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Investigation of Influence of Different Implant Size and Placement on Stress Distribution With 3-Dimensional Finite Element Analysis Mehmet Cudi Balkaya, DMD, PhD
n totally edentulous patients, dental implants have become a successful and predictable treatment procedure for fixed partial dentures (FPDs). However, the rehabilitation of the edentulous mandible presents a challenge to clinicians due to the presence of anatomical limitations such as the inferior alveolar nerve and the reduced bone volume.1,2 Successful long-term integration of an implant depends on a number of factors including bone quality and quantity and optimal load distribution on the supporting bone tissues.3,4 Insufficient vertical and horizontal bone volume does not allow implant placement particularly in the bone atrophy distal to the mental foramen.5 In this instance, small-diameter implants may be used where there is not enough bone width in the alveolar ridge; however, large-diameter implants have been proposed to increase the osseointegrated implant interface.3,6,7 In addition, shorter implants can be a preferable choice in regions with an insufficient residual bone height, instead of long implants,1,2 but clinical studies have shown
I
Associate Professor, Department of Prosthetics, Faculty of Dentistry, Istanbul University, Istanbul, Turkey.
Reprint requests and correspondence to: Mehmet Cudi Balkaya, DMD, PhD, Department of Prosthetics, Faculty of Dentistry, Istanbul University, 34390 Istanbul, Turkey, Phone: +90-212-4142020 ext 30261, Fax: +90-212-5312230, E-mail: mbalkaya@istanbul. edu.tr ISSN 1056-6163/14/02306-716 Implant Dentistry Volume 23 Number 6 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000158
Purpose: The purpose of this study was to analyze the biomechanical behavior of implants with varying number, inclination, and size, using 3-dimensional finite element (FE) analysis. Material and Methods: A total of 10 FE models were constructed to simulate 5 implant placement configurations: 4 and 5 axial implants, 4 implants inclined with 17.5 degrees mesial and 35 degrees distal, and 2 axial and 2 distal implants inclined with 17.5 and 35 degrees, using implants of 3.5 and 5.5 mm diameters. A vertical load of 300 N was applied to the distal portion of a standardized size metal framework. Results: Increasing the number of implants decreased the stress in the
bone for axially placed implant models. The short implants with a large diameter resulted in lower stress values in the bone, but higher stresses in the implant/abutment assembly than the long implants with a small diameter. Increasing diameter of implants decreased high stress concentration in the cortical bone, resulting from increasing cantilever length. Conclusions: Decreasing cantilever length with distal implant inclination decreases the stress values in the implant, cortical bone, and framework. (Implant Dent 2014;23:716–722) Key Words: implant, edentulous mandible, implant-supported fixed partial denture, finite element model, stress
unfavorable outcomes compared with longer length implants.8,9 Implants placed in a specified angle may become a good solution to increase the bone support of the implant system. A single-unit inclined implant demonstrates higher stress in the implant-bone interface compared with an axial implant when a vertical load was applied to both the implants. This is because the load is not equally distributed along the long axis of the inclined implant, and the stresses increase due to an irregular distribution of the load.1,10 However, the stress in the bone surrounding a splinted implant is
lower than that in a single-unit implant,11 and if the implant placed at an angle or exposed to an inclined load is splinted with a rigid superstructure, the stress values demonstrate a decrease in the bone surrounding the implants.1,10 In the edentulous mandible, the placement of inclined implants between the mental foramina makes it possible to use the FPDs with short distal cantilevers and long implants, eliminating the possibility of damage to the inferior alveolar nerve, and results in an increase in the distance between the implants.12,13 Allon-4 treatment concept involves the use
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014
Table 1. Lengths of Implants Placed With Different Inclinations in This Study Implant Length (mm) Diameter (mm) 3.5 5.5
Axial
17.5 degrees
35 degrees
12 8
14 10
16 12
Implant lengths were increased with increasing implant inclination.
of a full-arch FPD with 2 distal cantilevers supported with 2 mesial axial implants and 2 inclined distal implants. This concept provides particularly greater bone support and higher survival rates in the immediately loaded FPD.12,14 The purpose of this study was to analyze the influence of varying number, inclination, and sizes of implants placed into the interforaminal region of the edentulous mandible on the stress distribution, under a static vertical load with 3-dimensional (3D) finite element analysis (FEA).
MATERIALS
AND
METHODS
Three-dimensional FE models of the implants, abutments, a framework with standardized size, and an edentulous mandible were designed using a software application (ABAQUS version 6.7; Hibbitt, Karlsson, & Sorensen, Inc., Pawtucket, RI). The FE models of the implants and the abutments (Sky Implant
System; Bredent Medical GmbH & Co. KG, Senden, Germany) were performed according to the implant sizes in the manufacturer’s manual. The implant body lengths were selected as 12, 14, and 16 mm for 3.5-mm-diameter implants and 8, 10, and 12 mm for 5.5-mm-diameter implants. The variability in the inclinations of the implants was considered when choosing the implant lengths, and these are listed in Table 1. Straight 17.5- and 35-degree inclined abutments (Sky Implant System; Bredent Medical) of 4 mm in diameter were used to support the framework. The implant and the abutment were assumed as a single unit and defined as fully bonded. A total of 10 FE models were constructed to simulate 5 different implant placements: the axial, inclined, and a combination of axial and inclined implants, using 2 different implant diameters. In each FE model, the implant diameters were identical, and the implant lengths were progressively increased with increasing inclination of the implants (Table 2). The edentulous mandible was modeled by considering the data on anatomical properties reported in the literature.15,16 The implants were fully inserted buccolingually to the middle of the crest in the interforaminal region of the edentulous mandible. The mandibular model was composed of a cancellous
Table 2. Implant Lengths Selected According to Implant Diameter and Inclination in 5 Different Implant Placement Designs (From D1 to D5)
Implant Placement Designs D1: 5 axial implants D2: 4 axial implants D3: 2 axial and 2 distal inclined (17.5-degree) implants D4: 2 axial and 2 distal inclined (35-degree) implants D5: 4 inclined implants (17.5-degree mesial and 35-degree distal inclined implants)
Length According to Implant Location (mm)
Groups
Diameter Distal Mesial Mesial Mesial Distal (mm)
1 2 3 4 5 6
3.5 5.5 3.5 5.5 3.5 5.5
12 8 12 8 14 10
12 8 12 8 12 8
7 8
3.5 5.5
16 12
9 10
3.5 5.5
16 12
Axially placed implants were selected as shorter than inclined implants.
12 8
12 8 12 8 12 8
12 8 12 8 14 10
12 8
12 8
16 12
14 10
14 10
16 12
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bone surrounded by a cortical bone layer 2 mm in thickness. The distance between the mental foramina was assumed as 50 mm,15 and the axially placed distal implants were inserted 5 mm anterior to the mental foramen. Fully bonded interaction was modeled along the implant-bone interface to simulate a completely osseointegrated implant that directly bonded to the surrounding bone. Implant Placement Configurations
In the axial implant designs (D1 and D2), the 5 and 4 implants were placed perpendicular to the occlusal plane, parallel to each other and at equal separation distances. The location of the distal implants was the same in all the axial implant models. In the 2 axial and 2 inclined implant designs (D3 and D4), 2 mesial implants were placed perpendicular to the occlusal plane and 2 distal implants were inclined at angles of 17.5 and 35 degrees, respectively. In the 4 inclined implants design (D5), 2 distal implants were placed at an angle of 35 degrees and the mesial implants were placed at an angle of 17.5 degrees (Fig. 1). In all the inclined implant designs, the distal implants were inclined distally with an angle of either 17.5 or 35 degrees from a point along the long axis of the distal implants in the axial implant models. The distance between the implants was modeled at no less than 3 mm separation. This separation distance ensures the viability of the remaining bone tissue after the implant placement.1 A simplified metal framework with standardized size was designed to simulate a full-arch implant-supported FPD with cantilevers. The framework was attached as fully bonded to the abutments with the following dimensions: 63.0 mm in length, 4.8 mm in width, and 4.0 mm in height. The bilateral cantilever extensions tested were 14, 9, and 4 mm in the models with the axial and the distal implants inclined 17.5 and 35 degrees. A vertical load of 300 N with continuum distributing coupling constraint was applied to a point at 2.5 mm distance from the distal end of the
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Fig. 1. Three-dimensional FE models and stress distributions under load application of implant-abutment-framework (left), and implant-abutment assemblies (right). Red color refers to the highest stress level. Typical placement designs of implants: D1, 5 axial implants; D2, 4 axial implants; D3, 2 axial and 2 distal implants inclined with 17.5 degrees; D4, 2 axial and 2 distal implants inclined with 35 degrees; D5, 4 implants inclined with mesial 17.5 degrees and distal 35 degrees.
cantilever in all models, because the coupling distribution allows more control of the distribution of load and uniformly distributes load over the surface. The Young modulus (E) and Poisson ratio (n) of the materials used in this study were obtained from the literature (Table 3).17,18 The FE program ABAQUS (version 6.7; Hibbitt, Karlsson, & Sorensen, Inc.) was used to perform the analyses. The bottom of the mandible was restrained against movement in the x, y, and z directions (Ux ¼ Uy ¼ Uz ¼ 0 boundary conditions). A typical view of the boundary conditions used in the analyses is shown in Figure 2A. The mandible, framework, and the implants/abutment assembly were modeled as isotropic solids, using C3D4 type 4-node linear tetrahedron elements (Fig. 2B). A total of approximately 983,370 elements were used throughout the mesh to model the mandible, implants, abutments, and the framework. A finer FE mesh was used around the implants where rapidly varying stresses and strains were expected. The results of the FEA were compared using the maximum von Mises stress values.
RESULTS
Fig. 2. Boundary conditions (A) and typical mesh (B) of a 3D FE model with implants placed into interforaminal region of edentulous mandible.
Fig. 3. Von Mises stress distributions within cortical and cancellous bones surrounding 3.5mm-diameter implants from group 1 to 5 and 5.5-mm-diameter implants from group 6 to 10. Color scale shows stress concentrations from maximum (red) to minimum (blue).
The maximum von Mises stress values for all the models are presented in Table 4. The different implant placement designs and diameters had an influence on the stress values in the implant, bone, and framework. There were differences in the stress distributions among the FE models. The highest maximum von Mises stress value (361.2 MPa) was observed in the distal implant/abutment assembly close to the loading area, in the 5 axial implants model of 5.5 mm in diameter. In the cortical bone, the highest maximum von Mises stress value was 74.4 MPa in group 3 (4 axial implants of 3.5 mm in diameter) and was located on the distolingual aspect of the implant close to the loading point (Fig. 3). The lowest maximum von Mises stress value was 33.2 MPa in group 10 (4 inclined implants of 5.5 mm in diameter). In the cancellous bone, the maximum von Mises stress values were lower than those in the cortical bone. The highest maximum von Mises stress
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014
Table 3. Young Modulus (E) and Poisson Ratio (n) of Materials and Bone Used in This Study Materials
Content
Young Modulus, E (MPa)
Poisson Ratio, n
Reference
Implant and abutment Framework
Titanium
110.000
0.33
17
Ni Cr alloy
210.000
18
13.700 1.370
17 17
Cortical bone Cancellous bone
719
group 8 (2 axial and 2 distal 35-degree inclined implants of 5.5 mm in diameter) (Fig. 1). In the axially placed implants, as the implant number increased from 4 to 5, the stress values demonstrated a minimal decrease in the cortical bone and the framework but an increase in the implant/abutment assembly. The implant sizes had an effect on the stress distributions. The short implants with
Table 4. Maximum Von Mises Stress Values (MPa) Obtained With FEA of Different Implant Diameters and Placement Designs
Implant Placement
Five Axial
Four Axial
Implant diameters 3.5 5.5 3.5 5.5 Von Mises (MPa) Implant/abutment 291.8 361.2 284.5 340.3 Framework 173.8 169.3 186.8 170.3 Cortical bone 66.0 35.4 74.4 37.2 Cancellous bone 8.3 6.5 9.4 7.8
Two Axial and Two Distal Inclined 17.5-degree Implants
Two Axial and Two Distal Inclined 35-degree Implants
3.5
5.5
3.5
5.5
268.5 93.3 44.5 9.8
323.7 83.4 34.8 8.8
145.8 64.3 43.5 10.5
223.7 50.2 33.6 9.8
Four Inclined 3.5
5.5
150.4 216.6 68.7 50.9 42.6 33.2 12.5 11.5
Increasing implant diameter decreased stress values in framework and cortical and cancellous bones. Increasing implant inclination decreased stress values in implant/abutment assembly, framework, and cortical bone.
value was 12.5 MPa in group 9 (4 inclined implants of 3.5 mm in diameter) and was located on the distal surface of the neck of the implant close to the loading point (Fig. 3). Increased distal implant inclination increased the stress values in the cancellous bone. The lowest maximum von Mises stress value was 6.5 MPa in group 2 (5 axial implants of 5.5 mm in diameter). In the implant/abutment assembly, the highest maximum von Mises stress value was 361.2 MPa in group 2 (5 axial implants of 5.5 mm in diameter) and was located at the distal side of the shoulder of the abutment close to the loading point. As the implant placement inclination increased, the location of the maximum stresses shifted toward the distolingual surface of the implant neck (Fig. 1). The lowest maximum von Mises stress value was 145.8 MPa in group 7 (2 axial and 2 distal 35-degree inclined implants of 3.5 mm in diameter). In the framework, the highest maximum von Mises stress value was 186.8 MPa in group 3 (4 axial implant model of 3.5 mm in diameter) and was located at the distolingual area of the framework in contact with the shoulder of the abutment close to the loading area. As the implant placement inclination increased,
the maximum stresses concentrated on the occlusal surface of the framework. The lowest maximum von Mises stress value was 50.2 MPa in
large diameter resulted in lower stress values in the cortical and cancellous bones and the framework but higher stresses in the implant/abutment assembly
Fig. 4. A, Maximum von Mises stress values in cortical bone for different implant diameters and placement designs. Stress decreased with increasing implant diameter and distal implant inclination. B, Maximum von Mises stress values in cancellous bone for different implant diameters and placement designs. Stress increased with increasing distal implant inclination but decreased with increasing implant diameter. C, Maximum von Mises stress values in implant/abutment assembly for different implant diameters and placement designs. Stress increased with increasing implant diameter but decreased with increasing distal implant inclination. D, Maximum von Mises stress values in framework for different implant diameters and placement designs. Stress decreased with increasing implant diameter and distal implant inclination.
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compared with the long implants with small diameter (Fig. 4, A–D). When the framework cantilever length was decreased from 14 to 4 mm as a result of increasing the inclination of the implants, the stress values decreased in the implant/abutment assembly, the cortical bone, and the framework but increased in the cancellous bone. However, the increasing diameter of the implants decreased high stress concentration resulting from the increasing cantilever length. The 4 implant placements with inclined distal implants provided lower stress values compared with the axially placed 5 implants.
DISCUSSION The load distribution of the osseointegrated implant-bone interface is completely different than the natural teeth that have micromovements caused by the periodontal ligaments, because the implants transmit the functional loads directly to the bone tissue.19 Therefore, irregular load distribution or excessive loading may result in bone resorption and osseointegration failure, increasing stress in the bone, and consequently, may compromise the longevity of the implant and the prosthesis.19–21 In this study, the highest maximum von Mises stress in the cortical bone was located on the distolingual aspect of the distal implant close to the loading point for all the implant models. This result reveals that a vertical load applied on the cantilever is not distributed evenly throughout the entire surface of the implants. The cantilever extensions result in an angulated load or a torque moment on the implants and more stress and frequently resorption in the cortical bone around the distal implant neck than in the other regions.5 In this study, the inclination of the mesial implants had a minor effect on the load distribution because the mesial implants were subjected to lower stresses than the distal implants. Therefore the FE models (groups 9 and 10) with inclined mesial implants demonstrated similar stress values in the cortical bone compared with those (groups 7 and 8) with the axially placed mesial implants. Increasing the number of the implants supporting a denture increases
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the bone-to-implant contact surface per unit area and prevents overloading, providing a decrease in the load transmission to the bone.5 In this study, increasing number of implants resulted in a decrease of 11% and 5% in the stress values of the cortical bone surrounding the implants of 3.5 and 5.5 mm in diameters, respectively, when 4 axial implants were compared with 5 axial implants. This result is supported by an in vivo strain gauge study6 in which a decreased implant number led to an increase in the compressive forces on the supporting implants. Several strain gauge studies have found that the bending moments affecting on the FPD may be decreased with the increasing number of implants.22,23 In this study, the stress values in the framework showed a relatively minimal decrease with the increasing number of implants, as all the FE models with 4 and 5 axially placed implants had the same cantilever length. If the implant number is decreased due to anatomical limitations, the implant placement designs or inclinations can be effective to provide a reduction in the stress concentration on the surrounding bone. In cases of posterior residual ridge resorption, if the edentulous mandible is rehabilitated with the implant-supported FPD, the All-on-4 Concept with only 4 implants can provide a lower periimplant bone stress, with a shorter cantilevered prosthesis and longer implants, and a high rate of clinical success compared with 5 axial implants placed in the interforaminal region.12,13 In this study, the FE models of 4 implants placed with different distal implant inclinations demonstrated similar or lower stress values in the cortical bone, the implant/abutment assembly, and the framework than those of 5 axial implants. In an FEA study, Naini et al24 evaluated von Mises stress distribution in periimplant bone in 4 axial implants and All-on-4 implant placements in the interforaminal region of an edentulous mandible and suggested that the distal implant inclination resulted in an increase of 9% in the stress values resulting from the inclination and the proximity to the loading area of the distal implants during posterior loading.
These results are in disagreement with the findings of this study in which the stress values in the cortical bone decreased notably with the distal implant inclination and the decreased cantilever length, when the small-diameter implant (3.5 mm) was used. The differences in the results of this study and the one by Naini et al24 may result from the differences in the material properties of the bone and framework, the cortical bone thickness, and the implant lengths according to inclination. The implant-supported FPDs in the edentulous mandible are designed generally with bilateral cantilever extensions with the implant placement in the interforaminal area. However, the compressive stress leading to bone resorption increases as the cantilever length increases. There is a direct correlation between the cantilever length and the stress in the bone.13,18 In this study, the cantilever lengths were changed from 14 to 4 mm with different inclinations of the distal implants, using the framework with standardized size. As the cantilever lengths were decreased from 14 to 9 mm, with 17.5-degree inclination of the distal implants, the maximum von Mises stress values demonstrated a decrease of 40% and 7% in the cortical bone, 50% and 51% in the framework, and 8% and 46% in the implant/abutment assembly for the implants of 3.5 and 5.5 mm diameters, respectively. As the cantilever lengths decreased from 9 to 4 mm, the stress values demonstrated a minimal decrease in the cortical bone. The models of the distal implants inclined 17.5 and 35 degrees had generally similar stress values in the cortical bone for each implant diameter. These findings show that the 9 mm or lower cantilever length may be adequate to decrease the stress concentrated on the cortical bone surrounding the distal implant and can be used clinically in the oral rehabilitation with full-arch implant-supported FPDs of the edentulous mandible. An FEA study found that there was no significant difference between 4 and 12 mm cantilever lengths for 30- and 40-degree inclined distal implants under distributed loads.25 In this study, there was a decrease in the magnitude of stresses of 31% and 40% in the framework and
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014 46% and 33% in the implant/abutment assembly for the implants of 3.5 and 5.5 mm in diameters, respectively. These findings showed that the proximity of the implants to the loading area influenced the stress values in the implant/ abutment assembly and the framework. In this study, when the implants of 3.5 mm in diameter were used, the axial implant models demonstrated higher stress values than the inclined implant models. The results revealed that the decreased cantilever length with the increasing implant inclination resulted in a decrease in the stress values in the implants, the cortical bone, and the framework. However, when the implants of 5.5 mm in diameter were used, the stress values in the axial and the inclined implant models were similar. This result showed that the increasing implant diameter may decrease high stress concentrations resulting from increasing cantilever length. The implant diameter and length are important factors to achieve optimum load distribution in the bone.7 In this study, the short implants with large diameter and the long implants with small diameter were used to evaluate the effect of different implant size and inclinations on the load distributions. The implant lengths were increased with increasing implant inclinations to simulate the clinical conditions in the inclined implant models because the stress in the cancellous bone may decrease with an increase in the implant length.6 In the axial implant models, the long implants with small diameter (groups 1 and 3) showed higher stress values in the cortical bone than the short implants with large diameter (groups 2 and 4). These findings are in agreement with the FEA studies in which the increasing implant diameter and length can result in a decrease in the stress concentration in the bone; however, diameter may have a more significant effect than length,6,26,27 and therefore, increasing implant diameter is especially important in decreasing the stress and strain values in the bone when there is limited bone height of residual ridge26 or where short implants are subjected to lateral loads.3 It should also be considered that the results of standardized experiments and
FE simulations may be different from those of clinical studies. When considering previous retrospective studies,8,28,29 Olate et al8 evaluated the influence of implant diameter and length on early failure of implants at time period between first and second surgical phases and found that the implant diameter had no significant effect on the early loss with failure rates of 5.1%, 3.8%, and 2.7% in small-, regular-, and large-diameter implants, respectively. Mijiritsky et al28 analyzed the treatment outcome at the first 2 years of prosthetic loading and concluded that implant diameter did not affect the clinical success with survival rates of 98.2%, 98.7%, and 98.5% in small- (,3.75 mm), regular- (3.75–5 mm), and large-diameter (.5 mm) implants, respectively. However, a retrospective study reported the cumulative survival rates up to 5 and 7 years of large- and regulardiameter implants after prosthetic loading, respectively, and found that the large-diameter implants had lower clinical success than the regular-diameter implants, with survival rates of 80.9% and 96.8% in 64 large-diameter (5 mm) and 64 regular-diameter (3.75–4 mm) implants, respectively. They suggested that buccolingual bone dimensions may influence the survival rate of the implants placed in the posterior region.29 In the narrow resorbed ridge areas, the large-diameter implants may result in crestal bone loss30 and demonstrate lower clinical success rates29,31 because the thickness of the bone surrounding the implants may not be adequate to ensure viability of the remaining buccal and lingual bone after the implant placement.1 In this study, all the FE models with the large-diameter implants had lower stress levels in the cortical and cancellous bones than those of the small-diameter implants. The stress values in the cortical bone were higher than those in the cancellous bone because the cortical bone has higher elastic modulus and is subjected to higher stresses.3 Therefore, the increasing implant diameter provides more cortical bone-to-implant contact surface, which minimizes the crestal bone resorption, decreasing the load transfer acting from implants to the surrounding bone, compared with the small-diameter
721
implants.7 The result of this study is supported by a recent meta-analysis study in which the larger diameter implants than 3.3 mm demonstrated higher survival rates compared with the smaller diameter implants and the small-diameter implants had 3.92 times higher failure than the larger implants.32
CONCLUSIONS Increasing the number of implants decreased the stress values concentrated in the cortical bone for the axially placed implants. The short implants with large-diameter demonstrated lower stresses than the long implants with small diameter. The decreasing cantilever length decreased the stress values in the cortical bone, the implant/ abutment assembly, and the framework. Increasing diameter of implant may decrease high stress concentration resulting from increasing cantilever length.
DISCLOSURE The author claims to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The author thanks Dr Müge Balkaya, Istanbul Technical University, for her valuable help with the FEM program ABAQUS.
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5. Duyck J, Van Oosterwyck H, Vander Sloten J, et al. Magnitude and distribution of occlusal forces on oral implants supporting fixed prostheses: An in vivo study. Clin Oral Implants Res. 2000;11:465–475. 6. Baggi L, Cappelloni I, Di Girolamo M, et al. The influence of implant diameter and length on stress distribution of osseointegrated implants related to crestal bone geometry: A three-dimensional finite element analysis. J Prosthet Dent. 2008; 100:422–431. 7. Chun HJ, Cheong SY, Han JH, et al. Evaluation of design parameters of osseointegrated dental implants using finite element analysis. J Oral Rehabil. 2002; 29:565–574. 8. Olate S, Lyrio MC, de Moraes M, et al. Influence of diameter and length of implant on early dental implant failure. J Oral Maxillofac Surg. 2010;68:414–419. 9. Herrmann I, Lekholm U, Holm S, et al. Evaluation of patient and implant characteristics as potential prognostic factors for oral implant failures. Int J Oral Maxillofac Implants. 2005;20:220–230. 10. Bevilacqua M, Tealdo T, Pera F, et al. Three-dimensional finite element analysis of load transmission using different implant inclinations and cantilever lengths. Int J Prosthodont. 2008;21:539–542. 11. Bergkvist G, Simonsson K, Rydberg K, et al. A finite element analysis of stress distribution in bone tissue surrounding uncoupled or splinted dental implants. Clin Implant Dent Relat Res. 2008; 10:40–46. 12. Malo P, Rangert B, Nobre M. “All-on-Four” immediate-function concept with Brånemark system implants for completely edentulous mandibles: A retrospective clinical study. Clin Implant Dent Relat Res. 2003;5:2–9. 13. Takahashi T, Shimamura I, Sakurai K. Influence of number and inclination angle of implants on stress distribution in mandibular
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24. Naini RB, Nokar S, Borghei H, et al. Tilted or parallel implant placement in the completely edentulous mandible? A three-dimensional finite element analysis. Int J Oral Maxillofac Implants. 2011;26: 776–781. 25. Malhotra AO, Padmanabhan TV, Mohamed K, et al. Load transfer in tilted implants with varying cantilever lengths in an all-on-four situation. Aust Dent J. 2012; 57:440–445. 26. Anitua E, Tapia R, Luzuriaga F, et al. Influence of implant length, diameter, and geometry on stress distribution: A finite element analysis. Int J Periodontics Restorative Dent. 2010; 30:89–95. 27. Ding X, Liao SH, Zhu XH, et al. Effect of diameter and length on stress distribution of the alveolar crest around immediate loading implants. Clin Implant Dent Relat Res. 2009;11:279–287. 28. Mijiritsky E, Mazor Z, Lorean A, et al. Implant diameter and length influence on survival: Interim results during the first 2 years of function of implants by a single manufacturer. Implant Dent. 2013;22: 394–398. 29. Shin SW, Bryant SR, Zarb GA. A retrospective study on the treatment outcome of wide-bodied implants. Int J Prosthodont. 2004;17:52–58. 30. Degidi M, Piattelli A, Iezzi G, et al. Wide-diameter implants: Analysis of clinical outcome of 304 fixtures. J Periodontol. 2007;78:52–58. 31. Attard NJ, Zarb GA. Implant prosthodontic management of partially edentulous patients missing posterior teeth: The Toronto experience. J Prosthet Dent. 2003;89:352–359. 32. Ortega-Oller I, Suárez F, GalindoMoreno P, et al. The influence of implant diameter on its survival: A meta-analysis based on prospective clinical trials. J Periodontol. 2014;85:569–580.
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BALKAYA
Investigation of Influence of Different Implant Size and Placement on Stress Distribution With 3-Dimensional Finite Element Analysis Mehmet Cudi Balkaya, DMD, PhD
n totally edentulous patients, dental implants have become a successful and predictable treatment procedure for fixed partial dentures (FPDs). However, the rehabilitation of the edentulous mandible presents a challenge to clinicians due to the presence of anatomical limitations such as the inferior alveolar nerve and the reduced bone volume.1,2 Successful long-term integration of an implant depends on a number of factors including bone quality and quantity and optimal load distribution on the supporting bone tissues.3,4 Insufficient vertical and horizontal bone volume does not allow implant placement particularly in the bone atrophy distal to the mental foramen.5 In this instance, small-diameter implants may be used where there is not enough bone width in the alveolar ridge; however, large-diameter implants have been proposed to increase the osseointegrated implant interface.3,6,7 In addition, shorter implants can be a preferable choice in regions with an insufficient residual bone height, instead of long implants,1,2 but clinical studies have shown
I
Associate Professor, Department of Prosthetics, Faculty of Dentistry, Istanbul University, Istanbul, Turkey.
Reprint requests and correspondence to: Mehmet Cudi Balkaya, DMD, PhD, Department of Prosthetics, Faculty of Dentistry, Istanbul University, 34390 Istanbul, Turkey, Phone: +90-212-4142020 ext 30261, Fax: +90-212-5312230, E-mail: mbalkaya@istanbul. edu.tr ISSN 1056-6163/14/02306-716 Implant Dentistry Volume 23 Number 6 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000158
Purpose: The purpose of this study was to analyze the biomechanical behavior of implants with varying number, inclination, and size, using 3-dimensional finite element (FE) analysis. Material and Methods: A total of 10 FE models were constructed to simulate 5 implant placement configurations: 4 and 5 axial implants, 4 implants inclined with 17.5 degrees mesial and 35 degrees distal, and 2 axial and 2 distal implants inclined with 17.5 and 35 degrees, using implants of 3.5 and 5.5 mm diameters. A vertical load of 300 N was applied to the distal portion of a standardized size metal framework. Results: Increasing the number of implants decreased the stress in the
bone for axially placed implant models. The short implants with a large diameter resulted in lower stress values in the bone, but higher stresses in the implant/abutment assembly than the long implants with a small diameter. Increasing diameter of implants decreased high stress concentration in the cortical bone, resulting from increasing cantilever length. Conclusions: Decreasing cantilever length with distal implant inclination decreases the stress values in the implant, cortical bone, and framework. (Implant Dent 2014;23:716–722) Key Words: implant, edentulous mandible, implant-supported fixed partial denture, finite element model, stress
unfavorable outcomes compared with longer length implants.8,9 Implants placed in a specified angle may become a good solution to increase the bone support of the implant system. A single-unit inclined implant demonstrates higher stress in the implant-bone interface compared with an axial implant when a vertical load was applied to both the implants. This is because the load is not equally distributed along the long axis of the inclined implant, and the stresses increase due to an irregular distribution of the load.1,10 However, the stress in the bone surrounding a splinted implant is
lower than that in a single-unit implant,11 and if the implant placed at an angle or exposed to an inclined load is splinted with a rigid superstructure, the stress values demonstrate a decrease in the bone surrounding the implants.1,10 In the edentulous mandible, the placement of inclined implants between the mental foramina makes it possible to use the FPDs with short distal cantilevers and long implants, eliminating the possibility of damage to the inferior alveolar nerve, and results in an increase in the distance between the implants.12,13 Allon-4 treatment concept involves the use
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014
Table 1. Lengths of Implants Placed With Different Inclinations in This Study Implant Length (mm) Diameter (mm) 3.5 5.5
Axial
17.5 degrees
35 degrees
12 8
14 10
16 12
Implant lengths were increased with increasing implant inclination.
of a full-arch FPD with 2 distal cantilevers supported with 2 mesial axial implants and 2 inclined distal implants. This concept provides particularly greater bone support and higher survival rates in the immediately loaded FPD.12,14 The purpose of this study was to analyze the influence of varying number, inclination, and sizes of implants placed into the interforaminal region of the edentulous mandible on the stress distribution, under a static vertical load with 3-dimensional (3D) finite element analysis (FEA).
MATERIALS
AND
METHODS
Three-dimensional FE models of the implants, abutments, a framework with standardized size, and an edentulous mandible were designed using a software application (ABAQUS version 6.7; Hibbitt, Karlsson, & Sorensen, Inc., Pawtucket, RI). The FE models of the implants and the abutments (Sky Implant
System; Bredent Medical GmbH & Co. KG, Senden, Germany) were performed according to the implant sizes in the manufacturer’s manual. The implant body lengths were selected as 12, 14, and 16 mm for 3.5-mm-diameter implants and 8, 10, and 12 mm for 5.5-mm-diameter implants. The variability in the inclinations of the implants was considered when choosing the implant lengths, and these are listed in Table 1. Straight 17.5- and 35-degree inclined abutments (Sky Implant System; Bredent Medical) of 4 mm in diameter were used to support the framework. The implant and the abutment were assumed as a single unit and defined as fully bonded. A total of 10 FE models were constructed to simulate 5 different implant placements: the axial, inclined, and a combination of axial and inclined implants, using 2 different implant diameters. In each FE model, the implant diameters were identical, and the implant lengths were progressively increased with increasing inclination of the implants (Table 2). The edentulous mandible was modeled by considering the data on anatomical properties reported in the literature.15,16 The implants were fully inserted buccolingually to the middle of the crest in the interforaminal region of the edentulous mandible. The mandibular model was composed of a cancellous
Table 2. Implant Lengths Selected According to Implant Diameter and Inclination in 5 Different Implant Placement Designs (From D1 to D5)
Implant Placement Designs D1: 5 axial implants D2: 4 axial implants D3: 2 axial and 2 distal inclined (17.5-degree) implants D4: 2 axial and 2 distal inclined (35-degree) implants D5: 4 inclined implants (17.5-degree mesial and 35-degree distal inclined implants)
Length According to Implant Location (mm)
Groups
Diameter Distal Mesial Mesial Mesial Distal (mm)
1 2 3 4 5 6
3.5 5.5 3.5 5.5 3.5 5.5
12 8 12 8 14 10
12 8 12 8 12 8
7 8
3.5 5.5
16 12
9 10
3.5 5.5
16 12
Axially placed implants were selected as shorter than inclined implants.
12 8
12 8 12 8 12 8
12 8 12 8 14 10
12 8
12 8
16 12
14 10
14 10
16 12
717
bone surrounded by a cortical bone layer 2 mm in thickness. The distance between the mental foramina was assumed as 50 mm,15 and the axially placed distal implants were inserted 5 mm anterior to the mental foramen. Fully bonded interaction was modeled along the implant-bone interface to simulate a completely osseointegrated implant that directly bonded to the surrounding bone. Implant Placement Configurations
In the axial implant designs (D1 and D2), the 5 and 4 implants were placed perpendicular to the occlusal plane, parallel to each other and at equal separation distances. The location of the distal implants was the same in all the axial implant models. In the 2 axial and 2 inclined implant designs (D3 and D4), 2 mesial implants were placed perpendicular to the occlusal plane and 2 distal implants were inclined at angles of 17.5 and 35 degrees, respectively. In the 4 inclined implants design (D5), 2 distal implants were placed at an angle of 35 degrees and the mesial implants were placed at an angle of 17.5 degrees (Fig. 1). In all the inclined implant designs, the distal implants were inclined distally with an angle of either 17.5 or 35 degrees from a point along the long axis of the distal implants in the axial implant models. The distance between the implants was modeled at no less than 3 mm separation. This separation distance ensures the viability of the remaining bone tissue after the implant placement.1 A simplified metal framework with standardized size was designed to simulate a full-arch implant-supported FPD with cantilevers. The framework was attached as fully bonded to the abutments with the following dimensions: 63.0 mm in length, 4.8 mm in width, and 4.0 mm in height. The bilateral cantilever extensions tested were 14, 9, and 4 mm in the models with the axial and the distal implants inclined 17.5 and 35 degrees. A vertical load of 300 N with continuum distributing coupling constraint was applied to a point at 2.5 mm distance from the distal end of the
718
INFLUENCE
OF
DIFFERENT IMPLANT SIZE
AND INCLINATION
BALKAYA
Fig. 1. Three-dimensional FE models and stress distributions under load application of implant-abutment-framework (left), and implant-abutment assemblies (right). Red color refers to the highest stress level. Typical placement designs of implants: D1, 5 axial implants; D2, 4 axial implants; D3, 2 axial and 2 distal implants inclined with 17.5 degrees; D4, 2 axial and 2 distal implants inclined with 35 degrees; D5, 4 implants inclined with mesial 17.5 degrees and distal 35 degrees.
cantilever in all models, because the coupling distribution allows more control of the distribution of load and uniformly distributes load over the surface. The Young modulus (E) and Poisson ratio (n) of the materials used in this study were obtained from the literature (Table 3).17,18 The FE program ABAQUS (version 6.7; Hibbitt, Karlsson, & Sorensen, Inc.) was used to perform the analyses. The bottom of the mandible was restrained against movement in the x, y, and z directions (Ux ¼ Uy ¼ Uz ¼ 0 boundary conditions). A typical view of the boundary conditions used in the analyses is shown in Figure 2A. The mandible, framework, and the implants/abutment assembly were modeled as isotropic solids, using C3D4 type 4-node linear tetrahedron elements (Fig. 2B). A total of approximately 983,370 elements were used throughout the mesh to model the mandible, implants, abutments, and the framework. A finer FE mesh was used around the implants where rapidly varying stresses and strains were expected. The results of the FEA were compared using the maximum von Mises stress values.
RESULTS
Fig. 2. Boundary conditions (A) and typical mesh (B) of a 3D FE model with implants placed into interforaminal region of edentulous mandible.
Fig. 3. Von Mises stress distributions within cortical and cancellous bones surrounding 3.5mm-diameter implants from group 1 to 5 and 5.5-mm-diameter implants from group 6 to 10. Color scale shows stress concentrations from maximum (red) to minimum (blue).
The maximum von Mises stress values for all the models are presented in Table 4. The different implant placement designs and diameters had an influence on the stress values in the implant, bone, and framework. There were differences in the stress distributions among the FE models. The highest maximum von Mises stress value (361.2 MPa) was observed in the distal implant/abutment assembly close to the loading area, in the 5 axial implants model of 5.5 mm in diameter. In the cortical bone, the highest maximum von Mises stress value was 74.4 MPa in group 3 (4 axial implants of 3.5 mm in diameter) and was located on the distolingual aspect of the implant close to the loading point (Fig. 3). The lowest maximum von Mises stress value was 33.2 MPa in group 10 (4 inclined implants of 5.5 mm in diameter). In the cancellous bone, the maximum von Mises stress values were lower than those in the cortical bone. The highest maximum von Mises stress
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014
Table 3. Young Modulus (E) and Poisson Ratio (n) of Materials and Bone Used in This Study Materials
Content
Young Modulus, E (MPa)
Poisson Ratio, n
Reference
Implant and abutment Framework
Titanium
110.000
0.33
17
Ni Cr alloy
210.000
18
13.700 1.370
17 17
Cortical bone Cancellous bone
719
group 8 (2 axial and 2 distal 35-degree inclined implants of 5.5 mm in diameter) (Fig. 1). In the axially placed implants, as the implant number increased from 4 to 5, the stress values demonstrated a minimal decrease in the cortical bone and the framework but an increase in the implant/abutment assembly. The implant sizes had an effect on the stress distributions. The short implants with
Table 4. Maximum Von Mises Stress Values (MPa) Obtained With FEA of Different Implant Diameters and Placement Designs
Implant Placement
Five Axial
Four Axial
Implant diameters 3.5 5.5 3.5 5.5 Von Mises (MPa) Implant/abutment 291.8 361.2 284.5 340.3 Framework 173.8 169.3 186.8 170.3 Cortical bone 66.0 35.4 74.4 37.2 Cancellous bone 8.3 6.5 9.4 7.8
Two Axial and Two Distal Inclined 17.5-degree Implants
Two Axial and Two Distal Inclined 35-degree Implants
3.5
5.5
3.5
5.5
268.5 93.3 44.5 9.8
323.7 83.4 34.8 8.8
145.8 64.3 43.5 10.5
223.7 50.2 33.6 9.8
Four Inclined 3.5
5.5
150.4 216.6 68.7 50.9 42.6 33.2 12.5 11.5
Increasing implant diameter decreased stress values in framework and cortical and cancellous bones. Increasing implant inclination decreased stress values in implant/abutment assembly, framework, and cortical bone.
value was 12.5 MPa in group 9 (4 inclined implants of 3.5 mm in diameter) and was located on the distal surface of the neck of the implant close to the loading point (Fig. 3). Increased distal implant inclination increased the stress values in the cancellous bone. The lowest maximum von Mises stress value was 6.5 MPa in group 2 (5 axial implants of 5.5 mm in diameter). In the implant/abutment assembly, the highest maximum von Mises stress value was 361.2 MPa in group 2 (5 axial implants of 5.5 mm in diameter) and was located at the distal side of the shoulder of the abutment close to the loading point. As the implant placement inclination increased, the location of the maximum stresses shifted toward the distolingual surface of the implant neck (Fig. 1). The lowest maximum von Mises stress value was 145.8 MPa in group 7 (2 axial and 2 distal 35-degree inclined implants of 3.5 mm in diameter). In the framework, the highest maximum von Mises stress value was 186.8 MPa in group 3 (4 axial implant model of 3.5 mm in diameter) and was located at the distolingual area of the framework in contact with the shoulder of the abutment close to the loading area. As the implant placement inclination increased,
the maximum stresses concentrated on the occlusal surface of the framework. The lowest maximum von Mises stress value was 50.2 MPa in
large diameter resulted in lower stress values in the cortical and cancellous bones and the framework but higher stresses in the implant/abutment assembly
Fig. 4. A, Maximum von Mises stress values in cortical bone for different implant diameters and placement designs. Stress decreased with increasing implant diameter and distal implant inclination. B, Maximum von Mises stress values in cancellous bone for different implant diameters and placement designs. Stress increased with increasing distal implant inclination but decreased with increasing implant diameter. C, Maximum von Mises stress values in implant/abutment assembly for different implant diameters and placement designs. Stress increased with increasing implant diameter but decreased with increasing distal implant inclination. D, Maximum von Mises stress values in framework for different implant diameters and placement designs. Stress decreased with increasing implant diameter and distal implant inclination.
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DIFFERENT IMPLANT SIZE
compared with the long implants with small diameter (Fig. 4, A–D). When the framework cantilever length was decreased from 14 to 4 mm as a result of increasing the inclination of the implants, the stress values decreased in the implant/abutment assembly, the cortical bone, and the framework but increased in the cancellous bone. However, the increasing diameter of the implants decreased high stress concentration resulting from the increasing cantilever length. The 4 implant placements with inclined distal implants provided lower stress values compared with the axially placed 5 implants.
DISCUSSION The load distribution of the osseointegrated implant-bone interface is completely different than the natural teeth that have micromovements caused by the periodontal ligaments, because the implants transmit the functional loads directly to the bone tissue.19 Therefore, irregular load distribution or excessive loading may result in bone resorption and osseointegration failure, increasing stress in the bone, and consequently, may compromise the longevity of the implant and the prosthesis.19–21 In this study, the highest maximum von Mises stress in the cortical bone was located on the distolingual aspect of the distal implant close to the loading point for all the implant models. This result reveals that a vertical load applied on the cantilever is not distributed evenly throughout the entire surface of the implants. The cantilever extensions result in an angulated load or a torque moment on the implants and more stress and frequently resorption in the cortical bone around the distal implant neck than in the other regions.5 In this study, the inclination of the mesial implants had a minor effect on the load distribution because the mesial implants were subjected to lower stresses than the distal implants. Therefore the FE models (groups 9 and 10) with inclined mesial implants demonstrated similar stress values in the cortical bone compared with those (groups 7 and 8) with the axially placed mesial implants. Increasing the number of the implants supporting a denture increases
AND INCLINATION
BALKAYA
the bone-to-implant contact surface per unit area and prevents overloading, providing a decrease in the load transmission to the bone.5 In this study, increasing number of implants resulted in a decrease of 11% and 5% in the stress values of the cortical bone surrounding the implants of 3.5 and 5.5 mm in diameters, respectively, when 4 axial implants were compared with 5 axial implants. This result is supported by an in vivo strain gauge study6 in which a decreased implant number led to an increase in the compressive forces on the supporting implants. Several strain gauge studies have found that the bending moments affecting on the FPD may be decreased with the increasing number of implants.22,23 In this study, the stress values in the framework showed a relatively minimal decrease with the increasing number of implants, as all the FE models with 4 and 5 axially placed implants had the same cantilever length. If the implant number is decreased due to anatomical limitations, the implant placement designs or inclinations can be effective to provide a reduction in the stress concentration on the surrounding bone. In cases of posterior residual ridge resorption, if the edentulous mandible is rehabilitated with the implant-supported FPD, the All-on-4 Concept with only 4 implants can provide a lower periimplant bone stress, with a shorter cantilevered prosthesis and longer implants, and a high rate of clinical success compared with 5 axial implants placed in the interforaminal region.12,13 In this study, the FE models of 4 implants placed with different distal implant inclinations demonstrated similar or lower stress values in the cortical bone, the implant/abutment assembly, and the framework than those of 5 axial implants. In an FEA study, Naini et al24 evaluated von Mises stress distribution in periimplant bone in 4 axial implants and All-on-4 implant placements in the interforaminal region of an edentulous mandible and suggested that the distal implant inclination resulted in an increase of 9% in the stress values resulting from the inclination and the proximity to the loading area of the distal implants during posterior loading.
These results are in disagreement with the findings of this study in which the stress values in the cortical bone decreased notably with the distal implant inclination and the decreased cantilever length, when the small-diameter implant (3.5 mm) was used. The differences in the results of this study and the one by Naini et al24 may result from the differences in the material properties of the bone and framework, the cortical bone thickness, and the implant lengths according to inclination. The implant-supported FPDs in the edentulous mandible are designed generally with bilateral cantilever extensions with the implant placement in the interforaminal area. However, the compressive stress leading to bone resorption increases as the cantilever length increases. There is a direct correlation between the cantilever length and the stress in the bone.13,18 In this study, the cantilever lengths were changed from 14 to 4 mm with different inclinations of the distal implants, using the framework with standardized size. As the cantilever lengths were decreased from 14 to 9 mm, with 17.5-degree inclination of the distal implants, the maximum von Mises stress values demonstrated a decrease of 40% and 7% in the cortical bone, 50% and 51% in the framework, and 8% and 46% in the implant/abutment assembly for the implants of 3.5 and 5.5 mm diameters, respectively. As the cantilever lengths decreased from 9 to 4 mm, the stress values demonstrated a minimal decrease in the cortical bone. The models of the distal implants inclined 17.5 and 35 degrees had generally similar stress values in the cortical bone for each implant diameter. These findings show that the 9 mm or lower cantilever length may be adequate to decrease the stress concentrated on the cortical bone surrounding the distal implant and can be used clinically in the oral rehabilitation with full-arch implant-supported FPDs of the edentulous mandible. An FEA study found that there was no significant difference between 4 and 12 mm cantilever lengths for 30- and 40-degree inclined distal implants under distributed loads.25 In this study, there was a decrease in the magnitude of stresses of 31% and 40% in the framework and
IMPLANT DENTISTRY / VOLUME 23, NUMBER 6 2014 46% and 33% in the implant/abutment assembly for the implants of 3.5 and 5.5 mm in diameters, respectively. These findings showed that the proximity of the implants to the loading area influenced the stress values in the implant/ abutment assembly and the framework. In this study, when the implants of 3.5 mm in diameter were used, the axial implant models demonstrated higher stress values than the inclined implant models. The results revealed that the decreased cantilever length with the increasing implant inclination resulted in a decrease in the stress values in the implants, the cortical bone, and the framework. However, when the implants of 5.5 mm in diameter were used, the stress values in the axial and the inclined implant models were similar. This result showed that the increasing implant diameter may decrease high stress concentrations resulting from increasing cantilever length. The implant diameter and length are important factors to achieve optimum load distribution in the bone.7 In this study, the short implants with large diameter and the long implants with small diameter were used to evaluate the effect of different implant size and inclinations on the load distributions. The implant lengths were increased with increasing implant inclinations to simulate the clinical conditions in the inclined implant models because the stress in the cancellous bone may decrease with an increase in the implant length.6 In the axial implant models, the long implants with small diameter (groups 1 and 3) showed higher stress values in the cortical bone than the short implants with large diameter (groups 2 and 4). These findings are in agreement with the FEA studies in which the increasing implant diameter and length can result in a decrease in the stress concentration in the bone; however, diameter may have a more significant effect than length,6,26,27 and therefore, increasing implant diameter is especially important in decreasing the stress and strain values in the bone when there is limited bone height of residual ridge26 or where short implants are subjected to lateral loads.3 It should also be considered that the results of standardized experiments and
FE simulations may be different from those of clinical studies. When considering previous retrospective studies,8,28,29 Olate et al8 evaluated the influence of implant diameter and length on early failure of implants at time period between first and second surgical phases and found that the implant diameter had no significant effect on the early loss with failure rates of 5.1%, 3.8%, and 2.7% in small-, regular-, and large-diameter implants, respectively. Mijiritsky et al28 analyzed the treatment outcome at the first 2 years of prosthetic loading and concluded that implant diameter did not affect the clinical success with survival rates of 98.2%, 98.7%, and 98.5% in small- (,3.75 mm), regular- (3.75–5 mm), and large-diameter (.5 mm) implants, respectively. However, a retrospective study reported the cumulative survival rates up to 5 and 7 years of large- and regulardiameter implants after prosthetic loading, respectively, and found that the large-diameter implants had lower clinical success than the regular-diameter implants, with survival rates of 80.9% and 96.8% in 64 large-diameter (5 mm) and 64 regular-diameter (3.75–4 mm) implants, respectively. They suggested that buccolingual bone dimensions may influence the survival rate of the implants placed in the posterior region.29 In the narrow resorbed ridge areas, the large-diameter implants may result in crestal bone loss30 and demonstrate lower clinical success rates29,31 because the thickness of the bone surrounding the implants may not be adequate to ensure viability of the remaining buccal and lingual bone after the implant placement.1 In this study, all the FE models with the large-diameter implants had lower stress levels in the cortical and cancellous bones than those of the small-diameter implants. The stress values in the cortical bone were higher than those in the cancellous bone because the cortical bone has higher elastic modulus and is subjected to higher stresses.3 Therefore, the increasing implant diameter provides more cortical bone-to-implant contact surface, which minimizes the crestal bone resorption, decreasing the load transfer acting from implants to the surrounding bone, compared with the small-diameter
721
implants.7 The result of this study is supported by a recent meta-analysis study in which the larger diameter implants than 3.3 mm demonstrated higher survival rates compared with the smaller diameter implants and the small-diameter implants had 3.92 times higher failure than the larger implants.32
CONCLUSIONS Increasing the number of implants decreased the stress values concentrated in the cortical bone for the axially placed implants. The short implants with large-diameter demonstrated lower stresses than the long implants with small diameter. The decreasing cantilever length decreased the stress values in the cortical bone, the implant/ abutment assembly, and the framework. Increasing diameter of implant may decrease high stress concentration resulting from increasing cantilever length.
DISCLOSURE The author claims to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The author thanks Dr Müge Balkaya, Istanbul Technical University, for her valuable help with the FEM program ABAQUS.
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