The Role of Sacral Slope in the Progression of a Bilateral Spondylolytic Defect at L5 to Spondylolisthesis: A Biomechanical Investigation Using Finite Element Analysis.

STUDY DESIGN: A biomechanical study using finite element analysis. OBJECTIVES: The main objective of this study was to investigate the role of sacral slope in the progression of a L5 bilateral spondylolytic defect to spondylolisthesis. METHODS: A 3-dimensional model of lumbosacral spine was built using computed tomography (CT) data procured from an anonymized healthy male subject. The segmented CT data was manipulated to generate 3 more models representing L5 bilateral spondylolytic defect with normal sacral slope (SS), sacral slope increased by 10° (SS+10), and sacral slope decreased by 10° (SS-10). The 3D models were imported into finite element modelling software Strand7 for preprocessing, running nonlinear static solves, and postprocessing of the results. RESULTS: Directional biomechanical instabilities were induced in the lumbosacral spine as a result of changes in the L5-S1 disc shape secondary to the changes in sacral slope. Compared with the normal L5 lytic model, wedging of the L5-S1 disc (SS+10) resulted in a significantly greater range of motion in flexion (18% ↑) but extension motion characteristics were similar. Conversely, flattening of the L5-S1 disc (SS-10) resulted in a significantly greater range of motion in extension (16% ↑) but flexion motion characteristics were similar to that of the normal L5 lytic model. CONCLUSIONS: Variations in sacral slope while preserving the L5-S1 mid-disc height and orientation of the L5 vertebra resulted in variations in the L5-S1 disc shape. The results suggest that for such extremities in the L5-S1 disc shape different pathomechanisms exist for the progression of the L5 lytic defect to spondylolisthesis.

Mild (not severe) disc degeneration is implicated in the progression of bilateral L5 spondylolysis to spondylolisthesis.

BACKGROUND: Spondylolytic (or lytic) spondylolisthesis is often associated with disc degeneration at the index-level; however, it is not clear if disc degeneration is the cause or the consequence of lytic spondylolisthesis. The main objective of this computed tomography based finite element modelling study was to examine the role of different grades of disc degeneration in the progression of a bilateral L5-lytic defect to spondylolisthesis. METHODS: High-resolution computed tomography data of the lumbosacral spine from an anonymised healthy male subject (26 years old) were segmented to build a 3D-computational model of an INTACT L1-S1 spine. The INTACT model was manipulated to generate four more models representing a bilateral L5-lytic defect and the following states of the L5-S1 disc: nil degeneration (NOR LYTIC), mild degeneration (M-DEG LYTIC), mild degeneration with 50% disc height collapse (M-DEG-COL LYTIC), and severe degeneration with 50% disc height collapse(S-COL LYTIC). The models were imported into a finite element modelling software for pre-processing, running nonlinear-static solves, and post-processing of the results. RESULTS: Compared with the baseline INTACT model, M-DEG LYTIC model experienced the greatest increase in kinematics (Fx range of motion: 73% ↑, Fx intervertebral translation: 53%↑), shear stresses in the annulus (Fx anteroposterior: 163%↑, Fx posteroanterior: 31%↑), and strain in the iliolumbar ligament (Fx: 90%↑). The S-COL LYTIC model experienced a decrease in mobility (Fx range of motion: 48%↓, Fx intervertebral translation: 69%↓) and an increase in normal stresses in the annulus (Fx Tensile: 170%↑; Fx Compressive: 397%↑). No significant difference in results was noted between M-DEG-COL LYTIC and S-COL LYTIC models. CONCLUSIONS: In the presence of a bilateral L5 spondylolytic defect, a mildly degenerate index-level disc experienced greater intervertebral motions and shear stresses compared with a severely degenerate index-level disc in flexion and extension bending motions. Disc height collapse, with or without degenerative changes in the stiffness properties of the disc, is one of the plausible re-stabilisation mechanisms available to the L5-S1 motion segment to mitigate increased intervertebral motions and shear stresses due to a bilateral L5 lytic defect.

Global and segmental kinematic changes following sequential resection of posterior osteoligamentous structures in the lumbar spine: An in vitro biomechanical investigation using pure moment testing protocols.

Lumbar spinal surgeries may compromise the integrity of posterior osteoligamentous structures implicating mechanical stability. Circumstances necessitating a concomitant surgery to achieve restabilisation are not well understood. The main objective of this in vitro study was to quantify global and segmental (index and adjacent levels) kinematic changes in the lumbar spine following sequential resection of the posterior osteoligamentous structures using pure moment testing protocols. Six fresh frozen cadaveric kangaroo lumbar spines (T12-S1) were tested under a bending moment in flexion-extension, bilateral bending, and axial torsion in a 6-degree-of-freedom Kinematic Spine Simulator. Specimens were tested in the following order: intact state (D0), after interspinous and supraspinous ligaments transection between L4 and L5 (D1), further after a total bilateral facetectomy between L4 and L5 (D2). Segmental motions at the cephalad, damaged, and caudal levels were recorded using an infrared-based motion tracking device. Following D1, no significant change in the global range of motion was observed in any of the bending planes. Following D2, a significant increase in the global range of motion from the baseline (D0) was observed in axial torsion (median normalised change +20%). At the damaged level, D2 resulted in a significant increase in the segmental range of motion in flexion-extension (+77%) and axial torsion (+492%). Additionally, a significant decrease in the segmental range of motion in axial torsion (-35%) was observed at the caudal level following D2. These results suggest that a multi-segment lumbar spine acts as a mechanism for transmitting motions, and that a compromised joint may significantly alter motion transfer to adjacent segments. We conclude that the interspinous and supraspinous ligaments play a modest role in restricting global spinal motions within physiologic limits. Following interspinous and supraspinous ligaments transection, a total bilateral facetectomy resulted in a significant increase in axial torsion motion, both at global and damaged levels, accompanied with a compensatory decrease in motion at the caudal level.

Computational Aerodynamic Analysis of a Micro-CT Based Bio-Realistic Fruit Fly Wing.

The aerodynamic features of a bio-realistic 3D fruit fly wing in steady state (snapshot) flight conditions were analyzed numerically. The wing geometry was created from high resolution micro-computed tomography (micro-CT) of the fruit fly Drosophila virilis. Computational fluid dynamics (CFD) analyses of the wing were conducted at ultra-low Reynolds numbers ranging from 71 to 200, and at angles of attack ranging from -10° to +30°. It was found that in the 3D bio-realistic model, the corrugations of the wing created localized circulation regions in the flow field, most notably at higher angles of attack near the wing tip. Analyses of a simplified flat wing geometry showed higher lift to drag performance values for any given angle of attack at these Reynolds numbers, though very similar performance is noted at -10°. Results have indicated that the simplified flat wing can successfully be used to approximate high-level properties such as aerodynamic coefficients and overall performance trends as well as large flow-field structures. However, local pressure peaks and near-wing flow features induced by the corrugations are unable to be replicated by the simple wing. We therefore recommend that accurate 3D bio-realistic geometries be used when modelling insect wings where such information is useful.

Pedicle screw-based posterior dynamic stabilizers for degenerative spine: in vitro biomechanical testing and clinical outcomes.

Dynamic stabilization in a degenerate symptomatic spine may be advantageous compared with conventional fusion procedures, as it helps preserve motion and minimizes redistribution of loads at instrumented and adjacent segments. This article presents a systematic review of biomechanical and clinical evidence available on some of the pedicle screw based posterior dynamic stabilization (PDS) devices. Using Medline, Embase, and Scopus online databases, we identified four pedicle-screw-PDS devices for which both, biomechanical testing and clinical follow-up data are available: Graf artificial ligaments, Isobar TTL, Polyetheretherketone rods, and Dynesys. The current state-of-the-art of pedicle-screw-PDS devices is far from achieving its desired biomechanical efficacy, which has resulted in a weak support for the posited clinical benefits. Although pedicle-screw-PDS devices are useful in salvaging a moderately degenerate functionally suboptimal disc, for severe disc degeneration cases fusion is still the preferred choice. We conclude that a pedicle-screw-PDS device should aim at restoring load sharing amongst spinal elements while preserving the qualitative and quantitative nature of spinal motion, especially minimize posterior shift of the helical axis of motion. More precise and objective assessment techniques need to be standardized for in vivo evaluation of intervertebral motion and load sharing amongst spinal elements across different pedicle-screw-PDS devices.

Micromechanics of Sea Urchin spines.

The endoskeletal structure of the Sea Urchin, Centrostephanus rodgersii, has numerous long spines whose known functions include locomotion, sensing, and protection against predators. These spines have a remarkable internal microstructure and are made of single-crystal calcite. A finite-element model of the spine's unique porous structure, based on micro-computed tomography (microCT) and incorporating anisotropic material properties, was developed to study its response to mechanical loading. Simulations show that high stress concentrations occur at certain points in the spine's architecture; brittle cracking would likely initiate in these regions. These analyses demonstrate that the organization of single-crystal calcite in the unique, intricate morphology of the sea urchin spine results in a strong, stiff and lightweight structure that enhances its strength despite the brittleness of its constituent material.

An experimentally validated micromechanical model of a rat vertebra under compressive loading.

In recent years, finite element analysis (FEA) has been increasingly applied to examine and predict the mechanical behaviour of craniofacial and other bony structures. Traditional methods used to determine material properties and validate finite element models (FEMs) have met with variable success, and can be time-consuming. An implicit assumption underlying many FE studies is that relatively high localized stress/strain magnitudes identified in FEMs are likely to predict material failure. Here we present a new approach that may offer some advantages over previous approaches. Recently developed technology now allows us to both image and conduct mechanical tests on samples in situ using a materials testing stage (MTS) fitted inside the microCT scanner. Thus, micro-finite element models can be created and validated using both quantitative and qualitative means. In this study, a rat vertebra was tested under compressive loading until failure using an MTS. MicroCT imaging of the vertebra before mechanical testing was used to create a high resolution finite element model of the vertebra. Load-displacement data recorded during the test were used to calculate the effective Young's modulus of the bone (found to be 128 MPa). The microCT image of the compressed vertebra was used to assess the predictive qualities of the FE model. The model showed the highest stress concentrations in the areas that failed during the test. Clearly, our analyses do not directly address biomechanics of the craniofacial region; however, the methodology adopted here could easily be applied to examine the properties and behaviour of specific craniofacial structures, or whole craniofacial regions of small vertebrates. Experimentally validated micro-FE analyses are a powerful method in the study of materials with complex microstructures such as bone.

The visualization of hepatic vasculature by X-ray micro-computed tomography.

In this study, X-ray micro-computed tomography (CT) was used to reconstruct the fine structure macro- and microvasculature in three dimensions in contrast-enhanced rat liver samples. The subsequent application in the experimental CC531s colorectal cancer model was concurrent with results obtained from confocal microscopy in earlier studies. The en bloc stains osmium tetroxide in combination with uranyl acetate provided an excellent contrasting result for hepatic tissue after a trial of several contrasting agents. X-ray micro-CT allowed us to image the large blood vessels together with the branching sinusoids of hepatic tissue in three dimensions. Furthermore, interruption of the microvasculature was noted when rats were injected with CC531s colorectal cancer cells indicating the presence of hepatic metastases.

A three-dimensional fractal model of tumour vasculature.

We constructed a three-dimensional fractal model of the vascular network in a tumour periphery. We model the highly disorganised structure of the neoplastic vasculature by using a high degree of variation in segment properties such as length, diameter and branching angle. The overall appearance of the vascular tree is subjectively similar to that of the disorganised vascular network which encapsulates tumours. The fractal dimension of the model is within the range of clinically measured values.