Biomechamical Analysis of Lumbosacral Reconstruction Techniques for Spondylolisthesis

· (a - Medtronic Sofamor Danek) *Orthopaedic Biomechanics Laboratory, Union Memorial Hospital, Baltimore, MD, USA and † Washington University, St. Louis, MO, USA

INTRODUCTION: The current study was undertaken to evaluate the comparative biomechanical properties of four different lumbosacral fixation methods used in the surgical management of spondylolisthesis.

METHODS: Twenty-four fresh frozen porcine lumbosacral spines were utilized in this investigation and divided into four groups based on reconstruction technique: 1) L7-S1 pedicle screws alone (n=6), 2) L7-S1 pedicle screws and interbody cage (titanium mesh) (n=6), 3) L7-S1 pedicle screws and iliac screws (n=6), 4) L7-S1 pedicle / iliac screws and interbody cage (n=6). Following intact analysis, the specimen was destabilized by resecting the posterior spinal column at L7-S1 to produce a spondylolisthesis model. Non-destructive testing quantified peak lumbosacral range of motion (ROM) under four loading parameters. Each specimen underwent a total of 1500 cycles followed by repeating the static analysis. Data were normalized to the intact spine and analyzed.

RESULTS: 1) Axial compression: no differences in lumbosacral flexibility among the four treatment groups (p › 0.05). 2) Axial rotation: iliac screws, with or without cages, improved stability at the lumbosacral joint over that observed with pedicle screws alone (p ‹ 0.05). On the other hand, interbody cage reconstructions did not reduce motion. 3) flexion extension: iliac screws with interbody cages offered the optimal biomechanical environment for the lumbosacral junction (p ‹ 0.05). 4) Lateral bending: the iliac screw constructs resulted in significantly less lumbosacral motion compared to both pedicle screw constructs (with or without cages). With destructive flexural testing, the L7-S1 pedicle screw constructs failed by S1 pullout, while the L7-S1 iliac screw constructs failed by dislocation of the proximal adjacent facets.

DISCUSSION: Iliac screw constructs improved stability levels in 3 out of 4 loading modalities - axial rotation, flexion extension, and lateral bending compared to pedicle screw constructs. The addition of interbody cages increased stability for the iliac screw treatments under flexion-extension loading and pedicle screw constructs under axial rotation. Both iliac screws and cages enhance pedicle screw stability at the lumbosacral junction, but the iliac screws are more valuable.