Comparison of the effects of bilateral posterior dynamic and rigid fixation devices on the loads in the lumbar spine: a finite element analysis

Abstract

A bilateral dynamic stabilization device is assumed to alter favorable the movement and load transmission of a spinal segment without the intention of fusion of that segment. Little is known about the effect of a posterior dynamic fixation device on the mechanical behavior of the lumbar spine. Muscle forces were disregarded in the few biomechanical studies published. The aim of this study was to determine how the spinal loads are affected by a bilateral posterior dynamic implant compared to a rigid fixator which does not claim to maintain mobility. A paired monosegmental posterior dynamic implant was inserted at level L3/L4 in a validated finite element model of the lumbar spine. Both a healthy and a slightly degenerated disc were assumed at implant level. Distraction of the bridged segment was also simulated. For comparison, a monosegmental rigid fixation device as well as the effect of implant stiffness on intersegmental rotation were studied. The model was loaded with the upper body weight and muscle forces to simulate the four loading cases standing, 30 degrees flexion, 20 degrees extension, and 10 degrees axial rotation. Intersegmental rotations, intradiscal pressure and facet joint forces were calculated at implant level and at the adjacent level above the implant. Implant forces were also determined. Compared to an intact spine, a dynamic implant reduces intersegmental rotation at implant level, decreases intradiscal pressure in a healthy disc for extension and standing, and decreases facet joint forces at implant level. With a rigid implant, these effects are more pronounced. With a slightly degenerated disc intersegmental rotation at implant level is mildly increased for extension and axial rotation and intradiscal pressure is strongly reduced for extension. After distraction, intradiscal pressure values are markedly reduced only for the rigid implant. At the adjacent level L2/L3, a posterior implant has only a minor effect on intradiscal pressure. However, it increases facet joint forces at this level for axial rotation and extension. Posterior implants are mostly loaded in compression. Forces in the implant are generally higher in a rigid fixator than in a dynamic implant. Distraction strongly increases both axial and shear forces in the implant. A stiffness of the implant greater than 1,000 N/mm has only a minor effect on intersegmental rotation. The mechanical effects of a dynamic implant are similar to those of a rigid fixation device, except after distraction, when intradiscal pressure is considerably lower for rigid than for dynamic implants. Thus, the results of this study demonstrate that a dynamic implant does not necessarily reduce axial spinal loads compared to an un-instrumented spine.

Effect of the cord pretension of the Dynesys dynamic stabilisation system on the biomechanics of the lumbar spine: a finite element analysis

“…The values were always below the magnitude of the intact spine for the three cord pretension cases under extension; however, the FCF was 22.6% higher than the intact spine for the case of 300-N cord pretension under torsion. These results indicate that Dynesys can alleviate facet loading on the implanted level under extension, and they follow a similar trend to the results presented by Rohlmann [11]. Although FCF was noticeably increased at the adjacent levels, it remains much lower than the values of the fusion model (?169% under extension; ?28.9% under torsion) [20].…”

“…This FE analysis showed that the insertion of Dynesys reduced the ROM of the implanted level in terms of flexion and lateral bending, but less so in extension. The results are in good agreement with most previous biomechanical studies in which the Dynesys reduced the ROM below the magnitude of the intact spine for the implanted level under flexion and lateral bending [8,9,11,13]. For the adjacent levels, the present study revealed that the ROM increased by at least 22.4% in flexion and 15.6% in lateral bending; these trends conflict with the findings of some in vitro tests under the load control method [9,21].…”

“…No differences were found in motion and intradiscal pressure at the adjacent segments between the Dynesys and rigid fixation systems [9,10]. In 2007, Rohlmann et al [11] indicated that, other than after distraction, the mechanical effects of the Dynesys are similar to those of a rigid fixation system. Zander et al [12] evaluated the manner in which a Dynesys adjacent to a rigid fixation system affects the mechanical behaviour of the lumbar spine.…”

Molecular MR imaging for the evaluation of the effect of dynamic stabilization on lumbar intervertebral discs

“…Indeed, for this reason, dynamic stabilization could provide a reduction of the negative effect of fusion on the adjacent spinal segments and on the global functioning of the spine [6]. Up to now, promising clinical results have been described using this technique [7][8][9][10][11][12][13], but still not definitive. Also, the longterm effects of dynamic stabilization are still under discussion, with a specific focus on the remodeling of the disc cartilage matrix, both at the implanted and at the adjacent levels, caused by the spinal biomechanics which is modified by the implanted device.…”

Kinematic evaluation of one- and two-level Maverick lumbar total disc replacement caudal to a long thoracolumbar spinal fusion

“…Stiffness or hybrid protocols, rather than flexibility protocols, must be used to study the effects of surgical interventions on the biomechanics of the adjacent (non-operated) spinal levels [28,40]. With these methods, segmental kinematics are evaluated for the intact nonfusion, fusion, and TDR situations at an identical spinal posture [14,26,27,29,36,40,41]. However, some of the studies that have been performed to study TDR's adjacent to fusions [10,25,34] did not apply hybrid testing methods [10,34], or applied bending using an offset shear force that resulted in combined bending and shear loading [25].…”