Design, development, manufacturing and biomechanical testing of Stand-alone cage for posterior lumbar interbody fusion

Introduction: The most common method of spinal fusion includes pedicle screws instrumentation, either with or without interbody cage fusion. This thesis aimed to develop and test a novel stand-alone intervertebral device that eliminates the need for pedicle screws and rods. Method: The stand-alone cage was designed in collaboration with spinal surgeons and engineers using computer assisting drawings, and manufactured in titanium by 3D printing. Biomechanical testing comparing the stand-alone cage with standard posterior lumbar interbody fusion (PLIF) in sawbones (n=6) and cadavers (n=8). Result: Compared to PLIF, the stand-alone cage demonstrated no significant difference in range of flexion, lateral bend or axial rotation in sawbones; however, significant increase in range of extension was observed. Among cadavers, the stand-alone cage demonstrated a significant increase in range of motion (ROM) for flexion, extension, lateral bending to the right and total lateral bend ROM; but no significant increase to ROM in axial rotation. Conclusion: Due to the increased ROM associated with the stand-alone cage, this devise is not advisable to use as a fusion implant. Keywords Lumbar spine, anatomy, biomechanics, Posterior lumbar fusion, interbody fusion

Multi-objective design optimization of a mobile-bearing total disc arthroplasty considering spinal kinematics, facet joint loads, and metal-on-polyethylene contact mechanics

Total disc arthroplasty (TDA) is a motion-preserving surgical technique used to treat spinal disorders, when more conservative medical therapies fail. Unfortunately, a high incidence of revision surgery exists due to postoperative complications including abnormal kinematics, facet joint arthritis, and implant failures. However, TDA is still an attractive option, since an optimally designed artificial disc is expected to reproduce native segmental biomechanics. Correspondingly, it would mitigate the development of adjacent segment diseases (a major concern of spinal fusion) caused by altered segmental biomechanics. Design optimization is a process of finding the best design parameters for a component/system to satisfy one/multiple design requirements using optimization algorithms. The shape of a candidate design is parametrized using computer-aided design, such that design parameters are manipulated to minimize one/multiple objective functions subject to performance constraints and design space bounds. Optimization algorithms typically require the gradients of the objective/constraint functions with respect to each design variable. In the traditional design optimization, due to the high computational cost to calculate the gradients by performing finite element analysis in each optimization iteration, it often results in a slow process to seek the optimal solution. To address the problem, an artificial neural network (ANN) was implemented to derive the analytical expressions of the objective/constraint function and their gradients. By incorporating analytical gradients, we successfully developed a multiobjective optimization (MOO) framework considering three performance metrics simultaneously. Furthermore, a new mobile-bearing TDA design concept featuring a biconcave polyethylene (PE) core was proposed, to strengthen the PE rim, where a high risk of fracture exists. It was hypothesized that there is a trade-off relationship among postoperative performance metrics in terms of spinal kinematics, facet joint loading, and metal-on-polyethylene contact mechanics. We tested this hypothesis by refining the new TDA to match normal segmental biomechanics and alleviate PE core stress. After performing MOO, the best-trade-off TDA design was determined by the solved three-dimensional Pareto frontier. The novel MOO framework can be also used to improve existing TDA designs, as well as to push the cutting edge of surgical techniques for the treatment of spinal disorders

Multi-Scale Vertebral-Kinematics Based Simulation Pipeline of the Human Spine With Application to Spine Tissues Analysis

This study developed an analytical tool for understanding spine tissues’ behavior in response to vertebral kinematics and spine pathology over a range of body postures. It proposed a novel pipeline of computational models based on predicting individual vertebral kinematics from measurable body-level motions, using musculoskeletal dynamics simulations to drive the vertebrae in corresponding spine FEMs. A reformulated elastic surface node (ESN) lumbar model was developed for use in MSD simulations. The ESN model modifies the lumbar spine within an existing MSD model by removing non-physiological kinematic constraints and including elastic IVD behavior. The model was scaled using subject-specific anthropometrics and validated to predict in vivo vertebral kinematics and IVD pressures during trunk flexion/extension. The ESN model was integrated into a novel simulation pipeline that automatically maps it to a kinematics-driven FEM (KD-FEM). The KD-FEM consisted of lumbar vertebrae scaled to subject-specific geometries and actuated by subject-specific vertebral kinematics from the ESN model for different activities. The pipeline was validated for its ability to predict in vivo IVD pressures at L4-L5 level during flexion and load carrying postures. A detailed multi-layered multi-phase lumbar canal FE model was integrated into the KD-FEM to quantify risks to canal tissues due to vertebral kinematics and progressive canal narrowing (stenosis). This enabled distinct computation of proposed stenosis measures, including cerebrospinal fluid pressure, cauda equina deformation and related stresses/pressure/strains, among others. Model outputs included measures during flexion and comparison of three clinically relevant degrees of progressive stenosis of the bony vertebral foramen at L4 level

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

MRI characterisation of the lumbar spine in lower trunk rotation

Statistical data indicates that the percentage of prevalence of spine-related pain is considered to be high, and even up to 84%. The spinal manipulation technique, which is based on applying external forces to the shoulder and pelvis to twist the human spine can decrease lower back pain. Better understanding of the biomechanical behaviour of the normal lumbar spine during each rotational position of the lower trunk will provide valuable translational information to guide better physiotherapy in the future. It will also provide normal variant data that will help healthcare professionals and specialist in artificial spine implants to understand certain aspects of spinal pain. Therefore, this study proposes an MRI study of the lumbar spine during different lower-trunk rotational positions to investigate their effect on the normal spine structures with consideration of the shoulder and pelvis girdles’ motion. To control the angle of the lower-trunk rotation, an MRI holder and an adaptive goniometer have been developed to position the subject and obtain accurate pelvis angle of rotation during the scan. Before starting the MRI scan, the position of the subject on the MRI holder was checked by calculating specific parameters. Standard supine and four lower-trunk rotational positions with unrestricted left and right shoulder movements were performed. T2 Sagittal, T2 coronal and T2 Axial 3D acquisition cuts were performed for the lumbar spine with a 1.5-T MRI scanner. The MR images were collected from volunteers and analysed using Image J software depending on the determination of particular anatomical landmarks and image processing techniques. The results show that there is a significant difference between the position of the right and left scapula during lower trunk rotation, while there is no significant relation between the angle of rotation of L5 and the rotational angle of the posterior superior iliac spines relative to the horizontal plane in three tested sections of the sacroiliac joints. In addition, there is no significant difference in the angle of rotation of the examined sections of the sacroiliac joints during different rotational positions of the lower trunk. The effect of different lower trunk rotational positions on the angle of rotation of the lower lumbar segment and spinal canal depth was measured and it was found that the second rotational lower trunk rotational position caused the highest relative motion of the lower lumbar vertebra, while the first lower trunk rotational position caused the highest rotational torque between L5 and L3. In addition, the mean iii difference in the spinal canal depth increased significantly following the degree of the applied lower trunk rotational position. Lower trunk rotation caused morphologic changes in the intervertebral discs and intervertebral foramens at L3-L4, L4-L5, and L5-S1 levels. However, the significant change in the area, width and height of the intervertebral foramen and disc depended on the rotational positions of the lower trunk. A strong anatomical relationship was indicated between the posterior height of the intervertebral disc at both sides and the foraminal height. Finally, the degree of the lateral bending was the greatest at the L4-L5 level. The mean differences between the left and right superior articular processes according to their orientation angle and gapping distance at the L3-L4 level were higher than those of the other tested levels, while the L5 level recorded the lowest values. However, the mean differences did not achieve significant effects. These results may provide baseline information to enable the development of artificial implants of the right and left lumbar facet joints according to changes in lower trunk rotational positions. They can also help to explain the treatment benefits of manipulation therapy in spinal conditions

The Influence of Design Features in the Biomechanical Performance of a Fixator for the Lumbar Spine

PhDSpinal fixation systems using pedicular screws have gained popularity in manging the damaged spine. However, the loading to which individual components of a fixator are exposed are largely unknown. This thesis describes the use of a Corpectomy injury model to investigate the mechanical response of a commercial internal spinal fixator and the resultant loads acting on its rods and screws, under four separatelo ading regimens. The fixator was instrumentedw ith strain gaugesa nd tested using specially designed jigs. The results were then compared to theoretical models and any differences highlighted. An evaluation was also performed on a range of transpedicular screw designs under tensile loads. An increase in the tightening torque of the fixator clamps, ranging from 5 to 15Nm, and the inclusion of transverse elements across its vertical rods produced a combined increase in overall torsional rigidity of 89%. However, no such changes were found under axial compression and both simulated flexion and extension tests. The relative ineffectivenesso f the transversee lementsu nder sagittal loads was probably due to their spatial relationship with the fixator. The results from the instrumented fixator indicated several load response pathways, as predicted by the theoretical analysis. These pathways were influenced by several factors including, the screw angulation, the boundary conditions of the test and the addition of the transverse elements. Clamp design was critical in minimising rotational slippage of both screws and transverse elements. The results from the instrumented fixator revealed that the transpedicular screws were exposed to complex loads under each of the tests. Under tensile loads, both the increasei n screw insertion depth and a decreasein screw pitch were found to be the important parameters which affect screw performance. Analysis showed the state of stress and strain along the thread was the overriding factor in the tensile performance of these screws. This work hase mphasisedth e importance of a full biornechanicale valuation of any future designs of spinal fixators

In vivo lumbar spine biomechanics : vertebral kinematics, intervertebral disc deformation, and disc loads

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references.Knowledge of lumbar spine biomechanics in living human subjects is fundamental for understanding mechanisms of spinal injury and pathology, for improvement of corresponding clinical treatments, and for design of spinal prosthesis. However, due to the complicated spine anatomy and loading conditions as well as high risks in these direct measurements, it has been a challenge to determine the in vivo biomechanics of the lumbar spine. To address this problem, the overall objective of this thesis was to develop and implement a dual fluoroscopic imaging system to non-invasively study human lumbar spine biomechanics. In line with this objective, the first goal was to quantify the ability of the dual fluoroscopic imaging system to determine vertebral kinematics. The second goal was to implement this technique to investigate spinal motion in both healthy subjects and patients with pathology. The third goal was to explore the feasibility of using kinematic data obtained from this system as boundary conditions in finite element analysis to calculate the physiological loads on the intervertebral disc. The system was shown to be accurate and repeatable in determining the vertebral kinematics in all degrees of freedom. For the first time, six degree-of-freedom motion of different structures of the spine, such as the vertebral body, intervertebral disc, facet joint and spinous process were measured in vivo in both healthy subjects and subjects with pathology during functional activities. In general, the group of subjects with pathology showed a significantly abnormal kinematic response during various physiological functional activities. Preliminary studies have shown the applicability and high accuracy of finite element modeling to calculate disc loads using in vivo vertebral kinematics as displacement boundary conditions.by Shaobai Wang.Ph.D

Control of the Mechanical Properties of the Synthetic Anterior Longitudinal Ligament and its Effect on the Mechanical Analogue Lumbar Spine Model

The development and validation of an anatomically correct mechanical analogue spine model would serve as a valuable tool in helping researchers and implant designers understand and alleviate low back pain. Advanced composite ligaments were designed to mimic the tensile mechanical properties of human spinal ligaments. By changing the composite properties, the stiffness and deformation at toe were controlled in a repeatable manner. Five analogue spine models, with three different Anterior Longitudinal Ligament (ALL) stiffness configurations, were tested in bending and compression using displacement control in a MTS load frame. The bending stiffness and kinematic ranges of motion of the spines were compared for each ALL stiffness configuration. The ALL stiffness had a significant effect on the stiffness and peak moment in extension of the overall spine model. The study demonstrated that a change in the synthetic ligament properties successfully controls the biomechanics of the analogue spine model and the model effectively mimics the human cadaveric biomechanical response