The Effect of Cement Augmentation and Extension of Posterior Instrumentation on Stabilization and Adjacent Level Effects in the Elderly Spine

Abstract

An in vitro cadaveric study comparing different implant fixation techniques using a repeated measures design. To compare the effects of cement augmentation of pedicle screws and extension of posterior fixation on (i) 3-dimensional stabilization, and (ii) adjacent level effects in the aging spine. Device loosening and adjacent level effects are concerns in implant fixation in the elderly spine. Extension of posterior fixation and cement augmentation of pedicle screws have not been previously compared with respect to stabilization and adjacent level effects. Twelve T9 to L3 cadaveric specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) with applied pure moments of +/-5 Nm. A T11 corpectomy was reconstructed with a vertebral body replacement device and T10 to T12 posterior instrumentation. Further stabilization was provided by posterior rod extension to L1 (flexible or rigid rods) and/or cement augmentation of T12 and L1 screws. The effects of cement augmentation and posterior rod extension on intersegmental motion were compared using the hybrid flexibility-stiffness protocol. Two-way repeated measures ANOVA and SNK post hoc tests (99% significance level) were used. Range of motion at the corpectomy T10 to T12 levels significantly decreased after cement augmentation (AR 43%, LB 71%, FE 68%), and posterior rod extension (rigid rods: AR 26%, LB 64%, FE 57%) (flexible rods: AR 16%, LB 53%, FE 39%). Posterior rod extension significantly reduced range of motion at the rod extension level. Motion at the distal noninstrumented L1 to L2 level was increased significantly by posterior rod extension and cement augmentation. There were however, smaller magnitudes of increase in motion across L1 to L2 level with flexible rod and cement augmentation (AR 12%, LB 45%, FE 31%). Cement augmentation of pedicle screws resulted in the most stable vertebral reconstruction, whereas flexible rod extension minimized changes in range of motion at both adjacent rod extension and distal noninstrumented levels.