Closed Intratendinous Rupture of the Flexor Digitorum Profundus: A Rare Cause of Pediatric Trigger Finger.

Paediatric trigger finger (PTF) is a rare condition, far less common than paediatric trigger thumb, and often associated with anatomical anomalies or systemic disease. Unlike trigger thumb, which may resolve spontaneously, PTF usually requires surgical treatment. Trauma is an uncommon cause, and closed intratendinous rupture of the flexor digitorum profundus (FDP) has not previously been reported in a child following direct injury. We present the case of a healthy 12-year-old boy who developed triggering of the right middle finger eight weeks after blunt trauma during rugby. Examination revealed a palpable nodule at the A2 pulley and paradoxical distal interphalangeal (DIP) joint extension during fist-making (lumbrical plus phenomenon). Ultrasound and MRI demonstrated intact FDP and flexor digitorum superficialis (FDS) tendons without an obvious lesion. Ongoing symptoms led to surgical exploration, which revealed a partial intratendinous FDP rupture with a scar nodule beneath an intact A2 pulley. Management included partial A2 pulley release, nodule excision, FDP repair with 5/0 polypropylene suture, and lumbrical release. Early active mobilisation began within days postoperatively. At one year, the patient had a full, pain-free range of motion, normal grip strength, and no recurrence. This is the first reported paediatric case of PTF caused by a closed, trauma-induced partial FDP rupture. The intratendinous tear produced a scar mass that impinged beneath the A2 pulley, mimicking more typical A1 pulley pathology. The lumbrical plus sign, rarely described in children and usually linked to complete ruptures, here resulted from a partial tendon injury. Standard imaging did not identify the lesion, highlighting the limitations of ultrasound and MRI in subtle intratendinous injuries. Careful clinical examination, particularly eliciting paradoxical extension, was critical to diagnosis. Literature review indicates most PTF cases are idiopathic or related to systemic disease, with only ~5% linked to trauma. Surgical release yields higher resolution rates than conservative management (87% vs. 58%). Given the mechanical nature and delayed presentation of this lesion, surgery was both diagnostic and therapeutic. A comprehensive approach, pulley release, tendon repair, and lumbrical release, produced an excellent outcome. PTF after blunt trauma can arise from closed intratendinous FDP rupture, even when imaging is unremarkable. The lumbrical plus sign is a valuable but under-recognised diagnostic clue. Early surgical exploration should be considered when suspicion remains high, as timely intervention can restore full function and prevent long-term disability.