Robotic Simulation of Disc Arthroplasty Surgery: Influence of Surgical Placement on Motion Segment Dynamics

A variety of total disc replacement (TDR) designs exist for the treatment of disc pathologies. A key design parameter for a constrained ball and socket device is the location of the fixed center of rotation (COR). A previous study demonstrated that intact motion segment unit (MSU) mechanics and range of motion (ROM) were sensitive to the location of a prescribed sagittal plane rotational axis. Mal-alignment between the implant COR and the COR of the MSU may lead to an overloaded or over constrained condition. Two paradigms exist for the placement of a fixed COR TDR device relative to MSU anatomy: positioning the implant midline or posterior to midline. Presently, there are no data to indicate which paradigm may lead to better biomechanical/clinical outcome. This research attempts to evaluate changes in MSU mechanics and ROM as a result of variations in the size and placement of a simulated ball and socket TDR, like the ProDisc-L lumbar disc prosthesis. Six human cadaveric lumbar MSUs, L4-L5, were tested in flexion/extension using the Spine Robot to an end load limit of 8Nm. A fixed axis protocol was used to impose a pure rotation about a desired anatomical location. The Spine Robot was programmed to rotate the MSU about the COR of the implant. Subsequently, with the MSU held rigid, the implant was removed and rotation about the implant’s COR was repeated. Thereafter, simulated CORs were tested in different anatomical locations as defined by a customized grid pattern. The grid pattern consisted of 8 CORs which simulated the placement of a medium and large size constrained ball and socket device. Measurements of shear forces along the disc plane, axial force normal to the disc plane, segmental bending moment, and segmental ROM were analyzed at each grid point. Analysis of MSU mechanics and ROM for the ProDisc-L and Simulated Implant cases revealed that the two conditions were not comparable. Transfer of tissue pretension from the implant to the Spine Robot on removal of the implant, and dynamic contact forces at the implant surfaces were the contributing factors to the differences observed. Simulated COR testing demonstrated that the posterior tissue response was sensitive to varying placements of the simulated implant. For both implant sizes, posterior positioning of the COR required distraction of the disc space. During flexion, posterior positioning resulted in significantly higher shear and axial forces as well as a trend for reduced ROM. ROM in flexion may have been influenced by different starting positions within the neutral zone due to disc space distraction. During extension, the posterior placement of the COR reduced loading and increased rotation suggesting better alignment with, or separation of the facet joints. This novel study was able to delineate significant differences in spinal tissue response to varying simulated sizes and placements of an ideal fixed COR TDR device. The results of this study suggested that with both implant sizes the posterior placement of the COR will tend to distract the disc space and provide significantly increased ROM in extension at the expense of increased loads on posterior ligaments in flexion

A Kucha\v{r} Hypertime Formalism For Cylindrically Symmetric Spacetimes With Interacting Scalar Fields

The Kucha\v{r} canonical transformation for vacuum geometrodynamics in the presence of cylindrical symmetry is applied to a general non-vacuum case. The resulting constraints are highly non-linear and non-local in the momenta conjugate to the Kucha\v{r} embedding variables. However, it is demonstrated that the constraints can be solved for these momenta and thus the dynamics of cylindrically symmetric models can be cast in a form suitable for the construction of a hypertime functional Schr\"odinger equation.Comment: 5 pages, LaTeX, UBCTP-93-02

Application of multireflection grazing incidence method for stress measurements in polished Al–Mg alloy and CrN coating

Multi-reflection grazing incidence geometry, referred to as MGIXD, characterized by a small and constant incidence angle, was applied to measure low surface stresses in very thin layers of Al–Mg alloy and CrN coating. These two materials were selected in order to deal with the low and high levels of residual stress, respectively. The influence of different mechanical treatments on residual stresses was studied for Al–Mg samples. It was found that both rolling and mechanical polishing influence the distribution and amplitude of residual stress in surface layers. In the case of CrN coating, a very high compressive stress was generated during the deposition process. The stress distributions determined by the MGIXD method is in good agreement with the classic sin2 technique results for all studied samples. In performing stress measurements for a powder sample, it was found that the application of the Göbel mirror in the incident beam strongly reduces statistical and misalignment errors. Additionally, the root mean square values of the third order lattice strain within diffracting grains were determined

Wave-breaking-free passively mode-locked fiber laser using a hybrid regime of oscillation

We demonstrate the possibility of generating wave-breaking-free optical pulses from a mode-locked fiber ring laser coupled to a long passive resonator. By adjusting the delay line length to match the temporal separation of two consecutive spiking pulses, we have successfully obtained large stable pulses in the microsecond range for any available pump power. Our experimental results demonstrate a new method for energy scaling in passively mode-locked fiber lasers

Application of multireflection grazing incidence method for stress measurements in polished Al–Mg alloy and CrN coating

Multi-reflection grazing incidence geometry, referred to as MGIXD, characterized by a small and constant incidence angle, was applied to measure low surface stresses in very thin layers of Al–Mg alloy and CrN coating. These two materials were selected in order to deal with the low and high levels of residual stress, respectively. The influence of different mechanical treatments on residual stresses was studied for Al–Mg samples. It was found that both rolling and mechanical polishing influence the distribution and amplitude of residual stress in surface layers. In the case of CrN coating, a very high compressive stress was generated during the deposition process. The stress distributions determined by the MGIXD method is in good agreement with the classic sin2 technique results for all studied samples. In performing stress measurements for a powder sample, it was found that the application of the Göbel mirror in the incident beam strongly reduces statistical and misalignment errors. Additionally, the root mean square values of the third order lattice strain within diffracting grains were determined

Using 3D Stringy Gravity to Understand the Thurston Conjecture

We present a string inspired 3D Euclidean field theory as the starting point for a modified Ricci flow analysis of the Thurston conjecture. In addition to the metric, the theory contains a dilaton, an antisymmetric tensor field and a Maxwell-Chern Simons field. For constant dilaton, the theory appears to obey a Birkhoff theorem which allows only nine possible classes of solutions, depending on the signs of the parameters in the action. Eight of these correspond to the eight Thurston geometries, while the ninth describes the metric of a squashed three sphere. It therefore appears that one can construct modified Ricci flow equations in which the topology of the geometry is encoded in the parameters of an underlying field theory.Comment: 17 pages, Late

Multireflection grazing incidence diffraction used for stress measurementsin surface layers

The geometry based on the multireflection grazing incidence X-ray diffraction can be applied to measure residual stresses. Using this method, it is possible to perform a non-destructive analysis of the heterogeneous stresses for different and well defined volumes below the surface of the sample (range of several μm). As the result, the average values of stresses weighted by absorption of X-ray radiation are measured. In this work the stress profiles as a function of penetration depth were determined for mechanically polished Al sample. Measurements and verification of the method were performed using classical X-ray diffractometer and synchrotron radiation with different wavelengths

Cracking in asphalt materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.Peer ReviewedPostprint (author's final draft

Elastoplastic deformation and damage process in duplex stainless steels studied using synchrotron and neutron diffractions in comparison with a self-consistent model

In situ time of flight neutron diffraction and X-ray synchrotron diffraction methods were applied to measure lattice strains in duplex steels during a tensile test. The experimental results were used to study slips on crystallographic planes and the mechanical effects of damage occurring during plastic deformation. For this purpose the prediction of an elastoplastic self-consistent model was compared with the experimental data. The used methodology allowed to determine the elastic limits and parameters describing work hardening in both phases of studied polycrystalline materials. In the second part of this work the developed elastoplastic model was applied to study damage occurring in the ferritic phase. The theoretical results showed a significant reduction of stresses localized in the damaged phase (ferrite) and confirmed the evolution of the lattice strains measured in the ferritic and austenitic phases