Bone injury caused by a mechanism other than a single traumatic episode. Atraumatic fx may be due to underlying abnormal bone (insufficiency fx, atypical fx, or pathologic fx), or the bone may be normal (fatigue fx)

Fragility fx: traumatic fx in an elderly patient w osteoporosis and often sarcopenia caused by a low-velocity mechanism that would not normally be expected to cause fx in a patient w normal bone mineral density or mm mass. 

Stress fx

• Fatigue fx: abnormal stress on normal bone; injury to normal bone caused by repetitive submaximal force.

• Insufficiency fx: normal stress on abnormal bone; injury of an abnormal bone, weakened through a metabolic process (most commonly but not necessarily osteoporosis).

  • Atypical fx: special type only occurring in the lateral cortex of the prox-to-mid femur

Pathologic fx: fx through a focal lesion; most commonly a neoplasm (benign or malignant) or less commonly through a region of osteomyelitis. 

• Can appear similar radiographically and histopathologically. Immature osteoid of stress fx may appear similar to bone-forming tumors like osteosarcomas

• When indeterminate, MRI is preferred imaging modality. 

  • T1WI is most helpful sequence w special attention to both homogeneity and margins of the SI abnormality around fx plane.

  • T2WI and C+ is less helpful bc marrow edema, inflammation, and hemorrhage are hyperintense and enhance in nonmass-like fashion. 

TUMOR VS INFECTION

• Penumbra sign: helpful in distinguishing subacute osteomyelitis and ST abscess from neoplasm. 

  • Corresponds to a thin layer of granulation tissue that lines the cavities of both bone and ST abscesses. Relatively T1WI hyperintense when compared w content of central abscess or the surrounding bone marrow and ST edema. 

  • Granulation tissue is vascularized and avidly enhances. 

• Extraosseous fat-fluid level: rare but specific finding for osteomyelitis. 

  • Occurs when septic necrosis of bone marrow adipose cells relases fatty globules that layer in the subjacent ST through a cortical breach. The fatty layer is antidependent to pus resulting in a fat fluid level that virtually excludes underlying neoplasm.

  • In pediatric patients, the fibrous layer of periosteum is often uncoupled from the underlying bone by pus which can result in formation of a subperiosteal abscess. 

    • Spread of abscess is limited at periphery of physes by perichondrium. 

    • Subperiosteal spreading of tumor typically does NOT stop at perichondrium and violation of this anatomic boundary should be considered neoplastic.

    • Equivocal cases of subperiosteal dz, contrast material is helpful bc subperiosteal abscess should not show solid masslike enhancement typical of tumors.

    • Ewing sarcoma is notoriously difficult to distinguish from osteomyelitis. AAs were found to be fore more likely to have osteomyelitis than have Ewing sarcoma. 50% of ES bx are nondiagnostic. Open surgical bx is more accurate for Ewing sarcoma but can still yield inconclusive rsults. Therefore, repeat surgical bx should be performed w/o delay.