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

Is there any advantage of using stand-alone cages? A numerical approach

Background: Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics. Methods: A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated. Results: A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates. Conclusions: A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence

Biomechanical comparison of a new stand-alone anterior lumbar interbody fusion cage with established fixation techniques – a three-dimensional finite element analysis

<p>Abstract</p> <p>Background</p> <p>Initial promise of a stand-alone interbody fusion cage to treat chronic back pain and restore disc height has not been realized. In some instances, a posterior spinal fixation has been used to enhance stability and increase fusion rate. In this manuscript, a new stand-alone cage is compared with conventional fixation methods based on the finite element analysis, with a focus on investigating cage-bone interface mechanics and stress distribution on the adjacent tissues.</p> <p>Methods</p> <p>Three trapezoid 8° interbody fusion cage models (dual paralleled cages, a single large cage, or a two-part cage consisting of a trapezoid box and threaded cylinder) were created with or without pedicle screws fixation to investigate the relative importance of the screws on the spinal segmental response. The contact stress on the facet joint, slip displacement of the cage on the endplate, and rotational angle of the upper vertebra were measured under different loading conditions.</p> <p>Results</p> <p>Simulation results demonstrated less facet stress and slip displacement with the maximal contact on the cage-bone interface. A stand-alone two-part cage had good slip behavior under compression, flexion, extension, lateral bending and torsion, as compared with the other two interbody cages, even with the additional posterior fixation. However, the two-part cage had the lowest rotational angles under flexion and torsion, but had no differences under extension and lateral bending.</p> <p>Conclusion</p> <p>The biomechanical benefit of a stand-alone two-part fusion cage can be justified. This device provided the stability required for interbody fusion, which supports clinical trials of the cage as an alternative to circumferential fixations.</p

Studies on Spinal Fusion from Computational Modelling to ‘Smart’ Implants

Low back pain, the worldwide leading cause of disability, is commonly treated with lumbar interbody fusion surgery to address degeneration, instability, deformity, and trauma of the spine. Following fusion surgery, nearly 20% experience complications requiring reoperation while 1 in 3 do not experience a meaningful improvement in pain. Implant subsidence and pseudarthrosis in particular present a multifaceted challenge in the management of a patient’s painful symptoms. Given the diversity of fusion approaches, materials, and instrumentation, further inputs are required across the treatment spectrum to prevent and manage complications. This thesis comprises biomechanical studies on lumbar spinal fusion that provide new insights into spinal fusion surgery from preoperative planning to postoperative monitoring. A computational model, using the finite element method, is developed to quantify the biomechanical impact of temporal ossification on the spine, examining how the fusion mass stiffness affects loads on the implant and subsequent subsidence risk, while bony growth into the endplates affects load-distribution among the surrounding spinal structures. The computational modelling approach is extended to provide biomechanical inputs to surgical decisions regarding posterior fixation. Where a patient is not clinically pre-disposed to subsidence or pseudarthrosis, the results suggest unilateral fixation is a more economical choice than bilateral fixation to stabilise the joint. While finite element modelling can inform pre-surgical planning, effective postoperative monitoring currently remains a clinical challenge. Periodic radiological follow-up to assess bony fusion is subjective and unreliable. This thesis describes the development of a ‘smart’ interbody cage capable of taking direct measurements from the implant for monitoring fusion progression and complication risk. Biomechanical testing of the ‘smart’ implant demonstrated its ability to distinguish between graft and endplate stiffness states. The device is prepared for wireless actualisation by investigating sensor optimisation and telemetry. The results show that near-field communication is a feasible approach for wireless power and data transfer in this setting, notwithstanding further architectural optimisation required, while a combination of strain and pressure sensors will be more mechanically and clinically informative. Further work in computational modelling of the spine and ‘smart’ implants will enable personalised healthcare for low back pain, and the results presented in this thesis are a step in this direction

Biomechanical comparison of multilevel lateral interbody fusion with and without supplementary instrumentation: a three-dimensional finite element study

Abstract Background Lateral lumbar interbody fusion (LLIF) is a popular, minimally invasive technique that is used to address challenging multilevel degenerative spinal diseases. It remains controversial whether supplemental instrumentation should be added for multilevel LLIF. In this study, we compared the kinematic stability afforded by stand-alone lateral cages with those supplemented by bilateral pedicle screws and rods (PSR), unilateral PSR, or lateral plate (LP) fixation using a finite-element (FE) model of a multi-level LLIF construct with simulated osteoporosis. Additionally, to evaluate the prospect of cage subsidence, the stress change characteristics were surveyed at cage-endplate interfaces. Methods A nonlinear 3-dimensional FE model of the lumbar spine (L2 to sacrum) was used. After validation, four patterns of instrumented 3-level LLIF (L2-L5) were constructed for this analysis: (a) 3 stand-alone lateral cages (SLC), (b) 3 lateral cages with lateral plate and two screws (parallel to endplate) fixated separately (LPC), (c) 3 lateral cages with bilateral pedicle screw and rod fixation (LC + BPSR), and (d) 3 lateral cages with unilateral pedicle and rod fixation (LC + UPSR). The segmental and overall range of motion (ROM) of each implanted condition were investigated and compared with the intact model. The peak von Mises stresses upon each (superior) endplate and the stress distribution were used for analysis. Results BPSR provided the maximum reduction of ROM among the configurations at every plane of motion (66.7–90.9% of intact spine). UPSR also provided significant segmental ROM reduction (45.0–88.3%). SLC provided a minimal restriction of ROM (10.0–75.1%), and LPC was found to be less stable than both posterior fixation (23.9–86.2%) constructs. The construct with stand-alone lateral cages generated greater endplate stresses than did any of the other multilevel LLIF models. For the L3, L4 and L5 endplates, peak endplate stresses caused by the SLC construct exceeded the BPSR group by 52.7, 63.8, and 54.2% in flexion, 22.3, 40.1, and 31.4% in extension, 170.2, 175.1, and 134.0% in lateral bending, and 90.7, 45.5, and 30.0% in axial rotation, respectively. The stresses tended to be more concentrated at the periphery of the endplates. Conclusions SLC and LPC provided inadequate ROM restriction for the multilevel LLIF constructs, whereas lateral cages with BPSR or UPSR fixation provided favorable biomechanical stability. Moreover, SLC generated significantly higher endplate stress compared with supplemental instrumentation, which may have increased the risk of cage subsidence. Further biomechanical and clinical studies are required to validate our FEA findings.http://deepblue.lib.umich.edu/bitstream/2027.42/136058/1/12891_2017_Article_1387.pd

Development of a direct fuel injector for a two-stroke gasoline engine

A prototype pressure-swirl injector was designed to suit an arbitrary two�stroke, gasoline direct injection engine requirement. A commercial computational fluid dynamics software FLUENT was used as a tool to analyze the performance of the conceptual design. A prototype injector was fabricated once the conceptual design met the requirements. The fabricated prototype injector was then subjected to a series of specification tests such as leak test, static flow rate test and discharge coefficient test. A dedicated test rig was set up to evaluate the performance of the prototype injector. The spray images at specified time frame were illuminated by Nd:Y AG laser sheet and captured using a high-speed digital camera. The stored images were analyzed to give data of spray angle, and droplet Sauter mean diameter at fuel-air pressure differential ranging from 1.0 to 5.0 MPa, with a step size of 1.0 MPa. From the tests, the prototype injector static flow rate, half spray cone angle and the droplet Sauter mean diameter at fuel injection pressure of 5.0 MPa was found to comply with the outlined requirements, which are approximately 475 cc/min, 32° and 19 11m respectively. Finally, the experimental data was compared with the calculated data. It was found that the measured data of static flow rate, discharge coefficient, and droplet Sauter mean diameter were higher than the computed data at fuel-air pressure differential between 1.0 and 3.0 MPa. In contrast, the calculated initial spray angle was overestimated by 3% at all tested fuel-air pressure differentials

Human lumbar spine biomechanics: study of pathologies and new surgical procedures

This thesis aims to shed light on the process that undergoes the lumbar spine as a result of intervertebral disc degeneration and different lumbar surgeries, paying special attention on the main risk factors and how to overcome them. Low back pain is the leading musculoskeletal disorder in all developed countries generating high medical related costs. Intervertebral disc degeneration is one of the most common causes of low back pain. When conservative treatments fail to relieve this pain, lumbar surgery is needed and, in this regard, lumbar fusion is the \textquotedblleft gold standard\textquotedblright technique to provide stability and neural decompression.Degenerative disc disease has been studied through two different approaches. An in-vivo animal model was reproduced and followed-up with MRI and mechanical testing to see how the water content decreased while the stiffness of the tissue increased. Then, degeneration was induced in a single disc of the human lumbar spine and the effects on the adjacent disc were investigated by the use of the finite element models. Further on, different procedures for segmental fusion were computationally simulated. A comparison among different intersomatic cage designs, supplemented with posterior screw fixation or placed in a stand-alone fashion, showed how the supplementary fixation drastically decreased the motion in the affected segment increasing the risk of adjacent segment disease more than a single placed cage. However, one of the main concerns regarding the use of cages without additional fixation is the subsidence of the device into the vertebral bone. A parametric study of the cage features and placement pointed to the width, curvature, and position as the most influential parameters for stability and subsidence.Finally, two different algorithms for tissue healing were implemented and applied for the first time to predict lumbar fusion in 3D models. The self-repairing ability of the bone was tested after simple nucleotomy and after instrumentation with internal fixation, anterior plate or stand-alone intersomatic cage predicting, in agreement with previous animal and clinical studies, that instrumentation may be not necessary to promote segmental fusion. In particular, the intervertebral disc height was seen to play an important role in the bone bridge or osteophyte formation.To summarize, this thesis has focused in the main controversial issues of intervertebral disc degeneration and lumbar fusion, such as degenerative process, adjacent segment disease, segment stability, cage subsidence or bone bridging. All the models described in this thesis could serve as a powerful tool for the pre-clinical evaluation of patient-specific surgical outcomes supporting clinician decisions. This thesis aims to shed light on the process that undergoes the lumbar spine as a result of intervertebral disc degeneration and different lumbar surgeries, paying special attention on the main risk factors and how to overcome them. Low back pain is the leading musculoskeletal disorder in all developed countries generating high medical related costs. Intervertebral disc degeneration is one of the most common causes of low back pain. When conservative treatments fail to relieve this pain, lumbar surgery is needed and, in this regard, lumbar fusion is the \textquotedblleft gold standard\textquotedblright technique to provide stability and neural decompression. Degenerative disc disease has been studied through two different approaches. An in-vivo animal model was reproduced and followed-up with MRI and mechanical testing to see how the water content decreased while the stiffness of the tissue increased. Then, degeneration was induced in a single disc of the human lumbar spine and the effects on the adjacent disc were investigated by the use of the finite element models. Further on, different procedures for segmental fusion were computationally simulated. A comparison among different intersomatic cage designs, supplemented with posterior screw fixation or placed in a stand-alone fashion, showed how the supplementary fixation drastically decreased the motion in the affected segment increasing the risk of adjacent segment disease more than a single placed cage. However, one of the main concerns regarding the use of cages without additional fixation is the subsidence of the device into the vertebral bone. A parametric study of the cage features and placement pointed to the width, curvature, and position as the most influential parameters for stability and subsidence. Finally, two different algorithms for tissue healing were implemented and applied for the first time to predict lumbar fusion in 3D models. The self-repairing ability of the bone was tested after simple nucleotomy and after instrumentation with internal fixation, anterior plate or stand-alone intersomatic cage predicting, in agreement with previous animal and clinical studies, that instrumentation may be not necessary to promote segmental fusion. In particular, the intervertebral disc height was seen to play an important role in the bone bridge or osteophyte formation. To summarize, this thesis has focused in the main controversial issues of intervertebral disc degeneration and lumbar fusion, such as degenerative process, adjacent segment disease, segment stability, cage subsidence or bone bridging. All the models described in this thesis could serve as a powerful tool for the pre-clinical evaluation of patient-specific surgical outcomes supporting clinician decisions. <br /