Biomechanical modelling of segmental instrumentation for surgical correction of 3D spinal deformities using Euler-Bernoulli thin-beam elastic deformation equations

A simplified computer-modelling technique intended to analyse 3D spinal deformity correction with segmental instrumentation is presented. The spine was modelled as a thin beam-composed structure linked by implants to two deformable rods. The Landau vector representation of Euler-Bernoulli beam elastic deformation equations was used to formulate the simulation approach. All types of essential deformation (bending, torsion, tension, compression) were considered. An iterative numerical method was proposed to obtain an appropriate load, able to deform the spine axial curve to the desired post-operative shape. A simulation based on the spine of a real scoliotic patient (thoracic and lumbar Cobb angles: 39° and 8°), corrected using surgical instrumentation intervention, is presented. Force loads within the range of 20–350N were able to deform the pre-operational spine axial curve to the post-operational one with a root mean square approximation error of 3.7 mm. Similarly good corrections were obtained using different force patterns. This highlights the uncertainty of which corresponding surgical instrumentation to use. Such uncertainty is related to the ‘ill-posed problems’ property of mechanical systems.     

Simulation of an anterior spine instrumentation in adolescent idiopathic scoliosis using a flexible multi-body model

Anterior spinal instrumentation is an alternative option to posterior instrumentation for surgical treatment of adolescent idiopathic scoliosis (AIS). However, optimal instrumentation configuration and strategies are not yet clearly defined. A biomechanical kinematic model using flexible mechanism was developed to study instrumentation strategies. Preoperative 3D reconstruction of scoliotic patient’s spine was used to define the patient-specific geometry of the model. Mechanical properties were adjusted to consider the discectomy and surgical manoeuvres were reproduced. Anterior spine surgeries of ten patients were simulated and results were compared to immediate post-operative data and showed differences of <5° for the Cobb angles. The validated model was used to find optimal instrumentation configurations for one patient prior to surgery. Six strategies were tested out of which the optimal one was identified while two were not recommended for surgery since screw forces exceeded published pullout forces. This study demonstrates the possibility to simulate anterior spine instrumentations.     

Electromyogram and kinematic analysis of lateral bending in idiopathic scoliosis patients

In adolescent idiopathic scoliosis (AIS), surgical planning currently relies on spinal flexibility evaluation using lateral bending radiographs. The aim was to evaluate the feasibility of non-invasive dynamic analysis of trunk kinematics and muscle activity in patients with AIS before surgical correction. During various lateral trunk bending tasks, erector spinae (18 sites) and abdominal (four sites) muscle activity was sampled using surface electrodes in ten AIS patients and in ten controls. Simultaneously, the spatial displacements of infrared emitting diodes located on the trunk were sampled. Parameters considered were the heterolateral-to-homolateral root-mean-square EMG ratios R at each site and total lateral bending and thoracic and lumbar curvature angle courses. Main alterations concerned apical muscle activity during left bending tasks. ANOVA results showed a significant effect of side (p=2.1×10⁻⁹), EMG recording site (p=1.9×10⁻¹⁶), pathology (p=3.9×10⁻¹⁶) and task (p=2.2×10⁻¹¹) on R ratios. The R ratio at T10 and L1 for a simple lateral bending task during left bending averaged 4.8 (SD 4.3) and 3.0 (SD 3.1) in AIS patients, and 2.3 (SD 2.8) and 1.3 (SD 0.4) in controls (p=6.4×10⁻⁴ and 2.5×10⁻³, LSD post hoc). This preliminary study allowed the development of a functional, noninvasive, non-irradiating dynamic tool for pre-operative evaluation in AIS.     

Computer simulation for the optimization of patient positioning in spinal deformity instrumentation surgery

Studies have shown that scoliosis curves correct when patients are positioned on the operating table prior to instrumentation. However, biomechanical aspects of positioning have not been widely studied. The objective of this study was to simulate patient positioning during instrumentation surgery and test various adjustment parameters of the trunk and recommend optimal patient positioning prior to, and during spine surgery based on the results of finite element simulations. A scoliotic patient was simulated using a finite element model and six different positioning parameters were modified while ten geometric measures were recorded. Statistical analysis determined which model parameter had a significant effect on the geometric measures. Geometric measures were individually and simultaneously optimized, while corresponding model parameters were documented. Every model parameter had a significant effect on at least five of the geometric measures. When optimizing a single measure, others would often deteriorate. Simultaneous optimization resulted in improved overall correction of the patient’s geometry by 75% however ideal correction was not possible for every measure. Finite element simulations of various positioning parameters enabled the optimization of ten geometric measures. Positioning is an important surgical step that should be exploited to achieve maximum correction.     

Computer simulation for the optimization of instrumentation strategies in adolescent idiopathic scoliosis

Recent studies reveal a large variability of instrumentation strategies in adolescent idiopathic scoliosis (AIS). Determination of the optimal configuration remains controversial. This study aims to develop a method to define the optimal surgical instrumentation strategy using a computer model implemented in a spine surgery simulator (S3). A total of 702 different strategies were simulated on a scoliotic patient using S3. Each configuration was assessed using objective functions that represented different correction objectives. Twelve geometric parameters were used in the three anatomic planes and mobility, and their relative weights were defined by a spine surgeon according to his objectives for correction of scoliosis. Six instrumentation parameters were manipulated in a uniform experimental design framework. An interpolation technique was used to build an approximation model from the simulation results and to locate instrumentation parameters minimizing the objective function. Small or no differences in the correction between the simulated optimal strategy and the real postoperative results of the instrumented segments were observed in the three planes. But the same overall correction was obtained by using fewer implants (only screws) and less instrumented levels. This study demonstrates the potential and feasibility of using a spine surgery simulator to optimize the planning of surgical instrumentation in AIS.     

Finding line of action of the force exerted on erect spine based on lateral bending test in personalization of scoliotic spine models

In multi-body models of scoliotic spine, personalization of mechanical properties of joints significantly improves reconstruction of the spine shape. In personalization methods based on lateral bending test, simulation of bending positions is an essential step. To simulate, a force is exerted on the spine model in the erect position. The line of action of the force affects the moment of the force about the joints and thus, if not correctly identified, causes over/underestimation of mechanical properties. Therefore, we aimed to identify the line of action, which has got little attention in previous studies. An in-depth analysis was performed on the scoliotic spine movement from the erect to four spine positions in the frontal plane by using pre-operative X-rays of 18 adolescent idiopathic scoliosis (AIS) patients. To study the movement, the spine curvature was considered as a 2D chain of micro-scale motion segments (MMSs) comprising rigid links and 1-degree-of-freedom (DOF) rotary joints. It was found that two MMSs representing the inflection points of the erect spine had almost no rotation (0.0028° ± 0.0021°) in the movement. The small rotation can be justified by weak moment of the force about these MMSs due to very small moment arm. Therefore, in the frontal plane, the line of action of the force to simulate the left/right bending position was defined as the line that passes through these MMSs in the left/right bending position. Through personalization of a 3D spine model for our patients, we demonstrated that our line of action could result in good estimates of the spine shape in the bending positions and other positions not included in the personalization, supporting our proposed line of action.     

后路椎弓根螺钉系统治疗特发性腰椎侧凸的有限元分析

目的通过有限元计算与刚体动力学相结合的方法,模拟后路椎弓根螺钉系统治疗脊柱侧凸的矫形过程,研究矫形过程中的生物力学特性,探讨不同矫形策略对临床结果的影响,为脊柱侧凸的手术规划提供依据。方法通过病体腰椎CT切片进行三维几何重构,利用ANSYS有限元软件建立了右凸40o腰椎L1-L5和椎弓根螺钉器械的三维有限元模型,联合ADAMS刚体动力学软件模拟了矫形手术中的反旋转与回弹,得到了矫形全过程中植入物所受载荷以及脊椎的应力应变场。结果不同矫形手术过程中,植入器械承受的最大反力范围约为1961099N,椎骨的极少数单元应力超过强度极限120MPa。结论后路椎弓根螺钉系统矫正腰椎侧凸具有较好的治疗效果,脊椎骨性结构的应力水平整体较低。在满足矫形效果的前提下,临床上可考虑选择不同的矫形策略,以减少需植入螺钉的腰椎节段数量并提高手术质量。     

Vertebral pedicle anatomy in relation to pedicle screw fixation: a cadaver study

New techniques to stabilize and correct the thoracic and lumbar spine have been developed in recent years. In view of the wide variety and complexity of fixation devices, the optimum configuration of spinal instrumentation systems needs to be defined. Linear and angular measurements of both vertebral pedicles were made in ten complete thoracic and lumbar cadaveric spines using callipers and a goniometer. The vertical interpedicular distance gradually increased along the spine up to L5. The transverse interpedicular distance was larger at both ends of the spine. Pedicular height gradually increased from T1 to L5, plateauing between T3 and T9, being widest at the thoracolumbar junction. Pedicular width was greatest at the three junctional regions of the spine. The sagittal pedicular angle decreased along the length of the spine to zero at L5. The transverse pedicular angle decreased from T1 to T12 and then increased to L5.Of the pedicular measurements only width limits the diameter of fixation screws. The vertical interpedicular distance determines the distance between the holes of plates, while the length of the transfixator is related to the transverse interpedicular distance. The pedicular angles enable triangulation of screws and determine the stability of the fixation.