Case Report: prolonged sinus tract formation after collagen-hydroxyapatite artificial bone grafting — diagnostic dilemma between infection and foreign body reaction

Prolonged sinus tract after artificial bone grafting presents a diagnostic and therapeutic dilemma. We report a case with 8-month longitudinal follow-up, propose a theoretical residual graft load (RGL)—inflammatory threshold (IT) balance framework to explain the recurrent “healing-rupture” pattern, and demonstrate a dual-phase dexamethasone strategy—as diagnostic tool and healing catalyst—after infection is excluded. A 38-year-old male developed fracture-related infection after left tibial plateau fracture fixation and was treated with the Masquelet technique. After second-stage grafting with artificial bone which was composed of bovine Type I collagen and hydroxyapatite mixed with autologous bone, the wound developed an aseptic sinus tract that persisted for 8 months. Repeated cultures were negative and inflammatory markers remained normal. At hardware removal, the graft showed “bean-curd-like” disintegration, suggesting foreign body reaction. Dexamethasone led to sinus healing, with recurrence after withdrawal confirming the diagnosis. As drainage gradually decreased, a final short course of dexamethasone resulted in complete wound closure. This case highlights that foreign body reaction, not infection, should be considered in such prolonged sinus tracts, and that the RGL—IT hypothesis provides a framework for understanding the healing-rupture cycle.