A bone island, technically known as enostosis, is usually asymptomatic, according to National Center for Biotechnology Information. Asymptomatic means enostosis has no known symptoms, and the condition reveals itself during a bone scan performed for other reasons. Bone islands are benign, notes Orthopaedics One.
Enostosis is marked by a spot of compact bone within that of spongy bone. The benign spot forms during the bone's development or congenitally, and bone islands are believed to occur during the layering of new bone tissue, notes Skeletal Radiology. Bone islands typically occur in the pelvis, femur, ribs and other long bones.
In radiological scans, bone islands appear as lesions between 2 millimeters and 2 centimeters in diameter, according to Orthopaedics One. MRIs typically show no destruction of surrounding tissues with regards to enostosis lesions. Doctors may monitor bone islands for growth later. If the spot enlarges 25 percent in six months or 50 percent in one year, a closer examination of the bone island may be needed.
Multiple bone islands in the same area are collectively called osteopoikilosis, notes Radiopaedia. Bone islands more than 2 centimeters in diameter are known as giant bone islands as these structures may show slow growth over time. Large enostoses may interfere with diagnosing malignancies in known cancer patients, and these large spots may be confused with other bony growths or bone tumors during the initial radiological scans.