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Results for “Polymethyl Methacrylate”

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PubMed1997

PMMA model of steep central islands induced by excimer laser photorefractive keratectomy.

We designed a polymethyl methacrylate (PMMA) model of refractive steep central islands (CIs) induced by PRK. A standardized photorefractive ablation procedure was performed using seven different excimer lasers on test PMMA specimens including 70 flat plates and 98 convex contact lenses. The resulting surface was analyzed by high-resolution confocal microscopy and computerized videokeratoscopy using both TMS-1 and CAS-2000 systems. A total of 50 (54.9%) CIs were observed using computerized videokeratoscopy. The rate of occurrence of CIs was significantly reduced by shock wave absorption (P = 0.0001), aspiration of fumes (P = 0.0044), and smaller diameter ablation (P = 0.0296). The diameter of the CIIs was significantly increased for broad-beam mode ablation (P = 0.016) and for larger ablation zones, (P = 0.042). The refractive power of CIs was significantly increased in the absence of a shock wave absorption system (P = 0.001). Only 20 (40%) of the CIs detected by the TMS-1 device were identified on CAS-2000 at a 0.5 diopter (D) scale resolution level. Shock wave induced deformation and subsequent dynamic alteration of convection forces applied to emitted particles may be the primary mechanism underlying the formation of CIs after PRK, regardless of any biological response of the ablated tissue. Reported rates and characteristics of CIs may largely depend upon the specific design of videokeratoscopes.

Astigmatism↗

PubMedDate not supplied

Fracture resistance of fiber-reinforced PMMA interim fixed partial dentures.

PURPOSE: To compare different fiber reinforcements on fracture toughness of interim polymethyl methacrylate materials and then use the best combination to determine the optimal position for fiber placement in an interim 3-unit fixed partial denture (FPD). MATERIALS AND METHODS: In the first stage of the study, five groups of notched fracture toughness specimens were fabricated and loaded to failure (Instron): (1) unreinforced (control); (2) reinforced with pre-impregnated silanized E-glass fibers (Fibrestick); (3) cold plasma-treated woven polyethylene fibers (Ribbond triaxial); (4) pre-impregnated silanized plasma-treated woven polyethylene fibers (Construct); and (5) 1.0-mm-diameter stainless steel wire. In the second stage, the optimal position (occlusal, middle, or cervical third of pontic) for reinforcement with glass fibers (regimen 2) was tested by loading a 3-unit FPD to failure. All groups were compared with analysis of variance (alpha < 0.05). RESULTS: The fracture toughness (in MPam(1/2)) for each reinforced group (Fibrestick 2.74 +/- 0.12, Construct fibers 2.59 +/- 0.28, Ribbond triaxial 2.13 +/- 0.20, and orthodontic wire 1.66 +/- 0.09) was statistically greater (p< 0.05) than for the unreinforced group (control = 1.25 +/- 0.006). Fracture loads for FPDs were greatest when the fiber reinforcements were placed in the cervical third (cervical = 1165 N). CONCLUSIONS: The use of fiber and, to a lesser extent, orthodontic wire is an effective method to reinforce interim restoration resins.

Analysis of Variance↗

PubMed2006

Experimental determination of particle range and dose distribution in thick targets through fragmentation reactions of stable heavy ions.

In radiation therapy with highly energetic heavy ions, the conformal irradiation of a tumour can be achieved by using their advantageous features such as the good dose localization and the high relative biological effectiveness around their mean range. For effective utilization of such properties, it is necessary to evaluate the range of incident ions and the deposited dose distribution in a patient's body. Several methods have been proposed to derive such physical quantities; one of them uses positron emitters generated through projectile fragmentation reactions of incident ions with target nuclei. We have proposed the application of the maximum likelihood estimation (MLE) method to a detected annihilation gamma-ray distribution for determination of the range of incident ions in a target and we have demonstrated the effectiveness of the method with computer simulations. In this paper, a water, a polyethylene and a polymethyl methacrylate target were each irradiated with stable (12)C, (14)N, (16)O and (20)Ne beams. Except for a few combinations of incident beams and targets, the MLE method could determine the range of incident ions R(MLE) with a difference between R(MLE) and the experimental range of less than 2.0 mm under the circumstance that the measurement of annihilation gamma rays was started just after the irradiation of 61.4 s and lasted for 500 s. In the process of evaluating the range of incident ions with the MLE method, we must calculate many physical quantities such as the fluence and the energy of both primary ions and fragments as a function of depth in a target. Consequently, by using them we can obtain the dose distribution. Thus, when the mean range of incident ions is determined with the MLE method, the annihilation gamma-ray distribution and the deposited dose distribution can be derived simultaneously. The derived dose distributions in water for the mono-energetic heavy-ion beams of four species were compared with those measured with an ionization chamber. The good agreement between the derived and the measured distributions implies that the deposited dose distribution in a target can be estimated from the detected annihilation gamma-ray distribution with a positron camera.

Algorithms↗

PubMed2001

Augmentation of mechanical properties in osteoporotic vertebral bones--a biomechanical investigation of vertebroplasty efficacy with different bone cements.

Recent clinical trials have reported favorable early results for transpedicular vertebral cement reinforcement of osteoporotic vertebral insufficiencies. There is, however, a lack of basic data on the application, safety and biomechanical efficacy of materials such as polymethyl-methacrylate (PMMA) and calciumphospate (CaP) cements. The present study analyzed 33 vertebral pairs from five human cadaver spines. Thirty-nine vertebrae were osteoporotic (bone mineral density < 0.75 g/cm2), 27 showed nearly normal values. The cranial vertebra of each pair was augmented with either PMMA (Palacos E-Flow) or experimental brushite cement (EBC), with the caudal vertebra as a control. PMMA and EBC were easy to inject, and vertebral fillings of 20-50% were achieved. The maximal possible filling was inversely correlated to the bone mineral density (BMD) values. Cement extrusion into the spinal canal was observed in 12% of cases. All specimens were subjected to axial compression tests in a displacement-controlled mode. From load-displacement curves, the stiffness, S, and the maximal force before failure, Fmax, were determined. Compared with the native control vertebrae, a statistically significant increase in vertebral stiffness and Fmax was observed by the augmentation. With PMMA the stiffness increased by 174% (P = 0.018) and Fmax by 195% (P = 0.001); the corresponding augmentation with EBC was 120% (P = 0.03) and 113% (P = 0.002). The lower the initial BMD, the more pronounced was the augmentation effect. Both PMMA and EBC augmentation reliably and significantly raised the stiffness and maximal tolerable force until failure in osteoporotic vertebral bodies. In non-porotic specimens, no significant increase was achieved.

Aged↗

PubMed1999

Intraocular lens tilt and decentration, anterior chamber depth, and refractive error after trans-scleral suture fixation surgery.

OBJECTIVES: To compare the extent of intraocular lens (IOL) tilt and decentration, as well as the anterior chamber depth after trans-scleral suture IOL fixation after either secondary out-of-the-bag or primary in-the-bag IOL implantation. DESIGN: Retrospective, comparative, nonrandomized, interventional study. PARTICIPANTS: Fifty-two eyes that underwent scleral suture fixation were compared with 51 eyes that underwent secondary out-of-the-bag implantation and 50 eyes that underwent in-the-bag implantation. INTERVENTION: One-piece polymethyl methacrylate IOL implantation by three different techniques. MAIN OUTCOME MEASURES: The tilt angle and decentration length of the IOL, as well as the anterior chamber depth, were measured by the Scheimpflug videophotography system. The spherical equivalent error from the predicted value was also examined. RESULTS: The mean tilt angle in the scleral suture fixation group was significantly greater than that in either the out-of-the-bag or the in-the-bag implantation group (P<0.0001). The mean decentration length was also largest in the suture group, followed by the out-of-the-bag group and the in-the-bag group (P<0.0001). The anterior chamber depth in the suture group and the out-of-the-bag group was significantly smaller than that in the in-the-bag group (P<0.0001). The spherical equivalent error in the suture group and the out-of-the-bag group was also greater than that in the in-the-bag group (P<0.0001). CONCLUSIONS: The extent of both tilt and decentration after scleral suture fixation was greater than that observed after either out-of-the-bag or in-the-bag implantation. The anterior chamber depth with the sutured or out-of-the-bag fixated IOL was shallower than that with the in-the-bag fixated IOL, which resulted in a significant myopic shift.

Adolescent↗

PubMed2001

A comparison of palatal adaptation in acrylic resin denture bases using conventional and anchored polymerization techniques.

PURPOSE: The study investigated the effects of palatal depth and a resin anchoring system on the adaptation of denture base resin to the master cast after compression molding and heat polymerization. MATERIALS AND METHODS: Forty-eight virtually identical polymethyl methacrylate dentures were fabricated on master casts with either a deep or shallow palatal vault. One half of the master casts of each palate type were altered by the addition of anchoring holes along the posterior land area, as well as perpendicularly in the midsaggital area. Anchoring holes were made with a #8 round bur to a depth of 5 mm. Twenty-four hours after polymerization, the bases on their casts were sectioned at the posterior border and evaluated for degree of adaptation using a traveling microscope. Maladaptation at the interface of the denture base and master cast was measured at predetermined mediolateral locations. A split-plot analysis of variance (alpha = 0.05) was performed followed by a post-hoc Dunn Multiple Comparison Test. RESULTS: In general, depth of the palatal vault did not significantly influence denture palatal discrepancy (p =.0780), but the use of the anchoring system significantly reduced mean gap distances (p =.000). At lateral and midpalate locations, gap distances between the denture bases and their casts were reduced from approximately 0.3 mm to approximately 0.1 mm when the anchoring system was used. CONCLUSIONS: Mean gap distances for steep palate dentures were significantly less than shallow palate dentures at vestibule and lateral palate locations, and anchoring holes placed in an edentulous master cast along the posterior land area and at the midline significantly improved the adaptation of denture bases.

Acrylic Resins↗

PubMed2004

In vivo aging test for a bioactive bone cement consisting of glass bead filler and PMMA matrix.

The degradation of a new bioactive bone cement (GBC), comprised of an inorganic filler (bioactive MgO-CaO-SiO(2)-P(2)O(5)-CaF(2) glass beads) and an organic matrix [high-molecular-weight polymethyl methacrylate (PMMA)], was evaluated in an in vivo aging test. Hardened rectangular specimens (20 x 4 x 3 mm) were prepared from two GBC formulations (containing 50% w/w [GBC50] or 60% w/w [GBC60] bioactive beads) and a conventional PMMA bone cement control (CMW-1). Initial bending strengths were measured with the use of the three-point bending method. Specimens of all three cements were then implanted into the dorsal subcutaneous tissue of rats, removed after 3, 6, or 12 months, and tested for bending strength. The bending strengths (MPa) of GBC50 at baseline (0 months), 3, 6, and 12 months were 136 +/- 1, 119 +/- 3, 106 +/- 5 and 104 +/- 5, respectively. Corresponding values were 138 +/- 3, 120 +/- 3, 110 +/- 2 and 109 +/- 5 for GBC60, and 106 +/- 5, 97 +/- 5, 92 +/- 4 and 88 +/- 4 for CMW-1. Although the bending strengths of all three cements decreased significantly from 0 to 6 months, those of GBC50 and GBC60 did not change significantly thereafter, whereas that of CMW-1 declined significantly between 6 and 12 months. Thus, degradation of GBC50 and GBC60 does not appear to continue after 6 months, whereas CMW-1 degrades progressively over 12 months. Moreover, the bending strengths of GBC50 and GBC60 (especially GBC60) were significantly higher than that of CMW-1 throughout. It is believed that GBC60 is strong enough for use under weight-bearing conditions and that its mechanical strength is retained in vivo; however, its dynamic fatigue behavior will need assessment before application in the clinical setting.

Animals↗

PubMed2002

Embolization of cranial/spinal tumours and vascular malformations with hydrogel microspheres. An experience of 69 cases.

OBJECTIVE: To evaluate polymethyl methacrylate hydrogel microspheres (HMs) as an embolization material in the management of hypervascular craniofacial and spinal lesions. MATERIAL AND METHODS: Sixty-nine patients with vascular lesions of the craniofacial and spinal regions underwent embolization with HM. The pathologies included craniofacial tumours (n=38), cranial arteriovenous malformations (AVMs) (n=7), dural AVM (n=1), cavernous carotid fistula (n=1), spinal tumours (n=7), spinal AVMs (n=6), orbital lesions (n=4) and scalp AVMs (n=5). Surgery was done in 56 patients. The blood loss at surgery, tumour shrinkage, surgical cleavage and ease of removal of tumour was noted from the records. Histopathology was available in 39 patients. A follow-up of 3 months to 6 years was available in 13 patients in whom embolization was the sole treatment. Surgical and histopathological findings were correlated with angio-embolization findings. RESULTS: HMs were easy to inject through the microcatheter with good control. Good devascularization was obtained in 61 patients and partial in 8 patients. Two patients developed complications due to presence of anastomoses between intra- and extracranial circulation and 1 patient due to tumor swelling. Histopathology showed presence of HMs in all the lesions, with minimal inflammatory reaction in 2 patients. Patients with spinal lesions treated with embolization only stabilised in neurologic deficits. CONCLUSION: HMs are highly suitable as embolization agents in the management of craniofacial and spinal hypervascular lesions.

Antimutagenic Agents↗

PubMed2000

Phantom materials for boron neutron capture therapy.

The aim of this work was to establish which reference phantom material is most suited for dosimetry under reference conditions of neutron beams for boron neutron capture therapy (BNCT). For this purpose, phantoms of dimensions 15 x 15 x 15 cm3 and 30 x 30 x 30 cm3, composed of water, tissue-equivalent (TE) liquid, polyethylene (PE), polymethyl methacrylate (PMMA) and water containing 10 microg g(-1) and 30 microg g(-1) 10B were irradiated using the Petten BNCT beam. Activation foils and a diode detector were used for the determination of the thermal neutron fluence rate. The gamma-ray dose rate and the fast neutron dose rate were determined using paired ionization chambers. In water, PMMA and TE liquid the absolute dose and fluence values agreed within 3% at a reference depth of 2 cm, with the exception of the gamma-ray dose rate in PMMA, which was 12% lower than in water. Due to a higher hydrogen concentration in PE compared with water, the dose and fluence values in PE differed more than 30% from those in water. Only minor differences were observed between the percentage depth dose curves for the various dose components in water, PMMA and TE liquid. The addition of 10 microg g(-1) and 30 microg g(-1) 10B to water resulted in a decrease in the absolute thermal neutron fluence at 2 cm depth of about 2% and 8%, respectively, and a decreased penetration of thermal neutrons at depth for the 30 microg g(-1) 10B concentration. For reference dosimetry of an epithermal neutron beam for BNCT, both water and TE liquid are suitable phantom materials. For practical reasons, water is therefore proposed as reference phantom material. For measurements requiring a solid phantom, PMMA is proposed. The lower gamma-ray dose in PMMA compared to water, however, needs to be taken into account.

Boron↗

PubMed2003

Quantification of posterior capsule opacification with round and sharp edge intraocular lenses.

PURPOSE: To quantitatively evaluate and compare intraocular lenses (IOLs) with a round or sharp optic edge design for posterior capsule opacification (PCO). STUDY DESIGN: Prospective comparative observational case series. PARTICIPANTS/MATERIALS: Photographs from 174 eyes were analyzed for PCO at the Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany. MAIN OUTCOME MEASURES: Part I: 121 eyes of 121 patients were analyzed for quantification of PCO. IOLs evaluated were Corneal ACR6 (n = 21), Alcon Acrysof (n = 20), Allergan AR40 (n = 27), Pharmacia 811 one-piece polymethyl methacrylate (PMMA) IOL (n = 24), and Pharmacia 911A silicone IOL (n = 29). Mean follow-up was 14.01 +/- 2.81 months; mean patient age was 73.2 +/- 7.3 years. The morphologic PCO formation was evaluated for the entire optic and in the central 3-mm zone. Part II: In 53 eyes of 46 patients aged 73.4 +/- 10.8 years with an Alcon Acrysof IOL, PCO formation and capsulorrhexis/optic overlapping were analyzed 34.2 +/- 4 months after cataract surgery using EPCO Software. RESULTS: Part I: The PCO values of the entire optic were for Corneal ACR6, 1.93 +/- 0.62; PMMA, 0.64 +/- 0.63; AR40, 0.55 +/- 0.28; Alcon Acrysof, 0.145 +/- 0.27; and 0.161 +/- 0.181 for the Pharmacia 911A IOL (P < 0.01). The PCO values of the central 3-mm zone were for Corneal ACR6, 1.64 +/- 0.96; PMMA, 0.49 +/- 0.39; AR40, 0.22 +/- 0.32; Alcon Acrysof, 0.08 +/- 0.21; and 0.06 +/- 0.11 for the Pharmacia 911A IOL (P < 0.01). Part II: Average overlapping of capsulorrhexis and Acrysof IOL optic was 40.5% +/- 12.4%. There was a significant correlation between PCO values and overlapping (r = -0.69, P < 0.001). CONCLUSIONS: The sharp-edge IOL types (Alcon Acrysof and Pharmacia 911A silicone IOL) resulted in statistically significantly lower PCO values for analysis of the entire optic area and central 3-mm zone. There was no statistically significant difference in PCO values between the two sharp-edge optic IOLs. An overlapping of capsulorrhexis rim and the anterior IOL optic surface of more than 20% resulted in significantly lower PCO values with the Acrysof IOL.

Acrylic Resins↗

PubMed1999

Effects of water immersion on mechanical properties of new esthetic orthodontic wire.

New fiber-reinforced plastic orthodontic wire (FRP wire) was fabricated with polymethyl methacrylate (PMMA) for the matrix and biocompatible CaO-P(2)O(5)-SiO(2)-Al(2)O(3) (CPSA) glass fibers for fibers that have not only high esthetics but also mechanical properties similar to those of metal wires. The purpose of this study was to evaluate the effects of water immersion on the mechanical properties of this new wire. The fiber-reinforced plastic orthodontic wire specimens were 0.5 mm in diameter with 29.1% to 60. 4% volume fraction of fibers that were 20 microm in diameter. A three-point flexural test was performed to obtain the elastic modulus and flexural load at the deflection of 1 mm under dry and wet conditions. Stress relaxation of the wires was tested under dry and wet conditions, and the wire diameters were measured before and at 20 days after immersion. The results showed that there were changes in the elastic modulus up until 10 days and in flexural load up until 20 days after immersion. The values of these two at 30 days after immersion were 93% and 87%, respectively, of those before immersion. Stress relaxation occurred rapidly from the start of immersion until about 60 minutes under dry conditions and about 120 minutes under wet conditions and then approached saturation. The swelling of hydrated fiber-reinforced plastic orthodontic wires affected the wire diameter, although this effect was not significant. The results of this study therefore suggest that the mechanical properties of fiber-reinforced plastic orthodontic wires are reduced by water immersion in the initial stage.

Dental Materials↗

PubMed2005

Intraocular lens factors that may affect anterior capsule contraction.

PURPOSE: To examine the various factors comprising intraocular lenses (IOLs) that may affect the degree of anterior capsule contraction after cataract surgery. DESIGN: Randomized controlled clinical trials. PARTICIPANTS: Three hundred thirty-one patients scheduled for bilateral cataract surgery were studied to compare the degree of anterior capsule contraction in fellow eyes that received different IOLs with regard to the following factors: (1) optic material-acrylic optic versus silicone optic; (2) optic design-round-edge optic versus sharp-edge optic; (3) haptic material-polymethyl methacrylate (PMMA) loop versus polyvinylidene fluoride loop; and (4) haptic material and design-single-piece acrylic haptic versus 3-piece PMMA haptic. The 2 IOLs implanted in the fellow eyes of each patient had almost the same material and design, except for the specific factor being compared. METHODS: All patients underwent phacoemulsification surgery with implantation of different IOLs in fellow eyes. MAIN OUTCOME MEASURES: The area of the anterior capsule opening was measured using the Scheimpflug videophotography system at 3 days or 1 week (baseline) and at 1, 3, and 6 months after surgery. The percent reduction in the anterior capsule opening area from baseline was determined at 1, 3, and 6 months after surgery. RESULTS: The mean percent reduction of the anterior capsule opening area was significantly greater in eyes with a silicone optic IOL than in eyes with an acrylic optic IOL. However, no significant difference was observed in the percent reduction between round-edge and sharp-edge optic IOLs, between PMMA and polyvinylidene fluoride loop IOLs, and between single-piece acrylic and 3-piece PMMA haptic IOLs. Strong associations were found in the percentage between round-edge and sharp-edge optic IOLs, between PMMA and polyvinylidene fluoride loop IOLs, and between single-piece acrylic and 3-piece PMMA haptic IOLs, whereas there was a relatively weak association between silicone and acrylic optic IOLs. CONCLUSIONS: Among the IOL factors examined, optic material affects most significantly the degree of anterior capsule contraction, whereas optic design and haptic material and design are not strongly related to anterior capsule contraction.

Acrylic Resins↗

PubMed2005

The sustaining effect of three polymers on the release of chlorhexidine from a controlled release drug device for root canal disinfection.

OBJECTIVES: The aim of this in vitro study was to evaluate the suitability of using chitosan, poly (lactide-co-glycolide) (PLGA), and polymethyl methacrylate (PMMA) to control the release of chlorhexidine digluconate (CHX) from a prototype of controlled release drug device for root canal disinfection. STUDY DESIGN: Four different prototypes with different formulations were prepared. Group A (n = 12): the device (absorbent paper point) was loaded with CHX as control. Group B (n = 12): same as group A, but the device was coated with chitosan (Texan MedTech). In Groups C and D, the device was treated in the same way as group A and then coated 3 times with 5% PMMA (Group C, n = 12, Aldrich), or coated 3 times with 3% PLGA (Group D, n = 12, Sigma). The devices were randomly allocated to experimental groups of 12 each. All the prototypes of controlled release drug device were soaked in 3 mL distilled water. The concentrations of CHX were determined using a UV spectrophotometer. The surface characteristics of each prototype were observed using a scanning electron microscope. RESULTS: The result showed that release rate of CHX was the greatest in the noncoated group, followed by the chitosan-coated group, the PLGA-coated group, and the PMMA-coated group (P < 0.05). Pores were observed on the surface of the prototypes that were coated with PLGA and PMMA. When the pore size was smaller, the release rate was lower. CONCLUSION: These data indicate that polymer coating can control the release rate of CHX from the prototypes of controlled release drug device.

Analysis of Variance↗

PubMed2003

The effect of length and concentration of glass fibers on the mechanical properties of an injection- and a compression-molded denture base polymer.

STATEMENT OF PROBLEM: Fiber-reinforcement has been used to overcome the mechanical limitations of denture base polymers. One major difficulty in the use of fiber reinforcement has been the addition of fibers during conventional processing methods. PURPOSE: This study evaluated the effect of various lengths and concentrations of chopped E-glass fiber-reinforcement on the transverse strength, modulus of elasticity, and impact strength of injection and compression-molded polymethyl methacrylate based denture base polymer. MATERIALS AND METHODS: Test specimens (n=10) of 4-, 6-, and 8-mm fiber length and 1%, 3%, and 5% weight fiber concentrations were prepared with either an injection or a compression-molded processing method. Denture base polymer specimens without any fiber reinforcement were used as control for both processing methods. Transverse strength test specimens (65 x 10 x 2.5 mm) were stored in water bath at 37 degrees C for 2 weeks. The transverse strength (MPa) and modulus of elasticity (GPa) was measured with the 3-point bending test. Impact strength (kJ/m(2)) test specimens (60 x 7.5 x 4 mm) were tested with the Charpy-type pendulum impact test setup. The data were analyzed with multifactorial analysis of variance and Tukey post hoc tests (alpha=.05). RESULTS: Injection-molded fiber-reinforced groups showed significantly higher transversal strength, elastic modulus, and impact strength compared with compression-molded groups (P <.001). In the injection-molded groups, fiber concentration increased all mechanical properties tested (P <.05), but fiber length only increased transverse strength and modulus of elasticity (P <.05). In the compression molded groups, fiber concentration affected modulus of elasticity and impact strength significantly (P <.05), but fiber length did not show any significant effect on the mechanical properties tested (P >.05). CONCLUSION: The transverse strength, elastic modulus and impact strength of injection-molded denture base polymer increased significantly with the use of chopped E-glass fibers, whereas the effect was not significant with the compression-molded polymer.

Analysis of Variance↗

PubMed2004

Contact lens-induced corneal swelling and surface changes measured with the Orbscan II corneal topographer.

PURPOSE: The purpose of this study was to measure central and topographical corneal swelling in response to contact lens wear and eye closure, to determine whether the swelling induced by soft and polymethyl methacrylate (PMMA) lenses is different, and to determine whether the anterior and/or posterior corneal shape alters with corneal swelling. METHODS: An Orbscan II corneal topographer was used to measure corneal swelling and the shape of the anterior and posterior corneal surfaces of 16 neophytes before and after wearing soft and PMMA contact lenses with near-zero oxygen transmissibility. The lens-wearing eye was patched for 3 h. RESULTS: The mean 15.1% +/- 3.8% (+/- SD) central corneal swelling with soft lenses was significantly >12.6% +/- 4.1% with PMMA lenses. Topographical corneal swelling was significantly greater with soft vs. PMMA lenses. However, the difference between central and peripheral corneal swelling was much greater with PMMA lenses. With both lenses, the cornea swelled significantly more in the center than the periphery. The anterior best-fit sphere radius remained unchanged in response to soft lenses (0.00 +/- 0.04 mm) and steepened slightly but significantly with PMMA lenses (-0.04 +/- 0.05 mm). The posterior best-fit sphere radius flattened significantly with both lenses (0.12 +/- 0.07 mm with soft and 0.14 +/- 0.08 mm with PMMA lenses). CONCLUSIONS: Corneal swelling (greater centrally than peripherally) flattens the posterior surface of the cornea and is independent of lens type. Although the anterior best-fit sphere radius steepened with PMMA, the magnitude is probably clinically unimportant. Both lens types produced greater central vs. peripheral corneal swelling. However, the soft lens induced significantly greater overall swelling than PMMA. Because their oxygen transmission was the same, these results suggest that there is lateral diffusion of oxygen from the peripheral area of the cornea (that is not covered by the lens) toward the center.

Adult↗

PubMed2002

The bond strength of light-curing composite resin to finally polymerized and aged glass fiber-reinforced composite substrate.

OBJECTIVE: This study examines the shear bond strength of visible light-curing composite resin (VCR) to aged glass fiber-reinforced composite (FRC) substrate with multi-phase polymer matrix. METHODS: Linear polymethyl methacrylate and dimethacrylate monomer preimpregnated unidirectional glass fiber reinforcement was used as an adhesion substrate for low-viscosity diacrylate veneering composite resin and restorative composite resin. A total of 60 test specimens were divided into three groups according to the brand and the use of an intermediate monomer resin (IMR). The used IMRs were either BisGMA-HEMA-resin, BisGMA-TEGDMA resin or the controls were left without the IMR treatment. Dry- and water-stored FRC-substrates were used for adhering the VCR with or without the IMR. The shear bond strength of the VCR to the substrate was measured for dry and thermocycled specimens and the results were analyzed with multi-variate ANOVA. RESULTS: The highest mean shear bond strength (23.9 +/- 4.8 MPa) was achieved with FRC/BisGMA-HEMA/VCR combination when the FRC substrate was water stored and the test specimen was thermocycled. FRC/BisGMA-TEGDMA/VCR combination resulted in 15.7 +/- 6.0 MPa with the water-stored FRC substrate and after thermocycling of the test specimens. The lowest shear bond strength (1.0 +/- 0.5 MPa) was obtained with FRC/VCR combination with water-stored substrate and after thermocycling of the test specimens. Significant differences were found between the mean values of three groups according to the use of IMR (p<0.001). The storage conditions of the FRC substrate were related to brand of the IMR or the composite (p<0.001). High mean values of the shear bond strength after thermocycling fatigue were related to the type of IMR (p<0.001). SIGNIFICANCE: The results suggest that the IMRs used in this study greatly influence the mean shear bond strength values when the test specimens are thermocycled.

Analysis of Variance↗

PubMed1999

A comparison of visual performance between a rigid PMMA and a foldable acrylic intraocular lens.

PURPOSE: To examine post-operative visual outcomes when using micro-incision surgery and a three-piece, foldable acrylic intraocular lens (IOL) and when using small-incision surgery and a one-piece, rigid polymethyl-methacrylate (PMMA) IOL. METHODS: Phacoemulsification and IOL implantation was conducted by one surgeon on 86 subjects (aged 50-89 years) with cataracts. Of these subjects, 67 completed the study. Subjects were implanted either with a foldable IOL (n = 38) or a rigid IOL (n = 29). Best corrected Snellen visual acuity and contrast sensitivity (2-28 cycles/degree) were measured approximately 2 months post-operatively. Comparisons were made between the pseudophakic groups and an additional group of 28 age-matched, normal subjects (aged 58-81 years). Non-parametric (Kruskal-Wallis and Mann-Whitney U) as well as parametric (ANOVA and Pearson) statistics tests were used in the analysis of results. RESULTS: All pseudophakes had corrected visual acuity of 6/9 or better. There were no significant differences in visual acuity (Kruskal-Wallis, p = 0.61) or contrast sensitivity (ANOVA, p = 0.33) between the three groups. The mean residual refractive error of the foldable IOL group was hypermetropic in comparison with the rigid IOL group but not significant at the 99% level of probability (Mann-Whitney U, p = 0.02). There was no significant difference in astigmatism between the groups. CONCLUSIONS: Implanting a foldable acrylic IOL gave no post-operative benefit in visual acuity and contrast sensitivity to pseudophakes over a less expensive rigid PMMA IOL, within this post-operative period. This study did not provide a comparison between the two lenses regarding post-operative complications, in particular the important long-term complication of posterior capsular opacification (PCO). Until and if any particular advantage of a given IOL in reduced incidence of PCO is confirmed, it appears more economical to implant the conventional rigid PMMA IOL using small-incision surgery.

Acrylic Resins↗

PubMed2003

Effect of thermocycling on tensile bond strength of six silicone-based, resilient denture liners.

STATEMENT OF PROBLEM: Adhesion failure between silicone resilient denture lining materials and denture base resin is commonly encountered in clinical practice. Adhesion failure results in localized unhygienic conditions at the debonded regions and often causes functional failure of the prosthesis. PURPOSE: The purpose of this study was to investigate the effect of thermocycling on the tensile bond strength of 6 soft lining materials. MATERIAL AND METHODS: Six commonly used silicone-based resilient denture liners (Ufigel C, Ufigel P, Mollosil, Molloplast B, Permafix, and Permaflex) were chosen for the investigation. The bond strength was determined, in tension, after processing to PMMA. The resilient denture liners for each group (n = 24) were 10 x 10 x 3 mm and were processed between 2 polymethyl methacrylate specimens according to manufacturer's instructions. Two PMMA specimens were prepared by investing brass dies with a 3-mm-thick spacer in a denture flask. Specimens were made by processing the resilient denture liners against the polymerized PMMA block. After polymerization, the brass spacer was removed from the mold, the 2 PMMA resin specimens were trimmed, and the surfaces to be bonded were smoothed. The PMMA block was placed back into the molds and the resilient denture liners were packed into the space made by brass spacer, trial packed, and polymerized according to the manufacturer's instructions. Half of the specimens for each group were stored in water for 24 hours, and the other half were thermocycled (5000 cycles) between baths of 5 degrees and 55 degrees C. All specimens were placed under tension until failure in a universal testing machine at a crosshead speed of 5 mm/min. The maximum tensile stress before failure and mode of failure were recorded. The mode of failure was characterized as cohesive, adhesive, or mixed mode, depending on whether the fracture surface was in the soft liner only, at the denture base-soft liner interface only, or in both. Failure strength was recorded in kg/cm(2). Results were tested by multiple analysis of variance (ANOVA) for mode of failure (adhesive, cohesive, and mix), 2-way ANOVA (storage-products), and 1-way ANOVA (storage-products interaction, before and after thermocycling). Duncan's test was used to determine whether significant changes in the tensile bond properties of the materials occurred during thermocycling. RESULTS: Varying degrees of bond strengths were found for soft lining materials and were significantly different (P<.05). Results of this study also indicated that the bond strengths of soft lining materials had significantly decreased after thermocycling except Ufigel C and Mollosil. CONCLUSIONS: Because the adequate adhesive value for soft lining materials is given 4.5 kg/cm(2), all of the materials were acceptable for clinical use.

Adhesiveness↗