Simple Bone Cyst Imaging
- Author: Eu-Leong Harvey Teo, MBBS; Chief Editor: Felix S Chew, MD, MBA, MEd more...
The simple bone cyst is a common, benign, fluid-containing lesion, usually occurring in the metaphysis of long bones. The cause of the lesion is unknown. Bloodgood recognized it as a different entity from other cystic bone lesions in 1910. Jaffe and Lichtenstein provided a detailed discussion of simple bone cysts in 1942. (See the images below.)
Plain radiography is the examination of choice because of its high diagnostic capability of simple bone cysts.[3, 4, 5] 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 often are 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 simple bone cysts.[6, 7, 8]
Suei et al studied the relationship between the radiographic findings and treatment outcome (healing or recurrence) in 31 cases of simple bone cysts 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 of and prognosis and that, therefore, radiographic examination should be used not only for discovering and diagnosing such lesions but also for helping predict their prognosis.
Yandow et al reported that in 5 patients who received 7 contrast injections of simple bone cysts, 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.
Radiographs demonstrate simple bone cysts as well-defined, geographic lesions with narrow transition zones. A thin sclerotic margin is a typical finding. Simple bone cysts usually are situated in the intramedullary metaphyseal region immediately adjacent to the physis. Occasionally, they may be diaphyseal. See the image below.
The long axis of the lesion parallels that of the long axis of the tubular bone. Simple bone cysts may cause expansion of the bone with thinning of the overlying cortex. Some may have a multilocular appearance. In long bones, simple bone cysts typically are centrally located within the medullary cavity. See the image below.
A pathologic fracture through a simple bone cyst 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 simple bone cyst and other nonlytic bone lesions. When present, the fallen fragment sign is pathognomonic of a simple bone cyst. See the image below.
Simple bone cysts 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.
Degree of confidence
Radiography usually is sufficient to confirm the diagnosis of simple bone cysts.
The fallen fragment sign in a cystic lesion is pathognomonic of a simple bone cyst. It indicates the internal contents of the lesion are nonsolid and fluid-filled.
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.
CT scanning often is not necessary in the evaluation of simple 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 features of a simple bone cyst observed on plain radiography also can be appreciated on CT. Occasionally, air and air-fluid levels may be seen within simple bone cysts. Fluid-fluid levels also may be noted. Dynamic CT scanning may help in differentiating a fluid-containing simple bone cyst, which is avascular, from other solid benign bone lesions that demonstrate varying degrees of vascularity. See the images below.
Degree of confidence
The presence of a fallen fragment sign on CT is diagnostic of a simple bone cyst. CT has high sensitivity and specificity for detecting simple bone cysts.
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, simple bone cyst, recurrent malignant fibrous histiocytoma of bone, osteosarcoma, or aneurysmal bone cyst.
Magnetic Resonance Imaging
MRI can confirm the presence of fluid within a simple bone cyst. Uncomplicated simple bone cysts 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. See the images below.
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.
Degree of confidence
Uncomplicated lesions are diagnosed easily on MRI. Lesions complicated by pathologic fractures may reveal areas of heterogeneous signal and irregular enhancement patterns after the administration of IV contrast. This lowered specificity and sensitivity makes diagnosis more difficult.
Simple bone cysts show little or no uptake of tracer material in radionuclide bone scans unless they have been traumatized. See the image below.
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