Purpose: The fillet-of-sole (FOS) flap, consists of the glabrous skin of the foot sole, plantar muscles, and fascia. Leveraging the "spare-parts" principle, it is particularly valuable when local tissues are inadequate for conventional reconstruction. Its durability and sensory capacity make it ideal for prosthetic use, especially in young, active trauma patients without peripheral vascular disease. Despite this, it remains underutilised and predominantly described in a pedicled form for BKA reconstruction. To our knowledge, this is the first case report detailing the use of a non-pedicled FOS free flap for an AKA in the aftermath of trauma. Case Report: A 20-year-old male sustained a high-energy above knee impalement injury to the distal posterior thigh in a motorbike accident resulting in a complete transection of the distal right superficial femoral artery and sciatic nerve. Revascularization was attempted however the degree of eventual muscle necrosis necessitated an amputation. There was insufficient viable posterior tissue to allow for a conventional AKA. As such, reconstruction using a FOS free flap with posterior tibial artery and vein anastomoses and neurorrhaphy of the tibial to sciatic nerve was performed. Total ischaemic time was 103 minutes. At 12 months, the flap remained healthy and sensate with no wound breakdown or phantom pain. 10g monofilament testing was intact over 60% of the flap surface. Conclusion: The FOS free flap offers a durable, sensate solution for complex AKA reconstruction where local tissue is insufficient. This case demonstrates its successful use in trauma, preserving limb length and enabling functional recovery, underscoring its potential in challenging reconstructive scenarios.