Localized nodular synovitis of the infrapatellar fat pad
A 24-year-old male presented with a 2-year history of pain in the left knee joint on terminal extension and extension limitation. The symptoms developed insidiously without any antecedent trauma. He reported discomfort when ascending stairs, and no pain at rest or in bed at night, nor any constitutional symptoms.\nAn initial clinical examination revealed mild effusion but no clicking or localized joint line tenderness. Tests for Lachman's sign, anterior drawer, pivot shift, and posterior drawer were all negative, indicating there was no knee instability. The knee joint was able to reach full flexion, but extension was restricted by 20°. Attempts to reach full extension resulted in severe anterior knee pain. There was dull tenderness on the anterior and medial aspect around the knee joint.\nRoentgenograms of the left knee showed no abnormalities including no mineralized density. Sequential magnetic resonance image (MRI) scans were performed, and T1-weighted images revealed an oval shaped well-defined mass of intermediate signal intensity that lay superior to the infrapatellar fat pad and in between the distal half of the retropatellar region and the distal femur trochlea []. The mass was approximately 2.5 × 2 × 1.5 cm in size. T2-weighted images showed that the mass was inhomogeneous and had relatively high signal intensity with hypointense areas. The mass was connected to a fibrotic thickening of the infrapatellar plica which traversed the fat pad [].\nAn arthroscopic examination and excision biopsy were performed. Initially, we created anteromedial and anterolateral portals. However, we found it very difficult to insert the arthroscope into the joint via those portals due thick soft tissue. We suspected that an extensively hypertrophied infrapatellar plica had adhered to the mass. To overcome the problems associated with those obstructions, we chose to create an alternative viewing portal—a superolateral portal. The superolateral portal view revealed a well-encapsulated, round pan-shape nodular brownish mass that extended upward and entered the patellofemoral joint upon joint extension so as to prevent full extension. That mass retracted when the joint was flexed greater than 30° [Figure and ]. Those arthroscopic findings were consistent with the severe pain experienced on terminal extension and the extension limitation of the joint. We observed no damage to the cruciate ligament or menisci, nor did we observe articular cartilage without villous fronds. To remove the mass, we placed the knee at maximum extension such that the mass entered the patellofemoral joint, and then excised it using superior triangulation (i.e., a superolateral viewing portal and a superomedial working portal, or vice versa). The cut surface revealed yellowish and whitish fibrous tissue []. There was no evidence of a cystic component or acute hemorrhage. After excision, we assessed whether the knee could be moved through a full range of movement, and simultaneously confirmed via arthroscopy that there was no longer any mass impingement into the joint or any joint locking signs.\nHistopathological examination of the mass revealed that it was composed of an admixture of osteoclast-type giant cells and mononuclear cells in a collagenous stroma. Nuclear pleomorphisms and mitotic figures were rare, and there were no villous projections of the synovium, either macroscopically or microscopically []. Those pathological features were consistent with localized nodular synovitis. There were no postoperative complications, and the patient was discharged on postoperative day 2. At a 22-month followup, the patient was found to be pain-free and had a full range of knee motion with no evidence of recurrence. The patient was informed that data concerning the case would be submitted for publication, and he consented.