BackgroundWe proposed a new method to evaluate the height of the posterior articular surface in displaced intra-articular calcaneal fractures (DIACFs) by drawing two lines on the lateral radiograph of hindfoot: one line was drawn from the apex of the posterior facet to the apex of the posterior tuberosity (the apex line), and the other line was drawn from the apex of the anterior process to the posterior process (the anterior posterior process line). The aim of this study was to analyze the relationship between these two lines in normal calcanei and their changes in DIACFs.MethodsLateral radiographs of normal calcanei and DIACFs between January 2018 and February 2020 were collected. We draw two lines on the lateral radiograph, one line was the apex line and the other line was the anterior posterior process line. The value of the angle formed by these two lines was recorded, if these two lines were parallel to each other, the angle was recorded as 0°. if these two lines formed an angle anteriorly this was recorded as positive, and if these two lines formed an angle behind the calcaneum this was recorded as negative.ResultsIn 135 normal calcanei lateral radiographs, the minimum angle was − 8°, and the maximum angle was 6°, with an average angle of -1 ± 3°. In 145 DIACFs, the minimum angle was 6°; the maximum angle was 37°, and the average angle was 17°±6°. The difference between normal calcanei and DIACFs was statistically significant (P༜.001).ConclusionThe relationship between the apex line and the anterior posterior process line on lateral radiograph can evaluate the height of the calcaneal posterior facet simply and accurately. The two lines are approximately parallel or angulated backward in the normal calcanei, in DIACFs, these two lines angulated forward.Level of Evidence: Level III-retrospective comparative study.