Review Question - QID 218690

The patient has a unicameral bone cyst of the proximal humerus through which she has sustained a pathologic fracture. Initial treatment should include immobilization for 4-6 weeks until signs of radiographic healing (Answer 4)

Unicameral bone cysts (UBC), or simple cysts of bone, are non-neoplastic, serous, fluid-filled lesions of bone most commonly found in the proximal humerus. They occur most frequently in males with a mean age of 9 years and can be classified as either active or latent. Active cysts are adjacent to the physis and migrate away from the growth plate with age, eventually becoming latent when normal bone is formed between the cyst and the physis. On radiographs, UBCs appear as central, lytic, well-demarcated metaphyseal lesions. When a pathologic fracture occurs through a UBC, a cortical piece of bone can often be seen resting inside of the cyst and is termed the "fallen leaf sign" (Illustration A). It is important to differentiate these benign lesions from aneurysmal bone cysts (ABC) and telangiectatic osteosarcoma, both of which are more aggressive and should be within the differential diagnosis. Treatment of pathologic fractures through a UBC in the proximal humerus can most often be treated with immobilization alone, and up to 15% of lesions will fill in with native bone after an acute fracture. Symptomatic lesions without concomitant fracture should be aspirated and subsequently injected with a steroid (methylprednisolone acetate) solution. Oftentimes, repeated injections are necessary, especially in very young patients. In patients who have not responded to multiple injections or those with lesions in the proximal femur, curettage and bone grafting with or without internal fixation should be considered. Proximal femur lesions, in particular, have a high rate of recurrence and/or malunion after sustaining a pathologic fracture through a UBC. Of note, internal fixation should be avoided in active lesions, as communication with the growth plate can result in growth arrest at the adjacent physis.

Donaldson, et al. reviewed 24 patients with simple bone cysts to evaluate whether or not they resolve with age. Of the 24 subjects, 15 (63%) were male, 18 (75%) cysts were located in the humerus and 4 (25%) were located in the femur. The authors concluded that despite the common assumption that simple bone cysts resolve with skeletal maturity, none of the studied cysts were graded as completely healed even though 87% of growth plates had closed by the termination of the study.

Wilkins provides a current concepts review of unicameral bone cysts (UBC) in the Journal of the American Academy of Orthopaedic Surgeons. The author notes that once diagnosed, a UBC continues to be a treatment dilemma. Traditional methods, such as steroid injections, usually involve multiple anesthetics and injections and are associated with recurrence, while major surgical procedures, such as curettage and bone grafting, though somewhat more effective, still carry the potential for significant morbidity. He concludes that newer techniques involving percutaneous grafting with allograft or bone substitutes are promising in light of their low complication rates and lower re-operation rates.

Figure A is an AP radiograph of a left shoulder demonstrating a UBC of the proximal humerus with a pathologic fracture and the pathognomonic "fallen leaf sign." Figure B is a histology slide showing the cyst with a thin fibrous lining containing fibrous tissue, giant cells, and some hemosiderin pigment. Findings include a uniform population of spindle cells without nuclear atypia, indicating a benign process. Illustration A is a labeled version of Figure A, with an arrow depicting the "fallen leaf sign," which represents a fallen cortical fragment from the pathologic fracture that is resting within the contents of the cyst.

Incorrect Answers:
Answer 1: Aspiration of the cyst and intra-lesional injection of steroid is a reasonable treatment option for active cysts without a pathologic fracture. This would be a reasonable option in this patient if the cyst remains after the fracture has healed, as fracture healing does not usually result in cyst resolution.
Answer 2: Surgical treatment with curettage, bone grafting, and internal fixation is a reasonable option for symptomatic latent lesions that have not responded to aspiration and injections, or those in areas with structural concern for pathologic fracture, such as in the proximal femur.
Answer 3: Aggressive curettage and bone grafting with the use of an adjuvant such as phenol or liquid nitrogen is more frequently used in cases of aneurysmal bone cysts (ABC), not in UBC.
Answer 5: Neoadjuvant chemotherapy, wide resection, and adjuvant chemotherapy is used in the setting of telangiectatic osteosarcoma, which should always be in the differential diagnosis when treating a suspected ABC.