Bone Cyst

a complicated bone cyst in 14-year-old boy... 

Introduction

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 1910.¹ Jaffe and Lichtenstein provided a detailed discussion of simple bone cysts in 1942.²

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

International

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 (Image 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. The 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 (Image 2). Simple bone cysts rarely are found in more than one location in a single patient. Patients with multiple bone cysts are generally in the higher age group and show a very high male predominance.

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 (Image 3). Simple bone cysts are situated in the diaphysis in only 4-12% of patients (Image 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. Epiphyseal simple bone cysts have a lower male-to-female ratio, a very high humerus-to-femur ratio, and a higher incidence of tibial location than the classic metaphyseal bone cysts. 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.

Presentation

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 (Images 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.

Radiography

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

Computed Tomography

Findings

CT 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 (Images 12-13). 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.

Degree of Confidence

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.

False Positives/Negatives

The presence of a fallen fragment sign on CT also is diagnostic of a simple bone cyst. CT has high sensitivity and specificity for detecting simple bone cysts.

Magnetic Resonance Imaging

Findings

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 (Images 14-16). 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.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. 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. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. 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 movingor straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

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.

False Positives/Negatives

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.

Nuclear Imaging

Findings

Simple bone cysts show little or no uptake of tracer material in radionuclide bone scans unless they have been traumatized (Image 17).

Intervention

The goal of treatment of simple bone cysts is to prevent pathologic fracture, promote cyst healing, and to avoid cyst recurrence or refracture.

Simple bone cysts can be treated with curettage and bone grafting, cryotherapy, intramedullary nailing, injection of methylprednisolone under image-intensifier guidance, injection of bone marrow, or a combination of the above methods.

Some authors have reported better healing rates and lower complication rates with steroid injections compared to surgery (Images 19-20). No apparent advantage is gained with the use of autogenous injection of bone marrow compared with injection of steroids in the management of simple bone cysts.

The mechanism of action of methylprednisolone injection is unclear. A possible theory is a reparative response to the minor injury caused by the injection process.

Advantages of methylprednisolone injection include shorter operating times, less bleeding, and minimum hospital stay and rehabilitation. However, the healing rate with methylprednisolone injection has been reported as unpredictable and usually is incomplete even after multiple injections. The failure rate in weight-bearing bones has been reported to be high.

Radiographic evidence of a good response to steroid injection includes reduction of the cavity, increased radio-opacification of the cyst, cortical thickening, and osseous remodeling.

Large lesions and lesions with radiographic evidence of septations are factors indicating poor response to treatment.