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Table of Contents
ORIGINAL ARTICLE
Year : 2021  |  Volume : 11  |  Issue : 1  |  Page : 53-59

Evaluation of stress distribution in maxillary anterior bone from three different tapered implant thread designs with two angulated abutments: A 3-dimensional finite element analysis study


1 Department of Prosthodontics Crown and Bridge and Implantology, Dr D.Y. Patil Vidyapeeth, Pune, Maharashtra, India
2 Department of Prosthodontics, Navodaya Dental College and Hospital, Raichur, Karnataka, India

Date of Submission01-Dec-2020
Date of Decision21-May-2021
Date of Acceptance22-May-2021
Date of Web Publication10-Jun-2021

Correspondence Address:
Dr. Vikas Attargekar
Department of Prosthodontics, Dr D.Y Patil Dental College and Hospital, Pune - 411 018, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jdi.jdi_32_20

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   Abstract 

Background: Tapered implants imitate the natural form of the root. They are known to enhance primary stability by providing pressure on the cortical bone of regions with poor bone qualities and also has a good survival rate as it directs stresses away from the crestal cortical bone while transferring it to the cancellous bone.
Purpose: Maxillary anatomic constraints sometimes make it necessary to surgically position implants at angles that are not optimal for prosthetic restorations or by positioning the implant in the area with the greatest available bone, with the intention of correcting the implant alignment at the time of prosthetic restoration. This is made possible, in carefully planned cases with the use of angulated abutments.
Materials and Methods: Three tapered implants with triangular, square, and buttress thread designs having a 15° and 25° angulated abutment were created. The implant models were positioned in anterior maxillary bone D2 and D3 and clinical loading conditions simulated. The maximum equivalent von Mises stress values were recorded and analyzed using ANSYS software.
Results: The finite element analysis carried out showed less stresses from tapered implant square thread design in D2 and D3 bone with 15° angulated abutment, while buttress thread design performed better in D2 and D3 bone with 25° abutment angulation on axial and nonaxial loading.
Conclusion: Bone type is an important factor that affects stress distribution. More stress occurred in D3 bone compared to D2 bone type. Thus, bone type should be carefully considered when choosing the most appropriate implant design.

Keywords: Abutment angulation, bone quality, finite element analysis, implant thread, von Mises stress


How to cite this article:
Attargekar V, Galagali G, Reddy E S, Nidawani P, Harsha R H, Naik S. Evaluation of stress distribution in maxillary anterior bone from three different tapered implant thread designs with two angulated abutments: A 3-dimensional finite element analysis study. J Dent Implant 2021;11:53-9

How to cite this URL:
Attargekar V, Galagali G, Reddy E S, Nidawani P, Harsha R H, Naik S. Evaluation of stress distribution in maxillary anterior bone from three different tapered implant thread designs with two angulated abutments: A 3-dimensional finite element analysis study. J Dent Implant [serial online] 2021 [cited 2021 Jun 13];11:53-9. Available from: https://www.jdionline.org/text.asp?2021/11/1/53/318071




   Introduction Top


The selection of implant that will provide adequate stability in bone of poor quality is important. Tapered implants imitate the natural form of root. They are known to enhance primary stability by providing pressure on the cortical bone of regions with poor bone qualities.[1] Tapered implant body has a good survival rate because it directs stresses away from the crestal cortical bone while transferring it to the cancellous bone.[2] In addition, bone perforation is less likely to occur due to the anatomical shape.[3]

The natural maxillary teeth are loaded at an angle because of their natural angulation compared with the mandibular anterior teeth.[4] Anatomic constraints sometimes make it necessary to surgically position implants at angles that are not optimal for prosthetic restorations or by positioning the implant in the area with the greatest available bone with the intention of correcting the implant alignment at the time of prosthetic restoration. This is made possible, in carefully planned cases, with the use of angulated abutments.[5] Angulated abutments may be considered as a suitable restorative option when implants are not placed in ideal positions.[5],[6] Angulated abutments are often used to restore dental implants placed in the maxillary anterior region due to the esthetic and spatial needs.[7],[8] The angulation of these abutments varies from 0° to 35° and the most commonly used angulation is 15° and 25°.[8],[9],[10],[11] Angled abutment was subjected to higher stress values around the cervical region than those observed for straight abutment.[12]

The successful osseointegration of implant depends not only on the bone quantity but also on the bone quality.[13] With the tapered implant body design and triangular, buttress, and square threads, it is not known how various threads contribute to stress distribution in anterior maxillary region. The purpose of the study is to evaluate and compare stress distribution in maxillary anterior bone for tapered implant with three different thread design types incorporating 15° and 25° angulated abutments using three-dimensional finite element study.


   Materials and Methods Top


The geometry of bone built is based on a computed tomography scan which was exported into STL format in Hypermesh V11 and converted to hard cortical bone and soft cancellous bone model. Two qualities of bone D2 (2 mm thick cortical bone) and D3 (1 mm thick cortical bone) were modeled[14] [Figure 1]. Three-dimensional finite element tapered implant models measuring 4 mm × 12 mm with triangular, square, and buttress thread design types incorporating 15° and 25° angulated abutments were built [Figure 2] using Solid Edge V20 software and transferred into three-dimensional finite element model of maxillary anterior bone using ANSYS V14.5 software (Ansys, Inc. Canonsburg, Pennsylvania, US). Abutments have a base diameter equal to implant diameter of 4 mm × 7 mm with occlusal taper. Twelve three-dimensional finite element models were created with the following configurations as shown in [Table 1].
Table 1: Model description

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Figure 1: Bone types

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Figure 2: Finite element implant models with angulated

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The analysis of stress distribution was carried out using the software ANSYS V14.5. The models were processed in ANSYS to generate a meshed structure. Meshing divides the entire model into smaller elements which are interconnected at specific joints called nodes. The number of elements and nodes used for each model is shown in [Table 2]. In the current study, the materials used for the models were presumed to be isotropic, homogeneous, and linear elastic.[15],[16] The material properties such as Poisson's ratio and Young's modulus of elasticity of the material were incorporated into the model as shown in [Table 3]. The material properties were determined from values obtained from the literature.[14],[15],[17]
Table 2: Distribution of elements and nodes for creating finite element models

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Table 3: Material properties

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Von Mises stresses were calculated by subjecting the implant models to a load of 178N axially and nonaxially on abutment based on studies involving masticatory loads in anterior maxilla.[8],[16],[18] The maximum equivalent Von Mises stresses were analyzed in all 12 models at the following areas; cortical bone, cancellous bone, implant body, and overall.


   Results Top


Von Mises stresses were calculated following loading axially and nonaxially for each tapered implant with surrounding D2 and D3 bone. Hence, a total of 12 finite element models Von Mises stress distribution was studied when the models were loaded axially and nonaxially. The stress distribution was analyzed at cortical bone, cancellous bone, implant body, and overall area for all the models under axial and nonaxial loading as shown in [Figure 3], [Figure 4], [Figure 5], [Figure 6]. The distribution of stresses is color coded with blue showing lowest stress and red the highest stress. The results of stress distribution analysis are shown in [Figure 7] (axial loading) and [Figure 8] (nonaxial loading).
Figure 3: Von Mises stress in cortical D2 (a) and D3 (b) bone type

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Figure 4: Von Mises stress at cancellous bone

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{Figure 4}
Figure 5: Von Mises stress implant body. (a) Triangular thread, (b) Square thread, (c) Buttress thread

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Figure 6: Overall Von Mises stress in D2 (a) and D3 (b) bone type on loading

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Figure 7: Bar graph showing Von Mises stress distribution scores on axial loading

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Figure 8: Bar graph showing Von Mises stress distribution on nonaxial loading

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   Discussion Top


Finite element modeling

Finite element analysis (FEA), with all its inherent limitations, is a valuable instrument in pursuing that goal. When associated with clinical findings and accumulation of reliable data on implant loading, bone-implant contact area and other factors could help us understand the problems encountered in daily practice. For the reasons above mentioned, the results of this and other FEA studies have to be seen with a critical eye, and the values should not be taken as absolute but should rather be used to compare the possible magnitudes of stress that bone and implant components undergo during function.[19]

The FEA offers several advantages including accurate representation of complex geometries, easy model modification, the internal state of stress, and other mechanical quantities.[20] The FEA study offers valuable preliminary information for implant planning for the clinicians. However, one limitation to this study is that the model cannot fully reflect the properties of living tissues. Although it is impossible to reflect oral conditions in the models, this study is valuable in terms of providing preliminary information to clinicians to guide clinicians before planning.[21] The finite element stress analysis method in implant biomechanics is better than other methods in terms of its ability to mimic complex clinical situations. It can be used to predict the distribution of stresses in jaw bones and dental implants.[22]

Bone quality

To achieve stable osseointegration for implant restoration, the generation of high stress concentration or distribution in bone should be avoided, since high stress concentration or distribution can induce severe resorption in the surrounding bone, leading to gradual loosening and finally, complete loss of the implant.[23] The bone density and the cortical/cancellous bone ratio may be important due to their influence upon the primary stability of an implant.[24]

From a biomechanical perspective, the implant-restored in anterior maxilla is often the weakest region of the mouth. In the maxilla, the dominant bone type is trabecular bone, and a thin layer of cortical bone can make it difficult to achieve primary stability, which is a prerequisite for successful osseointegration.[25],[26] With bone loss, both bone quality and implant dimensions have a major impact on implant lifetime.[27],[28] The long-term success of implant treatment is ensured by keeping stresses and strains with safe limits, choosing adequate implants and dealing only with Types I-III bone.[29] It is important to consider the jaw bone types to make the most accurate plan due to the importance of the bone type in implant prognosis.

In the literature, there are several studies on the effects of cortical bone thickness in implant stability and they state that there is strong correlation between implant stability and cortical bone thickness. Increased bone density improves the mechanical properties of implants. Holmes and Loftus[30] reported that an increase in bone density provides a decrease in implant micromotion, which decreases the stress levels at the bone implant interface. These studies indicate that cortical bone thickness affects the jaw bones' stress.

Gümrükçü et al.[21] found higher stress values in type D3 maxilla models in comparison to type D2 maxilla and confirms that the stress in the bone around the implant is a result of the thickness of the cortical bone. In the present study also, we found higher stress values in D3 bone in comparison to D2 bone.

Tapered body implants

Tapered-screw implants are suggested, because they can fit to the extraction socket better, and primary stability can be achieved easily.[31] Tapered implants have been used to improve esthetics and facilitate implant placement between adjacent natural teeth. It was initially designed specially to serve for immediate implant placement after tooth extraction. The theory behind the use of tapered implants is to provide for a degree of compression of the cortical bone in a poor bone implant site. Tapered implants distribute forces into the surrounding bone, thereby creating a more uniform compaction of bone in adjacent osteotomy walls compared with parallel-walled implants. Thus, when inserted, it creates a lateral compression of the bone.

The advantages can be seen especially with anatomic constraints, including ridges with concavities or narrow ridges. A study demonstrated higher resonance frequency analysis and insertion torque values for tapered implants than for nontapered implants, suggesting increased stability in tapered implants.[32] A study carried out by Huang et al.[2] showed that tapered implant body decreased stresses in both cortical and trabecular bone compared to the square straight implant body design.

Tapered roughened-surface implants immediately restored with single provisional crowns at surgery and definitive crowns 8 weeks later were prosthodontically and esthetically successful as conventionally restored two-stage implants during the 1st year of service.[33]

Implant thread design

Primary stability at the time of implant placement is related to the level of primary bone contact. The level of bone contact with implant is affected by many factors such as bone quality, thread design, and surgical procedure. A square thread design has been suggested to reduce the shear component of force by taking the axial load of the prosthesis and transferring a more axial load along the implant body to compress the bone. However, the square thread design is the exception with success rates in the maxilla and mandible being equal and also the highest success rates reported.[34]

It is widely accepted fact that the thread geometry has a significant effect on implant biomechanics.[35] In particular, implants with external threads find high use as they provide better initial stability. The thread shape plays an important role in load transfer from dental implant to the surrounding bone. Square threads reduce the bone level stresses.[36],[37] The square thread configuration showed the lowest stresses for all degrees of osseointegration in the implant-cortical bone transition region as the critical zone for design purposes. They may point to the superiority of the square threads, because this design had the least stress concentration when compared with other thread shapes. Misch reported the advantage for square threads versus triangular (V) shaped and reverse buttress threads. This corresponds with reports that compressive stresses around implants with square threads were more evenly distributed than those around implants with V threads. Moreover, since Abuhussein et al.[38] indicated that implant threads should allow for better stability and more implant surface area, among the tested thread designs, the square threads will lead to the highest degree of bone-to-implant contact since they have the greatest perimeter in a section.[39]

In fact, thread geometry in terms of pitch, depth, and width seems to affect the stress distribution forces all around the implant contacted bone. Implant stability and osseointegration are the final clinical objectives to be reached, but they are critically depending on several factors. Misch stated that “the deeper the threads the wider the surface area of the implant” with a greater advantage in the areas of softer bone because of a higher functional surface area in contact with the bone itself.[13],[38],[40]

The material properties and dental implant thread designs influenced the stress distribution and stress transfer to the surrounding bone. Implants made of titanium performed better than zirconia with optimal stress transfer to the bone. The buttress thread design had the highest stress irrespective of the material that it was made of.[41] A study carried out among the three thread shapes, the contact area of the square thread was the highest, followed by the buttress one, with that of the symmetrical one the least. This indicates that the implant with square threads possesses higher total contact area at the implant-bone interface compared to the other two types.[42]

First, for thread shape, the square thread profile may provide the best primary stability in an immediate loading situation.[43] Due to increase of its contact area with the bone, the implant with deeper thread leads to an increase of the implant's stability.[44]

In our study, we found in D2 and D3 bone with 15° abutment square thread (A2 and B2 models) performed better with regard to stress distribution compared to buttress and triangular thread. In D2 and D3 bone with 25° abutment, buttress thread (A6 and B6 models) showed less concentration of stresses when compared to triangular and square thread designs.

Angulated abutments

The anterior maxilla morphology often imposes the use of an angled-abutment in the case of an upper lateral incisor restoration. This particular implant orientation leads to a biomechanical behavior of the prosthetic solution that strongly differs from the one encountered in the case of a straight one. According to Sethi et al.,[6] angled abutments can be used without compromising the survival rate of implants at 5 years. Eger et al.[5] reported in their preliminary investigation no significant difference in probing depths, gingival level, gingival index, and mobility between implants restored with angled or standard abutments.[45]

To achieve prosthetically desired parallelism between implant or teeth, the clinician can place an angled abutment. Numerous types of prefabricated abutments are available at specific angles. Pre-angled abutments with angulations varying from 15° to 35° often are commercially available.[46] Angled abutments result in increased stress on the implants and adjacent bone. These increased stresses usually are within physiological tolerances. The use of angled abutments has not decreased the survival rate of implants or prostheses in comparison with that of straight abutments. Nor has the use of angled abutments resulted in an increased amount of bone loss.[47] Saab et al.[7] stated that using an angled abutment may decrease the strain on bone when restoring implants in anterior maxilla. Angled abutments may result in decreased stress on surrounding bone of single-unit dental implants when implants are not placed in the ideal axial position.[48] In a study by Clelland et al.[12] used a three-dimensional finite element model of the maxilla and confirmed that stresses and strains became larger as abutment angle increased which also is seen in our study.

Limitations of the study

FEA even though is a precise and accurate method for analyzing stress distribution, the present study had limitations. First, the cortical bone, cancellous bone, and the implant were considered to be isotropic, and second the static load that was applied will defer from the dynamic loading during function. Hence, further studies need to be carried out with improvements in finite element models, wherein dynamic loading can be applied.


   Conclusion Top


Within the limitations of this study, the following conclusions can be drawn. Bone type is important factor that affects stress distribution. More stress occurs in D3 bone, while less stress occurs in D2 bone. Thus, bone type should be carefully considered when choosing the most appropriate implant design. Maximum stresses were seen at the cortical bone compared to the cancellous bone. As the abutment angulation increased, more stress was seen in cortical and cancellous bone. Stresses, which were transferred more to the implant than to the bone, promote bone preservation. Square thread design showed less concentration of stresses in D2 and D3 bone with 15° abutment angulation, while buttress thread design performed better in D2 and D3 bone with 25° abutment angulation. It may be inferred that, in view of the above conclusions, the thread selection depending on the conditions can bring better results.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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