Mayr, Eckart, et al. “The Frontal Pelvic Plane Provides a Valid Reference System for Implantation of the Acetabular Cup: Spatial Orientation of the Pelvis in Different Positions”. Acta Orthopaedica, vol. 76, no. 6, Jan. 2005, pp. 848-53, doi:10.1080/17453670510045471.