Angle Mismatches Research Articles

Does the interfacing angle between pedicle screws and support rods affect clinical outcomes after posterior thoracolumbar fusion? A retrospective clinical study

BACKGROUND CONTEXTProper alignment and tightening of the pedicle screw/rod assembly after instrumented posterior fusion of the lower spine is known to be crucial in order to achieve satisfactory clinical results. Such interfacing angle mismatches indicate stress overloading of the implant system. PURPOSEThe objective of this study is to investigate the incidence of postoperative screw/rod interfacing angle mismatch and to analyze the impact of mismatches on clinical outcome in terms of (1) revision surgery, (2) adjacent segment degeneration (ASD), and (3) pain. STUDY DESIGNThis is a monocentric retrospective observational study. PATIENT SAMPLEPatients underwent fusion surgery with pedicle screw/rod systems for predominantly degenerative pathologies. OUTCOME MEASURESPedicle screw/rod interfacing angle mismatch (mismatch is the angular deviation from 90° formed by the rod axis and the pedicle screw head axis as an indicator for missing form-fit) revision rate, ASD at the immediately adjacent cranial segment and VAS pain. METHODSRevision refers to subsequent procedures in which all or part of the original implant configuration is changed or removed. Radiographic parameters are evaluated using a/p and lateral radiographs at final follow-up. The interfacing angle mismatch between pedicle screw and rod is measured as the angle between two parallel lines on either side of each pedicle screw head and a line laterally along the associated rod. Multiple comparisons are counteracted by Bonferroni correction, adjusted significance level is at *p<.01. RESULTSPedicle screw and rod interfacing angle mismatch was found in 171/406 (42.1%) of patients undergoing fusion surgery, affecting 613/3016 (20.3%) screws. The overall revision incidence was 11.8% (48/406), and a new ASD occurred in 12.1% of all patients (49/406) with an average follow-up of 5 years. Mean VAS pain score at final follow-up was 2.0. Comparison of the two groups with and without mismatches revealed statistically significantly higher (1) numbers of revision procedures performed (26.9% vs 0.9%), (2) numbers of new ASD developed (27.5% vs 3.8%), and (3) higher VAS pain scores (2.8/10 vs 1.4/10) for cases with mismatch. When comparing patients who underwent intraoperative correction and/or reduction with those who did not, statistically significant more screw mismatches (63.4% vs 39.7%) and revision surgeries (29.3% vs 9.9%) were noted in patients who had these forceful maneuvers. CONCLUSIONSPedicle screw/rod interfacing angle mismatch is a frequent occurrence after fusion surgery. Mismatches indicate that the construct was assembled under mechanical stress. All preventable mechanical stresses, for example, unintentional uncontrolled forces on the instrumentation, should be avoided as much as possible, as they can negatively influence the clinical outcome.

The effect of manufacturing tolerances on the mechanical environment of taper junctions in modular TKR

Taper design is known to influence corrosive behavior in taper junctions used in modular orthopaedic devices. Manufacturing tolerance of bore-cone tapers is a critical design parameter due to the effect on taper fit, but the effect of variations in manufacturing tolerance on the mechanics of taper junctions has not been well characterized, particularly in modular total knee replacement (TKR). The purpose of this study was to investigate the effect of manufacturing tolerance on stress and micromotion of modular TKR taper junctions. A 3D finite element (FE) model of a modular TKR taper junction was developed and assigned elastoplastic material properties. Model taper geometry was varied by perturbing the angle mismatch by 0.05° between ±0.25° and represented expected variation in manufacturing tolerance. Stress and micromotion were calculated during dynamic FE simulations for each taper junction geometry under varying activity loads and material combinations. Although an increase in angle mismatch generally resulted in higher stress and micromotion, plastic material behavior disrupted this trend for larger angle mismatches. Model predictions corresponded with corrosion behavior evident in vitro. If the FE results obtained here apply in vivo, the absence of elastoplastic material properties in a taper model may grossly overestimate the micromotion and underestimate corrosion behavior and ion release. It is recommended that manufacturing tolerances of bore-cone tapers in modular TKR designs should produce angle mismatches within 0.1° at the taper junction.

Wave-propagation simulation and dark-field computed tomography imaging study to elucidate the contrast-loss problem in X-ray diffraction-based transcranial brain imaging

It is crucial to overcome the skull-based contrast loss of cerebral structures in the development of high-resolution X-ray phase-contrast transcranial brain imaging. The present study aimed to determine the consequence of microporous skull-based scattering and its correlation with the contrast loss in a dark-field imaging-computed tomography transcranial brain imaging setup and to suggest methods for visualizing interior structures in analyzer-based X-ray transcranial brain CT imaging. Virtual-skull brain phantoms that mimic the microporous structure of the skull and internal structures including gray/white matter, glioma edema, and the central necrosis of glioma were prepared for wave-propagation simulation of dark-field computed tomography imaging. Reconstruction of refraction angle images and sinogram analysis of angle deviation showed microporous skull-mediated increases in the right-left angle mismatch and out-of-range values in curve fitting with a rocking curve, which was correlated with the contrast loss for internal structures. Correcting the angle mismatch and artificially widening the rocking curve both restored contrast to the internal structures in the reconstructed images. In conclusion, the contrast loss in analyzer-based X-ray diffraction for transcranial brain imaging was related to increases in microporous skull-mediated angle mismatches and out-of-range values when calculating refraction angles using the analyzer rocking curve. This contrast loss may be resolved by using refraction angle measurements from an analyzer crystal with a wider rocking curve or by using an algorithm such as a neural network that can distinguish the left/right refraction angle for a given intensity in a rocking curve.

Synchronization of Multiple Pulsed Alternators Discharging Into an EM Launcher

As the energy level in the projectile increases, it becomes necessary to use multiple pulsed alternators discharging in parallel into the electromagnetic launcher. Another reason for having more than one machine is that one needs to compensate for the torque and gyroscopic effects of the pulsed alternator. This requires that machines be built in counter-rotating pairs. These machines are identical in all respects except for their direction of rotation. A study was conducted to determine how the machines can be motored so that they stay in lock step in speed and phase as they are motored to full speed. The methods of connecting these multiple machines are discussed. The aim of the connection scheme is to allow the machines to naturally stay locked in speed and phase throughout its operating range. Sensitivity of the performance of these machines to small variations in the machine parameters, which is to be expected in the machines, is discussed. Sensitivity of the discharge performance to small phase angle mismatches due to tolerances is also discussed. To verify the conclusions of the paper, an experiment was performed on two identical 50-kVA machines discharging into a low impedance load. The motoring system that is discussed in this paper was implemented in this experiment, which kept the machines in lock step. This motoring system is described. Thereafter, discharges were made at various speeds and field current levels. Phase angle mismatches were introduced between the two machines to see how it affected current sharing. The results and conclusions of these tests are presented in this paper.

Norm-Constrained Capon Beamforming Using Multirank Signal Models With Kalman Filter Implementation

The performance of the Capon beamforming is known to degrade dramatically in the presence of model mismatches, especially when the desired signal is present in the training data. To improve the robustness, the diagonal loading technique was introduced. However, the major drawback is the selection of the diagonal loading level, which is related to the unknown signal powers. Another way to alleviate performance degradation is to use a more accurate signal model of the array response. In this paper, a norm-constrained Capon beamforming using multirank signal models is proposed. Based on the pseudo-observation method, the quadratic constraints can be easily constructed. The problem is solved by a nonlinear Kalman filter which can be implemented on-line. The simulation results show that the design of the norm-constraint value is less sensitive to the signal powers, small angle mismatches, and number of sensors with a standard linear array. Further, it is shown that the use of a multirank signal model and Kalman filter technique result in less self-cancellation and performance degradation than that of the rank-1 signal model and the estimation of sample matrix.

MIMO Space-Time Adaptive Processing with Uncertainty in the Knowledge of Target Parameters

In this article, colocated multiple-input multiple-output tracking radar is studied; such radar needs robust space-time adaptive processing against target azimuth angle and Doppler frequency mismatches at the beginning of the tracking procedure, and sometime after the tracking begins, higher spatial resolution is required to track the target accurately. First, the target, noise, jammers and clutter signals considering internal clutter motion and covariance matrices are modeled. Then, by imposing two quadratic constraints, such that the STAP response exceeds unity, on a squared surface, and also using Karush-Kuhn-Tucker conditions, the closed-form solution for robust STAP is derived. To suppress the signal cancellation effect, the iterative diagonally loaded method is employed. Simulation results are presented to demonstrate the signal to interference pulse noise ratio performance of the proposed robust MIMO STAP. The results show that MIMO radar has better spatial resolution as well as better signal to interference pulse noise ratio performance in low-velocity target detection. KeywordsClutter covariance matrix; Karush-Kuhn-Tucker (KKT) conditions; Multiple-input multiple-output (MIMO) radar; Robust optimization; Space-time adaptive processing (STAP)

Dependence of image quality on geometric factors in breast tomosynthesis

Accurate and precise knowledge of the geometric relationships between the physical components (x-ray source, pivot point, and elements of the x-ray detector) critically influences the quality of reconstructed images in digital breast tomosynthesis (DBT). The sensitivity of image reconstruction to geometric inaccuracies is investigated by simulation of image formation and reconstruction for a DBT system. A mathematical simulation of a partial isocentric system is described. A block "phantom" containing small calcific particles is used to evaluate the effect of three linear and three angular parameters on localization of structures within the reconstructed image and on lesion contrast. Two types of geometric errors are studied: fixed offset inaccuracies and random interprojection inaccuracies in the context of a filtered back projection reconstruction algorithm. It is shown that, in general, fixed offset errors lead to little degradation of image quality. However, a lack of precision in interprojection geometric parameters can cause a loss in lesion contrast and introduce artifacts. For example, projection mismatches of the gantry angle of 0.14 degrees (standard deviation) can reduce reconstructed lesion intensity by 20%. Reconstruction is particularly sensitive to detector yaw angle mismatches; even small fixed offset errors (0.31 degrees) in detector yaw can reduce lesion intensity by 20%. Interprojection variations in geometric parameters can also cause localization errors. For example, if detector yaw variations between projections occur and these are not accounted for, a standard deviation of 0.34 degrees can be expected to induce 1 mm root-mean-square error shift in lesion location. In a simulation of image acquisition in DBT, the sensitivities in image quality to six geometric parameters were evaluated. Image reconstructions are relatively tolerant of fixed offset errors except for detector yaw. However, uncorrected variations in interprojection geometric parameters induce losses in lesion contrast and localization. Lesion contrast is affected more strongly by these errors compared to lesion localization in tomosynthesis.

Ultracold fermions in a graphene-type optical lattice

Some important features of the graphene physics can be reproduced by loading ultracold fermionic atoms in a two-dimensional optical lattice with honeycomb symmetry and we address here its experimental feasibility. We analyze in great detail the optical lattice generated by the coherent superposition of three coplanar running laser waves with respective angles $2\ensuremath{\pi}/3$. The corresponding band structure displays Dirac cones located at the corners of the Brillouin zone and close to half-filling this system is well described by massless Dirac fermions. We characterize their properties by accurately deriving the nearest-neighbor hopping parameter ${t}_{0}$ as a function of the optical lattice parameters. Our semiclassical instanton method proves in excellent agreement with an exact numerical diagonalization of the full Hamilton operator in the tight-binding regime. We conclude that the temperature range needed to access the Dirac fermions regime is within experimental reach. We also analyze imperfections in the laser configuration as they lead to optical lattice distortions which affect the Dirac fermions. We show that the Dirac cones do survive up to some critical intensity or angle mismatches which are easily controlled in actual experiments. In the tight-binding regime, we predict, and numerically confirm, that these critical mismatches are inversely proportional to the square root of the optical potential strength. We also briefly discuss the interesting possibility of fine tuning the mass of the Dirac fermions by controlling the laser phase in an optical lattice generated by the incoherent superposition of three coplanar independent standing waves with respective angles $2\ensuremath{\pi}/3$.

Multiple scattering from assemblies of dislocation walls in three dimensions. Application to propagation in polycrystals

The attenuation of ultrasound in polycrystalline materials is modeled with grain boundaries considered as arrays of dislocation segments, a model valid for low angle mismatches. The polycrystal is thus studied as a continuous medium containing many dislocation "walls" of finite size randomly placed and oriented. Wave attenuation is blamed on the scattering by such objects, an effect that is studied using a multiple scattering formalism. This scattering also renormalizes the speed of sound, an effect that is also calculated. At low frequencies, meaning wavelengths that are long compared to grain boundary size, then attenuation is found to scale with frequency following a law that is a linear combination of quadratic and quartic terms, in agreement with the results of recent experiments performed in copper [Zhang et al., J. Acoust. Soc. Am. 116(1), 109-116 (2004)]. The prefactor of the quartic term can be obtained with reasonable values for the material under study, without adjustable parameters. The prefactor of the quadratic term can be fit assuming that the drag on the dynamics of the dislocations making up the wall is one to two orders of magnitude smaller than the value usually accepted for isolated dislocations. The quartic contribution is compared with the effect of the changes in the elastic constants from grain to grain that is usually considered as the source of attenuation in polycrystals. A complete model should include this scattering as well.

Single-mode fibre coupler with the 3-dB splitting ratio simultaneously at 0.83 and 1.3 μm

The problem of designing a two-wave single-mode-fibre 3-dB coupler is considered. The method is proposed for adjusting the parameters of the coupler for simultaneous use of radiation at 0.83 and 1.3 μm in one fibre interferometer. The dependence of the coupling efficiency on the wavelength is studied experimentally for different angle mismatches of fibres in the coupler. The results of experiments and numerical calculations are presented.