In vitro evaluation of a lateral expandable cage and its comparison with a static device for lumbar interbody fusion: a biomechanical investigation

Abstract

Object Through in vitro biomechanical testing, the authors compared the performance of a vertically expandable lateral lumbar interbody cage (EC) under two different torque-controlled expansions (1.5 and 3.0 Nm) and with respect to an equivalent lateral lumbar static cage (SC) with and without pedicle screw fixation. Methods Eleven cadaveric human L2–3 segments were evaluated under the following conditions: 1) intact; 2) discectomy; 3) EC under 1.50 Nm of torque expansion (EC-1.5Nm); 4) EC under 3.00 Nm of torque expansion (EC-3.0Nm); 5) SC; and 6) SC with a bilateral pedicle screw system (SC+BPSS). Load-displacement behavior was evaluated for each condition using a combination of 100 N of axial preload and 7.5 Nm of torque in flexion and extension (FE), lateral bending (LB), and axial rotation (AR). Range of motion (ROM), neutral zone stiffness (NZS), and elastic zone stiffness (EZS) were statistically compared among conditions using post hoc Wilcoxon signed-rank comparisons after Friedman tests, with a significance level of 0.05. Additionally, any cage height difference between interbody devices was evaluated. When radiographic subsidence was observed, the specimen's data were not considered for the analysis. Results The final cage height in the EC-1.5Nm condition (12.1 ± 0.9 mm) was smaller (p < 0.001) than that in the EC-3.0Nm (13.9 ± 1.1 mm) and SC (13.4 ± 0.8 mm) conditions. All instrumentation reduced (p < 0.01) ROM with respect to the injury and increased (p ≤ 0.01) NZS in flexion, extension, and LB as well as EZS in flexion, LB, and AR. When comparing the torque expansions, the EC-3.0Nm condition had smaller (p < 0.01) FE and AR ROM and greater (p ≤ 0.04) flexion NZS, extension EZS, and AR EZS. The SC condition performed equivalently (p ≥ 0.10) to both EC conditions in terms of ROM, NZS, and EZS, except for EZS in AR, in which a marginal (p = 0.05) difference was observed with respect to the EC-3.0Nm condition. The SC+BPSS was the most rigid construct in terms of ROM and stiffness, except for 1) LB ROM, in which it was comparable (p = 0.08) with that of the EC-1.5Nm condition; 2) AR NZS, in which it was comparable (p > 0.66, Friedman test) with that of all other constructs; and 3) AR EZS, in which it was comparable with that of the EC-1.5Nm (p = 0.56) and SC (p = 0.08) conditions. Conclusions A 3.0-Nm torque expansion of a lateral interbody cage provides greater immediate stability in FE and AR than a 1.5-Nm torque expansion. Moreover, the expandable device provides stability comparable with that of an equivalent (in size, shape, and bone-interface material) SC. Specifically, the SC+BPSS construct was the most stable in FE motion. Even though an EC may seem a better option given the minimal tissue disruption during its implantation, there may be a greater chance of endplate collapse by over-distracting the disc space because of the minimal haptic feedback from the expansion.

An In Vitro Biomechanical Evaluation of a Lateral Lumbar Interbody Fusion Device With Integrated Lateral Modular Plate Fixation

“…Adjunctive unilateral PSF (UPSF), facet screw fixation (FSF), facet wedging, integrated lateral screws, lateral plating, and ISPF have all been studied. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The latter 2 fixation modalities, lateral plating and ISPF, were of particular interest in this study given their synergistic capabilities. While Fogel et al 8,9 and Reis et al 18 have previously assessed LLIF þ ISPF in vitro; plating was traditional independent fixation only.…”

“…This is consistent with the literature, in which diminished AR rigidity is common with a stand-alone LCage. [3][4][5][6][7][8][10][11][12][13][14][15][16][17][18] However, given the modest amount of axial angular motion inherent to the lumbar spine, ROM reduction in AR is typically of secondary concern. While the authors acknowledge the validity of such assertions around axial plane rigidity, consideration must still be given to the potential for cage subsidence and the amount of slip reduction necessary when performing stand-alone LLIF.…”

“…Greater reductions in AR ROM have been captured in the literature with LLIF þ BPSF (single-level ROM reduction: 15% to 72.1%); although, such values are still comparable to those in this study. [3][4][5][6][7][11][12][13] Not explored in this study, facet fixation has been shown to be advantageous when seeking rigidity in the axial plane, achieving reductions of 26.7% to 81.9%. 3,6,12,13 These trends can be largely attributed to the robust locking of the middle column with facet fixation.…”

“…Given such inherent structural benefits, it has been proposed that extensive and/or invasive posterior fixation could be unnecessary with LLIF. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Accordingly, increased consideration has been given to alternative, less invasive means to achieving stability with the lateral access technique. Two techniques of particular interest have been lateral plating and interspinous process fixation (ISPF).…”

Biomechanical analysis of an expandable lateral cage and a static transforaminal lumbar interbody fusion cage with posterior instrumentation in an in vitro spondylolisthesis model

“…16 It is advantageous to use an expandable cage because it provides increasing stability when compared with that of a static cage. 12 A stand-alone lateral procedure is advantageous because there is no need for separate surgical sites or for repositioning the patient, which increase operative times and cost expenditures. However, failure of a stand-alone lateral construct can lead to continued pain and disability and to reoperation.…”