Finite element stress analysis on the effect of splinting in fixed partial dentures

The Use of Splinted Versus Nonsplinted Prosthetic Design in Dental Implants: A Literature Review

The choice of a splinted or nonsplinted implant-supported prosthesis should be based on solid scientific evidence that considers the conditions and needs of each patient. This review elaborates on the factors that directly influence clinical decisions between splinted or nonsplinted dental implants. Digital and manual searches of the published literature were conducted to identify studies that examined splinted prostheses (SPs) and nonsplinted prostheses (NSPs). The search terms used, alone or in combination, were "splinting prosthesis," "nonsplinting prosthesis," "prosthetic design," "stress distribution in dental implant," "implant loading," "implant occlusion," and "crestal bone resorption." Ninety-four studies were selected to compare and address the details emphasized in this study. Thirty-four reported articles were not directly related to restoration design but were reviewed to better understand the influence of mechanical risk factors, finite element analysis limits, and criteria for implant survival and treatment success. There are advantages and disadvantages of splinting implants together. NSPs are the ideal choice because they resemble natural teeth. Splinting a restored implant will cause the implant to appear as part of one unit and is indicated in more compromised situations, unfavorable conditions, or when pontic spaces and cantilevers are needed in implant prostheses.

Biomechanical Implications of Mandibular Flexion on Implant-Supported Full-Arch Rehabilitations: A Systematic Literature Review

BACKGROUND

Mandibular flexion (MF) is a complex biomechanical phenomenon, which involves a deformation of the mandible, due mainly to the contraction of the masticatory muscles, and it can have numerous clinical effects. The deformation of the lower jaw caused by mandibular flexion is generally very small, and it is often overlooked and considered irrelevant from a clinical point of view by many authors; however, it should be important to remember that median mandibular flexure (MMF) has a multifactorial aetiology. The main aim of the current systematic review is to highlight the different factors that can increase MF in order to help clinicians identify patients to whom they should pay more attention. As a secondary outcome, we wanted to analyse the preventive measures and suitable techniques to be adopted to minimise the negative effects of this phenomenon on oral fixed rehabilitations.

METHODS

The review, which was carried out in accordance with the "Preferred Reporting Items for Systematic reviews and Meta-Analyses" (PRISMA) flowchart, was recorded in the "International Prospective Register of Systematic Reviews" (PROSPERO). As research questions, "Patient/Population, Intervention, Comparison and Outcomes" (PICO) questions were employed. Using the ROBINS-I technique, the risk of bias in non-randomised clinical studies was evaluated.

RESULTS

The initial electronic search identified over 1300 potential articles, of which 54 studies were included in this systematic review. Information regarding the relationship between MF and individual factors, mandibular movements, impression taking, and fixed rehabilitations were obtained.

CONCLUSIONS

The studies included in this systematic review showed that MF is greater during protrusive movements, in the posterior areas of the lower jaw, and in patients with brachial facial type, greater jaw length; small gonial angle; and less density, length, and bone surface of the symphysis. The biomechanical effects of mandibular flexion on fixed restorations are debated. Prospective clinical and radiological observational studies should be conducted to evaluate the potential short-, medium-, and long-term consequences of MF.

Clinically Based Classification and Positioning Indication for Single-Piece Compressive Implants Placement in Regard to Extraction Socket

(1) Background: Dental implantology has been rapidly developing over the last decades. The introduction of new materials, surface modifications and implant designs has brought the need to rethink and systematize our knowledge regarding dental implants. Thus, the aim of this paper is to introduce a new classification and implant positioning indications that can be used to maximize the survival rate and the aesthetic outcome of single-piece compressive screw implants. (2) Materials and methods: This classification was based on a multicenter clinical and radiological observation of 151 patients, in whom 1057 implants were placed with a success rate of 98.5% (1041). The follow-up period was up to 82 months with a mean of 22.34 months. (3) Results: it seems that, in the case of single-piece implants, diameter and length of the implant have influence on their survival rate, whereas smoking and hypertension do not. (4) Conclusions: this paper provides clinicians with comprehensive information about the rationale, criteria and implementation of the new classifications based on a large number of implants and long-term observations.

Digitally Fabricated Dentures for Full Mouth Rehabilitation with Zirconia, Polyetheretherketone and Selective Laser Melted Ti-6Al-4V Material

CAD/CAM technologies have been embedded into the fabrication of removable partial denture (RPD). Various materials such as zirconia and polyetheretherketone (PEEK) are developed for subtractive manufacturing. As for additive manufacturing, dental professionals have begun to use selective laser melting (SLM) techniques for fabricating metallic RPD frameworks. This report demonstrates a case rehabilitated with a maxillary telescopic crown-retained combining PEEK and zirconia material denture and a mandibular Kennedy Class I RPD fabricated with SLM techniques. First, a conventional impression was performed and the master cast was mounted with a centric relation record. Digital models were obtained using tabletop scanners and then the telescopic primary zirconia crowns were designed and milled. After transferring the intraoral distribution of primary crowns using pick-up impression, secondary PEEK crowns and framework were designed, milled, and veneered with composite resin. Mandibular framework was designed and constructed using SLM technique with Ti-6Al-4V. Definitive prostheses for both jaws were finished and delivered. Delivered prostheses functioned well for a one-year period. The was patient satisfied with the improvements in chewing function and esthetics. Both substrative and additive manufacturing techniques are suitable for framework fabrication. Further investigation is needed for improving the mechanical performance and long-term prognosis of digitally made prostheses.

Number of dental abutments influencing the biomechanical behavior of tooth‒implant-supported fixed partial dentures: A finite element analysis

Background. Local or systemic issues might prevent installing a sufficient number of dental implants for fixed prosthetic rehabilitation. Splinting dental implants and natural teeth in fixed dentures could overcome such limitations. Therefore, this study aimed to evaluate the influence of the number of dental abutments in the biomechanics of tooth‒implant-supported fixed partial dentures (FPDs). The null hypothesis was that increasing the number of abutment teeth would not decrease the stress over the abutments and surrounding bone. Methods. Left mandibular lateral incisor, canine, premolars, and molars were reconstructed through computed tomography and edited using image processing software to represent a cemented fixed metal‒ceramic partial denture. Three models were set to reduce the number of abutment teeth: 1) lateral incisor, canine, and first premolar; 2) canine and first premolar; 3) the first premolar. The second premolar and first molar were set as pontics, and the second molar was set as an implant abutment in all the models. Finite element analyses were performed under physiologic masticatory forces with axial and oblique loading vectors. Results. After simulation of axial loads, the stress peaks on the bone around the implant, the bone around the first premolar, and prosthetic structures did not exhibit significant changes when the number of abutment teeth decreased. However, under oblique loads, decreasing the number of abutment teeth increased stress peaks on the surrounding bone and denture. Conclusion. Increasing the number of dental abutments in tooth‒implant-supported cemented FPD models decreased stresses on its constituents, favoring the prosthetic biomechanics.

Interdisciplinary interface between fixed prosthodontics and periodontics

Although periodontal factors do not usually have a direct effect on the survival of a fixed prosthesis, harmony between the prosthesis and the periodontium is critical otherwise esthetics, the longevity of the prosthesis and the periodontium will be compromised. A close interdisciplinary relationship between periodontics and prosthodontics is therefore necessary to avoid an unsatisfactory treatment outcome, requiring extensive and expensive retreatment. The design of the prosthesis, the number and quality of the abutment teeth, the preparation and the pontic, the occlusion and the material need to be considered when planning prosthodontic treatment. The location of the preparation margin and the contour and emergence profile of the prosthesis will influence the response of the gingival tissues to the prosthesis. Pontic design and cleansibility also contribute to the response of the gingival tissues as well as to the clinical and esthetic outcome. Even an optimal pontic design will not prevent inflammation of the mucosa adjacent to the pontic if pontic hygiene is not maintained by removal of plaque. Case selection and the patients' ability to carry out adequate oral hygiene are therefore essential for longevity of the prosthesis, and regular reviews provide an opportunity for early detection and treatment of failures.

Clinical importance of median mandibular flexure in oral rehabilitation: a review

The mandible has a property to flex inwards around the mandibular symphysis with change in shape and decrease in mandibular arch width during opening and protrusion of the mandible. The mandibular deformation may range from a few micrometres to more than 1 mm. The movement occurs because of the contraction of lateral pterygoid muscles that pulls mandibular condyles medially and causes a sagittal movement of the posterior segments. This movement of mandible can have a profound influence on prognosis and treatment outcome for various restorative, endodontics, fixed, removable and implant-related prosthesis. The review unfolds the causes, importance and clinical implications of median mandibular flexure in oral rehabilitation. This review also highlights the appropriate preventive measures and techniques that should be adopted by clinicians to minimise the effect of flexural movement of the jaw during oral rehabilitation. This would not only help clinicians to achieve a good prosthesis with accurate fit and longevity but also maintain the health of the surrounding periodontal or periimplant gingival tissues and bone.

Effects of differing thickness and mechanical properties of cement on the stress levels and distributions in a three-unit zirconia fixed prosthesis by FEA

PURPOSE

This study analyzed the impact of cement layer thickness (CLT) and Young's modulus of the cement on the stress distribution in a three-unit zirconia fixed dental prosthesis (FDP) and in the bonding interfaces by means of finite element method.

MATERIALS AND METHODS

A 3D finite element model was created from a stylized three-unit FDP-cement-tooth/socket system. The pulp and the periodontal ligament were not modeled. Two CLTs (50 and 150 μm) and two values of Young's modulus of the cement (4.9 for simulation of resin cement, 20.1 GPa for glass ionomer cement) were evaluated. A 500 N static vertical load was applied at the central fossa of the pontic to calculate maximum displacement in the framework and maximum principal stresses in both framework and bonding interfaces.

RESULTS

The simulated results showed that the Young's modulus affected stress occurrence only in the cement interface. Lower moduli were associated with less stress. The thickness of the cement layer influenced the maximum principal stress in both the FDP and in the cement layer itself. Thicker cement layers led to higher stresses in the framework but lower stresses in the cement layer. Maximum displacement was less dependent of the investigated variables. During all trials, the location of the maximum principal stress did not change. Maximum stress concentrations were observed at the lower embrasures of the connector areas and in the bonding layer at the cervical margin of the preparation.

CONCLUSIONS

Choosing cements with a preferably low Young's modulus in combination with a CLT as small as possible might increase the clinical survival rate.

The effect of type of restoration on the stress field developed in terminal abutments with severely reduced periodontal support and coronal structure

STATEMENT OF PROBLEM

Periodontally compromised teeth (PCT) that serve as terminal abutments (TAs) are often challenging depending on the post-and-core treatment, the type of partial fixed dental prosthesis (PFDP), and the periodontal support.

PURPOSE

The purpose of this study was to investigate the biomechanical impact of 3 types of PFDP supported by cast post-and-cores on PCT serving as terminal abutments.

MATERIAL AND METHODS

A 3-dimensional (3D) model of a human mandible was fabricated by using computed tomography (CT) images and parameterized in a computer-aided design (CAD) environment as follows: Right premolar preparation geometries were designed. The second premolar was assembled with 7-mm or 10-mm cast post-and-core models. Both premolar-models were designed to support single, splinted, or 1-unit cantilever splinted crowns. In each situation, their periodontium geometries were designed to be reduced by 10%, 50%, and 70%. All models were imported into a 3D finite element analysis (FEA) environment and loaded; von Mises stress values and distribution patterns were evaluated.

RESULTS

Insertion of the post primarily affected the apical areas of both the root and post; the type of PFDP and periodontal support mainly affected stress distribution. In patients with a normal periodontium, splinting the teeth did not contribute to their stress relief. By extending the post length, a stressful area close to the apex of the post was developed. Splinting mitigated the stress field of the coronal part of the 50% PCT (up to 98.9%); the 30% PCT experienced a substantial decrease (up to 215.9%) in stress in the radical part as well. The increase in the length of the post produced negligible stress-related differences in the apical part of the 50% PCT (0.2% to 2.6%). The use of the 7-mm post effectively relieved the radical part of the splinted 30% PCT. The magnitude of the stress on the radical part of post-restored PCT was considerably increased in the presence of a cantilever.

CONCLUSIONS

Splinted crowns supported by a 7-mm cast post-and-core are a favorable biomechanical approach for the restoration of PCT with severe loss of coronal structure. The use of a cantilever greatly aggravates the biomechanical response, especially of post-restored PCT.

Stress distribution among periodontally compromised abutments: A comparative study using three-dimensional finite element analysis

Aims:

The aim of the study was to evaluate the stress distribution patterns in teeth and supporting structures of fixed prosthesis and design modifications in a fixed prosthesis with either normal or reduced bone support of an additional abutment. Study was also undertaken to disprove Ante's law.

Materials and Methods:

Main models and variations of main models (modification 1, 2, 3, 4, 5, 6, 7, 8) were subjected to 200 N at angulations of 90° and 15° on functional cusps. Results for each loading were obtained as stress distribution color images and numerical values were recorded. A three-dimensional finite element analysis study of variations of normal models was performed using two finite element softwares, namely PRO-Engineer wildfire version 1.0 manufacturer: Parametric technology corporation, Needham, MA 02494 U.S.A.

Results:

When periodontal compromised abutment teeth was splinted with an additional abutment an increase of stress was observed in periodontally compromised abutments so an additional abutment is not required. Eventhough the pericemental area of compromised abutments with an additional abutment (canine) was more than the combined pericemental area of pontics to be replaced, stress generated was more on abutments. This disproves Ante's law. Hence, it may be a reference, but should not be the ultimate criterion in determining the number of multiple abutments.

Conclusions:

When periodontal compromised abutment teeth was splinted with an additional abutment an increase of stress was observed in periodontally compromised abutments so an additional abutment is not required. Even though the pericemental area of compromised abutments with an additional abutment (canine) was more than combined pericemental area of pontics to be replaced, stress generated was more on abutments. This disproves Ante's law. Hence, it may be a reference, but should not be the ultimate criterion in determining the number of multiple abutments.

Finite element analysis of stresses in fixed prosthesis and cement layer using a three-dimensional model

Context:

To understand the effect of masticatory and parafunctional forces on the integrity of the prosthesis and the underlying cement layer.

Aims:

The purpose of this study was to evaluate the stress pattern in the cement layer and the fixed prosthesis, on subjecting a three-dimensional finite element model to simulated occlusal loading.

Materials and Methods:

Three-dimensional finite element model was simulated to replace missing mandibular first molar with second premolar and second molar as abutments. The model was subjected to a range of occlusal loads (20, 30, 40 MPa) in two different directions – vertical and 30° to the vertical. The cements (zinc phosphate, polycarboxylate, glass ionomer, and composite) were modeled with two cement thicknesses – 25 and 100 μm. Stresses were determined in certain reference points in fixed prosthesis and the cement layer.

Statistical Analysis Used:

The stress values are mathematic calculations without variance; hence, statistical analysis is not routinely required.

Results:

Stress levels were calculated according to Von Mises criteria for each node. Maximum stresses were recorded at the occlusal surface, axio-gingival corners, followed by axial wall. The stresses were greater with lateral load and with 100-μm cement thickness. Results revealed higher stresses for zinc phosphate cement, followed by composites.

Conclusions:

The thinner cement interfaces favor the success of the prosthesis. The stresses in the prosthesis suggest rounding of axio-gingival corners and a well-established finish line as important factors in maintaining the integrity of the prosthesis.

Reliability of a digital image method for measuring medial mandibular flexure in dentate subjects

Medial mandibular flexure (MMF) is the functional approximation of hemi-mandibles in jaw opening and protrusion, which may affect conventional or implant-supported arch prostheses.

OBJECTIVE

This study evaluated the intra- and inter-rater reliability of a digital image method for linear measurement of MMF in dentate subjects.

MATERIAL AND METHODS

Mandibular occlusal impressions in vinyl polysiloxane during rest and maximum opening were obtained from seven adult volunteers. Each impression and the Mitutoyo digital calipers with a 10mm opening were scanned at a 200% magnification. The images were processed using Adobe Photoshop software to get reference points on contralateral first molars. Linear intermolar measurements were taken using the Image Tool software (UTHSCSA), which was calibrated with the standard opening of the digital calipers. Intermolar distance was measured in triplicate for each image by two independent examiners. MMF was calculated by subtracting the intermolar distance in maximum opening (or protrusion) from the distance in rest position. Data were analyzed by intraclass correlation (ICC) tests for estimating intra- and inter-rater reliability.

RESULTS

MMF values ranged from -0.21 to 0.44mm. Intra-rater ICCs were 0.982 and 0.993, and inter-rater ICC was 0.696.

CONCLUSION

The present results show excellent intra-rater and good inter-rater reliability of this digital image method for measuring MMF in dentate subjects.

Digital image correlation analysis of the load transfer by implant-supported restorations

This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal-ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0Hz under a load from 0 to 250N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower stress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.

Fatigue of zirconia and dental bridge geometry: Design implications

Zirconia is currently used as a framework material for posterior all-ceramic bridges. While the majority of research efforts have focused on the microstructure and corresponding mechanical properties of this material, clinical fractures appear to be largely associated with the appliance geometry.

OBJECTIVE

The objective of this study was to estimate the maximum stress concentration posed by the connector geometry and to provide adjusted estimates of the minimum connector diameter that is required for achieving 20 years of function.

METHODS

A simple quantitative description of the connector geometry in an all-ceramic 4-unit bridge design is used with published stress concentration factor charts to estimate the degree of stress concentration and the maximum stress.

RESULTS

The magnitude of stress concentration estimated for clinically relevant connector geometries ranges from 2 to 3. Using previously published recommendations for connector designs, adjusted estimates for the minimum connector diameter required to achieve 20 years of clinical function are presented.

SIGNIFICANCE

To prevent clinical fractures the minimum connector diameter in multi-unit bridges designs must account for the loads incurred during function and the extent of stress concentration posed by the connector geometry.

Effect of varying the vertical dimension of connectors of cantilever cross-arch fixed dental prostheses in patients with severely reduced osseous support: a three-dimensional finite element analysis

STATEMENT OF PROBLEM

Inadequate dimensioning of the connectors in a cantilever cross-arch fixed dental prosthesis (FDP) in perioprosthetic patients jeopardizes the prognosis of the restoration.

PURPOSE

The purpose of this study was to investigate the effect of increasing the vertical dimension (VD) on the maximum stress developed within the connectors during the static loading of a cross-arch FDP extended as a 1- and 2-unit cantilever.

MATERIAL AND METHODS

Six digital models were developed, derived from a 3-dimensional (3-D) initial model. In the initial model, the teeth were prepared for metal ceramic restorations and splinted with a cross-arch FDP, extended as a 1- or 2-unit cantilever. The VDs of the connectors proximal to the retaining abutment were 3, 4, or 5 mm. A 3-D finite element analysis (FEA) was performed.

RESULTS

The VD increase, from 3 to 4 mm and from 3 to 5 mm, of the connector distal to the retaining abutment, for each FDP, presented a maximum stress value decrease of approximately 25% and 48%, respectively. The similar VD increase of the connector mesial to the retaining abutment, for each FDP, resulted in relatively smaller stress changes. For the 2-unit cantilever restoration, the stress decreases were approximately 9% and 15%, respectively, whereas in the 1-unit cantilever restoration, the decrease was about 10% for the 4-mm connector. Further increase of the VD to 5 mm did not relieve the peak stress. The highest stress value was measured on the 3-mm connector distal to the retaining abutment in the 2-unit cantilever restoration. Despite the VD increase, the connectors proximal to the retaining abutment still developed the highest stress values of all the connectors for every model.

CONCLUSIONS

The connector with the highest risk of failure is the 3-mm connector distal to the retaining abutment of the 2-unit cantilever restoration. Increasing the vertical dimension is beneficial for the connector distal to the retaining abutment, while the resultant stress changes are not substantial for the connectors mesial to the retaining abutment. (J Prosthet Dent 2010;103:91-100).

Estudo comparativo de diferentes prescrições de braquetes pré-ajustados em modelos virtuais pelo Método de Elementos Finitos

OBJETIVOS: comparar os efeitos de diferentes prescrições de braquetes pré-ajustados na movimentação dentária dos dentes anterossuperiores, por meio da utilização do Método de Elementos Finitos (MEF). MÉTODOS: foi utilizado um modelo tridimensional composto por um incisivo central, um incisivo lateral e um canino, todos do hemiarco superior esquerdo. Esses elementos foram submetidos à atuação de vetores de força que simularam a ação dos pré-ajustes de angulação e torque dos braquetes das prescrições de Alexander, Andrews, Capelozza, MBT, Ricketts e Roth. Após a simulação, os dados referentes ao deslocamento espacial dos pontos incisais e apicais foram registrados de forma que o novo posicionamento admitido pelos longos eixos dentários e seus pontos constituintes fossem determinados. RESULTADOS: a maioria das prescrições se comportou similarmente em relação aos parâmetros avaliados. As diferenças significativas encontradas no estudo se referem ao incisivo lateral superior que continha o braquete com a prescrição de Ricketts, que apresentou uma menor inclinação vestibular de seu longo eixo. Além disso, o canino superior da mesma prescrição apresentou uma maior inclinação de seu longo eixo para mesial e uma maior inclinação vestibular de seu longo eixo em relação às outras. A avaliação qualitativa da movimentação do incisivo central superior indicou um maior deslocamento distal dos ápices radiculares para as prescrições cujos autores estipularam maiores valores de angulação (Andrews, Capelozza, Roth e Alexander), e um maior deslocamento vestibular da coroa para as prescrições que estipularam os maiores valores de torque para os incisivos centrais (Ricketts, MBT, Alexander e Roth).

Biomechanical interactions in tooth-implant-supported fixed partial dentures with variations in the number of splinted teeth and connector type: a finite element analysis

OBJECTIVE

The aim of this study was to investigate the biomechanical interactions in tooth-implant-supported fixed partial dentures (FPDs) under several loading conditions with different numbers of splinted teeth and connector types (rigid and non-rigid) by adopting the three-dimensional (3D) non-linear finite element (FE) approach.

MATERIAL AND METHODS

A 3D FE FPD model was constructed containing one Frialit-2 implant in the mandibular second-molar region splinted to the first and second premolars. Frictional contact elements were used to simulate realistic interface conditions within the implant system and the non-rigid connector function. The main effects for each level of the three investigated factors (loading condition, number of splinted teeth and connector type) in terms of the stress values and dissimilar mobility of the natural teeth and implant were computed for all models.

RESULTS

The results indicated that load condition was the main factor affecting the stress developed in the implant, bone and prosthesis when comparing the type of connector and the number of splinted teeth. The stress values were significantly reduced in centric or lateral contact situations once the occlusal forces on the pontic were decreased. However, the prosthesis stress for the non-rigid connections was increased more than 3.4-fold relative to the rigid connections. Moreover, the average tooth-to-implant displacement ratios (R(TID)) with a non-rigid connection were obviously larger than those for rigid connections under axial loading forces. Adding an extra tooth to support a three-unit tooth-implant FPD only exploited its function when the prosthesis withstood lateral occlusal forces.

CONCLUSIONS

The load condition is the main factor affecting stress distribution in different components (bone, prosthesis and implant) of tooth-implant-supported FPDs. Minimizing the occlusal loading force on the pontic area through selective grinding procedures could reduce the stress values obviously. A non-rigid connector may more efficiently compensate for the dissimilar mobility between the implant and natural teeth under axial loading forces but with the risk of increasing unfavorable stresses in the prosthesis.

Optimum Design for Fixed Partial Dentures Made of Hybrid Resin with Glass Fiber Reinforcement by Finite Element Analysis: Effect of Vertical Reinforced Thickness on Fiber Frame

By means of finite element analysis, the optimal thickness of fiber framework placed in a fiber-reinforced composite bridge replacing the mandibular first molar was obtained. Test results demonstrated that more than 30% maximum principal stress was reduced by reinforcing with fiber framework in a thickness of up to 0.6 mm for 1.5-mm occlusal clearance. Indeed, maximum principal stress generated in lower embrasure of connectors was reduced from 107 MPa to 70 MPa by maximizing reinforcement effect.

A comparison of 2D and 3D finite element analysis of a restored tooth

The finite element method is widely used in dental research. The decision to use two-dimensional (2D) or three-dimensional (3D) modelling is dependent on many interrelated factors. The purpose of the present study was to compare and contrast 2D and 3D finite element analysis (FEA) in investigating the mechanical behaviour of a maxillary premolar restored with a full crown under similar conditions of axial and lateral occlusal loading. The 2D analysis required modelling both a buccolingual and mesiodistal section of the restored premolar and for comparison sections of a 3D model were examined. Differences in the results for displacement and maximum principal stress distribution within the component structures and interfaces of the 2D and 3D models were, in general, attributable to differences in geometry represented in the models. Maximum principal stresses tended to be greater under lateral rather than axial occlusal loading. It was concluded that 2D FEA may find application in investigating key aspects of the mechanical behaviour of a dental restoration in a single tooth unit, but that in certain situations combinations of 2D and 3D FEA may offer the best understanding of the biomechanical behaviour of complex dental structures. Sophisticated FE models are required to better understand the mechanical behaviour of restored tooth units.

Stress distribution in fiber-reinforced composite inlay fixed partial dentures

STATEMENT OF PROBLEM

Fiber-reinforced composite inlay fixed partial dentures (FRCIFPDs) may be a reliable prosthetic solution. Clinical procedures involved in their fabrication have been defined, but little is known about their mechanical behavior.

PURPOSE

This in vitro study used the finite element (FE) method to investigate 3-dimensional (3-D) stress and strain distribution produced in a 3-unit FRCIFPD.

MATERIAL AND METHODS

A 3-D FE model (227,768 3-D tetrahedral elements) of a 3-unit FRCIFPD cemented onto box-shaped prepared teeth was developed. Stress and strain distribution generated by a maximum load of 196 N applied vertically or laterally to the FRCIFPD centrally, on an area of 4 +/- 0.1 mm 2 , was analyzed. The specimen used to acquire the geometry of the model was also used for mechanical compressive tests, with vertical and lateral loads, to validate the numerical model.

RESULTS

The peak values of stress, calculated on the outer and inner surfaces of the FRCIFPD, were localized in the connector areas. When a vertical load was applied, stress on the prepared teeth was concentrated at the cervical margin of the abutment preparation.

CONCLUSION

Within the limitations of this in vitro study, the results suggest that within the FRCIFPD, stress concentrates at the connector areas, and that in the prepared teeth, peak stress is at the cervical margin of the box of the preparation.

Finite element analysis of fixed partial denture replacement

The purpose of this study was to investigate, by means of the finite element method the mechanical behaviour of three designs of fixed partial denture (FPD) for the replacement of the maxillary first premolar in shortened dental arch therapy. Two-dimensional, linear, static finite element analyses were carried out to investigate the biomechanics of the FPDs and their supporting structures under different scenarios of occlusal loading. Displacement and stress distribution for each design of FPD were examined, with particular attention being paid to the stress variations along the retainer-abutment--and the periodontal ligament-bone interfaces. The results indicated that displacement and maximum principal stresses in the fixed-fixed, three-unit FPD were substantially less than those in the two-unit cantilever FPDs. Of the two cantilever FPDs investigated, the distal cantilever design was found to suffer less displacement and stresses than the mesial cantilever design under similar conditions of loading. The highest values for maximum principal stress in the cantilever FPDs were found within the connector between the pontic and the retainer, and within the periodontal ligament and adjacent bone on the aspect of the retainer away from the pontic.

Biomechanics of cantilever fixed partial dentures in shortened dental arch therapy

PURPOSE

The purpose of the present study was to investigate, by means of 3-dimensional finite element analysis, aspects of the biomechanics of cantilever fixed partial dentures replacing the maxillary canine in shortened dental arch therapy. The null hypothesis was that no differences would be identified by finite element analysis in the mechanical behavior of the 2 designs of cantilever fixed partial denture under different scenarios of occlusal loading.

MATERIALS AND METHODS

Single- and double-abutted cantilever fixed partial dentures were modeled and analyzed using the finite element packages PATRAN and ABAQUS. Displacement and maximum principal stresses (magnitude and location) within the fixed partial dentures, supporting structures, and the periodontal ligament/bone and abutment/retainer interfaces were examined under 20 different scenarios of axial and lateral occlusal loading.

RESULTS

The results indicate that more displacement occurred in the 2 rather than the 3-unit cantilever fixed partial denture, with the greatest displacement having occurred under lateral loading. The maximum principal stresses observed in the periodontal ligament/bone interfaces were greatest buccocervically, with the highest value being observed in the 2-unit fixed partial denture under lateral loading. The highest maximum principal stresses observed in the retainer/abutment interfaces were located cervically in relation to the distal margin of the retainer of the 2-unit fixed partial denture under axial loading.

CONCLUSIONS

It was concluded that in adopting a cantilever fixed partial denture approach for the replacement of a missing maxillary canine in shortened dental arch therapy, there may be merits, in terms of mechanical behavior, in selecting a double-rather than a single-abutment design. Furthermore, prostheses' displacement and functional stresses may be minimized by reducing lateral loading and avoiding pontic only loading.

Effect of splinting and interproximal contact tightness on load transfer by implant restorations

STATEMENT OF PROBLEM

To circumvent the difficulty of achieving a passive framework fit, some authors have suggested that multiple adjacent implants be restored individually. This protocol requires that each unit be able to withstand mastication forces. Non-splinted restorations have numerous interproximal contacts that require adjustments prior to placement, with an unknown outcome relative to load transfer.

PURPOSE

This in vitro simulation study examined the effect of splinting and interproximal contact tightness on passivity of fit and the load transfer characteristics of implant restorations.

MATERIAL AND METHODS

A photoelastic model of a human partially edentulous left mandible with 3 screw-type implants (3.75 x 10 mm) was fabricated. For non-splinted restorations, individual crowns were fabricated on 3 custom-milled titanium abutments. After the units were cemented, 5 levels of interproximal contact tightness were evaluated: open, ideal (8 microm shim stock drags without tearing), light (ideal +10 microm), medium (ideal + 50 microm), and heavy (ideal + 90 microm). For splinted restorations, five 3-unit fixed partial dentures were fabricated, internally adjusted with silicone disclosing material, and cemented to the model. Changes in stress distribution under simulated non-loaded and loaded conditions (6.8 kg) were analyzed with a polariscope.

RESULTS

In the simulated alveolar structures, non-splinted restorations with heavier interproximal contacts were associated with increased tensile stresses between implants; occlusal loads tended to concentrate around the specific loaded implant. Splinted restorations shared the occlusal loads and distributed the stresses more evenly between the implants when force was applied. The load-sharing effect was most evident on the center implant but also was seen on the terminal abutments of the splinted restorations.

CONCLUSION

The results of this in vitro study suggest that excessive contact tightness between individual crowns can lead to a non-passive situation. In this experiment, splinted restorations exhibited better load sharing than non-splinted restorations.