Can Variations in Intervertebral Disc Height Affect the Mechanical Function of the Disc?

Study Design The finite element method was used to investigate the effect of variations in disc height on the mechanical behavior of the intervertebral disc. Objectives The effect of disc height on the mechanical behavior of a human lumbar spine segment in terms of axial displacement, intradiscal pressure, posterolateral disc bulge, tensile stress in the peripheral anulus fibers, and longitudinal stress distribution at the endplate-vertebra interface was evaluated. Summary of Background Data Disc height varies with individuals, disc level, abnormal conditions, and clinical management. Methods A three-dimensional finite element model of L2-L3 disc body unit was developed. Parametric studies were undertaken by studying discs of three different heights: 8 mm, 10 mm, and 12 mm, whereas disc cross-sectional area, finite element mesh density, and all other parameters were kept constant. The model accounted for geometric nonlinearity but assumed that the material properties were linear. Results Variations in disc height had a significant influence on the axial displacement, the posterolateral disc bulge, and the tensile stress in the peripheral anulus fibers, but the effect on the intradiscal pressure and the longitudinal stress distribution at the endplate-vertebra interface was minimal. Conclusions Variations in disc height may compromise the general conclusions reached from experimental work and analytic studies, in which geometric parameters (especially disc height and disc cross-sectional area) are not taken into consideration.