An intramedullary screw and fixation system for intraosseous bone fusion of the subtalar joint includes a lag screw member, a reamer for reaming an internal surface of each of the calcaneus and talus medullary canals, and targeting guide assembly for alignment with the subtalar joint. The lag screw member includes an elongated body, a first threaded portion at a first end and a bulbous portion at a second end. The targeting guide assembly receives a cutting blade in an elongated rod for reaming the articulating surfaces of the calcaneus and talus intramedullary canals, where the cutting blade is inserted through a percutaneous incision into the elongated rod.