Bone Island

Bone islands (enostoses) are osteoblastic benign bony lesions. They consist of focal regions of mature bone cortex at the site of trabecular bone. Bone islands are thought to result from failure of bone resorption during endochondral ossification process or due to congenital etiology or failure of the development during childhood. The prevalence of bone islands is variable; the incidence of bone islands in the spine ranges between 1% and 15% on imaging studies. Clinically, bone islands are asymptomatic and mostly diagnosed incidentally on imaging done for other indications. This chapter focuses on the imaging features of bone islands involving the spine and their differential diagnosis with osteoblastic metastases. Meticulous analysis of the imaging features allows final characterization of bone islands in most cases. Although the lesions may be detected on conventional radiography, this examination is less sensitive for detection because of the complex anatomy of the spine. Computed tomography is better suited for detection and plays a pivotal role in the characterization of dense bone islands based on morphological (spicular margins) and quantitative parameters (Hounsfield units). On magnetic resonance imaging, dense bone islands are hypointense on all pulse sequences. As bone islands need no further intervention or treatment, it is of utmost importance to avoid unnecessary biopsy and potential harmful treatment.