Unicameral bone cyst
Synonyms and keywords: Simple bone cyst; Traumatic bone cyst, UBC

# Overview
Unicameral bone cyst (UBC) constitute approximately 3% of all bone tumors. Adolescents and children are most affected by unicameral bone cyst. The age distribution of unicameral bone cyst is between 5-15 years. Men are more commonly affected compared to women, with a 2:1 ratio. In 1942, Jaffe and Lichenstein later recognized it as a distinct entity. The exact pathogenesis of unicameral bone cyst is not fully understood. Unicameral bone cyst may have association with genetic abnormalities on chromosome 4, 6, 8, 16, 21, and both chromosomes 12. The bones often involved are proximal femur, distal tibia, ilium, calcaneus, and occasionally metacarpals, phalanges, or distal radius. Unicameral bone cyst typically occur in the metaphysis adjacent to the physis of the long bones. The hallmark of UBC is pain, swelling and pathological fracture. The mainstay of treatment for UBC is surgery in form of curettage and bone grafting.

# Historical Perspective
- In mediveal times, Lagier et al identified a unicameral bone cyst in the femur from the remains of a child.
- In 1891, Virchow reported it as cystic structures that resulted due to anomalies in the local circulation.
- In 1942, Jaffe and Lichenstein later recognized it as a distinct entity.

# Classification
Unicameral bone cyst can be classified based on imaging findings.

## Enneking (MSTS) Staging System
- The Enneking surgical staging system (also known as the MSTS system) for benign musculoskeletal tumors based on radiographic characteristics of the tumor host margin.
- It is widely accepted and routinely used classification.

# Pathophysiology
- The exact pathogenesis of unicameral bone cyst is not fully understood.
- Various theories have been proposed concerning the pathogenesis of unicameral bone cyst:
Blockage in the venous drainage is the most favored mechanism which occurs in a rapidly growing and remodeling portion of cancellous bone.
Increased internal pressure of involved bone as compared to normal pressure of bone marrow.
Lower partial pressure of oxygen of cyst fluid than arterial or venous blood suggesting a venous obstruction.
Increased levels of lysosomal enzymes in the cyst than serum. Enzymes include prostaglandins, interleukin 1, nitrate and nitrites levels, proteolytic enzymes, tumor necrosis factor  and interleukins 1 and 6.
- Blockage in the venous drainage is the most favored mechanism which occurs in a rapidly growing and remodeling portion of cancellous bone.
- Increased internal pressure of involved bone as compared to normal pressure of bone marrow.
- Lower partial pressure of oxygen of cyst fluid than arterial or venous blood suggesting a venous obstruction.
- Increased levels of lysosomal enzymes in the cyst than serum. Enzymes include prostaglandins, interleukin 1, nitrate and nitrites levels, proteolytic enzymes, tumor necrosis factor  and interleukins 1 and 6.
- Unicameral bone cyst typically occur in the metaphysis adjacent to the physis of the long bones.
- The bones often involved are proximal femur, distal tibia, ilium, calcaneus, and occasionally metacarpals, phalanges, or distal radius.

## Genetics
- Unicameral bone cyst may have association with genetic abnormalities on chromosome 4, 6, 8, 16, 21, and both chromosomes 12.
- Translocation t (16; 20) (p11.2; q13) has been found in cases with unicameral bone cyst.

# Causes
There are no established causes for unicameral bone cyst.

# Differentiating Unicameral Bone Cyst from Other Diseases
Unicameral bone cyst must be differentiated from following bone disorders:

# Epidemiology and Demographics
- Unicameral bone cyst constitute approximately 3% of all bone tumors.
- Adolescents and children are most affected by unicameral bone cyst.
- The age distribution of unicameral bone cyst is between 5-15 years.
- The mean age of the patients with unicameral bone cyst is 9 years.
- Men are more commonly affected than women, with a 2:1 ratio.
- There is no racial predilection to unicameral bone cyst.

# Risk Factors
There are no established risk factors for unicameral bone cyst.

# Screening
There is insufficient evidence to recommend routine screening for unicameral bone cyst.

# Natural History, Complications, and Prognosis
- Common complications of unicameral bone cyst (UBC) include:
Pathological fracture
Premature epiphyseal closure
Limb-length discrepancy
Angular deformity
Malignant transformation
- Pathological fracture
- Premature epiphyseal closure
Limb-length discrepancy
Angular deformity
- Limb-length discrepancy
- Angular deformity
- Malignant transformation
- Prognosis is generally excellent for Unicameral bone cyst.
As a patient approaches skeletal maturity, a unicameral bone cyst will often decrease in size and may heal after growth is complete.
Fracture healing usually does not lead to cyst resolution.
It requires close follow up while in active phase due to recurrence and risk of fracture or growth arrest.
- As a patient approaches skeletal maturity, a unicameral bone cyst will often decrease in size and may heal after growth is complete.
- Fracture healing usually does not lead to cyst resolution.
- It requires close follow up while in active phase due to recurrence and risk of fracture or growth arrest.

# Diagnosis

## Diagnostic Study of Choice
- Biopsy is the diagnostic study of choice for the diagnosis of unicameral bone cyst.
- Biopsy findings include:
Cyst are lined with thin fibrous lining containing fibrous tissue, giant cells, and hemosiderin pigment.
Inflammatory cells such as lymphocytes may be found in small numbers.
Uniform amount of spindle cells without nuclear atypia.
- Cyst are lined with thin fibrous lining containing fibrous tissue, giant cells, and hemosiderin pigment.
- Inflammatory cells such as lymphocytes may be found in small numbers.
- Uniform amount of spindle cells without nuclear atypia.

## History and Symptoms
- The majority of patients with unicameral bone cyst have a positive history of:
Pain
Swelling
Pathological fracture
Neurologic conditions occur often due to compression of either the spinal cord or nerve roots, and leading to paralysis and spinal stiffness
- Pain
- Swelling
- Pathological fracture
- Neurologic conditions occur often due to compression of either the spinal cord or nerve roots, and leading to paralysis and spinal stiffness

## Physical Examination
- Common physical examination findings of unicameral bone cyst include:
Deformity
Decreased range of motion, weakness, or stiffness
Torticollis
Occasionally, bruit over the affected area
Warmth over the affected area
- Deformity
- Decreased range of motion, weakness, or stiffness
- Torticollis
- Occasionally, bruit over the affected area
- Warmth over the affected area

## Laboratory Findings
There are no diagnostic laboratory findings associated with unicameral bone cyst.

## Electrocardiogram
There are no ECG findings associated with unicameral bone cyst.

## X-ray
- Findings on an x-ray suggestive of unicameral bone cyst include include:
Central, lytic, well-demarcated metaphyseal lesion.
Cystic expansion with symmetric thinning of cortices.
"Fallen leaf" is a pathognomic sign seen in pathologic fracture with fallen cortical fragment in base of empty cyst.
Trabeculated appearance after multiple fractures.
- Central, lytic, well-demarcated metaphyseal lesion.
- Cystic expansion with symmetric thinning of cortices.
- "Fallen leaf" is a pathognomic sign seen in pathologic fracture with fallen cortical fragment in base of empty cyst.
- Trabeculated appearance after multiple fractures.

## Echocardiography or Ultrasound
There are no echocardiography/ultrasound findings associated with unicameral bone cyst.

## CT scan
- CT scan confirms the x-ray findings.
- In addition, CT demonstrates internal septation such as calcified rim, giving an eggshell appearance, which may be completely or partially intact.
- Free fluid levels can also be seen.

## MRI
MRI findings of unicameral bone cyst include:
- Very dark on T1
- Very bright on T2
- Gadolinium shows classic rim enhancement of a cystic lesion

## Other Imaging Findings

## Bone Scan
- Increased uptake is observed around the lesion of unicameral bone cyst.
- Findings often demonstrate a halo effect of increased radionuclide uptake surrounding an area of little uptake.

## Other Diagnostic Studies

## Angiography
- Angiography demonstrates a hypervascular area around the unicameral bone cyst.
- An intense diffuse area of persistent contrast accumulation may be visualized without main afferent or efferent vessels observed.

# Treatment
The mainstay of therapy for unicameral bone cyst is surgery.

## Medical Therapy

## Immobilization
Indications
- Proximal humerus lesions with pathologic fracture
- Inaccessible lesions

## Aspiration and Methylprednisolone Acetate Injection
Indications
- Active cysts which are communicating with physis.
Technique
- Usually requires several injections, especially in very young children.
- Bone marrow injections have recently been reported to be effective.

## Surgery
Surgery is the mainstay of treatment for unicameral bone cyst.

## Curettage and bone grafting with internal fixation
Indications
- Symptomatic latent cysts that have not responded to steroid injections.
- Latent cysts located in areas such as proximal femur where structural integrity is a concern and at risk for fracture and osteonecrosis.
- Lesions with a pathologic fracture that have a higher rate of re-fracture and malunion when treated nonoperatively.
Contraindications
- Active lesions which are in communication with physis, which may result in growth arrest.

## Treatment Response Evaluation
Neer rating system for the purposes of evaluating treatment response are as follows:
- Excellent  Complete obliteration of the cyst.
- Residual defect  One or more static cyst like areas with good reestablishment of bone strength.
- Reoperation  Subsequent operation required because of recurrence.

## Primary Prevention
There are no established measures for the primary prevention of unicameral bone cyst.

## Secondary Prevention
There are no established measures for the secondary prevention of unicameral bone cyst.