Cynthia S. Petrie

Dimensional accuracy and surface detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under dry, moist, and wet conditions☆

STATEMENT OF PROBLEM A major limitation of vinyl polysiloxane (VPS) impression materials is their hydrophobicity. There are 2 aspects to this problem, the wettability of the polymerized impression by dental gypsum materials and the ability of the unpolymerized material to wet intraoral tissues. To address this problem, manufacturers have added surfactants and labeled the new products as hydrophilic vinyl polysiloxane. PURPOSE The purpose of this investigation was to compare dimensional accuracy and surface detail reproduction of 2 hydrophilic VPS impression materials, when used under dry, moist, and wet conditions. MATERIALS AND METHODS A total of 102 impressions were made of stainless steel metal dies similar to those described in American Dental Association (ADA) specification 19. The dies had 2 vertical and 3 horizontal lines inscribed on their superior surfaces. Impressions were made under dry, moist, and wet conditions. Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression to the same line on the metal die, by use of a measuring microscope with an accuracy of 0.001 mm. A 2-way analysis of variance and least significant difference post hoc test were used to compare mean dimensional changes (alpha=.05). Surface detail reproduction was evaluated in 2 ways: (1) by use of criteria similar to ADA specification 19 for detail reproduction, continuous replication of at least 2 of the 3 horizontal lines, and (2) by use of a method developed for this study that categorized the impressions as satisfactory or unsatisfactory based on their surface characteristics: presence of pits, voids, or roughness. Pearson chi(2) (alpha=.05) was used to compare detail reproduction results. RESULTS Conditions (dry, moist, and wet) did not cause significant adverse effects on the dimensional accuracy of either material. The mean dimensional change and SD were 0.005% +/- 0.002% or less. With both surface detail analyses, dry, moist, and wet conditions had a significant effect on the detail reproduction of both materials (P<.05). Only under dry conditions did both impression materials continuously replicate at least 2 of the 3 horizontal lines 100% of the time. Under moist conditions, 82% of the Aquasil impressions and 100% of the Reprosil impressions were judged satisfactory, while under wet conditions, only 47% Aquasil and 11% Reprosil impressions were satisfactory. With the additional surface detail characterization, only under dry conditions were impressions produced with clinically acceptable surface quality (Aquasil 77% and Reprosil 100% satisfactory). CONCLUSIONS Dimensional accuracy of both materials tested was well within ADA standards. Best surface detail results were obtained only under dry conditions for both materials.

Comparison of recordings obtained with computerized axiography and mechanical pantography at 2 time intervals.

PURPOSE The purpose of this study was to compare recordings of mandibular movements obtained with a Denar mechanical pantograph to those obtained with a Denar computerized axiograph (Cadiax compact). MATERIALS AND METHODS Pantographic recordings and computerized axiograms were collected on 10 subjects. All of the subjects had intact dentition and no clinical signs or symptoms of temporomandibular dysfunction. Tracings produced with the 2 instruments were compared using pantographic reproducibility indices (PRIs). Recordings with each instrument were repeated twice and assessed for consistency. Pearsons correlation coefficient was used to compare PRI scores obtained with the pantograph and the axiograph. Recordings with both instruments were repeated 2 weeks to 1 month later, and new PRI scores were calculated. Consistency between initial and second PRI scores for each instrument was assessed using a coefficient of stability. RESULTS The coefficients of stability between the initial and second recordings were r = 0.91 for mechanical pantography and r = -0.06 for computerized axiography. Comparison of recordings made by mechanical pantography and computerized axiography at the initial and second recordings showed poor concordance, with coefficients of 0.23 and -0.11, respectively. CONCLUSIONS PRI scores recorded with the mechanical pantograph were consistent between the initial and second recordings. In contrast, recordings with the computerized axiograph appeared highly inconsistent across time. PRI scores calculated from the axiographs were significantly higher than PRI scores calculated from pantographic tracings for the same subject.

SHAPE OPTIMIZATION OF DENTAL IMPLANT DESIGNS UNDER OBLIQUE LOADING USING THE p-VERSION FINITE ELEMENT METHOD

The aim of this study was to introduce design optimization of an endosseous dental implant in order to minimize cortical bone strains at the crest of the implant/bone interface. Linear elastic p-version finite element analysis was used to find the optimal design in four different two-dimensional finite element models of the mandibular bone: model 1 (medium density cancellous bone and 2 mm thickness of cortical bone), model 2 (low density cancellous bone and 2 mm thickness of cortical bone), model 3 (medium density cancellous bone and 1 mm thickness of cortical bone), model 4 (low density cancellous bone and 1 mm thickness of cortical bone). Shape of the implant was evaluated with respect to its length and diameter, as well as the presence or absence of taper in the neck. Implants with maximum length, maximum diameter, and no taper had the lowest crestal strains in models 1, 3, and 4. Taper in the neck of the implant increased the strains in models 1, 3, and 4. In cases of poor quality of bone, increasing the diameter of the implant appeared to be the most effective means of reducing crestal bone strains.

Correlation of Impression Removal Force with Elastomeric Impression Material Rigidity and Hardness

PURPOSE Difficult impression removal has been linked to high rigidity and hardness of elastomeric impression materials. In response to this concern, manufacturers have reformulated their materials to reduce rigidity and hardness to decrease removal difficulty; however, the relationship between impression removal and rigidity or hardness has not been evaluated. The purpose of this study was to determine if there is a positive correlation between impression removal difficulty and rigidity or hardness of current elastomeric impression materials. MATERIALS AND METHODS Light- and medium-body polyether (PE), vinylpolysiloxane (VPS), and hybrid vinyl polyether siloxane (VPES) impression materials were tested (n = 5 for each material/consistency/test method). Rigidity (elastic modulus) was measured via tensile testing of dumbbell-shaped specimens (Die C, ASTM D412). Shore A hardness was measured using disc specimens according to ASTM D2240-05 test specifications. Impressions were also made of a custom stainless steel model using a custom metal tray that could be attached to a universal tester to measure associated removal force. Within each impression material consistency, one-factor ANOVA and Tukeys post hoc analyses (α = 0.05) were used to compare rigidity, hardness, and removal force of the three types of impression materials. A Pearsons correlation (α = 0.05) was used to evaluate the association between impression removal force and rigidity or hardness. RESULTS With medium-body materials, VPS exhibited significantly higher (p ≤ 0.05) rigidity and hardness than VPES or PE, while PE impressions required significantly higher (p ≤ 0.05) removal force than VPS or VPES impressions. With light-body materials, VPS again demonstrated significantly higher (p ≤ 0.05) hardness than VPES or PE, while the rigidity of the light-body materials did not significantly differ between materials (p > 0.05); however, just as with the medium-body materials, light-body PE impressions required significantly higher (p ≤ 0.05) removal force than VPS or VPES. Moreover, there was no positive correlation (p > 0.05) between impression removal force and rigidity or hardness with either medium- or light-body materials. CONCLUSIONS The evidence suggests that high impression material rigidity and hardness are not predictors of impression removal difficulty.

A preliminary three-dimensional finite element analysis of mandibular implant overdentures.

A treatment protocol that may lead to reduced mandibular posterior residual ridge resorption in patients with overdentures retained and supported by two interforaminal implants was investigated. The treatment included the addition of short implants in the posterior edentulous mandible for the presumed purpose of favorable provision of mechanical load stimulus to alveolar bone. Three-dimensional finite element analysis was used to model cited effective strains that may stimulate bone remodeling in two selected models. Based on this laboratory study, the addition of posterior short implants has a favorable effect in maintaining bone mass under implant retained overdentures.

Effect of airborne-particle abrasion and aqueous storage on flexural properties of fiber-reinforced dowels.

PURPOSE A great range of clinical failures have been observed with fiber-reinforced dowels, often attributed to fracture or bending of the dowels. This study investigated flexural properties of fiber-reinforced dowels, with and without airborne-particle abrasion, after storage in aqueous environments over time. Scanning electron microscopy (SEM) was used to analyze the mode of failure of dowels. MATERIALS AND METHODS Two dowel systems (ParaPost Fiber Lux and FibreKor) were evaluated. Ten dowels of each system were randomly assigned to one of six experimental groups: 1--control, dry condition; 2--dowels airborne-particle abraded and then stored dry; 3--dowels stored for 24 hours in aqueous solution at 37°C; 4--dowels airborne-particle abraded followed by 24-hour aqueous storage at 37°C; 5--dowels stored for 30 days in aqueous solution at 37°C; 6--dowels airborne-particle abraded followed by 30-day aqueous storage at 37°C. Flexural strength and flexural modulus were tested for all groups according to American Society of Testing and Materials (ASTM) standard D4476. One failed dowel from each group was randomly selected to be evaluated with SEM equipped with energy dispersive spectroscopy (EDS) to characterize the failure pattern. One intact dowel of each system was also analyzed with SEM and EDS for baseline information. RESULTS Mean flexural modulus and strength of ParaPost Fiber Lux dowels across all conditions were 29.59 ± 2.89 GPa and 789.11 ± 89.88 MPa, respectively. Mean flexural modulus and strength of FibreKor dowels across all conditions were 25.58 ± 1.48 GPa and 742.68 ± 89.81 MPa, respectively. One-way ANOVA and a post hoc Dunnetts t-test showed a statistically significant decrease in flexural strength as compared to the dry control group for all experimental groups stored in water, for both dowel systems (p < 0.05). Flexural modulus for both dowel systems showed a statistically significant decrease only for dowels stored in aqueous solutions for 30 days (p < 0.05). Airborne-particle abrasion did not have an effect on flexural properties for either dowel system (p > 0.05). SEM and EDS analyses revealed differences in composition and failure mode of the two dowel systems. Failed dowels of each system revealed similar failure patterns, irrespective of the experimental group. CONCLUSIONS Aqueous storage had a negative effect on flexural properties of fiber-reinforced dowels, and this negative effect appeared to increase with longer storage times. The fiber/resin matrix interface was the weak structure for the dowel systems tested.