Biomechanical Modeling of Posterior Instrumentation of the Scoliotic Spine

Abstract

Scoliosis is a three-dimensional deformation of the spine that can be treated by vertebral fusion using surgical instrumentation. However, the optimal configuration of instrumentation remains controversial. Simulating the surgical maneuvers with personalized biomechanical models may provide an analytical tool to determine instrumentation configuration during the pre-operative planning. Finite element models used in surgical simulations display convergence difficulties as a result of discontinuities and stiffness differences between elements. A kinetic model using flexible mechanisms has been developed to address this problem, and this study presents its use in the simulation of Cotrel -Dubousset Horizon surgical maneuvers. The model of the spine is composed of rigid bodies corresponding to the thoracic and lumbar vertebrae, and flexible elements representing the intervertebral structures. The model was personalized to the geometry of three scoliotic patients (with a thoracic Cobb angle of 458, 498 and 398). Binary joints and kinematic constraints were used to represent the rod-implant-vertebra joints. The correction procedure was simulated using three steps: (1) Translation of hooks and screws on the first rod; (2) 908 rod rotation; (3) Hooks and screws look-up on the rod. After the simulation, slight differences of 0 -68 were found for the thoracic spine scoliosis and the kyphosis, and of 1-88 for the axial rotation of the apical vertebra and for the orientation of the plane of maximum deformity, compared to the real post-operative shape of the patient. Reaction loads at the vertebra-implant link were mostly below 1000 N, while reaction loads at the boundary conditions (representing the overall action of the surgeon) were in the range 7 -470 N and maximum torque applied to the rod was 1.8 Nm. This kinetic modeling approach using flexible mechanisms provided a realistic representation of the surgical maneuvers. It may offer a tool to predict spinal geometry correction and assist in the preoperative planning of surgical instrumentation of the scoliotic spine.

Flexible non-fusion scoliosis correction systems reduce intervertebral rotation less than rigid implants and allow growth of the spine: a finite element analysis of different features of orthobiom™

“…Studying the mechanical effect of a scoliosis correction system is very difficult since there is great variety in the shape and stiffness of scoliotic spines, and it is very difficult to obtain cadaveric scoliotic spine specimens for experimental studies. Finite element analyses have been performed to study scoliosis correction by orthotic [11] and by surgical treatment [2,3,6,7,12]. Mostly the CotrelDubousset instrumentation procedure was simulated with numerical analyses.…”

“…In this study we hypothesized that (1) intervertebral rotation is more increased by mobile connectors than by polyaxial screws and that (2) placing pedicle screws at every second vertebrae instead of every level additionally increases intervertebral rotations. Further aims of this study were to determine the effect of the non-fusion orthobiom TM implant system on intervertebral rotations, and to predict the amount of translation of the mobile connectors as they glide along the rods.…”

Use of finite element analysis of a Lenke type 5 adolescent idiopathic scoliosis case to assess possible surgical outcomes

“…However, in the orthopedic stress test, the stress on most regions around the screw holes with solution A1 exceeded the ultimate strength of bone (120 Mpa) (Figure 6), resulting in a higher risk of loosening for the screws due to spinal fractures, thereby leading to fixation failure. Factors contributing to this were the overconcentration of stress resulting from the smaller number of fixed points in anterior short segment fusion and the stronger force involved, and that the scoliosis in this particular Lenke 5 AIS case was more rigid (flexibility was 36.7% on the concave side of the bending imaging, i.e., the angle formed by the connecting line of the L4 vertebral body and the spina iliaca was approximately 14 ), which contrasts with the guiding principles for short segment fusion (where general requirements for flexibility are >50%). Solution B2 lost too many lumbar movement segments.…”

“…For Lenke 5 AIS, during the posterior derotation correction, the peak-value load was only generated from the convex-side orthopedic screws and rods, while the concave-side orthopedic rod represented only a secondary stiffness with a minor role in the correction [14][15]. During the simulation of derotation, the concave-side orthopedic rod can exert an effect on the interface stress between the screws and bones, so the model of the B1/B2 correction complex only included the convex side.…”

Development of a Computer Simulation Tool for Application in Adolescent Spinal Deformity Surgery

“…Post-operative complications (such as screw pullout) or suboptimal correction can occur due to inappropriate choice of surgical levels or the application of excessive corrective force during the procedure. Biomechanical computer models of the spine have the potential to help optimise surgery outcomes and reduce complications, and patient-specific finite element (FE) models have been utilized previously to investigate the biomechanics of AIS surgery [1,2]. The current study aims to develop more anatomically detailed FE models of scoliosis patients, for subject-specific prediction of the loading and deformation of individual spinal structures (eg ligaments and implants) during surgery.…”