This video presents a pathology case focusing on the microscopic examination of tissue removed from a major joint. The initial low-power view reveals a massive polypoid lesion projecting into a synovial space. The lining is identified as synovium—a loose layer of histiocyte-like cells, distinct from true epithelium—surrounding extensive mature adipose tissue. The defining features of the lesion are the multiple, frond-like projections of benign fat into the joint space, leading to the diagnosis of Lipoma Arborescens (Synovial Lipomatosis). Histological review also highlights the presence of numerous dilated vessels and mixed inflammatory cells within the fatty tissue, and the diagnosis is differentiated from reactive conditions like papillary synovial hyperplasia.
Pathology Case Review: Lipoma Arborescens
0:16 Identifying Context: The examination begins at 2X magnification, showing dense regular connective tissue (collagen, referred to as "ramen noodles") adjacent to the lesion.
0:38 Identifying Synovium: The lining is identified as synovium, characterized by a layer of histiocyte-like cells with rounded nuclei and pink cytoplasm, which forms a loose layer unlike a tight epithelial lining.
1:23 Location Confirmation: The presence of a synovial line space and the massive size of the tissue slab confirm the location is a large joint space.
1:37 Lesion Description: The lesion is composed of mature adipose tissue that is extensively pushed into the joint space, forming multiple large, polypoid, and frond-like projections.
2:05 Diagnosis: The mass is diagnosed as Lipoma Arborescens (Synovial Lipomatosis), an unusual but benign condition involving the growth of fat that makes tree-like projections into the synovium.
2:28 Associated Conditions: The condition has been observed concurrently with other pathologies, such as Pigmented Villonodular Synovitis (PVNS).
2:52 Differential Diagnosis: The diagnosis must be differentiated from Papillary Synovial Hyperplasia, which is a reactive process seen in conditions like osteoarthritis but lacks the significant adipose tissue component.
3:56 Key Histological Features: The adipose tissue in Lipoma Arborescens typically contains a notable amount of dilated vessels and scattered mixed inflammatory cells.
3:35 Whole Slide Image: The whole slide image dramatically illustrates the massive size and frond-like configuration of the lesion. A link to the slide is provided for further review.