There are some anatomical restrictions in which implants are not possible to be inserted in their conventional configuration. Offset placement of implants in relation to the prosthetic unit could be a treatment solution. The aim of this study was to evaluate the effect of the offset placement of implant-supported prosthesis on the stress distribution around a dental implant using 3D finite element analysis. 3D finite element models of implant placement in the position of a mandibular molar with 4 configurations (0, 0.5, 1, 1.5 mm offset) were created in order to investigate resultant stress/strain distribution. A vertical load of 100 N was applied on the center of the crown of the models. The least stress in peri-implant tissue was found in in-line configuration (0 mm offset). Stress concentration in the peri-implant tissue increased by increasing the amount of offset placement. Maximum stress concentration in all models was detected at the neck of the implant. It can be concluded that the offset placement of a single dental implant does not offer biomechanical advantages regarding reducing stress concentration over the in-line implant configuration. It is suggested that the amount of offset should be as minimum as possible.

Introduction

Currently, the replacement of a single missing tooth using implant-supported fixed prosthesis is a widely accepted treatment modality. Although success and survival rates of single-implant restorations are considered to be high, there are still cases of failure.13  The clinical success and longevity of restoration with an implant-supported prosthesis are highly influenced by biomechanics of oral implants, load distribution at the implant-bone interface, and transfer of mechanical stresses to the surrounding bone.45 

Conventional placement of dental implants is not indicated in some anatomical restrictions such as proximity of the maxillary sinus or mandibular canal to the implant in molar region. However, in some cases, mild anatomical restriction could be managed with inserting the implant in an offset configuration in relation to the prosthetic unit. In addition, the offset placement of dental implants can lead to achieve an optimal aesthetic restoration by reduction of emergence profile and to facilitate the dental hygiene in the posterior mandible region.6  However, there are some concerns regarding the biomechanics of offset implants and stress distribution in their surrounding tissue due to the nonaxial forces applied to these implants.711  Thus, it is important to ensure whether stress distribution occurs in a safe manner around the offset implants.

Stress distribution around dental implants has been assessed using a wide variety of methods. Three-dimensional finite element analysis (3D-FEA) has been frequently used in the dental implant field to predict stress distribution patterns in the implant components and the surrounding bone.1215  FEA model enables researchers to simulate various clinical scenarios and to determine the best option from a biomechanical perspective. The latter is of paramount importance since stress distribution in peri-implant bone is considered a key factor for the success of dental implants.12 

Offset implant placement has been widely reported in the case of prosthesis supported by 3 implants.1622  Although it has suggested that an offset implant placement in the case of prosthesis supported by 3 implants due to its tripod effect would offer biomechanical advantageous,16,17  studies investigating biomechanical effects of this offset configuration using finite element analysis have demonstrated contradictory results.1822  However, the biomechanics of a restoration supported by a single implant is quite different with those of fixed partial dentures supported by 3 or more implants. In the case of 3 or more implants, a single load applying at the splinted restoration can induce varying degrees of stresses or strains in all implants and adjacent bone. Accordingly, the outcomes of the studies evaluating biomechanical properties of fixed partial dentures supported by 3 or more implants might not necessarily be extrapolated to the situations that a restoration is supported by a single implant. Unlike the cases of the offset implant placement in 3-unit fixed implant-supported partial dentures, very little information is available regarding the biomechanical effects of an offset implant placement when treating a single implant. Therefore, the aim of this study was to assess the effect of the offset implant placement on the stress distribution around a dental implant using 3D finite element analysis.

Materials and Methods

Four 3-dimensional (3D) finite element models were designed of a segment of mandible. Each model contained a 1-piece implant, cortical and spongy bone, and a metal-ceramic crown on the implant. The models were the same except for the exact location of the crown in relation to the implant long axis. The center of the crown coincided with the implant long axis in model A and shifted mesially 0.5 mm, 1.0 mm, and 1.5 mm in models B, C, and D, respectively (Figure 1). A symmetrical crown to the implant long axis was simulated in model A. Asymmetrical crowns were simulated in models B through D where the implant was not exactly at the mid span between the 2 remaining teeth. The implant dimension was 4 × 10 mm. SolidWorks 2010 software package (SolidWorks Corp, Concord, Mass) was selected for the modeling phase. The models were then transferred to the ANSYS Workbench version 11.0 (ANSYS Inc, Canonsburg, Pa) for analysis. Mechanical properties of the materials used were applied (Table 1). Models were meshed with 85 551 to 86 663 nodes; 52 304 to 52 913 elements. All nodes at the base of the model were restrained so that all rigid body motions were prevented. A vertical force of 100 N was applied on the center of the crown. Contact elements were defined between parts. The von Mises stress of the bone in the distal side of the implant from the crestal down to the apical was recorded in both cortical and spongy bone.

Figure 1.

Views of the meshed models. In-line (a), 0.5 mm (b) 1 mm (c), and 1.5 mm (d) offset placement.

Figure 1.

Views of the meshed models. In-line (a), 0.5 mm (b) 1 mm (c), and 1.5 mm (d) offset placement.

Table 1

Mechanical properties of the materials used in modeling

Mechanical properties of the materials used in modeling
Mechanical properties of the materials used in modeling

Results

The stress concentration in the peri-implant tissue was in proportion to the amount of offset placement. As the amount of offset placement increased, stress concentration increased (Table 2). By increasing the amount of offset placement from model A to model B, C, and D, stress concentration around the implant neck increased 28.88%, 59.23%, and 211.60%, respectively.

Table 2

The Von Mises stress (MPa) evaluated in the bone

The Von Mises stress (MPa) evaluated in the bone
The Von Mises stress (MPa) evaluated in the bone

During loading, maximum stress concentration in each model was detected around the neck of the implant, which was minimum in model A (8.67 MPa) and maximum in model D (18.9 MPa). In all models, stress concentration in peri-implant tissue decreased slightly from the neck to the apex of implant (Table 2). Additionally, as the distance from the implant increased, stress distribution in bone decreased.

By descending in implants from neck to apex, the stress concentration discrepancy between 4 models decreased (Figures 2 and 3). After the fourth thread, the stress distribution in all 4 groups was very close to each other (≤1 MPa), and the difference was not considerable.

Figure 2.

The von Mises stress distribution of models. In-line (a), 0.5 mm (b), 1 mm (c), and 1.5 mm (d) offset placement.

Figure 2.

The von Mises stress distribution of models. In-line (a), 0.5 mm (b), 1 mm (c), and 1.5 mm (d) offset placement.

Figure 3.

The von Mises stress along the implant (from crest to the apex) of different groups.

Figure 3.

The von Mises stress along the implant (from crest to the apex) of different groups.

Discussion

The present study aimed to investigate the stress distribution around a single dental implant placed offset in relation to the axis of the prosthetic unit in the mandibular posterior region. Three dimensional finite element analysis, simulating a vertical force of 100 N applied on the center of the crown of the models, was used to achieve this purpose. The key finding of the present study was that stress concentration in the peri-implant tissue increased by increasing the amount of offset placement.

A main factor influencing the final success or failure of implant-supported restorations is the manner in which stresses are transferred to the implant and surrounding bone.12,23  The present results showed that stress concentration in peri-implant tissue in different configurations of offset placement was more than that of in-line configuration. It can be attributable to the increased length of the prosthetic beam leading to inappropriate stress distribution in the fixture and surrounding bone, which is not directed to the long axis of the fixture.

The present findings are in disagreement with those of Weinberg and Kruger who investigated mathematically the biomechanical advantages of an offset placement of a single implant using a 2-dimensional model.24  They reported that the lingual offset of the mandibular implant decreased the torque at the gold screw. However, they investigated the effect of buccal or lingual offset placement configuration, but not the mesial or distal offset. The present results are also partially in disagreement with those of Anitua and Orive who compared the effect of the distal offset configuration of a single implant-supported prosthesis on bone stress distribution with that of straight alignment using Finite Element Analysis.6  They indicated that the offset implant placement produced less stress compared to the straight configuration. This disagreement can be explained by the fact that conditions they evaluated were quite different from those of the present study. In Anitua and Orive study, static loads of 200 N and 230 N were applied on the mesial and distal borders of the prosthetic unit, respectively. On the other hand, in the present study, a vertical force of 100 N was applied on the center of the prosthesis. Since the load transfer from implants to surrounding bone largely depends on the type and direction of loading,12  the difference in the direction of the loads between the 2 studies could be partially responsible for the disagreement. Moreover, it should be noted that material properties of mandible are other key factors that could greatly affect the predictive accuracy of the FEA studies.12,25,26 

The results of the present study revealed that the maximum stress concentration in all groups was located around the neck of the implant, and it decreased slightly from the neck to the apex. The present finding is in line with the results of other studies that have reported overloading occurred in the superior region of the cortical bone around the contact area with the implant.6,19,21,2729  This finding could explain why bone loss initiates mostly from the alveolar crest. Failure to achieve appropriate stress distribution would lead to increased stress concentration around the implant neck. This stress could surpass the elastic limit of the bone and cause microfracture and bone loss at the neck of the implant.22,30  The present finding also highlights the importance of the presence of good quality cortical bone around the implant neck that can tolerate the maximum stress concentration. Moreover, it was found that the stress values after the third thread were quite similar in all four groups, indicating that most of the stress in bone is located in the first 3 threads of the dental implants independently of the offset or in-line configuration of the implant.

In another way, the offset placement of the implant could be considered as a kind of implant-supported cantilevered fixed partial dentures (ICFPD). Accordingly, the present findings might be comparable with those of studies reporting stress concentration around implants with cantilever extension is more than implants without cantilever extension.811,31  This stress is shown to be proportional to the length of cantilever,23  and it might lead to problems such as screw loosening,32  bone resorption, and compromised longevity of implant-supported prosthesis.33,34  The present results also showed that the stress concentration in peri-implant tissue increased as the amount of offset placement increased. Accordingly, this study is in accordance with studies which investigated cantilevers and reported that the amount of stress concentration in bone surrounding the implant increases as the length of cantilever extension increases.35,36 

Although bone loss and stress concentration in implant and supporting bone increase in ICFPDs,811,31  the 5- and 10-year ICFDP cumulative survival rates are reported to be 94.3% and 88.9%, respectively; therefore, ICFDPs are considered as a reliable treatment option.31  These high survival rates could be attributable to several rules and guidelines such as splinting implants and using anterior-posterior spread, which are followed by clinicians for the safe length of cantilever.15,3537  Following these guidelines would reduce the harmful biomechanical effects and result in acceptable outcomes. However, small hidden cantilevers in the prosthetic reconstructions such as offset implant configurations could produce similar high stress as the long cantilevers since no guideline are followed to manage them. In a study evaluating the influence of cantilever length on the stress distribution,37  increasing the cantilever length from 10 mm to 15 mm led to 30% increase in the stress concentrated at the implant-abutment surface. This amount of increase in the stress at the neck of the implant was found in the current study (32.8%) when the amount of offset placement increased only half a millimeter (from 1 mm to 1.5 mm). Based on these results, it is suggested that the amount of offset should be as minimum as possible.

It should be noted that there are some limitations associated with the present study. As in previous studies, it was assumed that implants were completely osseointegrated; however, 100% ratio of osseointegration between the implant and bone does not occur in the clinical setting. Nevertheless, since this assumption was applied to all models, it most probably would not influence the final conclusion. In addition, in the present study, a vertical load was applied on the center of the crown in all models. However, in the clinical setting, complex patterns of force vectors such as oblique and horizontal forces are applied on a dental implant. Therefore, although finite element analysis closely simulates the clinical condition, these limitations should be taken into account when a clinical application is envisaged.

Conclusion

Within the limitations of the present study, it can be concluded that the offset placement of a single dental implant does not offer biomechanical advantages regarding reducing stress concentration over the in-line implant configuration. The stress concentration in peri-implant tissue in different configurations of offset placement was more than that of in-line configuration.

In the offset configuration, the amount of stress concentration in the peri-implant tissue increased by increasing the amount of offset placement. Therefore, it is suggested that the amount of offset should be as minimum as possible.

Abbreviations

     
  • 3D

    three-dimensional

  •  
  • 3D-FEA

    three dimensional finite element analysis

  •  
  • ICFPD

    implant-supported cantilevered fixed partial dentures

References

References
1
Salinas
TJ
,
Eckert
SE.
In patients requiring single-tooth replacement, what are the outcomes of implant- as compared to tooth-supported restorations?
Int J Oral Maxillofac Implants
.
2007
;
22
Suppl
:
71
95
.
2
Weber
HP
,
Sukotjo
C.
Does the type of implant prosthesis affect outcomes in the partially edentulous patient?
Int J Oral Maxillofac Implants
.
2007
;
22
Suppl
:
140
172
.
3
Jung
RE
,
Pjetursson
BE
,
Glauser
R
,
Zembic
A
,
Zwahlen
M
,
Lang
NP.
A systematic review of the 5-year survival and complication rates of implant-supported single crowns
.
Clin Oral Implants Res
.
2008
;
19
:
119
130
.
4
Bidez
MW
,
Misch
CE.
Force transfer in implant dentistry: basic concepts and principles
.
J Oral Implantol
.
1992
;
18
:
264
274
.
5
Kim
Y
,
Oh
TJ
,
Misch
CE
,
Wang
HL.
Occlusal considerations in implant therapy: clinical guidelines with biomechanical rationale
.
Clin Oral Implants Res
.
2005
;
16
:
26
35
.
6
Anitua
E
,
Orive
G.
Finite element analysis of the influence of the offset placement of an implant-supported prosthesis on bone stress distribution
.
J Biomed Mater Res B Appl Biomater
.
2009
;
89
:
275
281
.
7
Yokoyama
S
,
Wakabayashi
N
,
Shiota
M
,
Ohyama
T.
The influence of implant location and length on stress distribution for three-unit implant-supported posterior cantilever fixed partial dentures
.
J Prosthet Dent
.
2004
;
91
:
234
240
.
8
Romeo
E
,
Tomasi
C
,
Finini
I
,
Casentini
P
,
Lops
D.
Implant-supported fixed cantilever prosthesis in partially edentulous jaws: a cohort prospective study
.
Clin Oral Implants Res
.
2009
;
20
:
1278
1285
.
9
Jacques
LB
,
Moura
MS
,
Suedam
V
,
Souza
EA
,
Rubo
JH.
Effect of cantilever length and framework alloy on the stress distribution of mandibular-cantilevered implant-supported prostheses
.
Clin Oral Implants Res
.
2009
;
20
:
737
741
.
10
Stegaroiu
R
,
Sato
T
,
Kusakari
H
,
Miyakawa
O.
Influence of restoration type on stress distribution in bone around implants: a three-dimensional finite element analysis
.
Int J Oral Maxillofac Implants
.
1998
;
13
:
82
90
.
11
Kunavisarut
C
,
Lang
LA
,
Stoner
BR
,
Felton
DA.
Finite element analysis on dental implant-supported prostheses without passive fit
.
J Prosthodont
.
2002
;
11
:
30
40
.
12
Geng
JP
,
Tan
KB
,
Liu
GR.
Application of finite element analysis in implant dentistry: a review of the literature
.
J Prosthet Dent
.
2001
;
85
:
585
598
.
13
Geramy
A
,
Morgano
SM.
Finite element analysis of three designs of an implant-supported molar crown
.
J Prosthet Dent
.
2004
;
92
:
434
440
.
14
Petrie
CS
,
Williams
JL.
Comparative evaluation of implant designs: influence of diameter, length, and taper on strains in the alveolar crest. A three-dimensional finite-element analysis
.
Clin Oral Implants Res
.
2005
;
16
:
486
494
.
15
Akça
K
,
Iplikçioğlu
H.
Finite element stress analysis of the effect of short implant usage in place of cantilever extensions in mandibular posterior edentulism
.
J Oral Rehabil
.
2002
;
29
:
350
356
.
16
Rangert
B
,
Sennerby
L
,
Meredith
N
,
Brunski
J.
Design, maintenance and biomechanical considerations in implant placement
.
Dent Update
.
1997
;
24
:
416
420
.
17
Rangert
BR
,
Sullivan
RM
,
Jemt
TM.
Load factor control for implants in the posterior partially edentulous segment
.
Int J Oral Maxillofac Implants
.
1997
;
12
:
360
370
.
18
Sütpideler
M
,
Eckert
SE
,
Zobitz
M
,
An
KN.
Finite element analysis of effect of prosthesis height, angle of force application, and implant offset on supporting bone
.
Int J Oral Maxillofac Implants
.
2004
;
19
:
819
825
.
19
Huang
HL
,
Lin
CL
,
Ko
CC
,
Chang
CH
,
Hsu
JT
,
Huang
JS.
Stress analysis of implant-supported partial prostheses in anisotropic mandibular bone: in-line versus offset placements of implants
.
J Oral Rehabil
.
2006
;
33
:
501
508
.
20
Akça
K
,
Iplikçioğlu
H.
Finite element stress analysis of the influence of staggered versus straight placement of dental implants
.
Int J Oral Maxillofac Implants
.
2001
;
16
:
722
730
.
21
Abu-Hammad
O
,
Khraisat
A
,
Dar-Odeh
N
,
Jagger
DC
,
Hammerle
CH.
The staggered installation of dental implants and its effect on bone stresses
.
Clin Implant Dent Relat Res
.
2007
;
9
:
121
127
.
22
Sato
Y
,
Shindoi
N
,
Hosokawa
R
,
Tsuga
K
,
Akagawa
Y.
A biomechanical effect of wide implant placement and offset placement of three implants in the posterior partially edentulous region
.
J Oral Rehabil
.
2000
;
27
:
15
21
.
23
Bevilacqua
M
,
Tealdo
T
,
Menini
M
,
et al
.
The influence of cantilever length and implant inclination on stress distribution in maxillary implant-supported fixed dentures
.
J Prosthet Dent
.
2011
;
105
:
5
13
.
24
Weinberg
LA
,
Kruger
B.
An evaluation of torque (moment) on implant/prosthesis with staggered buccal and lingual offset
.
Int J Periodontics Restorative Dent
.
1996
;
16
:
252
265
.
25
Misch
CE.
Clinical biomechanics
.
In
:
Misch
CE
,
ed
.
Contemporary Implant Dentistry. 1st ed
.
St Louis
:
Mosby;
1993
:
298
300
.
26
O'Mahony
AM
,
Williams
JL
,
Spencer
P.
Anisotropic elasticity of cortical and cancellous bone in the posterior mandible increases peri-implant stress and strain under oblique loading
.
Clin Oral Implants Res
.
2001
;
12
:
648
657
.
27
Rungsiyakull
C
,
Rungsiyakull
P
,
Li
Q
,
Li
W
,
Swain
M.
Effects of occlusal inclination and loading on mandibular bone remodeling: a finite element study
.
Int J Oral Maxillofac Implants
.
2011
;
26
:
527
537
.
28
Bozkaya
D
,
Muftu
S
,
Muftu
A.
Evaluation of load transfer characteristics of five different implants in compact bone at different load levels by finite elements analysis
.
J Prosthet Dent
.
2004
;
92
:
523
530
.
29
Alikhasi
M
,
Siadat
H
,
Geramy
A
,
Hassan-Ahangari
A.
Stress distribution around maxillary anterior implants as a factor of labial bone thickness and occlusal load angles: a 3D-finite-element analysis
.
J Oral Implantol
.
2014
;
40
:
37
42
.
30
Clift
SE
,
Fisher
J
,
Watson
CJ.
Finite element stress and strain analysis of the bone surrounding a dental implant: effect of variations in bone modulus
.
Proc Inst Mech Eng H
.
1992
;
206
:
233
241
.
31
Aglietta
M
,
Siciliano
VI
,
Zwahlen
M
,
et al
.
A systematic review of the survival and complication rates of implant supported fixed dental prostheses with cantilever extensions after an observation period of at least 5 years
.
Clin Oral Implants Res
.
2009
;
20
:
441
451
.
32
Wood
MR
,
Vermilyea
SG.
A review of selected dental literature on evidence-based treatment planning for dental implants: report of the Committee on Research in Fixed Prosthodontics of the Academy of Fixed Prosthodontics
.
J Prosthet Dent
.
2004
;
92
:
447
462
.
33
Isidor
F.
Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys
.
Clin Oral Implants Res
.
1996
;
7
:
143
152
.
34
Barbier
L
,
Vander Sloten
J
,
Krzesinski
G
,
Schepers
E
,
Van der Perre
G.
Finite element analysis of non-axial versus axial loading of oral implants in the mandible of the dog
.
J Oral Rehabil
.
1998
;
25
:
847
858
.
35
Sertgöz
A
,
Güvener
S.
Finite element analysis of the effect of cantilever and implant length on stress distribution in an implant-supported fixed prosthesis
.
J Prosthet Dent
.
1996
;
76
:
165
169
.
36
White
SN
,
Caputo
AA
,
Anderkvist
T.
Effect of cantilever length on stress transfer by implant-supported prostheses
.
J Prosthet Dent
.
1994
;
71
:
493
499
.
37
Rubo
JH
,
Capello Souza EA. Finite-element analysis of stress on dental implant prosthesis
.
Clin Implant Dent Relat Res
.
2010
;
12
:
105
113
.