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

Biomechanics Laboratory, Orthopaedic Hospital, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12203 Berlin, Germany.
European Spine Journal (Impact Factor: 2.47). 09/2007; 16(8):1223-31. DOI: 10.1007/s00586-006-0292-8
Source: PubMed

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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Study Design. An in vitro biomechanical study.Objectives. Perform in vitro biomechanical testing on a lumbar spine using a 6 degree of freedom machine. To compare the range of motion, intradiscal pressure, and facet force of different three-level dynamic stabilization constructs to traditional rigid constructs. To determine the effect of decreasing the stiffness of the dynamic construct on the various parameters.Summary of Background Data. Dynamic stabilization systems are a surgical option that may minimize the development of adjacent segment disease.Methods. Seven T12-S1 specimens were tested at ±7.5 Nm in flexion-extension, lateral bending, and axial rotation. The testing sequence was 1) intact, 2) intact with facet sensors (IFS), 3) L3-S1 rigid (3R), 4) L3-L4 dynamic and L4-S1 rigid (1D-2R A), 5) L3-L5 dynamic and L5-S1 rigid (2D-1R A), and 6) L3-S1 dynamic (3D A). Constructs 1D-2R A, 2D-1R A, and 3D A were tested again with the specialized designs of B and C of decreased stiffness. Range of motion, intradiscal pressure, and facet force were measured.Results. In all loading modes there was a trend of increasing motion with decreased stiffness. Significant differences were seen with more dynamic stabilization levels but no significance was seen with only decreasing the stiffness. 3R Facet force at the caudal instrumented level significantly decreased compared to intact and dynamic stabilization constructs during axial rotation.Conclusions. Biomechanical testing resulted in a trend of increased ROM across instrumented levels as the stiffness was decreased. Dynamic stabilization increase the ROM across instrumented levels compared to rigid rods. These results suggest that decreasing the stiffness of the construct may lessen the probability of adjacent level disease. Although the specialized devices are not commercially available, clinical data would be necessary for a clearer understanding of adjacent level effects and to confirm the in vitro biomechanical findings.
    Spine 08/2013; · 2.16 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Pedicle screw-based dynamic constructs either benefit from a dynamic (flexible) interconnecting rod or a dynamic (hinged) screw. Both types of systems have been reported in the literature. However, reports where the dynamic system is composed of two dynamic components, i.e. a dynamic (hinged) screw and a dynamic rod, are sparse. In this study, the biomechanical characteristics of a novel pedicle screw-based dynamic stabilisation system were investigated and compared with equivalent rigid and semi-rigid systems using in vitro testing and finite element modelling analysis. All stabilisation systems restored stability after decompression. A significant decrease in the range of motion was observed for the rigid system in all loadings. In the semi-rigid construct the range of motion was significantly less than the intact in extension, lateral bending and axial rotation loadings. There were no significant differences in motion between the intact spine and the spine treated with the dynamic system (P>0.05). The peak stress in screws was decreased when the stabilisation construct was equipped with dynamic rod and/or dynamic screws.
    Computer Methods in Biomechanics and Biomedical Engineering 04/2014; · 1.39 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Surgeons often use spinal fixators to manage spinal instability. Dynesys (DY) is a type of dynamic fixator that is designed to restore spinal stability and to provide flexibility. The aim of this study was to design a new spinal fixator using topology optimization [the topology design (TD) system]. Here, we constructed finite element (FE) models of degenerative disc disease, DY, and the TD system. A hybrid-controlled analysis was applied to each of the three FE models. The rod structure of the topology optimization was modelled at a 39 % reduced volume compared with the rigid rod. The TD system was similar to the DY system in terms of stiffness. In contrast, the TD system reduced the cranial adjacent disc stress and facet contact force at the adjacent level. The TD system also reduced pedicle screw stresses in flexion, extension, and lateral bending.
    Medical & Biological Engineering 04/2014; · 1.76 Impact Factor

Full-text (2 Sources)

Available from
May 16, 2014