Mean Bond Angles Research Articles

TO STUDY THE EFFECT OF COMBINED ANTEVERSION IN TOTAL HIP REPLACEMENT

Introduction: In normal day-to-day life, the hip joint is the type of joint that undergoes a lot of stress during a persons normal daily activities. Since the hip joint is a vital joint that supports all joints in the body, normal and proper functioning of this joint is essential for maintaining a pain-free daily life.The overall success rate of total hip arthroplasty is largely determined by its ability to maintain hip stability and mobility while reducing pain in associated hip pathologies. The total joint replacement situation is in a state of constant development. Phillip Wiles was the first total hip replacement surgeon in London in 19381. Accurate positioning of the femoral and acetabular components is considered a prerequisite for successful total hip arthroplasty4-8. The optimal position of the stem and cup is an average anteversion angle of approximately 37 degrees (range: 25 to 50 ◦) 9-11 to avoid impingement and/or dislocation. Cup-to-stem or bone-to-bone impingement causes dislocation, accelerated wear, and pain in patients undergoing total hip replacement 12. McKibbin 13 first introduced the term in a study of infant cadavers and defined a total anteversion of 30 to 40 with his 15 anteversion of the femur as normal. Lewinnek8 defined his zone of safe cup alignment as his tilt of 40° ± 10° and his anteversion (AV) of 15° ± 10°.The Ranawat test is a visual assessment of compound anteversion when the femoral neck and head are flush with the acetabular opening. The angle of internal rotation to make the head and socket coplanar is combined anteversion.14 Therefore, dislocation is an important and critical complication which has to be avoided. In our study we are calculating combined version in total hip arthroplasty which will helpful in assessing the cup position post operatively with this we can assess the relation of stability of hip and functional outcome. Getting combined anteversion in safe zone of 40◦ ± 10◦ is important for surgeon to give patients a comfortable daily activities of living. Result: In our study, the mean total score was 90.10 ± 7.59in the postoperative 6 months period. Postoperative functional outcomes by Harris Hip Score at the end of the 6 months follow-up study were excellent in 17 patients (56.7%) and good in 11 patients(36.7%). Majority of the patients had good to excellent Harris Hip Score at final outcome with mean combined version 25-50%. Conculsion:The outcome of total hip arthroplasty performed depends on several factors, including component design, patient selection, mean bond angle, and surgical technique, determined by the outcome of the procedure should be evaluated in long-term studies. Our study suggests a favorable prognosis and outcome in the 20–60-year-old group with a mean age of 33.4. Of the 30 patients tested, 29 had a favorable postoperative outcome with normal range of combined version whose functional outcome is measured by the Harris Hip Score. The prosthesiss success and longevity depend on the restoration of the hip joints biomechanics .The goal was always to get the center of rotation back, limb length, medial and vertical offsets. Although this study was not without complications, the overall functional and clinical outcomes were excellent. Our study will require further follow-up and new patients as we were unable to obtain concrete results due to the small sample size.

Molecular dynamics simulations of cylindrical polyelectrolyte brushes in monovalent and multivalent salt solutions

Molecular dynamics simulations are applied to investigate the cylindrical polyelectrolyte brushes in monovalent and multivalent salt solutions. By varying the salt valence and concentration, the brush thickness, shape factor of grafted chains, and distributions of monomers and ions in the solutions are studied. The simulation results show that the single osmotic pressure effect in the brush leads to changes in conformation in the presence of monovalent salt, while the ion exchange effect induces the collapse of the brushes in the multivalent salt solutions. Furthermore, the snapshots combined with the distributions of the end-monomers and the mean bond angles demonstrate a nonuniform stretching picture of the grafted chains, which is different with the chains tethered on the planar surface. The charge ratios between the ions trapped in the brush and the monomers are also calculated to elucidate the details of ion exchange process.

Borosilicate glass potentials for radiation damage simulations

Three borosilicate glass (SiO2–B2O3) fixed charge potentials from the literature are compared (Delaye and Ghaleb, 1996; Kieu et al., 2011; Rushton, 2006) and their suitability for use in simulations of radiation damage is assessed.For a range of densities, we generate glass structures by quenching at 5×1012K/s using constant volume Molecular Dynamics. In each case, the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds are calculated, and where possible compared to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli and densities that are higher than experimental data.The displacement energy thresholds are generally lower than those for ionic crystalline materials, but show a wider spread of values. However, the barriers for atomic rearrangements, after atoms have been displaced in the equilibrium structures, are very high. This indicates, that the radiation damage produced in the ballistic phase of a collision cascade, is likely to persist for extended time scales. This is in contrast to crystals, where interstitials and vacancies can diffuse rapidly between successive radiation events.

Structure, molecular dynamics, and stress in a linear polymer

We present a study correlating uniaxial stress in a polymer with its underlying structure when it is strained. The uniaxial stress is significantly influenced by the mean-square bond length and mean bond angle. In contrast, the size and shape of the polymer, typically represented by the end-to-end length, mass ratio, and radius of gyration, contribute negligibly. Among externally set control variables, density and polymer chain length play a critical role in influencing the anisotropic uniaxial stress. Short chain polymers more or less behave like rigid molecules. Temperature and rate of loading, in the range considered, have a very mild effect on the uniaxial stress.

THE CORRELATION BETWEEN BOND ANGLE DISTRIBUTION AND THE COORDINATION OF SILICA LIQUID UNDER PRESSURE

Molecular dynamics simulation is carried out for liquid SiO 2 at pressure ranged from zero to 30 GPa and by using BKS, Born–Mayer type and Morse–Stretch potentials. The constructed models reproduce well the experimental data in terms of mean coordination number, bond angle and pair radial distribution function. Furthermore, the density of all samples can be expressed by a linear function of fractions SiO x. It is found that the topology of units SiO x and linkages OSi y is unchanged upon compression although the liquid undergoes substantial change in its network structure. Consequently, the partial bond angle distribution for SiO x and OSi y is identical for all samples constructed by the same potential. This result allows to establishing a simple expression between total bond angle distribution (BAD) and fraction of SiO x and OSi y. The simulation shows a good agreement between the calculation and simulation results for both total O–Si–O and Si–O–Si BADs. This supports a technique to estimate amount of units SiO x and linkages OSi y on base of total Si–O–Si and O–Si–O BADs measured experimentally.

Thermally Induced Structural Modification of Silica Nanoparticles Investigated by Raman and Infrared Absorption Spectroscopies

We report an experimental investigation by Raman and infrared (IR) absorption spectroscopies on the structural modifications induced by isochronal thermal treatments on amorphous SiO2 nanoparticles (fumed silica). In particular, three different commercial types of this material, characterized by particle mean diameters of 7, 14, and 40 nm, were subjected to thermal treatments from 100 up to 1000 °C. We found that some properties of fumed silica, such as the SiOSi mean bond angle, ring size distribution, and surface adsorbed water content, are drastically different from those of common bulk silica materials and intimately related to the particles’ dimension. The SiOSi mean bond angle, probed by the main Raman line peaked at about 440 cm−1, is modified by thermal treatments above 400 °C and tends toward typical values of bulk silica materials, whereas the three-membered ring population, probed by the D2 line peaked at about 600 cm−1, changes but does not reach bulk silica features. The surface adsorbed wate...

Elastic behavior of ring polymer chains

AbstractIn this article, the conformational properties and elastic behaviors of ring polymers in the process of tensile elongation are investigated with the Monte Carlo method and the bond fluctuation model. The ratio of the mean‐square diameter <d2> to the mean‐square radius of gyration <S2> increases with the elongation ratio, λ, and the instantaneous shape of ring polymers is more symmetric than that of linear chains in the process of tensile elongation. Here <d2> for ring polymers rather than the mean‐square end‐to‐end distance <R2> for linear polymers is defined as the average of squared distances between two segments separated by N/2 bonds, where N represents the total number of bonds. Local quantities, that is, the mean‐square bond length <b2> and the mean bond angle <θ> increase with λ, especially for short ring chains. The <d2> and <S2> have the same relationship with the chain length, N, that is, <d2> ∼ N1.130±0.020 and <S2> ∼ N1.160±0.013 for a different λ. Some thermodynamics properties are also addressed here. The average energy per bond <U> decreases with λ and the average Helmholtz free energy and elastic force f increase with λ, especially for short ring chains. Comparisons with linear chains are also made. These investigations may provide insight into the elastic behaviors of ring polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 223–232, 2005

Structure and phase transition of a two-dimensional dusty plasma

The structure and phase transition of a two-dimensional (2D) dusty plasma have been investigated in detail by molecular dynamics simulation. Pair correlation function, static structure factor, mean square displacement and bond angle correlation function have been calculated to characterize the structural properties. The variation of internal energy, shear modulus, particle trajectories and structural properties with temperature has been monitored to study the phase transition of the 2D dusty plasma system. The simulation results are in favour of a two-step continuous transition for this kind of plasma.

Transpolyacetylene chains in hydrogenated amorphous carbon films free of nanocrystalline diamond

The microstructure of distributed electron cyclotron resonance plasma-deposited hydrogenated amorphous carbon films (a-C:H) was investigated using electron diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Experimental evidence of the existence of transpolyacetylene (TPA) chains in a-C:H films free of nanocrystalline diamond is presented. The values of the mean bond angles and lengths and first neighbor numbers are consistent with the TPA data. The Raman spectra were fitted using the G and D bands and the bands centered at 1140, 1233, and 1475 cm−1 assigned to TPA chains modes. The relative intensity of the latter decreases while hydrogen content decreases. A significant sp2-CH olefinic mode contribution to the infrared stretching band is observed for the low-density films (∼1.2 g/cm3). TPA chains growth is enhanced when ion current density and energy decrease.

Structural Study on Amorphous Tellurium Using Reverse Monte Carlo Simulation

We calculated the best fitting model of 6002 tellurium atomic random chains using the reverse Monte Carlo (RMC) simulation. The coincidence between the experimental and calculated structure factors is within 4.5%. In this analysis, we adopted a novel convergence since the normal convergence yielded a false homometric structure without triangular molecules. The mean dihedral angle is 95.78°, the mean bond angle of triangular molecules is 98.12°, and the mean bond length of triangular molecules is 2.84 Å. The obtained structure consists of 1320 (21.99%) parallel bonds, and 690 (11.9%) chainlike and ringlike bonds are 690 in number (11.49%), the ratio of which is 2.73/1. Three-fold coordinated atoms make up 50%. Positional errors for convergent parameters ra's below 0.125 were less than 0.03%. The obtained model gave the same structural parameters as the above convergence.

Monte Carlo Simulation Studies of the Size and Shape of Ring Polymers

The present work gives a comparison between the properties of athermal ring polymers and those of linear chains. Based on bond fluctuation (BF) and pivot algorithm (PIV) for the construction of molecules, a new algorithm was developed, which proved most efficient due to large acceptance fractions and small (integral) auto-correlation times of global properties, in addition having the advantage of a large set of different bond vectors. While the topological state of ring polymers remains unchanged by exclusive use of BF, knotted structures (which were identified with the help of Alexander polynomials) can by formed and removed by the use of PIV. In accordance with previous work, it turned out that the probability of unknotted rings (in principle) exponentially decreases with an increasing number of segments, however, so slowly that the appearance of knotted structures (ca. 0.1% for N = 512) is a rare event in the range of chain-lengths evaluated (N = 32 – 8192). The chain-length dependence of global quantities of ring polymers are described by the use of scaling relations with proper short chain corrections, in analogy to linear chains. The instantaneous shape of ring polymers is more symmetric than that of linear chains. Local quantities, i.e., mean squared bond lengths and mean bond angles are the same for both systems, at least in the limit of an infinite number of segments.

Reverse Monte Carlo study of structural relaxation in vitreous selenium

Vitreous selenium is used extensively for light detection. Its photoconducting properties are altered by structural relaxation near room temperature. X-ray diffraction, densiometry, and calorimetry are used to determine the change of the glass state during relaxation. Annealing for 24 h at 300 K increases the density by 0.14% and lowers the enthalpy by 105 J/mol. The structure factor of cast samples was measured before and after annealing, and reverse Monte Carlo models of the structure of selenium glass were generated to match the data for the quenched and annealed states. Atoms in amorphous selenium are arranged in randomly oriented chains. In the model clusters annealing effects are found by analyzing inter- and intrachain distances, bond angle distributions, and dihedral angle distributions. The average bond length remains unchanged upon annealing, while the distribution of bond lengths becomes narrower by 1.2%. Distances between atoms in adjacent chains decrease by about 0.03%, i.e., the selenium chains move closer together, and the distribution of interchain distances becomes narrower. Bond angles within the chains are affected slightly. The mean bond angle decreases by 0.016°, which indicates a greater folding of the selenium chains and represents an evolution towards the angles found in crystalline selenium phases. The bond angle distribution for the annealed state is 0.006° narrower.

Raman spectra and structure of amorphous Si

In 1985, Beeman, Tsu, and Thorpe established an almost linear relation between the Raman transverse-optic (TO) peak width \ensuremath{\Gamma} and the spread in mean bond angle \ensuremath{\Delta}\ensuremath{\theta} in $a\ensuremath{-}\mathrm{Si}.$ This relation is often used to estimate the latter quantity in experiments. In the last decade, there has been significant progress in the computer generation of sample networks of amorphous silicon. Exploiting this progress, this paper presents a more accurate determination of the relation between \ensuremath{\Gamma} and \ensuremath{\Delta}\ensuremath{\theta} using 1000-atom configurations. Also investigated and quantified are the relations between the TO peak frequency and the ratio of the intensities of the transverse-acoustic (TA) and TO peaks, both as functions of \ensuremath{\Delta}\ensuremath{\theta}. As \ensuremath{\Delta}\ensuremath{\theta} decreases, the TA/TO intensity ratio decreases and the TO peak frequency increases. These relations offer additional ways to obtain structural information on $a\ensuremath{-}\mathrm{Si}$ from Raman measurements.

Scaling and labeling the high-resolution isotropic axis of two-dimensional multiple-quantum magic-angle-spinning spectra of half-integer quadrupole spins

(obs) ! of the center of gravity of an MQ-filtered central-transition peak in the two dimensions allows us to determine the true isotropic chemical shift of an absorption line, which is related to the mean bond angle in a compound. Only the isotropic chemical shift and the second-order quadrupole interaction for a sample rotating at the magic angle at a high spinning rate are considered during the free precession of the spin system. On the other hand, only the first-order quadrupole interaction for a static sample is considered during the pulses. The hypercomplex detection method is used to obtain a pure 2D absorption spectrum. The pulse program and the successive stages of data processing are described. For simplicity, only the density matrix for a spin I5 3 at the end of the first pulse of phase w is calculated in detail, which allows us to deduce the phase cycling of the pulse sequence that selectively detects the 63-quantum coherences generated by the first pulse. The positions of the echo and antiecho relative to the second pulse, and that of the MQ-filtered central-transition peak relative to the carrier frequency (v 0) along the F1 dimension are derived for the four half-integer quadrupole spins. The frequency offset of v 0 relative to an external aqueous solution in the F1 dimension is linearly related to that in the F2 dimension. The shearing transformation, whose main interest is to shift the beginning of the acquisition period from the end of the second pulse to the echo position and to yield a high-resolution spectrum along the F1 dimension, is presented in great detail. In the literature, two conventions are used to define the evolution period. Convention Cz takes the experimental evolution period as the evolution period, whereas convention Ck considers the echo position relative to the first pulse as the evolution period. The true isotropic chemical shift, the second-order quadrupole shift of the center of gravity of a peak, and the parameter CQh relating the quadrupole coupling constant with the asymmetry parameter are functions of d G1 (obs) and d G2 (obs) ; their relationships depend on the conventions. In fact, convention Ck introduces a scaling factor ~depending on the spin and the coherence order! that modifies not only the spectral width and the linewidth, but also the positions of the spinning sidebands in the F1 dimension. Convention Cz is recommended for linewidth comparison. The analytical expressions are checked using 87 Rb (I5 3 2 ) in the extensively studied RbNO3 powder with two carrier frequencies: 98.2 and 163.6 MHz. Our data are in agreement with those of the literature.

Devitrification of sodium disilicate glass: a NMR study

Several crystallographically non-equivalent structures of sodium disilicate are known to exist. Successive heat treatments of the sodium disilicate glasses show α-, β-, γ- and δ-Na2Si2O5 as the devitrification products, depending upon the nature of heat treatment. At higher temperatures and for longer treatment times, only α-Na2Si2O5 is observed but for intermediate temperature and time, α and β forms of the polymorphs are produced. For shorter times, all the polymorphs are formed. The results are verified using magic angle spinning nuclear magnetic resonance as well as X-ray diffraction. A way of estimating the unknown Si–O–Si mean bond angle of the polymorphs, with the help of the known values of the angles, is also presented.

Structure of bulk amorphous GaSb: A temperature-dependent EXAFS study

We have studied the structure of bulk amorphous GaSb using the extended x-ray-absorption fine-structure (EXAFS) technique over the temperature range from 80 K to 293 K. The possibility of the presence of chemical disorder has been explored throughout the temperature range. The configurational (static) contribution to the Debye-Waller broadening of the distances has been extracted from EXAFS data. The bond-angle distribution for $a$-GaSb was calculated from the results of EXAFS analysis. The mean bond angle has been calculated from Sb $K$-edge data and found to be 108.9\ifmmode^\circ\else\textdegree\fi{}.

Second-order quadrupole effects on Hahn echoes in fast-rotating solids at the magic angle

In solid-state NMR, knowledge of the quadrupole coupling constant and the asymmetry parameter from a powder spectrum allows the determination of the true chemical shift of a line, which can be correlated with the mean bond angle. However, broad powder patterns are distorted by the dead time of the receiver. The common way to recover the lost signals is to apply a Hahn echo sequence. As the quadrupole interaction is a single-spin multiple-energy level interaction, the effects of multiple-quantum ~MQ! transitions during the Hahn echo sequence are still not well understood. We apply the density-operator approach, combined with the order of coherence description, to predicting the echo locations of half-integer quadrupole spins ~I5 3 2, 5 2, 7 2, and 9 2! in a single crystal, the amplitudes of the echoes, and the excitation conditions for obtaining quantitative results on the spin population ratio. For these purposes, the expression of the MQ transition frequency or line shift is derived. Between the two pulses and during the detection period, the rotation of the crystal at the magic angle dramatically reduces the homonuclear magnetic-dipole interaction and completely removes anisotropic spectral broadening due to the heteronuclear magnetic-dipole and the first-order quadrupole interactions; the spin system is submitted to the true isotropic chemical shift and the second-order quadrupole interactions only during these two periods. We treat in a unified way the Hahn echoes @including those involved in MQ-magicangle spinning methodology# of the central transition representing the refocusing of single-quantum and MQ on-resonance coherences generated by the first pulse. During the pulses the crystal is assumed to be static and only the first-order quadrupole inteaction is considered; the echo amplitudes representing the refocusing of MQ coherences do not provide us with quantitative results on the spin population ratio. Only that associated with the refocusing of 1Q coherence gives quantitative results when the two pulse durations are short. Application to powders requires high-speed computer averaging of the echo amplitude versus one of the two pulse durations to extract the quadrupole parameters. @S0163-1829~97!00714-5#

Reverse Monte Carlo modeling of amorphous germanium.

The reverse Monte Carlo (RMC) method has been used to generate a model for the atomic structure of amorphous germanium (a-Ge). Fitting to experimental neutron diffraction data and applying coordination number and ``triplet'' constraints, the positions of 3000 ``atoms'' in a box, with full periodicity, were altered until the associated model structure factor, S(Q), and pair correlation function, g(r), agreed with the analogous experimental data, within the errors. The model generated is then analyzed to obtain coordination number, bond angle, and ring size distributions. These, in turn, are compared to the results obtained from random network models and other RMC studies. The effects of increasing the number density from the experimentally determined bulk value are also investigated. The results are consistent with an atomic structure which has a characteristic disordered tetrahedral network. For the highest density RMC model, the mean bond angle is 109.4\ifmmode^\circ\else\textdegree\fi{} and the average coordination number is 3.49. \textcopyright{} 1996 The American Physical Society.

Computer model of tetrahedral amorphous diamond.

We computer generate a model of amorphous diamond using the Wooten-Weaire method, with fourfold coordination everywhere. We investigate two models: one where four-membered rings are allowed and the other where the four-membered rings are forbidden; each model consisting of 4096 atoms. Starting from the perfect diamond crystalline structure, we first randomize the structure by introducing disorder through random bond switches at a sufficiently high temperature. Subsequently, the temperature is reduced in stages, and the topological and geometrical relaxation of the structure takes place using the Keating potential. After a long annealing process, a random network of comparatively low energy is obtained. We calculate the pair distribution function, mean bond angle, rms angular deviation, rms bond length, rms bond-length deviation, and ring statistics for the final relaxed structures. We minimize the total strain energy by adjusting the density of the sample. We compare our results with similar computer-generated models for amorphous silicon, and with experimental measurement of the structure factor for (predominantly tetrahedral) amorphous carbon.

Neutron diffraction studies of network structure in amorphous phosphorus

Neutron diffraction studies have been made for three samples of amorphous phosphorus. The structure factor data have been transformed to give the real-space correlation function, d(r). All samples were found to contain a small proportion of hydrogen and corrections were made before the data was analysed. The violet form was found to give a typical amorphous pattern which had a sharp main diffraction peak at 1.1 Å−1. On heattreating the sample, this peak increased in intensity but the rest of the pattern was unaltered. The red-brown form exhibited some Bragg peaks, but the overall shape of the pattern was similar to that of the violet material. The d(r) function exhibits a sharp peak at 2.21 Å corresponding to the PP-bondlength and a second peak at 3.45 Å defining a mean bondangle of 102.6° with a spread of 4°. At higher values there is a distinct three-peak characteristic which is not reproduced in current models based on a three-bonded CRN scheme.