Enostosis

Last revised by Dr Joachim Feger on 20 Jul 2022

Enostoses, also known as bone islands, are common benign sclerotic bone lesions that usually represent incidental findings. They constitute a small focus of compact bone within cancellous bone. Enostoses can be seen on radiographs, CT, and MRI, and are considered one of the skeletal “don’t touch” lesions.

Diagnosis is usually made based on radiographic features. CT can sometimes provide superior detail of the 'brush-like' periphery of enostoses. 

The etiology of enostoses is not definitively known. They are most likely congenital or developmental and are thought to represent either hamartomatous lesions or failure of osteoclastic activity during bone remodeling 1.

Although they have at times previously been referred to as benign bone tumors (in part due to occasional increased activity on bone scintigraphy), enostoses have been removed from the WHO classification of bone tumors 2.

Bone islands may occur anywhere in the skeleton, although there is some predilection for the pelvis, long bones, spine, and ribs.

Enostoses are commonly seen as small round or oval foci of homogeneously dense bone within the medullary space. They do not cause cortical destruction or periosteal reaction. The appearance of radiating spicules - "thorny radiation" or "fingers" - at the margins that blend with the surrounding trabeculae is classic, giving a "brush-like" or "stellate" periphery. This is usually best appreciated on CT.

In diaphyseal bone, the long axis of a bone island typically parallels the long axis of the involved bone. In the metaphysis and other regions where trabeculation is not as linearly organized, the bone islands are typically more spherical.

The size of a bone island is typically <1 cm, although large bone islands may occur, particularly in the pelvis, and when greater than 2 cm in maximal dimension are called giant bone islands 4,8. Sometimes a slow increase in size is seen in bone islands over time. 

Enostoses may be solitary or multiple (monostotic or polyostotic). The term osteopoikilosis is applied when there are multiple enostoses, usually concentrated in the epiphyses or metaphyses. 

One study has suggested that enostosis can be differentiated from bone metastases with a mean attenuation threshold of >885 HU and maximum attenuation of >1060 HU with high sensitivity and specificity 2, but exclusive use of attenuation values in the assessment of sclerotic bone lesions has been discouraged 9.

Enostoses are low signal intensity on all sequences (compatible with compact bone), with no surrounding edema. They do not enhance following contrast administration. 

There is usually no technetium-99m MDP uptake, and a normal bone scan can exclude osteoblastic metastases or osteosarcoma. However, low-grade scintigraphic activity has sometimes been reported in histologically proven enostoses, particularly if the lesion is >1 cm 3.

Consider biopsy if the lesion grows 50% in one year or 25% in six months 4.

In the vast majority of cases, bone islands have a pathognomonic appearance. Larger lesions may sometimes pose a diagnostic dilemma, particularly in the setting of known malignancy.  

Imaging differential considerations include:

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Cases and figures

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  • Case 7: in distal radius
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  • Case 8 : MRI
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  • Case 9: MRI
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