Photoelastic stress analysis of load transfer to implants and natural teeth comparing rigid and semirigid connectors

Biomechanical analysis of rigid and non-rigid connection with implant abutment designs for tooth-implant supported prosthesis: A finite element analysis

BACKGROUND/PURPOSE

The design of the connectors and implant abutments could affect the stress distribution of the tooth-implant supported prosthesis (TISP) entire system after loading. Therefore, the purpose of this study was to investigate the stress distribution of the TISP in different connectors and different implant abutments after loading.

MATERIALS AND METHODS

The TISP design used in this study was divided into six models. R1, R2 and R3 represented the tooth and the one-piece, two-piece and three-piece abutment implant system connected by a rigid connector, respectively, while NR1, NR2 and NR3 were the corresponding tooth-abutment implant systems connected by a non-rigid connector. A vertical occlusal load of 50 N was applied at a right angle on the 6 occlusal points of the occlusal surface.

RESULTS

As a result, regarding the maximum average stress distribution, R1 and NR1 appeared on the implant fixture, and the other four models were on the implant abutment. On the other hand, regardless of the abutment implant system, the maximum von Mises stress generated by the rigid connector was greater than the corresponding non-rigid connector in the cortical bone around implant. In addition, the three-piece abutment implant system had lower von Mises stress than the one-piece and two-piece implant systems in the cortical bone.

CONCLUSION

It is concluded that by adding a flexible non-rigid connector and three-piece abutment device design to TISP, the occlusal load of the implant was dispersed, and the stress could be gradually introduced into the relatively strong implant abutment.

Influence of Different Implants on the Biomechanical Behavior of a Tooth-Implant Fixed Partial Dentures: A Three-Dimensional Finite Element Analysis

This study analyzed the biomechanical behavior of rigid and nonrigid tooth-implant supported fixed partial dentures. Different implants were used to observe the load distribution over teeth, implants, and adjacent bone using three-dimensional finite element analysis. A simulation of tooth loss of the first and second right molars was created with an implant placed in the second right molar and a prepared tooth with simulated periodontal ligament (PDL) in the second right premolar. Configurations of two types of implants and their respective abutments-external hexagon (EX) and Morse taper (MT)-were transformed into a 3D format. Metal-ceramic fixed partial dentures were constructed with rigid and nonrigid connections. Mesh generation and data processing were performed on the 3D finite element analysis (FEA) results. Static loading of 50 N (premolar) and 100 N (implant) were applied. When an EX implant was used, with a rigid or nonrigid connection, there was intrusion of the tooth in the distal direction with flexion of the periodontal ligament. Tooth intrusion did not occur when the MT implant was used independent of a rigid or nonrigid connection. The rigid or nonrigid connection resulted in a higher incidence of compressive forces at the cortical bone as well as stress in the abutment/pontic area, regardless of whether EX or MT implants were used. MT implants have a superior biomechanical performance in tooth-implant supported fixed partial dentures. This prevents intrusion of the tooth independent of the connection. Both types of implants studied caused a greater tendency of compressive forces at the crestal area.

Combined Implant and Tooth Support: An Up-to-Date Comprehensive Overview

Objectives. This article presents a review on the concerned topics and some considerations related to the concept of splinting teeth and implants in the rehabilitation of partial edentulism. Study Selection. An electronic PubMed/MEDLINE and manual search of identified articles and reviews as well as clinical, laboratory, and finite element studies was performed in this project. Due to the shortage in within-subject, long term, randomized, controlled clinical trials regarding the subject a meta-analysis was not possible. Results. Although surrounded with some controversy, joining teeth and implants during the rehabilitation of partial edentulism provides the clinicians with more treatment options where proprioception and bone volume are maintained and distal cantilevers and free end saddles are eliminated. It makes the treatment less complex, of less cost, and more acceptable for the patient. Conclusions. Whenever suitable and justified, combining implant and tooth support might be recommended as an alternative during rehabilitation of partial edentulism. Based on the literature, clinical tips and suggestions were recommended to increase the success of this treatment.

Biomechanical outcomes of tooth-implant-supported fixed partial prostheses (FPPs) in periodontally healthy patients using root shape dental implants

Background: Connecting an osseointegrated implant and a natural tooth is a treatment alternative for partially edentulous patients in some clinical situations. The main issue of a connected tooth-implant system is derived from the dissimilar mobility patterns of the osseointegrated fixtures and natural abutments causing potential biomechanical problems within the entire system. Purpose: The aim of this review was to multilaterally analyze and discuss the main biomechanical factors that may question the reliability of splinted tooth-implant system and the long-term success of fixed partial prostheses (FPPs) supported by both teeth and implants with an emphasis on the disparity of mobility of these two different abutments. Material and methods: An electronic MEDLINE (PubMed) search supplemented by manual searching was performed to retrieve relevant articles. An assessment of the identified studies was performed, the most valuable articles were selected and biomechanical outcomes of tooth-implant splinting system were analyzed. Results: 3D FEM stress analyses and photoelastic studies show uneven load distribution between the tooth and the implant and stress concentration in the crestal bone around the implant neck when connected to a natural tooth by FPPs. However, clinical studies demonstrate good results for both the implants and FPPs supported by splinted implant-tooth abutments. Conclusion: Connecting implants to natural teeth is not a preferable treatment option because of possible inherent biomechanical complications. Whenever possible, this treatment option should be avoided.

Effects of rigid and nonrigid extracoronal attachments on supporting tissues in extension base partial removable dental prostheses: a nonlinear finite element study

STATEMENT OF PROBLEM

Resilient (nonrigid) and non-resilient (rigid) attachments are used in extension base partial removable dental prostheses for retention. However, the biomechanical effects of these 2 types of retainers on the terminal abutment and supporting tissues, which may influence clinical treatment planning, have not been compared.

PURPOSE

The purpose of this study was to compare the mechanical effects of 2 types of extracoronal attachments (rigid and nonrigid) in distal extension removable partial prostheses on the alveolar ridge and abutment tooth periodontal ligament.

MATERIAL AND METHODS

A finite element model of a human left mandible edentulous arch distal to the second premolar was fabricated. The second premolar was the terminal abutment for an attachment-retained denture. Two types of attachments (rigid and nonrigid) were modeled in the study. For the nonrigid attachment, there was movement between the patrix and matrix component of the attachment, but there was no movement between the 2 component parts for the rigid attachment. Six levels of loading (100, 150, 200, 250, 300, and 350 N) were applied from 3 directions (axial, buccolingual, and mesiodistal) on the central fossa of the first and second molars. Denture motion and stress distributions of denture supporting tissues were observed. Maximum equivalent stress values (SEQV) were recorded for 6 regions (cervical bone, cervical and apical periodontal ligaments, mesial and distal ridges, and mucosa). The data were divided into 2 groups according to the attachment type. Paired t tests were used to compare the values of the 2 groups. Factorial ANOVA was used to test the difference between the loading directions (α=.05). Multiple linear regression was used to analyze the interactions among the factors of region, direction, and level (α=.05).

RESULTS

Stress distributions in the rigid and nonrigid attachment models were similar but the magnitudes were different. For all 3 loading directions, significantly different stresses in the alveolar ridge and periodontal tissue of the terminal abutment were found between the rigid and nonrigid groups (P<.05). There were significant differences among the 3 loading directions (P<.05). In the nonrigid group, the stress ratio of the mesial to the distal area was higher than that of the rigid group from axial and mesiodistal loading (P<.05). Linear interactions were found between the direction and level and region and level combinations (P<.05). Movement between the patrix and matrix components increased as loading increased. The most obvious movement of attachment occurred when loading was in the buccolingual direction.

CONCLUSIONS

Stress on the terminal abutment can be reduced by the use of an extracoronal resilient attachment that allocates more loads onto the distal edentulous ridge. The level of loading influenced the extent of reduction. A resilient attachment with a universal hinge had the most movement when loading was in the buccolingual direction. Interactions were found between direction and level, as well as region and level combinations (P<.05).

Tooth-implant connection: a review

The aim of this review was to assess the long-term outcomes of restorations supported by implants and natural teeth with regard to complications associated with implants, teeth, and restorations, as well as the influence on these parameters of the connector type used. A net-based search in PubMed was combined with a manual search. Clinical studies, reviews, and biomechanical studies were included. Information on survival rate, complication rate, incidence of tooth intrusion, and, where applicable, type of connector used, was retrieved from the clinical studies. Force distribution and types of connectors used were retrieved from the biomechanical study. A summary of outcomes was retrieved from the reviews. A total of 25 articles were selected for inclusion in this review, including clinical studies (15), biomechanical studies (7), and reviews (3). Implant success rates ranged from 79.5%-100%. Tooth complications occurred in 5.4%-11.8% of cases. Complications in the suprastructure were observed in 5%-90% of cases. Tooth intrusion presented in a total of 0%-66% of all cases, more often in cases with nonrigid connection (0%-66%) than in cases with rigid connection (0%-44%). Biomechanical studies show a large difference in stress distribution and in dependence on the type of connector used, with most studies demonstrating that nonrigid connectors drastically reduce stress on the suprastructure while increasing forces on supporting teeth and implants. Long-term success rates for tooth-implant connections are lower than for solely implant-supported restorations with regard to prognosis for teeth, implants, and suprastructure. Use of rigid connectors leads to more favorable clinical outcomes in terms of long-term stability, occurrence of complications, and tooth intrusion.

Photoelastic analysis of stress distribution in oral rehabilitation

The purpose of this study was to present a literature review about photoelasticity, a laboratory method for evaluation of implants prosthesis behavior. Fixed or removable prostheses function as levers on supporting teeth, allowing forces to cause tooth movement if not carefully planned. Hence, during treatment planning, the dentist must be aware of the biomechanics involved and prevent movement of supporting teeth, decreasing lever-type forces generated by these prosthesis. Photoelastic analysis has great applicability in restorative dentistry as it allows prediction and minimization of biomechanical critical points through modifications in treatment planning.

Influence of the connector and implant design on the implant-tooth-connected prostheses

PURPOSE

The purpose of this study was to evaluate stress transfer patterns between implant-tooth-connected prostheses comparing rigid and semirigid connectors and internal and external hexagon implants.

MATERIALS AND METHODS

Two models were made of photoelastic resin PL-2, with an internal hexagon implant of 4.00 x 13 mm and another with an external hexagon implant of 4.00 x 13 mm. Three denture designs were fabricated for each implant model, incorporating one type of connection in each one to connect implants and teeth: 1) welded rigid connection; 2) semirigid connection; and 3) rigid connection with occlusal screw. The models were placed in the polariscope, and 100-N axial forces were applied on fixed points on the occlusal surface of the dentures.

RESULTS

There was a trend toward less intensity in the stresses on the semirigid connection and solid rigid connection in the model with the external hexagon; among the three types of connections in the model with the internal hexagon implant, the semirigid connection was the most unfavorable one; in the tooth-implant association, it is preferable to use the external hexagon implant.

CONCLUSIONS

The internal hexagon implant establishes a greater depth of hexagon retention and an increase in the level of denture stability in comparison with the implant with the external hexagon. However, this greater stability of the internal hexagon generated greater stresses in the abutment structures. Therefore, when this association is necessary, it is preferable to use the external hexagon implant.

Should we extract teeth to avoid tooth-implant combinations?

The controversy over combining teeth and implants for support of fixed partial dentures still remains after almost three decades of debate. The aim of this review was to evaluate what support that could be found in the literature for extracting teeth in favour of implants, and to elucidate whether tooth-implant prostheses were inferior to solely implant supported constructions in terms of survival and complications. The methods for gathering relevant information entailed electronic searches on PubMed using relevant key words, as well as complementary manual searches in the retrieved publications. The results showed that there was no support for extracting teeth in favour of placing implants. On the contrary, the healthy tooth had a survival that was life-long, which is yet to be shown for the dental implant. Also the use of teeth as abutments in combination with dental implants for support of fixed dental prostheses could be endorsed in certain situations with solid albeit limited scientific support. In a wider sense, such prostheses could be used as a reliable therapy in all regions of the jaws. However the status of the abutment teeth in terms of periodontal support, pulpal status and risk for carious lesions and biomechanical complications should always be considered in relation to the long-term prognosis of the prosthesis. The conclusion was that teeth should not be extracted in favour of placing dental implants without a specific indication, and that tooth-implant supported prostheses should be considered as a viable prosthetic option.

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.

An investigation of tooth/implant-supported fixed prosthesis designs with two different stress analysis methods: an in vitro study

PURPOSE

Tooth/implant-supported fixed prostheses (TIFPs) present biomechanical design problems, because the implant is rigidly anchored within the alveolus, and the tooth is attached by the periodontal ligament that allows movement. While TIFP designs with rigid connectors (RCs) are preferred by many clinicians, the designs containing non-rigid connectors (NRCs) are suggested as a method to compensate for these mobility differences. However, studies have failed to show the advantage of one design over the other. This study examined stresses formed around the implant and natural tooth abutments under occlusal forces, using two dimensional finite element (2D-FEM) and photoelastic stress analysis methods (PSAM).

MATERIALS AND METHODS

Connection of TIFP designs were investigated in distal extension situations using stress analysis interpreted with the 2D-FEM and PSAM. Three TIFP (screw type implant, 3.75 mm x 13 mm) models with various connection designs (i.e., rigidly connected to an abutment tooth, connected to an abutment tooth with an NRC, connected to an abutment implant with an NRC) were studied. The stress values of the three models loaded with vertical forces (250 N) were analyzed.

RESULTS

The highest level of stresses around the implant abutment was noted on the TIFPs with the RC. On the other hand, NRCs incorporated into prostheses at the site of the implant abutment reduced the level of stresses in bone.

CONCLUSION

It could be suggested that if tooth and implant abutments are to be used together as fixed prostheses supports, NRCs should be placed on the implant abutment-supported site.

Stress Distribution after Installation of Fixed Frameworks with Marginal Gaps over Angled and Parallel Implants: A Photoelastic Analysis

PURPOSE

The objective of this work was to compare by photoelastic analysis the stress distribution along a fixed framework placed over angled or parallel implants with different gap values between the framework and one of the implants.

MATERIALS AND METHODS

Two photoelastic models were created: (i) with parallel implants; (ii) with a 30 degrees angled central implant. In both cases, three implants were used, and CP titanium frameworks were constructed with commercial components. A plane polariscope was used to observe the photoelastic fringes generated after initial framework assembly, and also when an axial load of 100 N was applied over the central implant. For both models, stress analysis was conducted on well-fitting frameworks and on another with a 150 microm vertical gap between the framework and the central implant.

RESULTS

The photoelastic analysis indicated that in the model with parallel implants, stress distribution followed the implant axis, and in the model with an angled implant, a higher and nonhomogeneous stress concentration was observed around the apical region of the lateral implants. The placement of an ill-fitting framework resulted in increased preload stress patterns.

CONCLUSION

Stresses were generated after screw tightening of the frameworks, increasing when a load was applied and when a vertical gap was present. Angled implants resulted in oblique stress patterns, which were not transferred with homogeneity to the polymeric model.

Implant-tooth-supported fixed partial prostheses: correlations between in vivo occlusal bite forces and marginal bone reactions

PURPOSE

To evaluate maximal occlusal bite forces (MOF) and marginal bone level (MBL) changes in patients with implant-tooth-supported fixed partial prostheses (FPP).

MATERIAL AND METHODS

Twenty nine partially edentulous patients consecutively who received 34 three-occlusal unit FPP with terminal implant and tooth support were subjected to quantification of MOFs using a sub-miniature load cell connected to a data acquisition system and measurement of the MBL changes around implants in digitalized periapical radiographs obtained at prostheses delivery and 24-month follow-up.

RESULTS

MOFs for implant support (mean: 353.61 N) significantly differed from tooth support (mean: 275.48 N) (P < 0.05), while gender did not influence MOFs (P > 0.05). MBL changes at mesial and distal sites of the implants at 24 months of functional loading were 0.28 and 0.097 mm respectively.

CONCLUSION

Although MOFs under functional loading might indicate an increase in load participation for supporting implant, the rigid connection between implant and natural tooth via three-occlusal unit FPP does not jeopardize the time-dependent MBL stability of the implant under functional loads.

Numerical simulation on the biomechanical interactions of tooth/implant-supported system under various occlusal forces with rigid/non-rigid connections

The aim of this study was to analyze the biomechanics in an implant/tooth-supported system under different occlusal forces with rigid/non-rigid connectors by adopting a 3D non-linear finite element (FE) approach. A 3D FE model containing one Frialit-2 implant splinted to the mandibular second premolar was constructed. Contact elements (frictional surface) were used to simulate the realistic interface condition within the implant system and the sliding keyway stress-breaker function. The stress distributions in the splinting system and dissimilar mobility between natural tooth and implant with rigid and non-rigid connectors were observed for six loading types. The simulated results indicated that the lateral occlusal forces significantly increased the implant (sigma(I, max)), alveolar bone (sigma(AB, max)) and prosthesis (sigma(P, max)) stress values when compared with the axial occlusal forces. The sigma(I, max) and sigma(AB, max) values did not exhibit significant differences regardless of the connector type used. However, the sigma(P, max) values with a non-rigid connection increased more than two times those of the rigid connection. The sigma(I, max), sigma(AB, max) and sigma(P, max) stress values were significantly reduced in centric or lateral contact situations once the occlusal forces on the pontic were decreased. Moreover, the vertical-tooth-to-implant displacement ratios with a non-rigid connection were 23 and 9.9 times that for axial and lateral loads, respectively, applied on the premolar. However, the compensated non-rigid connector capabilities were not significant when occlusal forces acted on the complete prosthesis. The non-rigid connector (keyway device) only significantly exploited its function when the occlusal forces acted on a natural tooth. Minimizing the occlusal loading force on the pontic area through occlusal adjustment procedures to redistribute stress in the maximum intercuspation or lateral working position for an implant/tooth-supported prosthesis is recommended.

Photoelastic stress analysis of implant-tooth connected prostheses with segmented and nonsegmented abutments

STATEMENT OF PROBLEM

There is some question about whether implant abutment selection affects the transfer of load between connected implants and natural teeth.

PURPOSE

The purpose of this study was to compare stress transfer patterns with either 1 or 2 posterior implants connected to a single anteriorly located simulated natural tooth with either 1 or 2 segmented and nonsegmented implant abutments under relevant functional loads by use of the photoelastic stress analysis technique.

MATERIAL AND METHODS

A model of a human left mandible, edentulous posterior to the first premolar, with two 3.75-mm x 13-mm screw-type implants embedded within the edentulous area, was fabricated from photoelastic materials. The implants were in the first and second molar positions. Two fixed partial denture prosthetic restorations were fabricated with either segmented conical abutments or nonsegmented UCLA abutments. Vertical occlusal loads were applied at fixed locations on the restorations. The photoelastic stress fringes that developed in the supporting mandible were monitored visually and recorded photographically. The stress intensity (number of fringes), stress concentrations (closeness of fringes), and their locations were subjectively compared.

RESULTS

Loading on the restoration over the simulated tooth generated apical stresses of similar intensity (fringe order) at the tooth and the first molar implant for both abutment types. Low-level stress was transferred to the second molar implant. Loading directed on the implant-supported region of the restoration demonstrated low transfer of stress to the simulated tooth. Nonvertical stress transfer with slightly higher intensity was observed for the nonsegmented abutment.

CONCLUSION

Within the limitations of this simulation study, stress distribution and intensity for the 2 implant conditions was similar for segmented and nonsegmented abutment designs. Magnitude of stresses observed for both abutment designs was similar for the single implant condition. Vertical loading produced more nonaxial stresses away from the force applied for the 1 implant condition with the nonsegmented abutment. Direct loading results were similar for both abutment designs. Specific recommendations for selection of implant abutment and application should be based on clinical criteria.

Freestanding and tooth-implant connected prostheses in the treatment of partially edentulous patients

A total of 123 patients were followed between January 1983 and July 1998 with 140 tooth-implant connected prostheses. The age of the patients at prosthesis installation ranged from 20 to 79 years (mean 51.8). 339 (Brånemark(R) system) implants were connected to 313 teeth. The loading time ranged from 1.5 to 15 years (mean: 6.5). 123 patients were randomly selected as a control group with freestanding implant-supported prostheses only. The age of the patients at prosthesis installation ranged from 22 to 78 years (mean 52.3). The loading time for the 329 freestanding (Brånemark(R) system) implants ranged from 1.3 to 14.5 years (mean: 6.2). Evolution of the marginal bone stability around the implant in the tooth-implant connected as well as the freestanding group was studied with respect to the prognosis of the implants. Over the period from 0 to 15 years, there was significantly more marginal bone loss (0.7 mm) in tooth-implant connected versus freestanding prostheses. No significant difference in marginal bone loss was found between the non-rigid tooth-implant connected prostheses versus freestanding prostheses. However, there was a significant difference in marginal bone loss for rigid and multi-connected tooth-implant connected prostheses versus freestanding ones. The results of this study indicate that more bone is lost around implants which are rigidly connected to teeth. This suggests that bending load, which is increased in tooth-implant connected prostheses, might be responsible for this phenomenon. These observations favor the use of freestanding prostheses whenever possible. However, the clinical significance of greater bone loss in rigid versus non-rigid connections might outweigh the annoying phenomenon of tooth intrusion in the case of non-rigid tooth connection, when connection is considered.