Histopathologic evaluation of bone marrow lesions in early stage subchondral insufficiency fracture of the medial femoral condyle.

Subchondral insufficiency fracture (SIF) of the medial femoral condyle has been proposed to be a primary event in so-called 'spontaneous osteonecrosis of the knee'. SIF is also known to be associated with bone marrow lesions (BML), but the detailed histopathology of the BML has not been fully clarified. We thus investigated the pathophysiology of BML based on MRI and histology in the 4 consecutive patients diagnosed with SIF, whose onset was within 4 months. In all cases, BMLs were enhanced on T1 Gd-enhanced MRI. Histologically, BMLs comprised serous exudate, fibrous tissue, and vascular-rich granulation tissue in the marrow space. In addition, a lower signal intensity line was observed within the BML in all cases on T1 MRI. Histologically, this line showed thickened bone trabeculae accompanied by fibrovascular tissue in two cases, while the other two cases showed formation of woven bone trabeculae around the original fractured bone trabeculae indicating the presence of another fracture in the bone marrow space. In summary, BML in SIF was considered to be a secondary phenomenon resulting from a subchondral fracture.