Extradigital Glomus Tumor of the Knee Mimicking Osteoarthritis: A Case Report.

Introduction: Glomus tumors are rare benign vascular neoplasms that usually arise in the subungual region. Around the knee, they remain exceptional and may be mistaken for degenerative or cystic pathology. This case highlights that, in patients with focal, exquisitely tender, cold-sensitive pain that is disproportionate to radiographic osteoarthritis, orthopedic clinicians should broaden the differential diagnosis to include soft tissue pathology and consider magnetic resonance imaging (MRI) to facilitate timely and accurate diagnosis. Case Report: A 78-year-old Hispanic male presented with a 15-year history of medial left knee pain. Conservative treatment for presumed osteoarthritis provided partial symptomatic relief during the first 7 years; however, during the subsequent 8 years before presentation to our facility, the pain became progressively refractory to conservative measures and increasingly localized to a small palpable, discolored, superficial medial parapatellar nodule. Radiographs demonstrated mild-to-moderate osteoarthritic changes and a discrete medial soft-tissue radiodensity. Upon evaluation at our clinic, the above history, together with persistent focal tenderness to light touch and cold hypersensitivity, prompted MRI, which demonstrated a 1.3 cm well-circumscribed subcutaneous nodule in the upper medial parapatellar soft tissues, hypointense on T1-weighted images and hyperintense on T2-weighted images, without intra-articular extension or osseous involvement. Complete local excision was performed with a 2 cm gross soft-tissue cuff to the retinacular level, followed by layered closure. Histopathologic examination demonstrated uniform glomus cells arranged around dilated vascular channels. Immunohistochemical staining was positive for muscle actin and alpha-smooth muscle actin and negative for epithelial membrane antigen, S100, Cluster of differentiation 34 (CD34), and desmin, confirming a glomus tumor. Margins were free of tumor. The patient experienced immediate complete pain relief and remained symptom-free without recurrence at 12-month follow-up. Conclusion: A small extradigital glomus tumor can masquerade as osteoarthritic knee pain. When tenderness is focal, exquisite, cold sensitive, and not explained by radiographs, orthopedic evaluation should extend beyond arthritis. The clinician should consider MRI and biopsy with histopathologic confirmation to prevent prolonged ineffective treatment.