vascular and Interventional Radiology, University of Chicago Hospital; Consulting Staff, Department of General Radiology, Department of Vascular and Interventional Radiology, Weiss Memorial Hospital Sidney P Regalado is a member of the following medical societies: American College of Radiology, Radiological Society of North America, and Society of Interventional Radiology Coauthor(s): Gregory Scott Stacy, MD, Assistant Professor, Department of Radiology, University of Chicago Hospitals Editors: Amilcare Gentili, MD, Clinical Professor of Radiology, University of California at San Diego; Consulting Staff, Department of Radiology, Thornton Hospital; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Murali Sundaram, MBBS, FRCR, FACR, Consulting Staff, Department of Diagnostic Radiology, The Cleveland Clinic Foundation; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Felix S Chew, MD, MBA, EdM, Professor, Department of Radiology, Vice Chairman for Radiology Informatics, Section Head of Musculoskeletal Radiology, University of Washington Author and Editor Disclosure Synonyms and related keywords: enostosis, enostoses, giant bone island, osteopoikilosis INTRODUCTIONSection 2 of 10
  BackgroundA bone island, also known as an enostosis, is a focus of compact bone located in cancellous bone.1, 2 This is a benign entity that is usually found incidentally on imaging studies; however, the bone island may mimic a more sinister process, such as an osteoblastic metastasis (for example, from prostate cancer).3 PathophysiologyAlthough the exact etiology of bone islands is not clear, they are almost certainly developmental in nature, likely representing cortical bone that has failed to undergo medullary resorption during the process of endochondral ossification. Histologically, bone islands are intramedullary foci of normal compact bone with haversian canals and "thorny" radiations that merge with the trabeculae of surrounding normal cancellous bone. FrequencyUnited StatesThe exact frequency is unknown; however, reports have described a frequency of 1-14%. Mortality/MorbidityBone islands are considered to be benign lesions without associated morbidity or mortality. RaceNo racial predilection is recognized. SexThe prevalence of bone islands is approximately equal in men and women. AgeBone islands are common in the adult population and are rare in children. AnatomyBone islands are rare in the calvaria, a fact that lends additional support to the theory of a minor endochondral defect as a potential etiology. Bone islands can be found in any osseous site; however, they are most commonly identified in the pelvis, long bones, ribs, and spine. Clinical DetailsBone islands are almost invariably asymptomatic lesions that do not result in laboratory abnormalities. A single case report in the English literature describes a patient with a symptomatic, histologically proven bone island of the tibia; the symptoms resolved after curettage.4 Another report described an enlarging mandibular bone island that caused inclination of the adjacent teeth of a young girl.5 A third case report described a symptomatic bone island in the sacrum.6 Preferred ExaminationBone islands, which are usually found incidentally on imaging studies, demonstrate characteristic radiographic findings. Limitations of TechniquesIn the correct clinical context, findings on radiographs are considered diagnostic. However, if the lesion is large or demonstrates increased scintigraphic activity, or if the patient is symptomatic or has a history of malignancy, clinical follow-up and/or biopsy may be warranted. DIFFERENTIALSSection 3 of 10
Bone Infarct Enchondroma and Enchondromatosis Fibrous Dysplasia Osteoblastoma Osteoid Osteoma Osteosarcoma, Classic Osteosarcoma, Variants Other Problems to Be ConsideredSclerotic metastasis RADIOGRAPHSection 4 of 10
FindingsThe radiographic appearance of a bone island is well described in the literature.7 Bone islands are round or ovoid, intramedullary, sclerotic foci that do not extend beyond the cortex. The long axis of a bone island typically parallels the long axis of the involved bone. Bone islands appear homogeneously sclerotic, with "thorny" radiating bone spicules that extend from the center of the lesion and blend with the trabeculae (Image 1).1 Bone islands usually are 1 mm to 2 cm in diameter, with their size typically remaining stable. However, reports have described bone islands that have increased or decreased in size; complete disappearance has also been reported. They can be found at any osseous site, with the pelvis and long bones (especially the proximal femur) most commonly involved. Other involved sites include the ribs, the carpal and tarsal bones, and the thoracolumbar vertebral bodies. When bone islands are larger than 2 cm, they are classified as giant bone islands.8, 9, 10 With the exception of size, giant bone islands demonstrate the same radiographic features that smaller ones do. Giant bone islands are most commonly found in the pelvis; islands of up to 10 cm have been reported.4 CT SCANSection 5 of 10
FindingsBone islands demonstrate computed tomography (CT) scan findings that correlate with their plain film appearance. They are sclerotic and hyperdense foci with "thorny" radiations that blend with surrounding trabeculae (Images 3-4).1 MRISection 6 of 10
FindingsBecause bone islands are composed of cortical bone, on T1- and T2-weighted magnetic resonance imaging (MRI) scans they demonstrate low signal intensity that is characteristic of cortical bone (Images 2, 6).1 NUCLEAR MEDICINESection 7 of 10
FindingsBone islands do not usually demonstrate increased radiotracer activity on bone scans (Image 7). Thus, the bone scan has been used to help differentiate bone islands from more aggressive lesions, such as metastases or primary bone tumors, which demonstrate increased scintigraphic activity.11 However, several reports exist in the literature of biopsy-proven bone islands that have uncharacteristically demonstrated increased radiotracer activity on bone scans.7 An additional case report described a bone island that could be detected only on single-photon emission CT (SPECT) scans, not on conventional planar images.12 The mechanism of increased scintigraphic activity is unclear but has been hypothesized to be related either to increased metabolic activity or to osteoblastic bone remodeling associated with the growth of bone islands.13 According to several reports, giant bone islands are more likely to have increased scintigraphic activity (appearing "warm" rather than "hot") on bone scans than are smaller bone islands.8, 9, 10 INTERVENTIONSection 8 of 10
When radiographic findings that are characteristic of a bone island are demonstrated, the diagnosis should be made with confidence, and no follow-up is required. However, if the bone island is unusually large, shows rapid growth, demonstrates increased scintigraphic activity, or is found in a symptomatic patient or a patient with a history of malignancy that could produce osteoblastic metastases, follow-up and/or biopsy may be indicated. Follow-up can be performed at 3, 6, and 12 months. Open biopsy can be performed if growth exceeds 25% of the lesion's diameter within 6 months or 50% within 1 year; histology will establish the diagnosis. Medical/Legal Pitfalls
Related Medscape topic: Special Concerns
MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
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