AUTHOR INFORMATION	Section 1 of 10
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Author: Eu-Leong Harvey Teo, MBBS, FRCR, Consulting Staff, Department of Diagnostic Imaging, Kandang Kerbau Women's and Children's Hospital Coauthor(s): Wilfred CG Peh, MBBS, MD, FRCPE, FRCPG, FRCR, Clinical Professor, Faculty of Medicine, National University of Singapore; Senior Consultant Radiologist, Programme Office, Singapore Health Services

Eu-Leong Harvey Teo, MBBS, FRCR, is a member of the following medical societies: Royal College of Radiologists, and Society for Pediatric Radiology

Editor(s): Giuseppe Guglielmi, MD, Assistant Professor, Department of Radiology, Scientific Institute Hospital; Bernard D Coombs, MBChB, PhD, Assistant Professor, Department of Radiology, University of Colorado Health Sciences Center; Murali Sundaram, MBBS, FRCR, Department of Radiology, Mayo Clinic of Rochester; Robert M Krasny, MD, Visiting Assistant Professor of Radiology, University of California at Los Angeles Medical Center; Consulting Staff, Tower Imaging, Los Angeles, California; and Felix S Chew, MD, EdM, Vice-Chair for Education, Section Head of Musculoskeletal Radiology, Professor, Department of Radiology, Wake Forest University School of Medicine

	INTRODUCTION	Section 2 of 10
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Background: 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.

Pathophysiology: The pathogenesis of simple bone cysts is unknown.
Evidence exists that venous obstruction and blockage of interstitial fluid drainage, in an area of rapidly growing and remodeling cancellous bone, may play an important role in the formation of unicameral bone cysts. Despite this evidence, the pathogenesis of simple bone cysts has yet to be firmly established.

On gross examination, the cyst expands the cortex of the bone. An intact periosteum covers this thin cortical shell. The cyst usually contains clear serous fluid. Occasionally, blood products may be found within the fluid if a previous fracture has occurred. A membrane of varying thickness lines the inner wall of the cyst. Fibrous septa may form after a fracture and create a multilocular appearance.

Histologically, mesothelial cells line simple bone cysts. The inner wall of bone adjacent to the mesothelial membrane consists of well-vascularized new bone produced by the overlying periosteum. Multinucleated giant cells occasionally may be present within the cyst wall.

Frequency:

Internationally: Simple bone cysts are found in 3% of all biopsies of primary osseous neoplasms.

Mortality/Morbidity:

Half of all simple bone cysts present as pathologic fractures (Picture 1).

Growth arrest from a simple bone cyst occurs in approximately 10% of patients. The cause of growth arrest is uncertain and may be the result of fracture associated with the cyst, iatrogenic (eg, surgical curettage of a simple bone cyst abutting the physis and damaging the physis in the process), or direct extension of the cyst through the physeal plate into the epiphysis. The latter process can be well demonstrated by MRI.

Sex: Simple bone cysts occur more frequently in boys than in girls. Male-to-female ratio is 2:1.

Age:

Most cysts occur in the first and second decades of life, with most occurring in children aged 4-10 years.

The site of occurrence depends on the age of the patient. In patients younger than 20 years, simple bone cysts are found in the humerus in 55-65%, in the femur in 25-30%, and rarely in the tibia, fibula, radius, and ulna. In patients older than 20 years, simple bone cysts are found more commonly in flat bones such as the iliac bone and calcaneus (Picture 2). Simple bone cysts rarely are found in more than one location in a single patient.

Anatomy: Simple bone cysts are found in tubular bones in 90-95% of patients. Within the long bones, most simple bone cysts are situated in the proximal metaphysis (Picture 3). Simple bone cysts are situated in the diaphysis in only 4-12% of patients (Picture 4).

Involvement of the epiphysis is rare. Epiphyseal involvement may represent a distinct clinical and radiographic entity. Patients from this group are older compared to those with simple bone cysts without epiphyseal involvement. The mean age of patients with epiphyseal involvement is 20.1 years. The epiphyseal plates are closed in 50% of these patients. The biologic behavior of these lesions is thought to be less aggressive with a better prognosis compared to simple bone cysts occurring in the metaphyseal regions. However, because of the close proximity to the physeal plate, a greater association with growth arrest exists.

Clinical Details: Simple bone cysts usually are asymptomatic unless complicated by fracture. Consider the possibility if the patient presents with pain or limited limb movement following minor trauma.
Simple bone cysts enlarge during skeletal growth and become inactive, or latent, after skeletal maturity (Pictures 5-8).

Preferred Examination: Plain radiography is the examination of choice because of its high diagnostic capability of simple bone cysts.
CT and 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.

	DIFFERENTIALS	Section 3 of 10
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Aneurysmal Bone Cyst
Chondromyxoid Fibroma
Enchondroma and Enchondromatosis
Fibrous Dysplasia
Giant Cell Tumor

	X-RAY	Section 4 of 10
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Findings: 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.

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 (Picture 9). In long bones, simple bone cysts typically are centrally located within the medullary cavity (Picture 10).

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 (Picture 11). 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.

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: 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.

False Positives/Negatives: The radiograph usually is sufficient to confirm the diagnosis of simple bone cysts. 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.