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At least 469 records · Page 26Linked to original sources
PubMed2004

Rigid ureteroscopy for the treatment of ureteral calculi in children.

PURPOSE: We assess the safety and efficacy of rigid ureteroscopy for the treatment of pediatric ureterolithiasis. MATERIALS AND METHODS: The records of 33 children with an average age of 7.4 years (range 9 months to 15 years) treated with rigid ureteroscopy between May 1995 and July 2003 were reviewed. In 35 ureteral units use of a rigid 6.9 to 10Fr ureteroscope was planned for treating stones at various levels of the ureter. Stones were located in the upper ureter in 6 cases, middle ureter in 3 and lower ureter in 26. Dilatation of the ureteral orifice was necessary in 11 cases. RESULTS: Stone size varied from 3 to 10 mm (mean 5.3). In 33 patients (94%) all stone fragments were removed successfully. Stones were fragmented with pneumatic lithotripsy in 20 cases and removed by forceps without fragmentation in 13. In 1 child an upper ureteral stone migrated up to the kidney during ureteroscopy but following extracorporeal shock lithotripsy therapy she was rendered stone-free. In another child it was not possible to remove the stone. In a 9-month-old female patient with bilateral stones it was not possible to enter the left ureter because of a tight orifice resistant to balloon dilation. At the end of the procedure a 3 or 4Fr ureteral or a 4.8Fr Double-J (Medical Engineering Corp., New York, New York) stent was left in place for 3 days to 3 weeks in 12 cases. There were no cases of ureteral perforation. Of the patients 31 were followed for 1 to 36 months. No incidence of vesicoureteral reflux was detected in 9 who underwent postoperative cystography. CONCLUSIONS: After becoming experienced and meticulously working with finer instruments in adults, rigid ureteroscopy can be a safe and efficient treatment for ureteral stones in every location in children.

Adolescent↗
PubMed2005

Adverse events and discontinuations with rigid and soft hyper Dk contact lenses used for continuous wear.

PURPOSE: The purpose of this study was to assess the relative clinical success of hypertransmissible rigid and soft contact lenses in terms of the rates of adverse events and clinical discontinuations. METHODS: One hundred subjects (50 experienced wearers and 50 neophytes) were fitted with either the Z-alpha lens (Menicon) or the Focus Night & Day lens (CIBA Vision), which were worn on a 30-day continuous wear basis. The rates of adverse events and clinical discontinuations were monitored over 12 months. RESULTS: There were 20 adverse events that were categorized as being serious or significant--four with rigid lenses and 16 with soft lenses (p = 0.003). There was no significant difference between the experienced (10) and the neophyte (10) groups (p = 1.00). There were 23 clinical discontinuations--13 with rigid lenses and 10 with soft lenses (p = 0.46). Fewer experienced wearers (five) discontinued than neophytes (18) (p = 0.002). CONCLUSIONS: Practitioners should anticipate fewer adverse events with rigid versus soft lenses prescribed for continuous wear. Experienced wearers are less likely to discontinue compared with neophytes.

Adult↗
PubMed2004

Comparison of canine mandibular bone regeneration by distraction osteogenesis versus acute resection and rigid external fixation.

The present study was performed (1) to explore the mechanism of skeletal healing following distraction osteogenesis of the mandible and to evaluate whether the same process is involved following acute mandibular resection and rigid external fixation, and (2) to examine the role of the periosteum in skeletal healing in both models. The study was performed using 16 mongrel dogs divided into two equal groups. In the first group, distraction of 20 mm was performed at a rate of 1 mm/day. In the second group, bone resection of 20 mm was performed, followed by rigid external fixation. The buccal periosteum was stripped in four dogs from each group, and the periosteum was left intact in the remaining four dogs. Dogs were euthanized after a survival period of either 2 or 3 months, and the new bone regenerate was evaluated. Analysis consisted of three-dimensional computed tomography scanning, histometric analysis, and immunostaining. Analysis of bone mineral content in the residual gap was conducted. Bone mineral content was increased in 3- versus 2-month survival for all groups (p < 0.05). The distracted groups had greater bone mineral content than their acutely resected counterparts, with the difference achieving statistical significance by 3-month survival (p < 0.05). Although periosteal preservation resulted in increased bone mineral content over time for all groups (p = 0.044), periosteal preservation had no significant effect on bone mineral content in the distracted groups. After periosteal stripping, however, bone mineral content was significantly increased in dogs that underwent distraction rather than acute resection and rigid external fixation (p = 0.022). Regarding histometric analysis, analysis of fibrous tissue content in the bone regenerate demonstrated that by 3 months the distracted groups had significantly less fibrous tissue in the new bone regenerate than did the acutely resected groups (p < 0.001). Regarding immunostaining, diffuse localization of transforming growth factor-beta1 was observed in all groups at 2 months, returning to nearly baseline levels by 3 months. These data demonstrate that significant bone formation in a segmental gap can be achieved after acute mandibular resection and rigid external fixation if the periosteum is preserved. However, after periosteal injury or stripping, significant bone formation can only be achieved by distraction osteogenesis. In both processes, bone formation is preceded by up-regulation of transforming growth factor-beta1.

Animals↗
PubMedDate not supplied

Mechanical induction of osteogenesis: the importance of pin rigidity.

Eight dogs, divided into two groups of four by varying pin rigidity, underwent 15% left tibial lengthening by the Ilizarov method. In group I, "tensioned" 1.6-mm wires maintained a rigidity approaching that of 4.0-mm pins. In group II, the wires, maintained at half the tension, averaged 45% of the rigidity measured in group I. All dogs in group I filled the experimental gap with de novo osteogenesis, whereas all of the dogs in group II prematurely bridged the gap, arresting the process of osteogenesis. From these experimental results, clinical trials have been started using commercially available external fixation devices utilizing pins with equivalent rigidity.

Adolescent↗
PubMed1999

Directed rigidity and bootstrap percolation in 1+1 dimensions.

We study directed rigidity percolation (equivalent to directed bootstrap percolation) on three different lattices: square, triangular, and augmented triangular. The first two of these display a first-order transition at p=1, while the augmented triangular lattice shows a continuous transition at a nontrivial p(c). On the augmented triangular lattice we find, by extensive numerical simulation, that the the directed rigidity percolation transition belongs to the same universality class as the directed percolation. The same conclusion is reached by studying its surface critical behavior, i.e., the spreading of rigidity from finite clusters close to a nonrigid wall. Near the discontinuous transition at p=1 on the triangular lattice, we are able to calculate the finite-size behavior of the density of rigid sites analytically. Our results are confirmed by numerical simulation.

Journal Article↗
PubMed1999

Negative electrostatic contribution to the bending rigidity of charged membranes and polyelectrolytes screened by multivalent counterions.

Bending rigidity of a charged membrane or polyelectrolyte screened by monovalent counterions is known to be enhanced by electrostatic effects. We show that in the case of screening by multivalent counterions the electrostatic effects reduce the bending rigidity. This inversion of the sign of the electrostatic contribution is related to the formation of two-dimensional strongly correlated liquids (SCL) of counterions at the charged surface due to strong lateral repulsion between them. When a membrane or a polyelectrolyte is bent, SCL is compressed on one side and stretched on the other so that thermodynamic properties of SCL contribute to the bending rigidity. Thermodynamic properties of SCL are similar to those of Wigner crystal and are anomalous in the sense that the pressure, compressibility and screening radius of SCL are negative. This brings about substantial negative correction to the bending rigidity. For the case of DNA this effect qualitatively agrees with experiment.

Cations↗
PubMed2001

Rigidity transition in two-dimensional random fiber networks.

Rigidity percolation is analyzed in two-dimensional random fibrous networks. The model consists of central forces between the adjacent crossing points of the fibers. Two strategies are used to incorporate rigidity: adding extra constraints between second-nearest crossing points with a probability p(sn), and "welding" individual crossing points by adding there four additional constraints with a probability p(weld), and thus fixing the angles between the fibers. These additional constraints will make the model rigid at a critical probability p(sn)=p(sn)(c) and p(weld)=p(weld)(c), respectively. Accurate estimates are given for the transition thresholds and for some of the associated critical exponents. The transition is found in both cases to be in the same universality class as that of the two-dimensional central-force rigidity percolation in diluted lattices.

Journal Article↗
PubMed2002

Local rigidity in sandpile models.

We address the problem of the role of the concept of local rigidity in the family of sandpile systems. We define rigidity as the ratio between the critical energy and the amplitude of the external perturbation and we show, in the framework of the dynamically driven renormalization group, that any finite value of the rigidity in a generalized sandpile model renormalizes to an infinite value at the fixed point, i.e., on a large scale. The fixed-point value of the rigidity allows then for a nonambiguous distinction between sandpilelike systems and diffusive systems. Numerical simulations support our analytical results.

Journal Article↗
PubMed2003

Network rigidity at finite temperature: relationships between thermodynamic stability, the nonadditivity of entropy, and cooperativity in molecular systems.

A statistical mechanical distance constraint model (DCM) is presented that explicitly accounts for network rigidity among constraints present within a system. Constraints are characterized by local microscopic free-energy functions. Topological rearrangements of thermally fluctuating constraints are permitted. The partition function is obtained by combining microscopic free energies of individual constraints using network rigidity as an underlying long-range mechanical interaction, giving a quantitative explanation for the nonadditivity in component entropies exhibited in molecular systems. Two exactly solved two-dimensional toy models representing flexible molecules that can undergo conformational change are presented to elucidate concepts, and to outline a DCM calculation scheme applicable to many types of physical systems. It is proposed that network rigidity plays a central role in balancing the energetic and entropic contributions to the free energy of biopolymers, such as proteins. As a demonstration, the distance constraint model is solved exactly for the alpha-helix to coil transition in homogeneous peptides. Temperature and size independent model parameters are fitted to Monte Carlo simulation data, which includes peptides of length 10 for gas phase, and lengths 10, 15, 20, and 30 in water. The DCM is compared to the Lifson-Roig model. It is found that network rigidity provides a mechanism for cooperativity in molecular structures including their ability to spontaneously self-organize. In particular, the formation of a characteristic topological arrangement of constraints is associated with the most probable microstates changing under different thermodynamic conditions.

Entropy↗
PubMed2005

Rigidity percolation in dispersions with a structured viscoelastic matrix.

This paper deals with rigidity percolation in composite materials consisting of a dispersion of mineral particles in a microstructured viscoelastic matrix. The viscoelastic matrix in this specific case is a hydrocarbon refinery residue. In a set of model random composites the mean interparticle surface-to-surface distance was controlled, changing particle volume fraction phi and particle number density independently. This was achieved by mixing two sets of monodisperse particles with widely differing radii (0.35 microm and 17.5 microm) with the matrix. A scaling exponent of 3.9 +/- 0.6 for the storage modulus G' vs phi- phi(c) was observed above a threshold phi(c) , in good agreement with theoretical values for rigidity percolation. It is found that at the rigidity-percolation threshold the pore structure, as characterized by the mean surface-to-surface distance for the filler, rather than the filler volume fraction, is similar for different types of composites. This behavior is explained from the internal structure of the viscoelastic matrix, which consists of fractal solid aggregates dissolved in a viscous medium; the effective radius of these aggregates and the mean surface-to-surface distance together determine whether or not the aggregates are capable of providing rigidity to the composite. The explanation is further supported by a qualitative comparison with effective-medium calculations. These indicate that the observed breakdown of time-temperature superposition near phi(c) is due to the appearance of a time scale characteristic for the mechanical interplay between the viscous binder phase and the purely elastic solid particles.

Journal Article↗
PubMed2006

Stabilization of unstable rigid rotation of spiral waves in excitable media.

Depending on the parameters of two-dimensional excitable or oscillatory media rigidly rotating or meandering spiral waves are observed. The transition from rigid rotation to meandering motion occurs via a supercritical Hopf bifurcation. To stabilize rigid rotation in a parameter range beyond the Hopf bifurcation, we propose and successfully apply a proportional control algorithm as well as time delay autosynchronization. Both control methods are noninvasive. This allows for determination of the parameters of unstable rigid rotation of spiral waves either for a model or an experimental system. Using the Oregonator model for the light-sensitive Belousov-Zhabotinsky reaction as a representative example we show that quite naturally some latency time appears in the control loop, and propose an efficient method to overcome its destabilizing influence.

Journal Article↗
PubMed2005

Negative thermal expansion in ZrW2O8: mechanisms, rigid unit modes, and neutron total scattering.

The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material has been investigated by reverse Monte Carlo (RMC) modeling of neutron total scattering data. We obtain, for the first time, quantitative measurements of the extent to which the and polyhedra move as rigid units, and we show that these values are consistent with the predictions of rigid unit mode theory. We suggest that rigid unit modes are associated with the NTE. Our results do not support a recent interpretation of x-ray-absorption fine structure spectroscopy data in terms of a larger rigid structural component involving the Zr-O-W linkage.

Journal Article↗
PubMedDate not supplied

Regional "rigidity" of background EEG activity in the epileptogenic zone.

Spectral analysis of interictal background EEG activity recorded through foramen ovale (FO) electrodes during monitoring (mean 5.8 days per patient) was studied in 10 medically refractory complex partial seizure (CPS) patients who were candidates for epilepsy surgery. Data of the spectral analysis was plotted as compressed spectral array (CSA) with spectral edge frequency (SEF) markers. For each patient, time-varying fluctuations of the SEF markers were compared visually and by a computer-assisted method between two symmetrical FO electrode EEG channels recording from both mesiobasotemporal lobes (MTL). The amount of asymmetrical variations of the SEF markers ("rigidity" phenomenon) was first determined visually and then quantified by the computer-assisted method. These findings were correlated with the results of other clinical tests, including FO electrode-recorded seizure onset (FO ict), positron emission tomography with [18F]fluorodeoxyglucose (FDG-PET), and magnetic resonance imaging (MRI) to investigate whether the rigidity phenomenon could lateralize the primary epileptogenic zone. The rigid side had 80, 70, and 60% coincidence rates with the pathologic side indicated by FDG-PET, FO ict, and MRI, respectively, in a single test. We conclude that the rigidity phenomenon of FO electrode-recorded interictal background EEG activity is a valuable sign for lateralization of the primary epileptogenic zone in MTL epilepsy. The relative invariance of SEF may be caused by interictal deafferentation of epileptic neurons.

Adult↗
PubMed1997

A protocol to evaluate semi-rigid pedicle screw systems.

The objective of the current study was to develop an in vitro testing protocol to evaluate semi-rigid pedicle screw devices. A corpectomy model protocol exists to evaluate rigid spinal implants; however, semi-rigid devices are contraindicated for this condition. This paper describes a technique that simulates more closely the conditions a semi-rigid device would see in vivo. Finally, the new testing protocol is used to evaluate the DDS pedicle screw-cable system. Benefits and shortcomings of the new protocol are discussed.

Bone Screws↗
PubMed2000

Comparison of rigid and elastic matching of dynamic magnetic resonance mammographic images by mutual information.

The present study aims at (i) the evaluation of the performance of a rigid and of an elastic matching algorithm for the coregistration of dynamic magnetic resonance (MR) images visualizing the female breast, and (ii) the evaluation of the mutual information (MI) as a matching criterion. To this end, ten patient data sets were analyzed. The comparison was performed with respect to the achieved increase in the MI and by visual inspection of the dynamic image series in a continuous film sequence ((cine mode). In most cases, the achieved increase in MI by elastic image registration is much higher than that achieved by rigid registration. Only for three of the ten data sets could the MI be increased by rigid image registration to a similar or even larger degree than by elastic image registration. Taking into account the results of the visual inspection of the rigid and elastic matched data sets, however, the elastic match leads to equal or better results for all data sets. Therefore, elastic matching is the gold standard for the registration of dynamic MR mammographic images of the female breast. The comparison of the increase in MI and the visual inspection further shows that the visual impression agrees in most cases with the result of the calculation of the MI. Therefore, the MI proves to be a suitable matching criterion for the type of data sets studied.

Algorithms↗
PubMed1998

Comparison of electrohydraulic lithotripters with rigid and pressure-release ellipsoidal reflectors. I. Acoustic fields.

The most common lithotripter, a Dornier HM-3, utilizes an underwater spark to generate an acoustic pulse and a rigid ellipsoidal reflector to focus the pulse on the kidney stone to be comminuted. The pulse measured in water with a PVDF membrane hydrophone at the external focus of the ellipsoid was a 1-microsecond positive-pressure spike followed by a 3-microsecond negative-pressure trough. When we replaced the rigid reflector in our experimental lithotripter with a pressure-release reflector, the pulse was a 1.6-microsecond trough followed by a 0.6-microsecond positive spike. The waveforms are nearly time inverses (i.e., their spikes and troughs are reversed). The frequency spectra, the maximum peak positive pressures P+ (42 MPa, rigid and 43 MPa, pressure-release), and the maximum peak negative pressures P- (-12 MPa and -14 MPa) are comparable. The maximum P- occurred 20 mm closer to the reflector than did the maximum P+, for both reflectors. However, the spatial maxima of the peak pressures (P+ and P-) produced by the pressure-release reflector were located 20 mm nearer to the reflector than those produced by the rigid reflector. Qualitative explanation of the waveforms and the location of pressure maxima as well as comparison to previous theoretical and experimental results is given. The alternate waveform produced by the pressure-release reflector may be a tool in determining the role of cavitation in lithotripsy because cavitation is highly sensitive to waveform.

Acoustics↗
PubMed1999

An audit of positive findings in flexible and rigid check cystoscopy.

Both flexible and rigid cysto-urethroscopy are routinely used in the surveillance of transitional cell bladder tumours. This study addressed the issue of patient selection for either rigid or flexible cystoscopy. What proportion of positive findings at flexible cystoscopy and negative findings at rigid cystoscopy are acceptable? Standards were set of 10% for the former and 50% for the latter and our practice was then audited. A retrospective analysis of 800 patients undergoing check cystoscopy revealed a positive finding rate of 8.3% using the flexible instrument and 48.1% using the rigid instrument.

Carcinoma, Transitional Cell↗
PubMed2003

Mechanisms and consequences of large artery rigidity.

In this review paper, the classical and more recently described mechanisms responsible for the structural and functional characteristics of large artery rigidity are described. Mostly important, these characteristics appear to be non-specific to the primary disease process involved in arterial hypertension, diabetes mellitus, dyslipidemia, congestive heart failure, chronic uremia, and perhaps senescence, including vascular dementia. Nonspecific in terms of aetiology, the vasculopathy encountered in these diseases exhibits common structural and functional abnormalities. The identification of such abnormalities could well become the target of potent nonpharmacological and (or) pharmacological interventions capable of preventing or retarding morbidity and mortality. The structural characteristics responsible for large artery rigidity include smooth muscle cell hypertrophy, matrix collagen deposition, and recently described, dysfunction in proteoglycan metabolism. Functional abnormalities, such as bradykinin-dependent hyper-reactivity of smooth muscle cells and vasa vasorum microcirculation network disturbances, also appear to alter aortic wall rigidity. The physiopathology of target organ damage is then revisited, based on endothelial dysfunction, documented in large and resistance arteries, as well as in microcirculation networks, where altered permeability to macromolecules leads to interstitial matrix disorganization and cell damage. The clinical evaluation of large artery rigidity is described, and one of the noninvasive methods, evaluation of pulse-wave velocity, is validated in normal conditions and in disease processes. Finally, non-pharmacological and pharmacological therapeutic measures are presented, and includes physical exercise to reduce insulin resistance, and renin-angiotensin-II-aldosterone modulators.

Animals↗