Simon, E., F. Saur, M. Buerge, R. Glaab, and M. Roos. “Inter-Observer Agreement of Ultrasonographic Measurement of Alpha and Beta Angles and the Final Type Classification Based on the Graf Method”. Swiss Medical Weekly, vol. 134, no. 4546, Nov. 2004, pp. 671-7, doi:10.4414/smw.2004.10764.