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At least 595 records · Page 33Linked to original sources
PubMed1995

Self-reinforced composite poly(methyl methacrylate): static and fatigue properties.

A novel material called 'self-reinforced composite poly(methyl methacrylate)' (SRC-PMMA) is described. This composite material consists of high strength, high ductility PMMA fibres embedded in a matrix of PMMA. Tensile tests, three-point flexural tests, fracture toughness tests and flexural fatigue tests were carried out on unidirectional continuous fibre SRC-PMMA materials. Commercial sheet PMMA and bone cement were also tested for comparison purposes. Two PMMA fibre sizes (40 and 120 microns diameters) with different mechanical properties were used to make the SRC-PMMA materials. The results of this study show that the tensile strength, tensile modulus and tensile strain-to-failure were significantly greater for the SRC-PMMA compared with commercial PMMA (P < 0.05). The flexural strength was not increased in the SRC-PMMA compared with PMMA alone but was greater than that in bone cement (P < 0.05). There were no differences in flexural modulus between any group. The flexural strain-to-failure (30-35% for SRC-PMMA) was about three times greater in SRC-PMMA compared with bone cement and PMMA. Fracture toughness of these SRC-PMMA materials was also significantly greater than PMMA and bone cement (P < 0.001). Fracture toughness values of 3.2 MPa m1/2 were found in the 40 microns SRC-PMMA compared with 2.3 MPa m1/2 for the 120 microns SRC-PMMA and 1.3 MPa m1/2 for PMMA and bone cement. The fatigue strength of both SRC-PMMA samples was significantly greater (P < 0.001) at 80 MPa (10(6) cycles) compared with bone cement and PMMA, both of which had fatigue strengths of about 18 MPa. Fatigue damage in the form of fibre splitting and fibre-matrix interface failure was observed in the SRC-PMMA samples while the PMMA and bone cement showed only smooth fractures. During cyclic fatigue testing, the ongoing damage processes were periodically monitored using several novel computer-based and analysis algorithms. The measured cyclic loads and displacements are used to determine the creep-fatigue displacements, the sample stiffness (or modulus) and the hysteresis damage energy as functions of the number of applied cycles associated with the fatigue loading. The hysteresis damage energy to failure was about 25 times greater in the SRC-PMMA samples (2000 J at 10(6) cycles) compared with bone cement or PMMA at the same number of cycles to failure (80 J) indicating much greater fatigue damage tolerance in these materials. This material, SRC-PMMA, may be applicable for use in several medical and/or dental applications.

Bone Cements↗
PubMed1995

Mechanical analysis of maxillofacial miniplates.

The effect of compression and tensile forces on different types of maxillofacial miniplates was measured, in both as supplied and after bending states. Tests showed all plates showed values of tensile force at failure in excess of that found in most clinical situations; twisting through 90 degrees about the long axis had little effect. In flatwise bending tests there was a wide scatter of values of bending stiffness and ultimate load, indicating that some plates may be stronger than necessary. Repeatedly bending the plates reduced their bending stiffness, particularly in the continuous holed plates, and increased their ultimate load to failure. Values of bending stiffness and ultimate load were much larger when bending in the edgewise direction, with large differences between manufacturers. The data generated in the current study may assist the surgeon in deciding which type of plate to select for a particular clinical situation.

Bone Plates↗
PubMed1995

Postoperative computed tomography scan study of the pterygomaxillary separation during the Le Fort I osteotomy using a micro-oscillating saw.

PURPOSE: The objective of the study was to evaluate the type of pterygomaxillary separation that occurs with use of a micro-oscillating saw. PATIENTS AND METHODS: Sixteen patients underwent a postoperative computed tomography scan. RESULTS: Ideal or near-ideal separations occurred on 26 of 32 sides (81%), while low-level fractures occurred in 6 of 32 sides (19%). No high-level fractures of the pterygoid plates, or fractures extending to the base of the skull or orbit, were seen. There was a striking difference in the number of ideal separations on the right-hand side (94%), compared with the left-hand side (50%), probably because of the greater difficulty of a right-handed surgeon positioning the saw blade correctly on the left side without bending the thin flexible saw blade backward. CONCLUSION: In view of the high percentage of ideal pterygomaxillary separations achieved using a micro-oscillating saw, and the absence of high-level pterygoid plate fractures extending to the base of the skull, this technique is recommended for the pterygomaxillary dysjunction.

Adolescent↗
PubMed1995

Mechanical behaviour and structure of light-cured special tray materials.

OBJECTIVES: This study relates some structural characteristics of representative special tray visible light cured (VLC) resin composites to their creep-recovery, hardness and flexural strength properties. VLC prosthodontic materials have not yet achieved the same acceptance as the VLC resin composites, perhaps because less is known of their structural and performance characteristics. METHODS: The VLC resin composites used were Triad, Convertray and Palatray. The inorganic composition was analysed quantitatively by heating known amounts of resin composite to 800 degrees C, and weighing the ash. Elemental composition was determined using X-ray microanalysis. Mechanical properties, including hardness, flexural behaviour and creep recovery, were obtained by standard methods. RESULTS: The inorganic filler content for Palatray was higher than that of Convertray or Triad, and the flexural modulus and flexural strength of Palatray were superior to that of Triad or Convertray. The mean maximum creep strain for the Palatray specimens was only 2.45% after 5 h loading under 37 MPa and there was a permanent set of 1.5% after 7 h recovery. CONCLUSIONS: Palatray exhibited superior overall properties and this evidently related primarily to the high volume fraction of filler.

Aluminum↗
PubMed1995

Crowding-induced organization of cytoskeletal elements. III. Spontaneous bundling and sorting of self-assembled filaments with different flexibilities.

The typical cell contains ca. 25 vol.-% protein, of which ca. 10% forms cytoskeletal filaments and ca. 90% is non-aggregating globular protein. It has previously been theoretically predicted that, under such highly crowded conditions, rigid filaments will coalesce into tight bundles coexisting with an isotropic solution of globular proteins. In the present work we show that such spontaneous bundling will occur even when filament flexibility is taken into account because the persistence length of the filaments is much longer than the diameter of the globular proteins. The theoretical results are consistent with experimentally observed bundling of F-actin (the most flexible of the three most common types of cytoskeletal filaments) in the presence of globular macromolecules.The main effect of increased filament flexibility on bundling is to cause somewhat looser packing. In mixtures of filaments, differences in flexibilities can lead to segregation. This segregation is accentuated when the stiffer filament is also wider. The results suggest that actin filaments and microtubules will spontaneously form segregated bundles in the presence of cellular concentrations of globular proteins. While cross-linking proteins may serve to stabilize these bundles, their more important function in bundling may be to fine tune the structure (e.g., polarity and registration of filaments).

Cytoskeletal Proteins↗
PubMed1996

Coping with chronic pain: flexible goal adjustment as an interactive buffer against pain-related distress.

In the present study, Brandtstädter's (1992) distinction between assimilation and accommodation as two fundamental means of coping is applied to the field of chronic pain. Assimilative coping involves active attempts (e.g. instrumental activities, self-corrective actions, compensatory measures) to alter unsatisfactory life circumstances and situational constraints in accordance with personal preferences. Conversely, accommodative coping (e.g. downgrading of aspirations, positive reappraisal, self-enhancing comparisons) is directed towards a revision of self-evaluative and personal goal standards in accordance with perceived deficits and losses. Our research is based on the assumption that chronic pain can be described as a major source of threat or impediment to personal goals. When goals are no longer perceived to be attainable through active-assimilative coping efforts, accommodative coping should become increasingly important in dealing with chronic pain. In a study of 120 chronic pain patients, dispositional differences in assimilative (tenacious goal pursuit) and accommodative coping tendencies (flexible goal adjustment), as well as measures of pain-related coping and adjustment (depression, pain-related disability, pain intensity) were assessed. The results suggest that accommodative coping functions as a protective resource by preventing global losses in the psychological functioning of chronic pain patients and maintaining a positive life perspective. Most important, the ability to flexibly adjust personal goals attenuated the negative impact of the pain experience (pain intensity, pain-related disability) on psychological well-being (depression). Furthermore, pain-related coping strategies led to a reduction of disability only when accompanied by a high degree of flexible goal adjustment. The theoretical and clinical implications of these findings for coping research and the treatment of chronic pain patients are discussed.

Adaptation, Psychological↗
PubMed1996

Mechanical properties of normal skin and hypertrophic scars.

The non-linear viscoelastic properties of skin tissue were characterized by modulus of elasticity E, which represents stiffness, and percentage extension (strain, xi) at load intensities of 20, 40 and 100 gm. The latter property is a measure of the extensibility for both normal skin and hypertrophic scar. A quasi-static extensometer applies a standard rate of extension to the skin and its mechanical properties were obtained from 15 Chinese patients with burn injuries of superficial to full skin thickness burns. Clinical evaluation of hypertrophic scar is qualitative and depends on colour, thickness and hardness or firmness. Using mechanical properties assists in the characterization by providing a quantitative indicator. Higher scar grading is synonymous with increased stiffness and decreased extensibility. Correlation with clinical assessment was achieved with these in vivo viscoelastic properties.

Adolescent↗
PubMed2004

Accuracy of third-order bends of nickel-titanium wires and the effect of high and low pressure during memorizing heat treatment.

This study evaluated the accuracy of third-order bends of nickel-titanium wires and determined the effect of high and low pressure for maintaining the wire shape during memorizing heat treatment. A computer-aided bending machine was used to incorporate 200 randomly determined torsional angles between 0 degrees and 60 degrees into 30 linear 0.016 x 0.022-in NeoSentalloy F80 (GAC International, Central Islip, NY) wires. The torsional bendings were randomized into 2 groups. Bends assigned to group 1 (n = 100) received heat treatment of 1.6 MPa (16 bar) pressure, and bends assigned to group 2 (n = 100) received heat treatment of 50 MPa (500 bar) pressure. Cross-sectional cuts from the bent wires were prepared by using standard metallurgical techniques, and the torsional angles were analyzed under computer control. The results of our study show that third-order bends </= 30 degrees can be made with adequate clinical accuracy with an error of 1.89 degrees +/- 1.75 degrees (0 degrees to 10 degrees ) and 3.57 degrees +/- 1.57 degrees (20 degrees to 30 degrees ), irrespective of the pressure applied. With bends > 30 degrees but </= 40 degrees , the method with the higher pressure offers fundamentally greater precision than that with the lower pressure. With torque bends > 40 degrees , the bending error with both methods is clinically unacceptable. In addition to the variability in the dimension and composition of nickel-titanium wires, the scale of the incorporated plastic deformations makes a substantial contribution to the bending error. As far as permitted by the clinical situation, then, the bend should be distributed over the maximum possible wire length. Third-order bends in the first rectangular pseudoelastic nickel-titanium wires represent an efficient means of effecting torque at an early stage. This individualization allows the full therapeutic potential of these archwires to be exploited right from the initial phase of treatment.

Dental Alloys↗
PubMed2004

Force-deflection comparison of superelastic nickel-titanium archwires.

This in vitro study compared the force-deflection behavior of 6 superelastic nickel-titanium orthodontic archwires (0.016 x 0.022 in) under controlled moment and temperature. To simulate leveling, maxillary canine brackets and first molar tubes were bonded in such a manner as to remove the tip and angulation from the system. The wires (n = 10) were passively self-ligated into stainless steel brackets attached to an acrylic jig to simulate the maxillary arch. A testing machine recorded deactivations of 3 distances (5, 4, and 3 mm) at 37 degrees C in the canine position. Force-deflection measurements were recorded from the deactivations only. Forces produced during deactivation, at deflections of 2.5, 2.0, and 1.5 mm, were compared by analysis of variance. Significant differences (P < 0.0001) in forces were observed among the wires at the various deflections. All wires exhibited superelastic behavior, and rankings were derived according to statistically significant differences for each deflection distance.

Analysis of Variance↗
PubMed2006

In-vitro evaluation of the material characteristics of stainless steel and beta-titanium orthodontic wires.

INTRODUCTION: The exact composition and material properties of the metal alloys used in orthodontics are usually not identified by or even available from manufacturers. This makes meaningful comparisons between wires impossible and is unacceptable with regard to biocompatibility issues. The aim of this study was to investigate the material characteristics of contemporary stainless steel (SS) and beta-titanium (beta-Ti) wires, also known as titanium-molybdenum alloy (TMA), for comparison. METHODS: Twenty-two different SS and beta-Ti wires, preferably straight wires sized 0.43 x 0.64 mm, (0.017 x 0.025 in) were tested blindly for wire dimensions, chemical compositions, bending and tensile properties, and surface characteristics. RESULTS: Four chemical compositions were found for the beta-Ti wires: titanium-11.5, molybdenum-6, zirconium-4.5 tin; titanium-3, aluminum-8, vanadium-6, chromium-4, molybdenum-4, zirconium; titanium-6, aluminum-4, vanadium, and titanium-45 niobium. The SS wires were of AISI type 304 or the nickel-free variant BioDur 108. All beta-Ti wires showed high surface roughness values. TMA 02 significantly had the highest E-modulus, TMA 02 and TMA 11 had the highest 0.2% yield strength, TMA 02 had the highest hardness, and TMA 12 was the most ductile wire of the beta-Ti wires. All SS wires showed high 0.2% yield strength, SS 10 significantly had the lowest E-modulus and was the most ductile wire, and SS 08 significantly showed the lowest hardness values of all SS wires. CONCLUSIONS: Significant differences were found between SS and beta-Ti wires, but there was little or no difference between the mechanical and physical characteristics tested in each subgroup. However, the morphological analysis clearly demonstrated that the finishing phase (annealing, polishing) of the wires' production process lacks the quality one would expect with regard to good mechanical properties and biocompatibility issues. Accurate specifications are urgently needed concerning the quality of orthodontic wires on the market.

Analysis of Variance↗
PubMed2006

The effects of friction and flexural rigidity of the archwire on canine movement in sliding mechanics: a numerical simulation with a 3-dimensional finite element method.

INTRODUCTION: The purposes of this article were to clarify the combined effect of friction and an archwire's flexural rigidity on canine movement in sliding mechanics, and to explain how to select a suitable archwire and force level for efficient bodily movement. METHODS: A numerical simulation was carried out by using a 3-dimensional finite element method. RESULTS: As the frictional force decreased, both the net force acting on and the moving speed of the canine increased. The elastic deformation of the archwire increased, and the moving pattern of the canine changed from bodily movement to tipping, although there was no clearance between the archwire and the bracket slot. When a light wire was used, wire deformation increased, and the canine experienced greater tipping. These effects of friction and wire properties on canine movement (tipping and rotational angles) are combined by using a single parameter, EI/P, where EI is the flexural rigidity of the archwire, and P is the net force acting on the canine. A suitable combination of EI and P will cause canine bodily movement. The EI is determined from the archwire's size and material. CONCLUSIONS: We propose a method for estimating a suitable combination in this article.

Computer Simulation↗
PubMed2006

Experimental composite brackets: influence of filler level on the mechanical properties.

INTRODUCTION: Despite their popularity in satisfying esthetic demands, plastic brackets still have some disadvantages because of decreased wear resistance and hardness. Our aim in this study was to investigate the mechanical properties of an alternative bracket-polymer urethane-dimethacrylate (UDMA) reinforced by silicon dioxide fillers with several filler levels. METHODS: Three bracket groups were created: (1) unfilled UDMA, (2) UDMA with a filler content of 40% by volume, and (3) UDMA with a filler content of 70% by volume. Flexural strength, fracture force, and Vickers hardness of the bracket groups were tested. In addition, a 3-medium wear test was performed. RESULTS: The filler-reinforced brackets showed increased hardness, greater stiffness, and improved wear resistance compared with unfilled brackets. Higher filler concentrations in a polymer matrix result in greater stiffness of the polymer. Despite this, the filled composite brackets were not statistically more susceptible to fractures than the unfilled plastic brackets. CONCLUSIONS: Experimental UDMA brackets reinforced with silicon dioxide fillers showed an obvious trend for improved mechanical properties compared with unfilled UDMA brackets.

Composite Resins↗
PubMed2004

[Range of motion of the scoliotic spine in adults].

OBJECTIVE: To describe the movement of the trunk in adults with untreated adolescent idiopathic scoliosis (AIS) and to evaluate their relationship with the age of the patients, the location and angle of the curve, and the number of involved intervertebral segments. MATERIALS AND METHODS: Two hundred adult subjects with untreated AIS, without associated signs, were analyzed with rachimetry. The flexion, extension, bending, and rotation of the trunk were determined for each patient. The results were compared with a different sample: healthy subjects without spinal disease/other groups with more severe scoliosis (treated). RESULTS: In the absence of discopathy, regardless of the angle, location of the curve, or age of the patient, adults with untreated AIS demonstrated a 24% increase in truncal flexion, and a 30% decrease in hip flexion. The other movements studied with rachimetry were comparable to normal subjects. Lesions of the intervertebral segments increased in number and severity with age. These lesions progressively limited the mobility of the spine, initially in extension; and then in bending and flexion. CONCLUSIONS: The measurement of the mobility of the spine and the pelvis is part of the systematic examination of patients with adult scoliosis. This information can be used to improve the patients' health and spinal function; independent of their age and degree of spinal deformity.

Adolescent↗
PubMed2005

[The palmar nail syndrome. Case report].

The congenital ectopic nail is very rare and usually present as an ectopic abnormal nail alongside the normal fingernail. Ridder described in 1992 the "congenital palmar nail syndrome" sharing a palmar ectopic nail, a loss of flexion and a small distal phalanx on x-ray. We report a case of a congenital palmar nail syndrome of both little fingers on a boy 13 years old.

Adolescent↗
PubMed2005

The development of a quantitative flexibility test for body armour and comparison with wearer trials.

In this paper a mechanical flexibility test is developed which can be used to assess multi layer body armour systems. This is compared with a subjective manual test, and then with the results of wearer trials conducted using the recently approved ISO body armour standard ISO 14876-1 (2002). A series of trials was conducted on six different ballistic and/or stab resistant body armour types with a variety of protection levels and constructions. These were tested using the mechanical test system in which the armour was forced through a 200 mm hole by a 100 mm hemispherical plunger. The results of this test were then compared to a second set of trials in which flexibility of the same armour was assessed by manual handling and flexing of the armour. Finally an ergonomic wearer trial was conducted with four armours according to ISO 14876-1 (2002) each armour being assessed by four volunteers and the results compared to flexibility data collected in the first two trials. It was shown that the mechanical flexibility test produced results which were in good agreement with a purely subjective flexibility assessment. These results in turn showed reasonable but not exact correlation with the wearer trials. The ISO wearer trials addressed other factors such as overall comfort and fit of the systems and so the results were not purely a function of flexibility.

Ergonomics↗
PubMed2004

The effects of Pilates training on flexibility and body composition: an observational study.

OBJECTIVE: To assess claims regarding the effects of Pilates training on flexibility, body composition, and health status. DESIGN: An observational prospective study. SETTING: A community athletic club. PARTICIPANTS: A sample of 47 adults (45 women, 2 men) who presented for Pilates training. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Fingertip-to-floor distance, truncal lean body mass by bioelectric impedance, health status by questionnaire and visual analog scale were assessed at baseline, 2, 4, and 6 months (+/-1 wk). RESULTS: Thirty-two of 47 enrolled subjects met the protocol requirements of missing no more than 1 weekly 1-hour session Pilates mat class during each 2-month period. Investigators were blinded to measurements from previous time points. Median (interquartile range [IQR]) fingertip-to-floor distance improved from baseline by 3.4 cm (1.3-5.7 cm), 3.3 cm (0.3- 7.8 cm), and 4.3 cm (1.5-7.6 cm) at 2, 4, and 6 months, respectively (paired nonparametric analysis, all P<.01). There were no statistically significant changes in truncal lean body mass, height, weight, or other body composition parameters. Self-assessment of health also did not change in a statistically significant manner from its baseline median (IQR) value of 77 mm (69-85 mm). CONCLUSIONS: Pilates training may result in improved flexibility. However, its effects on body composition, health status, and posture are more limited and may be difficult to establish. Further study might involve larger sample sizes, comparison with an appropriate control group, and assessment of motor unit recruitment as well as strength of truncal stabilizers.

Adult↗
PubMedDate not supplied

Measurement of basilar artery bending and elongation by magnetic resonance cerebral angiography: relationship to age, sex and vertebral artery dominance.

Elongation and bending of the basilar artery are frequently observed in both normal and pathological subjects. However, their mechanism of generation remain unclear. In the present study, we measured basilar arteries in patients and normal subjects using magnetic resonance angiography (MRA) and estimated the relationships between these anatomical changes and some factors such as age, sex and vertebral artery dominance. A total of 510 subjects underwent MRA. Basilar artery length (BAL), bending length (BL), or side of bending of basilar artery was estimated on the plain images including the basilar artery in two groups with and without vertebral union. BAL was longer in males than in females (P = 0.0009) and correlated with age (P < 0.0001) in the population with vertebral union, while BL did not differ between genders and was not correlated with age in this population. Both BAL and BL were longer in subjects with vertebral artery dominance than in subjects with equivalent-sized vertebral arteries in the population with vertebral union (P < 0.0001 and P = 0.0005). No difference in the frequency of basilar artery bending was observed between males and females and no correlation was observed between the frequency of basilar artery bending and ages in subjects without vertebral artery union. A significant negative correlation between the side of bending of the basilar artery and dominant side of vertebral arteries was found for group of all patients (P < 0.0001). BAL growth may mainly depend on aging, and the presence of vertebral artery dominance may contribute to the growth of BAL while BL growth may depend on vertebral artery dominance mainly in the contralateral direction.

Adolescent↗