Unicameral bone cyst

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Rohan A. Bhimani, M.B.B.S., D.N.B., M.Ch.[2]

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.

Stages	Description
1	Latent: Well demarcated borders
2	Active: Indistinct borders
3	Aggressive: Indistinct borders

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 1β, nitrate and nitrites levels, proteolytic enzymes, tumor necrosis factor ⍺ and interleukins 1β 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:

Disease	Bubbly lytic lesion on x-ray	Lakes of Blood on histology	Diagnosis	Treatment is curretage and bone grafting
Unicameral bone cyst	+	-	Radiology and biopsy	-
Aneurysmal bone cyst	+	+	Radiology and biopsy	+
Non ossifying fibroma	+	-	Radiology and biopsy	-
Giant cell tumor	-	-	Radiology and Biopsy	+
Chondroblastoma	-	-	Biopsy	+
Chondromyxoid Fibroma	-	-	Radiology and biopsy	+
Osteoblastoma	-	-	Radiology and biopsy	+
Telangiectatic osteosarcoma	-	+	Radiology and biopsy	-

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
• 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.

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.

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

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

X-ray showing unicameral bone cyst of left proximal femur.Source: Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org, rID: 10361

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.

CT scan showing unicameral bone cyst of left proximal femur.Source: Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org, rID: 10361

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

MRI scan showing unicameral bone cyst of left proximal humerus.Source: Case courtesy of Dr Yasser Asiri, Radiopaedia.org, rID: 65130

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.

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.

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.

References

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