Fig. 1.

To assess the degree of height loss, first, the actual disc height has to be determined. For this purpose, the anterior and posterior edges of the adjacent vertebral bodies (small white circles) are defined as those points having the largest distance to the centre of the vertebral body (black points). Then, the distance of each of these four edges to the midplane of the disc (dashed line) is measured. Finally, the sum of the two anterior distances is defined as actual anterior disc height, and the sum of the two posterior distances is defined as actual posterior disc height. This procedure is meant to support the estimation of actual disc height, but does not have to be carried out using drawings or digitisation. In a second step, this actual height is compared to the respective height before degeneration, which is estimated based on the normal values reported by Frobin et al. [15](Table 2 )