Osteoid osteoma of the triquetrum: A rare cause of ulnar-sided wrist pain in an adolescent female.

Osteoid osteoma is a common benign osteoblastic tumor typically affecting males in their second or third decade of life, with a predilection for the long bones of the lower extremities. Its occurrence in atypical anatomical sites, such as the carpal bones, is rare. Specifically, triquetral involvement is exceedingly infrequent, often leading to diagnostic challenges as the lesion can mimic more common inflammatory or infectious pathologies. We present the case of an 11-year-old girl with a 9-month history of persistent, localized left wrist pain. Initial magnetic resonance imaging demonstrated extensive bone marrow edema and joint effusion, raising a high suspicion for septic arthritis. Following a negative infectious workup and clinical persistence of symptoms, specifically nocturnal pain responsive to salicylates, a high-resolution computed tomography scan was performed. The computed tomography revealed a pathognomonic 5-mm radiolucent nidus within the triquetrum. The patient underwent surgical excision, which provided immediate and dramatic symptomatic relief. Histopathological analysis confirmed the diagnosis of osteoid osteoma. In pediatric patients presenting with chronic, localized wrist pain that worsens at night, clinicians must maintain a high index of suspicion for osteoid osteoma, even in rare sites like the triquetrum. This case underscores the "magnetic resonance imaging masquerade" effect, where extensive reactive edema can mislead the clinician toward an infectious diagnosis. Early utilization of high-resolution computed tomography is essential to identify the nidus, prevent diagnostic delay, and avoid unnecessary hospitalization for suspected infection.