Biomechanical modelling of the whole human spine for dynamic analysis

Developing computational models of the human spine has been a hot topic in biornechanical research for a couple of decades in order to have an understanding of the behaviour of the whole spine and the individual spinal parts under various loading conditions. The objectives of this thesis are to develop a biofidefic multi-body model of the whole human spine especially for dynamic analysis of impact situations, such as frontal impact in a car crash, and to generate finite element (FE) models of the specific spinal parts to investigate causes of injury of the spinal components. As a proposed approach, the predictions of the multi-body model under dynamic impact loading conditions, such as reaction forces at lumbar motion segments, were utilised not only to have a better understanding of the gross kinetics and kinematics of the human spine, but also to constitute the boundary conditions for the finite element models of the selected spinal components. This novel approach provides a versatile, cost effective and powerful tool to analyse the behaviour of the spine under various loading conditions which in turn helps to develop a better understanding of injury mechanisms

In Vitro Biomechanical Testing and Computational: Modeling in Spine

Two separate in vitro biomechanical studies were conducted on human cadaveric spines (Lumbar) to evaluate the stability following the implantation of two different spinal fixation devices interspinous fixation device (ISD) and Hybrid dynamic stabilizers. ISD was evaluated as a stand-alone and in combination with unilateral pedicle rod system. The results were compared against the gold standard, spinal fusion (bilateral pedicle rod system). The second study involving the hybrid dynamic system, evaluated the effect on adjacent levels using a hybrid testing protocol. A robotic spine testing system was used to conduct the biomechanical tests. This system has the ability to apply continuous unconstrained pure moments while dynamically optimizing the motion path to minimize off-axis loads during testing. Thus enabling precise control over the loading and boundary conditions of the test. This ensures test reliability and reproducibility. We found that in flexion-extension, the ISD can provide lumbar stability comparable to spinal fusion. However, it provides minimal rigidity in lateral bending and axial rotation when used as a stand-alone. The ISD with a unilateral pedicle rod system when compared to the spinal fusion construct were shown to provide similar levels of stability in all directions, though the spinal fusion construct showed a trend toward improved stiffness overall. The results for the dynamic stabilization system showed stability characteristics similar to a solid all metal construct. Its addition to the supra adjacent level (L3- L4) to the fusion (L4- L5) indeed protected the adjacent level from excessive motion. However, it essentially transformed a 1 level into a 2 level lumbar fusion with exponential transfer of motion to the fewer remaining discs (excessive adjacent level motion). The computational aspect of the study involved the development of a spine model (single segment). The kinematic data from these biomechanical studies (ISD study) was then used to validate a finite element model

A Virtual Model of the Human Cervical Spine for Physics-based Simulation and Applications

Utilizing recent advances in computer technology, Our Biomechanics Laboratory have made an effort to integrate computer animation and engineering analysis software into biomedical research, specifically towards simulation and animation of in vitro experimentation of the human cervical spine in the virtual world. The objectives of this study were to develop a virtual model of the human cervical spine for physics-based simulation and to apply the virtual model to studies of different surgical procedures and instrumentation. A process for creating an accurate virtual model of the human cervical spine was developed. The model consisted of seven vertebrae (C2-T1) connected with soft tissue components: intervertebral joint, facet joints, and ligaments. The soft tissue components were assigned nonlinear viscoelastic properties. The evaluation of the model included the percent contribution of rotation relative to global rotation, coupling behaviors, helical axes of motion pattern, global rotational stiffness curves, and animations of the disc and facet forces. This model was used to evaluate different mounting configurations for axial rotation testing and to identify a set of end constraint conditions that produced physiologic responses during axial rotational loading. This model was also used to simulate the biomechanical responses of single-level cervical fusion. The single-level fusion was found to produce increased motion compensation at the adjacent segments during flexion and extension. Greater increases in the disc forces were found in the spinal level superior to the fusion during flexion and inferior to the fusion during flexion extension. This model was also used to study of the biomechanical effects of different design features for cervical disc arthroplasty. A constrained spherical joint placed at the disc level significantly increased facet loads during extension. Lowering the rotational axis of the spherical joint into the subjacent body also caused a marginal increase in facet loading during flexion, extension, and lateral bending. Un-constraining the spherical joint to a plane at the disc level minimized facet load build up. The virtual model bridges the gap between the cadaveric-based in vitro tests and clinicalbased experimental studies to further the research and educational knowledge of cervical spine biomechanics

Development and application of methods for the biomechanical characterization of spine ligaments and intervertebral discs

The spine is one of the major organs subject to trauma or genetic problems. Today 30% of people suffer from back pain and every day a large number of surgical interventions on the spine are performed to treat those patients with severe spinal deformities (i.e. scoliosis or kyphosis). From a statistical analysis, the percentage of failures for this type of interventions is around 25-30%. The aim of my PhD thesis was the improvement of the knowledge of the strain distribution on biological tissues, in particular on ligaments and intervertebral discs of the human spine. The first part of this thesis aimed at improvement of the methodologies used to measure the strain distribution, simultaneously on hard (vertebrae) and soft tissues (ligaments and intervertebral discs), using Digital Image Correlation. The second part of my research studied the biomechanical behaviour of the intervertebral discs and of the different ligaments. The disc acts as a shock absorber for the spine, reducing shocks and impacts. The anterior longitudinal ligament (ALL), supraspinous and interspinous ligaments were studied analysing how they were deformed under different loading conditions. These ligaments limit the movement of the spine during flexion reducing the overload on the intervertebral disc. The ALL does not offer great mechanical strength during lateral bending and axial torsion. Summarizing, the study underlines the necessity of having a full-field strain analysis tool to enhance the knowledge of the biomechanics of the spine and the interaction between different types of tissue. Furthermore, the results reported in this thesis could be useful also to build better multibody spine models and to include more realistic properties in finite element models. These results could be a starting point for future works in which the effect of different surgical procedures and the use of new surgical devices could be investigated

Advancing Thoracic Spine Biomechanical Research

The long term objective of this research was to elucidate issues with current thoracic spine testing methods and develop more accurate ways to quantify the biomechanical impact of surgical procedures or medical devices. The ability to perform thoracic spine testing with a rib cage is limited by test machine variability and experimental design inconsistency, so surgeons are left with little reliable information on the biomechanical impacts of procedures and implants. This research sought to validate a novel spine test machine, provide biomechanical data to support the inclusion of an intact rib cage when testing the thoracic spine, and quantify the biomechanical impacts of sequential Ponte osteotomies. Specific Aim 1 validated the accuracy of the spine test machine for rigidity ranges that represent cadaveric specimen rigidities present in the spine. Cervical, thoracic, and lumbar spine specimens were modeled with synthetic rubber that represented the breadth of rigidities, and testing was conducted in bending and axial rotation. The maximum machine displacement error was less than 2° for lumbar and thoracic specimens, so it is suggested that researchers use an external motion-tracking system in conjunction with the test machine when high accuracy measurements are required. Specific Aim 2 quantified the biomechanical differences of testing full cadaveric thoracic spine specimens with and without an intact rib cage. While it was presumed that the rib cage provides structural stability to the thoracic spine, the extent to which the rib cage contributes to spinal motion had not been fully quantified. Testing quantified the motion and stiffness values of an intact thoracic spine specimen, and results showed that testing without a rib cage changes both motion and stiffness values. Specific Aim 3 quantified the biomechanical impact of sequential Ponte osteotomies in cadaveric thoracic spine specimens with intact rib cages. Overall and regional changes in motion due to Ponte osteotomies were analyzed, and results showed increased flexibility in the sagittal plane on both overall and regional levels. The results from this work could provide researchers and surgeons the tools they need to better understand and improve spine procedures and implants, which could ultimately improve the quality of life for patients

Finite Element Analysis of a Model of a Single-Level Degenerated Cervical Spine: Influence of Simulated Surgical Treatment Methods on Kinematics

The most common pathology of the cervical spine is degenerative disc disease(DDD) and the surgical methods most often used to treat symptomatic cases of this condition are anterior cervical discectomy (ACD), anterior cervical discectomy followed by fusion (ACDF), and implantation of a total disc replacement (TDR). Although there are many literature reports on finite element analysis (FEA) of models of the cervical spine subjected to simulated surgical treatment(s), few modeled the full spine (C1-C7), very few analyzed a model in which degeneration was simulated at a disc, none compared all three of these popular surgical methods,and very few focused on kinematics of the spine. Since the performance of many activities of daily living involves the motion of cervical spine units, it is useful to determine the kinematic reponse of these units. The purpose of the present study was to determine the influence of these three popular surgical treatment methods on the rotation of the motion at each of the functional units of a single-level degenerated cervical spine (C5-C6), under s series of clinically-relevant applied loading. Thus five FEA models were analyzed; namely, INTACT, DEGEN, ACD, ACDF and TDR models. With respect to the simulated surgical treatments, the principal finding was that rotation motion at the treatment level (C5-C6) as well as at each of the other levels of the spine model is best preserved when the TDR model was used. This suggests that TDR is an attractive surgical option, but this can only be confirmed from the results of well-planned clinical trials