Analysis of stress on a fixed partial denture with a blade-vent implant abutment

Noriaki Takahashi, D.D.S.,” Tetsuya Kitagami, D.D.S., Ph.D.,**. and Tomio Komori, D.D.S., Ph.D.*** Osaka Dental University, Osaka, Japan

D ental implants are being used more frequently, with many histologic studies having been reported,‘-’

but only a few studies have been made that deal with physical problems. Yet more importance should be

given to investigations dealing with physical forces

that stabilize dental implants in a functional state. Tesk and associates’ reported stress distribution in the bone arising from loading on endosteal implants,

and Weinstein and associates” reported stress anal- ysis of porous rooted implants. However, there is no

literature on stress analysis of dental implants with superstructures.

A study of stress distribution of dental implants imbedded in the bone is described, with special

reference to the model mandibular posterior fixed partial denture constructed on a natural tooth and a

blade-vent implant abutment. The results were compared with the findings of a fixed partial denture

constructed on two natural tooth abutments. A finite element method was employed for a stress

analysis. This method was invented to analyze aero- nautical structures in 1956,” and it has been a part of

many reports about mechanical problems in dental restorations.

MATERIALS AND METHODS

A blade-vent implant was imbedded at the site of the mandibular second molar, and a fixed partial

denture was constructed on the second premolar and the molar implant abutments. As a control a fixed partial denture was constructed on the second premolar and the second molar; each pontic was a sanitary type. The size of the abutment tooth and the thickness of the periodontium and periimplan-

*Senior, Graduate School of Osaka Dental University. **Assistant Professor, Department of Prosthodontics. ***Professor, Department of Prosthodontics.

tium were based on the previous literature.‘.” The

thickness of the periodontium was 0.4 mm at the cervical part, 0.2 mm at the mid part, and 0,25-0.30

mm at the apical part. The thickness of the periim- plantium was 0.4 mm. It was assumed that the bone

was isotropic and homogeneous.

A finite element model of the fixed partial denture constructed on the second premolar and implant

abutments consisted of 428 triangular elements and 259 nodal points (implant fixed partial denture) (Fig. 1). A model of the fixed partial denture

constructed on the second premolar and the second molar abutments consisted of 342 triangular

elements and 212 nodal points (natural tooth fixed partial denture) (Fig. 2).

The physical properties of each material, based on the previous literature, are given in Table I.“‘-” It

was considered that the peripheral lines of the bone were fixed for support. A vertical load and an

inclined load 45 degrees distal to the vertical axis were created at the pontic with a 1 kg weight.

Deflections and stresses under each condition were computed mathematically with a two-dimensional finite element method.

RESULTS

The deffection field of the implant fixed partial

denture under the vertical load is seen in Fig. 3. The fixed partial denture unit was depressed by loading.

A principal stress distribution under the same eondi- tion is shown in Fig. 4. Stress concentration was

markedly found in the pontic, at the implant neck, and at the mesial and distal parts of the premolar retainer. Stresses below 0.2 kg/mm2 were omitted in all principal stress distribution figures.

The deflection field of the natural tooth fixed partial denture under the vertical load is seen in Fig. 5. The fixed partial denture unit was depressed by

186 AUGUST 1978 VOLUME 40 NUMBER 2 0022-3913/78/024M)1~~.~/OQ 1978 The C. V. Mosty Co.

STRESS ANALYSIS ON BLADE-VENT ABUTMENT

Fig. 1. Mathematical model of a fixed partial denture constructed on the second premolar and implant abut- ments (implant fixed partial denture). Triangular elements, 428; nodal points, 259.

Fig. 2. Mathematical model of a fixed partial denture constructed oh the second premolar and second molar abutments (natural tooth fixed partial denture). Trian- gular elements, 324; nodal points, 212.

loading. Fig. 6 shows a principal stress distribution under that same condition. Stress concentration was markedly found in the pontic, at the mesial and distal parts of the premolar retainer, and at the mesial part of the posterior retainer. Higher stresses were induced in the premolar abutment than in the posterior abutment.

The deflection field of the implant fixed partial denture under the inclined load is shown in Fig. 7. Following rotational movement of the implant, the fixed partial denture unit was moved along the loading direction and the distal shoulder of the implant was elevated. A principal stress distribution under that condition is seen in Fig. 8. Stress concentration was markedly found in the pontic, at the implant neck, and at the mesial and distal parts of the premolar retainer.

The deflection field of the natural tooth restora- tion under the inclined load is depicted in Fig. 9. The

VI ‘\ ’ ------- UNLOADED

- LOADED

Fig. 3. Deflection field of an implant fixed partial denture under vertical load.

Fig. 4. Principal stress distribution of an implant fixed partial denture under vertical load (stresses below 0.2 kg/ mm2 were omitted).

Table I. Physical properties of each material

M&&d

Implant metal

Casting metal Bone Dentin Periodontium Periimplantium

FObSOI?‘S

Y4l&S “) rath,

2oooo.00 0.33

9500.00 0.33

2oc!o.00 0.30

1200.00 0.30

1.00 0.45

1.00 0.45

fixed partial denture unit was moved along the loading direction. Fig. 10 shows a principal stress distribution under the same condition, Stress was markedly concentrated in the pontic, the me&al and distal parts of the premolar retainer, and the mesial part of the posterior retainer. Higher stresses were induced in the premolar abutments than in the posterior abutment.

THE JOURNAL OF FROSTHETJC DENTISTRY 187

TAKAHASHI, KITAGAMI, AND KOMORI

Fig. 5. Deflection field of a natural tooth fixed partial denture under vertical load.

Fig. 6. Principal stress distribution of a natural tooth fixed partial denture under vertical load (stresses below 0.2 kg/mm2 were omitted).

Table II shows absolute quantities of deflection at five reference points under each condition, i.e., the

premolar cusp, the center of the premolar cervix, the apex of the premolar, and the central fossa of the

posterior retainer. The implant restoration revealed less deflection than the natural tooth restoration in the vertical and inclined load.

Table III shows equivalent stresses* at the surrounding bone around the premolar under each

*Equidmt stress: It is f&wed by the shearing-strain energy theory and given by following formula:

Equivalent stress = J(S: - S, S, + S:) where S, = maximum principal stress and S, = minimum principal stress.

\ \ I U ------- UNLOADED - LOADED

Fig. 7. Deflection field of an implant fixed partial denture under inclined load.

Fig. 8. Principal stress distribution of an implant fixed partial denture under inclined load (stresses below 0.2 kg/ mm’ were omitted).

condition. The surrounding bone was divided into three parts-the mesial, apical, and distal. Equiva-

lent stresses placed on the implant restoration tended to be less than those placed on the natural tooth

restoration. This trend was found in the vertical and inclined loads at each part of the surrounding bone.

Table IV shows equivalent stresses at the surrounding bone around the posterior abutment

under each condition (dotted area in Fig. 11). The surrounding bone was divided into three parts-the mesial, apical, and distal parts. In the apical part equivalent stresses placed on the implant abutment were less than those placed on the natural tooth abutment. But in the mesial and distal parts of the bone, as well as the total bone, equivalent stresses were higher than those placed on the natural tooth

188 : AUGUST 1978 VOLUME 40 NUMBER 2

STRESS ANALYSIS ON BLADE-VENT ABUTMENT

Fig. 9. Deflection field of a natural tooth fixed partial denture under inclined load.

Table II. Quantities of deflection at the Table III. Equivalent stresses at the surrounding reference points (pm) bone around the premolar (kg/mm’)

Reference points

Inclined Vertical load load

Natural Implant Natural Implant posterior posterior posterior posterior abutment abutment abutment abutment

Inclined Vertical load load

Areas of Natural Implant Natural Implant surrounding posterior poswior posterior posterior

bone dw&ment abutment t abutarent

Bicuspid cusp 4.4 3.2 5.2 4.7 Center of biscus-

pid cervix 3.8 3.5 6.2 5.6 Apex of bicuspid 3.5 2.8 2.1 2.0 Center of pontic 8.3 6.3 7.0 6.3 Central fossa of

posterior retainer 6.7 3.6 6.1 5.1

abutment; such findings were found with both the vertical and inclined loads. These results indicated

that stress became higher around the posterior abut-

ment and lower around the premolar abutment in

the implant restoration as compared to the natural tooth restoration.

DISCUSSION

In previous reports 13. ‘-I stresses and deflections of a blade-vent implant without superstructures were studied and some results obtained about the basic behavior of an implant under loading. However, it is

clinically necessary to construct superstructures, e.g., fixed partial dentures on an implant abutment.

When a fixed partial denture is constructed on a root implant and natural tooth abutments it is important to investigate stresses induced in the surrounding bone supporting an implant fixed partial denture as

Fig. 10. Principal stress distribution of a natural tooth fixed partial denture under inclined load (stresses below 0.2 kg/mm? were omitted).

Mesial part 3.748 2.786 3.123 2.789 Apical part 0.2400 0.196 0.171 0.160 Distal part 0.220 0.201 0.343 0.130

Total 4.208 3.183 3.637 3.259

compared to stresses produced with a conventional fixed partial denture constructed OR natural tooth

abutments.

In this study the physical properties of materials

given in Table I were used. Only a few studies about the periodontium and periimplantium have been

reported from a physical viewpoint, while there have

been many such reports about other materials. It is necessary to consider the physical properties of the

periodontium and periimplantium in relation to their mechanical behavior.‘”

The fixed partial dentures were loaded at the center of the pontic with a 1 kg weight. It is possible

to proportionally calculate deflections and stresses under any magnitudes of load because this stress

analysis was made within the proportional limit. Therefore it was suggested that the results obtained be qualitatively reviewed and that a quantitative review be avoided.

From Table II it is obvious that deflections of the

THE JOURNAL OF PROSTHETIC DENTISTRY 189

Fig, 11. Surrounding bone for comparison of stresses under each condition (dotted area in Fig. 11). The left side of the model was omitted.

implant fixed partial denture were less than those of

the natural tooth fixed partial denture in each loading. It was suggested that these findings might

be caused by the wide base of a blade-vent implant.

But a more advanced three-dimensional analysis is desirable at this point because the thickness of a

blade implant greatly differs from that of the natural

tooth.

Regarding Tables II and III, equivalent stresses induced in the surrounding bone became higher around the posterior abutment and became lower

around the premolar abutment in the implant fixed

partial denture than in the natural tooth fixed partial denture. Therefore it was suggested that

occlusal forces have to be more concentrated on the premolar abutment in the implant restoration to

relieve stress in the bone around the implant abut- ment. It is recommended that two premolar abut-

ments be used and the occlusal contacts of a poste- rior retainer be reduced.

CONCLUSION

Using a two-dimensional finite element method, a

study was made that compared the behavior of a model mandibular posterior fixed partial denture constructed on the second premolar abutment and a

blade-vent implant imbedded at the site of the second molar with the behavior of a fixed partial denture constructed on the second premolar and second molar abutments.

The following were the results: 1. Deflections of the implant fixed partial denture

were less than those of the natural tooth fixed partial denture in vertical and inclined loads.

2. Stress concentration was markedly found in the

TAKAHASHI, KITAGAMI, AND KOMOKI

Table IV. Equivalent stresses at the surrounding

bone around the posterior abutment (kg/mm’)

Vertical load inclined load

Areas of Natural Implant Natural Implant surrounding posterior posterior posterior posterior

bone abutment abutment abutment abutment

Mesial part 0.090 0.436 0.098 0.245

Bottom 0.861 0.665 0.406 0.304

Distal part 0.229 0.490 0.107 0.236

Total 1.180 1.591 0.611 0.875

pontic and the mesial and distal parts of the

premolar retainer in both restorations and the

implant neck in the implant fixed partial denture. 3. In the implant fixed partial denture, stresses

induced in the surrounding bone became higher

around the posterior abutment and became lower around the premolar retainer than the stresses

produced with the natural tooth fixed partial

denture.

4. Therefore it was suggested that, to relieve stress to the surrounding bone around the implant abut-

ment, occlusal forces loaded to the implant fixed partial denture have to be more concentrated on the

premolar abutment than do forces loaded to the natural tooth fixed partial denture.

REFERENCES

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AUGUST1978 VOLUME40 NUMBER2

STRESS ANALYSIS ON BLADE-VENT ABUTMENT

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Rcprmt requests to. DR. N~RIAKI TAKAHASEU

l-47 KYCBASHI

HIGASHI-KU

OSAKA, JAPAN

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