ABSTRACT

Background: Replacement of tooth loss has been a topic of constant research in dentinstry,

given the challenges that dentists face during the treatment through dental implants. Amongst

such challenges is sinus lift surgery and controlling the stress encountered during the process.

Alternatives to the sinus lift surgery include the use of short straight implants and tilted implants

that allow dentists to control stress distribution and make a successful implant.

The Aim: The aim was to identify an alternative to sinus lift surgery in order to minimize its

drawbacks through a comparison between two alternatives to sinus lift surgery (short vs tilted)’

around the maxillary sinus.

Material and Methods: A human jaw model of cortical bone layer, trabecular bone layer and

commercially pure titanium implants was utilized in this regard and the analysis conducted on

the maxilla region through a MSC NASTRAN FEA model. A 3D simulation of a human adult

jaw was utilized to test the short and tilted implants using loads of 300N at a 25o inclination

(oblique occlusal) to the bucco-palatinal axis of the implants in both models. The FE model of

this study was derived from a C.T scan of an adult male patient to simulate the maxilla,

maxillary sinus and alveolar bone.

Results: Through the simulation, Von Mises principal stress distribution was charted along with

Cortical bone and Trabecular bone minimum and maximum principal stress distribution for both

the models i.e. short straight implants and tilted dental implants. The use of the oblique occlusal

loads increased the stress on the Trabecular bone, and had lower stress in the Cortical bone

during the simulation of the tilted implants in the maxilla region.

Conclusion: It was found that the use of tilted implants is a better alternative over short

implants, given its success rate in stress distribution as alternative to sinus lift surgery in the

maxillary region given the simulation results. It is recommended that future research within this

research area through the application of FE analysis be supported with a qualitative modelling

for better and more reliable results.

Introduction to Tooth Loss & its Effect

The replacement of missing teeth has been one of the most important aspects of dentistry in the

past decades (Irish, 2004). In order to reach a proper treatment plan, one must have a full

understanding of dental anatomy; the science that studies appearance, development and

morphology of teeth. Teeth are bone like structures composed of two parts, a root that is

implanted in the jawbone and a crown that is exposed to the oral cavity (Saber et al., 2015).

Jawbones are composed of two parts, a fixed part; the maxilla responsible for stability and a

lower mobile jaw; the mandible. The two jaws perform the action of mastication. (American

Dental Association, 2010). The alveolar bone is the part that surrounds the roots and it forms the

socket, similar to all bones in the body they are continuously undergoing through cyclic

resorption and reformation. (Linkow and Chercheve, 1970)

Tooth loss is multi-factorial and is mainly encountered in older patients. The case of complete

loss of the dentition is called edentoulism, while the loss of a few numbers of teeth is called

partial edentoulism (Ruth et al, 2001). There are various causes of tooth loss which includes

Physical trauma or injury; Tooth decay; and Periodontal disease (Saber et al., 2015). The failure

in replacing missing teeth has many drawbacks. Patients may suffer from aesthetical as well as

functional problems (Shahrul et al, 2011). After the tooth loss, the alveolar bone undergoes a

continuous regeneration. There is continuous osteoblastic (bone forming) and osteoclastic (bone

resorbing) activity (Saber et al., 2015). However, this cycle is affected by the force transmitted

by teeth from the mechanical action of chewing. After tooth loss this stimulus is absent, leading

to a disturbed equilibrium. Bone loss becomes more prominent, which eventually decreases bone

height and width leading to deformation (Verri et al., 2015).

The occlusion harmony is also disturbed by the movement of neighbouring teeth into the empty

space; this action shifts the direction of force distribution from the long axis of the tooth. The

opposing teeth lose their function due to the absence of mastication, their periodontal ligament

becomes thinner and looser. Hence, these teeth will gradually creep into the empty space.

(Shahrul et al, 2011)

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Replacing Missing Teeth and Applied Approaches

To replace the tooth loss and the missing teeth, there are different approaches that are practiced

by dentists. While the approaches have evolved over the years, there are two commonly used

methods practiced widely: Fixed Denture and Removable Denture. As the name indicates, a

fixed denture or a fixed bridge is cemented or bonded to the surface while a removable denture

allows removability.

Dental Implants

Dental implants are applied to solutions for total or partial tooth loss, however, given the varying

anatomy of positions as well as structures, they cannot be placed in the most appropriate position

as analysed (Sarfaraz et al., 2015). Sometimes, they are used in a titled position and sometime in

a straight position. The straight implants are commonly used in tiled position considering the

effect of the anatomy (often referred to as the maxillary sinus). The application of dental

implants has been and is subjected to numerous studies examining the most suitable methods to

investigate the distribution of stress and the logical effects that arise of the treatment.

Research conducted earlier highlight on the point that there is no clinical difference in the use of

tilted and straight implants (Rammelsberg et al., 2012; Guven et al., 2015). In those studies, the

biomechanical characteristics were considered for evaluation and used in comparison against the

design for straight implants (7, 8). Past research also indicates that the survival rates vary based

on the type of implants applied in clinical studies (Guven et al., 2015; Saleh Saber et al., 2015;

Verri et al., 2015). One of the most widely applied framework for analysis pertaining to dental

implant stress effects is the finite element analysis (Verri et al., 2015). Given the lower

availability of data on the stress distribution on the maxillary posterior region through short or

titled implants, this research area was identified.

In the past few years, an increase in the use of FEA tool has been found to predict dental

implants stress level within the implant as well as the surrounding area of the bone. Using the

FEA, more realistic and accurate structures can be assessed, with the models further being

divided parts that are mathematically accurate (Geng et al., 2008). With the simulation process,

the aspects covering strain, stress as well as displacement can be studied and calculated in detail

(Beer, 1981). The use of oblique and vertical loads apply a mastication affect leading to actions

such as bending leading to stress within the bone as well as the implant. In dental implants,

success of the implants is tested in the form of gradient of stress that is transferred to the bone

(Guven et al., 2015; Geng et al., 2008). The impact or density of the stress relates to the interface

between the bone and the implant structure, the applied location (oblique or vertical), the type of

prosthesis, implant length/diameter, shape and also, the implant surface characteristics. Another

important factor is the quality /quantity of the bone area (Beer, 1981). To measure the stress, von

Mises stress metric is utilized which combines shear and normal stress in all directions.

LITERATURE REVIEW:

Background on Dental Implants

History

Dental implants date back to ancient Egypt; wherein sea shells and stones were used to replace

missing teeth, while noble metals were used by others to act like the natural roots (Abraham,

2014). In the 1930's, some archaeological excavations discovered that the Mayans used curved

stones, shells, and gold to replace missing teeth over 2000 years ago (Jemat et al., 2015). In more

recent times, teeth from the same individual (autograft), and teeth from a different individual of

the same species (allograft) were used. However, its use was limited as a result of infectious

diseases that sometimes led even to death. (Lee JH et al, 2005)

In 1957, a Swedish orthopaedic surgeon by the name Peringor Branemark discovered that bone

could grow in proximity with Ti, without being rejected. This phenomenon was called

“osseointegration’’ (Searson, 2005; Sullivan, 2001). The first dental implant was placed in 1965

by Branemark into a 34 year old male patient; it served for more than 40 years until the patient

passed away. Branemark presented his 15 years of research in 1982 at the Toronto conference.

The US Food and Drug Administration (FDA) approved the use of Titanium (Ti) dental implants

in the same year. Ever since, there has been continuous development and different types of

dental implant have been used.

The history of Ti dental implants began in 1940 (Adelle et al, 1981), Bothe et al. implanted a

mix of Ti and other metals into laboratory animals, Ti was well tolerated due to its high

corrosion resistance. In the early 1970’s, Ti and its alloys have been gradually accepted in the

U.S, after the development of pre-fabricated Ti blade implants (Hiroshi and Toru, 1996). Grade

four commercially pure Ti is typically used in dental implants because of its great strength and

resistance to corrosion. However, recently Ti alloys, mainly Ti6A14V, has become popular

because of its strength and fatigue resistance. (Le Guéhennec et al, 2007)

Abutment

They are made of Ti in different sizes and angulations specific to each case.. As seen in the

figure 11, the classification of abutments is into three types: Rotational, manufacture version and

retention of prosthesis. These are further classified into seven types.

Figure 1: Classification of Abutments

classification of abutments

Rotational device

one piece abutment

two piece abutment

Form obtained from the

manufacurer

machined form

custom cast using gold or plastic components

Retention of prothesis to abutment

screw retained

cement retained

attachments

Abutments are available in two types: Angulated and Straight, as seen in the figure 12. The

angulated abutments are further divided into 30˚Cad-cam milled structure, 6˚ is indicated for

overdenture and bridges; Upto 20˚for immediate provisional prosthesis and 25˚ for narrow

platforms and 28˚ for the rest of the platforms.

Figure 2: Types of Angulations

Abutment selection criteria

There is a particular selection criterion on which dental implants are planted. Due to the

advancement in medical field, it has opened many doors for performing a dental implant in a

variety of ways. The dental surgeon can specifically choose what material is to be used and what

exact method of implants is going to be implemented during the dental surgery. The dental

implants are also very stable in the long term if the procedure is done accurately (Cavallaro and

Greenstein, 2011). Aesthetics is quite a difficult field and a lot of challenges are present in a

successful restoration through aesthetics. It has been seen that anterior teeth are very hard to

restore through aesthetics. The tissue and bone loss determine the success of the process and if

these losses are kept at a minimum the procedure is considered successful. However it is even

more difficult to replace multiple teeth instead of a single tooth through aesthetics. A lot of

planning is needed and also a lot of emphasis is to be put on the overall betterment of the patient.

Angulation

Angulated

upto 30˚Cad-cam milled structure for

individual prosthesis

6˚ is indicated for overdenture and

bridges

Upto 20˚for immediate provisional prosthesis

25˚ for narrow platforms and 28˚ for the rest of the

platforms

Straight

Some of the commonly used abutment selection criteria include it being bio-compatible, accurate

fit to prevent loosening, long-term stability and aesthetics.

Angulated Implant placement

It was first used by Dr: Paulo Malo in 1933, when he placed two vertical implants in the anterior

and two in the posterior region at an angulation of 35˚-40˚ and named it “all in four”.

A two-dimensional safe distance between the vertical and tilted implant must be obtained, by

multiplying the known length of each implant by a constant derived from the sine of the insertion

angle. After complete assessment of the site of insertion, a three-dimensional guidance must be

preformed. Extreme angulation must be avoided as it increases forces exerted at the bone-

implant interface. For easier construction of the final prosthesis, inter-implant angulation must be

confined to a single three-dimensional plane. Single tooth restoration should be avoided on

angulated abutments (Geramipanah et al, 2010).

Maxillary sinus lift surgery

Sinus lift is a well-accepted technique to treat the loss of the VBH (vertical bone height) inside

posterior maxilla either performed through a lateral window (Summers 1994) or by an osteotome

sinus floor elevation technique with bone-graft material located within maxillary sinus region in

order to enhance the width and height of the bone available.

Table 1: A guideline towards sinus floor elevation

Alternatives to sinus lift surgery

There is an alternative present to the sinus augmentation technique, short implants have some

potential as they are brought forward by some researchers (Renouard et al, 2006). The implants

with a relatively small length of 5-8 mm are known as short implants. Although there is some

ongoing discussion on this topic as some authors also claim that 7-10 mm are the short implants

(Das et al, 2006). It has been seen through Finite element (FEA) that the occlusal forces are

distributed mainly to the crestal bone, they are not evenly spread over the surface area of an

implant. Therefore the short implants are viable and can be used (Lum et al, 1999).

The short implants faced a lot of questions as the implants previously used had a lot of history

and research behind them although over the years short implants have proven to be more

successful. The success rates of short implants have steadily climbed and in 2004 it was 95.1% to

be exact (Renouard et al, 2006). The patients with missing teeth have benefitted from this as the

survival rate of normal implants is somewhat similar to the survival rate of short implants.

The earlier writings and researches stated the short implants had low survival rate when

compared to standard ones and they failed more often than them. However in recent times where

the textured-surfaced implants are more common, the survival rate has climbed and is almost the

same to the standard implants. Research from different sources has determined that implants with

a length of 6 mm have had the same survival rate as the standard ones (Bruggenkate et al, 1998).

Although it is still recommended to not use shorter implants on their own but to synergize them

with the longer ones to make them more stable.

A system review was conducted where a total of 16344 implants were reviewed and the failure

rate was 4.8%. Implants that were 3.75 mm wide and 7 mm long failed more often as they had

the failure rate of 9.7% while the failure rate of 3.75-10mm implants was at 6.3% (Das et al,

2006). The difference in the failure rates can be noted as the 4 mm implant in diameter proved to

be more successful and had little failures. The long term effects of short implants as opposed to

bone augmentation are not known at this time. The residual bone was about 6 to 7 mm with a

standard implant while it was placed there at the same time with the sinus augmentation

procedure (Pieri et al, 2012). Outcome of both of these techniques were similar. The clinical and

radiographic review therefore proved the short implants are viable too.

Implants can also be placed in a disto-angulated direction as an alternative to the sinus surgery

which can in turn avoid disturbing maxillary sinus. Pterygomaxilla can also be used as a location

to place implants. Zygoma implants are another alternative to sinus augmentation procedure.

(Graves et al, 1994)

Finite Element analysis

Many methods were implemented in the study of stress distribution around dental implants eg:

strain gauge, two and three dimensional photo elasticity and finite element analysis (FEA) (Shen

et al, 2010). FEA is a computer analysis that involves numerical techniques in order to calculate

the strength as well as the behaviour of the structures engineered. It was developed in 1956 in the

aircraft industry and was then introduced in the field of dentistry in the1970’s, when W. Farah,

Thresher R.W, and Yattran A.L studied the stress in a tooth structure they modelled.

In the year 2000, Zhang L, et al conducted a study on the correlation between implant length and

diameter, with proper stress distribution using three- dimensional FEA. The uses of three

dimensional FEA can assess biomechanical problems before they occur. Each element is

assigned with the properties of the material being modelled. The model should mimic the exact

physical properties of the actual structure or as close as possible to ensure the quality of FEA.

The most difficult part of the simulation is to mimic the properties of a living tissue. Mostly the

properties are either drawn from a detailed anatomy book or a cone beam C.T scan of the jaws.

CBCT offers not only the anatomical structure, but also can give a better idea about the material

properties in relation to different bone density. Depending on the study intended, different bone

modelling is initiated, in some studies bone is modelled as simple rectangle ellipsoids, or u-

shaped (Sagat et al, 2010; Gröning et al, 2012).

As natural bone density occurs in different densities, specific density is modelled according to

the study conducted, a more detailed geometric might be required sometimes like in cancellous

bone, especially in the posterior maxilla as it has a wide variety of densities in the same jaw

(Georgiopoulos et al, 2007). As for implants, many ways were used to simulate both implants

and abutment materials, but the two main ways were well studied in the literature. One way is to

obtain all required information from the manufacturer eg: length, diameter, macro- micro thread

configuration, the second way is by obtaining a digital scan of the used implant and abutment

and to use the scanned model. While crowns are modeled relying on the morphology of the

natural tooth to be replaced after calculation of both mesiodistal and buccolingual dimensions

(Wheeler, 1963).

After completion of bone and implant simulation, masticatory forces are simulated and applied.

Masticatory forces are defined 1. Compressive forces that attempt to push materials towards each

other. 2. Tensile forces that pull objects apart. 3. Shear forces which cause sliding. Tensile and

shear forces are the forces that can increase stress around implant-bone interface and prosthetics.

In actual mastication two different types of forces are present, cyclic and static. Most of FEA

studies use static forces as they are more accurate to simulate (Misch et al, 2005). The stress is

then measured in a cross sectional area in neuton (force) per square meter (unit area), that’s

N/m2. Usually different colour figures are used to illustrate the amount of stress around the

implant and prosthetic structures (Geng et al, 2001).

MATERIALS AND METHODS

In this study a 3D finite element model was used to examine and compare the force distribution

on the bone-implant interface between short and tilted dental implants around the maxillary

sinus. The FE model of this study was derived from a C.T scan of an adult male patient to

simulate the maxilla, maxillary sinus and alveolar bone. Using I-DEAS Artisan 4.0 CAD-CAM

software (World Head quarters 200 East man drive, Milford, Ohio 45150).

General overview

The effect of implant tilting on stress distribution was studied in model 1 for a single cylindrical

Ti implant placed in the posterior maxilla. The implant was placed in the upper left second pre-

molar area (#25) with an angulation of 20o in respect to the occlusal plane (Fig 16). Another

single short cylindrical Ti implant was inserted in model 2. The implant was inserted in area of

#25 with 0 angulation in respect to the occlusal plane (Fig 17). In the present study two different

implant configurations (diameter/length) were modelled (Fig 18).

A detailed description of the models used in this study varying according to implant diameter,

length and inclination are given in table 4. Implant and abutment dimension was based on the

dimensions of commercially available Straumann implant system, but the geometry of both

components was simplified to that of a cylinder. A 20o angulated abutment by Struman was used

with implant in model 1 whereas a straight one used with the implant in model 2, (Fig 17 and Fig

18).

Table 2: Configuration and inclination of the implants used in this study.

Model Dimensions ConfigurationModel 1 4.1mm/12mm 20 tiltedModel 2 4.1mm/6mm Perpendicular to the

horizontal plane

Figure 3: Model 1: 25o tilted dental implant at area of #25

Figure 4: model 2 short dental implant 00 inclination

Figure 5: Struman dental implants

Figure 6: 20o angulated Struman abutment and Struman straight abutment

The maxilla was represented as a combination of cortical and cancellous bone. The cortical plate

(see above, Fig 16, 17 colour coded purple) was assumed around a cancellous core (see above

Figure 16, 17 colour coded blue). For implant longevity in the posterior region, it is important to

maintain at least 2mm of bone between the sinus wall and the implant. In this study, therefore

bone plate of 2mm was modelled in this region. The residual ridge height in the posterior region

was identical in both models.

The focus of this study was on the stress distribution of tilted vs straight implants in the bone-

implant interface. Therefore, only half of the maxilla was analysed. Inclusion of the entire

maxilla in the analysis would have resulted in a FE model that consists of a great number of

elements and nodes. By analysing only half of the maxilla, a finer FE mesh was obtained, which

probably led to a higher accuracy of the results.

Material Properties

All materials used in this study were considered to be isotropic, homogeneous and linearly

elastic. The material properties of all the materials used in this study are given in table 6.

Table 3: materials parameter

Elastic Modulus (MPa) Poisson’s Ratio

Cortical Bone 14800 0.30

Type 4 Trabecular Bone 1100 0.30

Implant 105000 0.33

Abutment 105000 0.33

Crown 68900 0.28

Interface conditions

In a continuous bond, the interfacing between the bone and the implant was modelled, which

leads to an ideal osseoitegration minus any relativity. It can be said that the implants were rigid

to the bone in terms of position with similar bonding applied at all interfaces.

Elements and nodes

Generally the total number of elements and nodes used in this study sum up to 865,276. All

structures were meshed using First order Tetrahedron free meshing. (Fig 20)

Figure 7: meshing of the model.

Number of Elements: 329303 (Short Implant Model)

Number of Nodes: 66309 (Short Implant Model)

Number of Elements: 392529 (Long Implant Model)

Number of Nodes: 77135 (Long Implant Model)

Loading conditions and Boundary conditions

The direction and magnitude of masticatory forces vary greatly between individuals and location

of force application in the mouth. In this study the models were restored with a crown cemented

on the abutment, the crown to implant ratio is 1.5: 1. An oblique occlusal bite force on 300N was

applied at a 25o inclination to the bucco-palatinal axis of the implants in both models. The

boundary conditions applied to both models are shown in Fig 21 and. As restraint condition, the

top of the maxilla was totally fixed.

Figure 8: Figure 20: boundary conditions.

Software and evaluation

MSC NASTRAN/PATRAN Version FE, software was used.

RESULTS

The results of loading of the two models are presented in terms of Von Mises’ equivalent stress,

maximum tensile, and maximum compressive stresses. In this study the effect of the load on

each implant was individually assessed. Each model separately analysed for oblique occlusal bite

force on 300N at a 25o inclination to the bucco-palatinal axis: model 1 and model 2. To

appreciate the situation considered, first of all, the results for maximum tensile, maximum

compressive stresses and Von Mises’ equivalent stresses are separately reported, and in details

explained. Thereafter, the two models were compared.

Table 4: Von Mises, maximum principle, and minimum principle stress distribution value for group 1 and 2.

Model Section Von Misses Stress (Equivalent Stress)

(MPa)

Maximum Principal Stress (Maximum

Tension) (MPa)

Minimum Principal Stress (Maximum

Compression) (MPa)

Group 1 Abutment 226.69 186.91 -247.25

Crown - 132.92 -252.90

Implant 496.73 502.51 -529.30

Cortical bone - 313.54 -550.42

Trabecular bone - 16.21 -24.84

Group 2 Abutment 246.16 217.91 -262.41

Crown - 119.64 -253.96

Implant 541.61 501.77 -667.68

Cortical bone - 382.95 -526.61

Trabecular bone - 57.54 -44.67

The Von Mises stress distribution computed for the dental implants evaluated under oblique load

were 496.73 MPa for group 1 and 541.61 MPa for group 2 (Table 7; Figure 22 and 23). For the

cortical bone-implant interface on the side of the stress concentration under oblique load, the

maximum principle stress values were 313.54 and 382.95 MPa and the minimum principle stress

values were -550.42 and -526.61 MPa for groups 1 and 2, respectively (Table 7; Figure 24to 27).

For trabecular bone, the maximum principle stress, and minimum principle stress values of

39

groups 1 and 2 were estimated as 16.21 and 57.54 MPa, -24.84 and -44.67 MPa, respectively

(Table 7; Figure 28 to 31).

Figure 9: Model 1Von Mises stress distribution On implant (group 1)

Figure 10: Model 2 Von Mises stress distribution On the implant (group 2)

Figure 11: Cortical bone maximum principal stress (model1)

Figure 12: Figure 24 Cortical bone maximum principal stress (model 2)

Figure 13: Figure 25 Cortical bone minimum principal stress ( model 1)

Figure 14: Figure 26 Cortical bone minimum principal stress (model 2)

Figure 15: Figure 27 Trabecular bone maximum principal stress (model1)

Figure 16: Figure 28 Trabecular bone maximum principal stress (model2)

Figure 17: Figure 29 Trabecular bone minimum principal stress (model 1)

Figure 18: Figure 30 Trabcular bone minimum principal stress (model 2)

DISCUSSION

Overview

In recent years, field of implant dentistry has benefited from the computer-aided designs with

associated finite element analysis in achieving the proper implant and prosthesis design. Finite

element analysis (FEA) method is used in many biomechanical studies to investigate the stress

distributions in implant- bone complex (Akca et al., 2001, Menicucci et al., 2002, Mailath et al.,

1991). This method can precisely model the complex geometries mathematically and stress and

stain distributions may be obtained from a solution of equilibrium equations. Compared with the

other techniques (photo elastic model studies, strain gauge analysis on physical models) FEA

seems to be a superior tool in evaluating biomechanical loads on implants because it is capable

of simulating both isolated vertical and horizontal forces, as well as combined oblique loads

(Tepper et al., 2002).

The accuracy of a finite element study relies on the simulation of the model which represents the

natural tissues. In a finite element study, a two dimensional or a three dimensional model can be

used for an accurate prediction. Different patterns of stress values and magnitude were calculated

around the maxillary implants when an anatomic and a non-anatomic model are used (Simon et

al., 1977, Ismail et al., 1987). In 2-D system, it is assumed that out-of-plane deformations, strains

and stresses are negligible (DeTolla et al., 2000). Such models may lead to inconclusive data

with numerical failure. Therefore in this study a complex 3-D anatomical finite element model is

used to investigate the effect of tilted Vs short implants on stress distributions in edentulous

maxilla in the area of maxillary sinus. The complexity of the model in the study depends on two

factors: the spatially complex geometry of the maxilla, and oblique loads applied.

Findings

The model of the present study is derived from a model whose geometrical data are based on the

C.T scan of an adult male patient. The anatomical model of this study consists of the entire

maxilla, maxillary sinus, and cylindrical implants. However the reality is too complex to be

simulated completely in a numerical model (McElhaney et all., 1970, Carter & Spengler, 1978).

45

Therefore, in a finite element study several assumptions are made. The early attempts at

modelling dental implants resulted in unrealistic assumptions (DeTolla et al., 2000). Complex

reality is simplified assuming that proportion and relative effects reflect reality. For the

convenience of the study, material properties, loads, boundary and interface conditions are

simplified in the models. The structures are all assumed to be homogeneous and isotropic and to

possess linear elasticity. However in fact, the maxilla is inhomogeneous and subjected to

functional elastic deformations originating from masticatory forces, like other living tissues. In

reality the loads from mastication are dynamic and oblique relative to the occlusal surface of the

implants. The approximation of masticatory forces, loads and bone properties of bone has been

attributed to insufficient computer capacities (Ladd & Kinney, 1998). But recent improvement in

computer programming methods, computational power and digital imaging techniques have

allowed FEM to better analyse the biological structures (Ulrich et al, 1998).

The implants in the present study are simulated so that they are rigidly bounded to bone over the

entire surface and fully (100 %) osseointegrated, as in many other studies (Tepper et al., 2002,

Kawasaki et al. 2001, Van Oosterwyck et al 2002). However histomorphometric studies have

demonstrated that there is never 100 % osseointegration in bone. Simulating such an interface

would cause redistribution of the strains in the models. All these assumptions imply a certain

degree of uncertainty in all FE studies (del Valle et al, 1997). For this reason, only larger changes

in the strain levels are the interest of the study.

In the present study loads are applied to the occlusal surfaces of the superstructure in order to

simulate real masticatory movements. But with a finite element analysis, precise calculations

cannot be made, because there is great variation in the magnitude of the mechanical factors for

bone. And in addition, masticatory movements and their magnitude vary enormously between

the individuals (Glantz & Nilner, 1998). Theoretically, the problem of predicting loads on the

implants is a statistically indeterminate problem in mechanics. In most cases occlusal loads lie

between 100 N and 2400 N. Furthermore the masticatory loads are dynamic and oblique relative

to occlusal surfaces of the implants. However, in this study a static load of 300 N is applied with

250 inclinations (representing oblique loads). Simulating such a loading condition can be

considered as a realistic masticatory pattern. But when this is compared with the natural

mastication pattern, loads with inclinations relative to implants axis and sagittal plane may be

insufficient for complete simulation of oblique masticatory forces (Yaicioglu et al, 2015).

These variations in important biomechanical factors may lead to imprecise calculations.

Although the results of some other finite element studies (Holmes & Loftus 1997, Hoshaw et al,

1994; Spiekermann, 1994) demonstrated that the areas of higher stress concentrations (such as

implant neck) coincided with stress analysis predictions, the exact stress values causing

biological changes are not known (Lanyon, 1984).

In this study, similar to other studies using simplified models of human jaw, the model

compromises a cortical bone layer, trabecular bone layer and commercially pure titanium

implants. It is assumed that a homogeneous external layer of 0.75 mm cortical layer exists

around the cancellous bone. However, in reality the spatial distribution of the cortical and

trabecular bone is very different and inhomogeneous. The inhomogeneity of the cancellous bone

influences strongly the local elastic parameters. It is very important to choose the proper

parameters in stress analysis with FEM (Rong, 2002). The elastic modulus of cortical and

trabecular bone shows considerable variability. Therefore the elastic modulus of cortical and

trabecular bone are taken as an average of the several values cited in literature. These data are

either gained from mechanical testing or ultrasonic testing from human and bovine material.

However, even such a model may lead to failures in numerical data. Therefore, the inherent

limitations of the finite element stress analysis must be acknowledged. Since the reality is more

complex than the simulated models, a qualitative comparison among the models is advisable

rather than focusing on quantitative data from the finite element analysis (Stegaroiu et al., 1998).

Tilting of implant

Patients suffering from hard and soft tissue deficiencies are invariably the most difficult group of

patients to treat with osseointegrated implants. Severe atrophy of the maxilla, insufficient bone

quality and quantity of the arches complicate the use of osseointegrated implants in the maxilla

and usually necessitates advanced surgical techniques. However complicated surgical techniques

are not always practical because of patient related factors and the increased risk of

complications. Based on these considerations, tilting of dental implants and commercially

available dental implants are suggested as an alternative treatment method.

The benefits of tilting dental implants can be summarized are as follows (Krekmanov, 2000).

Firstly, it provides an end- support for the prosthetical restoration. This additional distal

anchorage helps to withstand occlusal loads by distributing them throughout the arch. It allows

the use of longer implants and reduces the cantilever length and broadens the prosthetic base. It

improves the cortical anchorage, indirectly the primary stabilisation of the implant, because the

implant follows a dense, bony structure.

According to the results of the present study, placement of short implant in the maxillary sinus

area, will affect the success rate of the implant, as all the stress will be absorbed by the trabecular

bone around the dental implant, and less on the cortical bone. Bone resorbtion will occur in the

trabecular bone all around the dental implant, which will cause failure of the implant. Hence, the

success rate is affected. (minimum principle stress in cortical bone is -550.42 MPa, minimum

principal stress in trabecular bone is -24.84 MPa).

Another result that was obtained by this study is that the use tilted implants in the maxillary sinus

area, will affect the survival rate of the implant, as bone reorbtion will be more in the cortical

bone leading to exposure of some threads but the implant remain anchoroged to the bone, while

less amount of stress in absorbed by the trabeular bone. (minimum principle stress in cortical

bone is -526.61MPa, minimum principal stress in trabecular bone is -44.67 MPa).

Conclusion

In conclusion, it is identified that tilted implants are a better alternative over straight implants in

the maxillary sinus area. The simulation of the model indicates that the short straight implants

have a compromised success rate, while tilted implants may have a decreased survival rate. The

application of a FE analysis comes with its own limitative as the study is limited to a computer

based simulation. It is further limited with restriction imposed in terms of material properties,

models, application type and load values. These factors together alter the results and hence, are

of limited nature in comparison with those in clinical testing. While the use of the FE analysis

has allowed in comparing the simulation of the short straight implants vs the tilted implants in

the bone in various situation with biomechanical aspects, it will be much helpful if it is supported

by clinical tests performed with qualitative data.

49

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