Biomechanical Evaluation of Flexible Reconstruction with Non-Fusion Instrumentation of the Lumbar Spine

M. Mesbah, M. Bendoukha, A. Berkaoui
University of Mostaganem,
Algeria

Keywords: flexible reconstruction, finite element, hybrid surgery, topping off fusion, adjacent segment disease

Summary:

The purpose of this study is to examine the behavior of flexible reconstruction of the lumbar spine using transitional and semi-rigid stabilization devices and their influence on the mobility of adjacent segments. A 3D finite element (FE) model the Lumbar spine L2–S1, validated based on in vitro data was used. seven different models were analyzed and compared according to conventional and hybrid surgery using polymeric PEEK, composite OstaPek and Dynesys rods. The instrumentation was composed of PEEK cage inserted through the L4/L5 intervertebral disc window, two fixed screws at L4 and two mobile screws at L3 and L5 connected by 5.5 mm rods. The topping off fusion assembly was simulated using pedicle screw-based PEEK, OstaPek Rods or hybrid Dynesys DTO (Dynesys Transition Optima) rods. Pure moments of ±10 Nm in flexion-extension, axial rotation, and lateral bending were applied to the intact and instrumented models while under a follower load of 100 N. The residual Range of motion (ROM), intradiscal pressure, implant forces were calculated for all modalities, stress distribution at the pedicles and the interpedicular displacement with respect to changes of bone/screw quality were investigated. Referring to the intact spinal condition, flexion/extension results for the adjacent level L3/4 were normalized. All implants had the same stabilizing effect in the fused segment L4/5 and reduced the angular motion by approximately 80% in comparison with the destabilized backbone. The results indicate that the unfused segment is subjected to additional stresses and acquires hypermotion over time, about 10 and 16% respectively in L2/3 and L3/4 for PEEK or Ostapek fusion 43 and 55% respectively in L2/3 and L3/4 in extension. hybrid instrumentation reduced the ROM following discectomy and gradual facetectomy, 57% of ROM was controlled for DynaPEEK and DynostaPek in L3/4 extension versus 70% that of an intact segment. The hybrid systems are highly stressed, while the rest of the structure’s motion is preserved. Loads in the semi-rigid fixator PEEK and OstaPek are all higher than in the dynamic, the forces are calculated for flexion-Extension (90~132 N), however, the dynamic constructs had a minor effect on axial implant forces (20~58 N). The topping off semi-rigid fusion system with a stiffness equivalent to that of an intact spinal segment is sufficient to stabilize the spine while preserving 60% of its intact ROM. The current FE investigation suggested that substitution of the superior level fusion with the flexible polymeric or composite devices in multi-level fusion procedures may be able to offer similar biomechanical outcomes and stability while reducing the likelihood of ASD and instrument failures such as screw toggling and loosening. As newly innovated techniques, long term prospective studies must be designed to achieve their effectiveness. However, the stiffness values utilized in the device were determined to be important design parameters for stabilization devices manufacturing.