Reliability and validity of measurements on digital study models and plaster models.

OBJECTIVE: To compare manual plaster cast and digitized model analysis for accuracy and efficiency. MATERIAL AND METHODS: Nineteen plaster models of orthodontic patients in permanent dentition were analyzed by two calibrated examiners. Analyses were performed with a diagnostic calliper and computer-assisted analysis after digitization of the plaster models. The reliability and efficiency of different examiners and methods were compared statistically using a mixed model. RESULTS: Statistically significant differences were found for comparisons of all 28 teeth (P < 0.001), mandibular intermolar width (IMW, P = 0.0453), and overjet (P < 0.001 to P = 0.0329). Single-tooth measurements tended to have larger values when measured manually and the SD was between 0.06 and 1.33mm. Digital analyses gave significantly higher values for mandibular IMW and overjet. Less time was needed for digital measurements. CONCLUSION: Clinical significance of the differences between the methods compared did not appear significant. 3D laser-scanned plaster model analysis appears to be an adequate, reliable, and time saving alternative to analogue model analysis using a calliper.

Load-bearing capacity of artificially aged zirconia fixed dental prostheses with heterogeneous abutment supports.

Aim of this in vitro study was to investigate the effect of artificial ageing and differential abutment support on the load-bearing capacity of zirconia posterior four-unit fixed dental prostheses (FDPs). Thirty-six FDPs were fabricated using CAD/CAM technology and divided into three groups. Specimens in the first group were cemented onto tooth analogues with simulated periodontal resilience, in the second group onto a dental implant and a tooth analogue, but in the third group only onto implants. Half of the samples in each group underwent artificial ageing. Afterwards, all FDPs were loaded until bulk fracture in a universal testing machine. Load-displacement curves and forces at fracture were recorded and results were statistically analysed using ANOVA. Load-bearing capacities within the different test groups averaged as follows (control/artificially aged): tooth-tooth supported (2,009/1,751 N), tooth-implant supported (2,144/1,935 N) and implant-implant supported (2,689/2,484 N). Artificial ageing as well as differential abutment support did have a significant influence on the fracture strength of the zirconia FDPs. Implant-retained prostheses demonstrated the highest load-bearing capacity, while resilient support was demonstrated to be unfavourable. According to these in vitro results, zirconia four-unit prostheses may be promising for application in posterior areas with all three support scenarios (implant-assisted, tooth-retained, or implant-tooth-interconnected prostheses). However, the restorations' mechanical strength may expected to be significantly influenced in situ by ageing of the material on the long term.

Load-bearing capacity of implant-supported, tooth-supported, and combined zirconia-fixed dental prostheses.

AIM: The aim of this in vitro study was to investigate the influence of heterogeneous abutment supports on the load-bearing capacity of zirconia posterior four-unit fixed dental prostheses. METHODS: A total of 18 specimens were fabricated and divided into 3 groups. Specimens in the first group were supported by teeth with simulated periodontal resilience, in the second group by the combination of a tooth with simulated periodontal resilience and an implant, whereas in the last group by implants only. The bridgeworks were loaded in a universal testing machine until fracture, and the load-bearing capacity was recorded. Statistical analysis was performed by one-way analysis of variance, with the level of significance set at 0.05. RESULTS: Implant-supported zirconia restorations exhibited the highest loads at fracture (2689.3 N), followed by combined restorations (2144.2 N) and tooth-retained prostheses (2009.4 N). Statistical analysis revealed a significant influence of abutment support on the load-bearing capacity (P = 0.014). Visual inspection of the tested specimens showed different fracture patterns in the 3 groups. CONCLUSION: For all support scenarios, the recorded loads were much higher than maximum natural bite forces, so that all the tested prosthesis designs could be recommended for application in posterior areas. The long-term prognosis of such restorations should however be further evaluated within in vivo studies to recommend their use in the clinic.

Effect of implant-abutment connection design on load bearing capacity and failure mode of implants.

PURPOSE: In this in vitro study, six implant-abutment connection designs were compared and evaluated regarding load bearing capacities and failure modes. MATERIALS AND METHODS: Five implants of Astra Tech, Bego, Camlog, Friadent, Nobel Biocare, and Straumann were separately embedded in stainless steel tubes using polyurethane, for a total of 30 specimens. Specimens were statically loaded under an angle of 30° with respect to the implant axis in a universal testing machine using a test setup according to ISO 14801. Failure was indicated by a load drop of 100 N in force. Load-displacement curves were analyzed, and maximum force and force at which permanent deformation occurred were recorded. Statistical analysis was performed using one-way ANOVA with the level of significance set at 0.05. RESULTS: Statistical analysis revealed that the type of implant-abutment connection design has a significant influence on load bearing capacity (p < 0.001). The mean maximum forces ranged between 606 N (Straumann) and 1129 N (Bego); the forces where plastic deformation set in ranged between 368 N (Friadent) and 955 N (Bego). Failure modes differed between the various implant-abutment connection types tested. CONCLUSIONS: Implant-abutment connection design has a significant influence on load bearing capacity and failure mode of implants; however, all implant-abutment connection designs tested would be expected to withstand clinically relevant forces.

Influence of preliminary damage on the load-bearing capacity of zirconia fixed dental prostheses.

PURPOSE: The objective of this investigation was to evaluate the influence of differently shaped preliminary cuts in combination with artificial aging on the load-bearing capacity of four-unit zirconia fixed dental prostheses (FDPs). MATERIALS AND METHODS: Forty frameworks were fabricated from white-stage zirconia blanks (InCeram YZ, Vita) by means of a computer-aided design/computer-aided manufacturing system (Cerec inLab, Sirona). Frameworks were divided into four homogeneous groups with ten specimens each. Prior to veneering, frameworks of two groups were "damaged" by defined saw cuts of different dimensions, to simulate accidental flaws generated during shape cutting. After the veneering process, FDPs, with the exception of a control group without preliminary damage, were subjected to thermal and mechanical cycling (TMC) during 200 days storage in distilled water at 36°C. Following the aging procedure, all specimens were loaded until fracture, and forces at fracture were recorded. The statistical analysis of force at fracture data was performed using two-way ANOVA, with the level of significance chosen at 0.05. RESULTS: Neither type of preliminary mechanical damage significantly affected the load-bearing capacity of FDPs. In contrast, artificial aging by TMC proved to have a significant influence on the load-bearing capacity of both the undamaged and the predamaged zirconia restorations (p < 0.001); however, even though load-bearing capacity decreased by about 20% due to simulated aging, the FDPs still showed mean load-bearing capacities of about 1600 N. CONCLUSIONS: The results of this study reveal that zirconia restorations have a high tolerance regarding mechanical damages. Irrespective of these findings, damage to zirconia ceramics during production or finishing should be avoided, as this may nevertheless lead to subcritical crack growth and, eventually, catastrophic failure. Furthermore, to ensure long-term clinical success, the design of zirconia restorations has to accommodate the decrease in load-bearing capacity due to TMC in the oral environment.

Influence of cyclic fatigue in water on the load-bearing capacity of dental bridges made of zirconia.

The humid atmosphere and permanent occurrence of chewing forces in the oral environment lead to degradation of ceramics used for prosthetic restorations. The aim of this in vitro study was to evaluate the influence of artificial aging on the load-bearing capacity of four-unit bridges, with both undamaged and predamaged zirconia frameworks. Additionally, different parameters for chewing simulation have been investigated and a finite element analysis was made to predict the location of highest tensile stresses within the bridges. A total of 60 frameworks were milled from presintered zirconia and divided into six homogeneous groups. Prior to veneering, frameworks of two groups were "damaged" by a defined saw cut similar to an accidental flaw generated during shape cutting. After veneering, FPDs were subjected to thermal and mechanical cycling - with the exception of control groups. The load-bearing capacity of tested FPDs was significantly reduced by artificial aging. In comparison to unaged specimens, fracture resistance decreased by about 40%, whereas preliminary damage did not have a significant effect. Increasing number of cycles and increasing upper load limit failed to show any additional effect on fracture force. To predict the progression of degradation under the terms of in vitro simulation for even longer periods, further aging experiments are required.

Comparative three-dimensional analysis of initial biofilm formation on three orthodontic bracket materials.

INTRODUCTION: The purpose of the present study was to investigate and compare early biofilm formation on biomaterials, which are being used in contemporary fixed orthodontic treatment. METHODS: This study comprised 10 healthy volunteers (5 females and 5 males) with a mean age of 27.3 +-3.7 years. Three slabs of different orthodontic materials (stainless steel, gold and ceramic) were placed in randomized order on a splint in the mandibular molar region. Splints were inserted intraorally for 48 h. Then the slabs were removed from the splints and the biofilms were stained with a two color fluorescence assay for bacterial viability (LIVE/DEAD BacLight-Bacterial Viability Kit 7012, Invitrogen, Mount Waverley, Australia). The quantitative biofilm formation was analyzed by using confocal laser scanning microscopy (CLSM). RESULTS: The biofilm coverage was 32.7 ± 37.7% on stainless steel surfaces, 59.5 ± 40.0% on gold surfaces and 56.8 ± 43.6% on ceramic surfaces. Statistical analysis showed significant differences in biofilm coverage between the tested materials (p=0.033). The Wilcoxon test demonstrated significantly lower biofilm coverage on steel compared to gold (p=0.011). Biofilm height on stainless steel surfaces was 4.0 ± 7.3 µm, on gold surfaces 6.0 ± 6.6 µm and on ceramic 6.5 ± 6.0 µm. The Friedman test revealed no significant differences between the tested materials (p=0.150). Pairwise comparison demonstrated significant differences between stainless steel and gold (p=0.047). CONCLUSION: Our results indicate that initial biofilm formation seemed to be less on stainless steel surfaces compared with other traditional materials in a short-term observation. Future studies should examine whether there is a difference in long-term biofilm accumulation between stainless steel, gold and ceramic brackets.

Enhancement of the adhesion between cobalt-base alloys and veneer ceramic by application of an oxide dissolving primer.

OBJECTIVES: Uncontrolled formation of an oxide layer on base metal alloy surface impairs adhesion between the alloy and veneer ceramic. The aim of this study was to investigate the influence of an oxide dissolving primer on the adhesion between cobalt-base alloys and a veneer ceramic. METHODS: Combinations of two cobalt-base alloys (Bärlight/BL, Cara Process/CP) and one veneering ceramic (HeraCeram) were investigated. 40 rectangular specimens of each alloy were covered with the veneer ceramic; half of the alloy samples were treated with an oxide dissolving primer (NP-Primer) prior to veneering (n=20). Subsequently, the veneering surface was ground flat and notched using the single-edge V-notched-beam method. Then specimens were loaded in a four-point bending test and the critical load to induce stable crack extension at the adhesion interface was determined, in order to calculate the strain energy release rate (G, J/m(2)). Finally, fracture surfaces of the specimens were evaluated by scanning electron microscopy (SEM). RESULTS: Strain energy release rates averaged between 24.1J/m(2) and 28.8 J/m(2). While application of the primer statistically significantly increased adhesion between alloy and ceramic with the BL specimens (p=0.035), no significant influence was found for the CP specimens (p=0.785). For both material combinations, SEM analysis revealed enhanced wetting of the alloy surfaces with ceramic after application of the primer. SIGNIFICANCE: Application of an oxide dissolving primer increases the wettability of cobalt-base alloy surfaces and thus improves adhesion to veneering ceramics. This may enhance the long-term stability of bilayer restorations made from these materials.

Load-bearing capacity of screw-retained CAD/CAM-produced titanium implant frameworks (I-Bridge®2) before and after cyclic mechanical loading.

UNLABELLED: Implant-supported screw-retained fixed dental prostheses (FDPs) produced by CAD/ CAM have been introduced in recent years for the rehabilitation of partial or total endentulous jaws. However, there is a lack of data about the long-term mechanical characteristics. OBJECTIVE: The aim of this study was to investigate the failure mode and the influence of extended cyclic mechanical loading on the load-bearing capacity of these frameworks. MATERIAL AND METHODS: Ten five-unit FDP frameworks simulating a free-end situation in the mandibular jaw were manufactured according to the I-Bridge®2-concept (I-Bridge®2, Biomain AB, Helsingborg, Sweden) and each was screw-retained on three differently angulated Astra Tech implants (30º buccal angulation/0º angulation/30º lingual angulation). One half of the specimens was tested for static load-bearing capacity without any further treatment (control), whereas the other half underwent five million cycles of mechanical loading with 100 N as the upper load limit (test). All specimens were loaded until failure in a universal testing machine with an occlusal force applied at the pontics. Load-displacement curves were recorded and the failure mode was macro- and microscopically analyzed. The statistical analysis was performed using a t-test (p=0.05). RESULTS: All the specimens survived cyclic mechanical loading and no obvious failure could be observed. Due to the cyclic mechanical loading, the load-bearing capacity decreased from 8,496 N±196 N (control) to 7,592 N±901 N (test). The cyclic mechanical loading did not significantly influence the load-bearing capacity (p=0.060). The failure mode was almost identical in all specimens: large deformations of the framework at the implant connection area were obvious. CONCLUSION: The load-bearing capacity of the I-Bridge®2 frameworks is much higher than the clinically relevant occlusal forces, even with considerably angulated implants. However, the performance under functional loading in vivo depends on additional aspects. Further studies are needed to address these aspects.

Influence of the interface design on the yield force of the implant-abutment complex before and after cyclic mechanical loading.

PURPOSE: The aim of this in vitro study was to evaluate the implant-abutment assembly of Astra Tech (AST), Bego (BEG), Camlog (CAM), Friadent (FRI), Nobel Biocare (NOB) and Straumann (STR) with respect to yield force before and after cyclic fatigue, using a static overload test with a test set-up according to ISO 14801. METHODS: Ten specimens of each type were split into two homogenous groups: one half was tested for static yield force without any further treatment (control), whereas the other one underwent one million cycles of mechanical loading with 100N as the upper load limit. For load-to-failure testing, specimens were then placed in a stainless steel jig and loaded in a universal testing machine under an angle of 30° with respect to the implant axis until failure. Load-displacement curves were analyzed and the yield forces at which non-linear behaviour set in (Fp) were recorded. Statistical analysis was performed using one-way ANOVA and t-test, respectively, with the level of significance set at 0.05. RESULTS: Statistical analysis revealed that the type of implant-abutment connection has a significant influence on Fp (p<0.001). Furthermore, dynamic loading proved to significantly influence Fp of BEG and CAM (p<0.001). CONCLUSION: None of the implant-abutment types tested would be expected to fail under clinically relevant forces, but the type of implant-abutment connection significantly influences the yield force Fp.

The influence of simulated aging on the mechanical properties of orthodontic elastomeric chains without an intermodular link.

OBJECTIVE: Orthodontic elastomeric chains are a main component in orthodontic therapy with fixed vestibular or lingual appliances. The objective of this study was to investigate the influence of artificial aging on the mechanical properties of orthodontic elastomeric chains (power chains, PCs) without an intermodular link using a test setup according to DIN EN ISO 21606:2007. MATERIAL AND METHODS: In this study, 11 types of PCs supplied by seven manufacturers were investigated. Four groups with 10 specimens each were randomly generated for each type. Samples were separately mounted in a universal testing machine and extended by 300% (four times their initial length) at a crosshead rate of 100 mm/min and were held at this position for five seconds. The chain length was then reduced to three times the initial length (extension by 200%) and kept for 30 s. Then, within the control group (t = 0) and the force Fmin was recorded before extension until failure was performed and force (Fmax) and length (Lmax) at failure were determined. After prestretching, specimens of the three other groups were stored in water at 37°C with its three times initial length for one day (t = 1), 14 days (t = 2), and 28 days (t = 3), respectively. The specimens were then placed in the universal testing machine and the residual force (Fmin) measured, so as to subsequently extend them until failure at Fmax and Lmax. Data were statistically analyzed by one-way analysis of variance; the level of significance was set at p = 0.05. RESULTS: Statistical analysis revealed significant differences in Fmax, Fmin, and Lmax in each group (t = 0 to t = 3) between the various manufacturers (p < 0.001). Moreover, artificial aging significantly influenced Fmax, Fmin, and Lmax (p < 0.001). CONCLUSION: The orthodontist should consider both the mechanical properties of PCs and the duration of these appliances' application when treating patients. Artificial aging had a significant influence on the parameters we determined (p <0.001).

Influence of the supporting structure on stress distribution in all-ceramic FPDs.

PURPOSE: The aim of this study was to investigate the influence of the design and material composition of the supporting structure of a zirconia four-unit fixed partial denture (FPD) on stress distribution during in vitro loading. MATERIALS AND METHODS: A three-dimensional finite element model of an all-ceramic FPD ranging from the maxillary left first premolar to second molar was constructed. The supporting structures were modeled in four versions. In version 1, the socket and rigidly fixed abutment teeth were made of a nickel-chromium (Ni-Cr) alloy. Version 2 was similar to version 1 but abutment teeth were embedded resiliently. Version 3 replaced the Ni-Cr alloy with polyurethane as the material for the socket and abutment teeth. Version 4 was designed according to the in vivo situation with a simulated periodontal ligament, the socket consisting of spongiosa, and abutment teeth composed of dentin. An occlusal force of 1,630 N was distributed over the marginal ridges of the pontics. RESULTS: The highest tensile stresses were located within the framework underneath the connector between the second premolar and first molar and ranged between 289 and 633 MPa, according to the model version. The resilient support of abutment teeth resulted in considerably higher maximum tensile stresses. CONCLUSIONS: The choice of material for abutment teeth and the socket, as well as the type of tooth support, significantly influence stresses generated in FPDs during in vitro load tests. To achieve realistic results, FPDs should be supported by resiliently embedded abutment teeth made of a moderately rigid material (eg, polyurethane). In clinical practice, risk of failure is likely to rise with an increasing resilience of the abutment teeth if occlusal contacts are directed over the pontic/connector region rather than being spread over the retainers. Int J Prosthodont 2010;23:63-68.

Tensile properties of orthodontic elastomeric chains.

AIM: Orthodontic elastomeric chains are a main component in orthodontic therapy employing labial or lingual appliances. The aim of this study was to investigate the tensile properties of orthodontic elastomeric chains with a test setup according to ISO 21606:2007. MATERIAL AND METHODS: Orthodontic elastomeric chains of eight manufacturers with and without an intermodular link were obtained from commercially-available stock, yielding 23 groups with ten specimens each. Samples were mounted in a universal testing machine and extended at a rate of 100 mm/min to four times the initial length and held for 5 seconds. After 5 seconds, the chain lengths were reduced to an extension of three times the initial length and held for 30 seconds before extension until failure. Forces at four times the test length (Fmax1), three times the initial length (Fmin) as well as force (Fmax2) and length (Lmax) at failure were recorded. Data were statistically analyzed by one-way analysis of variance using SPSS® 17. The level of significance was set at p = 0.05. RESULTS: Statistical analysis revealed significant differences in Fmax1, Fmin, Fmax2 and Lmax among the various manufacturers. Fmax1 ranged between 9.1 N and 23.2 N, Fmin ranged between 1.5 N and 3.0 N, Fmax2 ranged between 15.7 N and 34.0 N, and Lmax ranged between 425% and 629% of the initial length. CONCLUSIONS: The tensile properties of different manufacturers of elastomeric orthodontic chains differ statistically significantly. Hence the amount of orthodontic force that is applied depends significantly on the type of elastomeric chains used in combination with labial or lingual appliances.

Finite element analysis of a four-unit all-ceramic fixed partial denture.

All-ceramic restorations are known to be prone to brittle fracture. However, a previously performed in vitro study indicates that four-unit fixed partial dentures (FPDs) with a zirconia framework are sufficiently strong to withstand occlusal forces in the posterior region. The aim of this study was to determine the stress distribution in such a four-unit FPD made of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), under an occlusal load. A three-dimensional finite element model was constructed and a stress analysis performed with a force of 1630 N applied at the centre of the middle connector area. The location of maximum tensile stress according to finite element analysis coincided with the fracture origin of all 10 specimens fractured within the previous in vitro study. The maximum tensile stress in the area of the middle connector amounted to 633 MPa. It increased with the load being applied from the oral towards the buccal side (648 MPa) and decreased with the load being applied from the buccal towards the oral side (570 MPa). These stresses are of the same order as the flexural strength of Y-TZP, determined under standardized test conditions to be 600-1000 MPa. The model presented is intended to be used for further investigations, including thermally induced stresses during veneering.

Stresses and distortions within zirconia-fixed dental prostheses due to the veneering process.

Changes in the marginal and internal fit during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). Hence, the aim of this study was to investigate the distortion of four-unit zirconia bridges induced by the veneering process with in vitro and finite element analyses (FEA). Ten all-ceramic FDPs with zirconia frameworks were prepared. The marginal and internal fit of the restorations were determined prior to and after veneering by means of a replica technique. Additionally, a three-dimensional finite element model of the restoration was constructed and cooling after the veneering process was virtually simulated. Statistical analysis revealed significant changes in the marginal and internal fit due to the veneering process. FEA verified these observations and displayed tensile stresses (up to 65MPa) within the framework and compressive stresses (up to 10MPa) within the veneering layer. The present study showed that stresses and distortions, occurring due to the veneering process, may influence the marginal and internal fit and therefore the clinical success of dental restorations.

Load-bearing capacity of screw-retained CAD/CAM-produced titanium implant frameworks (I-Bridge®2) before and after cyclic mechanical loading

Implant-supported screw-retained fixed dental prostheses (FDPs) produced by CAD/ CAM have been introduced in recent years for the rehabilitation of partial or total endentulous jaws. However, there is a lack of data about the long-term mechanical characteristics. OBJECTIVE: The aim of this study was to investigate the failure mode and the influence of extended cyclic mechanical loading on the load-bearing capacity of these frameworks. MATERIAL AND METHODS: Ten five-unit FDP frameworks simulating a free-end situation in the mandibular jaw were manufactured according to the I-Bridge®2-concept (I-Bridge®2, Biomain AB, Helsingborg, Sweden) and each was screw-retained on three differently angulated Astra Tech implants (30º buccal angulation/0º angulation/30º lingual angulation). One half of the specimens was tested for static load-bearing capacity without any further treatment (control), whereas the other half underwent five million cycles of mechanical loading with 100 N as the upper load limit (test). All specimens were loaded until failure in a universal testing machine with an occlusal force applied at the pontics. Load-displacement curves were recorded and the failure mode was macro- and microscopically analyzed. The statistical analysis was performed using a t-test (p=0.05). RESULTS: All the specimens survived cyclic mechanical loading and no obvious failure could be observed. Due to the cyclic mechanical loading, the load-bearing capacity decreased from 8,496 N±196 N (control) to 7,592 N±901 N (test). The cyclic mechanical ...