--.-~
-
--~
s and strain rein James T. White, University
D.D.S., M.S.*
of North Carolina,
School of Dentistry,
Chapel Hill,
T
he purpose of this article is to present a method of observing strain patterns produced in the supporting structures of a removable partial denture. The strain patterns produced by three different removable partial dentures were compared by means of a photoelastic analysis technique.
REVIEW
OF THE LITERATURE
The photoelastic analysis technique has been used to study the tooth and related stressesin the basic sciences.‘-” The visualization of stressesproduced during orthodontic tooth movement has been studied and a photoelastic-histopathologic correlation carried out.s+ Analyses of the stress distribution properties of endodontic postsand the stresstransfer to the dental pulp by retention pins have also been reported.“, Ii’ Determining the effect of the cavity preparation on the stressdistribution in the amalgam restoration was one of the earliest applications of stressanalysis.l’-l’ The effects of the preparation design on the stressinduced in a veneer crown have also been reported.‘i‘17 An extensive study of experimental stressanalysis of dental restorations related to castings and fixed prosthodontics has been conducted.lx-” A photoelastic analysis of the internal stressesin the periodontium created by occlusal forces has led to far-reaching conclusions.‘“-3” The stressesinduced through impression removal have been visualized.‘“, ” The pressureson the teeth and bone supporting removable partial dentures and complete overdentures have been demonstrated,3”’ 3’ and stressanalysis of a maxillary complete denture during functional movements has been reported.“”
METHODS
AND MATERIALS
Internal stresses due to the effects of forces applied to a body are accompanied by internal strains within Read
before
Texas. *Assistant
the
Academy
Professor,
0022-3913/78/0240-0143$00.90/00
of Denture
Department
Prosthetics,
of Fixed
San
the body. Stress has been defined as “the internal force per unit of area which resistsa change in size or shapeof a body,” and strain has been defined as“the internal deformation per unit of length.““’ An analysis of internal strains should yield valuable criteria for successfuldesign.“’ These internal strains may be visualized and recorded with the photoelastic analysistechnique. “A photoelastic material is one which is capable of separating a polarized ray of light into two rays each polarized in a different direction.“‘” These materials become double refractive when in a state of strain, A circularly polarized transmission polariscope was used to visualize the separation of the polarized light by the photoelastic material while in a st.rained state. The colored lines produced by the white light are known as isochromatic lines. They represent “one of the frequencies being cancelled by interference at each point of double refraction (coinciding with maximum stresses),leaving a colored line that is white light minus one frequency.““” The isochromatic lines are close together in regions of high stress concentration and farther apart in regions of low stressconcentration. Replica fabrication. Three photoelastic: materials were used in the fabrication of the replica. PI-1 ,* a high-modulus epoxy resin, was used to simulate tooth structure. Pl-2,* a medium-modulus epoxy resin, was used to simulate the bony architecture. PI-~,* a low-modulus urethane, was used to simulate the periodontal
ligament.
Co
Silastic
382f
was molded
to simulate the gingiva around the teeth. Rubber base materialx equal to one thickness of baseplate wax was adapted to the ridges to simulate the mucosa. The ability periodontium “Photolastic.
Inc.,
of the replica to simulate the actual is a function of the replica’s design. Ii Malvern,
iDow Corning, Midland, $Permlastic, Kerr Sybron
Prosthodontics.
1978 The C. V. Moshy
Antonio,
N. C.
THE JOURNAL
Pa. Mich. Carp.,
Romulus.
OF PROSTHETIC
Mich
DENTISTRY
143
WHlTE
Fig. 1. The replica with the unilateral Dalbo removable partial denture in place. The geometric scaling was obtained by comparing the ratios of the E values of the tooth, periodontium, and bone to ratios of the E values of the simulated structures. The geometric scaling obtained using these materials was not as close as desired. This was the best scaling that could be attained during this project. The error should be taken into account when comparing this study to others. The replica was constructed to simulate a lower jaw requiring a bilateral distal-extension removable partial denture. The mandibular arch was intact from the first premolar to the first premolar. The second premolars and all molars were missing (Fig. 1). The first premolars and the canines were prepared to receive full cast gold crowns. Care was taken to not generate heat during the preparation of the teeth. Excessive heat generation can result in the formation of residual stressesin the replica, rendering it useless.Cast gold crowns were fabricated, and one bilateral and two unilateral removable partial dentures were constructed. Testing prostheses. The Dalbo attachment* and the May’s attachment? were usedin the construction of three testing removable partial dentures. The Dalbo attachment is a precision attachment that can be used in the fabrication of both a unilateral and bilateral distal-extension prosthesis. *APM-Sterngold, San LMateo, Calif. tDentalloy Dental Products Inc., Stanton, Calif.
144
The attachment is composed of a vertical T-rail (male portion) with a ball-shaped retentive piece projecting from the T-rail. The T-rail may be inserted into the wax pattern of the abutment and cast onto it. A slotted receptacle (female portion) accurately fits over the T-rail and has an open-ended cylinder that receives the retentive ball. A spring may be placed into the cylinder to provide vertical resilience to the attachment. The attachment provides both vertical and hinge stressbreaking features. A pin that is used to lock the attachment into a rigid state is also provided. Jaw records are made with the attachment locked.“” The May’s attachment is a precision attachment that can be used for the fabrication of a unilateral distal-extension removable prosthesis.The manufacturer provides a plastic pattern (male portion) that is luted to the wax pattern of the abutment tooth and cast in the same metal as the retainer. A manufactured slotted receptacle (female portion), with a hinge piece opening to the lingual, closesto engage a hole in the extension on the abutment crown, providing retention and a hinge movement to the removable prosthesis. The manufacturer states that a vertical action can be provided by altering the shape of the hole in the extension. The first removable partial denture was a bilateral distal-extension design incorporating two Dalbo attachments.* The principles described by Menso? were followed in the construction of the prosthesis. The ceramicor T-rails were incorporated into the distal surfacesof the first premolar crowns. A lingual plate was used as the major connector. A plaster occlusal index was made of the cusps of all the teeth. The index was used to position the denture teeth during the construction of the other two removable partial dentures. The occlusal plane and the point of force application could be reasonably duplicated by this procedure. The second removable partial denture was a unilateral prosthesis with a Dalbo attachment and no cross-arch support. The mandibular left second premolar and first molar were replaced with the prosthesis.The same abutment crown was used. The third removable partial denture was also a unilateral prosthesis with a May’s attachment. Retention was attained by engaging the hole in the abutment attachment with the latch. The castings were anchored to the photoelastic *63.03.2 unilateral Dalho attachment.
AUGUST
1978
VOLUME 40
NUMBER
2
VIWAUZATtON
IX STRAIN
PATTERNS
replica by cementation and cross-pinning with regular TMS pins.* Each removable partial denture was positioned, and a 3%pound force wa8 then applied to the central fossa of the mandibular left first molar. The straining frame used to apply the force was constructed to fit into the circular transmission polariscope. The resultant stress patterns were photographed with a 35 mm camera and type A fi1m.t To visualize the effects of using two splinted abutments, the crowns on the first premolars and canines were soldered to form bilateral double abutments. The splinted crowns were seated and crosspinned. The removable partial dentures were positioned, and a 35-pound force was then applied to the mandibular left first molar. Ali of the removable partial dentures were tested with both single and double abutments. The unilateral and the bilateral Dalbo partial dentures were tested in a fully active, less active, and rigid state. Fully active was defined as the attachment with no spring or pin in place, permitting free movement of the removable partial denture base. Less active was defined as the attachment with the spring in place, slightly restricting the vertical movement of the removable partial denture base. Rigid was defined as the attachment with the locking pin inserted, making the attachment nonmovable and thereby completely restricting the independent movement of the removable partial denture base. RESULTS Single abutment Repka before positioning prostheses. Slight residual stresses were observed in the replica before positioning the prostheses. A lightening of the photoelastic material at the apices of the canine and premolar teeth was apparent. The ridge was free of residual stress. Unilateral Dafbo attachment. The unilateral Dalbo attachment was tested first in its most active state, second in a less active state, and third in a rigid nonactive state. The most active state was achieved by removing both the spring and the pin from the attachment. As the 35-pound force was applied to the first molar, activity was observed in the ridge beneath the point of force application. The blue and red fringes repre*Whaledent, New York, N. Y. ?Eastman Kodak Co., Rochester,
THE fOlJRNAL
OF PROSTIiETIC
N. Y.
DENTISTRY
sented a region of stress concentration. The surrounding yellow fringe. represented a more distributed stress region. The first premolar had a yellow fringe band at the distal aspect of the apex. By placing the spring into the receptacle of the attachment, some of the freedom in the Dalbo attachment was removed. The stress pattern in the ridge region increased in both quantity and quality. The fringes penetrated deeper into the bone and closer to the first premolar root. The yellow fringe band associated with the distal aspect of the apex increased in size. Activity was observed on the mesial of the first premolar root at the midroot Jevel. The Dalbo attachment was rendered rigid by placing the locking pin into the attachment. This procedure greatly reduced the stress in the ridge. The stress pattern at the apex of the first premolar showed increased concentration directly below the apex. This represents a transfer of the force to the long axis of the root as opposed to being directed along the axis of the root configuration. Bilateral D&o attachment. The bilateral Dalba attachment prosthesis was tested in the same manner as the unilateral Dalbo prosthesis: most active, less active, and rigid. The springs and pins were removed from the attachments. The activity beneath the point of force application on the ridge presented a concentrated pattern. The yellow fringe band associated with the first premolar was concentrated directly below the root tip. Some activity was observed between the canine and first premolar in the interdental bone. The springs were placed into the receptacles of the Dalbo attachments. The stress pattern associated with the point of force application remained similar to that of the fully active sample. The stress pattern below the apex of the first premolar remained in the same position but increased in magnitude. A sharp yellow, red, and blue-green fringe was produced. Again, some activity was observed between the canine and first premolar in the interdental bone. The Dalbo attachments were made rigid by inserring the locking pins. The stress associated with the apex of the first premolar moved to the mesial side of the root tip and increased in magnitude. A distinct series of yellow, red, and blue fringes was observed. Summary statement. In general, the bilateral Dalbo prosthesis showed a more concentrated stress pattern in the ridge that was, however, of a similar magnitude as that produced with the unilateral Dalbo prosthesis. The stress associated with the apex
145
WHITE
Fig. 2. Stress pattern produced by the unilateral Dalbo removablepartial denturewith no pinsor springsin place. A, Singleabutment. B, Double abutment.
Fig. 3. Stress pattern produced
of the first premolar was greater in magnitude with the bilateral Dalbo prosthesis. With the May’s attachment, a stressdistribution pattern similar to that resulting with the unilateral Dalbo attachment with the locking pin inserted was observed. The ridge showed a moderately distributed stresspattern of moderate magnitude. The apex of the first premolar had a sharp yellow fringe band directly below it.
The spring was placed into the receptacle to reduce someof the freedom in the attachment. ‘I’he stresspattern beneath the point of force application remained very similar to that of the most active state. The first premolar presented a definite increase in fringe value in the apical region. At least one full fringe order was observed. The stress distribution was directly along the long axis of the root proper and was not associatedwith the distal curve at the root tip. The Dalbo attachment was rendered rigid by inserting the locking pin. The stresspattern beneath the point of force application was greatly reduced. With this reduction in stressthere was an increase in the stresspattern apical to the first premolar. There was a definite green band distal to the root in the region representing the alveolar crest. A definite fringe pattern was estabhshed in the interdental bone between the canine and the first premolar. Activity was also observed in the root of the canine abutment. Bilateral Dalbo attachment. With the springs and pins removed, the active Dalbo attachments permitted a poor distribution of stressalong the edentulous
Double abutment Replica before positioning prosthesis. The soldering and cross-pinning procedures resulted in a slight increase in the level of residual stress around the distal aspect of the apex of the first premolar. Unilateral Dalbo attachment. The most active state was achieved by removing both the spring and the pin from the attachment. Activity was observed beneath the point of force application. The resultant stresspattern was not distributed along the edentulous ridge. A yellow fringe band associatedwith the apex of the first premolar was observed. Slight changes in the character of the bone were also observed adjacent to the canine root. 146
by the unilateral Dalbo removable partial denture with the spring in place. .A Single abutment. B, Double abutment.
AUGUST
1978
VOLUME
40
NUMBER
2
VISUALJZATION
OF STRAIN
PATTERNS
Fig. 4. Stress pattern produced by the unilateral Dalbo removable partial denture with the locking pin in place. A, Single abutment. B, Double abutment.
Fig. 5. Stress pattern
ridge. The distribution of the pattern was associated with the point of force application. There was a region of stress concentration below the apex of the first premolar. Very little if any change in the appearance of the canine was observed. By placing the springs into the receptacles, the stress pattern in the edentulous ridge was markedly reduced both in distribution and quantity. The stress pattern below the apex of the first premolar increased sharply. The location of the stress pattern was along the long axis of the root. Very little activity was associated with the canine abutment. With the pins inserted, thus locking the Dalbo attachments in a rigid state, the stress in the edentulous ridge region was reduced and almost eliminated. The first premolar showed a marked increase of the stresses in the apical one third of the root. Two full fringe orders were observed. The distribution of the stress pattern was slightly mesial to the long axis of the root. Some activity was observed in the interdental bone between the. first premolar and canine abutments. The canine revealed a low-order stress pattern in the apical region to the distal aspect. May’s attachment. The stress patterns with the
May’s attachment were similar to those with the bilateral Dalbo attachments with no springs or pins in the receptacles; the values of the stresses were generally slightly greater. A lack of distribution of stress in the edentulous ridge was observed. Some activity was observed in the canine abutment. Cross-arch stress. Neither the single- nor the double-abutment bilateral Dalbo removable partial dentures transmitted stresses that could be demonstrated to the opposite side of the arch. The replica showed no changes in the character of light transmission on the side opposite the force application.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
produced by the bilateral Dalbo removable partial denture with no pins or springs in place. A, Single abutment. B, Double abutment.
Comparison of single to double abutments The relationship between the alterations in the attachments remained constant whether single or double abutments were used to support the removable partial dentures. The changes that were observed were relative to the change from single to double abutments. Unilateral Dalbo attachment. In the unilateral Dalbo attachment removable partial denture. the stress pattern associated with the point of force application in the edentulous ridge was decreased in 147
WHITE
Fig. 6. Stress pattern produced by the bilateral Dalbo removable partial denture with the springs in place. A, Single abutment. l3, Double abutment.
Fig. 7. Stress pattern produced by the bilateral Dalbo removable partial denture with the locking pins in place. A, Single abutment. 8, Double abutment.
distribution and magnitude by the use of two abutments. The stress patter11 below the apex of the first premolar increased with the use of two abutments (Fig. 2). Unilateral Dalbo with the spring in the receptacle. A decrease in the stress pattern in the edentulous ridge was observed when two abutments were used and the spring inserted into the receptacle. The stress pattern at the apex of the first premolar became more distinct (Fig. 3). Unilateral Dalbo with locking pin. When the pin was inserted in the unilatera1 Daibo removable partial denture, the edentulous ridge showed some decrease in the stress pattern when two abutments were used. The decrease was not in proportion to the increase in stress at the apex of the first premolar (Fig. 4). Bilateral Dalbo with no pins or springs. The bilateral Dalbo with no pins or springs in the receptacle showed a decrease in the stress pattern in the edentulous ridge with the use of two abutments. The stress pattern at the apex of the first premolar
increased in magnitude with the use of two abutments (Fig. 5). Bilateral Dalbo with the springs in the receptacle. By placing the springs in the receptacles, the stress to the edentulous ridge was greatly reduced when two abutments were used. The stress at the apex of the first premolar was greatly increased. The stress pattern was located directly below the long axis of the root and was not distributed mesiaily or distally along the curve of the root tip (Fig. 6). Bilateral Dalbo with locking pins. Of all the samples tested, the bilateral Dalbo removable partial denture with the pins in place showed the least stress transfer to the edentulous ridge in the doubleabutment sample. Associated with this decrease in stress was an increase in stress concentration at,the apex of the first premolar. Two full fringe orders were observed below the apex of the first premolar in the double-abutment sample (Fig. 7). May’s attachment. The patterns for the single and double abutments in the May’s attachment sample were similar to the previous findings (Fig. 8).
148
AUGUST
1978
VOLUME
QO
NUMBER
2
VISUALIZATION
OF STRAIN
PATTERNS
DISCUSSION As the Dalbo attachment was rendered less free, the stresses in the edentulous ridge supporting the distal-extension base decreased in magnitude. Cecconi:‘” also found this to be true. He reported that the ridge displacement was significantly decreased when the Dalbo stressbreaker was made rigid. The distribution of the stresses along the edentulous ridge was related to the point of force application, the central fossa of the first molar, and was not directly related to the attachment employed. The fully active Dalbo attachment resulted in the greatest distribution of stresses along the supporting edentulous ridge. Even with the ability of the denture base to move vertically without restriction, the stresses were not distributed along the full length of the supporting tissues. By placing the spring into the receptacle of the Dalbo attachment,‘the stresses to the ridge were more concentrated at the point of load application. The function of the spring is to maintain the denture base at the passive rest position when not in use.4” It is used to remove the “Class I lever action of nonmovable partial dentures or the shear and moment effect of the hinge-action or vertical slot-type of partial dentures on the anchor tooth and its underlying structures.“3R The increases in the stress below the apex of the first premolar and the decrease in the stress distribution along the edentulous ridge do not support the function of the spring. The attachment induces less stress when the spring is eliminated. One explanation for this behavior is that the spring requires too great an application of force to be compressed, thereby acting as a point of rotation. The similarities between the stress patterns of the May’s attachment and the Dalbo attachment with the springs inserted substantiate this premise. The function of a stressbreaking system is to reduce the torque to the abutment teeth induced by the distal-extension removable partial denture.:‘” The vertical stressbreaking effect of the fully active Dalbo attachment fulfilled this function. The forces were transmitted through the long axis of the root of the first premolar. All of the samples tested transmitted the forces through the long axis of the first premolar. Hayashi’ reported that the direction of force application to the teeth had a significant effect on the distribution of force in the supporting structures. The stress distribution for a given movement was independent of the force magnitude but related to the direction of the force application and the configuration of the root structure. Since all the attach-
THE JOURNAL
OF PR0STHEl-K
DENTISTRY
Fig. 8. Stress pattern produced by the May’s attachment removable partial denture. A, Single abutment. B, Double abutment. ments tested varied mostly in magnitude and not in distribution of stressrelated to the apex of the first premolar, similar force systemsmust be generated. By locking the Dalbo attachment with a pin, the removable ‘partial denture becomes a slot attachment partial denture behaving similarly to a rigid cantilever fixed partial denture. Hentierson~” conducted an in-depth study of the cantiiever type of posterior fixed partial denture. The horizontal axis of rotation for the fixed partial denture when two abutments were used was found to be in a region the two inferior to but almost midway between abutments. The horizontal axis of rotation in the replica when two abutments were used should have been in the root of the first premolar. ‘I’he high stress concentration seen at the apex of the first premolar ruled this region out asa center of rotation. Henderson also found that when two abutments were used the distal abutment resisted more of the resultant force than the other abutment. Also, the resultant force was found to be less for the single abutment than for the double abutment. The imrease of the
149
WHITE
resultant force over the applied force was explained by the system of levers inherent to the cantilever principle, The inactive Dalbo removable partial denture behaved similarly to the cantilever fixed partial denture studied by Henderson. Also, the resultant forces were less when a single abutment was used. The explanation for the increased stresses seen when the double abutment was used with the stressbreaking samples is not clear. The expected finding would have been more stress on the distal aspect of the canine abutment. Very little activity was seen in this region. Possibly the center of rotation is being moved anterior to the canine abutment when two abutments are used. If the canine abutment is closer to the center of rotation, it would explain the lack of stress associated with it. Henderson’s study does not support this idea. One of the reasons for using cross-arch stabilization is to distribute the stresses over a greater area. The increase in stress values seen when the bilateral Datbo removable partial denture was tested does not support this assumption. Contrary to a study conducted by Kratochvil and Caputo,“’ forces were not distributed to all the teeth in contact with the removable partial denture framework. No cross-arch stress patterns could be demonstrated. These forces must be in the major connector. Further studies should be conducted employing similar replicas and samples while varying the design of the major connector. Determining the location of the crossarch stresses may aid in the explanation of stress failures in removable partial dentures.
6. The use of two abutments (double abutments) resulted in greater stress concentrations in the distal abutment than the use of single abutments. 7. Forces in the single abutment were resisted along the long axis of the root. 8. Forces in the double abutments were resisted along the long axis of the distal abutment. The author would like to express his thanks to Dr. Matthew I‘ Wood, Professor of Removable Prosthodontics, t;niversity of North Carolina, School of Dentistry, for his assistance in conducting this project.
REFERENCES 1.
2. 3. .4.
6.
7.
8.
9.
CONCLUSIONS The following conclusions are based on the conditions set forth in this investigation and apply to forces acting through a vertical direction ‘of application only: 1. The unilateral removable partial denture produced no more stress than the bilateral removable partial denture. 2. The fully active Dalbo removable partial denture resulted in the greatest stressbreaking action. 3. The rigid Dalbo removable partial denture resulted in the least ridge displacement. 4. The rigid Dalbo removable partial denture resulted in the greatest stress concentrations in the distal abutment. 5. None of the attachments tested resulted in a distribution of the stresses along the edentulous ridge.
150
10.
11. 12. 13.
14.
15.
16.
17.
Mahler, D. B., and Peyton, F. A.: Photoelasticity as a research technique for analyzing stresses in dental structures. J Dent Res 34:831, 1955. Mahler, D. B., and Terkla, L. G.: Analysis of stress in dental structures. Dent Clin North Am, Nov., 1958, pp 789-798. Lehman, M. L.: Tensile strength of human dentin. .J Dent Res 46t197. 1967. Haines, I). J.: Physical properties of human tooth enamel and enamel sheath material under load. ,J Biomech 1:lli. 19.58. Chaconas, S. J,, Caputo, A. A., and Hayashi, R. K.: EfIects of wire size, loop configuration, and gabling on canineretraction springs. Am J Orthod 65:58, 1974. Caputo, A. A., Chaconas, S. J., and Hayashi, R. K.: Photoelastic visualization of orthodontic forces during canine retraction. Am J Orthod 65:252, 1974. Hayashi. R. K., Chaconas, S. J., and Caputo, A. A.: Effects of force direction on supporting bone during tooth movement.J Am Dent Assoc 90:1012, 1975. Brodsky, J. F., Caputo, A. A., and Furstman, 1,. I,.: Root tipping: A photoelastic-histopathologic correlation. Am .J Orthod 67:1, 1975. Standlee, J. P., Caputo, A. A., Collard, E. W., and Pollack, M. H.: Analysis of stress distribution by endodontic posts. Oral Surg 33:952, 1972. Trabert, K. C., Caputo, A. A., Collard, E. W., and Standlee. J. P.: Stress transfer to the dental pulp by retentive pins. J PROSTHET DE.NT 30:808, 1973. Noonan, M. A.: The use of photoelasticity in a study of cavity preparations. J Dent Child 16:24, 1949. Mahler, D. B.: An analysis of stresses in dental amalgam restoration. J Dent Res 37:516, 1958. Guard, W. F., Haack, D. C., and Ireland, R. L.: Photoelastic stress analysis of buccolingual sections of Class II cavityrestorations. J Am Dent Assoc 57:631, 1958. Johnson, C. R., CastaIdi, C. R., Gau, D. J., and Wysocki, G. P.: Stress pattern variations in operatively prepared human teeth, studied by three-dimensional photoelasticity. J Dent Res 47:548, 1968. Walton. C. B., and Leven, M. M.: A preliminary report of photoelastic tests of strain patterns within ,jacket crowns. J Am Dent Assoc 50:44, 1955. Lehman, M. I,., and Hampson, E. I,.: A study of strain patterns in jacket crowns on anterior teeth resulting from different tooth preparations. Br Dent J 113:337, 1962. Colin, L., Kaufman, E. C., Papimo. R.: Stress concentration in full crown restorations. NY State Dent J 29:370, 1963.
AUGUST1978
VOLUME40
NUMBER2
VISUALIZATION
18.
19.
20.
21.
22.
23.
24.
25 26
27
28
29
OF STRAIN
PATTERNS
Craig, R. G.. El-Ebrashi. Mw. K., LePeak, P. J., and Peyton, F. A.: I:xperimental stress analysis of dental restorations. Part I. Two-dimensional photoelastic stress analysis of inlays. .J PROSTHI.~ D~.wr 17:277, 1967. Craig, R. G.. El-Ebrashi. M. K.. and Peyton, F. A.: Experimental stress analysis of dental restorations. Part II. Twodimensional photoelastic stress analysis of crowns. J Pws‘r(u’r Dt.vl 17:292. 1967. El-Brashi, M. K., Craig. R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part III. The concept of the geometry of proximal margins. J PROWHF.T DI.~CT 22:333, 1969. El-Ebrashi, M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part IV. The concept of parallelism of axial walls. J PRosrw.r DEN.I 22:346, 1969. El-Brashi, M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part V. The concept of occlusai reduction and pins. .J PROSTI~E.T DF.NT 22:565, 1969. El-Ebrashi. M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part VI. The concept of proximal reduction in compound restorations. J PROSTHLT DENT 22:663, 1969. El-Ebrashi, M. K.. Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part VII. Structural design and stress analysis of fixed partial dentures. J PROSTHS.~ DINT 23:177, 1970. Farah, .J. W.. and Craig, R. G.: Reflection photoelastic stress analysis of a dental bridge. J Dent Res 50:1253, 1971. Fisher, D. W., Caputo, A. A., Shillingburg, H. T., and Duncanson, M. G.: Photoelastic analysis of inlay and onlay preparations. J PRosTH1.r DFwr 33:47> 1975. Hood, .J. ‘4.. Farah, .J. Mr., and Craig, R. G.: Stress and deflection of three different pontic designs. J PRosrw.T DE NT 33:55. 1975. Glickman, I.: Roeber, F. W., Brion, M., and Pameijer, J. H. S.: Photoelastic analysis of internal stresses in the periodontium created by occlusal forces. J Periodontol 41:30, 1970. Rodriquez. C. A., and Arrechea, G.: Periodontal distribution
THE JOURNAL
OF F’ROSTHETIC
DENTISTRY
30.
31.
32.
33.
34.
35.
36. 37.
38. 39.
40.
of occlusai forces photoelastic study. J Periotlontol 44:485, 1973. Rodriquez, C. A., and Vazques, S.: Experur~cntal approximation to the determination of the true axial forces. J Periodontol 45: 110, 1974. Collard, E. lv., Standlee, J. P.. and Capulo. .4. A: An acceptable photoelastic simulation of Merraptan rubber impression materials. .J South Calif Dent 4csoc 38:506. 1970. Collard. E. M’., Caputo, A. A., Standlee. J 1’ , and Trabert, K. C.: Dynamic stresses encountered in imprvsion removal. J PROSTIET DF.NT 29:498, 1973. Kratochvil, F. J.. and Caputo. A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J PROSTHE.T DENT 32:52, 1974. Warren, A. B., and Caputo, A. A.: Load transfer to alveolar bone as influenced by abutment designs for tooth-supported dentures. ,J PROSTHE.T DENT 33:137, 1975. Craig, R. G., Farah, J. W., and El-Tahawi. H. M.: l‘hreedimensional photoelastic stre’ess analysis of maxillary complete dentures. J PROXHE.~ DFNT 31:121. 1954. Photoelastic Stress Analysis. Kodak Job Sheet No. 13. Rochester, N. Y.. Eastman Kodak Co., 1969 Mehta. N. R., Roeber, F. W., Haddad, A. \%‘., and Glickman, I.: Photoelastic model for occlusal fwcr analysis. .J Dent Res 54: 1243, 1975. Mensor, M. C.: The rationale of resilient hinge-action stressbreakers. J PKOSTHFT DF.NT 20:204, 1968. Cecconi, B. T.. Kaiser, G., and Rahe, A.: Stressbreakers and the removable partial denture. ,J P~osrr3t.1 DFST 34:145, 1975. Henderson, D., Blevins, W. R., Wesley, R.
-
--~
s and strain rein James T. White, University
D.D.S., M.S.*
of North Carolina,
School of Dentistry,
Chapel Hill,
T
he purpose of this article is to present a method of observing strain patterns produced in the supporting structures of a removable partial denture. The strain patterns produced by three different removable partial dentures were compared by means of a photoelastic analysis technique.
REVIEW
OF THE LITERATURE
The photoelastic analysis technique has been used to study the tooth and related stressesin the basic sciences.‘-” The visualization of stressesproduced during orthodontic tooth movement has been studied and a photoelastic-histopathologic correlation carried out.s+ Analyses of the stress distribution properties of endodontic postsand the stresstransfer to the dental pulp by retention pins have also been reported.“, Ii’ Determining the effect of the cavity preparation on the stressdistribution in the amalgam restoration was one of the earliest applications of stressanalysis.l’-l’ The effects of the preparation design on the stressinduced in a veneer crown have also been reported.‘i‘17 An extensive study of experimental stressanalysis of dental restorations related to castings and fixed prosthodontics has been conducted.lx-” A photoelastic analysis of the internal stressesin the periodontium created by occlusal forces has led to far-reaching conclusions.‘“-3” The stressesinduced through impression removal have been visualized.‘“, ” The pressureson the teeth and bone supporting removable partial dentures and complete overdentures have been demonstrated,3”’ 3’ and stressanalysis of a maxillary complete denture during functional movements has been reported.“”
METHODS
AND MATERIALS
Internal stresses due to the effects of forces applied to a body are accompanied by internal strains within Read
before
Texas. *Assistant
the
Academy
Professor,
0022-3913/78/0240-0143$00.90/00
of Denture
Department
Prosthetics,
of Fixed
San
the body. Stress has been defined as “the internal force per unit of area which resistsa change in size or shapeof a body,” and strain has been defined as“the internal deformation per unit of length.““’ An analysis of internal strains should yield valuable criteria for successfuldesign.“’ These internal strains may be visualized and recorded with the photoelastic analysistechnique. “A photoelastic material is one which is capable of separating a polarized ray of light into two rays each polarized in a different direction.“‘” These materials become double refractive when in a state of strain, A circularly polarized transmission polariscope was used to visualize the separation of the polarized light by the photoelastic material while in a st.rained state. The colored lines produced by the white light are known as isochromatic lines. They represent “one of the frequencies being cancelled by interference at each point of double refraction (coinciding with maximum stresses),leaving a colored line that is white light minus one frequency.““” The isochromatic lines are close together in regions of high stress concentration and farther apart in regions of low stressconcentration. Replica fabrication. Three photoelastic: materials were used in the fabrication of the replica. PI-1 ,* a high-modulus epoxy resin, was used to simulate tooth structure. Pl-2,* a medium-modulus epoxy resin, was used to simulate the bony architecture. PI-~,* a low-modulus urethane, was used to simulate the periodontal
ligament.
Co
Silastic
382f
was molded
to simulate the gingiva around the teeth. Rubber base materialx equal to one thickness of baseplate wax was adapted to the ridges to simulate the mucosa. The ability periodontium “Photolastic.
Inc.,
of the replica to simulate the actual is a function of the replica’s design. Ii Malvern,
iDow Corning, Midland, $Permlastic, Kerr Sybron
Prosthodontics.
1978 The C. V. Moshy
Antonio,
N. C.
THE JOURNAL
Pa. Mich. Carp.,
Romulus.
OF PROSTHETIC
Mich
DENTISTRY
143
WHlTE
Fig. 1. The replica with the unilateral Dalbo removable partial denture in place. The geometric scaling was obtained by comparing the ratios of the E values of the tooth, periodontium, and bone to ratios of the E values of the simulated structures. The geometric scaling obtained using these materials was not as close as desired. This was the best scaling that could be attained during this project. The error should be taken into account when comparing this study to others. The replica was constructed to simulate a lower jaw requiring a bilateral distal-extension removable partial denture. The mandibular arch was intact from the first premolar to the first premolar. The second premolars and all molars were missing (Fig. 1). The first premolars and the canines were prepared to receive full cast gold crowns. Care was taken to not generate heat during the preparation of the teeth. Excessive heat generation can result in the formation of residual stressesin the replica, rendering it useless.Cast gold crowns were fabricated, and one bilateral and two unilateral removable partial dentures were constructed. Testing prostheses. The Dalbo attachment* and the May’s attachment? were usedin the construction of three testing removable partial dentures. The Dalbo attachment is a precision attachment that can be used in the fabrication of both a unilateral and bilateral distal-extension prosthesis. *APM-Sterngold, San LMateo, Calif. tDentalloy Dental Products Inc., Stanton, Calif.
144
The attachment is composed of a vertical T-rail (male portion) with a ball-shaped retentive piece projecting from the T-rail. The T-rail may be inserted into the wax pattern of the abutment and cast onto it. A slotted receptacle (female portion) accurately fits over the T-rail and has an open-ended cylinder that receives the retentive ball. A spring may be placed into the cylinder to provide vertical resilience to the attachment. The attachment provides both vertical and hinge stressbreaking features. A pin that is used to lock the attachment into a rigid state is also provided. Jaw records are made with the attachment locked.“” The May’s attachment is a precision attachment that can be used for the fabrication of a unilateral distal-extension removable prosthesis.The manufacturer provides a plastic pattern (male portion) that is luted to the wax pattern of the abutment tooth and cast in the same metal as the retainer. A manufactured slotted receptacle (female portion), with a hinge piece opening to the lingual, closesto engage a hole in the extension on the abutment crown, providing retention and a hinge movement to the removable prosthesis. The manufacturer states that a vertical action can be provided by altering the shape of the hole in the extension. The first removable partial denture was a bilateral distal-extension design incorporating two Dalbo attachments.* The principles described by Menso? were followed in the construction of the prosthesis. The ceramicor T-rails were incorporated into the distal surfacesof the first premolar crowns. A lingual plate was used as the major connector. A plaster occlusal index was made of the cusps of all the teeth. The index was used to position the denture teeth during the construction of the other two removable partial dentures. The occlusal plane and the point of force application could be reasonably duplicated by this procedure. The second removable partial denture was a unilateral prosthesis with a Dalbo attachment and no cross-arch support. The mandibular left second premolar and first molar were replaced with the prosthesis.The same abutment crown was used. The third removable partial denture was also a unilateral prosthesis with a May’s attachment. Retention was attained by engaging the hole in the abutment attachment with the latch. The castings were anchored to the photoelastic *63.03.2 unilateral Dalho attachment.
AUGUST
1978
VOLUME 40
NUMBER
2
VIWAUZATtON
IX STRAIN
PATTERNS
replica by cementation and cross-pinning with regular TMS pins.* Each removable partial denture was positioned, and a 3%pound force wa8 then applied to the central fossa of the mandibular left first molar. The straining frame used to apply the force was constructed to fit into the circular transmission polariscope. The resultant stress patterns were photographed with a 35 mm camera and type A fi1m.t To visualize the effects of using two splinted abutments, the crowns on the first premolars and canines were soldered to form bilateral double abutments. The splinted crowns were seated and crosspinned. The removable partial dentures were positioned, and a 35-pound force was then applied to the mandibular left first molar. Ali of the removable partial dentures were tested with both single and double abutments. The unilateral and the bilateral Dalbo partial dentures were tested in a fully active, less active, and rigid state. Fully active was defined as the attachment with no spring or pin in place, permitting free movement of the removable partial denture base. Less active was defined as the attachment with the spring in place, slightly restricting the vertical movement of the removable partial denture base. Rigid was defined as the attachment with the locking pin inserted, making the attachment nonmovable and thereby completely restricting the independent movement of the removable partial denture base. RESULTS Single abutment Repka before positioning prostheses. Slight residual stresses were observed in the replica before positioning the prostheses. A lightening of the photoelastic material at the apices of the canine and premolar teeth was apparent. The ridge was free of residual stress. Unilateral Dafbo attachment. The unilateral Dalbo attachment was tested first in its most active state, second in a less active state, and third in a rigid nonactive state. The most active state was achieved by removing both the spring and the pin from the attachment. As the 35-pound force was applied to the first molar, activity was observed in the ridge beneath the point of force application. The blue and red fringes repre*Whaledent, New York, N. Y. ?Eastman Kodak Co., Rochester,
THE fOlJRNAL
OF PROSTIiETIC
N. Y.
DENTISTRY
sented a region of stress concentration. The surrounding yellow fringe. represented a more distributed stress region. The first premolar had a yellow fringe band at the distal aspect of the apex. By placing the spring into the receptacle of the attachment, some of the freedom in the Dalbo attachment was removed. The stress pattern in the ridge region increased in both quantity and quality. The fringes penetrated deeper into the bone and closer to the first premolar root. The yellow fringe band associated with the distal aspect of the apex increased in size. Activity was observed on the mesial of the first premolar root at the midroot Jevel. The Dalbo attachment was rendered rigid by placing the locking pin into the attachment. This procedure greatly reduced the stress in the ridge. The stress pattern at the apex of the first premolar showed increased concentration directly below the apex. This represents a transfer of the force to the long axis of the root as opposed to being directed along the axis of the root configuration. Bilateral D&o attachment. The bilateral Dalba attachment prosthesis was tested in the same manner as the unilateral Dalbo prosthesis: most active, less active, and rigid. The springs and pins were removed from the attachments. The activity beneath the point of force application on the ridge presented a concentrated pattern. The yellow fringe band associated with the first premolar was concentrated directly below the root tip. Some activity was observed between the canine and first premolar in the interdental bone. The springs were placed into the receptacles of the Dalbo attachments. The stress pattern associated with the point of force application remained similar to that of the fully active sample. The stress pattern below the apex of the first premolar remained in the same position but increased in magnitude. A sharp yellow, red, and blue-green fringe was produced. Again, some activity was observed between the canine and first premolar in the interdental bone. The Dalbo attachments were made rigid by inserring the locking pins. The stress associated with the apex of the first premolar moved to the mesial side of the root tip and increased in magnitude. A distinct series of yellow, red, and blue fringes was observed. Summary statement. In general, the bilateral Dalbo prosthesis showed a more concentrated stress pattern in the ridge that was, however, of a similar magnitude as that produced with the unilateral Dalbo prosthesis. The stress associated with the apex
145
WHITE
Fig. 2. Stress pattern produced by the unilateral Dalbo removablepartial denturewith no pinsor springsin place. A, Singleabutment. B, Double abutment.
Fig. 3. Stress pattern produced
of the first premolar was greater in magnitude with the bilateral Dalbo prosthesis. With the May’s attachment, a stressdistribution pattern similar to that resulting with the unilateral Dalbo attachment with the locking pin inserted was observed. The ridge showed a moderately distributed stresspattern of moderate magnitude. The apex of the first premolar had a sharp yellow fringe band directly below it.
The spring was placed into the receptacle to reduce someof the freedom in the attachment. ‘I’he stresspattern beneath the point of force application remained very similar to that of the most active state. The first premolar presented a definite increase in fringe value in the apical region. At least one full fringe order was observed. The stress distribution was directly along the long axis of the root proper and was not associatedwith the distal curve at the root tip. The Dalbo attachment was rendered rigid by inserting the locking pin. The stresspattern beneath the point of force application was greatly reduced. With this reduction in stressthere was an increase in the stresspattern apical to the first premolar. There was a definite green band distal to the root in the region representing the alveolar crest. A definite fringe pattern was estabhshed in the interdental bone between the canine and the first premolar. Activity was also observed in the root of the canine abutment. Bilateral Dalbo attachment. With the springs and pins removed, the active Dalbo attachments permitted a poor distribution of stressalong the edentulous
Double abutment Replica before positioning prosthesis. The soldering and cross-pinning procedures resulted in a slight increase in the level of residual stress around the distal aspect of the apex of the first premolar. Unilateral Dalbo attachment. The most active state was achieved by removing both the spring and the pin from the attachment. Activity was observed beneath the point of force application. The resultant stresspattern was not distributed along the edentulous ridge. A yellow fringe band associatedwith the apex of the first premolar was observed. Slight changes in the character of the bone were also observed adjacent to the canine root. 146
by the unilateral Dalbo removable partial denture with the spring in place. .A Single abutment. B, Double abutment.
AUGUST
1978
VOLUME
40
NUMBER
2
VISUALJZATION
OF STRAIN
PATTERNS
Fig. 4. Stress pattern produced by the unilateral Dalbo removable partial denture with the locking pin in place. A, Single abutment. B, Double abutment.
Fig. 5. Stress pattern
ridge. The distribution of the pattern was associated with the point of force application. There was a region of stress concentration below the apex of the first premolar. Very little if any change in the appearance of the canine was observed. By placing the springs into the receptacles, the stress pattern in the edentulous ridge was markedly reduced both in distribution and quantity. The stress pattern below the apex of the first premolar increased sharply. The location of the stress pattern was along the long axis of the root. Very little activity was associated with the canine abutment. With the pins inserted, thus locking the Dalbo attachments in a rigid state, the stress in the edentulous ridge region was reduced and almost eliminated. The first premolar showed a marked increase of the stresses in the apical one third of the root. Two full fringe orders were observed. The distribution of the stress pattern was slightly mesial to the long axis of the root. Some activity was observed in the interdental bone between the. first premolar and canine abutments. The canine revealed a low-order stress pattern in the apical region to the distal aspect. May’s attachment. The stress patterns with the
May’s attachment were similar to those with the bilateral Dalbo attachments with no springs or pins in the receptacles; the values of the stresses were generally slightly greater. A lack of distribution of stress in the edentulous ridge was observed. Some activity was observed in the canine abutment. Cross-arch stress. Neither the single- nor the double-abutment bilateral Dalbo removable partial dentures transmitted stresses that could be demonstrated to the opposite side of the arch. The replica showed no changes in the character of light transmission on the side opposite the force application.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
produced by the bilateral Dalbo removable partial denture with no pins or springs in place. A, Single abutment. B, Double abutment.
Comparison of single to double abutments The relationship between the alterations in the attachments remained constant whether single or double abutments were used to support the removable partial dentures. The changes that were observed were relative to the change from single to double abutments. Unilateral Dalbo attachment. In the unilateral Dalbo attachment removable partial denture. the stress pattern associated with the point of force application in the edentulous ridge was decreased in 147
WHITE
Fig. 6. Stress pattern produced by the bilateral Dalbo removable partial denture with the springs in place. A, Single abutment. l3, Double abutment.
Fig. 7. Stress pattern produced by the bilateral Dalbo removable partial denture with the locking pins in place. A, Single abutment. 8, Double abutment.
distribution and magnitude by the use of two abutments. The stress patter11 below the apex of the first premolar increased with the use of two abutments (Fig. 2). Unilateral Dalbo with the spring in the receptacle. A decrease in the stress pattern in the edentulous ridge was observed when two abutments were used and the spring inserted into the receptacle. The stress pattern at the apex of the first premolar became more distinct (Fig. 3). Unilateral Dalbo with locking pin. When the pin was inserted in the unilatera1 Daibo removable partial denture, the edentulous ridge showed some decrease in the stress pattern when two abutments were used. The decrease was not in proportion to the increase in stress at the apex of the first premolar (Fig. 4). Bilateral Dalbo with no pins or springs. The bilateral Dalbo with no pins or springs in the receptacle showed a decrease in the stress pattern in the edentulous ridge with the use of two abutments. The stress pattern at the apex of the first premolar
increased in magnitude with the use of two abutments (Fig. 5). Bilateral Dalbo with the springs in the receptacle. By placing the springs in the receptacles, the stress to the edentulous ridge was greatly reduced when two abutments were used. The stress at the apex of the first premolar was greatly increased. The stress pattern was located directly below the long axis of the root and was not distributed mesiaily or distally along the curve of the root tip (Fig. 6). Bilateral Dalbo with locking pins. Of all the samples tested, the bilateral Dalbo removable partial denture with the pins in place showed the least stress transfer to the edentulous ridge in the doubleabutment sample. Associated with this decrease in stress was an increase in stress concentration at,the apex of the first premolar. Two full fringe orders were observed below the apex of the first premolar in the double-abutment sample (Fig. 7). May’s attachment. The patterns for the single and double abutments in the May’s attachment sample were similar to the previous findings (Fig. 8).
148
AUGUST
1978
VOLUME
QO
NUMBER
2
VISUALIZATION
OF STRAIN
PATTERNS
DISCUSSION As the Dalbo attachment was rendered less free, the stresses in the edentulous ridge supporting the distal-extension base decreased in magnitude. Cecconi:‘” also found this to be true. He reported that the ridge displacement was significantly decreased when the Dalbo stressbreaker was made rigid. The distribution of the stresses along the edentulous ridge was related to the point of force application, the central fossa of the first molar, and was not directly related to the attachment employed. The fully active Dalbo attachment resulted in the greatest distribution of stresses along the supporting edentulous ridge. Even with the ability of the denture base to move vertically without restriction, the stresses were not distributed along the full length of the supporting tissues. By placing the spring into the receptacle of the Dalbo attachment,‘the stresses to the ridge were more concentrated at the point of load application. The function of the spring is to maintain the denture base at the passive rest position when not in use.4” It is used to remove the “Class I lever action of nonmovable partial dentures or the shear and moment effect of the hinge-action or vertical slot-type of partial dentures on the anchor tooth and its underlying structures.“3R The increases in the stress below the apex of the first premolar and the decrease in the stress distribution along the edentulous ridge do not support the function of the spring. The attachment induces less stress when the spring is eliminated. One explanation for this behavior is that the spring requires too great an application of force to be compressed, thereby acting as a point of rotation. The similarities between the stress patterns of the May’s attachment and the Dalbo attachment with the springs inserted substantiate this premise. The function of a stressbreaking system is to reduce the torque to the abutment teeth induced by the distal-extension removable partial denture.:‘” The vertical stressbreaking effect of the fully active Dalbo attachment fulfilled this function. The forces were transmitted through the long axis of the root of the first premolar. All of the samples tested transmitted the forces through the long axis of the first premolar. Hayashi’ reported that the direction of force application to the teeth had a significant effect on the distribution of force in the supporting structures. The stress distribution for a given movement was independent of the force magnitude but related to the direction of the force application and the configuration of the root structure. Since all the attach-
THE JOURNAL
OF PR0STHEl-K
DENTISTRY
Fig. 8. Stress pattern produced by the May’s attachment removable partial denture. A, Single abutment. B, Double abutment. ments tested varied mostly in magnitude and not in distribution of stressrelated to the apex of the first premolar, similar force systemsmust be generated. By locking the Dalbo attachment with a pin, the removable ‘partial denture becomes a slot attachment partial denture behaving similarly to a rigid cantilever fixed partial denture. Hentierson~” conducted an in-depth study of the cantiiever type of posterior fixed partial denture. The horizontal axis of rotation for the fixed partial denture when two abutments were used was found to be in a region the two inferior to but almost midway between abutments. The horizontal axis of rotation in the replica when two abutments were used should have been in the root of the first premolar. ‘I’he high stress concentration seen at the apex of the first premolar ruled this region out asa center of rotation. Henderson also found that when two abutments were used the distal abutment resisted more of the resultant force than the other abutment. Also, the resultant force was found to be less for the single abutment than for the double abutment. The imrease of the
149
WHITE
resultant force over the applied force was explained by the system of levers inherent to the cantilever principle, The inactive Dalbo removable partial denture behaved similarly to the cantilever fixed partial denture studied by Henderson. Also, the resultant forces were less when a single abutment was used. The explanation for the increased stresses seen when the double abutment was used with the stressbreaking samples is not clear. The expected finding would have been more stress on the distal aspect of the canine abutment. Very little activity was seen in this region. Possibly the center of rotation is being moved anterior to the canine abutment when two abutments are used. If the canine abutment is closer to the center of rotation, it would explain the lack of stress associated with it. Henderson’s study does not support this idea. One of the reasons for using cross-arch stabilization is to distribute the stresses over a greater area. The increase in stress values seen when the bilateral Datbo removable partial denture was tested does not support this assumption. Contrary to a study conducted by Kratochvil and Caputo,“’ forces were not distributed to all the teeth in contact with the removable partial denture framework. No cross-arch stress patterns could be demonstrated. These forces must be in the major connector. Further studies should be conducted employing similar replicas and samples while varying the design of the major connector. Determining the location of the crossarch stresses may aid in the explanation of stress failures in removable partial dentures.
6. The use of two abutments (double abutments) resulted in greater stress concentrations in the distal abutment than the use of single abutments. 7. Forces in the single abutment were resisted along the long axis of the root. 8. Forces in the double abutments were resisted along the long axis of the distal abutment. The author would like to express his thanks to Dr. Matthew I‘ Wood, Professor of Removable Prosthodontics, t;niversity of North Carolina, School of Dentistry, for his assistance in conducting this project.
REFERENCES 1.
2. 3. .4.
6.
7.
8.
9.
CONCLUSIONS The following conclusions are based on the conditions set forth in this investigation and apply to forces acting through a vertical direction ‘of application only: 1. The unilateral removable partial denture produced no more stress than the bilateral removable partial denture. 2. The fully active Dalbo removable partial denture resulted in the greatest stressbreaking action. 3. The rigid Dalbo removable partial denture resulted in the least ridge displacement. 4. The rigid Dalbo removable partial denture resulted in the greatest stress concentrations in the distal abutment. 5. None of the attachments tested resulted in a distribution of the stresses along the edentulous ridge.
150
10.
11. 12. 13.
14.
15.
16.
17.
Mahler, D. B., and Peyton, F. A.: Photoelasticity as a research technique for analyzing stresses in dental structures. J Dent Res 34:831, 1955. Mahler, D. B., and Terkla, L. G.: Analysis of stress in dental structures. Dent Clin North Am, Nov., 1958, pp 789-798. Lehman, M. L.: Tensile strength of human dentin. .J Dent Res 46t197. 1967. Haines, I). J.: Physical properties of human tooth enamel and enamel sheath material under load. ,J Biomech 1:lli. 19.58. Chaconas, S. J,, Caputo, A. A., and Hayashi, R. K.: EfIects of wire size, loop configuration, and gabling on canineretraction springs. Am J Orthod 65:58, 1974. Caputo, A. A., Chaconas, S. J., and Hayashi, R. K.: Photoelastic visualization of orthodontic forces during canine retraction. Am J Orthod 65:252, 1974. Hayashi. R. K., Chaconas, S. J., and Caputo, A. A.: Effects of force direction on supporting bone during tooth movement.J Am Dent Assoc 90:1012, 1975. Brodsky, J. F., Caputo, A. A., and Furstman, 1,. I,.: Root tipping: A photoelastic-histopathologic correlation. Am .J Orthod 67:1, 1975. Standlee, J. P., Caputo, A. A., Collard, E. W., and Pollack, M. H.: Analysis of stress distribution by endodontic posts. Oral Surg 33:952, 1972. Trabert, K. C., Caputo, A. A., Collard, E. W., and Standlee. J. P.: Stress transfer to the dental pulp by retentive pins. J PROSTHET DE.NT 30:808, 1973. Noonan, M. A.: The use of photoelasticity in a study of cavity preparations. J Dent Child 16:24, 1949. Mahler, D. B.: An analysis of stresses in dental amalgam restoration. J Dent Res 37:516, 1958. Guard, W. F., Haack, D. C., and Ireland, R. L.: Photoelastic stress analysis of buccolingual sections of Class II cavityrestorations. J Am Dent Assoc 57:631, 1958. Johnson, C. R., CastaIdi, C. R., Gau, D. J., and Wysocki, G. P.: Stress pattern variations in operatively prepared human teeth, studied by three-dimensional photoelasticity. J Dent Res 47:548, 1968. Walton. C. B., and Leven, M. M.: A preliminary report of photoelastic tests of strain patterns within ,jacket crowns. J Am Dent Assoc 50:44, 1955. Lehman, M. I,., and Hampson, E. I,.: A study of strain patterns in jacket crowns on anterior teeth resulting from different tooth preparations. Br Dent J 113:337, 1962. Colin, L., Kaufman, E. C., Papimo. R.: Stress concentration in full crown restorations. NY State Dent J 29:370, 1963.
AUGUST1978
VOLUME40
NUMBER2
VISUALIZATION
18.
19.
20.
21.
22.
23.
24.
25 26
27
28
29
OF STRAIN
PATTERNS
Craig, R. G.. El-Ebrashi. Mw. K., LePeak, P. J., and Peyton, F. A.: I:xperimental stress analysis of dental restorations. Part I. Two-dimensional photoelastic stress analysis of inlays. .J PROSTHI.~ D~.wr 17:277, 1967. Craig, R. G.. El-Ebrashi. M. K.. and Peyton, F. A.: Experimental stress analysis of dental restorations. Part II. Twodimensional photoelastic stress analysis of crowns. J Pws‘r(u’r Dt.vl 17:292. 1967. El-Brashi, M. K., Craig. R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part III. The concept of the geometry of proximal margins. J PROWHF.T DI.~CT 22:333, 1969. El-Ebrashi, M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part IV. The concept of parallelism of axial walls. J PRosrw.r DEN.I 22:346, 1969. El-Brashi, M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part V. The concept of occlusai reduction and pins. .J PROSTI~E.T DF.NT 22:565, 1969. El-Ebrashi. M. K., Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part VI. The concept of proximal reduction in compound restorations. J PROSTHLT DENT 22:663, 1969. El-Ebrashi, M. K.. Craig, R. G., and Peyton, F. A.: Experimental stress analysis of dental restorations. Part VII. Structural design and stress analysis of fixed partial dentures. J PROSTHS.~ DINT 23:177, 1970. Farah, .J. W.. and Craig, R. G.: Reflection photoelastic stress analysis of a dental bridge. J Dent Res 50:1253, 1971. Fisher, D. W., Caputo, A. A., Shillingburg, H. T., and Duncanson, M. G.: Photoelastic analysis of inlay and onlay preparations. J PRosTH1.r DFwr 33:47> 1975. Hood, .J. ‘4.. Farah, .J. Mr., and Craig, R. G.: Stress and deflection of three different pontic designs. J PRosrw.T DE NT 33:55. 1975. Glickman, I.: Roeber, F. W., Brion, M., and Pameijer, J. H. S.: Photoelastic analysis of internal stresses in the periodontium created by occlusal forces. J Periodontol 41:30, 1970. Rodriquez. C. A., and Arrechea, G.: Periodontal distribution
THE JOURNAL
OF F’ROSTHETIC
DENTISTRY
30.
31.
32.
33.
34.
35.
36. 37.
38. 39.
40.
of occlusai forces photoelastic study. J Periotlontol 44:485, 1973. Rodriquez, C. A., and Vazques, S.: Experur~cntal approximation to the determination of the true axial forces. J Periodontol 45: 110, 1974. Collard, E. lv., Standlee, J. P.. and Capulo. .4. A: An acceptable photoelastic simulation of Merraptan rubber impression materials. .J South Calif Dent 4csoc 38:506. 1970. Collard. E. M’., Caputo, A. A., Standlee. J 1’ , and Trabert, K. C.: Dynamic stresses encountered in imprvsion removal. J PROSTIET DF.NT 29:498, 1973. Kratochvil, F. J.. and Caputo. A. A.: Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J PROSTHE.T DENT 32:52, 1974. Warren, A. B., and Caputo, A. A.: Load transfer to alveolar bone as influenced by abutment designs for tooth-supported dentures. ,J PROSTHE.T DENT 33:137, 1975. Craig, R. G., Farah, J. W., and El-Tahawi. H. M.: l‘hreedimensional photoelastic stre’ess analysis of maxillary complete dentures. J PROXHE.~ DFNT 31:121. 1954. Photoelastic Stress Analysis. Kodak Job Sheet No. 13. Rochester, N. Y.. Eastman Kodak Co., 1969 Mehta. N. R., Roeber, F. W., Haddad, A. \%‘., and Glickman, I.: Photoelastic model for occlusal fwcr analysis. .J Dent Res 54: 1243, 1975. Mensor, M. C.: The rationale of resilient hinge-action stressbreakers. J PKOSTHFT DF.NT 20:204, 1968. Cecconi, B. T.. Kaiser, G., and Rahe, A.: Stressbreakers and the removable partial denture. ,J P~osrr3t.1 DFST 34:145, 1975. Henderson, D., Blevins, W. R., Wesley, R.
