Posterior Knee TGCT (Formerly PVNS) Masquerading as a Popliteal Lesion: Combined Arthroscopic Synovectomy and Posterior Open Excision With 8-Year Disease-Free Follow-Up.

Tenosynovial giant cell tumor (TGCT), formerly pigmented villonodular synovitis (PVNS), may extend into the popliteal fossa and mimic a Baker cyst, complicating diagnosis and operative planning. A 53-year-old former endurance runner presented with posterior knee pain, swelling, intermittent locking, and a palpable popliteal mass. Examination demonstrated effusion and limited range of motion; weightbearing radiographs showed mild osteoarthritic change with effusion, and magnetic resonance imaging (MRI) revealed synovial hypertrophy with low T2 signal suggestive of hemosiderin deposition. Arthroscopic synovectomy via 4 anterior and 2 posterior portals was performed; histology confirmed diffuse, nonmalignant TGCT. Given persistent posterior disease, a staged posterior open approach (lazy-S incision) enabled en bloc excision of the extra-articular component. Adjuvant radiosynoviorthesis (intra-articular yttrium-90) was administered. At 8-year follow-up, there was no clinical or radiographic evidence of disease. This case emphasizes MRI recognition of hemosiderin-related low T2 signal, the value of combining arthroscopic synovectomy with posterior open excision in diffuse disease with popliteal extension, and selective use of adjuvant radiosynoviorthesis for durability.