Fatty lesions in intra-articular loose bodies: a histopathological study of non-primary synovial chondromatosis cases.

Intra-articular loose bodies (ILBs) are not uncommon conditions in patients with various joint diseases. Their morphological alterations have been investigated in detail, but little attention has been paid to their fatty lesions. In this study, we examined fatty bone marrow, fat cells without bone marrow structures (extramedullary fat cells), and their necrotic changes in 55 ILBs surgically removed from 42 patients, excluding primary synovial chondromatosis cases. The presence of viable re-vascularized vessels with or without enchondral ossification could discriminate 19 re-attached ILBs from 36 true free ILBs. Fatty bone marrow was found in 25 ILBs, and its necrosis was recognized in 11 (44.0%) of them. Extramedullary fat cells, characterized by single or clustered fat cells focally or multifocally distributed in fibrous or cartilaginous stroma, were identified in seven true free ILBs (7of 55, 12.7%), and all of them were focally necrotic. Unique lipomembranous changes were detected in 7 (12.7%) of 55 ILBs; they were found in 4 (36.4%) of 11 ILBs of necrotic bone marrow and were associated with 3 (42.9%) of 7 necrotic extramedullary fatty lesions. These changes were highlighted by periodic-acid Schiff and Masson's trichrome stain more clearly. We concluded that extramedullary fat cells represent lipometaplasia in ILBs with no blood supply. We considered that lipomembranous changes in ILBs can be a useful hallmark for necrotic bone marrow or necrosis of extramedullary lipometaplastic lesions.