A plain radiography may underestimate the steep posterior tibial slope of the medial condyle in elderly Japanese with varus knee osteoarthritis undergoing arthroplasties

•Radiographic PTS tended to underestimate steep medial slope.•Radiographic PTS better reproduces lateral slope in varus knees OA.•Outliers of PTS in radiograph is more in medial (42%) than lateral (12%).•Physical appearance of the medial PTS may be different from plain radiograph.•Not a few patients need reduction of the steep PTS during tibial cut.