Fig. 4

a Pelvic tilt is measured as the anterior tilt angle of the anterior pelvic plane in the sagittal plane toward the surgical table (α, α’). Pelvic tilt change is calculated by subtracting α from α’. Anterior tilt is given a positive value, and posterior tilt is negative. b Axial rotation is the lean of the anterior pelvic plane in the axal plane toward surgical side (β, β’). Lean toward the surgical side is given a positive value, and rise is negative. The difference in pelvic tilt toward the surgical side from registration to cup implantation is defined as change in axial rotation. The angle is calculated by subtracting β from β’