This video presents a pathology case review of an intra-articular knee mass in an 18-year-old male. The microscopic examination reveals a hypocellular, collagen-rich lesion composed of bland, spindle-shaped cells identified as fibroblasts or myofibroblasts. Key features distinguishing the lesion include the abundant purple cytoplasm of the cells (reminiscent of nodular fasciitis) and a highly collagenous, pink background, differentiating it from smooth muscle tumors. The definitive diagnostic feature for the final diagnosis—Fibroma of Tendon Sheath (FTS)—is the presence of thin, compressed, "cleft-like" vessels, often accompanied by cracking artifacts in the collagen. The video concludes by confirming the presence of adjacent normal tendon tissue and reviewing the differential diagnosis for pink spindle cell fascicles.
Fibroma of Tendon Sheath: Case Review of an Intra-Articular Knee Mass
0:08 Case Introduction: The case involves an 18-year-old male presenting with a large intra-articular knee mass (Case 2).
0:30 Initial Microscopic Findings: The lesion is nodular, hypocellular, very pink (indicating high collagen content), and composed of bland, thin spindle cells, which are categorized as fibroblasts or myofibroblasts.
1:10 Cellular Distinction: The speaker notes that fibroblasts and myofibroblasts exist on a spectrum and are often clinically lumped together. Neural lesions are also considered in the differential diagnosis for fibroblastic-appearing masses but are deemed unlikely here.
2:10 Cellular Morphology: A closer look reveals myofibroblastic cells with "juicy, abundant purple cytoplasm," similar to cells seen in nodular fasciitis. Crucially, the dominant pink color of the mass is confirmed to be collagen in the background, not the cellular cytoplasm, distinguishing it from smooth muscle tumors.
2:56 Diagnosis: The preferred diagnosis for this lesion, despite its proximal location in the knee, is Fibroma of Tendon Sheath (FTS), as opposed to a generic "intra-articular fibroma."
3:41 Key Diagnostic Feature (Vessels): FTS exhibits "thin, compressed elongated vessels" which the speaker refers to as "cleft-like" (avoiding the buzzword for Kaposi sarcoma).
4:15 Key Diagnostic Feature (Artifacts): The lesion also demonstrates cracking artifacts or "thin cleft-like spaces" within the densely collagenized background. The compressed vessels are confirmed to be real via the presence of a thin single lining of endothelial cells.
5:32 Locating Tendon: The video identifies adjacent normal tendon tissue (dense regular connective tissue) attached to the mass, supporting the FTS diagnosis.
6:04 Educational Triad: A key takeaway is the Triad for identifying pink spindle cells in a fascicle: 1) Dense Regular Connective Tissue (tendon/ligament/fascia), 2) Nerve, and 3) Smooth Muscle.