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 accounts for 10-14% of all benign bone tumors and 2-3% of all primary bone tumors peak incidence in 2nd decade of life Demographics 2-3:1 male to female ratio persons aged 5-25 years (70% present before age of 20) 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 scoliosis with lesion on concave side of curve thought to result from marked paravertebral muscle spasm growth disturbance epiphyseal involvement could result in elongation of affected limb and resultant limb length discrepancy 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 Giant cell tumor of bone* Stage 3 Aggressive lesions Giant cell tumor of bone* ABC* Giant cell tumor of bone and aneurysmal bone cyst can be either stage 2 (active) or stage 3 (aggressive) lesions depending on the amount of bone destruction, soft tissue, masses, and joint involvement Imaging Radiographs orthogonal views of affected bone or joint findings intensely reactive bone around radiolucent nidus because of intensely reactive sclerosis, may be possible to detect nidus only with CT or MRI CT indication compared to MRI, CT provides better contrast between lucent nidus and reactive bone considered imaging modality of choice characteristic findings of the nidus usually < 1.5 cm (otherwise think osteoblastoma) the area of reactive bone sclerosis may be greater than the diameter of the nidus target-shaped/bulls-eye appearance, representing calcified center of the nidus present 50% of the time 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 peripherally known as the 'double-density sign' MRI indications usually not recommended as bone marrow edema appreciated on MRI can mask typical features of tumors can be useful in illustrating nidus in cases of intra-articular lesions findings must be interpreted with reference to x-ray or CT scan 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 painful scoliosis due to paravertebral muscle spasm 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 Lesion does not infiltrate the surrounding bone 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, 3.0 - 3.5 cm) Site > 50 % in long bone diaphysis > 35% in posterior elements of the spine Location Proximal femur > tibia diaphysis > phalanges > spine Vertebral column > long bone diaphysis/metaphysis 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 that is not self-limiting. Central lesion less organized with greater vascularity. Symptoms Nocturnal pain, relieved by NSAIDS. If spine involvement, presents 75% of the time with painful scoliosis with lesion on concave side of curve Dull ache, partial response to NSAIDS. Neurologic symptoms common if spine involvement. Management of Spine lesions Nonsurgical management is indicated as first-line treatment, definitive treatment is percutaneous RFA/surgical resection Surgery is always indicated as they do not respond to nonsurgical treatment. Treatment Nonoperative clinical observation and NSAID administration indications NSAIDS can be effective for limiting pain In 50% of patients treated with NSAIDS, lesions burn out and resolve several years not ideal for every patient given harmful effects of NSAIDs also indicated for painful spine lesions without scoliosis Operative percutaneous radiofrequency ablation indications considered standard of care compared to surgery, RFA allows for access to more challenging anatomic sites, decreases procedure time and hospital stay length, and decreases costs RFA can be safe and effective for spinal osteoid osteoma based on motor response testing performed during procedure Safe for periarticular lesions if probe is >1cm away from articular surface, otherwise may risk damage to articular cartilage For lesions close to neurovascular structures, a distance of 1-1.5cm should be kept between probe and sensitive anatomy to prevent damage technique done under CT guidance radiofrequency electrode inserted into the nidus probe heated to 90 deg C for 4-6 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 compared with surgery and RFA, MR-HIFU is a non-invasive and non-ionizing treatment option comparable clinical response to RFA 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 surgical resection with curettage indications location of lesion is not amenable to CT guided percutaneous radiofrequency ablation e.g. close to skin or nerve en bloc resection indicated in spinal disease if osteoid osteoma associated with fixed spinal deformity, if RFA unsafe due to proximity to dura/nerve roots, and for lesions that reoccur after RFA 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 spinal deformity can resolve spontaneously if the lesion is resected within 15 months of onset of deformity Complications RFA related complications: recurrence: 10-15% recurrence rate cellulitis, thrombophlebitis, skin necrosis - increased risk if lesion is <1.0 cm from skin neurovascular damage - increased risk if lesion <1.0-1.5cm from neurovascular bundle elongation of affected limb and resultant limb length discrepancy may occur if osteoid osteoma effects the epiphysis Prognosis Pain from lesions usually resolves after an average of 3 years The lesion spontaneously resolves in 5-7 years overall good prognosis, as it is benign process with no potential for malignant progression