Bone cyst
A bone cyst, or geode, is a cyst that forms in bone. They are benign and can be found in various bones in the body. Bone cysts may or may not contain fluid. Although many types of bone cysts may be associated with trauma, the cause is often unknown. Types include simple bone cysts, aneurysmal bone cysts, traumatic bone cysts, and epidermoid bone cysts.
Causes
Bone cysts are most commonly found in adolescents. The simple bone cyst is a common, benign, fluid-filled lesion, most frequently found in the metaphysis of long bones, typically the proximal humerus or the top part of the femur. They are twice as common in males as in females. 1 out of 10,000 children is diagnosed with a simple bone cyst every year. Pathologic fractures are common as a result of the weaker bone, often with minor trauma. These cysts can commonly recur but are not typically associated with bone tumors. The cause is unknown.Aneurysmal bone cysts are benign, fluid-filled lesions that can affect any bone. More than half occur in the metaphysis of long bones and between 12 and 30% in the spine. Aneurysmal cysts are more likely to be diagnosed in females than males. The cause is also unknown, but may be associated with bone development due to their location near growth plates.
The traumatic bone cyst, also referred to as a hemorrhagic cyst, is a pseudocyst that most commonly affects the mandible of young individuals. It is a benign, typically empty cavity within the mandible body that does not have evidence of a true epithelial lining. This type of bone cyst is a condition found in the long bones and jaws. There is no definitive cause, though it relates to trauma in the oral region. There is no significant difference in incidence between males and females.
Epidermoid bone cysts are rare lesions as a result of trauma, located only in the phalanx or skull. In the hand, epithelial tissue from the nail bed forms a hard, fluid-filled cyst following trauma.
Signs and symptoms
Many bone cysts are incidental findings from routine exams or imaging for other reasons. Simple bone cysts are typically asymptomatic until the affected bone is broken. Individuals are at a higher risk of fracture, including from a minor injury, because the cyst causes the outer layer of bone to thin. Aneurysmal bone cysts are also often only diagnosed following a pathologic fracture. Some may experience pain, swelling, decreased range of motion. Bone cysts in the spine can cause back pain, limited mobility, and other neurological symptoms, such as a radiculopathy. Epidermoid bone cysts transilluminate when a flashlight is pointed at the affected area, such as the fingertip.Diagnosis
Aneurysmal bone cysts will appear as an empty space on X-ray with thinned surrounding bone. An MRI can better visualize the exact boundaries of the cyst and fluid inside indicative of an aneurysmal bone cyst. On CT scans, aneurysmal bone cysts have a radiodensity of 20 Hounsfield units or less and are osteolytic. In contrast, intraosseous lipomas have a lower radiodensity of -40 to -60 HU. A biopsy is necessary for diagnosis, either via a core needle biopsy or open biopsy.Simple bone cysts may appear similar to sarcomas on X-ray. Therefore, further imaging, needle aspiration, or a biopsy may be necessary for diagnosis.
Traumatic bone cysts appear on X-ray as a single bright area with an irregular but well-defined outline.