Figure 5

(a) The C-arm fluoroscopic ankle mortise radiograph is used during pin insertion. The low-risk zone on the simple radiograph was transferred to the MRI to measure the distance between its borders and the neurovascular structures. (b) On the axial image, the lateral end of the posterior malleolus at the joint level (α) and the anteromedial end of the anterior distal tibial articular surface (β) are marked. The marks can be shown in every oblique axial slice using Pictured Archives and Communication System (PACS). (c)The oblique axial slice, oriented on the trajectory of the pin fixation (from the center of the calcaneal tuberosity to the distal tip of the posterior malleolus: ④), was selected for the measurement. A line was drawn connecting the two ends (α, β). Then, a line was drawn from the center of the calcaneal tuberosity (c) bisecting (m) the line connecting the two ends (α, β), indicating the medial border of the low-risk zone (②). Also, for the low-risk zone’s lateral border (①), a line was drawn from the center of the calcaneal tuberosity (c) to the lateral end of the posterior malleolus (α). Similarly, for the most medial border (③), a line was drawn from the center of the calcaneal tuberosity (c) to the anteromedial end of the anterior distal tibial articular surface (β), allowing us to confirm the safety of aiming the pin toward the medial end of the anterior distal tibial articular surface during fixation. The three borders of the low-risk zone are observable in every oblique slice using PACS. The distance (d_s) between the sural nerve (closed circle) and the lateral border of the low-risk zone (①), (d_t1) the posterior tibial neurovascular structures (dotted circle) and the medial border of the low-risk zone (②), and (d_t2) the posterior tibial neurovascular structures and the most medial border of the low-risk zone (③) were measured. (d) Five additional consecutive oblique axial slices within 1 cm proximal from the distal tip of the posterior malleolus (④) were recorded to determine the safety of inserting the pin into the posterior malleolus 1 cm proximal from the distal tip (⑤).