EFFECT OF STRAIGHT AND ANGULATED ABUTMENTS ON …repository-tnmgrmu.ac.in/5058/1/240119617naveenkumar.pdf · V.VATSALA and my wife Dr. APARNA NAVEEN KUMAR, without her support I could

EFFECT OF STRAIGHT AND ANGULATED ABUTMENTS ON STRESS AND

STRAIN AROUND A PLATFORM SWITCHED IMPLANT PLACED IN ANTERIOR

MAXILLA-A FINITE ELEMENT ANALYSIS

Dissertation submitted to

THE TAMIL NADU Dr. M.G.R. MEDICAL UNIVERSITY

In partial fulfillment for the degree of

MASTER OF DENTAL SURGERY

BRANCH – I

PROSTHODONTICS AND CROWN & BRIDGE

APRIL – 2017

CERTIFICATE

This is to certify that the dissertation titled “EFFECT OF STRAIGHT AND

ANGULATED ABUTMENTS ON STRESS AND STRAIN AROUND A PLATFORM

SWITCHED IMPLANT PLACED IN ANTERIOR MAXILLA-A FINITE ELEMENT

ANALYSIS ” by Dr.Naveen kumar.T, post graduate student – MDS (Prosthodontics and

Crown & Bridge- Branch- I ), of KSR Institute of Dental Science and Research,

Tiruchengode, submitted to the Tamil Nadu Dr. M.G.R. Medical University, Chennai, in

partial fulfillment of the requirements for the MDS degree examination – April 2017- is a

bonafide research work carried out by him under our supervision and guidance.

GUIDED BY

DR.N. VIDYA SANKARI.MDS.

Professor,

Dept. of Prosthodontics and Crown Bridge,

KSR Institute of Dental Science and Research,

Tiruchengode – 637 215.

Dr. C. A. MATHEW, M.D.S.,

Professor & Head of the Department,

Dept. of Prosthodontics and Crown Bridge,

KSR Institute of Dental Science and Research,


Dr. G.S. KUMAR M.D.S.,

Principal,

KSR Institute of Dental Science

and Research,


DECLARATION BY THE CANDIDATE

TITLE OF DISSERTATION

Effect of straight and angulated abutments on stress and strain

around a platform switched implant placed in anterior maxilla-a

finite element analysis.

PLACE OF STUDY

K.S.R. Insititute of Dental Science and Research

DURATION OF COURSE

3 Years

NAME OF THE GUIDE

DR.N. VIDYA SANKARI

HEAD OF THE DEPARTMENT

DR. C.A. MATHEW

I hereby declare that no part of the dissertation will be utilized for gaining financial assistance

for research or other promotions without obtaining prior permission of the Principal, K.S.R

Institute of Dental Science and Research, Tiruchengode. In addition, I declare that no part of

this work will be published either in print or electronic without the guide who has been

actively involved in the dissertation. The author has the right to reserve publishing of work

solely with prior permission of the Principal, K.S.R Institute of Dental Science and Research,

Tiruchengode.

Head of the Department Guide Signature of the Candidate

ACKNOWLEDGEMENTS

I owe my deepest gratitude to God Almighty for all the blessings which he showers

upon me throughout my life and career.

I wish to express my heartfelt thanks to Thiru. Lion. K. S. Rangasamy, MJF,

Founder and Chairman, K.S.R. group of institutions, for giving me an opportunity to undergo

post-graduation in this prestigious institution.

I am extremely grateful to Dr. G. S. Kumar., MDS, Principal, KSR Institute of

Dental Science and Research for his invaluable guidance and constant support.

“A good teacher is a good mother”. I express my deep gratitude to my guide DR. N.

Vidya Sankari, MDS, Professor, department of prosthodontics. KSRIDSR, for her motherly

care. In fact, it was my good fortune to have a person who with sharp and concrete

suggestions at all junctures coupled with her tremendous knowledge, guided at all stages of

this difficult task. Often when I faced problems, which looked insurmountable, her moral

support gave me a lot of confidence without which this study would not have been

completed.

It is my privilege to express my regards to DR. C.A. Mathew, MDS, Professor &

HOD, Department of Prosthodontics. KSRIDSR. . He has been a continuous source of

inspiration and I am indeed indebted to him for selflessly sparing me his time and knowledge

throughout my post graduate course. His encouragement and affectionate guidance will

always be remembered.

I also thank Dr S. Suresh Kumar MDS., Dr. J. Muthu Vignesh MDS., and Dr. M

Maheshwaran MDS., Dr.Viswanathan.,MDS., and Dr. Raj Kumar MDS., for their

relentless encouragement and continuous support throughout the course of my study. I am

deeply indebted to them for their most valuable suggestions which were instrumental in

completing this dissertation.

Getting through my dissertation required more than academic support, and I have

many people to thank for listening to and, at times, having to tolerate me over the past three

years. I cannot begin to express my gratitude and appreciation for their friendship. Dr.

Brindha, Dr.Mohammed, Dr.Benny Thomas, Dr.Mithrarajan, Dr.Satheesh, Dr.Uma

maheshwari, Dr.Siva kumar, Dr.Yoganath, Dr. Sai Mahendran ,Dr. Kanmani ,Dr. Biju,

Dr.Kasthoori and Dr.Shanmugapriya have been unwavering in their personal and

professional support during my college hours.

I extend my heartfelt thanks to my friends, Dr.Sriram balagi , Mr.Vinosh kumar

and Mr.shiva shanker for their help, advice and support.

I take this opportunity to thank all other faculties, lab technicians and non-teaching

staffs of the Department of Prosthodontics for their invaluable assistance and support

throughout my post-graduation course.

Parents are next to God, and their silent sacrifices for me can’t be put into words. I

would like to express my heartfelt thanks to my parents, Mr. T.S.THANGAVELU and Mrs.

V.VATSALA and my wife Dr. APARNA NAVEEN KUMAR, without her support I could

not achieved this height.

Contents

CONTENTS

S. No. TITLE PAGE No.

1. INTRODUCTION 1

2. AIMS AND OBJECTIVES 4

3. REVIEW OF LITERATURE 5

4. MATERIALS AND METHODS 15

5. RESULTS 28

6. DISCUSSION 50

7. SUMMARY AND CONCLUSION 60

8. BIBLIOGRAPHY 61

LIST OF TABLES

TABLE No. TITLE PAGE No.

1.

(a):Number of elements and nodes

(b) :Material properties used in FEA Study 23

2.

The value of von Mises stress for the models with platform

switched abutments.(units in Mpa) 30

3.

The value of von Mises stress for the models with platform

switched abutments.(units in Mpa). 30

4.

The value of von Mises strain for the models with platform

switched abutment in micro strains (strain × 10 -6

). 31

5.

The value of von Mises strain for the models with platform

switched abutments in micro strains (strain × 10 -6

). 31

LIST OF GRAPHICAL PRESENTATIONS

GRAPH No. TITLE PAGE No.

1.

Bar diagram showing stress values (in Mpa) with in cortical

bone around the implant 32

2.

Bar diagram showing stress values (in Mpa) with in cancellous

bone around the implant 32

3.

Bar diagram showing strain values (in micro strains)with in

cortical bone around the implants 33

4.

Bar diagram showing strain values (in micro strains) with in

cancellous bone around the implants 33

LIST OF FIGURES

FIGURE NO. TITLE

PAGE

NO.

1.

A. Dimension of Implant 24

B. Bone model with implant and platform matched straight

abutment

24

2.

A. PS 0-an implant fixture (4.3mm) with platform switched 0°

(3.5 mm) straight abutment .

25

B. PS 15- an implant fixture (4.3mm) with platform switched

15° (3.5 mm) angulated abutment .

25

C. PS 20 - an implant fixture (4.3mm) with platform switched

20°(3.5 mm) angulated abutment .

26

D.PS 25 - an implant fixture (4.3mm) with platform switched 25°

(3.5 mm) angulated abutment .

26

3.

FEM mesh created by an analyst prior to finding a solution to a

problem using FEM software

27

4. A&B

Pictorial representation of stress values in straight abutment for

axial load

34

5. A&B

Pictorial representation of stress values in straight abutment for

off-axis load.

35

https://en.wikipedia.org/wiki/Polygon_mesh

6. A&B

Pictorial representation of stress values in 15° angled abutment

for axial load.

36

7. A&B


for off-axis load

37

8. A&B


for axial load.

38

9. A&B


for off-axis load

39

10. A&B


for axial load

40

11. A&B


for off-axis load

41

12. A&B

Pictorial representation of strain values in straight abutment for

axial load.

42

13. A&B

A pictorial representation of strain values in straight abutment for

off-axis load.

43

14. A&B

Pictorial representation of strain values in 15° angled abutment

for axial load.

44

15. A&B

A pictorial representation of strain values in 15° angled abutment

for off-axis load.

45

16. A&B


for axial load.

46

17. A&B


for off-axis load

47

18. A&B


for axial load.

48

19. A&B


for off-axis load

49

Introduction

Introduction

Page 1

INTRODUCTION

Dental implants are considered as one of the most successful treatment options

for replacing missing teeth after discovery of the osseointegration concept by Branemark in

the 1950s1. After the loss of teeth there will be a substantial amount of change in the

morphology of alveolar bone. After extraction in the anterior maxillary region there will

almost be twice the amount of horizontal bone resorption when compared to the vertical bone

resorption2.

This situation can be managed either by surgical management or by placing

implants in areas of maximum bone availability. This change in implant angulation can be

managed by placing angulated abutments during prosthetic rehabilitation and it is considered

as a valid treatment option3,4

.

The success of an implant restoration greatly depends upon the success of the

osseo integration. However crestal bone loss is observed after implant placement. Adell et al5

first reported the crestal bone loss by a retrospective 15 year study. A marginal bone loss of

1.5mm is evident from first thread during healing and in the first year after loading was noted

from his study. Thereafter an average 0.1mm bone loss was noted annually.

To minimize the marginal bone loss and for better esthetic outcome, platform

switched implants were introduced over conventional platform matched implants. In

conventional platform matched implants the abutment diameter is matched with the implant

diameter. In platform matched implant both the implant diameter and abutment diameter are

the same.

Introduction

Page 2

Platform switching concept implies the use of under sized prosthetic platform

than the implant platform. The prosthetic platform is shifted inwards from the perimeter of

implant platform, thereby creating a step, or angle, between the implant and abutment6.

The implant abutment junction (IAJ) in platform matched implant will be along

the implant perimeter, but in platform switched implants the IAJ gets shifted medially from

the implant perimeter. The micro gap between the implant and abutment in the IAJ harbors

lot of micro organisms which in turn leads to the collection of inflammatory cell infiltrate

(ICT) around the IAJ .This ICT leads to bone loss of 1.5mm around IAJ.As the IAJ gets

shifted medially in platform switched implants the bone loss will be coronal compared with

platform matched implants.

Implant manufacturers have introduced pre angled abutments available from 15°

to 35°.Custom made abutments can also be fabricated according to the individual situations.

Many clinical comparative studies have showed no significant difference in bone

loss and survival rates between platform matched straight and angled abutment3,7,8,9

.How ever

the photo elastic studies and strain gauge measurements 10

and finite element analyses11

revealed that platform matched angled abutment are subjected to more stress. Finite element

analysis by Xavier et al12

suggests that there was 15 % more strain in platform matched

straight abutments than platform matched angled abutments.

There are only few investigators who compared the straight and angulated

abutment conditions with respect to platform switched implant13,14,15

.A finite element

analysis by martini et al 13

states that implants with platform switched straight abutment

generates the highest stress value, but another study shows that platform switched angulated

abutments produce more stress on peri-implant bone than the straight abutments14

.

Introduction

Page 3

So far investigators have included only straight and 15° angled platform switched

abutments for their study. They have not included platform switched angled abutments with

more than 15°, but in clinical situations we may have to use more the 15° angulated

abutments, for better esthetic outcome. The purpose of the present study is to compare the

effect of straight (0°) and abutments of various degree angulation (15°, 20°, 25°) on stress

and strain distribution around a platform switched implant using three dimensional finite

element analysis.

Aim and objectives

Aim and objectives

Page 4

AIM

The aim of the present study is to compare the effect of straight (0°) and angulated abutments

(15°, 20°, 25°) on stress and strain distribution around a platform switched implant placed in

the anterior maxilla using three dimensional finite element analysis.

OBJECTIVES

The objectives of the present study are

1.To evaluate the von Mises stress and strain values in the cortical and cancellous bone,

around platform switched implants with straight abutments (0°) and abutments with various

angulations (15°,20°,25°).

2. To compare the von Mises stress and strain values between the cortical and cancellous

bone, around the platform switched implants with various angulations (0°,15°,20°,25°).

3.To compare the von Mises stress and strain values in cortical bone and cancellous, around

the platform switched implants between various abutment angulations (0°,15°,20°,25°).

4.To compare the von Miss stress and strain values around the implants in all of the above

mentioned conditions with 0° on axis load of 178 N along the long axis of abutment and off-

axis load of 178 N around 45° to the long axis of abutment.

Review of literature


Page 5

REVIEW OF LITERATURE

Atwood 22

in 1962 explained the physiology behind the resorption of residual alveolar ridges.

Atwood concluded that the rate of resorption of alveolar ridges varied among different

individuals. The factors related to the rate of resorption are divided in to anatomic, metabolic,

functional and prosthetic factor.

Adell et al 5

in 1981 studied the osseointegration of implants placed in both edentulous

maxilla and mandible. They followed the patients for 15 years and concluded that the mean

marginal bone loss after the first year of implant placement was 1.5mm.There after 0.1 mm of

bone was lost annually.

Charles A. Babbush ,and Mari Shimura,23

in 1993 evaluated patients who were treated

with IMZ system for five years. With statistical and clinical observation, they concluded that

larger diameter implants had higher survival rates than small diameter implants. The implants

in maxilla had lower survival rate than implants in the mandible.

Nancy L. Clelland, Amos Gilat, Edwin. McGlumphy, William A. Brantley 35

in 1993

conducted a photo elastic study and strain gauge measurements to determine the level of

stress and strain for angulated abutments. They concluded that there was a significant

increase in stress and strain for each, with increase in abutment angulation. Highest stresses

were found in regions closer to the fixture.


Page 6

Nancy L. Clelland, DMD, MSD, K. Lee, Olivier C. Bimbenet, MS, and William A.

Branthy36

in 1995 studied the effect of abutment angulation on stress and strain around the

implant using finite element study. They concluded that there was an increase in magnitude

of stress and strain as the abutment angulation increased.

Cany et al 11

in 1996 studied the stress distribution around the vertical and angled implant

with finite element study. They concluded that the angled implant showed more stress around

the implant in the cervical region.

George Papavasiliou, Phophi Kamposiora,Stephen C. Bayne, and David A. Felton 31

in

1996 did finite element analysis on stress distribution around single tooth implants and

concluded that there were no differences between types of veneering materials and the

absence of cortical bone increased the inter facial stresses. Oblique load increased stress by

15 times than axial load.

Balshi et al 7 in 1997 studied about the clinical outcome of angulated abutments. They used

angulated abutments and a combination of angled and standard abutments on 71 patients and

did a follow up for 3 years. They concluded that angulated abutments showed good

preliminary results and should be compared to the standard abutment as a predictable

modality in prosthetic rehabilitation.


Page 7

Brosh et al 10

in 1998 compared two experimental techniques for analyzing stress and strain

around implants. According to the author strain gauges were reliable to study the strain

around the implants, where as a photo elastic study can be regarded as a complimentary

method. They concluded that strain values were more for angled abutments than straight

abutments.

Ashok et al 3

in 2000 studied about the clinical success of angulated abutments between 0° to

45° .He concluded that angled abutments can be comfortably used in situations with

compromised bone. The esthetic and functional outcome was satisfactory. There was no

significant difference between the clinical outcome of straight and angulated abutments.

Dorthy et al 4

in 2000 compared the success of implants placed with standard and angulated

abutments. They compared the parameters like probing depth, gingival level, gingival index,

and mobility. They concluded that there was no significant difference in those parameters

between standard and angulated abutments. So it was suggested that angled abutments could

be a suitable restorative option.

Geng et al 16

in 2001 reviewed that Finite element analysis (FEA) has been used extensively

to predict the biomechanical performance of various dental implant designs as well as the

effect of clinical factors on implant success. This article reviewed the current status of FEA

applications in implant dentistry and discussed findings from FEA studies in relation to the

bone–implant interface, the implant–prosthesis connection, and multiple-implant prostheses.


Page 8

Rickard Brånemark,P-I Brånemark,Björn Rydevik, Robert R. Myers 1

in 2001 reviewed

about the concept of osseointegration and attempted to highlight the key developments in the

research and application of osseointegration. In this article the author defines osseointegration

and osseoperception. He explains in detail about the clinical applications of osseointegration.

Kaus et al 8

in 2002 described the concept of evaluation of angulated abutments ,which was

originally developed first for the external hex implants .Then the concept was evolved to use

in internal hex Morse taper connections .Authors have conducted a study for 151 months

and total of 3101 implants were placed with 0 degree to 45 degree angled abutments. They

concluded that the clinical out come of implants with angulated abutments were satisfactory

and could be successfully used in implant rehabilitation.

Seivimay et al 20

in 2005 studied about stress concentrations around implant supported

crowns in different bone qualities .Authors concluded that among the different qualities of

bone D3 and D4 bone produced more stress around the implant. The highest stress

concentration was at the in neck of the bone.

Richad.J.Lazzara,Stephan.S.Porter28

in 2006 reviewed about the biological dimensions of

hard and soft tissues around platform switched dental implants. Authors concluded that there

were many advantages in platform switched implants over platform matched implants and

supported the concept


Page 9

Jivraj et al 2 in 2006 explained the challenges in implant rehabilitation of the maxilla. They

stated that the amount and quality of available bone will be less after extraction of teeth in the

maxilla. Further esthetic concern is also an important .This article has compared the different

treatment options available for treatment of edentulous maxilla and explained about the

importance of diagnosis and treatment planning in such situations.

Xavier et al12

in 2007 studied the effect of abutment angulation on the strain on the bone

around an implant in the anterior maxilla. He concluded that the strain values were 15%

higher in implants placed with straight abutments compared to the implants with angulated

abutment.

Ming-Lun Hsu, Fang-Ching Chen,Hung-Chan Kao, Cheng-Kung Cheng 24

in 2007

conducted a finite element analysis on off-axis loading and concluded that to achieve a

favourable prognosis, axial loading is recommended. Off-axis loads produce more stress than

vertical loads.

Jose Henrique Rubo, Edson Antonio Capello Souza32

in 2008 conducted a finite element

study to find the stress distribution around the dental implants. They concluded that the stress

distribution was better with stiffer bone, longer abutments and implants with shorter

cantilevers. The use of co-cr alloy framework appears to contribute to better stress

distribution.


Page 10

Hung-Chan Kao, Yih-Wen Gung,Tai-Foong Chung,Ming-Lun Hsu, Dr Med Dent33

in

2008 investigated the micromotion between the implant and surrounding bone caused by the

implementation of an angled abutment for an immediately loaded single dental implant

located in the anterior maxilla and concluded that abutment angulation up to 25 degrees can

increase the stress in the peri-implant bone by 18% and the micromotion level by 30%.

Chun-Li Lin, Jen-Chyan Wang, Lance C. Ramp, Perng-Ru Liu34

in 2008 studied the

biomechanical response of implant system placed in the maxillary posterior region and

concluded that better stress/strain distribution is possible when implants are placed along the

axis of loading with good cortical contact.

Francesco Carinci ,Giorgio Brunelli, Matteo Danza30

in 2009 studied about the bone

platform switching and conventional implants. Bone platform switching involves an inward

bone ring formation in coronal part of implants, obtained by using a dental fixture with

reverse conical neck. They concluded that there was no difference in survival and success

rates between conventional vs reverse conical neck implant.

Matteo Danza et al 39

in 2009 concluded that lowest stress value was found in the system

with straight abutment loaded with vertical force while highest stress value was found in

implants with 15° angulated abutment loaded with angulated force.

Cavallaro et al 9

in 2011 reviewed the usage of angled abutments in implant rehabilitation.

They concluded that angled abutments not only had satisfactory clinical outcome, but they

also facilitated the paralleling of non aligned implants.


Page 11

Chun-Yeo Ha, Yung-Jun Lim, Myung-Joo Kim, Jung-Han Choi 37

in 2011 compared the

removal torque values of different abutments(straight, angled and gold premachined UCLA-

type) in external and internal hex implants after dynamic loading with clinical situation of the

anterior maxilla. They concluded that there was no significant difference in removal torque

value of internal hex implants.

Haibin, Zhiyong, Jinxin, Tao, Zaibo, Chuncheng 27

in 2011 compared the stress

distribution of non platform switched and platform switched abutment for implants supported

single crown with finite element analysis and concluded that when platform switched

abutment were used, the maximum Von mises stress with the surrounding bone was lower.

However, this value is higher with in the fixture and screw.

Alper Gultekin, Pinar Gultekin and Serdar Yalcin21

in 2012 explained in detail about the

application of finite element analysis in implant dentistry. They explained in detail about the

basics of finite element analysis and steps in analysing the stress strain pattern in detail.

Paula et al13

in 2012 studied about the stress around the platform switched straight abutment

and platform switched angled abutment with finite element analysis .They concluded that

platform switched straight abutment showed more stress around the implant than the platform

switched angled abutments. Further they stated that oblique load increased the stress than

axial load.


Page 12

Rohit Bahuguna et al 25

in 2013 evaluated the stress pattern in bone around dental implants

.Authors found that as the abutment angulation increased from 0° to 20°both compressive and

tensile stresses also increased around the implants.

Angel Alvarez-Arenal, Luis Segura-Mori,Ignacio Gonzalez-Gonzalez, Angel Gago 26

in

2013 concluded that platform switching reduced the stress values on the abutment and

retention screw. The stress on abutment screw gradually increased as the loading direction

changed from vertical to oblique.

Kumar et al19

in 2013 studied about stress distribution around implants with straight and

angulated abutments in different bone qualities and concluded that angled abutments

produced more stress than straight abutments. The stress in D4 quality bone was more when

compared to D1 quality bone .The high stress in the angled abutment at the cervical zone was

due to forces and momentum around the cortex.

Paul, et al 15

in 2013 studied about the strain generated in bone by platform switched and non

platform switched implants with straight and angulated abutments under vertical and

angulated load with finite element analysis. The results of this investigation indicated that the

ideal values of microstrain (50-3000 microstrain) could be exhibited by platform switching of

dental implants (with an abutment–implant diameter difference of 1 mm) and could be

considered as a better alternative for prevention of crestal bone loss when compared to non–

platform switched implant.


Page 13

Martini et al 14

in 2013 have done a finite element study to find out the influence of platform

switching and angulated abutments on surrounding bone. They concluded that platform

switched implant showed less stress in cortical bone around the fixture head than platform

matched implants. Further angulated abutments showed more stress than straight abutments.

Kalavathy et al6

in 2014 reviewed the concept of platform switching in implants. They

described about the factors that could lead to crestal bone loss. According to the authors

platform switching concept effectively reduced the crestal bone loss by reducing the stress

around the bone and shifting the implant abutment junction medially towards the centre of the

implant. There by the biological width of 1.5 mm could be maintained with reduced crestal

bone loss.

Pradeep Bholla, Liju Jacob Jo1, Kalepu Vamsi, Padma Ariga 38

in 2014 conducted a

finite element study about the stress pattern at bone implant interface by angulated abutments.

They concluded that von mises stresses were more concentrated in cortical bone and more

stress was seen in the crestal region. When the angulations were increased the stress around

the implants also increased .Oblique loads increased the stress around implants than vertical

loads.


Page 14

Yousuf Aseel KP, SripathiRao BH , Hassan Sarfaraz , Joyce Sequiera , Gunachandra

Rai , Jagadish Chandra 29

in 2015 studied radiographically about the crestal bone loss

around platform shifted and non platform shifted implants. They concluded that there was a

significant difference between the crestal bone loss among the two types of implants after 6

months of functional loading. Platform switched implants produced less bone loss than non

platform switched implants.

Mohamed A. Elsadek , Hesham A. Katamesh and Hanaa I. Sallam40

in 2016 evaluated

sthe effect of implant platform switching on strain developed around implants with straight

and angulated abutments using strain meter. They concluded that straight implants with

straight platform-switched abutments were associated with the least microstrain values.

Materials and methods


Page 15

MATERIALS AND METHODS

A three dimensional finite element study was under taken to create model and

analyse the situation. Finite element analysis was chosen to do this to determine the stress and

strain around the dental implant and to study the mechanical behavior of complex structures

easily by dividing the complex structures in to numerous small simple structures16

.

Bone model

Lekholm and Zarb17

have explained the classification system of bone as follows:

Based on its radiographic appearance and the resistance at drilling, bone quality has been

classified in four categories:

Type 1(D1) bone -the entire bone is composed of homogenous compact bone;

Type 2(D2) bone -a thick layer of compact bone surrounds a core of dense trabecular bone;

Type 3(D3) bone -a thin layer of cortical bone surrounds a core of dense trabecular bone; and

Type 4(D4) bone -a thin layer of cortical bone surrounding a core of low density trabecular

bone of poor strength. These differences in bone quality can be associated with different

areas in the upper and lower jaw.

In this study the bone properties approximating those of D3 bone was used since 65% of bone

found in the premaxillae is of D3 type12

Maxillary bone was modelled as a section simulating the pre maxillary area with

cortical bone thickness of 1.5 mm enclosing the trabecular bone core.

The bone block was modelled with 18 mm height from base to crestal bone and 8

mm length mesio distally and 8 mm width bucco lingually.


Page 16

Implant model

A solid tapered, screw type ,root form , commercially pure titanium implant of 13

mm length and 4.3 mm diameter is modelled and simulated to be placed in the section of

bone 15

[Figure 1]. The dimensions of implant fixture including thread design and pitch were

simulated with Noble Replace platform switching implants (Noble replace, Nobel Biocare,

Goteborg, Sweden). The dimensions were obtained by the noble replace implant manual18

.

The implant was modelled with collar diameter of 4.3 mm and tip diameter of

2.56mm.Collar height was designed as 1.5 mm and thread height is of 12.07 mm. The pitch

of the thread is of 0.71 mm.

Straight abutments(0°) and angulated abutments (15°,20°,25°) of 3.5 mm

(platform switched) diameter are simulated with 10° occlusal taper and 7 mm.

Three dimensional finite element models were constructed for the following

configurations.

PS 0-an implant fixture (4.3mm) with platform switched 0° (3.5 mm) straight abutment

(figure 2 a).

PS 15-an implant fixture (4.3mm) with platform switched 15° (3.5 mm) angled abutment

(figure 2 b).


(figure 2 c).


(figure 2 d).

[PS-Platform switched]


Page 17

Each of these implants were placed in four simulated premaxillary models of D3

bone quality.

Three dimensional models of the implant, bone and abutments have been

fabricated using Pro/Engineer Wildfire 2.0 software (Parametric Technology Corp, Needham,

MA, USA).Thus total numbers of four simulated premaxillary models with different

abutment angulations for platform switched implants were generated.

The analysis was performed using the software ANSYS Workbench 15.0(Santa

Monica, CA, U.S.A).The models were processed with ANSYS to generate a meshed

structure. Meshing divides the entire model in to smaller elements which are interconnected

at specific joints called nodes. The default number of elements and nodes used for each

model is shown in Table 1(a).

All the materials used in the models were considered to be isotropic,

homogenous, and linearly elastic. The osseointegration of implant was accepted as 100%.

Since there are no universally accepted properties of the biologic materials available in the

literature, a mean value of the material properties has been used in the present study19,20

and

have been tabulated in Table 1(b).

Young's modulus is the ratio of stress (which has units of pressure) to strain

(which is dimensionless), and so Young's modulus has units of pressure. Its SI unit is

therefore the pascal (Pa ). The practical units used are megapascals (MPa) or gigapascals

(GPa).

Poisson's ratio is the ratio of transverse strain to axial strain. In other words,

Poisson`s ratio is the amount of transversal expansion divided by the amount of axial

compression, for small values of these changes.

https://en.wikipedia.org/wiki/Stress_%28physics%29

https://en.wikipedia.org/wiki/Pressure

https://en.wikipedia.org/wiki/Strain_%28materials_science%29

https://en.wikipedia.org/wiki/Dimensionless_quantity

https://en.wikipedia.org/wiki/Pressure

https://en.wikipedia.org/wiki/SI

https://en.wikipedia.org/wiki/Pascal_%28unit%29

https://en.wikipedia.org/wiki/Strain_%28materials_science%29


Page 18

A simulated on axis load of 178N was applied at the centre of incisal edge, along

the long axis of each abutment and a simulated off-axis load of 178 N was applied at the

centre of incisal edge, 45° to the long axis of the abutment.

The amount of load selected in this study is based on the literature on average

biting force for incisors12,19

. The forces applied were static and von mises stress values

around the implants were recorded.

In finite element analysis overall stress state at a point are summarized with von

Mises stresses. All the materials including cortical bone, trabecular bone, titanium implant

and titanium abutment were assumed to be linear, elastic, homogenous and isotopic.19, 20

.

BASIC CONCEPT OF FINITE ELEMENT ANALYSIS

Finite element analysis is a practical application of finite element method, which

is used by researchers and scientists to create a complex mathematical problem and to solve

the problem by dividing the complex problem domain into numerous simple

domains(elements) and numerically solving the problem.

Finite element analysis was first introduced in aerospace industry in 1960s to

solve structural problems. In late 1980s it was introduced to implant dentistry by

Weinstein16,21

.

In finite element method the actual complex structure is divided in to numerous

small simple structures called as finite elements. The finite elements are the divided, smaller

and simpler parts of a complex domain. These elements inside the actual complex structure

are inter connected by numerous nodes. The collection of numerous elements and nodes

inside the complex structure is called mesh.


Page 19

The nodes lie on the boundary of the elements where adjacent elements are

connected. After meshing the next process is to define the boundary condition. In structural

analysis, boundary conditions are applied to the regions of the model where the

displacements and/or rotations are known21

. Such regions may be designed to remain fixed

(have zero displacement and/or rotation) during the simulation or may have specified,non-

zero displacements and/or rotations. The directions along which motion is possible are called

degrees of freedom21

.

The process of creating the mesh, elements and their respective nodes, and

defining boundary condition is termed as “discretization” of the problem domain. Then the

mechanical properties of desired materials are incorporated in the mesh to create a working

model.

After meshing and defining the boundary condition of the model, the loads to be

applied are defined and the results are reviewed.


Page 20

FUNDAMENTALS OF DENTAL IMPLANT BIOMECHANICS IN FEA

Photoelasticity is a method to determine the stress distribution in a material

experimentally. The method is mostly used where mathematical methods become quite

difficult. Unlike the analytical methods of stress determination, photoelasticity gives a fairly

accurate picture of stress distribution around discontinuities in materials. The method is an

important tool for determining critical stress points in a material, and is used for determining

stress concentration in irregular geometries. But it does not give accurate stress value at a

point.

A strain gauge is a device used to measure strain on an object. The strain gauge

consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is

attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed,

causing its electrical resistance to change. This resistance change, usually measured using a

Wheatstone bridge, is related to the strain by the quantity known as the factor. But this

method is not so accurate because numerous factors like temperature, humidity and

permanent deformation of material will affect the results and reproducibility of this method.

Since dental implant –bone system is considered as a complex structure, finite

element method is used to mathematically model the system. Finite element analysis has

been found to be the most suitable and predictable tool to evaluate the effect of stress and

strain around the implant and bone16

.

https://en.wikipedia.org/wiki/Stress_analysis

https://en.wikipedia.org/wiki/Discontinuity_%28mathematics%29

https://en.wikipedia.org/wiki/Deformation_%28mechanics%29#Strain

https://en.wikipedia.org/wiki/Electrical_insulation

https://en.wikipedia.org/wiki/Electrical_resistance

https://en.wikipedia.org/wiki/Wheatstone_bridge


Page 21

Finite element analysis has some advantages than other methods (photo elastic

analysis, strain gauge analysis) of stress analysis. They are

1. It is a non invasive technique.

2. The alveolar bone, tooth and the implant can be simulated according to the

material properties of these structures in order to achieve nearest possibly simulating in vitro

oral conditions

3. The actual stress experienced at any point can be measured.

4. Graphical visualisation of the actual implant displacement is possible.

5. Reproducibility does not affect the physical properties of involved materials and the study

can be repeated to any number of times.

6. Accurate modelling of complicated real shapes can be done.


Page 22

ARMAMENTARIUM

1. INTEL CORE i3-PROCESSOR

2.2.13 GHZ SPEED

3.3 GB RAM

4.320 GB HARD DISC DRIVE

5.52 x CD ROM

6.15 INCH COLOUR MONITOR

7. WINDOWS 10.64 -BIT BASED PROCESSOR

8. PRO-ENGINEERING WILD FIRE SOFTWARE

9. ANSYS WORKBENCH 15.0 FINITE ELEMENT SOFTWARE

10. KEY BOARD

11. MOUSE


Page 23

Table 1(a):Number of elements and nodes

Models Elements Nodes

PS 0 2239 6351

PS 15 2335 6756

PS 20 2366 6916

PS 25 2299 6588

Table 1 (b) :Material properties used in FEA Study

Material Youngs Modulus(GPa) Poisons ratio

Titanium abutment & implant 110 0.35

Dense trabecular bone

(D1,D2&D3)

1.37 0.3

Low density trabecular bone

(D4 Bone)

1.10 0.3

Cortical bone 13.7 0.3


Page 24

Figure 1 (a):

Implant diameter-4.3 mm and Implant height-13 mm

A. Collar height- 1.5 mm.

B. Thread pitch- 0.71 mm.

C. Major diameter-4.3 mm.

D. Minor diameter -3.67 mm.

E. Thread height -12.07 mm.

F. Overall length- 13.6 mm.

G. Tip diameter- 2.56 mm.

Figure 1(a): Dimension of Implant

fixture.

Figure 1 (b): bone model with

implant and platform matched

straight abutment


Page 25

FIGURE 2 (A): PS 0- AN IMPLANT

FIXTURE (4.3MM) WITH PLATFORM

SWITCHED 0° (3.5 MM) STRAIGHT

ABUTMENT.

FIGURE 2 (B): PS 15- AN IMPLANT


SWITCHED 15° (3.5 MM) ANGULATED

ABUTMENT.


Page 26

FIGURE 2 (C): PS 20 - AN IMPLANT



ABUTMENT.

FIGURE 2 (D): PS 25 - AN IMPLANT



ABUTMENT.


Page 27

Figure 3: FEM mesh created by an

analyst prior to finding a solution to a

problem using FEM software.

https://en.wikipedia.org/wiki/Polygon_mesh

Results

Results

Page 28

RESULTS

Stress distribution was represented numerically and it was colour coded. The maximum von

Mises stress around the platform switched straight abutment for axial load was 12.79 Mpa

and maximum von Mises stress value was increased with increase in abutment angulation

(Table-2). The maximum value around platform switched 25°abutments for axial load was

recorded as 40.12 Mpa. The von Mises stress value around the platform switched implant in

cancellous bone follows the same pattern and was less when compared with cortical bone, the

values ranged between 0.60 Mpa (for straight abutment) to 1.12 Mpa (for 25° angled

abutment) (Table-3).

The maximum von Mises stress around the platform switched straight abutment for 45° off-

axis load was 84.13 Mpa and maximum von Mises stress value was increased with increase

in abutment angulation (Table-2). The maximum value around platform switched

25°abutments for axial load was recorded as 157.32 Mpa. The von Mises stress value around

the platform switched implant in cancellous bone followed the same pattern and was less

when compared with cortical bone, the valued ranges between 2.03 Mpa (for straight

abutment) to 4.44 Mpa (for 25° angled abutment) (Table-3).

The maximum von Mises strain value for cortical bone in platform switched abutments for

axial load increased with increase in abutment angulation. Strain has no unit, but it can be

converted in to microns (10-6

) and expressed as microstrains, the microstrain value ranged

between 882 micro strains for straight abutment to 3220 microstrains for 25 ° abutment.

(Table 4).The strain value for cancellous bone was less when compared to cortical bone, and

ranged between 448 micro strains for straight abutment and 809 microstrains for 25°

angulated abutment. (Table 5).

Results

Page 29

The Maximum von Mises strain value in off-axis loading for platform switched abutments

increased with increase in abutment angulation and ranged between 6904 microstrains for

straight abutment to 13313 microstrains for 25° abutment (Table 4).The strain value for

cancellous bone was less when compared to cortical bone and ranged between 1505

microstrains for straight abutment and 3154 microstrains for 25° angulated abutments.

(Table 5).

The maximum von Mises stress and strain values for platform switched implant with 45° off-

axis load increased several folds when compared to platform switched implant with axial load

for all abutment angulations.

Results

Page 30

Table -2

The value of von Mises stress for the models with platform switched abutments.(units in

Mpa)

Abutment angulation Cortical bone (in Mpa)

Axis load Off-axis load

Straight 0° abutment 12.79 84.13

15 °angulated abutment 24.66 157.32


25 °angulated abutments 40.12 175.48

Table -3

The value of von Mises stress for the models with platform switched abutments.(units in

Mpa).

Abutment angulation Cancellous bone (in Mpa)


Straight 0° abutment 0.60 2.03



25 °angulated abutments 1.12 4.44

Results

Page 31

Table -4

The value of von Mises strain for the models with platform switched abutments in

micro strains (strain × 10 -6

).

Abutment angulation Cortical bone

(in microstrains)


Straight 0° abutment 982 6904

15 °angulated abutment 2672 12035


25 °angulated abutments 3220 13313

Table -5

The value of von Mises strain for the models with platform switched abutments in

micro strains (strain × 10 -6

).

Abutment angulation Cancellous bone

(in micro strains)


Straight 0° abutment 448

1505



25 °angulated abutments 809 3154

Results

Page 32

Bar diagram showing stress values (in Mpa) with in cortical bone around the implant

Bar diagram showing stress values (in Mpa) with in cancellous bone around the implant

0

20

40

60

80

100

120

140

160

180

0 °abutment 15° abutment 20°abutment 25°abutment

12.79

24.66 34.07

40.12

84.13

157.32 166.52

175.48

axial load

off-axis load

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 ° abutment 15°abutment 20°abutment 25°abutment

0.6 0.88

1.01 1.12

2.03

3.88 3.79

4.44

axial load

off-axis load

Results

Page 33

Bar diagram showing strain values (in micro strains) with in cortical bone around the

implants

Bar diagram showing strain values (in micro strains) with in cancellous bone around

the implants

0

2000

4000

6000

8000

10000

12000

14000

0 ° abutment 15 °abutment 20° abutment 25 °abutment

982

2672 3087 3220

6904

12035 12903

13313

axial load

off-axis load

0

500

1000

1500

2000

2500

3000

3500

0 °abutment 15 °abutment 20° abutment 25° abutment

448 677 755 809

1505

2562 2787

3154

axial load

off-axis load

Results

Page 34

FIGURE 4 A: PICTORIAL REPRESENTATION OF STRESS VALUES IN

STRAIGHT ABUTMENT FOR AXIAL LOAD.

FIGURE 4 B: THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL AND

CANCELLOUS BONE HAS BEEN SHOWN WITH TAGS.

Results

Page 35

FIGURE 5 A :PICTORIAL REPRESENTATION OF STRESS VALUES IN

STRAIGHT ABUTMENT FOR OFF-AXIS LOAD.

FIGURE 5 B: THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL AND


Results

Page 36

FIGURE 6 A :PICTORIAL REPRESENTATION OF STRESS VALUES IN 15°

ANGLED ABUTMENT FOR AXIAL LOAD.

FIGURE 6 B : THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL

AND CANCELLOUS BONE HAS BEEN SHOWN WITH TAGS.

Results

Page 37

FIGURE 7 A: PICTORIAL REPRESENTATION OF STRESS VALUE IN 15°

ANGLED ABUTMENT FOR OFF-AXIS LOAD.

FIGURE 7 B :THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL AND

CANCELLOUS BONE WAS SHOWN WITH TAG

Results

Page 38

FIGURE 8 A :PICTORIAL REPRESENTATION OF STRESS VALUES IN 20°




Results

Page 39

FIGURE 9A :PICTORIAL REPRESENTATION OF STRESS VALUES IN 20°


FIGURE 9B: THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL AND


Results

Page 40

FIGURE 10A :PICTORIAL REPRESENTATION OF STRESS VALUES IN 25°


FIGURE 10B: THE MAXIMUM VON MISES STRESS VALUES IN CORTICAL


Results

Page 41

FIGURE 11 A:PICTORIAL REPRESENTATION OF STRESS VALUES IN 25°




Results

Page 42

FIGURE 12 A :PICTORIAL REPRESENTATION OF STRAIN VALUES IN

STRAIGHT ABUTMENT FOR AXIAL LOAD.

FIGURE 12 B:THE MAXIMUM VON MISES STRAIN VALUES IN CORTICAL


Results

Page 43

FIGURE 13: A PICTORIAL REPRESENTATION OF STRAIN VALUES IN

STRAIGHT ABUTMENT FOR OFF-AXIS LOAD.

FIGURE 13 B: THE MAXIMUM VON MISES STRAIN VALUES IN CORTICAL


Results

Page 44

FIGURE 14 A PICTORIAL REPRESENTATION OF STRAIN VALUES IN 15°


FIGURE 14 B: THE MAXIMUM VON MISES STRAIN VALUES IN CORTICAL


Results

Page 45

FIGURE 15 :A PICTORIAL REPRESENTATION OF STRAIN VALUES IN 15°


FIGURE 15 B : THE MAXIMUM VON MISES STRAIN VALUES IN CORTICAL


Results

Page 46

FIGURE 16 A :PICTORIAL REPRESENTATION OF STRAIN VALUES IN 20°




Results

Page 47





Results

Page 48





Results

Page 49





Discussion

Discussion

Page 50

DISCUSSION

The close relationship between the tooth and alveolar process continues

throughout life. Wolff’s law (1892) states that bone remodels in relation to the forces applied.

Every time the function of bone is modified, a definite change will occur in the internal

architecture and external configuration17

.

The greater the magnitude of stress applied to the bone, greater will be the strain

observed. Bone modelling and remodeling are primarily controlled by the mechanical

environment of the strain. The density of alveolar bone evolves as a result of mechanical

deformation from the microstrain. In the theory of mechanostat, H. M. Frost15

proposed that

the bone mass is the direct result of the mechanical usage of skeleton. A model of four zones

for the compact bone as related to mechanical adaptation to the strain has been proposed: The

pathologic overload zone (greater than 3000 microstrain), mild overload zone (1500-3000),

adapted window (50-1500), and acute disuse window (0-50). Crestal bone loss will be often

evidenced during the early implant loading, as the result of bone in the pathologic overload

zone (excess stress and strain at the implant–bone interface). Stress is seen to be greatest at

the crest, compared with other regions in the implant body. An optimum strain environment

will exist for each specific anatomical area and the peak strains innate for that area should be

maintained to optimize the bone’s response15

.

Bone resorption is a common phenomenon in the residual alveolar ridge. After

extraction the pattern of bone loss cannot be predicted in anterior maxilla22

.Survival rate of

implant is less when placed in anterior maxilla than in anterior mandible23

. As the bone

volume is less in anterior maxilla than the mandible, long term prognosis will be less in

maxilla.

Discussion

Page 51

After extraction in the anterior maxillary region there will almost be twice the

amount of horizontal bone resorption when compared to the vertical bone resorption2. This

situation can be managed either by surgical management or by placing implants in areas of

greatest bone availability. This change in implant angulations can be corrected by placing

angulated abutments during prosthetic rehabilitation. There are wide ranges of pre angled

abutments available in the market. Additionally custom made abutments can also be made

according to the prosthetic situation.

In the present study a bone block of D3 type was modeled as a section simulating

the pre maxillary area, since 65% of bone found in the premaxillary area is of D3 type. As,

already discussed, after extraction the bone width will reduce rapidly in the premaxillary

area, so implants with wide diameter are not commonly used in the premaxillary region. The

available height of bone will be more when compared to posterior regions, because

anatomical limitations are less for selecting implant length in the premaxillary area. So with

the most commonly available width and height of bone in the premaxillary area, the implant

dimensions of 4.3 mm diameter with 13 mm length can be placed in the premaxillary area.

So, the implant dimension modeled in the study was selected as 4.3 mm in width and 13 mm

in length.

Even though only 1mm of bone around the implant is sufficient for implant

placement, for convenience to study the flow of stress pattern around wider areas, bone was

modeled with 8 mm width and 18 mm height.

The abutment diameter platform switch was modeled according to the

manufacturer instructions(Noble replace platform switch-Noble bio care, Goteborg ,Sweden)

.The thread dimensions for the implant was obtained from the noble replace implant

Discussion

Page 52

manual(Noble replace platform switch-Noble bio care ,Sweden) and has been discussed with

more details in the materials and methodology section.

For evaluating stress and strain around the implant, finite element analysis was

used. Even though finite element study values are quantitatively not reliable, three

dimensional finite element study is qualitatively reliable and is an excellent tool for

comparative study.

The mean value of the material properties are chosen from literature for titanium

alloy, cancellous and cortical bone12,19

. A simulated load of 178 N was applied at the centre

of the abutment. The load value was selected according to the average biting force of the

incisors and it was chosen from literature12,19

.

During incising, the load will be directed towards the long axis of the tooth .The

loading condition along the long axis of the abutment was simulated in accordance with that.

During eccentric movement lateral load will be applied to the long axis of the tooth, so the

off-axis loading condition was simulated in accordance with that24,25

.

Platform switching was introduced to reduce the crestal bone loss and increase

the implant survival. Platform switching concept implies the use of undersized abutment

diameter than the implant diameter. There by the implant abutment junction is shifted from

perimeter of implant to the central axis of implant6.

Platform switching concept works on the basis of reduced stress and strain

around the crest of the marginal bone. As the perimeter of implant shifts to central axis, the

stress concentration around the implant is reduced. These results were concluded in many

other studies14,26,27

.

Discussion

Page 53

Other than the reduction in stress concentration platform switched implants have

many other advantages to reduce the crestal bone loss. The important factor is shifting the

implant abutment junction medially from the implant perimeter. Implant abutment junction

(IAJ) will be located in the implant perimeter for platform matched abutment. Whatever may

be the seal between implant and abutment there will be small micro gap between the implant

and abutment. This micro gap leads to the collection of inflammatory cell infiltrate (ICT) in

the implant abutment junction. The toxins from the inflammatory cell infiltrate will induce

bone loss up to 1.5mm6, within one year after loading.

So in all two piece implants there will be a initial bone loss of around 1.5 mm to

2mm from the implant abutment junction. So this implant abutment junction will maintain a

biological width (junctional epithelium + connective tissue) of 1.5mm to 2mm from IAJ to

the crest of peri-implant bone. As the IAJ gets shifted medially in platform switched

implants, the biological width will be shifted coronally as compared to the platform matched

implants. This biological width along with the soft tissue thickness will maintain a minimum

soft tissue seal of 3mm28

.

More over the angle of spread of the toxins from implant abutment junction in

platform matched implant is vertical (180°) .In platform switched implant as the implant

abutment junction is shifted medially, the angle of spread of toxins is horizontal (90°).So this

horizontal angulation reduces the accessibility of the toxins to the bone. This is also an

important factor in platform switched implant for the reduction in crestal bone loss around the

implants28

.

This concept of platform switching starts from second stage of surgery during

conventional implant placement. This usage of undersized prosthetic platform should be

followed by placing a healing collar of reduced diameter, than the implant fixture head28

.

Discussion

Page 54

Even though the stresses in the bone around the platform switched implant is less

than platform matched abutment, finite element studies by Haiben et al27

and Alvarez-renal et

al27

showed that, the stress in the implant fixture and abutment screw was more for platform

switched implants than platform matched implants, in axial loading. The possible reason for

lower stress in the platform matched implants may be due to the greater diameter of

abutment, which distributes the loads better as a result of increased contact area between

implant and abutment. In platform switched implants the size of the contact area between

implant and abutment is less. Therefore, less stress from the abutment is transferred to the

surrounding bone and more stress is concentrated within the fixture and screw. This may

cause problems such as fixture and screw deformation or even fracture, if over the elastic

limit.

Apart from the finite element analysis studies, there are some radiographic studies

which conclude that there will be less crestal bone loss in platform switched implants than

platform matched implants after loading29

.

Bone platform switching30

is a concept which was introduced by using the reverse

conical neck implant fixture. These type of implants produce more amount of residual crestal

bone volume around the implants, thereby reducing the stress in crestal alveolar bone area,

repositioning of gingival papilla on the bone ring ,and a proper vascular supply to hard and

soft tissues in case of reduced inter implant distance. But it was shown that reverse conical

neck by itself is not enough to reduce the crestal bone loss. The medial shift in implant

abutment junction is needed to reduce the crestal bone loss significantly.

There are many studies on implant biomechanical behavior which conclude that

stresses are more concentrated in bone-implant interface at the level of crestal bone31, 32, 33,

34.After the loading of implants, crestal bone loss and early implant failure occurs due to

Discussion

Page 55

excess stress concentrated in the implant bone interface. This phenomenon is explained in

many finite element stress evaluation studies35, 36, 31, 10, 12,33,34,37

.

This study shows that the maximum von Mises stress values found at the crestal

bone in platform switched implants was 12.79 Mpa for axial load & 84.13 Mpa for off-axis

load (Table 2).

During incising maximum compressive load is applied on the incisors. If the

abutment is placed along the long axis of the implant then the stresses will be evenly

distributed in and around the implants. When the abutment is in an angulation with the

implant, then the stresses will be concentrated in the bone opposite to that of the abutment

angulation19

. Current study also shows the same phenomenon .The von Mises stresses in the

current study were concentrated in the area of bone, opposite to the side to which the

abutment was angulated.

The amount of maximum von Mises stress and strain around the platform

switched implants was increased when the abutment angulation increased (Table 2 and 4), so

comparatively 15° angulated abutments produced more stress and strain (24.66 Mpa & 2672

micro strains) than the straight (12.79 Mpa& 982 micro strains) abutment,20° angled

abutment (34.07 Mpa & 3087 micro strains) produced more stress and strain than the 15°

angled abutment & 25° angled abutment (40.12 Mpa & 3220 micro strains) produced more

stress and strain than the 20° angled abutment. The amount of stress and strain values around

the platform switched implants increased with an increase in abutment angulation. This result

can be compared with many finite element studies14, 19, 38, 39.

A strain gauge analysis by Mohamed A. Elsadek, et al40

also concluded that

straight implants with straight platform-switched abutments were associated with the least

Discussion

Page 56

micro-strain values than a non platform switched angled abutment. The result of his study

with strain gauge is in accordance with the result of this finite element study.

In the present study stress and strain increased with an increase in abutment

angulation from 0° to 25° angulated abutment in both axial and off axis load.

Apart from crestal bone loss other type of failures are reviewed in literature.

They are fracture of occlusal material3, fracture in parts of the frame work

3 and loosening or

fracture of abutment screws8. Most of the articles do not take abutment screw loosening,

fracture of occlusal material and framework in to account for calculating success rates. These

complications might not lead to implant failure but still are major concerns as far as the

biomechanical point of view is concerned.

Kao and colleagues33

conducted a finite element study on the influence of

abutment angulation in micro motion level for immediately loaded dental implants. The

authors concluded that abutment angulation up to 25° can increase the stress in the peri

implant bone by 18% and micro motion level by 30%.Increase in micro motion level leads to

deleterious effects on osseointegration, and leads to fibrous encapsulation around implants.

Lin and colleagues34

conducted an analysis on single implants with different

implant systems, position, bone type, and loading conditions and noted that strains on the

implant and cortical bone was higher when the implants were not placed along the long axis

of loading and strain on the bone was increased as the bone density decreased.

When the load of 178N is applied axially along the long axis of the platform

switched abutment, the maximum von Mises stress and strain value in the cortical bone

around the implant was recorded as 12.79 Mpa & 982 micro strains for straight abutment,

24.66 Mpa & 2672 micro strains for 15° angulated abutment, 34.07 Mpa & 3087 micro

Discussion

Page 57

strains for 20° angulated abutment and 40.12 Mpa & 3220 for 25° angled abutment. (Table

2 and 4).

When the load of 178 N was applied at 45° to the long axis of the platform

switched abutment the maximum von Mises stress and strain values in the cortical bone


157.32 Mpa & 12035 micro strains for 15° angulated abutment, 166.52 Mpa & 12903

micro strains for 20° angulated abutment and 175.48 Mpa & 13313 microstrains for 25°

angled abutment (Table 2 and 4).

When the load of 178N was applied axially along the long axis of the platform

switched abutment, the maximum von Mises stress and strain value in the cancellous bone


0.88 Mpa & 677 micro strains for 15° angulated abutment, 1.01 Mpa & 755 micro strains

for 20° angulated abutment and 1.12 Mpa & 809 for 25° angled abutment (Table 3 and 5).

When the load of 178 N was applied at 45° to the long axis of the platform

switched abutment the maximum von Mises stress and strain values in the cancellous bone


3.88 Mpa & 2562 micro strains for 15° angulated abutment, 3.79 Mpa & 2787 micro

strains for 20° angulated abutment and 4.44 Mpa & 3154 microstrains for 25° angled

abutment (Table 3 and 5).

Comparing the above values, the off-axis load produces more maximum stress

and strain than axial load in the bone, around platform switched implant. There are many

studies to support the concept of increased stress and strain during oblique (off-axis)

loading31, 33, 24, 25

. The increase in the stress during off axis loading was explained by Hsu et

Discussion

Page 58

al24

. They suggested that an off axis load could induce a bending moment and thus exert

stress gradients within the implant as well as adjacent bone.

In this study the maximum von Mises stress and strain is exerted in the cortical

bone than in cancellous bone. This result was seen in all the platform switched models

irrespective of the angulations, under axial or oblique load (Table 2,3,4,5).These results are

in accordance with other finite element studies 13,19,39,

.The pattern of stress distribution in

cancellous bone followed the same pattern as that of cortical bone ,except that the stress and

strain values recorded were less in cancellous bone than in the cortical bone.

The stress concentration in cortical bone (around the neck of the implant closest

to the load) is more than the stress concentration in cancellous bone. This is likely due to the

difference between modulus of elasticity in cortical and cancellous bone. Cortical bone

having higher modulus of elasticity is more resistant to deformation, so it will bear more load

than cancellous bone39

. Another reason for increased stress in cortical bone might be due to

the mechanical stress distribution which occurs primarily when bone comes in contact with

implant. As the amount of bone to implant contact is directly related to the density of bone,

and cortical bone being more denser there is more bone to implant contact. So more stress

concentration is seen in cortical bone19

.

The present study clearly states that, whatever may be the angulation, more stress

and strain values are recorded for oblique loading than axial loading. The stress concentration

was more in the cortical bone opposite to the abutment inclination, which is usually the

buccal bone. So clinically to prevent the crestal bone loss, sufficient thickness of the buccal

bone should be evaluated carefully while using angulated abutments. Insufficient bone

volume may result in buccal fenestration or dehiscence, which will lead to implant failure38

.

Discussion

Page 59

The oblique load produces more stress and strain than axial load in the platform

switched implant, so whenever possible oblique load should be avoided. To avoid off axis

(oblique) loading there should not be any contacts during eccentric loading38,31

.So whenever

possible, we should limit the biomechanical effect of restoration by limiting occlusal contacts

in centric occlusion and remove all the excursive contacts24

.

Limitations of the study:

1.This study has kept bone as the main determinant for implant success and analyzed the

stress and strain in the bone. The stress generated in the implants has not been taken in to

consideration in the present study.

2. Bone is a viscoelastic, anisotropic and heterogeneous material, but in the present study for

convenience the bone was assumed to be linearly elastic and homogenous in nature. The

resultant values may not be accurate quantitatively, but are generally accepted qualitatively.

3. The merging of the colours in the model makes it difficult to ascertain the definitive range.

So subjective variation cannot be eliminated.

4. Masticatory Forces are dynamic in nature, where as the present study was conducted under

static loads.

Conclusion

Conclusion

Page 60

CONCLUSION

A three dimensional finite element analysis model was constructed to investigate the effect of

angled abutments on stress and strain around the platform switched implants. Within the

limitations of this study, the following conclusions were drawn:

1. The stress and strain values in bone around the platform switched implant increased

with an increase in abutment angulation.

2. Stress and strain values increased in off-axis loading than in axial loading.

3. Maximum stress and strain values were found in cortical bone than the cancellous

bone around the neck of the implant.

4. The stress concentration was more in the cortical bone opposite to the abutment

inclination, which is usually the buccal bone. So clinically to prevent the crestal bone

loss, sufficient thickness of the buccal bone should be evaluated carefully while using

angulated abutments

As per the conclusions from this study, whenever possible in anterior maxilla, we have to

use platform switched implants with minimal use of angulated abutments and by avoiding

eccentric contacts. Clinical studies involving platform-switched implants should be

carried out for further research in this particular field.

References

References

Page 61

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