Pure shear properties of lumbar spinal joints and the effect of tissue sectioning on load sharing.

Abstract

STUDY DESIGN: An in vitro biomechanical study on lumbar intervertebral joints. OBJECTIVES: To examine the mechanical properties of lumbar motion segments under pure shear loading and establish whether a simple model for functional differentiation between the anterior column and the posterior elements is applicable. SUMMARY OF BACKGROUND DATA: Anteroposterior shear has been implicated as a major factor in spinal instability. There is a substantial amount of data on shear motion as a coupled part of flexion-extension; data on the pure shear properties of intervertebral joints is limited. METHODS: Eighteen human cadaver lumbar motion segments were subject to nondestructive testing under pure shear loads (anterior shear and posterior shear). An MTS standard testing machine was used to record the load-deformation characteristics of specimens subject to deformation at a constant rate to a maximum shear load of approximately 250 N. Tissue sectioning was then performed with the specimen mounted in the testing machine. Eight specimens were sectioned through the intervertebral disc, including the anterior and posterior longitudinal ligaments, and 8 specimens were sectioned through the pedicles to remove the posterior elements. The same deformation pattern applied to the intact specimen was then reapplied to the sectioned specimen, and the load-deformation characteristics following sectioning were evaluated. RESULTS: The shear stiffness of the intact segments were found to be higher in anterior shear (mean group A = 583.8, B = 607 N/mm) than in posterior shear (mean group A = 469, B = 438.4 N/mm). Section of the anterior column and adjacent longitudinal ligaments resulted in a mean stiffness decreased by 22.8% of the intact value under anterior shear and 23.9% under posterior shear. Much larger change in shear stiffness was seen, and the mean sectioned stiffness dropped by 77.7% in anterior shear and 79% in posterior shear after removal of the posterior elements. After the anterior column was sectioned, 12% and 18% increases in the deformation for anterior and posterior directions were seen, whereas a distinct increase in the deformations was found after posterior elements sectioned. CONCLUSIONS: The posterior elements of the lumbar spine are more efficient in resisting anterior and posterior shear loads. However, the anterior column will exhibit similar load-displacement characteristics if subject to greater deformations. The sum of the normalized mean shear loads of the anterior column and posterior elements sustained at maximum intact deformation is significantly different from the shear load sustained by the intact spine at the same deformation. A simple concept of load sharing between the anterior column and the posterior elements may not be valid.