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The effect of fatigue impact forces upon the retention of various designs of resin-retained bridgework
The effect of fatigue impact forces upon the retenbon of various designs of resinretained bridgework
W. P. S a u n d e r s Department of Conservative Dentistry, Dundee University, Scotland
Saunders WP. The effect of fatigue impact forces upon the retention of various designs of resin-retained bridgework. Dent Mater 1987: 3: 85-89. Abstract - A comparative study of the tensile fatigue limits of 3 designs of resin retained bridge (electrolytically etched, lost-salt crystal and Panavia EX) was undertaken using the staircase technique. Fixed-fixed designs of these resinretained bridges were attached to prepared bovine enamel tooth surfaces. Specimens were thermocycled between 5 ~ and 55 ~ for 48 h and then tested sequentially for 5000 cycles at different stress increments utilising a tensile impact loading to simulate masticatory forces. Those specimens that did not debond were subjected to a single debonding impact force and the tensile impact retentive strength calculated. A statistically valid value for mean fatigue limit was determined for each design. This showed that the mean fatigue limit was similar in all cases. Fatigue forces did not influence ultimate tensile impact retentive bond strength in those specimens tested.
The resin-retained bridge has now become a standard treatment option in fixed prosthodontics and a number of different designs are now in clinical use. These are based upon the same general design principle: a metal framework is attached to acid-treated tooth enamel by means of a composite resin. This framework supports an artificial tooth pontic. Retention of the resin to the metal surface may be achieved in a number of ways. A perforated framework was the first to be described (1), the composite resin flowing into the holes to create retentive "rivets" of resin. Other designs rely upon the retention of the composite to the metal framework by roughening the fitting surface of the latter. In the Maryland bridge, this roughness is achieved b y etching the metal electrolytically (2) and in another design, the surface is roughened by the addition of salt crystals to the surface of the resin pattern of the bridge. These are subsequently washed away prior to casting to give a microscopically pitted surface (3, 4). Recently, a number of resins have been introduced which are said to combine chemically to the metal surface of the framework to enhance retention.
Resin-retained bridges are expected to withstand the forces of mastication. These forces may be considered to have a cyclic component which is also impact in nature. This implies that the effect of loading upon the teeth and restorations is a function of volume under stress (5, 6). The effect of these forces may be to create a single impact force of a magnitude that may be sufficient to dislodge a resin-retained bridge (7, 8) but also repeated impact forces. It is known that when a component or structure is subjected to repeated stress cycles, it may fail at stresses well below the tensile strength by a process known as fatigue (9). The fatigue life of a resin-retained bridge may be defined as the number of cycles of stress the bridge withstands before it fails. At high stresses, the failure will occur after a low number of cycles and conversely, at low stresses failure will occur after a large number of stress cycles. Below a certain stress value, known as the fatigue limit, the bridge can be subjected to a very large number of cycles without failing. The effect of the fatiguing influence of masticatory load upon the retention of resin-retained bridgework has not been reported. The purpose of this
Key words: acid etching, dental bonding, dental
composite resins, fatigue testing, fixed partial denture. W. P. Saunders, Department of Conservative Dentistry, Dental School, The University, Dundee DD1 4HN, Scotland.
Received May 28, 1986; accepted August 15, 1986.
study, therefore, was to compare the effect of cyclic stress upon the retentive capacity of 3 designs of resin-retained bridgework. Material and m e t h o d s
Permanent bovine incisors were used as the tooth substrate in this study. These teeth have been shown to be suitable for use as a substitute for human teeth in adhesion tests (10-12). The teeth were prepared in the same way as for previous impact studies (7, 8). The pulps were removed from the sectioned apex and the labial face of each crown was flattened using a water-cooled diamond disc. The mesial and distal surfaces were then cut at 90 ~ to the labial face to give a flattened surface of enamel at least 4 mm by 4 ram. Pairs of teeth were then mounted in autopolymerising polymethyl methacrylate resin separated by a distance of 5 mm with each labial surface in the same plane. The specimens were stored in deionised water until ready for use. The bridges were all of the same dimensions, 13 mm long, 4 mm wide a 1 mm thick. These were constructed from resin patterns that were invested and cast in a nickel chromium alloy,
86
Saunde~
Biobond Plus*. Three groups of bridges were to be tested. In the first group, the fitting surfaces of the bridges were etched electrolytically using 0.5 N HNO 3 as the electrolyte with a current density of 250 mA/cm z for 5 min. Cleaning of the etched surface was carried out in an ultrasonic bath for 10 min using 18% hydrochloric acid. The fitting surfaces of the second group of bridges were sandblasted with A1203 of 50 ~tm particle size at 60 psi to roughen the surfaces. The third group were constructed using the lost-salt crystal technique (13). All bridges were subjected to 4 porcelain firing cycles immediately following casting and despruing. The design of each bridge was fixedfixed and the fitting surface to each tooth was 4 mm by 4 mm, making a total area of contact of 32 mm 2. The enamel surfaces were cleaned with plain pumice and water on a bristle brush in a slow-speed handpiece prior to etching. The teeth were washed, dried and etched with the proprietary acid gel provided with each composite. The acid was then washed off thoroughly with a water jet from a three-in-one syringe for 45 s. The enamel surface was then dried thoroughly with an air syringe and checked for the characteristic frosted appearance. Prior to cementation, all bridges were degreased in isopropyl alcohol for 10 min. Composite resin was mixed according to the manufacturers' instructions and placed on the fitting surfaces of the bridge. The bridge was then seated on to the teeth and pushed down with firm finger pressure to give as close a fit as possible. Excess material was removed with a probe. The bridge was held in position until the resin had polymerised. Each specimen was then thermocycled in water between 5 ~ and 55 ~ for approximately 4115 cycles over a 48-h period prior to testing. The composite used for the lost-salt crystal and electrolytically etched groups was Cgnclude t, an ultramicrofine BIS-GMA resin made specifically for use with resin retained bridgework. Prior to placement of these groups, each etched enamel surface was coated with an adhesion promotor, Scotchbond t, as a previous study (8) had shown that the use of the material
* Dentsply, York, PA, USA. t 3M Co, St Paul, Minnesota, USA. * Kuraray Co, Tokyo, Japan.
Fig. 1. The fatigue testing machine with control box A, steel beam B, striking pin P and loading weight W.
greatly increased the bond strength. The sandblasted group was cemented with Panavia EX*, a composite resin containing a phosphate ester within the monomer. After placement of this group of bridges, a gel, Oxyguard, supplied by the manufacturers, was applied to the margins of the bridge so that anaerobic polymerisation could take place. Fatigue testing was carried out using equipment designed for the purpose. The principle of operation of this apparatus was based upon the "walking beam" machine (14). This consists of a simple lever actuated by a motordriven cam to give a cyclic stress to a specimen by way of a striking pin. The apparatus is shown in Fig. 1. The operation of the machine is carried out from the control box (A) which houses the switch gear for the motor, a digital revolution counter and a cut-out device for the motor which is activated when the bridge is debonded. A steel beam (B) is attached to a metal base by means of a steel block and bolt. Rotation of the beam can occur in the vertical axis about this bolt. The free end of the beam is allowed to rest upon a cam which, in turn, is attached to an AC-powered drive motor. The free end of the beam is fitted with a platform on to which weights can be added during the testing sequence. A striking pin (P), 2 mm in diameter, is situated over the specimen holder such that the force applied to the bridge is tensile in nature. The pin is allowed to act on the pontic of the bridge half way between the 2 tooth abutments. The direction of the fatigue force is at 90 ~ to the flattened tooth enamel surfaces and acting to push the bridge away from those sur-
faces. The force acting on the beam could be adjusted in 3 kg wt (29.42 N) increments by the addition or removal of a f kg wt lead block from the end of the beam (W). The fatigue test was carried out using the staircase or "up and down" method which has been shown to provide a good measure of mean fatigue limit (15). By this method, a specimen is tested for a prescribed number of cycles at a stress close to the estimated mean fatigue stress. If failure occurs within this number of cycles, then the next specimen is tested at a stress level lowered by a fixed increment. If failure does not occur, then the next specimen is tested at a stress level increased by the same increment. Each specimen is tested sequentially and the stress applied depends upon whether failure or non-failure occurred with the previous specimen. The number of specimens required in the staircase method is less than with other fatigue tests. A minimum of 15 specimens is required for accurate data analysis (16). The speed of cycling was adjusted to 215 revolutions/min. The stress acting upon the bridge was calculated using the formula F/A where F is the force in Newtons and A is the cross sectional area of the striking pin, 3.142 mm ~. The stress increment was 18.73 N/mm 2 for the Maryland and Panavia EX groups and 9.36 N/mm 2 for the lost-salt crystal group. The prescribed number of cycles was set at 5000. Sixteen specimens were tested for the Maryland design and 15 specimens for each of the lostsalt crystal and Panavia EX designs. Those specimens that did not fail during the fatigue test were debonded using a single impact force using appa-
Resin-retained bridges & fatigue impact ratus described in detail elsewhere (8). The object was to determine whether the cyclic stress for 5000 cycles reduced tensile impact retentive strength. This was calculated by dividing the impact force required for debonding, in Newtons, by the surface area of contact of each bridge, 32 mm 2. Following debonding, a study was made of the mode of fracture of each specimen using a stereoscopic microscope at x 10 magnification.
Fatigue
LOST S A L T C R Y S T A L
Stress
X Failures
N/mm 2
9 N o n Failures = 8
46.825
-
37.46
-
28.08
-
18.73
-
9.36
-
X
X
J~=Xo + d
(A 1)
X
9
9
|
|
|
l
|
l
|
I
I
1
2
3
4
5
6
7
8
9
|
|
l
|
I
|
10 11 12 13 14 15
M A R Y L A N D SCOTCHBOND
Fatigue
Stress
X Failures
N/mm 2
= 7
9 Non Failures = 9
56.19
X
37.46
X
X
9
9
X
X
9
X
9
X
Table 1. Analysis of staircase test procedural data Stress N / m m 2
i
n~
in~
i2n~
46.825 37.46 28.095
2 1 0
3 3 1
6 3 1
12 3 1
A
=
Zin~
=9
B
Zi2n~ =15 =
9
9
|
1
9
I
9
I
|
9
i
l
I
I
I
I
I
I
2
3
4
5
6
7
8
9 1A2A 3A4A5A6A7A
I
I
I
I
I
8A
Specimen Number
Fig. 3. Sequence of failures and non-failures for Maryland group.
This formula is an approximation but when NB - A2/N 2 is larger than 0.3, it is sufficiently accurate. The standard deviations of fatigue limits in this study were calculated with this formula. The results for the fatigue tests are shown in Table 2. The results of mean impact failure stress for those speci-
on~
9
+
/ N B - A2 ) S = 1.62 d \ N2 + 0.029
-
9
X
Number of Specimens
where Xo is the lowest stress level considered in the analysis and d is the stress increment employed. The positive sign is used for non-failures and the negative sign for failures. The standard deviation was derived from the formula
=7
X
X
9
= 7
Fig. 2. Sequence of failures and non-failures for lost-salt crystal group.
18.73
N
9
X
Results Figs. 2, 3 and 4 show the sequence of non-failures and failures at different stress levels for each of the 3 designs of resin-retained bridge. The analysis of data is based upon the least frequent event. In the lost-salt crystal design, there were 8 non-failures and 7 failures. The mean fatigue limit was calculated by arranging the data as in Table 1. The lowest stress level at which a failure occurs is denoted by i = o, the next level i = 1 and so on. The mean fatigue limit was calculated using the formula
87
P A N A V I A EX.
Fatigue Stress
X Failures
9 Non Failures = 7
N/mm 2
56.19 -
X
9
X
37.46 -
18.73 -
= 8
9
I
1
X
X
9
9
l
2
I
3
9
X
X
9
X
9
I
4
I
l
5
6
l
7
I
8
I
9
X
!
l
I
I
I
!
10 11 12 13 14 15
Number of Specimens Fig. 4. Sequence of failures and non-failures for Panavia E X group.
88
Saunde~
Table 2. Results of impact fatigue limit
Table 3. Mean impact failure stress
Design of bridge Lost-salt crystal Maryland Panavia EX
Impact fatigue limit, N/mm2
SD
Design of bridge
35.46 36.15 38.8
7.87 8.31 8.19
Lost-salt crystal Maryland Panavia EX
mens debonded by a single impact force, following fatigue testing, are shown in Table 3, together with the mean impact failure stress of specimens tested without undergoing fatigue. A study of the fracture patterns showed that in the electrolytically etched design, 15 of the 16 specimens had complex patterns with composite remaining on both the tooth and metal surfaces. In the other specimen, failure was at the metal composite interface. In the lost-salt crystal group, the fracture patterns were complex through the bulk of the composite with composite resin remaining on both tooth enamel and fitting metal surfaces. The distribution of the composite resin on the metal surface" was sparse, however, and was confined to the depths of the roughened surface. The Panavia EX group also showed complex fracture patterns with composite resin remaining on both fitting surfaces in 13 cases. In the other 2 specimens, failure was entirely at the metal composite resin interface.
Discussion From the results, it would appear that the effect of cyclic fatigue stress was similar for the 3 designs of resin retained bridge. The figures recorded for the mean fatigue limit in this study may be applied, however, only to bridges of the dimensions tested. The reason for this is that calculations were based upon the stress applied to the bridge only and no account was taken of the surface area of the bridge on contact with the teeth. It would be imprudent to suggest that the mean fatigue limit would be the same if a different surface area had been covered with adhesive. This study was, thus, comparative in nature where the surface area of the teeth that was covered by each bridge was the same in the 3 groups, and bridges of the same dimensions were used. Comparative tests are frequently used in fatigue studies and are considered to be of benefit (17). Five thousand cycles is a small number when compared with the number of
No. of specimens
Impact failure stress Impact failure stress after fatigue, N/mm2 after no fatigue, N/mm2
8 9 6
stress cycles a bridge must withstand to give a satisfactory service life. It could be argued that to withstand fatigue stresses in excess of 5000 cycles, the applied stress should be tess than the mean fatigue limit. This study, however, subjected the bridge to the maximum effects of tensile fatigue stress but clinically masticatory forces may not always act in a way that provides these maximum fatigue effects. The first specimen of the Maryland group was tested at a stress of 18.73 N/mm 2. Debonding did not take place but cycling was continued. After 90,000 cycles, debonding had still not occurred and the tensile impact failure strength was determined. This was 33.75 N/mm z. All other bridges that had not failed after 5000 cycles were tested immediately to determine the impact rententive strength. From these results, it would appear that the effect of fatigue stress up to 5000 cycles may not influence the ultimate impact retentive bond strength. It could be that the specimens that did not survive the fatigue test would have had lower values for tensile impact retentive strength anyway and conversely, those specimens with a high fatigue tolerance would have high impact failure strengths. It could be argued that the action of fatigue forces upon resin retained bridgework are analogous to those forces which act upon an adhesive joint. There had been a general lack of research in the field of fatigue when applied to adhesive joints (18), but it has been concluded that the fatigue threshold limit is normally lower than the adhesive fracture energy and that this fatigue threshold is independent of cyclic frequency. The threshold value for adhesive fatigue cannot be predicted from the adhesive fracture energy (19). The method of staircase analysis for comparing the effect of fatigue upon various designs of resin-retained bridgework has shown that the cyclic application of stress affects these designs in a way that gives similar mean fatigue limits for each design. The fracture mechanics of the fatigued resin-retained
33.43 45.83 50.52
22.88 43.52 36.17
bridge are complex and, therefore, it would be unreasonable to postulate that the method of crack growth in all the systems was the same.
Acknowledgements - I wish to thank Profes-
sor A. R. Grieve for his continuing support, the Department of Medical Physics, Ninewells Hospital, Dundee for constructing the fatigue testing apparatus, and the Scottish Home and Health Department for financial support.
References 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J Prosthet Dent 1973: 30: 418-423. 2. Lividitis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin bonded retainers. J Prosthet Dent 1982: 47." 52-58. 3. Moon PC. The laboratory procedures for the Virginia resin bonded bridge, trends and techniques in the contemporary dental laboratory, July/August 1985: 22-28. 4. Moon PC, Knap FJ. Acid-etched bridge bond strength utilizing a new retention method. J Dent Res 1983: 62: Abstr. 296. 5. Brumfield RC. Fundamental mechanics of dental bridges. In: Tylman M, ed. Theory and practice of crown and bridge prosthodontics. St. Louis: Mosby, 1965: 1118-1136. 6. Johnson BE. Effect of impact loading upon Class II amalgam restorations. J Dent Child 1972: 39: 206-214. 7. Saunders WP. The retentive impact strengths of various designs of resin bonded bridges to etched bovine enamel. Br Dent J 1984: 156: 325-328. 8. Saunders WP. The effect of an adhesion promotor on the retentive strengths of various designs of resin-bonded bridges. Restorative Dent 1985: 1: 127-130. 9. Ashby MF, Jones DRH. Fatigue failure. In: Engineering materials: an introduction to their properties and applications. Oxford: Pergamon, 1980: 135-142. 10. Viohl J, Kops C. Bond strength of auto and photopolymerising restorative resin. J Dent Res 1982: 61: Abstr. 825.
Resin-retained bridges & fatigue impact 11. Nakamichi I, Iwaku M, Fusayama T. Bovine teeth as possible substitutes in the adhesion test. J Dent Res 1983: 62: 1076-1081. 12. NY KC, Chang R. An approach to the study of the mechanism of adhesion to teeth. US Public Health Serv Publ 1965: 1494: 103-155. 13. Hudgins JL, Moon PC, Knap FJ. Particle roughened resin bonded retainers. J Prosthet Dent 1985: 53: 471-476.
14. Gurney TR. Fatigue testing. In: Fatigue of welded structures. Cambridge, Cambridge Univ Press, 1965. 5. 15. Draughn RA. Compressive fatigue limits of composite restorative materials. J Dent Res 1979: 58: 1093-1096. 16. Dieter GE. In: Mechanical metallurgy. New York: McGraw-Hill, 1961: 446449. 17. Osgood CC. In: Fatigue design. New York: Wiley, 1970.
89
18. Anderson GP, Bennett SJ, De Vries KL. Analysis and testing of adhesive bonds. New York: Academic, 1977. 19. Mostovoy S, Ripling EJ. Fracturing characteristics of adhesive joints - final report. 1974: Mater Res Lab Contract No. N00019-73-C-0163.
W. P. S a u n d e r s Department of Conservative Dentistry, Dundee University, Scotland
Saunders WP. The effect of fatigue impact forces upon the retention of various designs of resin-retained bridgework. Dent Mater 1987: 3: 85-89. Abstract - A comparative study of the tensile fatigue limits of 3 designs of resin retained bridge (electrolytically etched, lost-salt crystal and Panavia EX) was undertaken using the staircase technique. Fixed-fixed designs of these resinretained bridges were attached to prepared bovine enamel tooth surfaces. Specimens were thermocycled between 5 ~ and 55 ~ for 48 h and then tested sequentially for 5000 cycles at different stress increments utilising a tensile impact loading to simulate masticatory forces. Those specimens that did not debond were subjected to a single debonding impact force and the tensile impact retentive strength calculated. A statistically valid value for mean fatigue limit was determined for each design. This showed that the mean fatigue limit was similar in all cases. Fatigue forces did not influence ultimate tensile impact retentive bond strength in those specimens tested.
The resin-retained bridge has now become a standard treatment option in fixed prosthodontics and a number of different designs are now in clinical use. These are based upon the same general design principle: a metal framework is attached to acid-treated tooth enamel by means of a composite resin. This framework supports an artificial tooth pontic. Retention of the resin to the metal surface may be achieved in a number of ways. A perforated framework was the first to be described (1), the composite resin flowing into the holes to create retentive "rivets" of resin. Other designs rely upon the retention of the composite to the metal framework by roughening the fitting surface of the latter. In the Maryland bridge, this roughness is achieved b y etching the metal electrolytically (2) and in another design, the surface is roughened by the addition of salt crystals to the surface of the resin pattern of the bridge. These are subsequently washed away prior to casting to give a microscopically pitted surface (3, 4). Recently, a number of resins have been introduced which are said to combine chemically to the metal surface of the framework to enhance retention.
Resin-retained bridges are expected to withstand the forces of mastication. These forces may be considered to have a cyclic component which is also impact in nature. This implies that the effect of loading upon the teeth and restorations is a function of volume under stress (5, 6). The effect of these forces may be to create a single impact force of a magnitude that may be sufficient to dislodge a resin-retained bridge (7, 8) but also repeated impact forces. It is known that when a component or structure is subjected to repeated stress cycles, it may fail at stresses well below the tensile strength by a process known as fatigue (9). The fatigue life of a resin-retained bridge may be defined as the number of cycles of stress the bridge withstands before it fails. At high stresses, the failure will occur after a low number of cycles and conversely, at low stresses failure will occur after a large number of stress cycles. Below a certain stress value, known as the fatigue limit, the bridge can be subjected to a very large number of cycles without failing. The effect of the fatiguing influence of masticatory load upon the retention of resin-retained bridgework has not been reported. The purpose of this
Key words: acid etching, dental bonding, dental
composite resins, fatigue testing, fixed partial denture. W. P. Saunders, Department of Conservative Dentistry, Dental School, The University, Dundee DD1 4HN, Scotland.
Received May 28, 1986; accepted August 15, 1986.
study, therefore, was to compare the effect of cyclic stress upon the retentive capacity of 3 designs of resin-retained bridgework. Material and m e t h o d s
Permanent bovine incisors were used as the tooth substrate in this study. These teeth have been shown to be suitable for use as a substitute for human teeth in adhesion tests (10-12). The teeth were prepared in the same way as for previous impact studies (7, 8). The pulps were removed from the sectioned apex and the labial face of each crown was flattened using a water-cooled diamond disc. The mesial and distal surfaces were then cut at 90 ~ to the labial face to give a flattened surface of enamel at least 4 mm by 4 ram. Pairs of teeth were then mounted in autopolymerising polymethyl methacrylate resin separated by a distance of 5 mm with each labial surface in the same plane. The specimens were stored in deionised water until ready for use. The bridges were all of the same dimensions, 13 mm long, 4 mm wide a 1 mm thick. These were constructed from resin patterns that were invested and cast in a nickel chromium alloy,
86
Saunde~
Biobond Plus*. Three groups of bridges were to be tested. In the first group, the fitting surfaces of the bridges were etched electrolytically using 0.5 N HNO 3 as the electrolyte with a current density of 250 mA/cm z for 5 min. Cleaning of the etched surface was carried out in an ultrasonic bath for 10 min using 18% hydrochloric acid. The fitting surfaces of the second group of bridges were sandblasted with A1203 of 50 ~tm particle size at 60 psi to roughen the surfaces. The third group were constructed using the lost-salt crystal technique (13). All bridges were subjected to 4 porcelain firing cycles immediately following casting and despruing. The design of each bridge was fixedfixed and the fitting surface to each tooth was 4 mm by 4 mm, making a total area of contact of 32 mm 2. The enamel surfaces were cleaned with plain pumice and water on a bristle brush in a slow-speed handpiece prior to etching. The teeth were washed, dried and etched with the proprietary acid gel provided with each composite. The acid was then washed off thoroughly with a water jet from a three-in-one syringe for 45 s. The enamel surface was then dried thoroughly with an air syringe and checked for the characteristic frosted appearance. Prior to cementation, all bridges were degreased in isopropyl alcohol for 10 min. Composite resin was mixed according to the manufacturers' instructions and placed on the fitting surfaces of the bridge. The bridge was then seated on to the teeth and pushed down with firm finger pressure to give as close a fit as possible. Excess material was removed with a probe. The bridge was held in position until the resin had polymerised. Each specimen was then thermocycled in water between 5 ~ and 55 ~ for approximately 4115 cycles over a 48-h period prior to testing. The composite used for the lost-salt crystal and electrolytically etched groups was Cgnclude t, an ultramicrofine BIS-GMA resin made specifically for use with resin retained bridgework. Prior to placement of these groups, each etched enamel surface was coated with an adhesion promotor, Scotchbond t, as a previous study (8) had shown that the use of the material
* Dentsply, York, PA, USA. t 3M Co, St Paul, Minnesota, USA. * Kuraray Co, Tokyo, Japan.
Fig. 1. The fatigue testing machine with control box A, steel beam B, striking pin P and loading weight W.
greatly increased the bond strength. The sandblasted group was cemented with Panavia EX*, a composite resin containing a phosphate ester within the monomer. After placement of this group of bridges, a gel, Oxyguard, supplied by the manufacturers, was applied to the margins of the bridge so that anaerobic polymerisation could take place. Fatigue testing was carried out using equipment designed for the purpose. The principle of operation of this apparatus was based upon the "walking beam" machine (14). This consists of a simple lever actuated by a motordriven cam to give a cyclic stress to a specimen by way of a striking pin. The apparatus is shown in Fig. 1. The operation of the machine is carried out from the control box (A) which houses the switch gear for the motor, a digital revolution counter and a cut-out device for the motor which is activated when the bridge is debonded. A steel beam (B) is attached to a metal base by means of a steel block and bolt. Rotation of the beam can occur in the vertical axis about this bolt. The free end of the beam is allowed to rest upon a cam which, in turn, is attached to an AC-powered drive motor. The free end of the beam is fitted with a platform on to which weights can be added during the testing sequence. A striking pin (P), 2 mm in diameter, is situated over the specimen holder such that the force applied to the bridge is tensile in nature. The pin is allowed to act on the pontic of the bridge half way between the 2 tooth abutments. The direction of the fatigue force is at 90 ~ to the flattened tooth enamel surfaces and acting to push the bridge away from those sur-
faces. The force acting on the beam could be adjusted in 3 kg wt (29.42 N) increments by the addition or removal of a f kg wt lead block from the end of the beam (W). The fatigue test was carried out using the staircase or "up and down" method which has been shown to provide a good measure of mean fatigue limit (15). By this method, a specimen is tested for a prescribed number of cycles at a stress close to the estimated mean fatigue stress. If failure occurs within this number of cycles, then the next specimen is tested at a stress level lowered by a fixed increment. If failure does not occur, then the next specimen is tested at a stress level increased by the same increment. Each specimen is tested sequentially and the stress applied depends upon whether failure or non-failure occurred with the previous specimen. The number of specimens required in the staircase method is less than with other fatigue tests. A minimum of 15 specimens is required for accurate data analysis (16). The speed of cycling was adjusted to 215 revolutions/min. The stress acting upon the bridge was calculated using the formula F/A where F is the force in Newtons and A is the cross sectional area of the striking pin, 3.142 mm ~. The stress increment was 18.73 N/mm 2 for the Maryland and Panavia EX groups and 9.36 N/mm 2 for the lost-salt crystal group. The prescribed number of cycles was set at 5000. Sixteen specimens were tested for the Maryland design and 15 specimens for each of the lostsalt crystal and Panavia EX designs. Those specimens that did not fail during the fatigue test were debonded using a single impact force using appa-
Resin-retained bridges & fatigue impact ratus described in detail elsewhere (8). The object was to determine whether the cyclic stress for 5000 cycles reduced tensile impact retentive strength. This was calculated by dividing the impact force required for debonding, in Newtons, by the surface area of contact of each bridge, 32 mm 2. Following debonding, a study was made of the mode of fracture of each specimen using a stereoscopic microscope at x 10 magnification.
Fatigue
LOST S A L T C R Y S T A L
Stress
X Failures
N/mm 2
9 N o n Failures = 8
46.825
-
37.46
-
28.08
-
18.73
-
9.36
-
X
X
J~=Xo + d
(A 1)
X
9
9
|
|
|
l
|
l
|
I
I
1
2
3
4
5
6
7
8
9
|
|
l
|
I
|
10 11 12 13 14 15
M A R Y L A N D SCOTCHBOND
Fatigue
Stress
X Failures
N/mm 2
= 7
9 Non Failures = 9
56.19
X
37.46
X
X
9
9
X
X
9
X
9
X
Table 1. Analysis of staircase test procedural data Stress N / m m 2
i
n~
in~
i2n~
46.825 37.46 28.095
2 1 0
3 3 1
6 3 1
12 3 1
A
=
Zin~
=9
B
Zi2n~ =15 =
9
9
|
1
9
I
9
I
|
9
i
l
I
I
I
I
I
I
2
3
4
5
6
7
8
9 1A2A 3A4A5A6A7A
I
I
I
I
I
8A
Specimen Number
Fig. 3. Sequence of failures and non-failures for Maryland group.
This formula is an approximation but when NB - A2/N 2 is larger than 0.3, it is sufficiently accurate. The standard deviations of fatigue limits in this study were calculated with this formula. The results for the fatigue tests are shown in Table 2. The results of mean impact failure stress for those speci-
on~
9
+
/ N B - A2 ) S = 1.62 d \ N2 + 0.029
-
9
X
Number of Specimens
where Xo is the lowest stress level considered in the analysis and d is the stress increment employed. The positive sign is used for non-failures and the negative sign for failures. The standard deviation was derived from the formula
=7
X
X
9
= 7
Fig. 2. Sequence of failures and non-failures for lost-salt crystal group.
18.73
N
9
X
Results Figs. 2, 3 and 4 show the sequence of non-failures and failures at different stress levels for each of the 3 designs of resin-retained bridge. The analysis of data is based upon the least frequent event. In the lost-salt crystal design, there were 8 non-failures and 7 failures. The mean fatigue limit was calculated by arranging the data as in Table 1. The lowest stress level at which a failure occurs is denoted by i = o, the next level i = 1 and so on. The mean fatigue limit was calculated using the formula
87
P A N A V I A EX.
Fatigue Stress
X Failures
9 Non Failures = 7
N/mm 2
56.19 -
X
9
X
37.46 -
18.73 -
= 8
9
I
1
X
X
9
9
l
2
I
3
9
X
X
9
X
9
I
4
I
l
5
6
l
7
I
8
I
9
X
!
l
I
I
I
!
10 11 12 13 14 15
Number of Specimens Fig. 4. Sequence of failures and non-failures for Panavia E X group.
88
Saunde~
Table 2. Results of impact fatigue limit
Table 3. Mean impact failure stress
Design of bridge Lost-salt crystal Maryland Panavia EX
Impact fatigue limit, N/mm2
SD
Design of bridge
35.46 36.15 38.8
7.87 8.31 8.19
Lost-salt crystal Maryland Panavia EX
mens debonded by a single impact force, following fatigue testing, are shown in Table 3, together with the mean impact failure stress of specimens tested without undergoing fatigue. A study of the fracture patterns showed that in the electrolytically etched design, 15 of the 16 specimens had complex patterns with composite remaining on both the tooth and metal surfaces. In the other specimen, failure was at the metal composite interface. In the lost-salt crystal group, the fracture patterns were complex through the bulk of the composite with composite resin remaining on both tooth enamel and fitting metal surfaces. The distribution of the composite resin on the metal surface" was sparse, however, and was confined to the depths of the roughened surface. The Panavia EX group also showed complex fracture patterns with composite resin remaining on both fitting surfaces in 13 cases. In the other 2 specimens, failure was entirely at the metal composite resin interface.
Discussion From the results, it would appear that the effect of cyclic fatigue stress was similar for the 3 designs of resin retained bridge. The figures recorded for the mean fatigue limit in this study may be applied, however, only to bridges of the dimensions tested. The reason for this is that calculations were based upon the stress applied to the bridge only and no account was taken of the surface area of the bridge on contact with the teeth. It would be imprudent to suggest that the mean fatigue limit would be the same if a different surface area had been covered with adhesive. This study was, thus, comparative in nature where the surface area of the teeth that was covered by each bridge was the same in the 3 groups, and bridges of the same dimensions were used. Comparative tests are frequently used in fatigue studies and are considered to be of benefit (17). Five thousand cycles is a small number when compared with the number of
No. of specimens
Impact failure stress Impact failure stress after fatigue, N/mm2 after no fatigue, N/mm2
8 9 6
stress cycles a bridge must withstand to give a satisfactory service life. It could be argued that to withstand fatigue stresses in excess of 5000 cycles, the applied stress should be tess than the mean fatigue limit. This study, however, subjected the bridge to the maximum effects of tensile fatigue stress but clinically masticatory forces may not always act in a way that provides these maximum fatigue effects. The first specimen of the Maryland group was tested at a stress of 18.73 N/mm 2. Debonding did not take place but cycling was continued. After 90,000 cycles, debonding had still not occurred and the tensile impact failure strength was determined. This was 33.75 N/mm z. All other bridges that had not failed after 5000 cycles were tested immediately to determine the impact rententive strength. From these results, it would appear that the effect of fatigue stress up to 5000 cycles may not influence the ultimate impact retentive bond strength. It could be that the specimens that did not survive the fatigue test would have had lower values for tensile impact retentive strength anyway and conversely, those specimens with a high fatigue tolerance would have high impact failure strengths. It could be argued that the action of fatigue forces upon resin retained bridgework are analogous to those forces which act upon an adhesive joint. There had been a general lack of research in the field of fatigue when applied to adhesive joints (18), but it has been concluded that the fatigue threshold limit is normally lower than the adhesive fracture energy and that this fatigue threshold is independent of cyclic frequency. The threshold value for adhesive fatigue cannot be predicted from the adhesive fracture energy (19). The method of staircase analysis for comparing the effect of fatigue upon various designs of resin-retained bridgework has shown that the cyclic application of stress affects these designs in a way that gives similar mean fatigue limits for each design. The fracture mechanics of the fatigued resin-retained
33.43 45.83 50.52
22.88 43.52 36.17
bridge are complex and, therefore, it would be unreasonable to postulate that the method of crack growth in all the systems was the same.
Acknowledgements - I wish to thank Profes-
sor A. R. Grieve for his continuing support, the Department of Medical Physics, Ninewells Hospital, Dundee for constructing the fatigue testing apparatus, and the Scottish Home and Health Department for financial support.
References 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J Prosthet Dent 1973: 30: 418-423. 2. Lividitis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin bonded retainers. J Prosthet Dent 1982: 47." 52-58. 3. Moon PC. The laboratory procedures for the Virginia resin bonded bridge, trends and techniques in the contemporary dental laboratory, July/August 1985: 22-28. 4. Moon PC, Knap FJ. Acid-etched bridge bond strength utilizing a new retention method. J Dent Res 1983: 62: Abstr. 296. 5. Brumfield RC. Fundamental mechanics of dental bridges. In: Tylman M, ed. Theory and practice of crown and bridge prosthodontics. St. Louis: Mosby, 1965: 1118-1136. 6. Johnson BE. Effect of impact loading upon Class II amalgam restorations. J Dent Child 1972: 39: 206-214. 7. Saunders WP. The retentive impact strengths of various designs of resin bonded bridges to etched bovine enamel. Br Dent J 1984: 156: 325-328. 8. Saunders WP. The effect of an adhesion promotor on the retentive strengths of various designs of resin-bonded bridges. Restorative Dent 1985: 1: 127-130. 9. Ashby MF, Jones DRH. Fatigue failure. In: Engineering materials: an introduction to their properties and applications. Oxford: Pergamon, 1980: 135-142. 10. Viohl J, Kops C. Bond strength of auto and photopolymerising restorative resin. J Dent Res 1982: 61: Abstr. 825.
Resin-retained bridges & fatigue impact 11. Nakamichi I, Iwaku M, Fusayama T. Bovine teeth as possible substitutes in the adhesion test. J Dent Res 1983: 62: 1076-1081. 12. NY KC, Chang R. An approach to the study of the mechanism of adhesion to teeth. US Public Health Serv Publ 1965: 1494: 103-155. 13. Hudgins JL, Moon PC, Knap FJ. Particle roughened resin bonded retainers. J Prosthet Dent 1985: 53: 471-476.
14. Gurney TR. Fatigue testing. In: Fatigue of welded structures. Cambridge, Cambridge Univ Press, 1965. 5. 15. Draughn RA. Compressive fatigue limits of composite restorative materials. J Dent Res 1979: 58: 1093-1096. 16. Dieter GE. In: Mechanical metallurgy. New York: McGraw-Hill, 1961: 446449. 17. Osgood CC. In: Fatigue design. New York: Wiley, 1970.
89
18. Anderson GP, Bennett SJ, De Vries KL. Analysis and testing of adhesive bonds. New York: Academic, 1977. 19. Mostovoy S, Ripling EJ. Fracturing characteristics of adhesive joints - final report. 1974: Mater Res Lab Contract No. N00019-73-C-0163.