Kinetic Monte Carlo simulations for heterogeneous catalysis: Fundamentals, current status, and challenges.

Kinetic Monte Carlo (KMC) simulations in combination with first-principles (1p)-based calculations are rapidly becoming the gold-standard computational framework for bridging the gap between the wide range of length scales and time scales over which heterogeneous catalysis unfolds. 1p-KMC simulations provide accurate insights into reactions over surfaces, a vital step toward the rational design of novel catalysts. In this Perspective, we briefly outline basic principles, computational challenges, successful applications, as well as future directions and opportunities of this promising and ever more popular kinetic modeling approach.

The role of oxygenated species in the catalytic self-coupling of MeOH on O pre-covered Au(111).

The oxidation of alcohols plays a central role in the valorisation of biomass, in particular when performed with a non-toxic oxidant such as O2. Aerobic oxidation of methanol on gold has attracted attention lately and the main steps of its mechanism have been described experimentally. However, the exact role of O and OH on each elementary step and the effect of the interactions between adsorbates are still not completely understood. Here we investigate the mechanism of methanol oxidation to HCOOCH3 and CO2. We use Density Functional Theory (DFT) to assess the energetics of the underlying pathways, and subsequently build lattice kinetic Monte Carlo (kMC) models of increasing complexity, to elucidate the role of different oxygenates. Detailed comparisons of our simulation results with experimental temperature programmed desorption (TPD) spectra enable us to validate the mechanism and identify rate determining steps. Crucially, taking into account dispersion (van der Waals forces) and adsorbate-adsorbate lateral interactions are both important for reproducing the experimental data.

A kinetic Monte Carlo approach to study fluid transport in pore networks.

The mechanism of fluid migration in porous networks continues to attract great interest. Darcy's law (phenomenological continuum theory), which is often used to describe macroscopically fluid flow through a porous material, is thought to fail in nano-channels. Transport through heterogeneous and anisotropic systems, characterized by a broad distribution of pores, occurs via a contribution of different transport mechanisms, all of which need to be accounted for. The situation is likely more complicated when immiscible fluid mixtures are present. To generalize the study of fluid transport through a porous network, we developed a stochastic kinetic Monte Carlo (KMC) model. In our lattice model, the pore network is represented as a set of connected finite volumes (voxels), and transport is simulated as a random walk of molecules, which "hop" from voxel to voxel. We simulated fluid transport along an effectively 1D pore and we compared the results to those expected by solving analytically the diffusion equation. The KMC model was then implemented to quantify the transport of methane through hydrated micropores, in which case atomistic molecular dynamic simulation results were reproduced. The model was then used to study flow through pore networks, where it was able to quantify the effect of the pore length and the effect of the network's connectivity. The results are consistent with experiments but also provide additional physical insights. Extension of the model will be useful to better understand fluid transport in shale rocks.

Beyond mean-field approximations for accurate and computationally efficient models of on-lattice chemical kinetics.

Modeling the kinetics of surface catalyzed reactions is essential for the design of reactors and chemical processes. The majority of microkinetic models employ mean-field approximations, which lead to an approximate description of catalytic kinetics by assuming spatially uncorrelated adsorbates. On the other hand, kinetic Monte Carlo (KMC) methods provide a discrete-space continuous-time stochastic formulation that enables an accurate treatment of spatial correlations in the adlayer, but at a significant computation cost. In this work, we use the so-called cluster mean-field approach to develop higher order approximations that systematically increase the accuracy of kinetic models by treating spatial correlations at a progressively higher level of detail. We further demonstrate our approach on a reduced model for NO oxidation incorporating first nearest-neighbor lateral interactions and construct a sequence of approximations of increasingly higher accuracy, which we compare with KMC and mean-field. The latter is found to perform rather poorly, overestimating the turnover frequency by several orders of magnitude for this system. On the other hand, our approximations, while more computationally intense than the traditional mean-field treatment, still achieve tremendous computational savings compared to KMC simulations, thereby opening the way for employing them in multiscale modeling frameworks.

The possible role of locally produced cytokines in the pathogenesis of peritrochanteric fractures in the elderly.

Sixteen patients with intertrochanteric femoral fractures were studied for possible involvement of the cytokines interleukin-6 (IL-6), interleukin-1 beta (IL-1beta), tumor necrosis factor-alpha, and the disease activity factors C-reactive protein and alpha1-antitrypsin as local bone-resorbing agents. Cytokine and disease activity factor levels were measured in gluteus medius muscle and serum samples and were compared to sera obtained from age- and sex-matched healthy controls. Interleukin-6 and IL-1beta levels were significantly higher (P=.0024 and P=.036, respectively) in the muscle samples from the fractured side than in the samples from the contralateral unaffected side. Levels of IL-6 and IL-1beta also were significantly higher in patients' sera than in the sera of healthy controls. These results support a new hypothesis that may contribute to the pathogenesis of fractures in the elderly: unilaterally locally over-produced IL-6 and IL-1beta may lead to local bone resorption in the intertrochanteric region, which subsequently weakens the femoral bone and increases the risk of unilateral peritrochanteric fractures.

Treatment of dysplastic and congenitally dislocated hips with the Zweymueller total hip prosthesis.

Uncemented Zweymueller total hip prostheses were implanted in 35 dysplastic or dislocated hips in 33 patients. Sixteen hips were dislocated and 19 hips were dysplastic; in 12 hips, an intertrochanteric or pelvic osteotomy was performed in early childhood. In all cases, the titanium screw socket was implanted at the level of the original cotyloid cavity. Osteotomy of the greater trochanter, shortening osteotomy, or roof acetabuloplasty were not performed. In cases in which the femoral cavity was too narrow for the Zweymueller stem, an anterolateral longitudinal window-shaped osteotomy was performed. In cases of severe dysplasia, cotyloid cavity bone grafts from the resected femoral head were placed medially to reinforce the acetabular bottom. Clinical and radiographic follow-up ranged from 3-8 years. Average Harris hip score improved from 47 points preoperatively to 86.2 points postoperatively. Complications included two primary anterior dislocations, two temporary femoral nerve pareses, and two deep vein thromboses. At longest follow-up evaluation, no revision was indicated in any of the hips. Satisfactory results in this series were attributed to careful patient selection, precise preoperative radiographic planning, and an operative technique that included implantation of the socket at the primary acetabulum and achievement of primary stability using press-fit fixation.

Medium- and long-term results of open reduction and internal fixation for unstable pelvic ring fractures.

Over a 10-year period, 74 patients with unstable pelvic injuries were treated with open reduction and internal fixation. Radiographic and clinical follow-up averaged 71 months (range: 38-141 months). Satisfactory (ie, good and very good) radiographic results were obtained in 90% of patients. Clinical results were superior in patients without associated injuries (P=.05-.001). Most of the complications in this series were due to associated injuries. Sepsis was mostly due to open pelvic injuries and malunion to either lack of patient cooperation or inadequate open reduction and internal fixation. Careful preoperative analysis of the nature of the pelvic injury and selection of the appropriate operative technique for open reduction and internal fixation result in a satisfactory outcome for the majority of operative patients.

Posterior stabilization of unstable sacroiliac injuries with the Texas Scottish Rite Hospital spinal instrumentation.

This article describes a new alternative technique for stabilization of unstable sacroiliac injuries using the Texas Scottish Rite Hospital (TSRH) instrumentation. The TSRH instrumentation was applied in 14 consecutive patients with unstable posterior pelvic ring disruptions. All patients underwent follow-up for an average of 29 months. Clinical results, using the rating system of d'Aubigne, were good in 9 and satisfactory in 5 patients. No pressure sores, infection, significant loss of correction, or hardware failures were observed postoperatively. The use of the TSRH instrumentation for stabilization of sacroiliac injuries offered sufficient stabilization of the posterior pelvic ring and permitted early mobilization without loss of correction.

Clinical, radiographic, and scintigraphic comparison of the mechanical stability of Mueller and Zweymueller total hip prostheses.

Twenty-two cemented Mueller and 25 uncemented Zweymueller total hip arthroplasties (THAs) were performed in the same period in 47 consecutive randomly selected patients with unilateral primary osteoarthritis. Patients were evaluated using the same clinical and radiographic protocol preoperatively; 3, 6, and 12 months postoperatively; and annually thereafter as well as with Tc 99m MDP three-phase bone scintigraphy 3, 6, 12, and 24 months postoperatively. At 11 periprosthetic regions of interest (ROIs) around the stem and socket of each prosthesis, several radiographic parameters and bone scintigram uptake grade were recorded and correlated to determine the significance of early scintigraphic uptake for prediction of the stability of cemented and uncemented THA components over time. Around stable Zweymueller and Mueller sockets, the radionuclide uptake was similar during all periods of observation. Radionuclide uptake around stable Zweymueller stems was significantly more than around stable Mueller stems at the medial femoral cortex 3 and 6 months postoperatively, at the lateral femoral cortex 3-24 months postoperatively, and near the tip of the stem 3 months postoperatively. Radionuclide uptake around stable Mueller stems was significantly more than around Zweymueller stems at the greater trochanter 6 and 12 months postoperatively and at the lesser trochanter 12 months postoperatively. Any significant deviation of radionuclide uptake at any ROI, time, and THA component during the first 2 years postoperatively in combination with radiographic findings of loosening should be considered a serious predictive sign for loosening of a Mueller or Zweymueller THA component.

Medium- and long-term results of high tibial osteotomy for varus gonarthrosis in an agricultural population.

High tibial valgus osteotomy for varus gonarthrosis was performed in 63 consecutive patients in a homogenous agricultural population using two different surgical techniques. Patients were divided into two groups. A two-level Mittelmeier osteotomy was performed in group A patients, and a lateral closed wedge high tibial osteotomy using the AO/ASIF L-plate was performed in group B patients. Operations were performed by two different groups of surgeons. Patients were evaluated postoperatively for correction of knee axis, functional result, subjective impression, and complications. In group A patients, 80% of the operated knees were corrected to the mechanical axis and in group B patients, 82% of the knees were corrected to 6 degrees-10 degrees valgus of the anatomical axis. Ninety percent, 70%, and 54% of group A and 91%, 73%, and 57% of group B patients were rated as satisfactory results at 5, 9, and 12 years postoperatively, respectively; these differences were not statistically significant. One year postoperatively, 91% of group A and 96% of group B patients reported their symptoms had improved. However, patient satisfaction decreased at 5, 7, and 12 years postoperatively, with 91%, 89%, and 66% of group A and 96%, 93%, and 68%, respectively, of group B patients reporting their symptoms had improved; these differences were not statistically significant. Postoperatively, most patients returned to full agricultural activity. Total knee arthroplasty, which was later required in 12% of the knees, was not significantly jeopardized by the previous osteotomy.

Mueller roof reinforcement rings. Medium-term results.

Short term results of cemented acetabular reconstruction with the use of the Mueller reinforcement ring in 30 hips in 29 patients have been reported previously. The indications for reinforcement were primary and postrevision segmental, cavitary, and combined acetabular deficiencies. The current study reports the medium term clinical and radiologic results of 20 of the 29 patients who were surgically treated (18 primary and eight postrevision hips) and who underwent followup with detailed clinical and radiographic analysis within an average of 9 years (range, 7-12 years). In the latest followup, there has been a statistically insignificant decrease in clinical scores compared with those obtained immediately after surgery; the changes probably are a result of the patients' aging. The radiologic scores at the latest followup were lower, although not statistically significant, than those at the short term followup. The causes of the three ring failures that occurred 5, 8, and 8.5 years after surgery and required revision arthroplasty were either acute trauma or tuberculous arthritis. Three additional hips (two primary and one revision) had evidence of loosening in the ring and socket associated with symptomatology and were considered as hips with impending revision. The rate of success of primary implantation was 94% at 7 years, 86% at 10 years, and 86% at 12 years and was not statistically different from that of revision implantation, which was 86% in all three followup periods. In this small series this surgical technique was successful and effective and followed by good medium term clinical and radiographic results in primary and revision implantation in segmental, cavitary, or complex acetabular deficiencies and in osteoporotic or deficient acetabular bone.

Segmental roentgenographic analysis of vertebral inclination on sagittal plane in asymptomatic versus chronic low back pain patients.

Two homogenous groups of 120 volunteers and 120 low back pain (LBP) patients, age range 20-79 years, underwent a prospective roentgenographic segmental vertebral analysis of the thoracic and lumbar spine to compare several roentgenographic parameters useful for planning spine surgery. The following roentgenographic parameters were measured: thoracic kyphosis, lumbar lordosis, sacral inclination, distal lordosis (L4-S1), inclination of each vertebra from T4-S1, and relative vertebral inclination between adjacent vertebrae. Thoracic kyphosis increased (p < 0.0001) and sacral inclination decreased (p < 0.05) with age in the control group. Increased thoracic kyphosis correlated with decreased lumbar lordosis after the sixth decade in the control group (p < 0.01), less so in the LBP group (p < 0.05). Increased thoracic kyphosis was seen more in the LBP group than in the controls, but significantly solely in the sixth decade (p < 0.01). Lumbar lordosis was more increased in the controls than in the LBP group but significantly solely in the sixth decade (p < 0.001). The L5-S1 segmental lordosis was much more in the LBP patients than in the controls (p < 0.001). Lumbar lordosis was strongly correlated with sacral inclination in both groups (p < 0.0001) and it was significantly greater in the controls, particularly in the sixth decade (p < 0.001). Sacral inclination was significantly more in the female than in male volunteers (p < 0.05). Distal lordosis (L4-S1) represents 55% and 49% of total lumbar lordosis in controls and low back patients, respectively. Spine surgeons frequently deal with sagittal spinal deformities and the deviations of sagittal spinal curvatures and vertebral inclination in the sagittal plane, both in normal subjects and LBP patients should be clinically helpful.

Disintegration performance of renal multivitamin supplements.

OBJECTIVE: Vitamins have traditionally been regulated as dietary supplements and have not been required to meet the same rigorous product quality performance standards as drug products. Impaired product performance, such as failure to disintegrate and/or dissolve in the gastrointestinal tract, could limit the absorption of vitamins. Furthermore, patients with renal disease have been reported to experience a wide range in gastrointestinal pH, which could influence a product's performance. The purpose of this study was to determine the effect of pH on the in vitro disintegration of renal multivitamin supplements. DESIGN: Products were studied using the United States Pharmacopeial Convention standard disintegration apparatus. Products were tested in simulated gastric fluid, neutral fluid, and intestinal fluid. Product failure was defined as two or more individual tablets or capsules failing to disintegrate completely within compendial limits. RESULTS: Of 11 products tested, 4 products failed the disintegration study test in all pH conditions. Sixty-four percent of the products showed statistically significant differences in disintegration time (DT) based on pH. As pH increased, time to disintegration increased. CONCLUSION: The DT of commercially available renal multivitamin supplements was highly variable. Poorest product performance was shown in simulated intestinal fluid. The pH significantly affected in vitro disintegration in greater than half the products tested. How this affects dissolution and in vivo performance has yet to be studied.

First experience with the use of compression cerclage Gundolf in orthopaedic and trauma surgery. A preliminary report.

In this prospective study we describe our experience with a new method of osteosynthesis of fissures, spiral and comminuted fractures of long bones of the extremities using the titanium compression cerclage of Gundolf (CCG). The operative technique is described in detail, and the results are presented for 50 operated patients who were followed up until fracture consolidation. The technique of application of the CCG is simple and quick and according to the results obtained ensures a permanent fixation at the site of the fracture. No cerclage-related complications such as stress shielding, corrosion, breakage of the band and impairment of the superficial cortical blood supply were radiologically observed in this series. We recommend the use of this titanium band in difficult cases of fractures of the long bones as well as in selected cases of primary and particularly revision hip arthroplasty.

Influence of pH on in vitro disintegration of phosphate binders.

Hyperphosphatemia, a common complication in patients with end-stage renal disease, is treated with oral phosphate-binding medications that restrict phosphorus absorption from the gastrointestinal (GI) tract. Impaired product performance, such as failure to disintegrate and/or dissolve in the GI tract, could limit the efficacy of the phosphate binder. Disintegration may be as important as dissolution for predicting in vitro product performance for medications that act locally on the GI tract, such as phosphate binders. Furthermore, patients with end-stage renal disease have a wide range in GI pH, and pH can influence a product's performance. The purpose of this study was to determine the effect of pH on in vitro disintegration of phosphate binders. Fifteen different commercially available phosphate binders (seven calcium carbonate tablet formulations, two calcium acetate tablet formulations, three aluminum hydroxide capsule formulations, and three aluminum hydroxide tablet formulations) were studied using the United States Pharmacopeia (USP) standard disintegration apparatus. Phosphate binders were tested in simulated gastric fluid (pH 1.5), distilled water (pH 5.1), and simulated intestinal fluid (pH 7.5). Product failure was defined as two or more individual tablets or capsules failing to disintegrate completely within 30 minutes. Results indicate that 9 of the 15 phosphate binders tested showed statistically significant differences in disintegration time (DT) based on pH. The percentage of binders that passed the disintegration study test in distilled water, gastric fluid, and intestinal fluid were 80%, 80%, and 73%, respectively. The findings of this study show that the disintegration of commercially available phosphate binders is highly variable. The pH significantly affected in vitro disintegration in the majority of phosphate binders tested; how significantly this affects in vivo performance has yet to be studied.

Augmentation of anterior transvertebral screws using threaded teflon anchoring.

This is a biomechanical study to investigate the effect of augmentation of single anterior transvertebral screws. Two subsequent experiments (pullout and caudal loading) were performed in 78 porcine vertebral bodies of equal bone mineral density using 6.5-mm transvertebral screws augmented or not with specially designed Teflon anchoring. Three different types of 35- and 45-mm unaugmented screws (Kaneda, TSRH, and Zielke) were inserted at 90 degrees laterally in the vertebral body and were tested for pullout force. The pullout load to loosen the three different unaugmented screws did not significantly differ. The pullout force to loosen the 35- and 45-mm Zielke screws was 507 +/- 22 and 860 +/- 50 N, respectively. The corresponding pullout load to loosen the construct Zielke screw-Teflon anchoring was 1,005 +/- 148 N and 1,306 +/- 135 N for the 35- and 45-mm screws, respectively. When comparing the pullout force needed for unaugmented versus the augmented Zielke screw of the same length, there was a statistically significant (p < 0.0001) difference. During the caudal loading test, the unaugmented 45-mm Zielke screw showed that a uniform slope up to the yield point occurred at 0.35 +/- 0.12 mm of displacement, with an average tilting force for the tested screw of 1,362 +/- 151 N, when the screw became loose. The caudal loading test for the augmented Zielke screw was interrupted at 2,000 N because the load applied exceeded the load capacity of the testing machine, and thus no loosening occurred. The findings of this in vitro study showed that the Teflon anchoring provides superior anchorage and stability of single transvertebral Zielke screws. However, further biomechanical and clinical studies are required before using this device or its modification.

Monoradiculopathy of the fifth lumbar nerve root due to lumbar disc herniation between lumbar one and lumbar two vertebrae.

An extremely rare case is reported of a 34-year-old man who had a drop foot due to a herniated disc between the first and second lumbar vertebrae with a monoradiculopathy of the fifth lumbar nerve root. The diagnosis was made on the basis of myelography and magnetic resonance imaging (MRI), which revealed a disc centrolateral herniation at the level between the first and second lumbar vertebrae. The patient underwent anterior discectomy and fusion with the use of iliac bone graft. Because of increasing local kyphosis and associated symptoms, a posterior TSRH instrumentation was added successfully. Postoperatively the patient had alleviation of his symptoms, and at the 6-year follow-up evaluation, he was completely symptomless. The spine surgeon should be aware of the possibility of this rare location of lower lumbar nerve root compression within the dural sac. In such a case, myelography and MRI seemed to be superior to the computed tomography scan.

Reciprocal angulation of vertebral bodies in the sagittal plane in an asymptomatic Greek population.

STUDY DESIGN: A prospective study conducted on several roentgenographic parameters of the standing sagittal profile of the spine in an asymptomatic Greek population. OBJECTIVES: To perform segmental analysis of the sagittal plane alignment of the normal thoracic, lumbar, and lumbosacral spines and to compare the findings with those derived from similar populations. SUMMARY OF BACKGROUND DATA: Until recently, little attention has been paid to the sagittal segmental alignment of the spine, and there are only a few studies (in French and American populations) in which radiographic analysis of sagittal spinal alignment is investigated. METHODS: Ninety-nine consecutive asymptomatic Greek volunteers (38 men, 61 women), an average age of 52.7 +/- 15 years old (range, 20-79 years), were included in this prospective study, on the basis of several inclusion criteria. These volunteers were divided into six distinct age groups. The radiologic parameters, which were measured (by Cobb's method) on the lateral standing roentgenograms of the whole spine were: thoracic kyphosis (T4-T12), lumbar lordosis (L1-L5), total lumbar lordosis (T12-S1), distal lumbar lordosis (L4-S1), sacral inclination (measured from the line drawn parallel along the back of the proximal sacrum and the vertical line), pelvic tilting, vertebral body inclination, and relative segmental inclination between pairs of adjacent vertebrae. RESULTS: Thoracic kyphosis and lumbar lordosis (T12-S1, L1-L5) were not gender related. Thoracic kyphosis increased with age (P < 0.001), the lumbar spine (L1-L5) gradually became less lordotic as the thoracic kyphosis increased (P < 0.003), and total lumbar lordosis was not age related. Sacral inclination correlated strongly with both thoracic kyphosis (P < 0.002) and L1-L5 lordosis (P < 0.001). Pelvic tilting correlated strongly with L1-L5 lordosis (P < 0.0075), but did not correlate with thoracic kyphosis and age. Vertebral body inclination showed a narrow variability in T6-T12 and in L4 and a wide variability in T4, T5, L1-L3, and S1. Distal lumbar lordosis represents the 68.6% of the total lumbar lordosis. CONCLUSIONS: In the results of this study, a reliable table of reference for roentgenographic parameters in the sagittal plane of the spine was established in an asymptomatic Greek population. The parameters are similar to those used in previous studies. Thus, these data should be considered in preoperative planning and postoperative evaluation of achieved correction during restoration procedures of the spine in the sagittal plane.

Unrecognized laceration of main bronchus caused by fracture of the T6 vertebra.

We report on an extremely rare case of a 58-year-old male polytraumatized patient who sustained a bursting fracture of the T6 vertebral body associated with laceration of the posterior wall of the right main bronchus from the spiculae of the vertebral body fragments. The patient was on admission asymptomatic until he underwent an endotracheal anesthesia for stabilization of associated fractures of the extremities, when he developed acute respiratory disturbances. The opening of the bronchus was repaired via a thoracotomy, while the spine was successfully supported with a custom-made brace for 3 months. In the 3-year follow-up evaluation, the patient did well and suffered no complaints of the spine or lungs. Spine surgeons should be aware of this rare complication, which may accompany bursting fractures in the midthoracic spine. Laceration of the bronchus should be suspected when fragments of the vertebral body are dislocated far anteriorly.