The Effect of Veneering Materials on Stress Distribution in Implant...

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The International Journal of Oral & Maxillofacial Implants 571

The Effect of Veneering Materials on Stress Distribution in Implant-Supported

Fixed Prosthetic Restorations Yalc ın Ciftc i, DDS, MS1/Senay Canay, DDS, MS2

In this study, the effect of various materials used in fabricating superstructures for implant-retained fixed partial dentures on stress distribution around implant tissues was investigated.Five different mathematical models consisting of 11,361 nodes and 54,598 elements were con-structed to study porcelain, gold alloy, composite resin, reinforced composite resin, and acrylicresin veneering materials using the 3-dimensional finite element analysis method. MARCK7.2/Mentat 3.2 software was used for the analysis. Reference points were determined on thecortical bone, where perpendicular, oblique, and horizontal forces were applied. Stress valuescreated by oblique and horizontal forces appeared to be higher than those created by verticalforces. Stress seemed to be concentrated at the cortical bone around the cervical region of theimplant. Gold alloy and porcelain produced the highest stress values in this region. Stresses cre-ated by acrylic resin and reinforced composite resin were 25% and 15% less, respectively, thanporcelain or gold alloy. Porcelain and gold alloy produced stress values at the lingual implantsites that reached the ultimate strength values of the cortical bone. (INT J ORAL MAXILLOFAC

IMPLANTS 2000;15:571–582)

Key words: dental implant, dental materials, dental stress analysis, finite element analysis, fixedpartial denture

Biomechanical factors play an important role inthe long-term survival of oral implants. The

selection of implant positions, prosthesis design,and superstructure material is critical for thelongevity and stability of the implant prosthesis.1–7

The nature and magnitude of loads necessary tocause implant loosening are unknown, so it hasbeen recommended that forces be kept to a mini-mum. The osseointegrated implant provides directcontact with bone and has no micromovement.Therefore, all stress waves or shocks applied aretransmitted to the implants.

Because of the lack of micromovement ofosseointegrated implants, most of the force distrib-ution is concentrated on the crest of the ridge, and

this may lead to bone resorption and subsequentloss of the implant. It has been suggested thatstress-absorbing or load-dampening systems beincorporated into the superstructures supported byosseointegrated implants, so as to reduce loads onthe implant that occur because of the lack of vis-coelasticity at the bone-implant interface.

The IMZ System (Interpore International,Irvine, CA) has an internal shock absorber made ofa medical-grade polyoxymethylene incorporatedinto its design.8–10 The manufacturer of the Bråne-mark System (Nobel Biocare, Göteborg, Sweden)has recommended acrylic resin for fabrication of theocclusal surfaces of prostheses.11 Other researchershave demonstrated that acrylic resin provides a gen-erous layer of material with sufficient cushioningeffect to dampen most commonly exerted oralforces and thus acts as a shock absorber.12–14 How-ever, acrylic resin wears at an accelerated rate whenopposed by natural teeth or porcelain.15,16 Suchchanges in the occlusal surfaces will reduce chewingefficiency and may alter maxillomandibular rela-tionships.17 This has led authors to encourage theuse of porcelain on occluding surfaces.18

1Assistant Professor, Department of Prosthodontics, Faculty ofDentistry, University of Hacettepe, Ankara, Turkey.

2Associate Professor, Department of Prosthodontics, Faculty ofDentistry, University of Hacettepe, Ankara, Turkey.

Reprint requests: Dr Senay Canay, Bagis sok. 17/10, 06660 Kocatepe Ankara, Turkey. Fax: 90 (312) 4184715. E-mail: [email protected]




572 Volume 15, Number 4, 2000

CIFTCI/CANAY

Ceramic materials are acceptable for veneeringimplant-supported prostheses, and their use greatlyimproves the esthetics of implant restorations.19

However, porcelain is not a stress-absorbing material,so that forces developed at the occlusal level will betransmitted directly to the prosthesis, the implant, orthe bone interface unless they are mediated in somefashion.10,20 The intensity of the resulting stresseswould be a function of the physical qualities of thedifferent veneering materials used. The thickness ofthe veneering material is greatly reduced when a par-tially edentulous arch is restored. The question arisesas to whether or not resin used as a veneering mater-ial provides a significant cushioning effect.

The finite element analysis (FEA) method hasproved to be a useful tool in estimating stress levelsaround implants. It involves the development of amathematical model of a continuous structuredivided into a system of discrete components or ele-ments. These components are connected at nodalpoints, where stresses and displacements are deter-mined. The accuracy of the 3-dimensional (3-D)method is proportional to the number of nodes andelements in the mathematical model.21,22

The purpose of this study was to analyze, with the 3-D FEA method, the force-absorbingbehavior of 5 restorative materials used to veneersuperstructures that were rigidly connected toimplants.

MATERIALS AND METHODS

In this study, the FEA method was used to evaluatestresses in the mandibular posterior quadrant, wherean implant-supported fixed partial prosthesis wasfabricated with different restorative materials. Theimplants were assumed to be placed in the region ofthe second premolar and second molar, and the firstmolar tooth was assumed to be lost. The model wasprovided with 2 spline hydroxyapatite-coated cylin-dric Calcitek dental implants (Sulzer Calcitek Inc,Carlsbad, CA) 4 mm in diameter and 13 mm inheight. A 3-D mathematical model including aframework, abutments, implants, a bone-implantinterface, and a section of the mandible was con-structed, representing the anatomic geometry of themandible (Fig 1).

Figs 1a to 1d Mathematical model including implants and superstructures.

Fig 1a Section with implants and corti-cal bone.

Fig 1b Section with implants, corticalbone, and trabecular bone. C1 = crest ofcortical bone; C2 = junction between corti-cal and cancellous bone.

Fig 1c Section with implants, corticalbone, trabecular bone, and copings.

Fig 1d Section with implants, corticalbone, trabecular bone, copings, andveneering materials.

1.5 mm

C2C1

Implant 1

Implant 2

13 mm





CIFTCI/CANAY

An actual human mandible was used in thepreparation of mathematical models. With the helpof laser scanners (3D Digitizer Model 3030 andEcho digitizing software, resolution 1 � 1 mm/30 s,Cyberware, Monterey, CA), the surface topographyof prosthetic superstructures and mandibles wasconverted into digital data. Data were convertedinto a 3-D solid mathematical model by using thesolid and crest modeling programs I-DEAS ArtisanSeries 3 (Structural Dynamics Research Corpora-tion, Milford, OH) and the MARC K7.2/Mentat3.2 FEA program (MARC Analysis Research Cor-poration, Palo Alto, CA). For the analysis, a directprofile was used. In this procedure, the cores ofshell structures were meshed with 4-node tetrahe-dron elements. The analysis program used in thestudy could easily automesh the complicated modelwhen the tetrahedron elements were used. Sincethese nodal points could move in 3 axes, theirdegree of freedom was set at 3. The displacement ofthese nodes assisted in the calculation of stress dis-tribution inside the structures.

To simulate the clinical situation, the model wasnot supported at the bottom; instead, it was fixedalong the points where the masticatory muscleswere inserted. A pilot study (ie, a trial run) showedthat the region where the analyses were done wasnot influenced by the location of the muscle inser-tions. Therefore, the mesh was refined in the areaof implant placement to provide additional stressresolution in this region. The mathematical modelwas divided into 54,598 elements connected at11,361 points, known as nodes.

Mandibular anatomy mandated placement of theposterior implant with a lingual inclination to takeadvantage of the bone height in the model, withmoderate (10-degree) inclination. The modeledsection of the mandible had an 18-mm buccolingualthickness, was 26 mm in height, and was sur-rounded by 1.5 mm of cortical bone.

A superstructure that represented the frameworkof an implant-supported fixed prosthesis was alsomodeled at 4 mm in height and 6 mm in width.The materials used as veneering materials wereType III gold alloy (Sjoding C-3); feldspathicporcelain (Vita VMK 68, Vident, Brea, CA); heat-polymerized polymethylmethacrylate resin(Biotone, Dentsply Co, York, PA); microfilled com-posite resin (Charisma, Heraeus Kulzer, SouthBend, IN); and glass-modified composite resin(Artglass, Heraeus Kulzer). Restorative materialsvaried, but the abutment designs were similar, sothat the resulting stress and stress distributioncould be attributed to the material differences. Thecement layer between the crown and abutment was

too thin to adequately model in the finite elementsimulation and was considered to be negligible.

All materials used in the models were consideredto be isotropic, homogeneous, and linearly elastic.To simulate ideal osseointegration, the implantswere rigidly anchored along their entire interface inthe bone model. Modulus of elasticity and Poisson’sratio values are presented in Table 1.

A wide range of magnitudes for chewing forces hasbeen reported in the literature. For the currentmodel, 3 forces from different directions wereselected: a horizontal bite force (Fh = 0 degrees), avertical bite force (Fv = 90 degrees), and an obliquebite force (Fo = 120 degrees). The proportion of theforce magnitude was Fh : Fv : Fo = 1 : 3.5 : 7.23 In thestudy, a vertical load of 500 N, a horizontal load of142 N, and an oblique load of 1,000 N wereapplied.24 The vertical and oblique loads were appliedequally at the 125 nodal points on the buccal inclina-tion of the lingual cusps. Hence, when the mandibu-lar implant opposes a natural maxillary tooth, the pri-mary contacting cusp becomes the maxillary lingualcusp opposing the mandibular implant crown, withthe mandibular buccal cusp of decreased height andwidth over the implant body.25 The horizontal loadswere applied to the nodal points on the buccal.

Loading forces on the models were static. Stresscontours were computed and plotted in the bone tis-sue. Then by giving a 0.1-mm displacement to thenodes defined in a specific period of time, stress dis-tributions formed on the cortical bone were analyzed.

Before loading, specific points at 2 levels alongthe bone-implant interface were selected for con-venience to directly compare models representingdifferent analysis variables.

Table 1 Mechanical Properties of Oral Tissuesand Prosthetic Materials in FEA Evaluations

Modulus ofelasticity Poisson’s

GPa ratio (v)

Cortical bone 13.7 0.30Cancellous bone 1.37 0.30Titanium 117 0.33Metal coping (Ceramco O)* 86.2 0.33Full-crown Type III gold alloy 100 0.33

(Sjoding C-3)Porcelain (Vita VMK 68) 70 0.19Glass-modified composite resin 10 0.30

(Artglass)Acrylic resin (Biotone) 2.26 0.35Composite resin (Charisma) 14.1 0.24

*Ceramco/Dentsply, Weybridge, Surrey, England.





CIFTCI/CANAY

1. The crest of the cortical bone (C1). Three pointswere chosen at the buccal and lingual aspects ofboth the first and second implants, which wereplaced in the second premolar and second molarregion, as reference (Fig 1b).

2. The junction between the cortical and cancellousbone (C2). Two points were chosen for the sec-ond implant, and 3 points were chosen for thefirst implant on the buccal and lingual aspects(Fig 1b).

For an evaluation of stress distribution, the mag-nitudes of the concentrations were presented inprincipal stresses. The principal stress offers thepossibility of making a distinction between tensilestress and compressive stress. Displacement compo-nents of specific points provide information aboutthe deformation of the model and facilitate interpre-tation of the results. The magnitude of the stressesfrom these reference points was evaluated for eachof the 5 different veneering materials. Principalstress values of fragile compact bone were comparedwith the ultimate compressive strength and ultimatetensile strength values.

The use of statistical analyses was very limitedfor the FEA studies because the results of the modelobservations were invariant.

RESULTS

Stress patterns appeared as contour lines with dif-ferent color connecting equistress points betweencertain limits. The stress value for each contour linewas presented as a positive or negative; positive val-ues identified tensile stresses, while negative valuesidentified compressive stresses. For an evaluation ofstress distribution, the magnitude of the concentra-tions was presented in minimum (compressivestress) and maximum (tensile stress) principal stress,together with their location in relation to theimplant. Maximum and minimum principal stressvalues that resulted from vertical and oblique load-ing conditions on the bone-implant interfaces andthe cortical bone surrounding the neck of theimplants with 5 different veneering materials areillustrated in Figs 2 to 5. Stress contours werecolor-coded and were explained on the left side ofeach figure for both loading magnitudes. Table 2summarizes the magnitude and type of stresses.

The intensity of compressive stress was higherwhen vertical and horizontal loads were transferredto the upper and lower crests of the cortical bonefound at the first implant’s lingual surface. Thesevalues were highest for gold alloy and lowest for the

acrylic resin veneering material. Lower values werealso found for composite resin and glass-modifiedcomposite resin. All the stress values seen at the C2level of the cortical bone show that there was areduction in stress from the C1 level. However,when oblique and horizontal loads were transferredto the C1 and C2 level of the cortical bone on thebuccal crest, compressive stresses were more likelyto be formed.

In this study, tensile and compressive stresseswere evaluated. However, in the regions where themaximum principal and minimum principal stressvalues were similar, the stresses could not bedefined as actually tensile or compressive; there-fore the term complex or mixed stress was used. Onthe C1 level of the cortical bone, when verticalload was transferred, complex stresses were formedonly on the gold alloy and porcelain. In contrast,when acrylic resin, composite resin, and glass-modified composite resin materials were used, ten-sile stress was more likely to occur. Tensile andcompressive stresses that existed on the secondimplant’s lingual and buccal aspects were greaterthan those on the first implant’s lingual and buccalsites. This was true for all 5 veneering materialsunder all 3 loading conditions. Higher stresseswere situated in the upper cortical layer than in thelower regions. Gold alloy had the highest averagetensile stress values observed under vertical,oblique, and horizontal loads, but with porcelain,composite resin, glass-modified composite resin,and acrylic resin, this rate decreased gradually. Onthe buccal aspect of the cortical bone, stresses werecomplex in nature under vertical loading but werepredominantly tensile under horizontal andoblique loading.

Cortical Bone Displacement ResultsIn the study, static analysis findings were considered,and stress rates under applied loads were recorded.After the same amount of load was applied verticallyon the prespecified nodal points, the highest stresslevels that formed on the upper crest of the corticalbone around both implants were observed in themodels where gold alloy and porcelain were used asveneering material, and the lowest stress levels wereobserved in the model in which acrylic resin wasused. Similar conclusions were drawn for the lowercrest of the cortical bone, which showed a decreasein the stress level compared to that of the uppercrest (Figs 6 and 7).





CIFTCI/CANAY

Figs 2a to 2e Maximum principal stress values (MPa) aroundimplants restored with 5 different veneering materials under ver-tical loading.

Fig 2a Porcelain.

Fig 2b Glass-modified composite resin. Fig 2c Acrylic resin.

Fig 2e Gold alloy.Fig 2d Composite resin.

Inc: 1 Time: 1.000e + 000

1.991e + 0011.731e + 0011.471e + 0011.211e + 0019.505e + 0006.903e + 0004.302e + 0001.701e + 000–9.008e + 001–3.502e + 000–6.104e + 000

Inc: 1 Time: 1.000e + 000

2.000e + 0011.745e + 0011.491e + 0011.236e + 0019.808e + 0007.295e + 0004.710e + 0002.161e + 000–3.877e + 001–2.937e + 000–5.486e + 000

Inc: 1 Time: 1.000e + 000

2.011e + 0011.760e + 0011.508e + 0011.256e + 0011.004e + 0017.524e + 0005.006e + 0002.488e + 000–2.954e + 002–2.547e + 000–5.065e + 000

Inc: 1 Time: 1.000e + 000

1.998e + 0011.742e + 0011.486e + 0011.230e + 0019.736e + 0007.176e + 0004.615e + 0002.054e + 000–5.061e + 001–3.067e + 000–5.627e + 000

Inc: 1 Time: 1.000e + 000

1.998e + 0011.737e + 0011.476e + 0011.215e + 0019.544e + 0006.934e + 0004.325e + 0001.716e + 000–8.932e + 001–3.502e + 000–6.112e + 000


CIFTCI/CANAY




Figs 3a to 3e Maximum principal stress values (MPa) aroundimplants restored with 5 different veneering materials underoblique loading.

Fig 3a Porcelain.



Inc: 2 Time: 2.000e + 000

1.031e + 0029.140e + 0017.970e + 0016.801e + 0015.632e + 0014.462e + 0013.293e + 0012.123e + 0019.538e + 000–2.157e + 000–1.385e + 001

Inc: 2 Time: 2.000e + 000

8.580e + 0017.523e + 0016.466e + 0015.409e + 0014.352e + 0013.294e + 0012.237e + 0011.180e + 0011.229e + 000–9.343e + 000–1.991e + 001

Inc: 2 Time: 2.000e + 000

8.229e + 0017.193e + 0016.158e + 0015.123e + 0014.088e + 0013.052e + 0012.017e + 0019.818e + 000–5.342e + 001–1.089e + 001–2.124e + 001

Inc: 2 Time: 2.000e + 000

8.867e + 0017.791e + 0016.716e + 0015.640e + 0014.565e + 0013.489e + 0012.413e + 0011.338e + 0012.624e + 000–8.131e + 000–1.889e + 001

Inc: 2 Time: 2.000e + 000

1.058e + 0029.396e + 0018.208e + 0017.020e + 0015.833e + 0014.645e + 0013.457e + 0012.269e + 0011.081e + 001–1.066e + 000–1.294e + 001





CIFTCI/CANAY

Figs 4a to 4e Minimum principal stress values (MPa) aroundimplants restored with 5 different veneering materials under ver-tical loading.

Fig 4a Porcelain.



Inc: 1 Time: 1.000e + 000

2.345e + 000–6.843e – 002–2.481e + 000–4.894e + 000–7.307e + 000–9.720e + 000–1.213e + 001–1.455e + 001–1.696e + 001–1.937e + 001–2.179e + 001

Inc: 1 Time: 1.000e + 000

2.234e + 000–3.710e – 001–2.976e + 000–5.581e + 000–8.186e + 000–1.079e + 001–1.340e + 001–1.600e + 001–1.860e + 001–2.121e + 001–2.381e + 001

Inc: 1 Time: 1.000e + 000

2.072e + 000–6.497e – 001–3.372e + 000–6.094e + 000–8.816e + 000–1.154e + 001–1.426e + 001–1.698e + 001–1.970e + 001–2.243e + 001–2.515e + 001

Inc: 1 Time: 1.000e + 000

2.284e + 000–2.822e – 001–2.849e + 000–5.415e + 000–7.982e + 000–1.055e + 001–1.312e + 001–1.568e + 001–1.825e + 001–2.081e + 001–2.338e + 001

Inc: 1 Time: 1.000e + 000

2.515e + 0008.171e – 002–2.352e + 000–4.785e + 000–7.219e + 000–9.653e + 000–1.209e + 001–1.452e + 001–1.695e + 001–1.939e + 001–2.182e + 001





CIFTCI/CANAY

Figs 5a to 5e Minimum principal stress values (MPa) aroundimplants restored with 5 different veneering materials underoblique loading.

Fig 5a Porcelain.



Inc: 2 Time: 2.000e + 000

7.599e + 000–5.813e + 000–1.922e + 001–3.264e + 001–4.605e + 001–5.946e + 001–7.287e + 001–8.628e + 001

–1.131e + 002–9.970e + 001

–1.265e + 002

Inc: 2 Time: 2.000e + 000

1.388e + 001–1.442e + 000–1.676e + 001–3.208e + 001–4.740e + 001–6.271e + 001–7.803e + 001–9.335e + 001–1.087e + 002–1.240e + 002–1.393e + 002

Inc: 2 Time: 2.000e + 000

1.761e + 0011.163e + 000–1.528e + 001–3.173e + 001–4.817e + 001–6.462e + 001–8.107e + 001–9.751e + 001–1.140e + 002–1.304e + 002–1.469e + 002

Inc: 2 Time: 2.000e + 000

1.230e + 001–2.588e + 000–1.748e + 001–3.237e + 001–4.726e + 001–6.215e + 001–7.704e + 001–9.193e + 001–1.068e + 002–1.217e + 002–1.366e + 002

Inc: 2 Time: 2.000e + 000

7.593e + 000–5.639e + 000–1.887e + 001–3.210e + 001–4.534e + 001–5.857e + 001–7.180e + 001–8.504e + 001–9.827e + 001–1.115e + 002–1.247e + 002


CIFTCI/CANAY




Ta

ble

2M

ag

nitu

de

an

dTy

pe

of P

rincip

al S

tresse

s a

rou

nd

the

Firs

t an

d S

eco

nd

Imp

lan

ts a

t C1

an

d C

2 L

ev

els

Maxim

um

prin

cip

al s

tress (M

Pa)

Min

imu

m p

rincip

al s

tress (M

Pa)

Typ

e o

f stre

ss

Vertic

al

Ob

liqu

eH

oriz

on

tal

Vertic

al

Ob

liqu

eH

oriz

on

tal

Vertic

al

Ob

liqu

eH

oriz

on

tal

LB

LB

LB

LB

LB

LB

LB

LB

LB

Implant 1Level C

1P

orcelain–0.60

4.55–0.20

54.100.50

64.15–17.75

–6.15–86.50

–22.75–95.80

–24.85C

MC

TC

TG

lass-modified

–0.552.75

2.4045.95

3.7055.35

–15.50–7.25

–78.05–24.50

–86.45–28.25

CC

CT

CT

composite resin

Acrylic

resin–0.45

1.955.35

40.257.10

49.10–10.55

–7.85–64.80

–21.50–74.55

–26.15C

CC

TC

TC

omposite resin

–0.503.00

1.8047.55

2.8557.00

–15.90–7.10

–79.55–26.85

–88.20–29.30

CC

CT

CT

Gold alloy

–0.554.95

–0.2055.35

0.1065.45

–18.00–6.00

–87.60–21.60

–97.00–23.45

CM

CT

CT

Level C2

Porcelain

1.334.70

9.4750.00

11.0058.50

–12.27–4.00

–66.67–17.10

–74.73–18.70

CM

CT

CT

Glass-m

odified 1.47

3.2010.13

42.5011.60

51.50–11.00

–4.70–61.53

–22.00–69.13

–24.50C

MC

TC

Tcom

posite resinA

crylicresin

1.402.30

10.2738.10

11.7345.40

–10.33–3.90

–55.60–18.70

–66.00–21.40

CM

CT

CT

Com

posite resin1.40

3.5010.07

43.8011.47

51.60–11.20

–4.60–62.40

–21.40–70.00

–23.70C

MC

TC

TG

old alloy1.27

5.0010.27

51.1011.93

59.60–12.40

–3.90–67.47

–16.10–75.60

–17.60C

MC

TC

T

Implant 2Level C

1P

orcelain2.53

10.27–2.13

63.40–9.33

76.07–20.13

–1.33–120.80

6.93–140.20

5.60C

TC

TC

TG

lass-modified

2.809.20

1.4055.00

1.0066.33

–17.87–2.93

–105.801.73

–123.130.47

CT

CT

CT

composite resin

Acrylic

resin2.00

10.93–11.27

49.67–13.47

64.33–17.60

–0.40–96.34

9.53–117.00

8.87C

TC

TC

TC

omposite resin

2.809.47

0.8056.33

0.3367.67

–18.20–2.67

–111.20–2.33

–124.80–5.07

CT

CT

CT

Gold alloy

2.407.40

–8.7364.60

–10.4076.40

–20.60–1.60

–127.138.80

–142.6010.20

CT

CT

CT

Level C2

Porcelain

3.006.50

8.4035.90

8.8043.40

–13.90–3.70

–70.80–1.40

–78.60–1.50

CM

CT

CT

Glass-m

odified 2.90

5.806.60

31.006.90

37.80–12.40

–4.60–64.20

–4.70–71.20

–4.90C

MC

TC

Tcom

posite resinA

crylicresin

3.003.47

9.8036.24

10.4031.2

–11.60–2.9

–56.40–3.1

–61.30–1.9

CM

CT

CT

Com

posite resin3.00

6.106.70

31.707.00

38.70–12.70

–4.40–65.20

–4.00–72.30

–4.20C

MC

TC

TG

old alloy3.00

6.608.70

36.809.30

44.40–14.30

–3.50–72.10

–1.10–80.00

–1.60C

MC

TC

T

L = lingual; B

= buccal; C

= com

pressive stress; T = tensile stress; M

= m

ixed stress.

DISCUSSION

This study used the 3-D FEA method to comparestress distribution in a mandibular posterior seg-ment in which an implant-supported prosthesis wasfabricated using different types of veneering materi-als. The bone quality and quantity in the cervicalregion may be critical to the long-term success ofdental implants. Its loss could endanger implant sta-bility. The type of veneering material is importantfor conducting the stress generated by static orimpact forces to the lower structures.5,12,25

Since the modulus of elasticity of the compositeresin (14.1 GPa) is higher than that of the glass-modified composite resin and acrylic resin, thismaterial is resistant to fracture during masticationand absorbs maximum energy. On the other hand,porcelain transfers nearly all of the load to the bonebecause of its higher modulus of elasticity (70 GPa).Glass-modified composite and acrylic resins, with alower modulus of elasticity (10 and 2.26 GPa,respectively), transfer minimal load to the bone andabsorb the load.

When findings from this study were analyzed, itwas seen that stress levels to the cortical bone aroundimplants were lower in the models that used acrylicresin, composite resin, or glass-modified compositeresin as veneering materials compared to those withporcelain or gold alloy. Davis and colleagues12 con-cluded that acrylic resin helps to reduce the stressesunder impact conditions, such as those that occurwhen a patient inadvertently bites on a hard object.Gracis and coworkers14 used strain gauges in theirstudy and concluded that microfilled light-cured andheat-cured composite resins and heat-cured poly-methylmethacrylate resin materials showed impactforces that were 50% lower than those of porcelainand or gold alloy. Brånemark11 recommended acrylicresin teeth for occluding surfaces of complete-archprostheses in completely edentulous patients treatedwith implants. He inferred that this material wouldcompensate for the resiliency normally provided bythe periodontium. However, other researchers con-cluded that changing the veneering material on theprosthesis had no significant effect on the stress levelor distribution at the bone-implant interface.5,26


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80

70

60

50

40

30

20

10

0Goldalloy

Porcelain Compositeresin

Glass-modifiedcomposite resin

Acrylicresin

59.50

24.50

56.00

23.20

40.10

17.05

36.50

15.6520.20

9.80

Str

ess

(MPa

)

C1

C2

Material

60

50

40

30

20

10

0Goldalloy

Porcelain Compositeresin

Glass-modifiedcomposite resin

Acrylicresin

39.67

15.31

37.33

14.50

26.73

10.66

24.33

9.7813.47

6.13

Str

ess

(MPa

)

C1

C2

Material

Fig 6 Normal principal stress values (for thedisplacement) around the first implant at bothC1 and C2 levels.

Fig 7 Normal principal stress values (for thedisplacement) around the second implant atboth C1 and C2 levels.

According to the present results under staticloading and impact conditions, the use of acrylicresin reduces the stress that is transmitted to theframework and the cortical bone. Another advan-tage of acrylic resin is the relative ease with which itcan be added to the framework and adjusted whennecessary. However, low resistance to abrasion andfracture are disadvantages to its use.

When the structured occlusal scheme and mor-phology cannot be maintained over time, undesir-able lateral forces may increase.17,27 Glass-modifiedcomposite resins represent an attempt to eliminatethe disadvantages of acrylic and composite resins.Glass-modified composite resins provide wear resis-tance similar to tooth enamel. Inorganic active silicaand microparticles of barium, aluminum, and siliconglass (Ba-Al-Si glass) were found among the glassparticles, which were higher in organic content.Thus glass-modified composite resin is stiffer andmore resistant to fracture.

In light of the high stress calculated for obliqueand horizontal forces in each model, it is importantto create an occlusal scheme that minimizes lateralforce components in planning and fabricating asuperstructure. Buccal cusp inclination was set at 30degrees in the present models, and force was equallydistributed at 125 predetermined nodal points onthe buccal inclination of the lingual cusps. The ulti-mate tensile and compressive strength values of cor-tical bone have been reported as 121.00 MPa and167.00 MPa, respectively.28 In models that usedmetal as the veneering material, compressive stressvalues of a maximum of 119.40 MPa were measuredon the lingual side of the cortical bone of the firstimplant and 157 MPa at the second implant. Thesevalues are very close to the ultimate strength valueof the cortical bone, which is 167 MPa.

This study has demonstrated that stress valuesfound at the C1 and C2 level of bone around theimplant are a function of the veneering material.The compressive stress values on the lingual aspectof the cortical bone around the second implant werehigher than those in the same region of the firstimplant. This may be because of the 10-degree lin-gual inclination of the second implant resultingfrom the natural angulation of the mandible.

Impact forces have more destructive effects onthe bone surrounding the implants and on thesuperstructures. Stress distribution is directlyrelated to the elastic modulus of the veneeringmaterial, ie, the stiffer the material, the more stresswill be transmitted to the bone.

A properly selected prosthetic material will mini-mize forces on implants and consequently reducestresses in the supporting bone. It is impossible to find

every desirable characteristic in a single veneeringmaterial. Acrylic resin reduces impact force; however,it absorbs water easily, which contributes to discol-oration. Porcelain has a higher modulus of elasticitythan acrylic resin, so if it is added to a framework, ascompared to acrylic resin or composite resin, morestress will be taken by the superstructure (under a sta-tic load). The greater wear resistance of porcelain,which may contribute to premature overload on theimplants over time, would indicate a need for cautioususe. Moreover, opposing dentition, as well as the per-sonal characteristics of the patient and potential para-functional mandibular movements, should be noted.

When metal alloy was used as the superstructurematerial, similar characteristics to porcelain in termsof load transfer to the implant and its surroundingswere seen; however, it is esthetically inappropriate.In contrast, composite resins or glass-modified com-posite resins allow the formation of low stress levelsin the bone around the implant, and their estheticqualities are very satisfactory. Particularly with theArtglass system, its increased bonding with themetal framework by the Kevloc system (HereausKulzer), ease of implant placement and working inthe mouth, and the present findings of minimalstress levels under all types of loading provide sig-nificant advantages. Glass particles incorporatedinto the structure add resilience. Thus, it is an alter-native material that can be used comfortably forimplant-supported fixed partial prostheses.

CONCLUSIONS

The mechanical behavior of an implant-supportedframework for which 5 different veneering materialswere used was examined using FEA. The followingconclusions were drawn:

1. For all veneering materials, stress was highestunder horizontal and oblique loading and lowestunder vertical loading.

2. For all models, the most extreme stress valueswere located within the implant collar immedi-ately below the bony crest.

3. Maximum compressive stresses were seen on thelingual aspect of the cortical bone, and these val-ues were very close to approximating the ulti-mate strength of the bone.

4. Resin materials are beneficial in reducing thestresses endured under different loading condi-tions. Acrylic resin or glass-modified compositeresin reduced the stress by 25% and 15%,respectively, when compared to equivalent thick-nesses of porcelain or metal.


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