B. J. Burger, J. D. Burger, C. F. A. Bos, J. Hermans, P. M. Rozing, and J. P. Vandenbroucke, “Frejka pillow and Becker device for congenital dislocation of the hip: Prospective 6-year study of 104 late-diagnosed cases”, ActaO, vol. 64, no. 3, pp. 305–311, Jan. 1993.