Stress analysis on the free-end distal extension of an implant-supported mandibular complete denture

Evaluation of immediately loaded mandibular four vertical versus tilted posterior implants supporting fixed detachable restorations without versus with posterior cantilevers

BACKGROUND

Distally inclining posterior implants may be technically challenging in certain situations. The presence of a posterior cantilever can also exert unfavorable forces on supporting implants. The aim of the present study was to evaluate and compare peri-implant soft and hard tissues around 4 mandibular interforaminal implants having tilted posterior implants with posterior cantilevers, versus vertical implants, 2 in the interforaminal region and 2 in the first molar regions, without posterior cantilevers. All implants supported full-arch fixed detachable restorations opposing complete dentures.

MATERIAL AND METHODS

A total of 80 implants were placed flapless in the mandibles of 20 edentulous participants. Four implants were placed for every participant, who were randomly assigned into 2 equal groups. Axial group implants were vertically aligned, with 2 implants in the interforaminal area and 2 in the molar area. Tilted group implants have 2 anterior axial and 2 posterior distally inclined implants. Interim screw-retained prostheses converted from pre-existing dentures were immediately fabricated and loaded on the same day of surgery. After awaiting period of 3 months, all participants received fixed detachable metal acrylic resin definitive restorations. A follow-up protocol of 3, 6, and 12 months was scheduled to assess the modified gingival index, modified plaque index, peri-implant probing depth, implant stability, and marginal bone level and bone density changes.

RESULTS

No statistically significant differences (P > .05) were found in the modified gingival index, modified plaque index, peri-implant probing depth, implant stability, bone density, and marginal bone level between the axial and tilted implant groups after the 1-year follow-up period.

CONCLUSION

Placing 4 flapless immediately loaded implants in mandibular edentulous patients that supported full-arch fixed restorations provided high implant and prosthodontic success rates whether posterior implants were tilted with posterior cantilevers or vertically aligned without posterior cantilevers.

TRIAL REGISTRATION

Pan African Clinical Trial Registry database, PACTR201907776166846. Registered 3 July 2019, www.pactr.org .

Influence of cantilever position and implant connection in a zirconia custom implant-supported fixed partial prosthesis: in silico analysis

Abstract Introduction A better tension distribution on implants and abutments in implant-supported fixed partial prosthesis is essential in the rehabilitation of posterior mandible area. Objective: To evaluate the influence of cantilever position and implant connection in a zircônia custom implant-supported fixed partial prosthesis using the 3-D finite element method. Material and method: Four models were made based on tomographic slices of the posterior mandible with a zirconia custom three-fixed screw-retained partial prosthesis. The investigated factors of the in silico study were: cantilever position (mesial or distal) and implant connection (external hexagon or morse taper). 100 N vertical load to premolar and 300 N to molar were used to simulate the occlusal force in each model to evaluate the distribution of stresses in implants, abutments, screws and cortical and cancellous bone. Result: The external hexagon (EH) connection showed higher cortical compression stress when compared to the morse taper (MT). For both connections, the molar cantilever position had the highest cortical compression. The maximum stress peak concentration was located at the cervical bone in contact with the threads of the first implant. The prosthetic and abutment screws associated with the molar cantilevers showed the highest stress concentration, especially with the EH connection. Conclusion: Morse taper implant connetions associated with a mesial cantilever showed a more favorable treatment option for posterior mandible rehabilitation.

Influence of implantoplasty on stress distribution of exposed implants at different bone insertion levels

This study evaluated the effect of implantoplasty on different bone insertion levels of exposed implants. A model of the Bone Level Tapered implant (Straumann Institute, Waldenburg, Switzerland) was created through the Rhinoceros software (version 5.0 SR8, McNeel North America, Seattle, WA, USA). The abutment was fixed to the implant through a retention screw and a monolithic crown was modeled over a cementation line. Six models were created with increasing portions of the implant threads exposed: C1 (1 mm), C2 (2 mm), C3 (3 mm), C4 (4 mm), C5 (5 mm) and C6 (6 mm). The models were made in duplicates and one of each pair was used to simulate implantoplasty, by removing the threads (I1, I2, I3, I4, I5 and I6). The final geometry was exported in STEP format to ANSYS (ANSYS 15.0, ANSYS Inc., Houston, USA) and all materials were considered homogeneous, isotropic and linearly elastic. To assess distribution of stress forces, an axial load (300 N) was applied on the cusp. For the periodontal insert, the strains increased in the peri-implant region according to the size of the exposed portion and independent of the threads' presence. The difference between groups with and without implantoplasty was less than 10%. Critical values were found when the inserted portion was smaller than the exposed portion. In the exposed implants, the stress generated on the implant and retention screw was higher in the models that received implantoplasty. For the bone tissue, exposure of the implant's thread was a damaging factor, independent of implantoplasty. Implantoplasty treatment can be safely used to control peri-implantitis if at least half of the implant is still inserted in bone.

Stress Distribution in an Implant-Supported Mandibular Complete Denture Using Different Cantilever Lengths and Occlusal Coating Materials

PURPOSE

The aim of this study was to assess stress distribution in the bone-implant interface of a mandibular implant-supported prosthesis with different cantilever lengths, aesthetic coating materials, and implant abutments.

MATERIALS AND METHODS

A photoelastic model of an edentulous mandible, containing 5 external hexagon implants, was constructed. Experimental models were divided into 6 groups: group 1-UCLA component and metal bar; group 2-UCLA component and acrylic resin coating; group 3-UCLA component and porcelain coating; group 4-abutment and metal bar; group 5-abutment and acrylic resin coating; and group 6-abutment and porcelain coating. Forces were applied to the most anterior implant, the most posterior implant, and different cantilever lengths.

RESULTS

The results showed a higher number of high-stress fringes as the cantilever length increased. Fringes were better distributed in groups with prostheses composed of acrylic resin and in groups that contained an abutment.

CONCLUSION

The stress distribution in the bone-implant interface is improved when the cantilever is eliminated and when abutments in an acrylic resin prosthesis are used.

Effect of cantilever length on stress distribution around implants in mandibular overdentures supported by two and three implants

Objective:

There is no definitive study comparing stress distribution around two versus three implants in implant-retained overdentures with different cantilever length. The purpose of this finite element study was to evaluate stress pattern around the implants of the 2 or 3 implant- supported mandibular overdenture with different cantilevered length.

Materials and Methods:

The models used in this study were 2 and 3 implant-supported overdenture with bar and clip attachment system on an edentulous mandibular arch. Each model was modified according to cantilever length (0 mm, 7 mm, and 13 mm); thus, 6 models were obtained. The vertical load of 15 and 30 pounds were applied unilaterally to the first molar and 15 pounds to the first premolar, and the stress in bone was analyzed.

Results:

With increasing cantilever length, no similar stress pattern changes were observed in different areas, but in most instances, an increase in cantilever length did not increase the stress around the implant adjacent to cantilever.

Conclusions:

Within the limitations of this study, it can be concluded that increasing of cantilever length in mandibular overdentures retained by 2–3 implants did not cause distinct increasing in stress, especially around the implant adjacent to cantilever, it may be helpful to use cantilever in cases of mandibular overdenture supported by splinted implants with insufficient retention and stability. Based on the findings of this study, optimal cantilever length in mandibular overdenture cannot be determined.

Stress Distribution in Bone and Implants in Mandibular 6-Implant-Supported Cantilevered Fixed Prosthesis: A 3D Finite Element Study

PURPOSE

The purpose of the study was to evaluate by a 3-dimensional finite element analysis the load transmission to periimplant bone by a framework supported by 6 implants placed in an edentulous mandible and to compare the stress distribution for varying cantilever lengths.

METHODOLOGY

A computerized model of the anterior segment of a mandible with a 6-implant-supported bridge was created in software. The length of the cantilever segment was considered as 10, 15, and 20 mm. A 150 N load was applied to the terminal point of the cantilever segment, and Von Mises stresses were analyzed along implants, framework, and bone.

RESULTS

When the cantilever length was increased from 10 to 20 mm, the stress increased 79.66% in the framework, 68.16% in implants, and 59.96% and 52.81% in cortical and cancellous bones, respectively.

CONCLUSION

The greatest amount of stress was seen around the distal-most region of the distal-most implant. The framework absorbed the maximum amount of stresses followed by the implants, cortical bone, and cancellous bone. Extension of the cantilever beyond 15 mm could lead to greater stress in the lingual cortical plate, which could compromise the integrity of the implants.

Optimal placement of the two anterior implants for the mandibular All-on-4 concept

The novelty of the All-on-4 concept for a mandibular implant-supported fixed dental prosthesis is the inclination of the posterior implants. Typically, the anterior implants are placed lingually relative to the canine/incisor teeth and perpendicular relative to the occlusal plane. According to the laws of elementary biomechanics, the long axis of the implant unit should be aligned to the axis of the occlusal loading forces during clenching in the maximal intercuspal position. When several implants are connected by a prosthesis, the mean axis of the overall occlusal loading must be taken into account. The objective of this report was to propose a different position for anterior implants by tilting them labially to counterbalance the distal inclination of the posterior implants.

Finite Element Analysis of the Influence of Implant Inclination on Stress Distribution in Mandibular Overdentures

The purpose of this finite element study was to evaluate the influence of implant inclination on the stress pattern in the bone surrounding the implants that support mandibular overdentures. The models used in this study were 3-implant-supported mandibular overdentures with a bar-and-clip attachment system. Each model was modified according to the distal implant inclination (0 and 20°). A unilateral vertical load was applied unilaterally to the first molar and first premolar of the overdenture, and the stress distribution in the bone was analyzed. Implant inclination decreased the stress distribution pattern in bone surrounding the implants when the load was applied on the molar site, but when applied at the premolar site, similar stress value changes were not found. Within the limitation of this study, it seems that the inclination of splinted implants in mandibular overdentures does not have any adverse effect on stress distribution pattern values around the implant.

Wear of double crown systems: electroplated vs. casted female part

OBJECTIVES

The wear of telescopic crowns is a common problem often reducing the patient's satisfaction with the denture and resulting in a renewal of the denture. The study aims to compare the wear behavior of conical crowns using electroplated copings (group E ) with standard telescopic crowns with cast female parts (group C).

MATERIAL AND METHODS

10 conical crowns were milled for each group of a cast gold alloy. The specimen of group E had a conicity of 2º. The cast secondary crowns of group C had a 0º design. The electroplated coping was established by direct electroforming. An apparatus accomplishing 10,000 wear cycles performed the wear test. The retentive forces and the correlating distance during insertion and separation were measured. The wear test was separated in a start phase, an initial wear phase and the long term wear period. The retention force value and the force-distance integral of the first 0.33 mm of each cycle were calculated.

RESULTS

The retentive forces were significantly higher for group E and the integrals were significantly lower for this group except the integral at cycle 10,000. The changes of retention force and integral did not differ significantly between both groups in all phases. The change of the integrals as well as the integral at the particular cycles showed higher interquartile distances for group C.

CONCLUSIONS

Within the limitations of this study the tested conical crowns showed clinically acceptable retentive properties. The values reached a range comparable to retentive elements tested in recent literature. The values of group C showed higher ranges. The force measured for group E was significantly higher than for group C but the integrals showed an opposite tendency. The results indicate that an exclusive analysis of the force is not sufficient as the integral is not equivalent to the force although it describes the retentive property of the system in a better way than the force over a distance is described. Both systems seem to be suitable for clinical practice.

Standard of disocclusion in complete dentures supported by implants without free distal ends: analysis by the finite elements method

OBJECTIVE

The occlusal patterns are key requirements for the clinical success of oral rehabilitation supported by implants. This study compared the stresses generated by the disocclusion in the canine guide occlusion (CGO) and bilateral balanced occlusion (BBO) on the implants and metallic infrastructure of a complete Brånemark protocol-type denture modified with the inclusion of one posterior short implant on each side.

MATERIAL AND METHODS

A three-dimensional model simulated a mandible with seven titanium implants as pillars, five of them installed between the mental foramen and the two posterior implants, located at the midpoint of the occlusal surface of the first molar. A load of 15 N with an angle of 45° was applied to a tooth or distributed across three teeth to simulate the CGO or BBO, respectively. The commercial program ABAQUS® was used for the model development, before and after the processing of the data. The results were based on a linear static analysis and were used to compare the magnitude of the equivalent stress for each of the simulations.

RESULTS

The results showed that the disocclusion in CGO generated higher stresses concentrated on the working side in the region of the short implant. In BBO, the stresses were less intense and more evenly distributed on the prosthesis. The maximum stress found in the simulation of the disocclusion in CGO was two times higher than that found in the simulation of the BBO. The point of maximum stress was located in the neck of the short implant on the working side.

CONCLUSIONS

Under the conditions of this study, it was concluded that the BBO pattern was more suitable than CGO for the lower complete denture supported by implants without free distal ends.

Biomechanical effects of two different collar implant structures on stress distribution under cantilever fixed partial dentures

OBJECTIVE

The purpose of the study was to compare the effects of two distinct collar geometries of implants on stress distribution in the bone around the implants supporting cantilever fixed partial dentures (CFPDs) as well as in the implant-abutment complex and superstructures.

MATERIALS AND METHODS

The three-dimensional finite element method was selected to evaluate the stress distribution. CFPDs which was supported by microthread collar structured (MCS) and non-microthread collar structured (NMCS) implants was modeled; 300 N vertical, 150 N oblique and 60 N horizontal forces were applied to the models separately. The stress values in the bone, implant-abutment complex and superstructures were calculated.

RESULTS

In the MCS model, higher stresses were located in the cortical bone and implant-abutment complex in the case of vertical load while decreased stresses in cortical bone and implant-abutment complex were noted within horizontal and oblique loading. In the case of vertical load, decreased stresses have been noted in cancellous bone and framework. Upon horizontal and oblique loading, a MCS model had higher stress in cancellous bone and framework than the NMCS model. Higher von Mises stresses have been noted in veneering material for NMCS models.

CONCLUSION

It has been concluded that stress distribution in implant-supported CFPDs correlated with the macro design of the implant collar and the direction of applied force.

Effect of the number of abutments on biomechanics of Branemark prosthesis with straight and tilted distal implants

Objective

This study aimed to evaluate the bending moments, and compressive and tensile forces in implant-supported prostheses with three, four or five abutments.

Material and Methods

Ten Pd-Ag frameworks were tested over two master models with: 1) parallel vertical implants, and 2) tilted distal implants. Strain gauges were fixed on the abutments of each master model to measure the deformation when a static load of 50 N was applied on the cantilever (15 mm). The deformation values were measured when the metallic frameworks were tested over three, four or five abutments, and transformed into force and bending moment values. Data were analyzed by ANOVA and Tukey’s test for multiple comparisons at 5% level of significance.

Results

Abutment #1 (adjacent to the cantilever) had the highest values of force and sagittal bending moment for all tests with three, four or five abutments. Independently from the number of abutments, axial force in abutment #1 was higher in the vertical model than in the tilted model. Total moment was higher with three abutments than with four or five abutments. Independently from the inclination of implants, the mean force with four or five abutments was lower than that with three abutments.

Conclusion

The results suggest that in the set-ups with four or five abutments tilted distal implants reduced axial force and did not increase bending moments.