Fig. 2.

Estimation of the neuroforaminal height by a trigonometric approach. A: The length of the vertebral end plates (d) and the bony height (a, b) of the neural foramen can accurately be measured in a sagittal view 2D x-ray of the spine. The height of the disc part (c) of the neural foramen is variable and depending on the amount of the lordotic angle of the cage (y) as well as by the size of the posterior aspect of the cage (c1). B: By assumption of an isosceles triangle and the consecutive four distinct steps (step 1 to 4), the height of the disc part (c) of the neural foramen can be estimated by use of the length of the vertebral endplates before surgery (d), the angle of the cage (y), and the height of the posterior aspect of the cage (c1). The total height of the neural foramen is then given by the simple sum of the bony (a, b) and disc parts of the neural foramen (c). Important remark: The formula is only valid if the cage is placed in the anterior two thirds of the intervertebral disc space assuming postoperative rigidity of the segment by posterior instrumentation. Panel A of the figure was partly created using Servier Medical Art, provided by Servier, licensed under a Creative Commons Attribution 3.0 unported license.