Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

Featured on Channel NewsAsia

Unicameral Bone Cyst

Basics

Description
  • A benign membrane-lined, fluid-filled lesion of bone that develops in childhood and fills in by maturity
  • Location:
    • 80% occur in the proximal humerus and proximal femur.
    • Other, less common, areas are the proximal tibia, distal tibia, distal femur, calcaneus, distal humerus, radius, fibula, ilium, ulna, and rib.
    • Usually centrally located adjacent to the physeal plate or in the metaphyseal or diaphyseal region
    • Rarely, it crosses the physis into the epiphysis.
    • May also occur in flat bones
  • Rarely seen in adults
  • Classification:
    • Active:
      • Usually seen in children <10 years old
      • A lytic or lucent area abuts the physis and may fill the entire metaphyseal region.
      • Typically, it has a thinner cortical wall, which makes the lesion more prone to fracture and recurrence (~50%).
    • Inactive:
      • Usually seen in children >10 years old
      • A lytic area is separated from the physeal plate by normal cancellous bone.
      • Typically, it has a thicker cortical wall, which makes it less prone to fracture and recurrence
  • Synonyms: Simple bone cyst; Solitary bone cyst
Epidemiology
All lesions are diagnosed in patients <20 years old.
Incidence
Male:Female ratio is 2:1.
Risk Factors
Genetics
No known relationship exists.
Pathophysiology
Pathological Findings
  • Gross:
    • Examination reveals a cystic cavity with a membrane lining of variable thickness (2–10 mm) usually containing yellowish fluid that may be blood-tinged or frankly bloody if the fracture is recent or has been previously aspirated and injected.
    • Septa may be present with loculations of fluid, particularly with a history of fracture.
  • Microscopic:
    • A membrane containing fibrous tissue and occasional spicules of bone is seen along with occasional osteoclasts, chronic inflammatory cells, and giant cells.
Associated Conditions
None are known.
Etiology
The origin is unknown, but suggested causes include an intraosseous hematoma, a necrotic lipoma, lymphatic or venous obstruction, or an intraosseous synovial rest.
Diagnosis
Signs and Symptoms
  • The patient usually is asymptomatic unless fracture occurs as a result of a thinned-out cortex.
  • Most cysts are discovered as a result of a fracture, which typically occurs with minimal trauma, such as throwing a ball.
  • The remaining lesions usually are found incidentally.
  • If symptomatic, the patient presents with localized pain and with swelling or stiffness of the adjacent joint.
Physical Exam
  • Usually, nothing is detectable on examination unless the patient is symptomatic.
  • With respect to recurrent fractures resulting from a unicameral bone cyst, patients must be examined for angular deformities resulting from malunion and for limb-length discrepancies secondary to growth arrest.
Tests
Imaging
  • Radiography:
    • The typical appearance is a centrally located, expansile, radiolucent lesion with a well-marginated border in the metaphyseal region, causing thinning of the adjacent cortex.
    • The fallen fragment or fallen leaf sign indicates the presence of a fracture with movement of a cortical piece of bone to a dependant portion of the fluid-filled cyst and suggests the presence of a cavity instead of a solid tumor.
    • The cyst moves away from the epiphysis as the lesion becomes inactive.
  • MRI:
    • May be performed if a question of a solid lesion remains
    • A bright uniform signal in the T2-weighted image is consistent with a high water content, suggesting a cyst.
    • An extraosseous soft-tissue mass never will be present.
Differential Diagnosis
  • Aneurysmal bone cyst
  • Fibrous dysplasia
  • Enchondroma
  • Giant cell tumor
  • EOG
Treatment
General Measures
  • If found incidentally, a unicameral bone cyst may be treated with observation or with dual-needle aspiration of the cyst followed by injection of methylprednisolone, bone marrow, or other substance.
  • If it is discovered as a result of a pathologic fracture, the bone may be allowed to heal.
    • ~15% of fractures are followed by spontaneous healing of the cyst.
    • The remainder usually are offered injections.
    • Curettage and grafting rarely are necessary.

P.477
Surgery
  • Needle aspiration may be performed by inserting 2 needles with stylets into the lesion under fluoroscopic guidance.
  • Removal of the stylets and attempted aspiration will prove whether the lesion is fluid-filled.
  • Proper needle placement may be confirmed with radiographic contrast agent.
  • If the contrast agent cannot be injected or no fluid appears on aspiration, then an open biopsy should be performed, assuming the possibility of a solid lesion.
  • If the lesion is fluid-filled: Injection:
    • It is thought that the injection may stimulate the cyst to heal.
    • If necessary, the injection may be repeated at 2-month intervals up to 3 times.
    • ~50% of patients require multiple injections.
    • Usually, 4200 mg of methylprednisolone
    • Newer protocols include injecting other substances, such as marrow or bone-grafting substitutes.
  • Persistent or recurrent cysts may require open curettage and bone grafting with autograft (cortical, cancellous, and bone marrow aspirate), allograft (cortical and cancellous), or injectable demineralized bone matrix.
  • In critical, high-stress regions such as the base of the femoral neck and femoral head, internal fixation with a plate may be indicated.
Follow-up
Prognosis
  • The unicameral bone cyst eventually heals spontaneously and fills in with bone, but it may involve recurrent fractures, injections, or curettage and bone grafting before resolution.
  • Results of treatment vary with the location or size of a cyst and the age of the patient.
  • Recurrence:
    • The rate of recurrence is higher when the cyst occurs in the proximal humerus than in the femur or tibia.
    • When a cyst is present in flat bones, recurrence is rare.
    • Smaller cysts have a lower rate of recurrence than do larger cysts.
    • Cysts presenting in patients in the 1st decade have a higher recurrence rate.
  • Malignant degeneration of the unicameral bone cyst never occurs.
Complications
  • Growth arrest may occur, giving rise to limb-length discrepancies and malunions that cause angular deformities.
  • AVN may occur as a result of fracture through proximal femoral lesions.
Miscellaneous
Codes
ICD9-CM
733.21 Unicameral bone cyst
Patient Teaching
  • Patients must be advised that the persistence rate is ~85% in cysts treated with observation after a fracture.
  • The risk of refracture remains as long as the cyst is present.
    • Activity modification may greatly reduce the risk of refracture, but it may not be practical for this specific age group, depending on the patient.
Activity
  • Children with unicameral bone cysts are allowed to participate in sports without restriction after partial or complete healing of the cyst.
  • A risk exists for repeat fracture with heavy activities or even activities of daily living.
Prevention
  • Prevention of fractures is difficult and often unpredictable.
  • The clinician and parents can restrict sports activities in patients with large cysts.
FAQ
Q: Do unicameral bone cysts have any malignant potential?
A: Unicameral bone cysts can be very troublesome because of recurrent fractures, but they never become malignant.
Q: Do unicameral bone cysts of the hip always need surgery?
A: Most children with unicameral bone cysts of the proximal femur require curettage, internal fixation, and bone grafting to prevent fracture and the associated risk of AVN of the femoral head.
Q: Is a biopsy necessary to confirm the diagnosis of unicameral bone cysts?
A: Unicameral bone cysts have such characteristic radiographic findings that a biopsy is not necessary to establish the diagnosis.

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