RESUMEN
BACKGROUND: Histomorphological change of endplate may affect the nntritional transmission of intervertebral disk, eventually leading to intervertebral disc degeneration. OBJECTIVE: To observe the effects of endplate concavity variation on mechanical response of lumbar motion segment. DESIGN, TIME AND SETTING: Repeated measurement design, biomechanical analysis of finite element models, performed at the Laboratory of Biomechanics, Department of Orthopedics, the Second Affiliated Hospital of Medical College of Zhejiang University between January 2005 and January 2007. MATERIALS: SOMATOM SENSATION 16 spiral CT machine (Siemens, German) and ANSYS (Inc. Pennsylvania, USA).METHODS: A three-dimensional nonlinear geometrical and mechanical accurate finite element model of lumbar L<,4-5> segment was developed. Parametric studies were undertaken by studying endplate of three different concave angles, whereas disc angle, the gap of facet joint, finite element mesh density, and all other parameters were kept constant. Biomechanical parameters of three kinds of finite element models were tested under 5 different loading conditions, including vertical compression, flexion, extension, and anterior and posterior shearing loading. MAIN OUTCOME MEASURES: Endplate-intervertebral disc interface strain, intervertebral disc stiffness, nucleus pulposus pressure, annular fiber stress, radial disc bulge, stress in the annulus ground substance, stresses in posterior structure and facet contact force. RESULTS: The decrease in the endplate concavity stimulated by an increasing endplate concave angle would result in decreased strains of endplate-intervertebral disc interface, increased disc stiffness and nucleus pulposus pressure, decreased annular fiber stress, radial disc bulge and stress in the annulus ground substance, and simultaneously produce decreased facet contact force and stresses in posterior structure. CONCLUSION: The decrease of endplate concavity enhances the protective effects of the disc on vertebral body breakage. Small endplate deformation results from decreased endplate concavity would contribute to the reduction of nutritional diffusion to the intervertebral disc.