Illustration of measurement of intervertebral disc height and disc depth. a: Anterior disc height (ADH), b: Posterior disc height (PDH), Disc height (DH) = 1/2 (ADH + PDH) = 1/2 (a + b). c: Superior disc depth (SDD), d: Inferior disc depth (IDD), Disc depth (DD) = 1/2 (SDD + IDD) = 1/2 (c + d).