Introduction Osteoid osteomas are bone tumors less than 2 cm in greatest dimension and usually occur in patients in their teens and twenties. In fact, 75% of patients are under age 25. Osteoid osteomas can arise in any bone but have a predilection for the appendicular skeleton.
50% of cases involve the femur or tibia, where they commonly arise in the cortex.
Osteoid osteomas are painful lesions. The pain is caused by excess prostaglandin E2 which is produced by the proliferating osteoblasts. It characteristically occurs at night and is dramatically relieved by aspirin.
Special features Failure to increase in size with time Spontaneous regression Replacement by scar tissue These features are unlike those of other benign tumors suggesting that the etiology still remains an enigma.
Incidence 10-11% of all benign bone tumors 2.5%-5% of all bone tumors First three decades of life Most common-second decade Most common-in men (2:1)
Sites of predilection Diaphysis of femur and tibia Medial side of neck of femur Posterior elements of spine Humerus Phalanges of hand Fibula Talus Ribs Skull
Symptoms Pain which has characteristic pattern described variously as sharp,dull,boring deep,or intense often worst at night and very frequently relieved by salicylates*(aspirin) Limp Muscular atrophy due to disuse Swelling and warmth if it is superficial
Adjacent joint stiffness Scoliosis In children overgrowth and angular deformities Nerve root compression or cord compression Point tenderness over the lesion
Gross Appearance Cherry red to gray red tissue Overlying cortex distorted Reactive periosteal new bone formation Nidus may vary from few mm to 1.5 cm in diameter. Surrounding reactive bone usually thick hard and extensive..
Microscopy Numerous osteoblasts forming highly irregular trabeculae of osteoid and woven bone Numerous osteoclasts Woven bone trabeculae variably mineralized Calcification more near centre of lesion.
At times no calcification of nidus Surrounding bone shows reactive bone formation which is lamellar bone in contrast to woven bone of nidus Thin zone of fibrovascular tissue between nidus and reactive bone .
Radiological findings Small to round to oval focus of decreased density called nidus .sometimes nidus also sclerotic. Surrounding area of sclerosis which is normal reactive bone .
Lesions usually in diaphysis Mostly cortical sometimes inside medullary canal or subperiosteally Periosteal reaction when occurs is large but smooth in contrast to “codman triangle” of malignant lesions
Bone scan Useful in detecting small lesions “Double density sign” which is a focal area of increased activity with a second smaller area of increased uptake superimposed on it is said to be diagnostic.
CT scan and MRI Sometimes required to localise the lesion accurately.
Osteoidosteoma. A lateral view (A) of the proximal tibia shows a very dense lesion in the posterior cortex. A darker central area contains a white nidus. This lesion in a 20-year-old man caused pain in this area, relieved by aspirin. B, A nuclear medicine bone scan in a different patient with an osteoidosteoma in the left lower tibia shows increased activity (arrows) at the site of the lesion.
Osteoidosteomas, especially those that arise beneath the periosteum, usually elicit a tremendous amount of reactive bone formation that encircles the lesion. The actual tumor, known as the nidus, manifests radiographically as a small round lucency that is variably mineralized Specimen radiograph of intracortical osteoid osteoma. The round radiolucency with central mineralization represents the lesion and is surrounded by abundant reactive bone that has massively thickened the cortex.
Why relief with aspirin !!!! High levels of prostaglandins present in osteoid osteoma which mediate pain receptor pathway Aspirin (salicylates) act as prostaglandin synthetase inhibitors
Treatment Surgical removal of lesion To relieve pain.secondary manifestations like synovitis ,scoliosis, nerve root compression
Principle of surgery Necessary to remove the “NIDAL” tissue
Surgical options Block resection of the nidus Increases risk of subsequent # if lesion is in cortical bone Alternative method is to shave the reactive bone with sharp osteotome until the nidus is exposed ,then curette the exposed nidus
Intraoperativelocalization of nidus possible with pre operatively injected technetium labelledmethylenediphosphonate and sterile wrapped geigercounter. Intraoperativexrays of excised specimen to document complete removal of nidus
Excision of nidus using CT assisted localization K-wire inserted into the nidus Biopsy punch inserted over k-wire Percutaneous CT guided resection using a trephine 2mm larger then the lesion to ensure complete removal.
Radiofrequency ablation Done percutaneously Initial core needle biopsy after which radiofrequency electrode is inserted through cannula of biopsy needle Temperature at the tip raised to 90 degrees centigrade for 6 minutes Results:claim to be equivalent to surgical excision Used only in extraspinal lesions that are away from neurovascular structures
Conclusion Osteoid osteomas’ are considered benign and are normally treated by conservative surgery. However there is a possibility of malignant transformation. This is rare except when treated with radiation, which promotes this complication.