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MRI findings in the subchondral bone marrow: a discussion of conditions including transient osteoporosis, transient bone marrow edema syndrome, SONK, and shifting bone marrow edema of the knee.

Bone marrow edema signal can be seen in many settings ranging from trauma and arthritis to less common conditions including transient osteoporosis, transient bone marrow edema syndrome, true osteonecrosis, spontaneous osteonecrosis, and shifting bone marrow edema. Terms such as spontaneous osteonecrosis of the knee (SONK) appear frequently in the radiology and orthopedics literature but are rarely described on true, histologic basis. This article reviews the less frequently encountered and less well understood entities and explores their potential pathophysiologies and significance.