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Remote Fellowship – Foot & Ankle Fundamentals – 11/2/2020

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Diagnosis

Avascular Necrosis 

Case Discussion

The talus is the 2nd largest of the tarsal bones, however only approximately 60 percent of its surface is covered by articular cartilage and the bone is void of muscular or tendinous attachments. The unique structure and precarious blood supply to a small portion of bone predispose the talus to avascular necrosis/osteonecrosis.

Osteonecrosis or avascular necrosis of the talus can be classified as a traumatic or atraumatic processes that impair the nutrient supply to the bone. Atraumatic causes for AVN include excess exogenous or endogenous corticosteroids, connective tissue disorders and other pro-thrombotic states such as SLE, antiphospholipid syndrome, blood clotting disorders such as Factor V Leiden, dyslipidemia, radiotherapy/chemotherapy, emoglobinopathies such as sickle cell anemia, and renal transplantation.

MRI is the most sensitive imaging modality for detecting AVN, detecting changes well before radiographic changes are evident.

A highly specific or pathognomonic sign of avascular necrosis on MRI is the “double line sign” or “band form sign” of serpiginous hypointensity with adjacent hyperintensity on water weighted sequences. The hypointense serpiginous line represents sclerosis, while the adjacent hyperintense serpiginous line represents reactive fibrovascular granulation tissue.

More advanced AVN may depict high grade surrounding high signal osteoedema (directly correlated to the degree of symptomatology), a surrounding fluid signal cleft indicating potential instability of the overlying fragment, and finally subchondral fragmentation and/or collapse.

Marrow pallor with AVN should always raise suspicion of either endogenous or exogenous hypercortisolism.  exaggerated red marrow with AVN should always raise suspicion of a hemoglobinopathy.

Treatment of talar AVN depends on the etiology and extent of AVN. If detected early, talar AVN may be treated with protected weight bearing. However, with late detection and in the presence of talar dome collapse, surgical intervention, including arthrodesis with or without concomitant partial or complete talectomy, may be required.

Key Images

Extensive multifocal avascular necrosis in a 45 year old woman with a history of leukemia complaining of anterolateral ankle pain for 2 weeks with no inciting event.

A. Sagittal T2 image of the right ankle shows a serpiginous hypointense line, consistent with extensive avascular necrosis in the posterior and superior aspects of the talar dome (red arrow).
B. Sagittal T2 FSE image of the right ankle shows extensive avascular necrosis in the posterior and superior aspects of the talar dome (red arrow) with associated extensive reactive osteoedema (yellow arrow) and capsulitis. An additional bone infarct/osteonecrosis is noted in the posterior calcaneus (green arrow).

References

  1. Pearce D, Mongiardi CN, Fornasier VL, Daniels TR. Avascular necrosis of the talus: A pictorial essay. RadioGraphics 2005; 25:399-410
  2. Murphey MD, Foreman KL, Klassen-Fischer MK, Fox MG, Chung EM, Kransdorf MJ. From the radiologic pathology archives imaging of osteonecrosis: Radiologic-pathologic correlation. RadioGraphics 2014; 34:1003-1028