Intracapsular Osteoid Osteoma

T1 coronal

T2 coronal

STIR-coronal

T1FS( contrast)-coronal

PDFS-axial

T1FS(contrast)-axial

STIR-coronal

A 27 year old female patient presented with long standing severe left hip pain. Contrast enhanced MRI showed cortical thickening and a small round well-defined area of signal alteration in anteromedial cortex of neck of left femur (isointense signal to skeletal muscle on T1, target apearance on T2/ STIR/PDFS) and  intense peripheral enhancement after contrast suggestive of osteoid osteoma. There is mild effusion in left hip joint.


A non-contrast MR scan performed 6 monthes earlier ( not shown here) showed significant left hip joint effusion and synovial thickening along with a poorly marginated large area of marrow oedema in the left femoral neck. A diagnosis of infective arthritis was made ( the intracapsular osteoid osteoma was overlooked). The patient was treated with antibiotics but showed persistence of pain with mild improvement.

INTRODUCTION: 
 Osteoid osteomas are benign bone forming tumours.

DEMOGRAPHICS:

• 10-25 years of age
• M>F( 3:1)
• Relatively common being 4-10%of primary bone tumours( excluding myeloma)

CLINICAL PRESENTATION:

• Severe pain which is worse at night and is relieved by salicylates or nonsteroidal anti-inflammatory drugs in 80% patients. It is intially intermittent and becomes unremitting later.
• Intracapsular lesions present with signs of synovitis, joint pain and reduced range of movement
• Premature osteoarthritis can occur with chronic intracapsular lesions ( in 50% cases)
• Intracapsular lesions can also cause growth abnormalities
• Muscle atrophy and neurologic signs are rare

IMAGING:
LOCATION:

• Cortical diaphyseal: commonest (65-70%) with femur being the commonest site
• Intramedullary : Rare and involves carpal and tarsal bones
• Intracapsular: usually in femoral neck, calcar. Has also been reported in elbow, foot, wrist, knee and facet joints
• Subperiosteal: Usually intracapsular( common in hip and talus)
• Spine:10% and involves posterior elements( Lumbar commonest site)
• Multifocal synchronous lesions: rare

X-RAY FINDINGS


Cortical diaphyseal:

• Oval lytic lesion with surrounding cortical sclerosis which may obscure lytic nidus
• The nidus may have central calcification which is seen as a raiopaque point called the Bell

Intracapsular( intra-articular):

• Oval lytic lesion in cortex with or without surrounding sclerosis.
• Sclerotic bone may be at distant site from the lesion
• Joint effusion

CT FINDINGS:

• Accurate in specifying location: cortical vs subperiosteal/ intramedullary
• Scerotic reaction seen
• Lytic nidus better seen
• Assciated pathologic fracture may be seen
• Helpful for guidance of percutaneous RF ablation

MR FINDINGS:
Nidus:

• Low to intermediate signal on T1
• Variable intensity on T2 depending on mineralization.
• Intense and dynamic enhancement( maximum in arterial phase)
• With increased spatial resolution, a partially mineralized nidus has a target appearance with mineralized portion appearing hypointense and non-mineralized portion appearing peripherally hyperintense.Nidus may be missed however in cortical bone.

Reactive response:

• Cortical thickening and sclerosis shows low SI on all sequences
• Reactive marrow oedema is seen in 63% cases( low signal on T1, high signal on fluid sensitive sequences)
• Periosteal elevation and adjacent soft tissue reactive changes are seen in 50% cases( high signal on fluid sensitive sequences)
• Effusion is seen in intracapsular lesions

Advantages of MR: It shows not only the nidus and surrounding sclerosis, but also the marrow oedema and articular changes.

NUCLEAR MEDICINE FINDINGS:

• Technetium Bone scan shows very intense round activity at nidus and is highly sensitive
• Double density sign: Very intense central activity at nidus surrounded by less intensity of reactive bone
• Round focus helps differentiate from stress fracture which has more linear activity.

DIFFERENTIAL DIAGNOSIS :

• Arthritis:( As in the index patient)
• Osteosarcoma/ Ewing's sarcoma
• Stress fracture
• Chronic osteomyelitis

TREATMENT:

• CT Guied radiofrequency ablation is treatment of choice
• Surgical resection

REFERENCES:

• Manaster BJ: Osteoid Osteoma: Diagnostic Imaging:Musculoskeletal non-traumatic disease. Amirsys.2-26, 2010
• Chai JW et al: Radiologic Diagnosis of Osteoid Osteoma: From Simple to Challenging Findings.  Radiographics; 30(3): 737:49,2010
• emdicine. medscape. com