In vitro comparison of master cast accuracy for single-tooth implant replacement

Comparative Evaluation of the Rigidity and Accuracy of Different Elastomeric Impression Materials with Open Tray Implant Level Impression Technique-An In vitro Study

Introduction

An implant-supported prosthesis requires a precise imprint to provide a good fit. It has been suggested that for implant/abutment level impressions, both the indirect (closed tray) and direct (open tray) methods are effective in achieving a passively fitting prosthesis.

Aim

With an open plate embed level impression technique, this in vitro investigation set out to evaluate three different elastomeric imprint materials for their relative rigidity and precision.

Materials and Methods

Sixty bespoke trays were created. Part I (rotational opposition assessment) of the investigation included selecting 30 uniquely shaped plates at random and using them to create embed-level open plate engraves. The second part of this investigation (evaluation of rotational discrepancy and vertical inconsistency) used open plate embed level impressions generated with the remaining thirty individualized plates to evaluate how well these materials recreated the patient's impressions.

Result

Within the bounds of the current investigation, vinylpolysiloxane had the highest degree of stiffness among the impression materials examined in comparison to polyether and vinylsiloxanether, as seen by its much greater rotational resistance to torquing. A rotational discrepancy was shown to be much lower in polyether open tray implant level impressions compared to vinylpolysiloxane and comparable to vinylsiloxane.

Conclusion

In this investigation, the firmest impression medium was vinylpolysiloxane, followed by polyether and vinylsiloxanether for taking imprints of implants. Polyether material, however, was more precise than vinylpolysiloxane and vinylsiloxanether.

An assessment of the passivity of the fit of multiunit screw-retained implant frameworks manufactured by using additive and subtractive technologies

STATEMENT OF PROBLEM

The lack of passive fit in implant-supported restorations can lead to mechanical and biological complications and compromise the longevity of the prosthesis. The manufacturing technique and evaluation site are factors that may affect the passive fit of multiunit screw-retained implant frameworks. However, scientific information regarding this issue is lacking.

PURPOSE

The purpose of this in vitro study was to investigate the effect of manufacturing technique and evaluation site on the passive fit of multiunit screw-retained implant frameworks.

MATERIAL AND METHODS

Two multiunit implant analogs were placed into the right second premolar and second molar sites of a mandibular typodont model. A total of 50 3-unit Co-Cr frameworks were fabricated with 3 indirect (conventional technique, polymethyl methacrylate milling, stereolithography) and 2 direct techniques (selective laser melting and soft alloy milling). The patterns obtained by indirect techniques were subsequently cast. The Sheffield test was used for the assessment. Digital images of the sites were obtained by using a stereomicroscope at ×40 magnification, and the measurement points (n=10 for each site) were examined to record the vertical marginal discrepancy values (μm) with the aid of a measuring software program. The collected data were subjected to the 2-way ANOVA and Tukey honestly significant difference test (α=.05).

RESULTS

The influence of the manufacturing technique (variable 1) on the vertical marginal discrepancy values was statistically significant (P<.001). However, the evaluation site (variable 2) (P=.097) and the interaction of the variables (P=.960) were not statistically significant. The lowest misfit values were observed for selective laser melting (74.2 ±20.5 μm) followed by stereolithography (92.8 ±23.9 μm), soft alloy milling (108.4 ±12.0 μm), polymethyl methacrylate milling (116.7 ±17.0 μm), and conventional technique (137.5 ±18.9 μm). The vertical marginal discrepancy values of the selective laser melting group were significantly lower than those of all other groups (P<.05).

CONCLUSIONS

The manufacturing technique significantly affected the passive fit. selective laser melting-fabricated frameworks demonstrated superior fitting accuracy. Among the indirect techniques, stereolithography-fabricated frameworks revealed the lowest misfit values. The vertical marginal discrepancy values of all manufacturing groups were within the range of clinical acceptability (<150 μm).

Influence of Implant Angulation and Implant Number on the Accuracy of Definitive Casts

Background:

The present study determined the effect of implant angulation and implant number on the dimensional precision of implant definitive casts.

Materials and Methods:

Three definitive casts with implant analogs placed in a triangular pattern were made from dental stone. Group I was control group, Group II in which implant numbers 1 and 3 were at 5° convergence to the implant number 2. Group III in which implant numbers 1 and 3 are at 5° divergence to the implant number 2. Group IV in which implant numbers 1 and 3 are at 10° convergence to the implant number 2. Implant analogs were secured in all the definitive casts with cyanoacrylate. Three open tray impressions of the definitive cast were obtained and poured in Type IV dental stone. Coordinates in the three planes were measured at implant analog top surface and base of the cast using a fine tip measuring stylus. The data were aligned and angular differences between implant analog vectors from definitive and duplicate casts were measured.

Results:

There was a significant correlation between dental implant number and dental implant angulation (P < 0.05). There was a significant result when comparing the effect of the parallel group from 5° divergence and 10° convergence groups. A significant results while comparing the parallel group with 5° divergence and 10° convergence was obtained whereas while comparing 5° divergence, 5° divergence, and 10° convergence a nonsignificant difference was obtained (P > 0.05).

Conclusion:

Close proximity of implant angulation toward right-angled direction results in higher precision of implant.

The impact of impression coping geometrical design on accuracy of implant impressions: an experimental study

AIM

This in vitro study aimed to evaluate the effect of implant impression coping geometrical designs on the accuracy of open and closed impression techniques and in the parallel and nonparallel implant positions.

MATERIAL AND METHODS

Three custom-made acrylic resin models of three tested implant systems (Straumann®, SIC Invent®, and Osstem®) with diverse coping geometrical designs were evaluated in simulated cases of two parallel and two nonparallel implants. The horizontal and vertical discrepancies were measured and analyzed.

RESULTS

No statistically significant differences between the two impression techniques in either parallel or nonparallel implants were observed. The high retentive design of the Osstem system showed a statistically significant difference.

CONCLUSION

The geometrical design of the impression copings did not affect the accuracy for either the open and closed tray techniques. However, the high retentive coping design of the Osstem implant affected the accuracy in the open tray technique.

Impact of design and length on the accuracy of closed tray transfer copings

Background

The aim of this study was to evaluate the accuracy of two closed-tray transfer copings for implant impressions (a new design vs. an old design) in two different lengths (short and long).

Material and Methods

Four groups of transfer copings (NS - new short, NL - new long, OS - old short and OL - old long) were tested. An epoxy resin model was prepared of missing teeth 1.4, 1.5 and 1.6. Two Alpha-Bio analogues were placed in position of teeth 1.4 and 1.6, at a 10o angulation. Two calibrated operators took 10 closed-tray impressions for each group with polyether in a Rim-Lock impression tray.

Results

After measuring and comparing impressions, a significant difference was found between the two new transfer copings and the old short transfer coping.

Conclusions

The new transfer coping design significantly improved impression accuracy. An adequate transfer coping design for the closed-tray impression technique can help to achieve clinically acceptable impressions for two-unit implant supported bridges.

Key words:Closed tray, impression coping, transfer coping, implant impression.

Accuracy of fit of implant-supported bars fabricated on definitive casts made by different dental stones

BACKGROUND

The purpose of this study was to evaluate the accuracy of fitting of an implant supported screw-retained bar made on definitive casts produced by 4 different dental stone products.

MATERIAL AND METHODS

The dental stones tested were QuickRock (Protechno), FujiRock (GC), Jade Stone (Whip Mix) and Moldasynt (Heraeus). Three external hexagon implants were placed in a polyoxymethylene block. Definitive impressions were made using monophase high viscosity polyvinylsiloxane in combination with custom trays. Then, definitive models from the different types of dental stones were fabricated. Three castable cylinders with a machined non-enganging base were cast and connected with a very small quantity of PMMA to a cast bar, which was used to verify the marginal discrepancies between the abutments and the prosthetic platforms of the implants. For that purpose special software and a camera mounted on an optical microscope were used. The gap was measured by taking 10 measurements on each abutment, after the Sheffield test was applied. Twelve definitive casts were fabricated for each gypsum product and 40 measurements were performed for each cast. Mean, minimum, and maximum values were calculated. The Shapiro-Wilk test of normality was performed. Mann-Whitney test (P<.06) was used for the statistical analysis of the measurements.

RESULTS

The non-parametric Kruskal-Wallis test revealed a statistically significant effect of the stone factor on the marginal discrepancy for all Sheffield test combinations: 1. Abutment 2 when screw was fastened on abutment 1 (χ2=3, df=35.33, P<0.01), 2. Abutment 3 when the screw was fastened on abutment 1 (χ2=3, df=37.74, P<0.01), 3. Abutment 1 when the screw was fastened on abutment 3 (χ2=3, df=39.79, P<0.01), 4. Abutment 2 when the screw was fastened on abutment 3 (χ2=3, df=37.26, P<0.01).

CONCLUSIONS

A significant correlation exists between marginal discrepancy and different dental gypsum products used for the fabrication of definitive casts for implant supported bars. The smallest marginal discrepancy was noted on implant supported bars fabricated on definitive casts made by Type III mounting stone. The biggest marginal discrepancy was noted on implant supported bars fabricated on definitive casts made by Type V dental stone. The marginal discrepancies presented on implant supported bars fabricated on definitive casts made by two types of Type IV dental stone were not significantly different. Key words:Dental implant, passive fit, dental stones, marginal discrepancy.

Evaluation of positional accuracy in multiple implants using four different splinting materials: An in vitro study

Background:

Prosthesis misfit plays an important role in complications such as occlusal and abutment screw loosening and fracture in implant restorations. Reproducing the intraoral relationship of implants through impression procedures is the first step in achieving an accurate, passively fitting prosthesis to ensure maximum accuracy. Hence, splinting of multiple implants with most accurate material may be a valid option.

Context:

The results of splinting multiple implants are always inconsistent, and there is limited literature available to compare the accuracy of impression techniques as well as materials. In such situation, more and more studies have to be done to prove the efficacy and accuracy of each splinting materials.

Aim:

The aim of the study was to evaluate the positional accuracy in multiple implants using acrylic resin, pattern resin, flowable composite, and bite registration paste.

Subjects and Methods:

An acrylic resin model was fabricated into which four implant analogs placed. The copings attached were splinted with dental floss onto which acrylic resin was added, which was sectioned and rewelded (Group A). Similarly, pattern resin, flowable composite, and bite registration material were added which were considered as Group B, C, and D, respectively. Impressions were made in vinyl polysiloxane and analogs were attached. The casts retrieved from each group were analysed for the positional accuracy of implants.

Statistical Analysis:

One-way ANOVA was done to analyze the significant difference between the four implant analogs of each group with the master model. The confidence interval was also calculated to assess the accuracy.

Results:

It was observed that all materials are equally effective for the accurate reproduction of implant positions. There was no significant difference between the centroids of implant analogues of master model and the mean of interimplant distance 1 and 2, 1 and 4, 3 and 4 and 2 and 4 in each group. The P-values were >0.05. The accuracy of splinting materials were analysed and it showed that splinting with flowable composite (Group C) as well as bite registration paste were in par with the conventionally used materials like pattern resin and acrylic resin.

Conclusions:

Results showed that flowable composite as well as bite registration material can be recommended as splinting material of choice for multiple implant cases, as these exhibited similar results like other groups (pattern resin and acrylic resin) which are conventionally used.

Torque resistance of impression copings after direct implant impression: An in vitro evaluation of impression materials with and without adhesive

STATEMENT OF PROBLEM

No data are available on the ability of an impression coping to resist the manual placement of an abutment replica (implant analog) during prosthodontic laboratory procedures after a direct (pick-up) impression.

PURPOSE

The purpose of this in vitro study was to evaluate the torque resistance of impression copings after a direct impression, that is, the amount of rotational torque sufficient to induce irreversible displacement of impression copings in the impression material bulk once the impression has been made.

MATERIAL AND METHODS

A reference model with 5 abutment replicas was constructed. Five impression copings were screwed onto the abutment replicas, and standardized impressions were made. A controlled twisting force was applied to each impression coping. A torque tester recorded the torque variation. Three elastomeric impression materials were tested. ANOVA and the Tukey test (α=.05) were performed using an average of 30 measurements per impression material, with and without adhesive.

RESULTS

ANOVA and the Tukey test results showed that the adhesive, cohesive, and mechanical bonds between the impression coping and the impression material depended greatly on the type of material and that the average rupture threshold of these bonds was statistically significantly different in pairwise comparisons (P<.05). The curve analysis showed that when the impression materials are used with adhesives, the deformation of the interface is irreversible beyond 5 Ncm of torque.

CONCLUSIONS

The polyether impression material is the direct impression material that showed the highest breakdown threshold for adhesive bonding when used without an adhesive. The use of an adhesive on impression copings leads to irreversible deformation of the interface at torque stresses well below the adhesive bond threshold of the same materials used without an adhesive.

Comparison of implant cast accuracy of multiple implant impression technique with different splinting materials: An in vitro study

Introduction:

An accurate and passive fit of implant framework prosthesis, as well as the successful surgical operation is suggested as one of the critical requirements for long-term implant success.

Objective:

The purpose of this in vitro study was to evaluate the accuracy of the master cast using open tray impression technique with conventional and novel splinting materials.

Methodology:

A mandibular reference model with four ADIN implants was done. Ten custom trays were fabricated using the light curable resin sheets. Medium body polyether impression material was used. These trays were randomly divided between the two groups, with five trays in each group. Impression techniques were divided into two groups namely: Group A: Direct impression technique with open tray impression copings splinted with autopolymerizing acrylic resin (GC pattern resin). Group B: Direct impression technique with open tray impression copings splinted with Pro-temp TM 4 (bis-GMA) syringable temporization material. Thus, final impressions were made. Total of 10 master casts were fabricated. Evaluation of casts using Dynascope-Vision Engineering, TESA microhite two- dimension and coordinate measuring machine were used.

Results:

Statistical comparisons were made using ANOVA test and post-hoc test. Same amount of deviation values obtained with resin splinted and bis-GMA splinted impression copings.

Conclusion:

The master cast obtained by both the splinting material exhibits no difference from the reference model. So bis-GMA can be used, which is easy to handle, less time consuming, less technique sensitive, rigid, and readily available material in clinics.

Three-dimensional accuracy of different impression techniques for dental implants

Background:

Accurate impression making is an essential prerequisite for achieving a passive fit between the implant and the superstructure. The aim of this in vitro study was to compare the three-dimensional accuracy of open-tray and three closed-tray impression techniques.

Materials and Methods:

Three acrylic resin mandibular master models with four parallel implants were used: Biohorizons (BIO), Straumann tissue-level (STL), and Straumann bone-level (SBL). Forty-two putty/wash polyvinyl siloxane impressions of the models were made using open-tray and closed-tray techniques. Closed-tray impressions were made using snap-on (STL model), transfer coping (TC) (BIO model) and TC plus plastic cap (TC-Cap) (SBL model). The impressions were poured with type IV stone, and the positional accuracy of the implant analog heads in each dimension (x, y and z axes), and the linear displacement (ΔR) were evaluated using a coordinate measuring machine. Data were analyzed using ANOVA and post-hoc Tukey tests (α = 0.05).

Results:

The ΔR values of the snap-on technique were significantly lower than those of TC and TC-Cap techniques (P < 0.001). No significant differences were found between closed and open impression techniques for STL in Δx, Δy, Δz and ΔR values (P = 0.444, P = 0.181, P = 0.835 and P = 0.911, respectively).

Conclusion:

Considering the limitations of this study, the snap-on implant-level impression technique resulted in more three-dimensional accuracy than TC and TC-Cap, but it was similar to the open-tray technique.

Accuracy of interchangeable implant impression systems: an in vitro pilot study

PURPOSE

To evaluate the accuracy of impressions on changing the implant component (coping and abutment replica) between Brånemark and BIOMET 3i and between Straumann and Allfit.

MATERIALS AND METHODS

Two master casts were constructed, one containing pair of Brånemark self-tap Mk II fixtures (Nobel Biocare) labeled Master cast I and the other containing pair of Straumann Standard plus implant, SLA fixtures (Straumann) labeled Master cast II. Ten impressions of each master cast were made by open tray technique using polyether impression material. Five out of 10 impressions made from master cast I contained Brånemark pair of coping and replica and the other 5 contained 3i (Implant Innovations). Five out of 10 impressions made from master cast II contained Straumann pair of copings and replica and the remaining 5 contained Allfit implants (Ihde Dental). Duplicated casts were prepared, and distances between copings were measured using traveling microscope and compared the distance in master casts.

RESULT

No significant difference was observed in mean value between 2 master casts and their duplicates.

CONCLUSION

The interchangeability of coping and abutment replica between Brånemark and BIOMET 3i and between Straumann and Allfit was found to be possible without compromising the quality of impressions.

In Vitro Implant Impression Accuracy Using a New Photopolymerizing SDR Splinting Material

PURPOSE

The study aims to evaluate three-dimensionally (3D) the accuracy of implant impressions using a new resin splinting material, "Smart Dentin Replacement" (SDR).

MATERIALS AND METHODS

A titanium model of an edentulous mandible with six implant analogues was used as a master model and its dimensions measured with a coordinate measuring machine. Before the total 60 impressions were taken (open tray, screw-retained abutments, vinyl polysiloxane), they were divided in four groups: A (test): copings pick-up splinted with dental floss and fotopolymerizing SDR; B (test): see A, additionally sectioned and splinted again with SDR; C (control): copings pick-up splinted with dental floss and autopolymerizing Duralay® (Reliance Dental Mfg. Co., Alsip, IL, USA) acrylic resin; and D (control): see C, additionally sectioned and splinted again with Duralay. The impressions were measured directly with an optomechanical coordinate measuring machine and analyzed with a computer-aided design (CAD) geometric modeling software. The Wilcoxon matched-pair signed-rank test was used to compare groups.

RESULTS

While there was no difference (p = .430) between the mean 3D deviations of the test groups A (17.5 μm) and B (17.4 μm), they both showed statistically significant differences (p < .003) compared with both control groups (C 25.0 μm, D 19.1 μm).

CONCLUSIONS

Conventional impression techniques for edentulous jaws with multiple implants are highly accurate using the new fotopolymerizing splinting material SDR. Sectioning and rejoining of the SDR splinting had no impact on the impression accuracy.

Digital evaluation of the accuracy of impression techniques and materials in angulated implants

OBJECTIVES

The aim of this study was to investigate the accuracy of 2 different impression techniques and 3 different impression materials in models simulating parallel and angulated implants.

METHODS

Three master models simulating partial edentulous mandible with 2 implants at the sites of second premolars (parallel) and second molars with different angulations (parallel, 10° or 20° angulated) were fabricated. Two different impression techniques [splinted direct (D), indirect (I)] and 3 different monophase impression materials [polyether (PE), vinyl polysiloxane (VPS), vinyl polyether silicone (VPES)] were used for each master model and a total of 180 impressions were made (n=10). Master model and casts were scanned by a modified laser scanner and data were transferred to VRMesh software. Master model and duplicate cast scans were digitally aligned observing the superposition of anatomic markers. Angular and coronal deviations between master and duplicated copings were calculated and data were statistically analyzed.

RESULTS

Mean angular and coronal deviations were in a range of 0.205-0.359° and 22.56-33.33μm, respectively. Statistical analysis revealed that the angulation of implant affected both coronal and angular deviations of the impression copings (P<0.05). According to statistical analyses, for parallel implants, the accuracy of impression materials and techniques were ranging as VPS-D=PE-D>VPS-I=PE-I>VPES-D>VPES-I from most accurate to the least. For 10° and 20° angulated implants the most accurate material and technique was VPS-D whereas the least accurate combination was VPES-I (P<0.05).

CONCLUSION

Angulation, impression technique and material were found to be effective on the accuracy of implant impressions.

CLINICAL SIGNIFICANCE

Clinicians may prefer VPS impression material and splinted direct technique for impressions of both parallel and up to 20° angulated implants.

Effects of impression levels and trays on the accuracy of impressions taken from angulated implants

BACKGROUND AND PURPOSE

It is crucial to keep the misfit of the abutment-fixture unit at the lowest possible rate. There are a few controversial studies on the accuracy of impression making of angulated implants, and much fewer (and controversial) studies on the abutment-level impression technique, which is a convenient and clinically favorable method. Besides, there are no studies on comparison of sectional vs. full-arch trays. We aimed to assess these.

METHODS

A trapezoidal model with four angulated implants installed at 20° and 30° buccal tilts was fabricated. Forty impressions were taken from this model, with two groups of full-arch and sectional custom trays (n = 2 × 20), each divided into two subgroups of implant-level and abutment-level techniques (n = 2 × 2 × 10 in four subgroups). Absolute and non-absolute linear and angular impression errors were estimated by comparing the fabricated casts with the model, using a coordinate measuring machine. The effects of sectional/full-arch trays and abutment-level and fixture-level techniques on impression accuracies were analyzed using one- and two-way analyses of variance (ANOVA), Tukey, Mann-Whitney, and one-sample t-tests (α = 0.05, Mann-Whitney's α using the Bonferroni Bonferroni method).

RESULTS

No significant differences between the absolute linear errors of the two trays (P = 0.100 [ANOVA]) and the two levels (P = 0.400 [ANOVA]) were observed. The assessment of absolute angular errors showed no significant differences (all P values ≥ 0.4 [ANOVA]). The difference between the linear errors in the full-arch vs. sectional trays was not significant in the fixture-level group (P = 0.290). However, in the abutment-level group, the linear error was significantly greater in the sectional tray compared to full-arch tray (P = 0.013, α = 0.025 [Mann-Whitney]).

CONCLUSIONS

Using sectional trays might not be advantageous over full-arch trays. Sectional trays are not recommended for taking abutment-level impressions. The abutment-level impression technique is as accurate as the studied fixture-level technique. Increasing the angle of implants' divergence from 40° to 60° might not usually lead to a significant increase in the errors, particularly when using abutment-level impressions.

Passive Fit in Screw Retained Multi-unit Implant Prosthesis Understanding and Achieving: A Review of the Literature

One of the considerable challenges for screw-retained multi-unit implant prosthesis is achieving a passive fit of the prosthesis' superstructure to the implants. This passive fit is supposed to be one of the most vital requirements for the maintenance of the osseointegration. On the other hand, the misfit of the implant supported superstructure may lead to unfavourable complications, which can be mechanical or biological in nature. The manifestations of these complications may range from fracture of various components in the implant system, pain, marginal bone loss, and even loss of osseointegration. Thus, minimizing the misfit and optimizing the passive fit should be a prerequisite for implant survival and success. The purpose of this article is to present and summarize some aspects of the passive fit achieving and improving methods. The literature review was performed through Science Direct, Pubmed, and Google database. They were searched in English using the following combinations of keywords: passive fit, implant misfit and framework misfit. Articles were selected on the basis of whether they had sufficient information related to framework misfit's related factors, passive fit and its achievement techniques, marginal bone changes relation with the misfit, implant impression techniques and splinting concept. The related references were selected in order to emphasize the importance of the passive fit achievement and the misfit minimizing. Despite the fact that the literature presents considerable information regarding the framework's misfit, there was not consistency in literature on a specified number or even a range to be the acceptable level of misfit. On the other hand, a review of the literature revealed that the complete passive fit still remains a tricky goal to be achieved by the prosthodontist.

Marginal Fit of Implant-Supported All-Ceramic Zirconia Frameworks

This study evaluated the effect of fabrication techniques and cyclic loading on the vertical marginal fit of implant-supported fixed partial denture (FPD) frameworks. Thirty implant-supported 3-unit FPD frameworks were fabricated on a model system, divided into 3 equal groups (n = 10). The first group (control) was constructed from base metal alloy; the other 2 test groups were constructed from all-ceramic zirconia using a computer-aided design/computer-aided manufacturing (CAD/CAM) Cerec 3 system and a copy milling (Zirkonzahn) system. A cyclic load of 200 N was applied to each framework for up to 50,000 cycles. Linear measurements were made in micrometers of the vertical gap between the framework and the implant-supported abutment at 16 predetermined points before and after cyclic loading. The frameworks were viewed using scanning electron microscopy to inspect any fractographic features. One-way analysis of variance was performed to compare the marginal discrepancy values of the control and the 2 test groups and for each group; a t test was applied to determine whether significant changes in the fit were observed after cyclic loading (α = 0.05). The CAD/CAM group showed significantly higher marginal gap mean values (80.58 μm) than the Zirkonzahn and control groups (50.33 μm and 42.27 μm, respectively) with no significant difference. After cyclic loading, the CAD/CAM group recorded the highest marginal gap mean value (91.50 ± 4.260 μm) followed by control group (72.00 ± 2.795 μm); the Zirkonzahn group recorded the lowest marginal gap (65.37 ± 6.138 μm). Cyclic loading significantly increased the marginal gap mean values in the control group only. A marginal chip was observed in one of the CAD/CAM ceramic frameworks. Within the limitations of this study, the fabrication technique influenced the marginal fit of the implant-supported 3-unit FPD frameworks. Cyclic loading failed to change the fit of all-ceramic zirconia frameworks, whereas significant changes were found in the metal frameworks.

A comparative study of the accuracy between plastic and metal impression transfer copings for implant restorations

PURPOSE

A precise transfer of the position and orientation of the antirotational mechanism of an implant to the working cast is particularly important to achieve optimal fit of the final restoration. This study evaluated and compared the accuracy of metal and plastic impression copings for use in a full-arch mandibular edentulous simulation with four implants.

MATERIALS AND METHODS

Metal and plastic impression transfer copings for two implant systems, Nobel Biocare™ Replace and Straumann SynOcta®, were assessed on a laboratory model to simulate clinical practice. The accuracy of producing stone casts using these plastic and metal impression transfer copings was measured against a standard prosthetic framework consisting of a cast gold bar. A total of 20 casts from the four combinations were obtained. The fit of the framework on the cast was tested by a noncontact surface profilometer, the Proscan 3D 2000 A, using the one-screw test. The effects of implant/system and impression/coping material on gap measurements were analyzed using repeated measures ANOVA.

RESULTS

The findings of this in vitro study were as follows: plastic copings demonstrated significantly larger average gaps than metal for Straumann (p = 0.001). Plastic and metal copings were not significantly different for Nobel (p = 0.302). Nobel had significantly larger average gaps than Straumann for metal copings (p = 0.003). Nobel had marginally smaller average gaps than Straumann (p = 0.096) for plastic copings. The system-by-screw location interaction was significant as well (p < 0.001), indicating significant differences among the four screw locations, but the location differences were not the same for the two systems. A rank transformation of the data was necessary due to the nonnormal distribution of the gap measurements. No adjustments were made for multiple comparisons.

CONCLUSIONS

The metal impression copings were more accurate than plastic copings when using the Straumann system, and there was no difference between metal and plastic copings for the Nobel Replace system. The system-by-screw location was not conclusive, showing no correlation within each system.

Three-Dimensional Accuracy of Implant and Abutment Level Impression Techniques: Effect on Marginal Discrepancy

Impression techniques should precisely represent the 3-dimensional status of implants to allow for the fabrication of passively fitting prostheses and subsequently the elimination of strain on supporting implant components and surrounding bone. The aim of this study was to compare the accuracy of an abutment level impression method with that of an implant level (direct and indirect) impression method using polyether impression material to obtain precise definitive casts and prostheses. A reference acrylic resin dentoform with 2 internal connection implants (Implantium) was made. A total of 21 medium-consistency polyether impressions of the dentoform, including 7 direct implant level, 7 indirect implant level, and 7 abutment level (after 2 straight abutments were secured), were made. Impressions were poured with American Dental Association (ADA) type IV stone, and the positional accuracy of the implant replica heads and abutment analogs in each dimension of x-, y-, and z-axes, as well as angular displacement (Δθ), was evaluated using a coordinate measuring machine. Noble alloy 3-unit castings were fabricated and seated on the abutments in 3 groups; marginal discrepancies were measured at 4 points between prostheses and abutments. Data were analyzed using Mann-Whitney U test, 1-way analysis of variance (ANOVA), and Kruskal-Wallis tests. In comparisons of different impression techniques, only significant statistical Δθ differences were noted between the abutment level method and other techniques (P &lt; .001). Results of this study reveal that although the implant level impression method could better transfer the angular position of the implants (Δθ), the impression method could not affect Δy, Δx, and Δz coordinates of the implants or marginal discrepancy of the 3-unit fixed partial dentures (FPD).

Prosthodontic considerations designed to optimize outcomes for single-tooth implants. A review of the literature

BACKGROUND

The aim of this study was to review the literature on the restoration of single-tooth implants, and to develop evidence-based conclusions to optimize aesthetic, biologic and patient-related outcomes.

METHODS

An electronic and hand search was conducted using the search terms 'dental implants, single-tooth; dental restoration, temporary; dental impression materials; dental impression technique; dental prosthesis, implant-supported; dental prosthesis design; dental abutments; dental occlusion; maintenance; survival; and survival analysis'. Resultant titles were screened, and full text was obtained where relevant. The authors selected the most appropriate articles, giving preference to systematic reviews and long-term, patient-based outcome data.

RESULTS

Thirty-nine articles were selected and critiqued by the authors.

CONCLUSIONS

There was strong suggestion by several authors that peri-implant soft tissue aesthetics can be sculpted through provisional restoration contour, but there are no clinical outcome studies to define or support this claim. Laboratory studies demonstrate that pick-up type impression copings in conjunction with elastomeric impressions are the most accurate means for transferring implant position to a dental cast. Laboratory and finite-element analysis studies suggest implants with an internal-type connection show improved stress distribution, but supportive clinical data are lacking. The authors of this review favour a screw-retained prosthesis for retrievability. Clinical and histological studies show that gold, titanium and zirconia ceramic abutment materials exhibit excellent biological responses, although there is insufficient data on the clinical service provided by zirconia as an implant-substructure material. The literature does not associate any particular occlusal scheme with superior clinical outcomes. Implant-borne single crowns offer comparable clinical service to tooth-borne fixed dental prostheses. However, single-tooth implant restorations are associated with an increased incidence of biological and technical complications.

Evaluation of impression accuracy for implant at various angulations

PURPOSE

The purpose of this study was to compare 2 splinted impression transfer techniques for implant-supported prostheses.

MATERIALS

A metal matrix (control) with 4 implants was used. The implants were positioned at 90, 80, 75, and 65 degrees in relation to the surface of the matrix. Squared impression copings were splinted with self-curing acrylic resin in group 1 (n = 10) and with condensation silicone in group 2 (n = 10). A computer software was used to measure the implants/analogs inclinations. Data were analyzed by 2-way analysis of variance and Tukey test (alpha <0.05).

RESULTS

There was significant difference between groups, implant/analog inclinations, and interaction between them (P < 0.05). Group 1 had no significant difference from the control (P > 0.05) nor from group 2 (P > 0.05) regardless implant/analog inclinations. Considering implant/analog inclinations, both techniques did not differ from control group (P > 0.05), except for 75 degrees implant/analog inclinations (P < 0.05).

CONCLUSION

The results suggest that condensation silicone may not be used as an alternative splinting material. Furthermore, implant inclination may affect master cast accuracy.

Accuracy of implant impressions with different impression coping types and shapes

BACKGROUND

Accurate recording of implant location is required so that definitive restorations are properly supported and do not place additional stresses on the implants. Movement of impression copings inside the impression material using an open-tray or close-tray impression technique during clinical and laboratory phases may cause inaccuracy in transferring the three-dimensional spatial orientation of implants intraorally to the definitive cast. Consequently, the restoration may require corrective procedures.

AIM

This in vitro study compared the accuracy of two different impression techniques with two different impression coping shapes using polyether impression material to obtain precise definitive casts.

MATERIALS AND METHODS

Two reference acrylic resin models (Technovits 4000, Heraeus Kulzer GmbH & Co., Wehrheim, Germany) with five internal connection implants having different shapes of impression copings (Implantium [Dentium, Seoul, South Korea] and Replace Select [Nobel Biocare AB, Göteborg, Sweden]) were fabricated. Twenty medium-consistency polyether impressions of these models were made with square and conical impression copings of each system using open-tray and close-tray techniques. Matching implant replicas were screwed into the impression copings in the impressions. Impressions were poured with type IV stone, and the positional accuracy of the implant replica heads in x-, y-, and z-axes (represented in [Δr]) and also rotational displacement (ΔΘ) were evaluated using a coordinate measuring machine (Mistral, DEA Brown&Sharpe, Grugliasco, Italy). These measurements (linear and rotational displacements) were compared with the measurements calculated on the reference resin models that served as control, and data were analyzed with a two-way analysis of variance at α = 0.05.

RESULTS

Less inaccuracy occurred in less retentive shape impression copings (Replace Select) compared with the more retentive one (Implantium) (p(r) < .001 and p(Θ) < .001), but there was no significant difference between direct and indirect impression techniques (p(r) and p(Θ) > .05).

CONCLUSION

The impression coping shape had more impact on impression inaccuracy than impression technique did. Understanding of the magnitude and variability of distortion when employing certain impression-making methods and impression coping shapes helps the clinician to select a better implant component and impression technique.

Effect of implant angulation, connection length, and impression material on the dimensional accuracy of implant impressions: an in vitro comparative study

BACKGROUND

With regard to implant-supported prostheses, to date no technique has been proven to guarantee a completely passive fit of prosthetic frameworks. Several clinical variables may affect the precision of impressions, particularly in the presence of implants.

PURPOSE

To compare the accuracy of implant impressions made with different materials, lengths of impression coping connections, and not parallel position of the implants.

MATERIALS AND METHODS

A calibrated testing device allowing reproducible standardized positions was used. Two control groups of master models and eight experimental groups with predetermined undercuts were used to make addition silicon and polyether implant impressions by means of the open-tray pick-up technique. Four reference distances were evaluated on each study cast by using a profile projector and a standardized measurement protocol. The data were statistically analyzed by means of three-factor analysis of variance.

RESULTS

The impressions made in the presence of angulated implants were significantly less accurate than the ones made with parallel implants. The tested addition silicon resulted advantageous in presence of nonparallel implants whereas the polyether achieved the best results with parallel implants and standard impression copings.

CONCLUSIONS

The angulation of the implants may cause strains of impressions, probably because of the higher forces required for the impression removal. Moreover, undercuts negatively affected the impression accuracy. More accurate casts were obtained using the tested addition silicon in the presence of nonparallel implants and using a standard length connection of the copings in the presence of parallel implants, respectively.

Accuracy of impression techniques for implants. Part 2 - comparison of splinting techniques

PURPOSE

The aim of this study was to compare splinting techniques for impression copings of osseointegrated implants with different angulations.

MATERIALS AND METHODS

Replicas (N = 24) of a metal matrix (control) containing two implants at 90 degrees and 65 degrees in relation to the horizontal surface were obtained by using four impression techniques: Technique 1 (T1), direct technique with square copings without union in open trays; Technique 2 (T2), square copings splinted with dental floss and autopolymerizing acrylic resin; Technique 3 (T3),square copings splinted with dental floss and autopolymerizing acrylic resin, sectioned and splinted again with autopolymerizing acrylic resin; Technique 4 (T4), square copings splinted with prefabricated acrylic resin bar. The impression material was polyether. The replicas were individually scanned to capture the images, which were assessed in a graphic computation program. The program allowed the angulation between the bases of the replicas and the reading screws to be measured. The images of the replicas were compared with the matrix image (control), and the differences in angulations from the control image were calculated. The analysis of variance and the Tukey test for comparisons (p < 0.05) were used for statistical analysis.

RESULTS

All groups showed significant differences in the implant angulations in comparison with the control group (p < 0.05). Group T1 showed the highest difference (1.019 degrees ) followed by groups T2 (0.747 degrees ), T3 (0.516 degrees ), and T4 (0.325 degrees ), which showed the lowest angular alteration compared to the control group. There were significant differences between inclined and straight implants in all the groups, except in group T4.

CONCLUSIONS

Based on the results, the splinting of pick-up impression copings is indicated for osseointegrated implant impressions. The square copings splinted with a prefabricated acrylic resin bar presented the best results among the pick-up impression techniques evaluated in this study.

Effect of subgingival depth of implant placement on the dimensional accuracy of the implant impression: an in vitro study

STATEMENT OF PROBLEM

In some instances, an implant needs to be placed deep subgingivally, which may result in a less accurate impression of the implant. PURPOSE.: The purpose of this study was to evaluate the effect of subgingival depth of implant placement on the accuracy of implant impressions.

MATERIAL AND METHODS

A stone master model was fabricated with 5 implant analogs (RN synOcta analog), embedded parallel to each other, at the center (E) and the 4 corners (A, B, C, and D). The vertical position of the shoulders of the implants was intentionally different among the implants: A and E were flush with the top surface of the model; B was 2 mm below, and C and D were 4 mm below the surface. The horizontal distances of implants A, B, C, and D from E were measured with a measuring microscope. A cross-shaped metal measuring bar was then fabricated and connected to E, with the arms of the casting designed to be 2 mm above the top surface of the model and incorporating a reference mark. With the measuring bar connected to E, the vertical distances from the apical surface of A, B, C, and D to the measuring reference marks were measured with a digital micrometer. The body of the impression coping for implant D was modified by adding 4 mm of additional impression coping, while standard impression copings were used for all other implants. Open tray impressions were made using medium-body polyether material (Impregum Penta) or a combination of putty and light-body vinyl polysiloxane (VPS) material (Elite HD+) (n=15). Then casts were poured with type IV dental stone. The vertical and horizontal distances of the casts were measured with the methods outlined above for the master model. The distortion values that were determined as differences between the measurements of the master model and those of the casts were collected for statistical analysis. Two-way and 1-way repeated measures ANOVA followed by Tukey's HSD test were performed to compare the distortion values (alpha=.05).

RESULTS

For vertical measurements, 2-way repeated measures ANOVA showed no significant depth (P=.36), material (P=.24), or interaction effects (P=.06). However, it showed significant depth effect for horizontal measurements (P=.01). Within the polyether group, 1-way repeated measures ANOVA showed significant differences in horizontal measurements among the implants with different depths (P=.03). The post hoc Tukey's test showed that the impression of 4-mm-deep implants with normal impression copings (C) was significantly less accurate than impressions of 0-mm-deep implants (A) (P=.02). Within the VPS group, there was no significant difference among the implants with different depths (P=.09).

CONCLUSIONS

There was no effect of implant depth on the accuracy of the VPS group. However, for the polyether group, the impression of an implant placed 4 mm subgingivally showed a greater horizontal distortion compared to an implant placed more coronally. Adding a 4-mm extension to the retentive part of the impression coping eliminated this difference.

Influence of fixation mode and superstructure span upon strain development of implant fixed partial dentures

PURPOSE

Implant-borne fixed partial dentures (FPDs) should fit passively in order to avoid complications ranging from screw loosening to loss of osseointegration. The aim of this study was to measure the strain development of three-unit and five-unit screw- and cement-retained implant-supported FPDs. Additionally, the influence of the parameters retention mechanism and FPD span were evaluated.

MATERIALS AND METHODS

Three Straumann implants were anchored in a measurement model based on a real-life patient situation and strain gauges (SGs) were fixed mesially and distally adjacent to the implants and on the pontics of the superstructures. During cement setting and screw fixation of 40 implant FPDs (10 samples from each group: three-unit cementable; five-unit cementable; three-unit screw-retained; five-unit screw-retained), strain development was recorded. For statistical analysis, multivariate two-sample tests were performed with the level of significance set at p= 0.1.

RESULTS

The mean strain values for the four FPD groups at the different SG sites ranged from 26.0 to 637.6 microm/m. When comparing the four groups, no significant differences in strain magnitude could be detected. Similarly, a comparison of the two FPD spans revealed no significant difference (p= 0.18 for cementable FPDs; p= 0.22 for screw-retained FPDs). A comparison of the two fixation modes also revealed no significant difference (p= 0.67 for three-unit FPDs; p= 0.25 for five-unit FPDs).

CONCLUSION

FPD span and retention mechanism appear to have only a minor influence on strain development in implant FPDs. As implant-supported restorations have proven to be successful over time, the question arises as to whether an "absolute" passive fit is a prerequisite for successful implant restorations.

Comparative analysis of 4 impression techniques for implants

PURPOSE

This in vitro study investigated 4 impression techniques to determine their dimensional accuracy in comparison with a standard technique.

MATERIALS AND METHODS

A master metal framework with 2 inner hex implants (SIN; Sistema de Implante Nacional Ltda., Sao Paulo, Brazil) was used as a standard for the comparisons. Sixty master casts were prepared to evaluate 4 impression techniques: (1) indirect impression technique with tapered transfer copings, (2) direct impression technique with unsplinted squared transfer copings, (3) direct impression technique with squared transfer copings splinted with acrylic resin, and (4) direct impression technique with squared transfer copings with acrylic resin splints sectioned 17 minutes after setting and welded with the same resin. A profile projector was used to measure the distance between the copings attached to the analogs. Mean distances (mm) were calculated from 3 measurements for each sample in the master casts and in the master metal framework.

RESULTS

Analysis of variance and the Tukey HSD test were used for statistical analysis of data (alpha = 0.05). The results for the direct technique with squared transfer copings with acrylic resin splints sectioned and welded after setting were not significantly different from results for the master metal framework.

CONCLUSION

Considering the methodology used and the results obtained, the direct impression technique with squared transfer copings with acrylic resin splints sectioned and welded after setting had better results than the other techniques studied.

Tensile Bond Strength between Custom Tray and Elastomeric Impression Material

The aim of this study was to investigate how to achieve sufficient and stable adhesive strength between impression material and tray. Impression materials were molded between autopolymerizing resin columns, and tensile strength was measured as a function of these factors: tray storage time (1, 2, 4, 7, and 10 days), adhesive drying time (0, 1, 5, 10, and 15 minutes), and tray surface roughness (air abrasion, bur-produced roughness, and no treatment). Tensile bond strength was not affected by tray storage time throughout the entire evaluation period of 10 days. As for tray adhesive drying time, Reprosil and Exaimplant yielded extremely low values for drying times of 10 minutes or less (P<0.05), while Imprint II and Impregum were not influenced by drying time. Vinyl polysiloxane achieved the highest adhesive strength with bur-produced roughness, which was significantly higher than with air abrasion or no treatment (P<0.05), whereas polyether achieved the lowest value with bur-produced roughness (P<0.05). It was concluded that surface treatment of custom tray should be adapted to the type of impression material used to achieve optimum bond strength.

A 4-Year Prospective Study to Assess Peri-Implant Hard and Soft Tissues Adjacent to Titanium Versus Gold-Alloy Abutments in Cemented Single Implant Crowns

PURPOSE

The purpose of this prospective clinical study was to compare titanium and gold-alloy abutments when used with cemented, implant-supported single-tooth crowns. For 4 years following prosthodontic rehabilitation, these abutments were evaluated with respect to peri-implant marginal bone levels and peri-implant soft tissue parameters.

MATERIALS AND METHODS

During the years 1998 to 2000, 20 patients were selected from a patient population receiving treatment in the Implantology Department at the University of Padova, Italy. They all presented with single-tooth bilateral edentulous sites in the premolar/molar region with adequate bone width, similar bone height on each side, and an occlusal scheme that allowed for the establishment of identical occlusal cusp/fossa contacts on each side. Each subject received two identical implants (one in each edentulous site). One was randomly selected to be restored with a titanium abutment and a cemented implant-supported single-tooth crown, and the other was restored with a gold-alloy abutment and a cemented implant-supported single tooth crown. Data on peri-implant marginal bone levels and soft tissue parameters were collected for 4 years after abutment and crown insertion placement and analyzed to determine whether there was a significant (p< .001) difference with respect to the type of abutments (titanium vs. gold alloy) used.

RESULTS

All subjects completed the study. All 40 implants survived, resulting in a cumulative implant success rate of 100%. Statistical analysis revealed no significant differences between the two groups with respect to peri-implant marginal bone levels and soft tissue parameters.

CONCLUSIONS

Within the limitations of this study, the results indicate that there was no evidence of different response with the peri-implant marginal bone and soft tissue when titanium or gold-alloy abutments were used in conjunction with the cemented, single-tooth implant restorations provided for this limited patient population. There was no evidence of different behavior of peri-implant marginal bone and of peri-implant soft tissue when titanium abutments or gold-alloy abutments were used for cemented single-tooth implant restorations in this limited patient population.

Strain Situation after Fixation of Three-Unit Ceramic Veneered Implant Superstructures

The passive fit of superstructures used in implant prosthodontics is affected by several variables. The objective of this study was to quantify the strain development in various fixed partial dentures (FPDs), both in the condition as cast and after ceramic veneering. Five different types of three-unit FPDs (cementable/repositioning technique impression; cementable/pick-up technique impression; screw-retained/plastic cylinder; screw-retained/gold cylinder; screw-retained/bonded) with 10 samples each, representing commonly used FPD-types, were investigated before and after ceramic veneering. Two ITI implants were anchored in a measurement model simulating a real-life patient situation and strain gauges were mounted close to the implants. The strain development was recorded during cement setting and screw fixation. For statistical analysis, multivariate two-sample tests were performed with the level of significance set at P = 0.1. All FPDs revealed measurable amounts of strain. Neither the impression technique nor the fabrication modes for conventional screw-retained FPDs had a significant influence on strain development. Ceramic veneering caused an increase in strain development for the conventional bridge types. Furthermore, cementing appears to be able to compensate fabrication inaccuracies better than screw retention. The lowest strains were found in FPDs bonded to gold cylinders on the measurement model for metal frames and ceramic-veneered FPDs. Conventional procedures are unable to produce superstructures with an absolute passive fit. The technique of bonding superstructures to prefabricated components in the oral cavity seems to compensate for various shortcomings in superstructure fabrication.

Static implant loading caused by as-cast metal and ceramic-veneered superstructures

STATEMENT OF PROBLEM

The passive fit of superstructures for implant-supported restorations is affected by each step of the fabrication process. In this context the question arises whether ceramic veneering would increase static implant loading.

PURPOSE

The purpose of this study was to quantify the strain development of various fixed partial dentures (FPDs) both in the as-cast condition and after ceramic veneering.

MATERIAL AND METHODS

Four different types (n = 10) of 5-unit FPDs (cementable, screw retained/plastic cylinder, screw retained/gold cylinder, screw retained/cemented) representing commonly used types of FPDs were investigated before and after ceramic veneering. Three implants were placed in a model simulating a patient situation, and strain gauges were mounted mesially and distally adjacent to the implants. The strain development was recorded during cement setting (provisional cement) and screw fixation. The data were analyzed statistically using multivariate 2-sample tests (alpha=.1).

RESULTS

All FPDs revealed measurable amounts of strain. Neither the type of retention nor the mode of fabrication for conventional screw-retained FPDs had a significant influence on strain development. Ceramic veneering caused an increase in strain development for the conventional fixed partial dentures tested. The lowest strains were found in FPDs cemented to gold cylinders on the model for the metal frames and the ceramic-veneered FPDs.

CONCLUSION

Conventional procedures were unable to produce superstructures with absolute passive fit. Ceramic veneering appeared to increase strain development and, thus, inaccuracy of the fit. The technique of cementing superstructures to prefabricated components directly on the implants may compensate for dimensional errors caused by impression making and superstructure fabrication.

An evaluation of impression techniques for multiple internal connection implant prostheses

STATEMENT OF PROBLEM

Movement of impression copings inside the impression material using an open-tray impression technique during clinical and laboratory phases may cause inaccuracy in transferring the 3-dimensional spatial orientation of implants intraorally to the definitive cast. Consequently the restoration may require corrective procedures.

PURPOSE

This in vitro study evaluated the accuracy of 3 different impression techniques using polyether impression material to obtain a precise definitive cast for a multi-unit implant restoration with multiple internal connection implants.

MATERIAL AND METHODS

A reference acrylic resin model with 4 internal connection implants (3i Implant Innovations) was fabricated. Forty-five medium-consistency polyether impressions (Impregum Penta) of this model were made with square impression copings using an open-tray technique. Three groups of 15 specimens each were made with different impression techniques: in the first group, nonmodified square impression copings were used (NM group); in the second group, square impression copings were used and joined together with autopolymerizing acrylic resin before the impression procedure (R [resin] group); and in the third group, square impression copings previously airborne-particle abraded and coated with the manufacturer-recommended impression adhesive were used (M [modified] group). Matching implant replicas were screwed into the square impression copings in the impressions. Impressions were poured with ADA type IV stone (New Fujirock). A single calibrated examiner blinded to the nature of the impression technique used examined all definitive casts to evaluate the positional accuracy (mum) of the implant replica heads using a profile projector (at original magnification x10). These measurements were compared to the measurements calculated on the reference resin model which served as control. Data were analyzed with a 1-way analysis of variance at alpha=.05, followed by the Student Newman-Keuls test (alpha=.05).

RESULTS

The data obtained with the profile projector revealed significant differences within the 3 impression techniques ( P <.001). The Student Newman-Keuls procedure disclosed significant differences between the groups, with group R casts being significantly more accurate than group NM and group M casts ( P =.05). The mean distance (+/-SD) between the posterior implants compared to the reference acrylic resin model was 18.17 mum (+/- 6.4) greater for group R casts, 41.27 mum (+/- 8.4) greater for group M casts, and 46.21 mum (+/- 8.9) greater for group NM casts. Distances between the anterior implants were also greater than those recorded on the reference model. The distance was 15.23 mum (+/- 5.9) greater on group R casts, 38.17 mum (+/- 8.3) greater on group M casts, and 43.23 mum (+/- 8.7) greater on group NM casts.

CONCLUSION

Within the limitations of this study, improved accuracy of the definitive cast was achieved when the square impression copings joined together with autopolymerizing acrylic resin were used to make an impression of multiple internal connection implants.

Cement fixation and screw retention: parameters of passive fit. An in vitro study of three-unit implant-supported fixed partial dentures

It is generally assumed that passively fitting superstructures are a prerequisite for long-lasting implant success. In the study presented, the strain development of three-unit implant fixed partial dentures (FPDs) was evaluated at the bone surrounding the implant and on the superstructure using a strain gauge technique. Six groups of three-unit FPDs representing the commonly used techniques of bridge fabrication were investigated with 10 samples each, in order to quantify the influence of impression technique, mode of fabrication and retention mechanism on superstructure fit. Two ITI implants (Straumann, Waldenburg, Switzerland) were anchored in a measurement model according to a real-life patient situation and strain gauges were fixed mesially and distally adjacent to the implants and on the bridge pontics. The developing strains were recorded during cement setting and screw fixation. For statistical analysis, multivariate two sample tests were performed setting the level of significance at P=0.1. None of the investigated bridges revealed a truly passive fit without strains occurring. About 50% of the measured strains were found to be due to impression taking and model fabrication, whereas the remaining 50% were related to laboratory inaccuracies. The two impression techniques used did not reveal any significant differences in terms of precision. Both modes of fixation--i.e. cement and screw retention--provoked equally high stress levels. In the fabrication of screw-retained FPDs, similar results were obtained from the use of burn-out plastic copings and the technique of casting wax moulds to premachined components. Bonding bridge frames onto gold cylinders directly on the implants significantly reduces strain development.

Evaluation of the accuracy of three techniques used for multiple implant abutment impressions

STATEMENT OF PROBLEM

Movement of pick-up type impression copings inside the impression material during clinical and laboratory phases may cause inaccuracy in transferring the spatial position of implants from the oral cavity to the master cast. As a consequence, the laboratory technician may fabricate a restoration that requires corrective procedures.

PURPOSE

This in vitro study evaluated the accuracy of 3 different impression techniques using polyether impression material to obtain a master cast for the fabrication of a prosthesis that would fit passively on multiple implants.

MATERIAL AND METHODS

A machined metal model with 6 implants and abutments and a corresponding, passively fitting, matching metal template were fabricated. A total of 45 medium-consistency polyether impressions (Impregum Penta) of this model were made with pick-up type square impression copings. Three groups of 15 each were made with different impression techniques: in group 1, nonmodified square impression copings were used; in group 2, square impression copings were used and joined together with autopolymerizing acrylic resin before the impression procedure; and in group 3, square impression copings previously airborne particle-abraded and coated with the manufacturer-recommended impression adhesive were used. The matching metal template, which had been passively fit to the metal model so that it encountered no visually perceptible resistance or rocking on the abutments, was used as the control for evaluation of the accuracy of passive fit. A single calibrated and blinded examiner visually evaluated each cast. Positional accuracy of the abutments was numerically assessed with an optical scanner at original magnification x 10, which provided measurements to within 2 microm of the variations of the casts with respect to the horizontal distances between the 2 most posterior abutments and the 2 most anterior abutments. Data were analyzed with a 1-way analysis of variance at alpha=.05, followed by the Student Newman-Keuls method (P=.05).

RESULTS

Visual examination of the casts from group 1 revealed discrepancies between 1 or more abutments and the metal template. Visual analysis of the master casts from groups 2 and 3 revealed close alignment of the metal template on all 6 abutments. One-way analysis of variance analyzed the numerical data obtained with the optical scanner and revealed significant differences among the 3 impression techniques (P<.001). The Newman-Keuls procedure disclosed significant differences between the groups, with group 2 and 3 casts being significantly more accurate than group 1 casts (P=.05). The distance between abutments 1 and 6 compared to the standard metal model was 33.83 microm (SD +/- 5.4) greater on group 2 casts, 31.72 microm (SD +/- 4.6) greater on group 3 casts, and 78.16 microm (SD +/- 22.14) greater on group 1 casts. Distances between the most anterior abutments were also greater than those recorded on the metal model. The distance was 31.42 microm (SD +/- 7.6) greater on group 2 casts, 30.34 microm (SD +/- 6.4) greater on group 3 casts, and 67.91 microm (SD +/- 15.34) greater on group 1 casts.

CONCLUSION

Within the limitations of this study, improved accuracy of the master cast was achieved when the impression technique involved square impression copings joined together with autopolymerizing acrylic resin or square impression copings that had been airborne particle-abraded and adhesive-coated.