Unicameral Bone Cyst

(Simple Bone Cyst)

Unicameral Bone Cysts (UBC), also known as Simple bone cysts (SBC) are cavities filled with serous fluid and covered by a thin membrane in the bone.

They are common in pediatric age group of 5-20 years, boys more affected than girls.

What is cause for SBC?

Most commonly affected area is metaphysis and bone is proximal aspect of humerus adjacent to growth plate (nearly 60 %), followed by proximal femur, around knee joint and rarely in heel bone (calcaneus).

Exact etiology is unknown but it is suspected that UBC appear as a focal defect in metaphysical remodeling with blockage to interstitial fluid drainage (by Cohen). The cyst fluid contains prostaglandins which stimulate osteoclasts to eat away the surrounding bone, leading to the accumulation of clear yellow/straw colored fluid.

How is SBC Identified?

Most of UBS’s are asymptomatic, meaning it is diagnosed more commonly when evaluated for an episode of trauma.

Pain primarily due to the lesion is uncommon.

It is also common to diagnose UBC in a child when evaluating a fracture due to episode of trauma. In these situations, the fracture itself acts as a stimulant to resolve and treat lesion in 8-20 % of situations with adequate immobilization depending on the bone involved.

Plain radiographs show a pure lytic lesion with a well-defined margin. The lesion does not usually extend beyond the original borders of the involved bone.

MRI may be performed when the clinical and radiological findings allow the surgeon to suspect other causes such as Aneurysmal bone cyst, Non-ossifying fibroma or Fibrous dysplasia.

 

What is treatment for SBC?

The natural history of these lesions are to spontaneously resolve in 33 %, increase in size during growth spurt in 33 % and reappear in 33 %. Usually most cysts heal by the time the patient reaches adulthood. It is common to note multiple episodes of fractures in individuals identified in early childhood and followed up until adulthood or maturity.

Surgical intervention is necessary when the UBC is situated adjacent to joints showing signs of growth (expansion), deformity, fracture, nonunion of fracture leading to weakening of the bone.

Various options are:

  1. Minimally invasive percutaneous breakage of cyst wall and 1-3 doses of steroid injections (depomedrol) under radiological guidance
  2. Percutaneous curettage and infiltration of Stem cells with bone matrix
  3. Curettage and filling of defect with bone graft or cement in large lesions or recurrent cases
  4. Percutaneous placement of nails to drain the cyst and stabilize fracture
  5. Curettage and augmentation with plate or screw and bone graft in cases of large lesion, weakened bone and proximal femur location.

These options are individualized and none of them have been proven to be superior over other methods in clinical studies.

In those cysts that respond to injection therapy, radiological signs of healing can be noticed anytime between 6 weeks to 6 months.

Results of surgical treatment are good especially when performed after the cyst has stopped growing and has become inactive. But it is not possible to wait until maturity in majority of the situations due to functional disability.