Osteoid Osteoma

Definition

  • Small (< 1cm diameter of central nidus), solitary, benign, painful bone-forming tumours

Epidemiology

  • Relatively common
  • 10% of all primary bone tumours
  • Classically children & adolescents
  • Aged 5-25 years
  • M:F – 2:1
  • Most common site is lower extremity long bones (60-70%) usually near diaphysis
    • Femur 30%
      • Femoral neck & Intertrochanteric area most common femoral location
    • Tibia 25%
    • Foot 10%
      • Talus & Calcaneum most common in foot
    • Humerus 10%
    • Forearm 3%
    • Hand 10%
      • Scaphoid most common in wrist
    • Spine
      • Can occur in spine (Osteoblastoma more commonly) in posterior elements
  • Not infrequently located at the site of previous trauma
    • ? pathogenesis associated with trauma

Clinical Features

  • Characteristic pain
    • Intense, unrelenting
    • Chronic pain months/ years
    • Nocturnal
      • Due to prostaglandin production
    • Relieved by aspirin (narcotics often unhelpful)
  • Local swelling
    • Exquisitely tender
  • Mild Leucocytosis may be present
  • If intra-articular/ juxta-articular may present as synovitis of joint
  • Spinal lesions will show irritative scoliosis often

Radiology

X-ray

  • Cortical location in long bone
    • Central lucent zone (Nidus)
    • Increased surrounding bone density with sclerosis
    • Fusiform shaped
    • May obscure nidus
    • Marked periosteal reaction maybe present
    • Subperiosteal location = marked sclerosis
    • Endosteal location moderate sclerosis
    • Subarticular location = often no sclerosis
    • Nidus can be obscured by the surrounding sclerosis
  • Medullary
    • Four features (~ 50% show all 4)
      • Sharply round or ovoid
      • < 1cm
      • Homogenous dense centre
      • 1mm peripheral radiolucent zone
  • Differential on XR includes
    • Stress fracture
    • Osteomyelitis

CT Scan

  • Demonstrates nidus better
  • Location in cortex clearer
  • Thin sections 1-2mm
  • Low attenuation nidus
  • Central mineralization
  • Surrounding endosteal & periosteal sclerosis

Bone Scan

  • Help with diagnosis & localisation of the tumour
  • Extensive ↑ uptake
  • Avidly take up the isotope due to osteoblastic activity in nidus
  • Also develops intense surrounding reaction
  • Can help localise intraoperatively

Pathology

Gross

  • Well demarcated cherry red nidus with gritty consistency
  • Surrounding dense bone with periosteal reaction & thickening

Microscopic

  • Maze of small spicules of immature bone
  • Haphazard
  • Delicate trabeculae of osteoid
  • Rimmed by numerous osteoblasts
  • Enclosed in vascular spindle cell stroma
  • Numerous vascular channels & capillary network
  • Rich nerve fibres
  • Giant cells maybe present
  • More mature lesions stroma sparsely cellular with intervening vascular spaces
  • No chondroid elements

Differential Diagnosis

  • Osteoblastoma
  • Brodie’s abscess
  • Stress fracture
  • Bone Island
  • Eosinophilic Granuloma
  • Osteosarcoma
  • Osteoma

Treatment

  • Generally remove the nidus
    • (May heal spontaneously but takes long time)
  • En Bloc Excision
    • Gold standard
    • Ensures all of lesion excised
    • Weakens bone
    • Need graft & protection
    • Send for intraoperative FFS & XR
  • “Burr Down” Technique
    • Excellent results
    • Intraoperative CT guidance
    • Direct incision over lesion
    • Shave cortex off with high sped burr to reactive bone
    • Scoop nidus out once hit hypervascular zone & sent for FFS
    • Burr 2mm zone out
    • Can leave strong reactive bone behind
  • Percutaneous Guided Reaming
    • CT-guided in XR suite
    • Doesn’t produce tissue for histology
  • Radiofrequency Ablation
    • CT-guided
    • No tissue for histology
    • Death occurs by thermal ablation
    • Uncertain results
  • Methods of localisation include
    • Image Intensifier
    • CT-guidance
    • Radioisotope probe
    • Intraop tetracycline fluoroscence
  • If complete excision performed then recurrence very rare
  • However incomplete excision not uncommon & often leads to recurrence & failure of pain relief

OsteoBlastoma