Biomechanics of stand-alone cages and cages in combination with posterior fixation: a literature review

Abstract

IntroductionInterbody cages are porous implants which are placed between two vertebral bodies to facilitate an intervertebral fusion. The concept for these cages was described first by Bagby [3] along with initial animal evaluation [10]. Clinical trials of these cages began in 1991 for degenerative problems of the lumbar spine. Early clinical results and the broader results of multicentre clinical trials have been encouraging [21,30]. However, some investigators have had less success [19,25]. Furthermore, recent studies have found that fusion assessment with these cages is virtually impossible with X-rays or CT scans [4,15], thereby putting into doubt some of the definitive fusion results from the clinical studies. All in all, the effectiveness of stand-alone interbody cages has been questioned, with some investigators claiming that supplementary posterior fixation is required to produce better long-term clinical results [6,25].Abstract Interbody cages in the lumbar spine have met with mixed success in clinical studies. This has led many investigators to supplement cages with posterior instrumentation. The objective of this literature review is to address the mechanics of interbody cage fixation in the lumbar spine with respect to three-dimensional stabilization and the strength of the cage-vertebra interface. The effect of supplementary posterior fixation is reviewed. Only three-dimensional stabilization evaluations in human cadaveric models are included. These studies involve the application of different loads to the spine and the measurement of vertebral motion in flexion-extension, axial rotation, and lateral bending. There are no published studies which detected any differences between different cage designs. However, it does seem that cages inserted from an anterior direction provide better stabilization to the spine than those inserted from a posterior direction. In general, anterior cages stabilize better than posterior cages in axial rotation and lateral bending. Cages from both directions stabilized well in flexion, but not in extension. Supplementary posterior fixation with pedicle or translaminar screws substantially improves the stabilization in all directions. The strength of the cage-vertebra interface from studies using human cadaveric specimens is also reviewed. The axial compressive strength of this interface is highly dependent upon vertebral body bone density. Other factors such as preservation of the subchondral bony end-plate and cage design are clearly less important in the compressive strength. Supplementary posterior instrumentation does not enhance substantially the interface strength in axial compression.

Lumbar lateral interbody cage with plate augmentation: in vitro biomechanical analysis

“…Posterior cage insertion reduces the mobility in flexion and in lateral bending, but not in extension or axial rotation [9]. Anterior cage insertion has a better stabilizing effect in axial rotation and in lateral bending than posterior insertion, but neither provides significant stabilization in extension [9,12,14,20]. Generally, the surgical approach destroys either the anterior or the posterior tension band of the spine [1], which causes destabilization, particularly in extension.…”

“…Theoretically, interbody fusion cages provide a primary stabilization to the spine, by distracting the annulus fibers to create a peripheral tension band [3,11,14,15,18]. However, this tension band effect decreases gradually along with the interbody fusion, which is a concomitant multi-factorial process: the bone creeping substitution [17], the cage settling into the vertebral endplates due to the repetitive load [6], the loosening of the tension of the annulus fibers due to the viscoelastic property [6], the bone density change [5,9], etc.…”

“…Generally, bony fusion may occur if stability is maintained [11,12,17], whereas loss of stability before the fusion will probably lengthen the fusion process or result in non-union. Many authors have suggested the utilization of supplementary fixations to reinforce the stability, especially in extension [9,14,15]. So far, no study has been performed to investigate the stabilizing effect of laterally inserted cages combined with a supplementary fixation.…”

Biomechanical comparison of two different concepts for stand alone anterior lumbar interbody fusion

“…Previous biomechanical studies have demonstrated that stand alone implantation of interbody cage implants shows a good stabilization in flexion and lateral bending, but stabilization in extension and rotation is poor [14].…”

“…The plate is integrated into the anterior aspect of the cage but remains biomechanically separate from the cage. This plate should provide the stability lost by resection of the anterior longitudinal ligament in extension, which is the main biomechanical problem of ALIF procedures in addition to rotational instability [14]. The plate and screw configuration are also designed to provide stability to the device during rotation and lateral flexion movements.…”