A 21-year-old man presents with 4 to 5 months of right upper arm pain. He works in a warehouse and lifting objects has become increasingly painful. The pain became severe at work, which prompted him to visit an urgent care center. Anteroposterior and lateral radiographs of the right humerus show a well-circumscribed cystic lesion in the proximal humerus. MRI shows a 4-cm intramedullary lesion within the right proximal humeral diaphysis consistent with a unicameral bone cyst. There is endosteal erosion consistent with an impending or early fracture.
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Unicameral bone cysts (UBCs), or simple bone cysts, are benign fluid-filled lesions that are most commonly found in proximal long bones such as the proximal humerus and femur. UBCs contain a single fluid-filled cavity that contains prostaglandins and other enzymes. UBCs are usually diagnosed incidentally on radiograph, although some may present symptomatically as a pathologic fracture. The cyst may encroach and thin corticol bone around the cyst, which increases fracture risk. Characteristics found on radiograph and MRI help differentiate UBCs from other cystic lesions, including an aneurysmal bone cyst, fibrous dysplasia, enchondroma, and possible malignancy.1,2
Patients with an asymptomatic UBC found incidentally on radiograph can be managed with observation if the risk of pathologic fracture is low. Risk factors for pathologic fracture include a cyst taking up more than 85% of the transverse diameter of the affected bone and cortical thinning with a cyst wall <0.5 mm thick. UBCs often start to decease in size when skeletal growth is complete. In general, patients who present with pathologic fracture can be treated conservatively with a sling for 4 to 6 weeks. Pathologic fracture may decompress the UBC; however, few, or <10% of lesions, will completely fill in with bone. The most popular treatment for active symptomatic UBCs that have failed conservative treatment includes a percutaneous intralesional injection of either a steroid (methylprednisolone) or bone graft. The technique is performed under general anesthesia, as a large bore needle such as an 11 gauge is necessary to drain the lesion. Open curettage and bone grafting is a more invasive technique and equivalent to higher recurrence rates of 35% to 45%.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Rougraff BT, Kling TJ. Treatment of active unicameral bone cysts with percutaneous injection of demineralized bone matrix and autogenous bone marrow. J Bone Joint Surg Am. 2002;84-A:921-929.
- Allen D. Unicameral bone cysts. Retrieved from https://www.orthobullets.com/pathology/8035/unicameral-bone-cyst