RESIN-BONDEDPROSTHESES

16. Cheung GS, Dimmer A, Mellor R, Gale M. A clinical evaluation of conventional bridgework. J Oral Rehabil 1990;17:131-6. 17. Freilich MA, Niekrash CE, Katz RV, Simonsen RJ. The effects of resin-bonded and conventional fixed partial dentures on the periodontium: restoration type evaluated. J Am Dent Assoc 1990;121:265-9. 18. Isidor F, Budtz-J$rgensen E. Periodontal conditions following treatment with distally extending cantilever bridges or removable partial dentures in elderly patients. A 5-year study. J Periodontol1990;61:21-6.

Three-dimensional finite element cantilever fixed partial denture Hania Yahia University

A. Awadalla, BDS, MDS,a Mohsen Azarbal, H. Ismail, DMD, PhD,C and Wael El-Ibiari, of Pittsburgh,

School

of Dental

Medicine,

Pittsburgh,

Reprint

requests to:

DR. FLEMMINC ISIDOR DEPARTMENT OF PROSTHETIC OGY ROYAL DENTAL COLLEGE VENNELYST BOULEVARD DK-8000, AARHUS C DENMARK

stress DMD, BSd

DENTISTRY

analysis

AND STOMATOGNATHIC

PHYSIOL

of a

MDS,b

Pa.

A three-dimensional mathematical model was generated, representing a three-unit cantilever fixed partial denture and its supporting mandibular structures. First and second premolars were used as abutments with one posterior cantilever pontic. A 5 lb vertical load was applied to the pontic. Vertical and horizontal stresses were analyzed by means of a three-dimensional finite element stress analysis technique. The results showed that a cantilever pontic creates considerable compressive stress on the abutment nearest to the pontic and produces tensile stress on the abutment farthest from the pontic. (J PROSTHET DENT 1992;68:243-8.)

R eplacement of missingposterior teeth in the absenceof distal abutments can be achieved by one of the following ways: (1) with removable partial dentures with distal extensionbases,(2) with fixed partial denturesusing implants as distal abutments, and (3) with fixed partial dentures with cantilever pontics. Conflicting opmionsexist regarding the useof cantilever fixed partial denturesto restore edentulousspaceswithout a distal abutment. The decision to use a cantilever fixed partial denture should be basedon a sound periodontal condition, good alveolar bone support, favorable toothto-tooth and arch-to-arch relationships, favorable root shape, a good crown-to-root ratio, and clinical experience.l This study evaluated stress distribution in the abutment teeth and their supporting alveolar bone in a mandibular posterior cantilever fixed partial denture using a three-dimensional finite element stress analysis technique. aGraduate Resident. bAssistant Professor, Department of Prosthodontics. CProfessor and Chairman, Department of Prosthodoutics. dEngineering Consultant. 10/l/37744

THE JOURNAL

OF PROSTHETIC

DENTISTRY

MATERIAL

AND

METHODS

In this study the finite elementstressanalysistechnique wasusedto evaluate the stressesin a mandibular posterior cantilever fixed partial denture. Two- and three-dimensional finite element stressanalyseshave beenusedextensively in dentistry. 2-gA three-dimensional stressanalysis technique is preferred over a two-dimensionalone because it is an actual representation of the stressbehavior in the supporting bone.7 A three-dimensionalmathematical model of the mandible was used in this study, representing the mandibular canine, first premolar, secondpremolar, and their supporting structures with a cantilever pontic and 7 mm of the edentulous ridge distal to the secondpremolar. A threeunit cantilever fixed partial denture was designedusing first and secondpremolarsasabutments and a first molar as a pontic. In making the finite element model, the anatomic form and dimensions,the modulusof elasticity, and Poisson’sratios equivalent to those structures being simulated were entered into the computer program. Components of the dental model were mandibular bone, periodontal ligaments,natural teeth, type 3 gold crowns,and full gold pontics. Both abutment teeth were restored with cast crowns, using type 3 gold with one cast gold cantilever pontic replacing the first molar.

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AWADALLA

i

‘4

I

-

1

Fig. 1, Mathematical model illustrating gual planes of reference.

seven faciolin-

The mathematical model was completed and the finite element model was developed by dividing the model into 1216 elements connected at 1614 points known as nodes. All other required data were entered into the computer using the GTSTRUDL (GTICES Systems Laboratory, “GTSTRUDL User Manual,” Georgia Institute of Technology, version 83.02) program. Five pounds of vertical occlusal load was applied to the occlusal surface of the pontic. Generated stresses were calculated rrumerically and plotted graphically in seven specific faciolingual cross sections of the model (Fig. 1). Principal vertical and horizontal stresses (both compressive and tensile) were considered in this study. RESULTS Vertical and horizontal stress values and contours are presented in the form of compressive and tensile stresses in seven specific faciolingual planes of reference (Fig. 1). Table I summarizes quantity of the stresses. At planes 1 and 2, which represent the canine tooth and surrounding bone, vertical stresses were low and tensile horizontal stresses were low and compressive. Vertical stresses in the bone between the canine and first premolar (plane 3) were low and tensile, and horizontal stresses were primarily low and compressive. Plane 4 represents the first premolar and its surrounding bone. Vertical stresses in the bone were low and tensile in nature, and were concentrated around the apex and middle third of the root (Fig. 2, right panel ). Vertical stresses inside the root of the first premolar were high and tensile, and were decreased in cervical and apical directions (Fig. 2, left panel). Horizontal stresses in bone surrounding the root of the first premolar were low and tensile in nature, appear244

ET AL

ing around the apex and in the crest of the alveolar bone. These stressescontinued to decrease,both anteriorly and laterally, until they changedto compressivestresses(Fig. 3, right panel). Horizontal stressesinside the root of the first premolar were low and tensile, occurring around the apex and decreasedcervically until they reached a zero value. They then changedto compressivestressat the cervical surface (Fig. 3, left panel). In the bone between the first and secondpremolar (plane 5), vertical stresseswere low and compressiveand wereobservedin the middle third of the section. Horizontal stresseswere low and tensile in nature, and were present in the upper third of the section. Plane6 representsthe secondpremolar and its surrounding bone. Vertical stressesin this area were moderate and compressivein nature, and decreasedgradually both laterally and anteriorly (Fig. 4, right panel). Vertical stresses inside the root of the secondpremolar were high and compressive in nature at the apical third of the root. These stressesdecreasedcervically until they reached a zero value, then they changed to tensile stresses(Fig. 4, left pane2). Horizontal stressesin the bone around the root of the secondpremolar were low and compressive,and were recorded around the entire root length. These stresses changed to low and tensile in a lateral direction (Fig. 5, right panel). Horizontal stressesinside the root of the second premolar were high and compressiveand were concentrated at the apical third of the root (Fig. 5, left panel). At plane 7, which representsbone under pontic, vertical low compressive stresses were observed while horizontal stresseswere low and tensile in nature. DISCUSSION Among the different methods of stress analysis, the three-dimensional finite element stress analysis was selected. This method allows closesimulation of the components of the dental m.odelunder investigation. The amount of biting force decreasesfrom the molar region to the incisor region. The averagebiting force in the molar region is 127 lb, and in premolar, canine, and incisor regions,is 65, 47, and 35 lb, respectively. The chewing force in the first molar, which wasrestored with the fixed partial denture, was37% of the natural dentition, or an averageof 55 lb.rO There is a linear relationship betweenthe applied load and the internal stressgenerated.i1 This means,for example, if a load of 5 lb produced a maximum of 124psi stress,a load of 55 lb will generatea maximum stressof 124 X 11 = 1364 psi. The highest value of tensile stressin the entire model (145.7psi) wasrecorded inside the root of the first premolar at the center. The highest value of all compressive stresses(125 psi) was recorded inside the root of the second premolar at the apical third. In bone, the most significant values of vertical tensile stresswere 14 and 14.5 psi, recorded between the canine and the first premolar, and around the first premolar, respectively.

AUGUST

1992

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3-D

STRESS

ANALYSIS

OF FPD

Fig. 2. Vertical

Table

I. Highest

stress contours

stress values recorded

in model

inside root of first premolar

(in pounds Vertical

Planes

Components

Piane

1

Plane

2

Plane

3

Bone Bone Root Bone Bone Root Bone Bone Root Bone

Plane 4 Plane

5

Plane

6

Plane

7

only

only

only

only

JOURNAL

OF PROSTHETIC

DENTISTRY

bone.

per square inch) stresses

Horizontal

Compressive

Tensile

Compressive

2.2 0 1.0 0 0 0 13.0 46.0 125.0 15.6

5.2 a.2 3.0 14.0 14.5 145.7 0 6.5 21.7 0

4.1 2.0 2.7 0 0 2.7 5.6 25.3 101.6 3.4

The most significant values of horizontal tensile stress were 9 and 18.2 psi, recorded around the root of t.he first and second premolars. The most significant values of vertical compressive stress were 13,15, and 46 psi, recorded between the first and second premolars, in the pontic area, and around the root of the second premolar, respectively. Finally, the most significant value of horizontal compressive stress in the bone was 25.3 psi and was recorded around the root of the second premolar. These stresses were generated as a result of applying 5 lb

THE

and surrounding

stresses Tensile

1.0 2.0 0 3.8 9.0 34.0 8.5 18.2 2.5 13.7

of vertical occlusal load. Since this magnitude of force is 11 times less than the average biting force on a restored side,1° these resultant stresses should be multiplied by a factor of 11 for a more accurate calculation of stresses generated by a cantilever fixed partial denture and of its effect on the supporting bone. By evaluating different kinds of stresses and their magnitude, it is evident that the abutment nearest the edentulous space and its supporting bone receives compressive stresses, and the abutment farthest from the pontic space and its bone support is under tensile stress.

245

AWADALLA

Fig.

3. Horizontal

stress

contours

inside

root

of first

premolar

and surrounding

bone.

of second

premolar

and surrounding

bone.

ET

AL

r------“-----i -200 -300 -460 -500 -----$-O0 ‘eoa 3 G

Fig. 4. Vertical

stress

contours

inside

root

AUGUST

1992

VQLWME

t38

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2

3-D

STRESS

ANALYSIS

OF FPD

Fig.

5. Horizontal

stress contours

inside root of second premolar

The effect of the stress on the bone has been discussed by many investigators. 2a12,13 Although bone is influenced by many systemic and local factors, most investigators studying bone tissue have accepted the concept that biomechanical forces represent the principal factor that regulates the course of bone remodeling through life. “Nothing is known about the magnitude of stress that is capable of initiating bone resorption and apposition. Clinically, however, it is known that relatively small force magnitudes are capable of tipping teeth (10 gm or less) on maxillary central incisors.“14 The duration of force application is more influential in bone resorption and deformation than its quantity. I5 Further study is needed to clarify the limit of bone tolerance to stresses without resorption.

SUMMARY

AND

CONCLUSIONS

The study of vertical forces in a cantilever fixed partial denture analyzed by means of a three-dimensional finite element stress analysis technique showed: 1. Vertical forces applied to abutment tooth through a cantilever pontic are resisted both vertically and horizontally. 2. The abutment nearest to the pontic receives more than 50% of the forces. These forces, which are mainly compressive, generate 101.6 to 125 psi compressive stress in the root and 25.3 to 46 psi in surrounding bone.

THE

JOURNAL

OF PROSTHETIC

DENTISTRY

and surrounding

bone.

3. The abutment farthest away from the pontic assumes tensile forces. These forces generate tensile stress of 34 to 145.7 psi in the root and 9 to 14.5 psi in surrounding bone. 4. To achieve the actual value of these stresses, which are the result of a 5 lb vertical load, they were multiplied by 11 times, because the average biting force on a posterior fixed partial denture is 55 lb. REFERENCES 1. Antonoff SJ. The status of cantilever bridges. Oral Health 1973;63:8-14. 2. Craig RG, Farah JW. Stresses from loading distal-extension removable partial dentures. J PROSTHET DENT 197&39:274-7. 3. Gupta KK, Knoell AC, Grenoble DE. Mathematical modeling and structural analysis of the mandible [Abstract]. J Biomat Med 1973;1:469. 4. Hadeed GJ, Ismail YH, Garrana H, Pahountis LN. Three-dimensional finite element stress analysis of Nobelpharma and Core-Vent implants and their supporting structures [Abstract]. J Dent Res 1988;

67:286. 5. Ismail YH, Michel

6. 7.

8. 9.

MC, Hadeed GJ, Pahountis LN. A three-dimensional finite element stress analysis of a Bladevent implant. J Dent Res 1986;65:304. Ismail YH, El-At&r MS, Zaki HS, Pahountis LN. Three-dimensional finite element stress analysis of TPS implant. J Dent Res 1986;65:304. Ismail YH, Pahountis LN, Fleming JF. Comparison of two-dimensional and three-dimensional finite element analysis of a blade implant. Int J Oral Implant 1987;4:25-31. Tesk JA, Widera 0. Stress distribution in bone arising from loading on endosteal dental implant. J Biomed Mater Res Sym 1973;4:251-61. Takahashi N, Kitagami T, Kimoro T. Behavior of teeth under various loading conditions with finite element method. J Oral Rehabil 1980:7:453-61.

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10. Craig RG, O’Brien NJ, Powers JM. Restorative dental Materials. 6th ed. St. Louis: CV Mosby Co, 1980. 11. Kakudo Y, Ishoda A, Yashimoto S. Strains in the dog’s jaw bones following implant insertion and a photoelastic study of implants and the jaw bone. J Osaka Dent University 1973;7:1-31. 12. Aydinlik E, Akay HU. Effect of the resilient layer in a removable partial denture base on stress distribution to the mandihle. J PROSTHET DENT

1980;44:1-17.

13. DeAngelis V. Observation on the response of the alveolar bone to orthodontic force. Am J Orthod 1970;58:284-94. 14. Graber TM, Swain BF. Orthodontics, current principles and techniques. St. Louis: CV Mosby Co, 1985:ZOl.

In vitro tubules

evaluation activated

15. Akerman J Orthod Reprint

JL, Cohen MI. The effects of quantified 19665234-6.

ET AL

pressure on bone. Am

requests to;

DR. MOHSEN AZARBAL SCHOOL OF DENTAL MEDICINE UNIVERSITY OF PITTSBURGH 3501 TERRACE ST. PITTSBURGH, PA 15261

of dynamic fluid displacement on pin placement

in dentinal

Hummer& DDS,” and David Kaiser, DDS, MSDb University of Texas Health Science Center, Dental School, San Antonio, Tex.

Thomas

The use of cemented, friction-lock, and self-threading pins for improving retention has been essential for treatment in restorative dentistry, with the self-threading pin considered to be the most retentive. Cavity varnish has been suggested to prevent microleakage around pins. This study investigated the insertion of a self-threading pin when the pinhole was filled with a liquid dye. The results showed that pinholes filled with dye before pin placement had measurable dye displacement. Pinholes without pins displayed no measurable dye displacement through the dentin; this was also true when the dye fluid was removed before the pin placement. However, the dye in pinholes was displaced toward the path of least resistance, and fluid in a pinhole can contribute to crazing of the dentin during placement. (J PROSTKET DENT 1992;68:248-55.)

he use of cemented, friction-lock, and self-threading pins for improving the retention of restorations has become routine.lm5 Cemented pins were described explicitly by Markley6, 7 in 1951 and 1958. Woehrlen8 also has published a comprehensive literature review with clinical considerations for using pins. The use of different cements and varnishes to retain pins was also reported,g, lo and optimal depths were evaluated. I1 I5 Friction-lock pins can cause stress in the dentinI and are inferior to the retention of self-threading pins. 11,17:l8 Self-threading pins are self-retentive, require no cement, and are superior in retention to cemented pins.lg Hanson et aLzO demonstrated that a self-threading pin 0.24 inch (0.61 mm] in diameter, cemented with zinc phosphate in a pinhole 0.25 inch (0.63 mm) in diameter, was one of the most retentive pins. CaOH21 has been recommended for pulpal protection and/or cavity varnish9 in controlling

aAssistant Clinical Professor, Department of Restorative tistry. “Associate Professor, Department of Restorative Dentistry; terim Head, Department of Prosthodontics. 10/l/36924

248

DenIn-

Fig.

1.

Occlusal view of sectioned pins and pinholes.

microleakage with pins. A fluid, such as cavity varnish, CaOH, or a cement, that fills a pinhole during insertion of a pin is subject to specific dynamic changes. The fluid will be displaced either through the open dentinal tubules with slight leakage at the access of the pinhole, or it can traumatize the approximating thin dentin with eventual pulpal involvement. If the fluid does not escape at the same AUGUST1992

VOLUME68

NUMBER2