Adhesive removable partial denture attachments made from zirconia ceramic: A finite element analysis and in vitro study.

STATEMENT OF PROBLEM: Whether adhesive zirconia ceramic removable partial denture attachments are feasible with current technology is unclear. PURPOSE: The purpose of this finite element analysis and in vitro study was to evaluate the effect of the lever arm, tooth preparation, and aging on the loading of the tooth-zirconia attachment interface. MATERIAL AND METHODS: Three different finite element analysis (FEA) models allowing for the loading of an adhesive attachment either directly or through a removable partial denture were used. Two models represented a human tooth with 2 different types of attachments, while the third model also included a removable partial denture. For the evaluation of bond strength, a combination of shear and hydrostatic stress was used. In addition, composite resin teeth were fabricated, and zirconia bars were bonded to them with varying tooth preparations and lever arm lengths. In 1 group the influence of aging was analyzed. Fracture load was determined by using a universal testing machine. Statistical analysis was based on the Shapiro-Wilk normality test, ANOVA, and Games-Howell test (α=.05). RESULTS: The maximum stress of 65 MPa occurring in the bonding area was reduced to 37 MPa by adding a retainer to the attachment. Loading of the denture resulted in a maximum stress of 9 MPa. Mean fracture loads ranged from 33.6 N to 209.1 N. Preparing a flat bonding surface showed a nonsignificant increase (P=.197), whereas aging led to a nonsignificant decrease in fracture load (P=.075). A lever arm extended by 2 mm significantly reduced fracture load (P=.002). The addition of an occlusal-distal (OD) cavity led to a nonsignificant increase (P=.186), which became significant when a mesial-occlusal-distal (MOD) preparation was applied (P=.001). CONCLUSIONS: Adhesive zirconia attachments should use a MOD cavity and have a cross section of at least 2.5×2.5 mm. The attachment should not extend more than 3 mm.

Effect of model parameters on finite element analysis of micromotions in implant dentistry.

Micromotion between dental implant and bony socket may occur in immediate-loading scenarios. Excessive micromotion surpassing an estimated threshold of approximately 150 µm may result in fibrous encapsulation instead of osseointegration of the implant. As finite element analysis (FEA) has been applied in this field, it was the aim of this study to evaluate the effect of implant-related variables and modeling parameters on simulating micromotion phenomena. Three-dimensional FEA models representing a dental implant within a bony socket were constructed and used for evaluating micromotion (global displacement) and stress transfer (von Mises equivalent stress) at the implant-bone interface when static loads were applied. A parametric study was conducted altering implant geometry (cylinder, screw), direction of loading (axial, horizontal), healing status (immediate implant, osseointegrated implant), and contact type between implant and bone (friction free, friction, rigid). Adding threads to a cylindrically shaped implant as well as changing the contact type between implant and bone from friction free to rigid led to a reduction of implant displacement. On the other hand, reducing the elastic modulus of bone for simulating an immediate implant caused a substantial increase in displacement of the implant. Altering the direction of loading from axial to horizontal caused a change in loading patterns from uniform loading surrounding the whole implant to localized loading in the cervical area. Implant-related and bone-related factors determine the degree of micromotion of a dental implant during the healing phase, which should be considered when choosing a loading protocol.

Parameters of implant stability measurements based on resonance frequency and damping capacity: a comparative finite element analysis.

PURPOSE: Contradictory results have been reported on the comparability of implant stability measurements performed with the Periotest and the Osstell Mentor devices. The purpose of the present finite element analysis was to simulate the influence of the parameters implant length, bone quality (cortical thickness and damping factor), bone loss, and quality of transducer fixation on resonance frequency analysis (RFA) and damping capacity measurements. MATERIALS AND METHODS: Three-dimensional finite element models of implants placed in human mandibular bone were designed for the simulation of Periotest (Periotest value; PTV) and RFA (implant stability quotient) measurements. Three values for each of the parameters implant length, damping capacity of cortical and trabecular bone, thickness of cortical bone, bone loss, and quality of transducer fixation were obtained. Measurements were simulated at four stages of osseointegration. RESULTS: For all parameters, an increase in implant stability was found with increasing levels of osseointegration. Implant stability was positively correlated with implant length and thickness of cortical bone, with slightly converging values at increased levels of osseointegration. Varying the damping factor of bone had no significant effect. Implant stability was negatively correlated with bone loss, with slightly converging values at increased levels of osseointegration. Linear changes in implant length and bone loss caused nonlinear effects in implant stability values. Stiffness of transducer fixation had an impact on RFA measurements when values below 10 GPa were applied. CONCLUSION: Although both measuring devices reacted similarly when different parameters of implant stability were changed, good correlation between Periotest values and implant stability quotients was observed only when measurement values of implants without bone loss were considered.

Bone loading caused by different types of misfits of implant-supported fixed dental prostheses: a three-dimensional finite element analysis based on experimental results.

PURPOSE: To show, by comparison of horizontal, vertical, and angular misfit in a three-dimensional finite element model, that clinical methods for the evaluation of implant framework fit cannot provide objective results. MATERIALS AND METHODS: Two three-dimensional finite element models were designed for the simulation of experimentally determined strain values of three-unit fixed dental prostheses supported by two implants. Horizontal, vertical, and angular misfits between implants and restorations were used to create predetermined strain levels. The magnitudes of misfit and resulting bone loading were recorded as von Mises equivalent stresses for the different types of misfit. RESULTS: A horizontal misfit of 36 µm and a vertical misfit of 79 µm had to be modeled to simulate the experimentally determined strain values. An angular misfit of 0.083 degree (equivalent to a gap of 3 µm on one aspect of the implant) resulted in comparable strain levels. Bone loading in the cortical area around both implants ranged from 50 to 90 MPa for horizontal and vertical misfit. In trabecular bone, loading of 2 to 5 MPa was found. For the angular misfit, bone loading up to 20 MPa in the cortical layer and 1 MPa in the cervical part of the trabecular bone occurred at the implant where the misfit had been introduced. Horizontal and vertical misfits led to comparable loading patterns around both supporting implants. Under angular misfit, bone loading mainly occurred around the implant where the misfit had been introduced. Almost no loading was observed in the circumference of the contralateral implant. CONCLUSIONS: Minimal angular misfits between implant abutments and restorations, which cannot be detected clinically, may lead to substantial bone loading.

Fixation of 5-unit implant-supported fixed partial dentures and resulting bone loading: a finite element assessment based on in vivo strain measurements.

PURPOSE: It is believed that implant-supported fixed partial dentures (FPDs) should display passive fit. The objective of this in vivo-based finite element analysis (FEA) was to quantify the magnitude of bone loading occurring on account of the fixation of cemented or screw-retained 5-unit superstructures. MATERIALS AND METHODS: Based on a patient situation with 3 implants, 4 different groups of restorations with 10 samples each were fabricated. Strain gauges on the pontics of the restorations were used for in vivo measurements. Using the values obtained, bone loading in 3-dimensional FE models was simulated as von Mises equivalent stress. RESULTS: The in vivo measured mean strain values ranged from 32 microm/m to 458 microm/m at the different sites. FEA revealed stresses between 5 and 30 MPa in the cortical area, while in trabecular bone values ranging from 2 MPa to 5 MPa were observed. Stress of a similar magnitude was found for axial implant loading with 200 N. DISCUSSION: Assuming that the axial loading of a single implant with 200 N is within the realm of the bone's adaptation ability, it would appear that the amount of stress resulting from the fixation of superstructures alone does not constitute a risk. CONCLUSIONS: The level of precision of fit which can be obtained in superstructure fabrication would appear to suffice to produce restorations that do not cause bone damage.

In vitro study on passive fit in implant-supported 5-unit fixed partial dentures.

PURPOSE: Fabrication and retention methods have an influence on the passivity of superstructure fit. The objective of the study was to quantify the strain development of various cemented and screw-retained fixed partial dentures (FPDs). MATERIALS AND METHODS: Forty samples of 4 different types of FPDs (10 of each type) were investigated. Each sample had 3 ITI implant abutments and 2 pontics. The 3 implants were anchored in a straight-line configuration in a measurement model simulating a real-life patient situation. Strain gauges were mounted close to the implants and on the pontics. The developing strains were recorded during cement setting and screw fixation. For statistical analysis, multivariate 2-sample tests were performed, with the level of significance set at P = .1. RESULTS: All FPDs investigated revealed a considerable amount of strain, with no significant difference between cement and screw retention. Furthermore, no significant difference was found between the conventional fabrication modes for screw-retained FPDs. The lowest strains were found in prostheses that were intraorally bonded onto gold cylinders. DISCUSSION: Because bonding of the superstructure in the oral cavity may compensate for impression and laboratory variables, restorations with the best possible passive fit can result from this retention technique. Before this technique can be recommended, the long-term stability of the adhesive layer should be investigated. CONCLUSIONS: As an absolute passive fit of superstructures is not possible using conventional clinical and laboratory procedures, and as clinical fit-evaluation methods often do not detect "hidden" inaccuracies, the more sensitive strain-gauge technique should be utilized for an objective accuracy test. Reference strain values from implant-supported prostheses that have served without complications could help define a "biologically acceptable fit.

Screw loading and gap formation in implant supported fixed restorations: Procera implant bridge vs. conventionally cast screw-retained restorations.

OBJECTIVES: It was the purpose of this finite element analysis to compare the implant-restoration connection created by Procera Implant Bridge (PIB) and conventional screw-retained prostheses with respect to screw loading and gap formation. METHOD AND MATERIALS: Finite element models representing a conventional screw-retained restoration and a PIB were set up. A horizontal load of 200N was applied on the restorations while resulting gap formation and screw loading was recorded. RESULTS: Increasing the preload of the retaining screws led to a decrease in gap formation. Smaller gaps were observed in the conventional restorations. Loading of the screws was of comparable magnitudes in both cases. CONCLUSION: The use of screw-retained implant-supported restorations representing butt-joint connections at the restorative interface may result in increased gap formation.

Micromotion of Dental Implants: Basic Mechanical Considerations.

Micromotion of dental implants may interfere with the process of osseointegration. Using three different types of virtual biomechanical models, varying contact types between implant and bone were simulated, and implant deformation, bone deformation, and stress at the implant-bone interface were recorded under an axial load of 200 N, which reflects a common biting force. Without friction between implant and bone, a symmetric loading situation of the bone with maximum loading and displacement at the apex of the implant was recorded. The addition of threads led to a decrease in loading and displacement at the apical part, but loading and displacement were also observed at the vertical walls of the implants. Introducing friction between implant and bone decreased global displacement. In a force fit situation, load transfer predominantly occurred in the cervical area of the implant. For freshly inserted implants, micromotion was constant along the vertical walls of the implant, whereas, for osseointegrated implants, the distribution of micromotion depended on the location. In the cervical aspect some minor micromotion in the range of 0.75 µm could be found, while at the most apical part almost no relative displacement between implant and bone occurred.

Dehydration-induced shrinkage of dentin as a potential cause of vertical root fractures.

Vertical root fractures (VRF) of endodontically treated teeth constitute a severe clinical condition frequently requiring removal of the affected tooth. Numerous attempts have been made to find the cause for VRF without reaching definitive conclusions. As changes in moisture content have been reported to appear as a consequence of root canal therapy, it is the goal of this paper to evaluate associated volume changes as a possible cause for VRF. Considering disk shaped horizontal cross sections of endodontically treated teeth with a moisture content of dentin decreasing from the root surface towards the root canal, both relative circumferential and relative radial stresses resulting from volume changes of dentin were calculated. It could be shown that the presence of a root canal itself increases radial and circumferential stresses acting on root dentin by a factor of two. Reduction in moisture content of dentin at the wall of the root canal results in shrinkage of the tooth structure and tensile stress. On the outer surface of the root, compressive stresses occur. Thus, VRF would start at the canal wall and propagate to the root surface. The theory presented appears to be consistent with previous reports on stress development as a consequence of dehydration of dentin and finite element analysis on root fractures. It may be concluded that dehydration of dentin induces cracks at the walls of a root canal which subsequently grow as a result of cyclic loading or traumatic overload.

Applicability of strain measurements on a contra angle handpiece for the determination of alveolar bone quality during dental implant surgery.

Alveolar bone quality is considered to be an important prognostic factor in dental implant stability. Although numerous methods have been described, no technique allows for reliable diagnostics. The purpose of this study was to determine if strain measurements on the shaft of a contra angle handpiece during implant bed preparation could be used for the determination of bone quality. Experiments in polyurethane foam and human cadaver bone were conducted to investigate whether strain measurements could be correlated with other diagnostic parameters, such as the surgeon's tactile sensation during drilling, implant insertion torque, implant stability, elastic modulus of bone and bone quality as assessed radiographically. Tests were also performed to determine if strain measurements could be used to distinguish various types of bone. As axial feed and contact pressure during the drilling process could not be standardized under simulated clinical conditions, substantial deviations in the time needed to complete the drilling occurred. Under controlled circumstances using polyurethane foam, this problem could be addressed by a normalization procedure, but great variations occurred in human cadaver bone. As bone quality could not be reliably determined, especially when a cortical layer was present, strain measurements on a contra angle handpiece appears to be inappropriate for this purpose.

In vitro validation of a novel diagnostic device for intraoperative determination of alveolar bone quality.

PURPOSE: Current methods for the evaluation of alveolar bone quality and dental implant stability have been shown to provide inconsistent results. The aim of this investigation was to validate a novel diagnostic device (BoneProbe) for the objective classification of alveolar bone during dental implant surgery. MATERIALS AND METHODS: A metal cylinder (diameter, 3.5 mm) split into six segments was used as a sensor that could be positioned in implant sockets and expanded while the force needed to expand the site was recorded. Simulating all surgical steps, implants were placed into polyurethane foam materials (n = 100 implants) and human cadaver bone (n = 110 implants). Bone quality (cortical bone thickness, trabecular bone density, drilling resistance, implant insertion torque, compressive testing) and primary implant stability (Osstell, Periotest) were evaluated as reference data. RESULTS: In polyurethane foam, significant correlations between all parameters (Pearson correlation coefficients) and a significant influence of the different polyurethane foam materials on all measurement results (multiple analysis of variance with Pillai trace) were found. In general, in human cadaver bone, weaker correlations between the different measurement techniques were seen (Pearson product-moment correlation). With the exception of compressive testing and radiographic assessment of trabecular bone density, all methods were able to differentiate between mandibular and maxillary bone. CONCLUSIONS: Based on these in vitro results, it appears that intraoperative testing of alveolar bone allows for a reproducible classification of bone quality. Because the proposed system is independent of any specific implant design, this device could be used for establishing a universally valid bone classification system.

Effect of geometric parameters on finite element analysis of bone loading caused by nonpassively fitting implant-supported dental restorations.

OBJECTIVE: Finite element analysis (FEA) has been frequently used to study the loading situation of dental implants and bone resulting from the fixation of nonpassively fitting restorations. The goal of the present investigation was to demonstrate the effect of geometric model parameters and mesh size on FEA results. METHOD AND MATERIALS: Five three-dimensional FEA models representing a three-unit fixed dental prosthesis (FDP) supported by two terminal implants were constructed. The models differed in terms of mesh size, bone geometry, implants, and restoration and were created either by joining virtual free-form objects or utilizing optical scans of existing components. By applying thermal changes in volume of specific elements in the area of the FDP pontic, a horizontal misfit of 10 Μm between implants and the restoration was introduced. The resulting loading situation of the bone around the implants was recorded as von Mises equivalent stress. RESULTS: Maximum stress magnitudes ranging from 13.1 to 24.9 MPa occurred in the cortical part of the implant site where the neck of the implant penetrates bone. In trabecular bone, loading magnitudes were lower by a factor of 20. Modeling implant threads did have a remarkable effect on the stress situation as well as different span lengths of the restorations modeled. All other parameters led only to small variations in maximum loading magnitudes. CONCLUSION: Simplistic FEA models based on virtual free-form objects with limited level of mesh refinement seem to allow for a basic evaluation of peri-implant bone loading resulting from the fixation of misfitting superstructures.

Bone adaptation induced by non-passively fitting implant superstructures: a finite element analysis based on in vivo strain measurements.

PURPOSE: Stress caused by a non-passively fitting implant superstructure may induce bone adaptation, thereby changing the magnitude of static implant loading. MATERIALS AND METHODS: In a previous investigation, repeated in vivo strain measurements were conducted on an implant-supported bar to evaluate changes in the magnitude of misfit resulting from bone remodeling processes. Both maximum (445 µm/m) and minimum (383 µm/m) strain values were simulated using a three-dimensional finite element model. The horizontal misfit needed to simulate experimentally determined strain values and the resulting stresses occurring in the restoration and the bone were quantified as von Mises equivalent stress. Additionally, different stages of osseointegration were modeled by altering the elastic modulus of bone immediately surrounding the implants. RESULTS: To simulate the maximum strain value, a horizontal misfit of 83.3 µm had to be introduced, whereas the minimum strain value could be simulated via a horizontal misfit of 71.5 µm. Maximum misfit caused stress magnitudes of 105 MPa in cortical bone and 5.3 MPa in trabecular bone. Minimum misfit caused stress magnitudes of 90 MPa in cortical bone and 4.6 MPa in trabecular bone. The difference between maximum and minimum horizontal misfit was 12 µm and led to a reduction in maximum stress levels of 15 MPa in cortical bone and 0.7 MPa in trabecular bone. Progressing osseointegration affected the stress situation of the supporting implants. CONCLUSIONS: Within the limitations of this investigation, it can be concluded that bone adaptation may lead to implant site displacement in the range of several micrometers. Early fixation of non-passively fitting superstructures on implants may lead to greater passivity of fit.

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.

Loading of bone surrounding implants through three-unit fixed partial denture fixation: a finite-element analysis based on in vitro and in vivo strain measurements.

Implant-borne fixed partial dentures (FPDs), whether cementable or screwable superstructures, ought to display a true passive fit. The objective of this in vivo-based finite-element analysis is, therefore, to quantify the degree of stress which occurs in the bone around the implants as a result of the fixation of cemented and screw-retained FPDs. On the basis of a simulated patient situation with two implants, six groups of implant-supported superstructures containing 10 samples each were fabricated. Strain gauges which were mounted on the pontics of the restorations were subsequently used to take in vivo measurements (Ethics Commission Approval No. 2315). Taking the values obtained as a basis, the von Mises equivalent stress was chosen to illustrate bone loading in three-dimensional finite-element models. Superstructure fixation caused residual interface stress as high as 30 MPa. Similar stress magnitudes can be observed for axial implant loading of 200 N. Assuming that the axial loading of a single implant with 200 N is within the bone's physiological range, it can be concluded that the degree of stress resulting from the fixation of superstructures alone does not constitute a risk.

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.