Does a herniated nucleus pulposus contribute significantly to a decrease in height of the intervertebral disc? Quantitative volumetric MRI

A lumbar intervertebral disc with a herniated nucleus pulposus (HNP) often exhibits a decrease in the height of the intervertebral space. Our purpose was to ascertain whether the loss of volume of an HNP is sufficient to caus e a perceptible decrease in the height of the intervertebral space. MRI of 44 patients with 51 HNPs were reviewed. The volumes of the herniated materi al and of the intervertebral discs were calculated for every level from L1- 2 to L5-S1. The average volume of the HNP was 503 +/- 301 mm(3). The averag e volumes of all 220 intervertebral discs and of the 127 normal-appearing d iscs were 14 442 +/- 4200 mm(3) and 17 476 +/- 2885 mm(3) respectively. The average volume of the HNP represented 3.5 % of the parent disc. An average HNP caused a decrease in intervertebral space height of 0.35 mm (0.56 pixe ls). Therefore, the loss of the volume of the HNP does not cause a signific ant decrease in the intervertebral space height. The average calculated dec rease in the disc height is less than that reported in normal diurnal varia tion.