Comparison between two different concepts of lumbar posterior osteosynthesis implants A finite-element analysis

Abstract

Summary: The present study is a numerical comparison using finite-element analysis (FEA) oftwo different concepts of spinal fixation devices when implanted. These implants are 1) the Easy ®, "rigid" Screw/Rod (0 6mm) system; z) the Twinflex ®, "dynamic" system (o 2 x 2.5 mm ELF). A parameterised 3D FEA model of an L3-sacrum segment, developed by Lavaste, Skalli & Robin, was used. Geometric and mechanical models of each implant were then constructed, before being inserted in the spinal segment model. Then, for model validation, these two L3-$2 instrumented segmental models were submitted to similar boundary conditions as used in a previous in vitro comparison of the same implants. Flexion loaddisplacement curves were then controlled using experimental results. Loads acting on screws and longitudinal elements were calculated and analysed for a better understanding of the intrinsic differences between both constructs. Load-displacement responses of both constructs were quite similar (L3 sagittal rotation at lo N.m = -1.5°), while loads in the implant were not. For example, the axial push-in forces at the $1 screws were equal to 3o N for the Twinflex®, and 15o N for the Easy® Screw/Rod system. The pull-out forces at the Sz screws were respectively looN and 20o N for the Twinflex and Screw/Rod Concept. At other levels, axial forces were all lower than 60 N, the Twinflex®, values being higher than the Easy ®ones. Bending moments along screws were respectively o.7 N.m and 1. 4 N.m at the L3 level for the Twinflex ® and the Easy ~ systems. At lower levels, values were all below o.6 N.m, again with a reversed proportion. Bending moments calculated along longitudinal elements were always lower than 0.3 Nm for the Twinflex®, and up to 2 N.m for the Easy® system. Axial forces in the Twinflex ® longitudinal elements were about 16o N, and about lOO N in the Easy ® rods. Although the numerical approach mainly provides tendencies, it clearly seems that reducing flexural stiffness of lumbar fixation induces more homogeneous load transmission along the construct, and greatly reduces axial push-in/pull-out forces at the SdSz levels, and all this without reducing the rigidity of the whole construct. Conversely, it has been shown that "rigid" longitudinal elements may concentrate stresses at the construct extremities, relieving loads at intermediate levels at the same time, which may be the sign of a stress-shielding-like phenomenon. These differences arise from a fundamental difference between both kinds of longitudinal elements in the way they transmit loads. The o 6 mm rods mainly oppose a bending reacfion torque to the applied flexion moment, whereas the Twinflex ® construct mainly balances the applied flexion torque by an anterior compression of the anterior column, combined with posterior traction on its longitudinal elements (ELF).

Biomechanical Evaluation of Pedicle Screw-Based Dynamic Stabilization Devices for the Lumbar Spine: A Systematic Review

“…The authors found that the dynamic system enabled more load to be transferred through the anterior column and the interbody graft, as compared with rigid instrumentation, without compromising stability. 33 These findings confirmed those reported by Templier et al 22 in 1998, suggesting that a dynamic device (Twinflex) could offer a more favorable environment for enhanced interbody fusion healing ( Figure 4 ). Reducing the flexural rigidity of lumbar fixation results in more homogeneous load transmission along the system without reducing the rigidity of the whole system.…”

“…Templier et al 22 evaluated the role of the longitudinal component in load transfer between the FSU and implant via finite element analysis (FEA) ( Table 4 ). Using a 3D geometric FE L3-sacrum model, the authors compared load transmission of rigid instrumentation versus the semirigid Twinflex system ( Figure 3 ) following the application of a flexion moment.…”

“… In flexion, predominant load transfers through the system depends on instrumentation stiffness: A dynamic system results in anterior compression and posterior traction while a rigid system results in axial pull-out forces at the ends of the construct. 22 …”

Pedicle-Screw-Based Dynamic Systems and Degenerative Lumbar Diseases: Biomechanical and Clinical Experiences of Dynamic Fusion with Isobar TTL

“…Some authors suggested that eliminating mechanical loads on an interbody bone graft may result in negative bone remodeling, pseudarthrosis, and osteoporosis [ 3 – 6 ]. This “stress shielding” phenomenon at the disk space level may result from the excessive stiffness of traditional rigid instrumentation.…”

“…In 1998, Templier et al [ 6 ] using a 3D geometric FE model of the lumbar spine postulated that the TWINFLEX semirigid device could offer a more favorable biomechanical environment for enhanced interbody fusion healing. They evaluated the role of the longitudinal component in load transfer between the FSU and implant and noted that by reducing the stiffness of lumbar fixation, there was more homogeneous load transmission throughout the FSU without significantly reducing the rigidity of the instrumented spinal segment.…”