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PubMed2004

Brownian dynamics simulations of needle chain and nugget chain polymer models--rigid constraint conditions versus infinitely stiff springs.

It is well known that orientational correlations appear in polymer chain models when the subunits are linked by ball-socket joints implemented as rigid constraint conditions. These correlations do not appear when the subunits are connected by springlike potential forces, even in the limit of infinitely stiff springs. In a widely used class of algorithms for Brownian dynamics simulations, inertia effects are ignored. However, in the recently introduced needle chain and nugget chain algorithms, the rigid constraint correlations depend on the mass and moment of inertia. This inconsistency does not appear in the bead-rod (Kramers) polymer chain model, which also has orientational correlations introduced by rigid constraint conditions. Explicit expressions for the correlation functions are given for thermodynamic equilibrium states. Analytical expressions for the associated forces ("metric forces") and simulation results showing how the rigid constraint correlations influence dynamical properties, are also presented. Further we discuss the physical relevance of these correlations and show via simulations that their influence on stationary and dynamical properties depend significantly on chain length. We further show that if the metric forces are removed, algorithms designed with rigid constraint conditions describe a chain of segments connected by infinitely stiff springs. Finally we show that the results presented here for needle chains are relevant also for the bead-rod (Kramers) chain model, making it possible to simulate a bead-spring chain with infinitely stiff springs.

Journal Article↗
PubMed2001

Clinical application of a new colonoscope with variable insertion tube rigidity: a pilot study.

BACKGROUND: Colonic loop formation can prolong colonoscopy, increase patient discomfort, and preclude complete examination. A colonoscope with variable insertion tube rigidity may facilitate colonoscopy. Our aim was to determine whether the use of a colonoscope with variable insertion tube rigidity reduces insertion time and improves patient acceptance of colonoscopy. METHODS: Fifty patients were randomly assigned to undergo colonoscopy with a conventional colonoscope or a variable rigidity colonoscope (VRC). Patient acceptance, dosage of medication, use of abdominal pressure, and patient repositioning were assessed. Statistical analysis was performed by the 2-sample Wilcoxon rank sum test and an extension of Fisher exact test. RESULTS: The groups were comparable with respect to age, gender, and medications required during colonoscopy. The cecum was reached in all 25 patients who underwent colonoscopy with the VRC, including 1 patient in whom the cecum was not reached at a previous colonoscopy with a conventional instrument. In the conventional colonoscopy group, the cecum was not reached in 4 patients (2 poor preparation, 2 loop formation). There was no significant difference between the 2 groups with respect to insertion time. In the group that underwent colonoscopy with the variable rigidity instrument, less abdominal pressure was required (p = 0.05), and nursing assessment of patient discomfort was more favorable (p = 0.05). There were no complications and no significant differences in the intubation time to cecum or in repositioning, patient acceptance, or patient assessment of abdominal pain. CONCLUSION: The use of a variable rigidity colonoscope reduced the frequency of abdominal pressure but did not affect intubation time to cecum, repositioning, patient acceptance, or patient assessment of abdominal pain.

Adult↗
PubMed2002

Rigid versus wire fixation for mandibular advancement: skeletal and dental changes after 5 years.

The bilateral sagittal split osteotomy (BSSO) is the most common surgical procedure for the correction of mandibular retrognathism. Commonly, the proximal and distal segments are fixated together with either wire or rigid screws or plates. The purpose of this study was to compare long-term (5 years) skeletal and dental changes between wire and rigid fixation after BSSO. In this multisite, prospective, randomized clinical trial, the rigid fixation group received three 2-mm bicortical position screws, and the wire fixation group received inferior border wires and 6 weeks of skeletal maxillomandibular fixation with 24-gauge wires. Cephalometric films were obtained 2 weeks before surgery and at 1 week, 8 weeks, 6 months, 1 year, 2 years, and 5 years after surgery. Linear cephalometric changes were referenced to a cranial base coordinate system. Before surgery, both groups were comparable with respect to linear and angular measurements of craniofacial morphology. Both groups underwent similar surgical changes. Skeletal and dental movements occurred in both groups throughout the study period. Five years after surgery, the wire group had 2.2 mm (42%) of sagittal skeletal relapse, while the rigid group remained unchanged from immediately postsurgery. Surprisingly, at 5 years, both groups had similar changes in overbite and overjet. This was attributed to dental changes in the maxillary and mandibular incisors. Although rigid fixation is more stable than wire fixation for maintaining the skeletal advancement after a BSSO, the incisor changes made the resultant occlusions of the 2 groups indistinguishable.

Adolescent↗
PubMed2001

Maintenance of soft tissue changes after rigid versus wire fixation for mandibular advancement, with and without genioplasty.

OBJECTIVE: This multisite prospective randomized clinical trial examined 2-year longitudinal soft tissue profile changes after bilateral sagittal split osteotomy for mandibular advancement by using rigid or wire fixation, with and without genioplasty. STUDY DESIGN: The study sample consisted of 127 subjects. The rigid-fixation group (n = 78) received 2-mm bicortical position screws, whereas the wire-fixation group (n = 49) received inferior border wires. In the rigid-fixation group, 35 subjects underwent genioplasty, whereas 24 subjects underwent genioplasty in the wire-fixation group. Soft tissue profile changes of labrale inferius, B-point, and pogonion were obtained from digitized cephalometric films taken immediately before surgery and up to 2 years after surgery. RESULTS: Regardless of fixation technique, subjects who had genioplasty in conjunction with the mandibular advancement had the largest surgical movement and the largest postsurgical change (P <.05). When all variables were constant, fixation technique was associated with maintenance of soft tissue change. Subjects who underwent rigid fixation maintained more soft tissue change than patients who underwent wire fixation. CONCLUSIONS: These findings suggest that subjects undergoing rigid fixation and genioplasty maintained the most soft tissue advancement.

Adult↗
PubMed1978

Energy conformation study of Met-enkephalin and its D-Ala2 analogue and their resemblance to rigid opiates.

Conformational similarities of Met-enkephalin and its D-Ala2 analogue to rigid opiates were studied by both empirical and quantum mechanical methods. By both methods, conformers with maximum resemblance to rigid opiates have the highest energies. Conformers with the lowest energy had no resemblance to rigid opiates. However, several low and intermediate energy conformers were identified in which at least the NH2-terminal tyrosine residue overlaps with the phenethylamine moiety of rigid opiates and which could equally well accommodate either Gly2 or D-Ala2. The conformer among these with greatest additional resemblance to other binding sites of rigid opiates is proposed as the most likely candidate for an induced fit at the receptor site.

Endorphins↗
PubMed1991

Rigid protein motion as a model for crystallographic temperature factors.

The extent to which the librations of rigid molecules can model the crystallographic temperature factor profiles of proteins has been examined. For all proteins considered, including influenza virus hemagglutinin, glutathione reductase, myohemerythrin, myoglobin, and streptavidin, a simple 10-parameter model [V. Schomaker and K. N. Trueblood (1968) Acta Crystallogr. Sect. B 24, 63-76] is found to reproduce qualitatively the patterns of maxima and minima in the isotropic backbone meansquare displacements. Large deviations between the rigid molecule and individual atomic temperature factors are found to be correlated with a region in hemagglutinin for which the refined structural model is unsatisfactory and with errors in the structure in a partially incorrect model of myohemerythrin. For the high-resolution glutathione reductase structure, better results are obtained on treating each of the compact domains in the structure as independent rigid bodies. The method allows for the refinement of reliable temperature factors with the introduction of minimal parameters and may prove useful for the evaluation of models in the early stages of x-ray structure refinement. While these results by themselves do not establish the nature of the underlying displacements, the success of the rigid protein model in reproducing qualitative features of temperature factor profiles suggests that rigid body refinement results should be considered in any interpretation of crystallographic thermal parameters.

Binding Sites↗
PubMed2003

RhoA is required for cortical retraction and rigidity during mitotic cell rounding.

Mitotic cell rounding is the process of cell shape change in which a flat interphase cell becomes spherical at the onset of mitosis. Rearrangement of the actin cytoskeleton, de-adhesion, and an increase in cortical rigidity accompany mitotic cell rounding. The molecular mechanisms that contribute to this process have not been defined. We show that RhoA is required for cortical retraction but not de-adhesion during mitotic cell rounding. The mitotic increase in cortical rigidity also requires RhoA, suggesting that increases in cortical rigidity and cortical retraction are linked processes. Rho-kinase is also required for mitotic cortical retraction and rigidity, indicating that the effects of RhoA on cell rounding are mediated through this effector. Consistent with a role for RhoA during mitotic entry, RhoA activity is elevated in rounded, preanaphase mitotic cells. The activity of the RhoA inhibitor p190RhoGAP is decreased due to its serine/threonine phosphorylation at this time. Cumulatively, these results suggest that the mitotic increase in RhoA activity leads to rearrangements of the cortical actin cytoskeleton that promote cortical rigidity, resulting in mitotic cell rounding.

Actin Cytoskeleton↗
PubMed2001

Flexible versus rigid endoscopes for outpatient hysteroscopy: a prospective randomized clinical trial.

To evaluate patient acceptance, optical properties and the clinical feasibility of flexible compared with rigid hysteroscopes, 142 patients undergoing outpatient hysteroscopy were included in a prospective, randomized clinical trial. The flexible hysteroscope was used in 70 patients, and the rigid instrument in 72. At different stages of the hysteroscopy the level of pain experienced by the women was assessed using a 10 cm visual analogue scale. Optical properties characterized by the parameters intrauterine visibility, hysteroscopic view and diagnostic accuracy were ranked by the surgeons using a 5-point scale (1 = excellent to 5 = insufficient), and duration of the hysteroscopy was measured. Hysteroscopy was successful in 87.5 and 100% of patients in the flexible and rigid groups respectively. With the use of rigid telescopes, discomfort at introduction and during the hysteroscopy was significantly greater (median 1.7 versus 0.7, P = 0.003; 3.1 versus 1.2, P < 0.001 respectively), but optical properties were judged to be far superior (P < 0.001 for all three comparisons) and procedure time was significantly shorter (median 70 versus 120 s, P = 0.003). In conclusion, outpatient hysteroscopy seems to be less painful when using flexible telescopes. However, rigid hysteroscopes provide superior optical qualities and permit a more rapid performance with higher success rates at much lower cost.

Adult↗
PubMed1993

Impact tolerances of the rigidly fixated maxillofacial skeleton.

A study was designed to determine how soon an athlete who undergoes rigid fixation of a facial fracture can return to full competition. The impact resistance of a rigidly fixated malar complex fracture was studied and compared with that of an intact malar complex. Twelve fresh human cadaver heads were used. A custom-designed impact device was used to deliver a blow of a specific energy to each intact malar complex. The subsequent fractures were rigidly fixated at three points using titanium miniplates and screws. A second impact of identical energy was delivered. The forces generated and the subsequent displacement of hard and soft tissues were recorded after each impact. It was concluded from this study that an impact to a rigidly fixated malar complex fracture produced less force and greater displacement of hard and soft tissues than an impact of identical energy to an intact malar complex. The potential for sustaining more severe maxillofacial injuries after an initial facial fracture should be seriously considered. The results suggest that sufficient time should be allowed for the bony healing of a facial fracture to occur, even after rigid fixation, before an athlete can resume full contact activities.

Age Factors↗
PubMed1991

Long-term effects of rigid fixation on the growing craniomaxillofacial skeleton.

Fixation of craniomaxillofacial bones with plates and screws is becoming an increasingly popular method of managing patients with maxillofacial fractures, congenital anomalies, and skeletal defects after tumor extirpation. The effects of rigid fixation on growth, however, are not well defined and remain controversial. This experimental work was designed to evaluate the effects of rigid fixation on the growing craniomaxillofacial skeleton. Eight 2-month-old beagle dogs were divided into two groups. Cephalometric analysis and computed tomography were obtained preoperatively to establish a baseline of the craniomaxillofacial skeleton. Rigid fixation using plates and screws was applied across the intact coronal and nasofrontal sutures. The contralateral side served as the control. The periosteum was elevated over the contralateral suture and replaced without any surgical intervention. The animals were killed one year after application of rigid fixation, when they had achieved full and mature skeletal growth. The skulls were evaluated both grossly and cephalometrically. There was bony growth over some of the plates, as well as sclerosis of the plated suture lines. Cephalometric analysis demonstrated consistent asymmetry between the plated and nonplated sides, with deviation of the midline toward the plated side in all of the animals. Although further studies are necessary to clarify this issue fully, we conclude from our study that rigid fixation during skeletal development can potentially alter the growth patterns of the craniomaxillofacial skeleton and should be used with caution.

Animals↗
PubMed1995

Effect of rigid microfixation on the craniomaxillofacial skeleton.

Rigid microfixation has enhanced the immediate results in pediatric craniomaxillofacial surgery. The effects of rigid fixation on the development of the craniofacial skeleton, however, remain uncertain. The long-term effects of rigid microfixation on the growing craniofacial skeleton in the rabbit model were evaluated in this study. Bifrontal craniectomies were preformed in 9-day-old rabbits. The frontal bones were replaced orthotopically and fixed both anteriorly and posteriorly with either wire osteosynthesis or titanium microplates and screws. The animals were allowed to grow to craniofacial maturity, and craniofacial development was evaluated with both cephalometric and craniometric analyses. All data were statistically analyzed through analysis of variance. Regional adverse growth effects on the craniofacial skeleton with the use of both wire and rigid fixation were observed. The adverse effects were greatest with the use of rigid fixation.

Analysis of Variance↗
PubMed1998

Effect of topical anesthetic use on initial patient satisfaction and overall success with rigid gas permeable contact lenses.

BACKGROUND: Rigid gas permeable (RGP) contact lenses have numerous benefits; however, RGP lens use is not increasing in the United States. An important factor for this trend has been initial comfort. Studies have demonstrated that how RGPs are presented to patients, in addition to lens design, can play an important role in the initial comfort process. Another important factor could be the use of a topical anesthetic during the fitting and dispensing visits. The purpose of this study was to use a multicenter format to determine if topical anesthetic use increased the likelihood of patient satisfaction and success. METHODS: A total of 80 subjects, with no previous rigid lens wear experience, was entered into this 1-month study, including 20 subjects from each of 4 institutions. Subjects were randomly divided into the following two groups: (A) anesthetic or (B) placebo, with the former group receiving one drop of a topical anesthetic before lens insertion at both the diagnostic fitting and dispensing visits, whereas the latter group received a placebo. Subjects completed a questionnaire on their perception of rigid lens wear both immediately before fitting and at the 1-month visit. After diagnostic fitting with rigid lenses, subjects completed an adaptation questionnaire after 15 min, 1 week, 2 weeks, and 1 month of lens wear. RESULTS: Seventy of the 80 subjects completed the study and, of the 10 subjects who discontinued, 8 were in the placebo group. In all categories evaluated, the anesthetic group experienced a more optimum adaptation experience at each visit vs. the placebo group. Specifically, overall comfort was rated significantly higher at both dispensing and 2 weeks. In addition, the anesthetic group exhibited significantly greater overall satisfaction with rigid lens wear at 2 and 4 weeks. Also, the anesthetic group perceived their adaptation, sensitivity, and adaptation time to be significantly better at the 1-month visit. There was no significant difference in corneal staining between these two groups at each visit, with the exception of a greater amount of staining in the central quadrant for the placebo group at the 1-month visit. CONCLUSIONS: The use of a topical anesthetic at the fitting and dispensing visits for first-time wearers of RGP lenses resulted in significantly fewer dropouts, improved initial comfort, an enhanced perception of the adaptation process, and greater overall satisfaction after 1 month of lens wear as compared to the use of a nonanesthetizing placebo at those visits. This result, in combination with both presenting RGP lenses in a nonthreatening manner and optimizing the lens design and fitting relationship, should result in a positive adaptation process and successful wear of RGP contact lenses.

Adaptation, Physiological↗
PubMed1990

The effect of rigid fixation on the survival of onlay bone grafts: an experimental study.

Much attention has recently been focused on rigid fixation as a method of improving fracture healing. Whether such fixation, when applied to onlay grafting, improves graft take and volume is unknown. To examine this question, we compared survival of both endochondral and membranous grafts fixed rigidly and nonrigidly in areas of low motion (snout) and high motion (femur) in a rabbit model. Gross morphology, histologic analysis, and graft volume kinetics were evaluated. Findings demonstrate that in areas of high motion, the application of rigid fixation improves graft survival, whereas in a low-motion region, no differences in graft volume retention as a function of fixation were observed. Histologically, no differences with the method of fixation employed were seen, and similar revascularization patterns were noted. By kinetic analysis, rigid fixation appears to exert its most profound effect early in the postgraft period. Membranous bone grafts remain superior to endochondral grafts under all circumstances. From these studies, we conclude that rigid fixation is the method of choice in all circumstances where onlay bone grafts may be exposed to motion, shear, and torsional forces.

Analysis of Variance↗
PubMed1991

Effects of rigidity of an internal fixation device. A comprehensive biomechanical investigation.

Internal fixation with instrumentation often accompanies surgical fusion to augment spinal stability, provide temporary fixation while the surgical fusion mass unites, and enhance postoperative mobilization of a patient. Some surgeons, however, feel that the existing plate-screw designs are too rigid and are the primary cause of "iatrogenic" adverse effects clinically observed. A three-part study, involving in vitro experimental protocol, analytical finite-element-based models, and an in vivo canine investigation, was undertaken to study the role of decreasing rigidity of a device on the biomechanical response of the stabilized segments. Two alternatives--the use of one variable screw placement (Steffee plate [unilateral, 1VSP model]) as opposed to two VSP plates (bilateral, 2VSP model) and two VSP plates with polymer washers placed in between the integral nut and plate (2MVSP model)--were considered for achieving a reduction in the rigidity of the conventional VSP system. The load-displacement data obtained from the in vitro experiments and the stress distributions within the stabilized and intact models predicted by the finite-element models revealed that the unilateral VSP system is less rigid and is likely to reduce stress shielding of the vertebral bodies compared with the 2VSP model. The undesirable effects associated with the use of the 1VSP plate system are the presence of coupled motions due to the inherent asymmetry and the likely inability to provide enough rigidity for decompression procedures requiring a complete excision of the disc. The use of two MVSP plates overcomes these deficiencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

Prevalence of laryngeal irritation signs associated with reflux in asymptomatic volunteers: impact of endoscopic technique (rigid vs. flexible laryngoscope).

OBJECTIVES: The objectives of this study were to 1) determine the prevalence of ENT findings in the normal asymptomatic population and 2) to compare findings between flexible and rigid laryngoscopes in an attempt to increase specificity of diagnosis of reflux in endoscopic laryngeal examinations. STUDY DESIGN: Prospective study. METHODS: Fifty-two nonsmoker volunteers (24 male, 28 female), mean age of 42.7 years, with no history of ENT abnormalities or gastroesophageal reflux disease, underwent both rigid and flexible videolaryngologic examinations with a digital endoscopic unit. A group of three expert judges reviewed the oral and transnasal examinations blindly and independently for physical signs of irritation/inflammation commonly associated with reflux. RESULTS: Atleast one sign of tissue irritation was detected in 93% and 83% of the population when using a flexible and a rigid laryngoscope, respectively. Results showed a high incidence of posterior commissure bar (53.2% and 51.9%), arytenoid complex edema/erythema (76.3% and 53.2%), and pseudosulcus (37.2% and 7.7%). Most signs were more frequently detected on flexible transasal examinations than with rigid transoral examinations: posterior pharyngeal wall (<0.01), interarytenoid irritation (<0.01), arytenoids complex irritation (<0.01), ventricular obliteration (<0.01), and pseudosulcus (<0.01). CONCLUSIONS: Several signs of posterior laryngeal irritation (e.g., interarytenoid bar, erythema of the medial wall of the arytenoids), which are generally considered to be signs of laryngopharyngeal reflux, are present in a high percentage of nonsymptomatic individuals, raising question about their diagnostic specificity. In addition, these signs were more often detected with flexible than with rigid laryngoscopes, suggesting that flexible laryngoscopy is more sensitive but less specific in identifying laryngeal tissue irritation.

Adult↗
PubMed2005

Comparison of flat and steep rigid contact lens fitting methods in keratoconus.

PURPOSE: The purpose of this article is to compare the safety and efficacy of flat- and steep-fitting rigid contact lenses in keratoconus. METHODS: The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a 16-center observational study. Cross-sectional results at baseline were generated for 1091 subjects with longitudinal results from the 871 subjects who completed 8 years of follow up. RESULTS: Of the 761 rigid contact lens-wearing patients at baseline, 41% had a scar at baseline compared with 24% of the nonrigid contact lens wearers (odds ratio [OR], 2.15; 95% confidence interval [CI], 1.35-3.43; p = 0.001). Eighty-seven percent were fitted with flat-fitting lenses, whereas 13% were fitted with steep-fitting lenses. Rigid lens fitting method was also associated with incident corneal scarring. A greater proportion of the corneas wearing flat-fitting contact lenses were scarred (43% compared with 26% for the steep-fitted eyes; OR,= 2.19; 95% CI, 1.37-3.51; p = 0.001). After controlling for corneal curvature, the association of rigid contact lens fit and corneal scarring at baseline did not persist (adjusted OR, 1.20; 95% CI, 0.70-2.06; p = 0.52). Thirty-two percent of unscarred eyes at baseline fitted flat had developed an incident corneal scar by the eighth year follow-up visit compared with 14% of eyes fitted steep (OR, 2.93; 95% CI, 1.34-6.42; p = 0.007). CONCLUSIONS: The data reported here indicate that, after controlling for disease severity in the form of corneal curvature, keratoconic eyes fitted with a rigid contact lens resulting in an apical touch fluorescein pattern did not have an increased risk of being scarred centrally at baseline. This "natural history" sample cannot determine causal proof that one method of fitting lenses is safer than another. To achieve this, a randomized clinical trial is needed.

Contact Lenses↗
PubMed2004

Value of rigid bronchoscopy in the management of critically ill children with acute lung collapse.

OBJECTIVE: To describe the potential uses of a rigid bronchoscopy in pediatric patients, as well as explaining the advantage of a rigid bronchoscopy versus a flexible bronchoscopy in pediatric patients with acute lung collapse. DESIGN: Case report. SETTING: Pediatric intensive care unit. PATIENT: A 16-month-old male infant with a 5-day history of cough, congestion, and "wheezing" that progressively worsened. INTERVENTIONS: A rigid bronchoscopy. MEASUREMENTS AND MAIN RESULTS: The evolution of the patient's acute lobar collapse to his final discharge is described. This includes the initial presentation, primary methods employed to improve the patient's condition, the use of a rigid bronchoscopy in removing a mucous plug, and the patient's results up to his final discharge. CONCLUSION: This report demonstrates the value of rigid bronchoscopy in pediatric patients with severe atelectasis and pneumonia. Although bronchoscopy may not be indicated as an initial procedure to remove respiratory tract secretions, it may provide a beneficial option in cases where less invasive methods prove ineffective in removing secretions and mucous plugs.

Acute Disease↗
PubMed2001

Rigidity of random networks of stiff fibers in the low-density limit.

Rigidity percolation is analyzed in two-dimensional random networks of stiff fibers. As fibers are randomly added to the system there exists a density threshold q=q(min) above which a rigid stress-bearing percolation cluster appears. This threshold is found to be above the connectivity percolation threshold q=q(c) such that q(min)=(1.1698+/-0.0004)q(c). The transition is found to be continuous, and in the universality class of the two-dimensional central-force rigidity percolation on lattices. At percolation threshold the rigid backbone of the percolating cluster was found to break into rigid clusters, whose number diverges in the limit of infinite system size, when a critical bond is removed. The scaling with system size of the average size of these clusters was found to give a new scaling exponent delta=1.61+/-0.04.

Journal Article↗