Unicameral Bone Cyst (Simple Bone Cyst) Imaging 

Updated: Jul 31, 2020
  • Author: Eu-Leong Harvey Teo, MBBS, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Practice Essentials

The unicameral bone cyst (UBC), or simple or solitary bone cyst (SBC), is a common, benign, fluid-containing lesion, usually occurring in the metaphysis of long bones. The cause of the lesion is unknown. Unicameral bone cysts occur almost exclusively in children and adolescents (85%). The reported peak is between 3 and 14 years of age, with the mean age at diagnosis being approximately 9 years. They are classified as active when they are within 1 cm of the physis and classified as latent as they progress to a diaphyseal location. The differential diagnosis includes aneurysmal bone cyst, fibrous dysplasia, enchondroma, and intraosseous ganglia. [1]  

Unicameral bone cysts have been diagnosed in almost every bone, but more than 95% occur in the long bones, such as the proximal humerus and the femur. [2]  About 6-10% of UBCs have been reported in adults in flat bones. [1, 3]

UBCs are asymptomatic unless there is a pathologic fracture. Signs include tenderness, swelling, and bruising at the site of the fracture. In the case of fracture, a bony fragment may become displaced, which is called a fallen fragment. A fallen fragment can be seen on radiographs and CT scans but less commonly on MRI. With a pathologic fracture, a rising bubble sign may also be seen on CT. This is a bubble of gas in the nondependent portion of the bone cyst indicating a pathologic fracture. This sign may also be seen on MRI. [4, 5, 6, 7, 8]

(See the images below.)

A 9-year-old girl with a simple bone cyst in the d A 9-year-old girl with a simple bone cyst in the distal diametaphyseal region of the tibia. Lateral radiograph reveals the simple bone cyst complicated by a pathologic fracture.

 

A 7-year-old girl with a simple bone cyst in the u A 7-year-old girl with a simple bone cyst in the upper right tibia. CT scan demonstrates the simple bone cyst within the medullary cavity of the tibia. There is thinning of the cortex in the anterolateral portion of the bone (arrow). Thickening and sclerosis is observed in the other parts of the cortex (arrowheads).
A nuclear medicine bone scan reveals mild uptake o A nuclear medicine bone scan reveals mild uptake of tracer material in the left upper tibia corresponding to the site of the simple bone cyst.

 

Preferred examination

Plain radiography is the examination of choice because of its high diagnostic capability for unicameral bone cysts. [9, 10, 11]  Computed tomography (CT) scanning and magnetic resonance imaging (MRI) usually are not required and should only be used for evaluation in anatomically complex areas such as the spine or pelvis. These areas are often difficult to evaluate accurately on plain film. Use CT and MRI to determine the extent of the lesion and whether complications such as a fracture are present. Nuclear medicine scans usually are not necessary in the evaluation of UBCs. In cases of pathologic fracture, the fallen fragment or rising bubble sign may be seen. [12, 13, 14, 4, 5, 6, 7, 8, 15]

Suei et al studied the relationship between the radiographic findings and treatment outcome (healing or recurrence) in 31 cases of UBCs of the jaw to identify whether radiography can predict prognosis. In 17 of 31 cases, radiographic findings included preserved lamina dura adjacent to the lesion, with a smooth margin, and no or smooth bone expansion. All 17 of these lesions healed after surgery. In the other 14 cases, there was resorption of the lamina dura, a scalloped margin, nodular bone expansion, root resorption, and a sclerotic mass or multiple cavities. In 9 of these cases, there was recurrence of bone cysts. From these findings, the authors concluded that there is a relationship between radiographic features and prognosis and that, therefore, radiographic examination should be used not only for discovering and diagnosing such lesions but also for helping determine prognosis. [16]

Yandow et al reported that in 5 patients who received 7 contrast injections of UBCs, large and rapid outflow veins from the solitary bone cysts occurred. Precordial Doppler was able to show increased signal in all 7 particulate injections (2 steroid injections, 5 bone marrow aspirates and cyst injections.) According to the authors, Doppler may be valuable for monitoring the potential harmful effects of such injections and lead to a better understanding of failure of cyst healing because of rapid outflow of material. [17]

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Radiography

Radiographs demonstrate unicameral bone cysts as well-defined, geographic lesions with narrow transition zones. A thin sclerotic margin is a typical finding. UBCs usually are situated in the intramedullary metaphyseal region immediately adjacent to the physis. Occasionally, they may be diaphyseal. [1, 9, 10, 16]  UBCs occurring in the ilium may be large and radiolucent, resembling fibrous dysplasia. Lesions occurring in the spine may be localized to the vertebral body or posterior elements, and diagnosis based solely on radiographic findings is difficult. Difficulty in diagnosis may arise when an enchondroma or fibrous dysplasia occurs in the metaphyseal region of a long bone in a patient in the first 2 decades of life.

(See the image below.)

A 7-year-old boy with a simple bone cyst in the up A 7-year-old boy with a simple bone cyst in the upper metaphyseal region of the right humerus. The cyst has a multilocular appearance.

The long axis of the lesion parallels that of the long axis of the tubular bone. UBCs may cause expansion of the bone with thinning of the overlying cortex. Some may have a multilocular appearance. In long bones, UBCs typically are centrally located within the medullary cavity.

(See the image below.)

A 9-year-old boy with a simple bone cyst in the up A 9-year-old boy with a simple bone cyst in the upper tibia. Lateral radiograph of the upper tibia reveals the cyst to be centrally located within the medullary cavity.

A pathologic fracture through a UBC is a common occurrence. This may lead to the "fallen fragment" sign, which describes the migration of a fragment of bone to a dependent portion of the fluid-filled cyst. It occurs in only a minority of patients. This sign is an important differentiating feature between a UBC and other nonlytic bone lesions. When present, the fallen fragment sign is pathognomonic of a UBC.

(See the image below.)

An 11-year-old boy with a simple bone cyst in the An 11-year-old boy with a simple bone cyst in the diametaphyseal region of the upper humerus. The lesion is complicated by a pathologic fracture. A fragment of bone is observed in the most dependent portion of the cyst (arrow). This finding is known as the ‘fallen fragment' sign and is pathognomonic of a simple bone cyst.

 

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Computed Tomography

CT scanning often is not necessary in the evaluation of unicameral bone cysts because of the high accuracy of diagnosis of radiography. CT occasionally is used in the evaluation of lesions observed in areas difficult to assess on plain radiography, such as the spine and pelvis. The role of CT is to determine the extent of the lesion as well as to detect subtle complications difficult to evaluate on plain radiography.

The presence of a fallen fragment sign on CT is diagnostic of a UBC. The presence of fluid-fluid levels within a bony lesion is not diagnostic of any particular tumor. This sign can be observed on CT in patients with fibrous dysplasia, UBC, recurrent malignant fibrous histiocytoma of bone, osteosarcoma, or aneurysmal bone cyst.

The features of a UBCs observed on plain radiography also can be appreciated on CT. Occasionally, air and air-fluid levels may be seen within UBCs. Fluid-fluid levels also may be noted. Dynamic CT scanning may help in differentiating a fluid-containing UBC, which is avascular, from other solid benign bone lesions that demonstrate varying degrees of vascularity.

(See the images below.)

CT scan reveals a simple bone cyst in the upper me CT scan reveals a simple bone cyst in the upper metaphyseal region of the right femur. The cyst is centrally located in the medullary cavity of the femur with expansion and thinning of the overlying bony cortex.
A 7-year-old girl with a simple bone cyst in the u A 7-year-old girl with a simple bone cyst in the upper right tibia. CT scan demonstrates the simple bone cyst within the medullary cavity of the tibia. There is thinning of the cortex in the anterolateral portion of the bone (arrow). Thickening and sclerosis is observed in the other parts of the cortex (arrowheads).

 

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Magnetic Resonance Imaging

MRI can confirm the presence of fluid within a unicameral bone cyst. Uncomplicated UBCs have low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Lesions that have a pathologic fracture have heterogeneous signal intensities on both T1- and T2-weighted images because of bleeding within the cyst. With gadolinium-diethylenetriamine pentaacetic acid (DTPA) enhancement, they demonstrate enhancement with focal, thick peripheral, heterogeneous, or subcortical patterns. Septations within the lesions may be observed on MRI and may not be visualized on radiographs. [18]

(See the images below.)

A 10-year-old girl with a simple bone cyst in the A 10-year-old girl with a simple bone cyst in the left upper femur. T1-weighted axial image reveals the cyst to be of low signal intensity and is located within the metaphyseal region of the left femur (arrow).
A T2-weighted coronal image with fat saturation de A T2-weighted coronal image with fat saturation demonstrates the cyst has high signal intensity.
A postgadolinium T1-weighted coronal image with fa A postgadolinium T1-weighted coronal image with fat saturation reveals enhancement of the periphery of the cyst.

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

MRI has also been shown useful for evaluating the efficacy of intracavital injection of steroids into bone cysts. MRI reveals the presence of thin reparative tissue lining the cyst wall. This tissue progressively thickens, and new bone formation is also observed. Residual cyst cavities may also be seen with no evidence of enhancing tissue, thus requiring further treatment.

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