Clinical History
The patient presented to the clinic complaining of nonspecific discomfort in the region of his left hip. The patient was an otherwise healthy person, denying any previous trauma to the painful area. The physical examination was unrevealing. Conventional radiography was obtained of the left hip (Fig. 1), which was further evaluated with MR imaging (Fig. 2).
Imaging Findings
The patient presented to the clinic complaining of nonspecific discomfort in the region of his left hip. He was an otherwise healthy person, denying any previous trauma to the painful area. The physical examination was unrevealing. Conventional radiography was obtained of the left hip (Fig. 1), which was further evaluated with MR imaging (Fig. 2).
Radiography of right proximal femur shows a lytic lesion with a clearly-defined nonsclerotic margins involving the medullary space of the intertrochanteric region, extending into the proximal diaphysis. Endosteal scalloping was observed, with marked thinning of the cortex laterally and medially. There was no evidence of cortical break-through or adjacent soft tissue mass and no definable matrix. Coronal T1-weighted MR imaging showed a uniformly hypointense mass and axial fat-suppressed T2-weighted MR imaging showed a fluid-fluid level with intermediate signal fluid below the horizontal line and a larger component of diffusely hyperintense signal above it, the latter representing less dense fluid. Coronal fat-suppressed contrast-enhanced T1-weighted MR imaging showed thin marginal enhancement of this uniformly hypointense process.
Discussion
Unicameral bone cysts or simple bone cysts are typically found in the long bones of pediatric patients. Most commonly found in the proximal femur or humerus, the etiology of these benign tumors is still controversial, with arguments for developmental, neoplastic, and vascular causes [1]. Often, these tumors of the proximal metaphyses of long bones are found incidentally or following a pathologic fracture [2]. Specifically, unicameral bone cysts were cited in one study as being the underlying lesion in as high as 40% of pediatric pathologic femoral neck fractures [3].
When symptomatic, the patient’s history often includes previously unreported pain or discomfort with extremity use. Clinical observation may reveal findings to include limp, failure to weight bear, or inability to use the extremity in a normal fashion [1].
Radiographic findings reveal a lytic lesion with well-defined margins without evidence of reactive sclerosis. Expansile properties of a cyst may also be observed [1,2]. Radiography is often diagnostic, particularly when a “fallen fragment” sign is seen, which is secondary to a cortical fragment that has been displaced internally within the fluid-filled cyst. CT may show fluid-fluid levels which are viewed as nonspecific and suggests hemorrhage associated with a pathologic fracture [4]. MR imaging usually shows an expansile process containing homogeneous fluid signal. Fluid-fluid levels when present are well visualized on MR examination as a horizontal line separating dependent blood products from less dense fluid. Heterogeneous signal intensity and areas of thick and nodular peripheral enhancement are most likely associated with pathologic fracture and early healing [2].
Treatment options for unicameral bone cysts have changed over time. Lysosomal bone destruction was recommended to be corrected by both surgical and pharmacologic means [5]. Open surgical options, though still effective in the treatment of other benign lesions, were replaced by steroid injections [1]. Typically, two or even three injections of methylprednisolone are given [1]. Recently however, reports have demonstrated that demineralized bone matrix injection delivered percutaneously may result in cyst resolution with one injection in 80% to 90% of patients [1]. It should be pointed out that despite treatment, unicameral bone cysts can recur, refracture, and rarely, cause growth arrest. Additionally, resistant lesions with high risk of fracture can be immobilized via internal fixation [1].
Differential Diagnosis List
