Undiagnosed Factor V Leiden Presenting as Free Flap Thrombosis in Mandibular Reconstruction: A Case Report

Microvascular free flap reconstruction remains the gold standard for repairing complex head and neck defects, yet vascular thrombosis continues to pose a serious threat to flap viability. Flap failure is often attributed to technical or local factors; however, the contribution of underlying hypercoagulable states is frequently underrecognized. We present the case of a 71-year-old male with no personal or family history of thrombosis who developed both intraoperative and postoperative arterial and venous thromboses of a fibula free flap used for mandibular reconstruction following extirpation of squamous cell carcinoma. Flap loss led to hematologic evaluation, revealing previously undiagnosed heterozygous Factor V Leiden (FVL). The patient subsequently underwent successful contralateral fibula reconstruction using a modified anticoagulation regimen that included intraoperative heparin boluses and continuous postoperative heparin for therapeutic anticoagulation in intensive care. This case highlights the potential for occult thrombophilia to cause flap failure in the absence of technical error or thrombotic history. It underscores the need for individualized risk assessment and tailored perioperative planning in complex microsurgical reconstruction when there is clinical suspicion of systemic coagulopathic dyscrasia.