Solitary Bone Cyst

Definition

Clinical Features

• Usually:
  • male
  • <20 years of age
  • advanced when first seen

Pathogenesis

• Believed to result from a local disorder of development and bone growth
• Synovial origin suggested as alternative pathogenesis1

Gross Pathology

• Location:
  • usually:
    • long bones
    • upper portion of shaft of humerus (Fig. 1
      

      Fig. 1: Typical solitary bone cyst of upper end of humerus abutting against epiphyseal plate in a 13-year-old boy.

      ) and femur
  • also:
    • short bones, particularly calcaneus2
• Most are centered in metaphysis (Fig. 2
  

  Fig. 2: Gross appearances of solitary bone cyst. A large lesion located in the upper metaphysis of the humerus.

  ):
  • natural evolution is to migrate away from epiphyseal line
• Thinned cortex
• Periosteal bone proliferation does not take place except in areas of fracture
• Bones often fracture, usually in proximal portion of cystic area
• Cyst:
  • contains clear or yellow fluid:
    • may be hemorrhagic if previous fracture
  • lined by smooth fibrous membrane that may be brown (Fig. 3
    

    Fig. 3: Gross appearances of solitary bone cyst. A triangular lesion located in the upper end of the tibia. There has been secondary hemorrhage, leading to an appearance not too dissimilar to that of an aneurysmal bone cyst.

    )

Histopathology

• Frequently:
  • well-vascularized connective tissue
  • hemosiderin (often within macrophages)
  • cholesterol clefts
• Bone surrounding cyst may have dense quality with irregular cement lines1,3

Diagnosis

• May be difficult if:
  • reparative changes following fracture
  • recurrent lesion after bone grafting
  • articular cartilage included in curettings
• Becomes clear if history and radiographs available

Differential Diagnosis

Management

• Curettement and replacement of cyst with bone chips:
  • treatment of choice
  • results correlate well with cyst ‘activity’ determined by location:
    • good when cyst has migrated away from epiphyseal line
    • recurrences common when cyst has not migrated away from epiphyseal line2