Measurement of the translation at the operated level. The flexion and extension radiographs were superimposed by aligning the inferior vertebrae. The tangent (in red) of the inferior endplate of the prosthesis was made, and the two lines perpendicular to the red line, touching the inferior anterior tip of the superior endplate of the prosthesis in flexion (line in white) and extension (line in black), were erected. The vertical distance between these two parallel lines was determined as the translation