Osteoid Osteoma - Pathology

Introduction

A small, discrete, painful, benign bone lesion
Epidemiology
- incidence
  - cause of painful scoliosis in the adolescent population
- demographics
  - 3:1 male to female ratio
  - persons aged 5-25 years (>80% present before age of 30)
- location
  - most common
    - lower extremity (>50%)
      - proximal femur > tibia diaphysis
      - usually found within the bone cortex
    - spine (10-15%)
      - thoracic and lumbar regions > cervical and sacral
      - majority involve the posterior elements
      - usually found on the side of concavity in scoliosis
    - hand (5-10%)
      - scaphoid and proximal phalanx
    - foot (<5%)
      - predominantly involves the talar neck
Pathophysiology
- pathoanatomy
  - nidus
    - central nodule of woven bone and osteoid with osteoblastic rimming
  - reactive zone
    - area of thickened bone and fibrovascular tissue
- cellular biology
  - pain
    - attributed to increased local concentration of prostaglandin E2 and COX1 & 2 expression
    - increased number and size of unmyelinated nerve fibers within the nidus
Associated conditions
- orthopaedic manifestations
  - painful scoliosis
  - growth disturbance
  - flexion contractures
Prognosis
- pain from lesions usually resolves after an average of 3 years
- the lesion spontaneously resolves in 5-7 years
- in the spine, early resection (within 18 months) leads to resolution of scoliosis in young children (<11years)

Classification

Enneking Classification of Benign Lesions
Stage	Grade	Examples	Images
Stage 1	Latent lesions	enchondroma, non-ossifying fibroma
Stage 2	Active lesions	osteoid osteoma, UBC, ABC, chondroblastoma, chondromyxoid fibroma
Stage 3	Aggressive lesions	giant cell tumor of bone

Symptoms

Symptoms
- pain
  - constant and progressive
  - worse at night and with drinking ETOH
  - relieved by NSAIDS
  - may be adjacent to joint and mimic arthritis
- hand lesions may present with painless swelling
Physical exam
- inspection
  - palpable bone
    - deformity, swelling, erythema, tenderness
  - proximity to a joint
    - effusion, contracture, limp, muscle atrophy
  - spine
    - postural scoliosis, paravertebral muscle spasm

Imaging

Radiographs
- recommended views
  - 3 views of affected bone or joint
- findings
  - intensely reactive bone around radiolucent nidus
CT
- indication
  - cross-sectional imaging is the study of choice
- findings
  - to help identify the location and size of nidus
    - usually < 1.5 cm (otherwise think osteoblastoma)
Bone scan
- indication
  - concerning features on radiograph or advanced imaging
- findings
  - intense hot area of focal uptake at the nidus
  - low uptake in reactive zone
    - known as the 'double-density sign'
MRI
- indications
  - usually not recommended as it can mimic aggressive lesions
- findings
  - must be interpreted with reference to x-ray or CT scan

Histology

Histology
- distinct demarcation between nidus and reactive bone
  - nidus
    - contains uniform osteoid seams of immature osteoid trabeculae (woven bone) with a sharp border of osteoblastic rimming
    - uniform plump osteoblasts have regularly shaped nuclei with abundant cytoplasm
  - reactive zone
    - region surrounding the sclerotic border

Differentials

Long bone osteoid osteomas need to be differentiated from
- stress fx
- osteomyelitis
- Ewing's sarcoma
Posterior spinal element lesions need to be differentiated from
- aneurysmal bone cyst
- osteoblastoma (see table below)

	Osteoid Osteoma	Osteoblastoma
Incidence	10% of benign tumors	3% of benign tumors
Size	< 2 cm (typically <1.5cm)	>2 cm (average, 3.5 - 4.0 cm)
Site	> 50 % in long bone diaphysis	> 35% in posterior elements of the spine
Location	Proximal femur > tibia diaphysis > spine	vetebral column > proximal humerus > hip
Natural History	Self-limited	Progressive
Histology	Benign appearance. No growth potential. Central nidus composed of more organized osteoid and lined by osteoblasts.	Benign appearance. Localized growth, with aggressive potential. Central lesion less organized, with greater vascularity.
Symptoms	Nocturnal pain, relieved by NSAIDS	Dull ache, partial response to NSAIDS. >50% of spine tumors have neurologic symptoms.
Management of Spine Lesions	Nonsurgical management is indicated as first-line treatment.	Surgery is always indicated as they do not respond to nonsurgical treatment.

Treatment

Nonoperative
- clinical observation and NSAID administration
  - indications
    - NSAIDS are 1st line and will lead to a dramatic decrease in symptoms
      - ~50% can be treated with NSAIDS alone
    - also indicated for painful spine lesions without scoliosis
    - fingertip lesions (distal phalanx) may not respond to NSAIDS
Operative
- percutaneous radiofrequency ablation
  - relative indications
    - failure of medical management
    - periarticular lesions, which increase the risk of cartilage injury and premature degenerative disease.
    - spinal lesions (controversial) - depends on the location of the lesion and proximity to neural elements
  - contraindications
    - lesions close to spinal cord or nerve roots
  - technique
    - done under CT guidance
    - probe at 80-90 deg C for 6 minutes to produce a 1cm zone of necrosis
  - outcomes
    - 90% of patients are successfully treated with 1-2 sessions of RFA
    - 10-15% recurrence rate
- MR-guided high-intensity focused ultrasound (MR-HIFU)
  - indications
    - failure of medical management
  - contraindications
    - any contraindication to anesthesia
    - lesions in anatomic area which could not be directly accessed by ultrasound energy (ex. vertebrae)
  - technique
    - based on MR imaging, therapeutic ultrasound energy transferred transcutaneously causing thermal ablation to the periosteal nerves, which penetrates the medullary bone and coagulates the osteoma nidus
  - outcomes
    - less invasive than other modalities
    - significant and persistent decreases in pain with low risk for adverse events
    - improvements in quality of life scores
    - decreased need for NSAID in treatment population
    - improvement in follow-up imaging showing decrease or total absence in osteoid osteoma nidus vascularity
- surgical resection with currettage
  - indications
    - location of lesion is not amenable to CT guided percutaneous radiofrequency ablation
      - e.g. close to skin or nerve
    - spine lesion associated with painful scoliosis
    - digital lesions
      - RFA carries risk of thermal skin necrosis and injury to digital neurovascular bundle
  - technique
    - successful treatment depends on complete marginal resection of nidus (sclerotic bone is normal and can be left behind)
      - percutaneous approach
      - open approach
  - outcomes
    - 94% success with local excision
    - associated scoliosis rarely requires treatment

Complications

Recurrence
- 10-15% recurrence rate with percutaneous radiofrequency ablation

Image Bank

	Location	Xray	CT	Bone scan	MRI	MRI	Histo
Case A	tibia
Case B	prox. femur
Case C	tibia
Case D	prox tibia