Benign intra-articular soft tissue tumors mimicking hip septic arthritis: an arthroscopic case series with tenosynovial giant cell tumor predominance

Benign intra-articular soft tissue tumors can mimic septic arthritis of the hip (SAH), posing a significant diagnostic challenge. Timely differentiation between these entities is crucial, as SAH requires urgent intervention to prevent adverse outcomes. The present study aims to (1) examine clinical presentations, imaging findings, laboratory results, and histopathological features associated with benign intra-articular soft tissue tumors mimicking SAH and (2) describe diagnostic challenges encountered in treating these patients. A retrospective review identified 23 patients who underwent arthroscopic irrigation and debridement for suspected SAH at a single institution between 2014–2023. Seven patients with negative cultures and intraoperative diagnoses of intra-articular tumors confirmed by histopathology were included in the primary analysis. Clinical presentations, imaging findings, laboratory results, arthroscopic findings, histopathology results, and follow-up data were analyzed. A comparison with the non-tumor cases ( n = 16) was performed as a secondary analysis. The mean age of the tumor group was 25.0 ± 8.1 years and a female predominance (6/7, 85.7%) was observed. All patients presented with acute hip pain, limited range of motion, and elevated C-reactive protein levels (80.0 ± 38.3 mg/L). Three patients presented with fever and three with leukocytosis. Synovial fluid white blood cell counts exceeded 50,000 cells/μL in five patients. Arthroscopy revealed solid tumors in all cases. Tumor necrosis was observed in all cases and tenosynovial giant cell tumor (TGCT) was confirmed histologically in six cases (6/7, 85.7%). Postoperative MRI in four patients showed no tumor recurrence. Compared to the non-tumor group, patients with tumors were significantly younger ( p = .016) and had lower preoperative CRP levels ( p = .027). Differentiating benign intra-articular tumors from SAH remains difficult, particularly among younger and female patient populations with acute presentations of TGCT which, as observed in the present study, may be explained by histopathologic evidence of tumor pedicle torsion and resultant necrosis. MRI can be valuable in identifying intra-articular lesions given that clinical and laboratory findings are frequently equivocal. The proposed diagnostic algorithm offers a framework for identifying cases where preoperative MRI may be beneficial, though it requires validation in larger studies.