Osteoid Osteoma - Pathology

summary
- Osteoid Osteomas are small, benign, osteogenic bone lesions most commonly found in the proximal femur. Patients typically present between ages 5 and 25 with regional pain that is worse at night and improves with NSAIDS.
- Diagnosis is made radiographically by a characteristic lesion that is less than 1.5 cm in diameter with a sclerotic margin and radiolucent nidus.
- Treatment is usually nonoperative with observation and NSAID for pain control. Radiofrequency ablation or surgical resection may be indicated in patients with progressive and severe symptoms.

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)
- Anatomic 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

Etiology
- 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

Classification

Enneking Classification of Benign Lesions
	Grade	Examples
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 vs. Osteoblastoma
                                        
                                            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
                                        
                                            Vertebral 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 in the fingers
    
    lesions close to nerves
    
    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 curettage
    
    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

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)

Osteoid osteoma vs. Osteoblastoma
	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	Vertebral 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.