Bone Island Imaging

Updated: Dec 06, 2015
  • Author: Sidney P Regalado, MD; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Overview

Overview

A bone island, also known as an enostosis, is a focus of compact bone located in cancellous bone (see the images below). [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]

Plain-film radiograph of a bone island. Bone islan Plain-film radiograph of a bone island. Bone islands typically appear as sclerotic, round-to-ovoid intramedullary foci. The long axis of the bone island is aligned parallel to the long axis of the bone.
T1- weighted, sagittal magnetic resonance imaging T1- weighted, sagittal magnetic resonance imaging (MRI) scan of the knee. Because bone islands are composed of cortical bone, they demonstrate low signal intensity on MRI scans.

Gould et al provided and in-depth review of 13 cases of bone tumor mimics (ie, a benign osseous lesion that radiologically resembles a bone tumor) and noted that this is not an uncommon finding. They reported that diagnosis and management of such lesions requires knowledge of the epidemiology, clinical presentations, anatomy, and imaging features of these benign lesions to prevent misdiagnoses. [3]

Achong reported on a case of increased uptake of radionuclide in a bone island of the lumbar vertebral body; the increased uptake was seen on single-photon emission computed tomography (SPECT) imaging but not conventional planar imaging. [4]

Preferred examination

Bone islands, which are usually found incidentally on imaging studies, demonstrate characteristic radiographic findings. In 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. [3, 5]

Osteopoikilosis

Osteopoikilosis, a skeletal dysplasia, manifests radiographically as multiple bone islands that typically are situated in a peri-articular distribution in the epiphyses (and often the metaphyses) of long and short tubular bones, as well as in the pelvis and scapulae (see the image below). The distribution is typically bilateral and symmetrical. The ribs, clavicles, spine, and skull are rarely involved. [6, 7, 8]

Osteopoikilosis. A plain film radiographic image o Osteopoikilosis. A plain film radiographic image of the knee demonstrates numerous bone islands in a peri-articular distribution that is characteristic of osteopoikilosis.

The process usually does not result in symptoms or abnormal laboratory values, although associations with mild arthralgias and/or dermatologic manifestations have been reported. As with solitary bone islands, the multiple bone islands of osteopoikilosis typically are not apparent on bone scintigraphy studies and can therefore usually be distinguished from multifocal osteoblastic metastases (see the images below). [9, 10, 11, 12]

Osteopoikilosis. A plain film radiograph of the pe Osteopoikilosis. A plain film radiograph of the pelvis with numerous bone islands that are characteristic of osteopoikilosis
Osteopoikilosis. A whole body bone scan for the ra Osteopoikilosis. A whole body bone scan for the radiograph of the same patient as in the previous image does not demonstrate increased scintigraphic activity. Osteopoikilosis typically does not appear "hot" on bone scans.
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Radiography

The radiographic appearance of a bone island is well described in the literature. [13] 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 (see the image below). [1]

Plain film radiograph of the hip. The right iliac Plain film radiograph of the hip. The right iliac bone demonstrates a bone island with "thorny" radiations that merge with the surrounding trabeculae.

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. [14, 15, 16, 17] 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. [18]

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Computed Tomography

Bone 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 (see the images below). [1, 19]

Plain film radiograph of the pelvis. A sclerotic f Plain film radiograph of the pelvis. A sclerotic focus that is compatible with a bone island is seen in the right iliac bone adjacent to the sacroiliac joint. Bone islands typically are 1 mm to 2 cm in diameter.
Computed tomography (CT) scan. A sclerotic focus t Computed tomography (CT) scan. A sclerotic focus that correlates with the radiograph in the previous image is seen in the right iliac bone adjacent to the sacroiliac joint. CT scan findings of bone islands correlate with their plain film appearance.
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Magnetic Resonance Imaging

Because 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 (see the images below). [1]

T1- weighted, sagittal magnetic resonance imaging T1- weighted, sagittal magnetic resonance imaging (MRI) scan of the knee. Because bone islands are composed of cortical bone, they demonstrate low signal intensity on MRI scans.
Magnetic resonance imaging (MRI) scan of the hip. Magnetic resonance imaging (MRI) scan of the hip. A T1-weighted, coronal MRI scan demonstrates a focus of low signal intensity. The bone marrow is abnormally hypo-intense, because normal fatty bone marrow has been replaced with red marrow in this patient, who also has sickle cell disease.
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Nuclear Imaging

Bone islands do not usually demonstrate increased radiotracer activity on bone scans (see the image below). 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. [20] However, several reports exist in the literature of biopsy-proven bone islands that have uncharacteristically demonstrated increased radiotracer activity on bone scans. [13] 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. [4]

Anteroposterior and posteroanterior views from a w Anteroposterior and posteroanterior views from a whole body bone scan do not demonstrate a corresponding focus of increased scintigraphic activity with the radiograph shown in the previous section on Radiography. Bone islands typically do not appear "hot" on bone scans.

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. [21] 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. [14, 15, 16]

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