- Email: [email protected]
Conservative preparation designs for Class II amalgam restorations
Conservative preparation designs for Class II amalgam restorations
J. R. Sturdevant, D. F. Taylor, R. H. Leonard, W. F. Straka, T. M. Roberson, A. D. Wilder Department of Operative Dentistry, University of North Carolina School of Dentistry, Chapel Hill, USA.
Sturdevant JR, Taylor DF, Leonard RH, Straka WF, Roberson TM, Wilder AD. Conservative preparation designs for Class II amalgam restorations. Dent Mater 1987: 3:144-148 Abstract - This study evaluates whether improvements in the physical properties of dental amalgam affect the use of Class II preparations without an occlusal dovetail. Four amalgam alloys were selected providing combinations of particle shape and copper content. The amalgams were characterized for their 24-h compressive and tensile strengths. High-copper amalgams were significantly stronger in compression, but weaker in tension than low-copper amalgams. Five Class II preparation designs were evaluated in cast metal replicas of prepared teeth. Controlling for the amalgam used, the proximal boxonly designs required significantly higher loads for restoration failure than did designs with occlusal dovetails. Of the two physical properties tested prior to the metal die tests, tensile strength was more predictive of load required for restoration failure than compressive strength. Neither copper content nor particle shape of the alloys had a discernible effect on load required for restoration failure. The results indicate that box-only restorations should be tested clinically for their suitability in routine operative dentistry.
The basic principles in the design of amalgam cavity preparations have been modified but not changed in essence over the last 90 years (1). Most of the modifications which have been adopted have served to reduce the extent of the preparation and, thus, increase the conservation of sound tooth structure (2). Two persistent design features in Class II preparations for amalgam have been the occlusal dovetail and proximal retention locks. In 1924 Ward recommended the use of both types of retention, while identifying flow or creep as a property closely associated with displacement (3). When a restoration including an occlusal dovetail is subjected to occlusal loads,'a tensile stress is generated in the isthmus portion of the restoration (4). Thus, the dovetail helps to prevent displacement of the proximal portion of Class II restorations. Proximal retention locks, normally in the form of grooves in the facial and lingual walls of the proximal box, serve the same purpose. The availability of high speed handpieces, carbide burs, and amalgams with high early strength combined in
the late 1950's to promote a renewed interest in cavity design. Since then many laboratory investigations have examined the effect of variations in different design features (5-9). Among other results these studies indicated that proximal retentive grooves are an important factor in retaining the proximal portion of a Class II restoration. One study showed that the retentive effect of the grooves was increased as the grooves were extended occlusally (6). Based in part upon these studies, it has been recommended that, when the occlusal surface is free of defects needing restoration, the dovetail portion of Class II restorations should be totally eliminated and retention afforded by the proximal grooves only (10). When this concept was tested in a laboratory study it was found that proximal boxonly preparations with retentive grooves (no occlusal dovetail) produced stronger restorations than preparations with an occlusal dovetail and no retention grooves in the proximal box. On the basis of these studies it would appear that one could proceed with confidence to a clinical study of box-
Key words: dental materials, dental amalgam, operative dentistry, dental restorations, permanent. Dr. John R. Sturdevant, Department of Operative Dentistry, University of North Carolina School of Dentistry, Chapel Hill, NC 27514, USA.
ReceivedAugust 11; accepted October 2, 1986.
only preparations. If it is true that proximal preparations with retentive grooves without occlusal dovetails are stronger than those with dovetails and no grooves (9), and that restorations with dovetails and no grooves give adequate clinical.performance when used with conventional filing alloys (11, 12), then a clinical evaluation of the conservative box-only preparation is both timely and well justified. Subsequent to the studies described above, another consideration has arisen which appears to make the question of broader significance. Most restorations at the present time appear to be made using high-copper alloys (13): These amalgams have an increased resistance to marginal deterioration in service and somewhat better polish retention. Numerous investigations of the mechanical properties of these amalgams have shown that in comparison to low-copper amalgams they exhibit higher early and final compressive strengths and a significantly reduced tendency toward creep (14-16). However, these high-copper amalgams occasionally show considerably reduced tensile strength in comparison to the
Conservative Class II amalgam restorations
low-copper amalgams (17). If it is concluded that creep is not a relevant property of dental amalgam in regard to proximal displacement (12, 18), and that on the basis of stress analysis the tensile strength must be of significance in regard to isthmus fracture (5), then there is a valid doubt as to whether the conclusions drawn above about the suitability of box-only preparations can be extended to apply to high-copper amalgams. In principle, it is desirable to employ the most conservative treatment which is compatible with preparations of clinical success. Where a tooth presents with proximal caries and the occlusal grooves are not faulty, a proximal preparation without an occlusal dovetail is clearly the most conservative. For a Class II preparation involving the occlusal as well as the proximal surfaces, an occlusal dovetail with proximal boxes but without retention grooves is somewhat more conservative than one with grooves. It is important to determine whether such preparations can safely be used with modern materials. In this study the materials selected permit a comparison between high-cooper and con-
ventional amalgam compositions and between filing and spherical alloy particle shapes. A t the same time the cavity preparation forms were chosen to permit an identification of the relative importance of the various retentive features. Material and methods
Four alloys were selected to provide combinations of particle shape and copper content. 1. Low-copper filing alloy: Velvalloy (F-~) 2. Low-copper spherical alloy: Spheraloy (F-2) 3. High-copper filing admix: Dispersalloy (F-3) F-1. Velvalloy, batch number 5648403X, SS White Division, Pennwalt Corporation, 900 1st Avenue, Philadelphia, PA 19102. F-2. Spheraloy, batch number 0530843013, Kerr Manufacturing Company A Division of Sybron Corporation, 28200 Wick Road, Romulus, Michigan, 48174. F-3. Dispersalloy, batch number 053184A-4C512, Johnson and Johnson Dental Products Division, 20 Lake Drive, East Windsor, N.J. 08520.
145
4. High-copper spherical alloy: Sybraloy (F-4) Amalgam characterization
Each of the amalgams was characterized for their 24-h compressive strength and tensile strength. The specimen design, specimen preparation, and test procedures followed the current A D A Specification I (19, 20), except for the tensile strength test which was determined by the diametral tensile strength test in A D A Specification I (1974) (21). Ten specimens were tested for each amalgam alloy. Mixing and preparation of the amalgams for test specimens followed the manufacturer's directions. The specimens were stored in air at 37~ until tested. Preparation testing
To avoid the inherent variability of extracted natural teeth, 5 different preparation designs were evaluated using metal dies (5). Three were Class II preparations with proximal boxes and occlusal dovetails, while 2 were Class II proximal boxes without occlusal dovetails (Fig. 1). 1. A Class II preparation with an occlusal dovetail and a proximal box but no retention grooves. 2. A Class II preparation with an occlusal dovetail and a proximal box in which proximal retention grooves are placed at the axiofacial and axiolingual line angles and extend to 1 mm from the occlusal surface. 3. A Class II preparation with an occlusal dovetail and a proximal box in which proximal retention grooves are similarly placed but extend through the occlusal surface. 4. A Class H preparation with a proximal box only (no occlusal dovetail) in which the proximal retention grooves extend to 1 mm from the occlusal surface. 5. A Class II preparation with a proximal box only (no occlusal dovetail) with proximal retention grooves that extend through the occlusal surface. All in vitro preparations were replicas of a mesioocclusal preparation cut into an ivorine replica of a maxillary second
Fig. 1. The 5 Class II preparation designs tested in this study. I0
Dental Materials 3:3, 1987
F-4. Sybraloy, batch number 053084-1081, Kerr Manufacturing Company A Division of Sybron Corporation, 28200 Wick Road, Romulus, Michigan, 48174.
146
Sturdevant et al.
premolar (F-5). All preparations were identical in size of the proximal box portion, and varied only in the addition or deletion of occlusal dovetails or proximal retention grooves. All preparations were prepared with a No. 245 carbide bur (F-6) such that the proximal box dimensions were as follows: 4.0 mm wide faciolingually at the level of the gingival floor, tapering to 3.0 mm faciolingually at the level of the mesial marginal ridge. The width of the gingival floor mesiodistally was 1.0 mm. The plane of the axial wall was parallel to the long axis of the tooth. The gingival floor was flat and perpendicular to the long axis of the tooth. The height of the proximal box occlusogingivally was 4.0 mm. To provide consistency in the size and shape of the proximal box portion of the cavity preparations, a polyvinylsiloxane impression (F-7) was made of the basic "box" preparation in the premolar described above and then 5 acrylic resin replicas (F-8) were made and modifications added to generate the 5 preparation forms previously described. In those preparations with an occlusal dovetail (preparations 1, 2, and 3), the dovetail was cut 1.5 mm deep pulpally with a No. 245 carbide bur. The isthmus of the dovetail was 1.0 mm wide faciolingually. In those preparations in which the retention grooves extended to the occlusal surface (Preparations 3 and 5), the grooves were nearly cylindrical in shape, located at the axiofacial and axiolingual line angles, and prepared with a No. 169L carbide bur (F-9) to a uniform depth of 0.5 mm in the facial or lingual wall of the proximal box. The retentive grooves prepared in Preparations 2 and 4 were tapering grooves extending from the axiofaciogingival or the axiolinguogingival point angles to a point approximately 1.5 mm below the occlusal surface. They were prepared with a 169L carbide bur so positioned that the tip produced a 0.5 mm deep groove at the point angle, which tapeF-5. Columbia Dentoform, Columbia Dentoform, 49th East 21st Street, New York, NY 10010. F-6. No. 245 bur, SS White, South Street, Holmdel, NJ 07733. F-7. Reprosil, LD Caulk Company- A division of Dentsply International, Inc., Milford, Delaware 19963. F-8. Duralay Resin, Reliance Dental Manufacturing Company, Worth, IL 60482. F-9. No. 169L bur, SS White, South Street, Holmdel, NJ 07733.
red to zero depth at a distance 1.5 mm gingivally from the occlusal surface. Cast replicas of the acrylic teeth were made in Ticonium base metal casting alloy (F-10). The internal surfaces of the cast metal teeth were polished to remove irregularities which might provide non-designed retentive features (F-11). Specimens for in vitro preparation testing were prepared by hand condensing amalgam into the dies with the aid of Tofflemire retainers and bands (F12). The manufacturer's recommendations for proportioning and trituration of the amalgam were used. Occlusal contours were carved to reproduce as nearly as possible the original occlusal form of the tooth. After condensation the matrix was removed and the proximal portion of the restoration was checked for voids and marginal defects. Incomplete restorations were discarded and replaced. The filled dies were stored in air at 37~ for 24 h prior to testing. Testing was performed on an Instron Universal Testing Machine (F-13). The F-10. Ticonium, Ticonium Company, P.O. Box 350, Albany, NY 12201. F-11. Ti-Lectra Polisher, Ticonium Company, P.O. Box 350, Albany, NY 12201. F-12. Toffiemire retainer, Teledyne Getz, Elk Grove Village, IL 60007. F-13. Instron Universal Testing Machine, Instron Corporation, 2500 Washington Street, Canton, MA 02021.
dies and restorations were positioned to provide a l0 ~ angle of inclination of the long axis of the tooth toward the proximal restored surface in order to generate a proximally directed force component. The load was applied to the marginal ridge of the restoration by a cylindrical carbide tip with a 1.5 mm radius of curvature. The long axis of the cylindrical tip was positioned perpendicular to the direction of loading and located relative to the restoration to be tested so that it extended across the restored marginal ridge in a mesiodistal direction (Fig. 2). The headspeed of the testing machine was adjusted to 0.1 ram/rain to produce an axial strain rate in the box portion of the restoration equivalent to the strain rate specified for compressive strength testing in the A D A specition (19). Loading was continued until the specimen failed by fracture or displacement. The load required, as well as the mechanism of failure was recorded. A total of 15 restorations were tested for each combination of material and preparation design (300 total). The results of the physical properties testing and the preparation testing were described with means and standard errors for each material. Pairwise comparisons between materials were made with the Wilcoxon rank-sum test. The effects of copper and particle
Fig. 2. The cast metal replicas of teeth were inclined 10~ toward the restored proximal sur-
face. The restorations were loaded by a 3.0 mm diameter carbide rod oriented perpendicular to the marginal ridge.
Conservative Class H amalgam restorations Table 1. 24-h physical properties testing Tensile strength Velvalloy Spheraloy Sybraloy Dispersalloy
Significant groupings*
55.7_+1.4 MPa 52.8+1.7 MPa 46.3_+2.5 MPa 46.9+1.8 MPa
] ] ] ]
Compressive strength
Significant groupings*
367.6_+ 1.2 MPa 335.6_+ 2.0 MPa 492.2_+11.4 MPa 448.1_+ 3.2 MPa
] ] ] ]
* Pairwise comparisons between each pair of materials were based on the Wilcoxon ranksum test. Value(s) within an individual bracket showed no significant differences at p = 0.05. Values within an individual bracket differ significantly from those in other brackets at p = 0.05. Table 2. Significance of copper content and particle shapet Tension High copper vs. low copper Filing vs. spherical particles
p chi-square p chi-square
Compression
< 0.01" (1 d.f.) = 2 8 . 5 7 > 0.10 (1 d.f.) = 0 . 1 0
p chi-square p chi-square
< 0.01" (1 d.f.) = 14.69 > 0.10 (i d.f.) = 1.80
* Chi-square statistics with 1 d.f. from stratified rank tests and their p-value status. * Significant with p < 0.01.
shape on physical properties were analyzed with the van Elteran stratified rank tests (22). The effects of amalgam and preparation on failure load were evaluated through a multiple linear regression model. This model simultaneously accounted for preparation and amalgam effects through a set of indicator variables. Comparisons for amalgams, preparations, copper content, particle shape, and mode of failure were undertaken through t-statistics for the corresponding estimated effects from the multiple linear regression model (23). Computations were made with the GLM procedure in SAS (24). In addition, chi-square tests were used for further evaluations of mode of failure.
Results The results of the 24-h physical properties tests are shown in Table 1. The mean compressive strength values ranged from a low of 335.6 MPa for
Spheraloy to a high of 492.2 MPa for Sybraloy. The Wilcoxon rank-sum tests indicated significant differences between each of the 4 amalgams in compressive strengths at p - 0 . 0 5 . In tension, the 2 low-copper amalgams, Velvalloy and Spheraloy, had mean values of 55.7 MPa and 52.8 MPa respectively. The 2 high-copper amalgams had lower mean tensile strengths of 46.9 MPa for Dispersalloy and 46.3 MPa for Sybraloy. The Wilcoxon ranksum tests indicated the tensile strengths of both the low-copper amalgams were significantly greater than those of the high-copper amalgams. Evaluations were made of the effects of copper content and particle shape on the strength of amalgam, both in tension and compression, by means of stratified rank tests (Table 2). These tests indicate that high-copper amalgams are significantly stronger than low-copper amalgams in compression, but are significantly weaker than lowcopper amalgams in tension. Particle shape of the alloys had no significant
effect on tensile or compressive strengths. Two modes of failure were observed in the metal die preparation tests. The predominant failure (79%) was a cohesive failure of the amalgam evidenced as a chipping or crushing of the marginal ridge with no evidence of dislodgement of the restoration. The second mode of failure was a proximal displacement or dislodgement of the box portion of the restoration which accounted for 21% of the failures. A significantly higher frequency of dislodgement failures was noted in Preparation 4 (chi-square (4 d.f.) = 37.937, p = 0.0001) (Fig. 3). No significant difference in failure mode was noted that could be related to amalgam used (chisquare (3 d.f.) = 0.808, p = 0.848). The regression analysis indicated a significant difference (p = 0.002) between the mean failure loads (averaged across preparations and amalgams) of those restorations that were dislodged and those that failed by marginal ridge fracture with mean values of 32.7 kg and 28.8 kg, respectively. The mean failure loads and standard errors of the metal die preparation tests are shown in Table 3.
Discussion As expected, the high-copper amalgams were significantly stronger than the low-copper amalgams when tested for compressive strength. Conversely, the low-copper amalgams were stronger than the high-copper amalgams when tested for diametral tensile strength. Since one might expect significant tensile stresses to be induced across the occlusal isthmus and in the proximal retention grooves of a restoration loaded toward the proximal, the hypothesis would be that the low-copper amalgams might have a mechanical advantage over the high-copper amalgams when isthmus or retention groove failures occurred. However the majority (79%) of the restorations in this
Table 3. Failure loads as a function of alloy and preparation design*
Velvalloy Spheraloy Dispersalloy Sybraloy Preparation Means
Preparation 1
Preparation 2
Preparation 3
Preparation 4
Preparation 5
Material Means
26.7_+2.6 27.2_+2.7 24.6_+i .4 20.7-+1.5 24.8_+1.2
33.8_+3.6 29.6+2.6 25.3_+2.8 23.3+2.7 28.0_+1.5
28.5_+3.0 27.5+2.4 26.0_+3.7 25.7+2.8 26.9_+1.5
39.0_+3.4 31.8_+2.5 33.7+_2.9 30.8-+3.5 33.8-+1.6
31.5-+2.8 37.7_+4.1 38.7+3.9 30.0-+3.6 34.5_+1.8
31.9-+1.4 30.8_ + 1.4 29.7_+1.5 26.1-+1.4
Failure loads expressed in kilograms _ standard error of the mean. 10"
147
148
Stur'devant et al.
The results indicating that box-only restorations tend to fail by displacement more often than restorations that have an occlusal dovetail seem to be in contradiction to the fact that the boxonly restorations accept higher loads prior to failure. One possible explanation could be that the occlusat morphology of the marginal ridges carved on the box-only restorations may have been slightly different than those carved on the box and dovetail restorations. Another possibility is that without the occlusal dovetail the box-only restorations are less firmly constrained within their preparations and are able to shift slightly during loading, thus are able to avoid early stress concentrations that might lead to a marginal ridge fracture. This hypothesis is compatible with the higher frequency of displacement of the restorations in the box-only preparations.
INCIDENCE OF DISPLACEMENT BY PREPARATION
PROXIMAL BOX & OCCLUSAL DOVETAIL & NO RETENTION LOCKS (PREPARATION 1)
i.S fi!iii i
ii]
PROXIMAL BOX & OCCLUSAL DOVETAIL & STANDARD RETENTION LOCKS (PREPARATION2) PROXIMAL BOX & OCCLUSAL DOVETAIL & FULL LENGTH RETENTION LOCKS (PREPARATION3) PROXIMAL BOX ONLY & STANDARD RETENTION LOCKS (PREPARATION4) PROXIMAL BOX ONLY & FULL LENGTH RETENTION LOCKS (PREPARATION5) 0
l
l
I
1
10
20
30
40
!
50
PERCENT DISPLACED
Fig. 3. Incidence of displacement of restorations during simulated occlusal loading.
study did not fail in the anticipated mode, but failed cohesively via marginal ridge fractures. These results are similar to those obtained by Crockett (9) when testing Class II restorations in metal dies. Averaged across preparations, there was no significant difference between the failure load values for Dispersalloy, Velvalloy, and Spheraloy. Sybraloy was found to give significantly lower failure load values than Spheraloy and Velvalloy (Table 4). Neither copper content nor particle shape of the alloys tested had a consistent effect on failure loads in tbe metal die testing. It must be noted that the relative order of the amalgams is the same both in the diametral tensile tests and in the metal die testing. Averaged across amalgams, the boxonly restorations (Preparations 4 and 5) required significantly higher loads for failure than restorations that included an occlusal dovetail (Preparations 1, 2, and 3) (Table 5). These relationships also applied when controlling for mode of failure. Within the boxonly preparations (Preparations 4 and 5), the length of the retention grooves had no significant effect on failure load. Within the 3 preparations that included occlusal dovetails (Preparations 1, 2, and 3), the presence, length, or absence of proximal retentive grooves did not have a significant effect on failure load. Even though there was no difference between the failure loads between the 2 box-only preparations, the difference in retentive groove length had an effect on mode of failure (Fig. 3). The boxonly preparation with shorter length retentive grooves (Preparation 4) had the
highest incidence of failures by displacement (47%). The box-only preparation with full-length retention grooves (Preparation 5) had half as many displacement failures as did Preparation 4 (23%). Preparations that included an occlusal dovetail and retention grooves (Preparations 2 and 3) had fewer displacement failures (7%) than the box-only preparations and had no difference in frequency of displacement which could be related to groove length. The complete absence of retention grooves in the preparation with an occlusal dovetail (Preparation 1) increased the frequency of failure by displacement (18%).
Conclusions
The physical property tests of the amalgams selected in this study indicate that high-copper amalgams have significantly higher compressive strengths than low-copper amalgams. In tension these same high-copper amalgams were weaker than the low-copper amalgams. Particle shape of the alloys did not have an effect on compressive or tensile strength. In the metal die tests, neither copper
Table 4. Differences between alloys in metal die tests * Spheraloy
Dispersalloy
Sybraloy
0.392 -
0.126 0.506 -
0.0006" 0.010" 0.055
Velvalloy Spheraloy Dispersalloy
* Comparisons between pairs of alloys (p-values) which were based on the multiple linear regression model. * Significant with p < 0.01. Table 5. Differences between preparations in metal die tests t Preparation 2
Preparation 3
Preparation 4
Preparation 5
Preparation
0.110
0.280
0.0001"
0.000l*
Preparation 2
_
0.602
0.003"
0.00l*
Preparation 3
_
-
0.0004"
0.0001"
Preparation 4
_
_
-
0.766
1
Comparisons between pairs of preparations (p-values) which were based on the multiple linear regression model. * Significant with p < 0.0l.
C o n s e r v a t i v e Class H a m a l g a m r e s t o r a t i o n s
content nor particle shape of the alloys had a discernible effect on the load required for failure of the restoration. O f the 2 physical properties tested prior to the metal die tests, tensile strength was m o r e predictive of load required for restoration failure than compressive strength. The metal die tests indicate that the box-only restorations require significantly higher loads for failure than restorations that include an occlusal dovetail. Length of retention grooves did not have a significant effect on failure loads of the box-only restorations, but it did have an effect on m o d e of failure, with more displacement failures occurring in preparations with shorter grooves. Within the 3 preparations that included occlusal dovetails the presence, length, or absence of proximal retentive grooves did not have a significant effect on failure load. The complete absence of retentive grooves did increase the n u m b e r of failures which occurred by displacement. I n v i v o failure mechanisms are very complex and difficult to predict in vitro. W h e t h e r the patterns of failure observed in this laboratory study would be observed in a carefully controlled clinical study remains to be seen. It may be that the forces required for the marginal ridge failures and the restoration displacements in the laboratory exceed the loads generated within the oral cavity. This study is intended to be a preliminary screening program for testing conservative preparation designs prior to clinical trials. If failures are seen later clinically, it will be possible to relate back to the physical properties testing and preparation screening to determine possible cause and effect relationships and the predictive ability of this laboratory test. The results from this study indicate that the box-only approach to Class II lesions is a viable, m o r e conservative m e t h o d to treat proximal caries when occlusal grooves are not faulty or when an existing occlusal restoration is not defective. The box-only approach also has applications in the repair of existing Class II amalgam restorations when only the proximal por-
tion is in need of replacement. These results indicate that the box-only restorations are equally as strong as restorations in conventional preparations, and should be tested clinically for their suitability in routine operative dentistry.
Acknowledgements - This study was sup-
ported by NIDR Grant No. 5-P50-DE02668-19. The authors wish to express appreciation to Dr. Gary Koch and his staff for their guidance and assistance in the statistical analysis of the results of this study, and to Dr. Henry Zaytoun for his assistance in the fabrication of the metal dies.
References 1. Black GV. Operative dentistry. Medico-Dental Publishing Company, 1890. 160-166. 2. Strickland WD, Wilder AD. Amalgam restorations for Class II cavity preparations. In: Sturdevant CM, ed. The art and science of operative dentistry. St. Louis: Mosby, 1985: 213-246. 3. Ward ML. Modern tests for the strength of amalgam. J A m Dent Assoc 1924: 11: 487-496. 4. Taylor NO. A survey of amalgam alloys. J A m Dent Assoc 1929: 16: 590-601. 5. Mahler DB, Terkla LG, Johnson LN. Evaluation of techniques for analyzing cavity design for amalgam restoration. J Dent Res 1961: 40(3): 497-503. 6. Mondelli J, Ishikiriama A, Navarro MF, Galon J, Coradazzi JL. Fracture strength of amalgam restorations in modern Class II preparations with proximal retentive grooves. J Prosthet Dent 1974: 32: 567-571. 7. Yates JL, Hembree JH, McKnight JP. A comparison of the effect of cavity design on the fracture strength of three dental alloy systems. J Dent Child 1976: 43(3): 187-191. 8. Amarim W, Navarro MF, Mondelli J, Samipo E. Influence of axiopulpal line angle and proximal retention on fracture strength of amalgam restorations. J Prosthet Dent 1978: 40: 169-173. 9. Crocket WD, Shepart FE, Moon PC, Creal A E The influence of proximal retention grooves on the retention and resistance of Class II preparations for amalgams. J A m Dent Assoc 1975: 91: 1053-1056.
149
10. Almquist TC, Cowan RD, Lambert RL. Conservative amalgam restorations. J Prosthet Dent 1973: 29: 524-528. 11. Terkla LG, Mahler DB. Clinical evaluation of interproximal retention grooves in Class II amalgam cavity design. J Prosthet Dent 1967: 17: 592-602. 12. Terkla LG, Mahler DB, Eysden JV. Analysis of amalgam cavity design. J Prosthet Dent 1973: 29: 204-209. 13. Leinfelder KF. Clinical evaluation of high-copper amalgams. J Gen Dent 1983: 31: 105-109. 14. Vrijhoef MMA, Vermeersch AG, Spanauf AJ. Diametral tensile strength of twenty-three hardened commercial amalgams. J Dent Res 1979: 6: 153-157. 15. Eames WB, MacNamara JF. Eight highcopper amalgam alloys and six conventional alloys compared. Oper Dent 1976: 1: 98-107. 16. Osborne JW, Gale EN, Chew CL, Rhodes BF, Phillips RW. Clinical performance and physical properties of twelve amalgam alloys. J Dent Res 1978: 57: 983-988. 17. Phillips RW. Dental amalgam: dimensional change, strength, and creep. In: Phillips RW, ed. Skinner's science of dental materials. Philadelphia: Saunders, 1982: 320-328. 18. Vrijhoef MMA, Vermeersch AG, Spanauf AJ. Is creep of dental amalgam a relevant property? Quintessence Int 1980: 11(5): 15-18. 19. American Dental Association Council on Dental Materials, Instruments and Devices. Revised Specification No. 1 for Alloy for Dental Amalgam. J A m Dent Assoc 1977: 95: 614-617. 20. American Dental Association Council on Dental Materials, Instruments and Devices. Addendum to American National Standards Institute/American Dental Association Specification No. 1 for Alloy for Dental Amalgam. J A m Dent Assoc 1980: 100: 246. 21. American Dental Association Council on Dental Materials, Instruments and Equipment. Guide to Dental Materials and Devices 7th Edition. A m Dent Assoc, 1974. 170-173. 22. Lehmann EL. Nonparametrics: statistical methods based on ranks. HoldenDay, 1975. 23. Neter J, Wasserman W, Kutner MH. Applied linear regression models. Homewood: Irwin, 1983. 24. SAS Institute, Inc. The GLM procedure. In: SAS users guide. SAS Institute 1985: 433-506.
J. R. Sturdevant, D. F. Taylor, R. H. Leonard, W. F. Straka, T. M. Roberson, A. D. Wilder Department of Operative Dentistry, University of North Carolina School of Dentistry, Chapel Hill, USA.
Sturdevant JR, Taylor DF, Leonard RH, Straka WF, Roberson TM, Wilder AD. Conservative preparation designs for Class II amalgam restorations. Dent Mater 1987: 3:144-148 Abstract - This study evaluates whether improvements in the physical properties of dental amalgam affect the use of Class II preparations without an occlusal dovetail. Four amalgam alloys were selected providing combinations of particle shape and copper content. The amalgams were characterized for their 24-h compressive and tensile strengths. High-copper amalgams were significantly stronger in compression, but weaker in tension than low-copper amalgams. Five Class II preparation designs were evaluated in cast metal replicas of prepared teeth. Controlling for the amalgam used, the proximal boxonly designs required significantly higher loads for restoration failure than did designs with occlusal dovetails. Of the two physical properties tested prior to the metal die tests, tensile strength was more predictive of load required for restoration failure than compressive strength. Neither copper content nor particle shape of the alloys had a discernible effect on load required for restoration failure. The results indicate that box-only restorations should be tested clinically for their suitability in routine operative dentistry.
The basic principles in the design of amalgam cavity preparations have been modified but not changed in essence over the last 90 years (1). Most of the modifications which have been adopted have served to reduce the extent of the preparation and, thus, increase the conservation of sound tooth structure (2). Two persistent design features in Class II preparations for amalgam have been the occlusal dovetail and proximal retention locks. In 1924 Ward recommended the use of both types of retention, while identifying flow or creep as a property closely associated with displacement (3). When a restoration including an occlusal dovetail is subjected to occlusal loads,'a tensile stress is generated in the isthmus portion of the restoration (4). Thus, the dovetail helps to prevent displacement of the proximal portion of Class II restorations. Proximal retention locks, normally in the form of grooves in the facial and lingual walls of the proximal box, serve the same purpose. The availability of high speed handpieces, carbide burs, and amalgams with high early strength combined in
the late 1950's to promote a renewed interest in cavity design. Since then many laboratory investigations have examined the effect of variations in different design features (5-9). Among other results these studies indicated that proximal retentive grooves are an important factor in retaining the proximal portion of a Class II restoration. One study showed that the retentive effect of the grooves was increased as the grooves were extended occlusally (6). Based in part upon these studies, it has been recommended that, when the occlusal surface is free of defects needing restoration, the dovetail portion of Class II restorations should be totally eliminated and retention afforded by the proximal grooves only (10). When this concept was tested in a laboratory study it was found that proximal boxonly preparations with retentive grooves (no occlusal dovetail) produced stronger restorations than preparations with an occlusal dovetail and no retention grooves in the proximal box. On the basis of these studies it would appear that one could proceed with confidence to a clinical study of box-
Key words: dental materials, dental amalgam, operative dentistry, dental restorations, permanent. Dr. John R. Sturdevant, Department of Operative Dentistry, University of North Carolina School of Dentistry, Chapel Hill, NC 27514, USA.
ReceivedAugust 11; accepted October 2, 1986.
only preparations. If it is true that proximal preparations with retentive grooves without occlusal dovetails are stronger than those with dovetails and no grooves (9), and that restorations with dovetails and no grooves give adequate clinical.performance when used with conventional filing alloys (11, 12), then a clinical evaluation of the conservative box-only preparation is both timely and well justified. Subsequent to the studies described above, another consideration has arisen which appears to make the question of broader significance. Most restorations at the present time appear to be made using high-copper alloys (13): These amalgams have an increased resistance to marginal deterioration in service and somewhat better polish retention. Numerous investigations of the mechanical properties of these amalgams have shown that in comparison to low-copper amalgams they exhibit higher early and final compressive strengths and a significantly reduced tendency toward creep (14-16). However, these high-copper amalgams occasionally show considerably reduced tensile strength in comparison to the
Conservative Class II amalgam restorations
low-copper amalgams (17). If it is concluded that creep is not a relevant property of dental amalgam in regard to proximal displacement (12, 18), and that on the basis of stress analysis the tensile strength must be of significance in regard to isthmus fracture (5), then there is a valid doubt as to whether the conclusions drawn above about the suitability of box-only preparations can be extended to apply to high-copper amalgams. In principle, it is desirable to employ the most conservative treatment which is compatible with preparations of clinical success. Where a tooth presents with proximal caries and the occlusal grooves are not faulty, a proximal preparation without an occlusal dovetail is clearly the most conservative. For a Class II preparation involving the occlusal as well as the proximal surfaces, an occlusal dovetail with proximal boxes but without retention grooves is somewhat more conservative than one with grooves. It is important to determine whether such preparations can safely be used with modern materials. In this study the materials selected permit a comparison between high-cooper and con-
ventional amalgam compositions and between filing and spherical alloy particle shapes. A t the same time the cavity preparation forms were chosen to permit an identification of the relative importance of the various retentive features. Material and methods
Four alloys were selected to provide combinations of particle shape and copper content. 1. Low-copper filing alloy: Velvalloy (F-~) 2. Low-copper spherical alloy: Spheraloy (F-2) 3. High-copper filing admix: Dispersalloy (F-3) F-1. Velvalloy, batch number 5648403X, SS White Division, Pennwalt Corporation, 900 1st Avenue, Philadelphia, PA 19102. F-2. Spheraloy, batch number 0530843013, Kerr Manufacturing Company A Division of Sybron Corporation, 28200 Wick Road, Romulus, Michigan, 48174. F-3. Dispersalloy, batch number 053184A-4C512, Johnson and Johnson Dental Products Division, 20 Lake Drive, East Windsor, N.J. 08520.
145
4. High-copper spherical alloy: Sybraloy (F-4) Amalgam characterization
Each of the amalgams was characterized for their 24-h compressive strength and tensile strength. The specimen design, specimen preparation, and test procedures followed the current A D A Specification I (19, 20), except for the tensile strength test which was determined by the diametral tensile strength test in A D A Specification I (1974) (21). Ten specimens were tested for each amalgam alloy. Mixing and preparation of the amalgams for test specimens followed the manufacturer's directions. The specimens were stored in air at 37~ until tested. Preparation testing
To avoid the inherent variability of extracted natural teeth, 5 different preparation designs were evaluated using metal dies (5). Three were Class II preparations with proximal boxes and occlusal dovetails, while 2 were Class II proximal boxes without occlusal dovetails (Fig. 1). 1. A Class II preparation with an occlusal dovetail and a proximal box but no retention grooves. 2. A Class II preparation with an occlusal dovetail and a proximal box in which proximal retention grooves are placed at the axiofacial and axiolingual line angles and extend to 1 mm from the occlusal surface. 3. A Class II preparation with an occlusal dovetail and a proximal box in which proximal retention grooves are similarly placed but extend through the occlusal surface. 4. A Class H preparation with a proximal box only (no occlusal dovetail) in which the proximal retention grooves extend to 1 mm from the occlusal surface. 5. A Class II preparation with a proximal box only (no occlusal dovetail) with proximal retention grooves that extend through the occlusal surface. All in vitro preparations were replicas of a mesioocclusal preparation cut into an ivorine replica of a maxillary second
Fig. 1. The 5 Class II preparation designs tested in this study. I0
Dental Materials 3:3, 1987
F-4. Sybraloy, batch number 053084-1081, Kerr Manufacturing Company A Division of Sybron Corporation, 28200 Wick Road, Romulus, Michigan, 48174.
146
Sturdevant et al.
premolar (F-5). All preparations were identical in size of the proximal box portion, and varied only in the addition or deletion of occlusal dovetails or proximal retention grooves. All preparations were prepared with a No. 245 carbide bur (F-6) such that the proximal box dimensions were as follows: 4.0 mm wide faciolingually at the level of the gingival floor, tapering to 3.0 mm faciolingually at the level of the mesial marginal ridge. The width of the gingival floor mesiodistally was 1.0 mm. The plane of the axial wall was parallel to the long axis of the tooth. The gingival floor was flat and perpendicular to the long axis of the tooth. The height of the proximal box occlusogingivally was 4.0 mm. To provide consistency in the size and shape of the proximal box portion of the cavity preparations, a polyvinylsiloxane impression (F-7) was made of the basic "box" preparation in the premolar described above and then 5 acrylic resin replicas (F-8) were made and modifications added to generate the 5 preparation forms previously described. In those preparations with an occlusal dovetail (preparations 1, 2, and 3), the dovetail was cut 1.5 mm deep pulpally with a No. 245 carbide bur. The isthmus of the dovetail was 1.0 mm wide faciolingually. In those preparations in which the retention grooves extended to the occlusal surface (Preparations 3 and 5), the grooves were nearly cylindrical in shape, located at the axiofacial and axiolingual line angles, and prepared with a No. 169L carbide bur (F-9) to a uniform depth of 0.5 mm in the facial or lingual wall of the proximal box. The retentive grooves prepared in Preparations 2 and 4 were tapering grooves extending from the axiofaciogingival or the axiolinguogingival point angles to a point approximately 1.5 mm below the occlusal surface. They were prepared with a 169L carbide bur so positioned that the tip produced a 0.5 mm deep groove at the point angle, which tapeF-5. Columbia Dentoform, Columbia Dentoform, 49th East 21st Street, New York, NY 10010. F-6. No. 245 bur, SS White, South Street, Holmdel, NJ 07733. F-7. Reprosil, LD Caulk Company- A division of Dentsply International, Inc., Milford, Delaware 19963. F-8. Duralay Resin, Reliance Dental Manufacturing Company, Worth, IL 60482. F-9. No. 169L bur, SS White, South Street, Holmdel, NJ 07733.
red to zero depth at a distance 1.5 mm gingivally from the occlusal surface. Cast replicas of the acrylic teeth were made in Ticonium base metal casting alloy (F-10). The internal surfaces of the cast metal teeth were polished to remove irregularities which might provide non-designed retentive features (F-11). Specimens for in vitro preparation testing were prepared by hand condensing amalgam into the dies with the aid of Tofflemire retainers and bands (F12). The manufacturer's recommendations for proportioning and trituration of the amalgam were used. Occlusal contours were carved to reproduce as nearly as possible the original occlusal form of the tooth. After condensation the matrix was removed and the proximal portion of the restoration was checked for voids and marginal defects. Incomplete restorations were discarded and replaced. The filled dies were stored in air at 37~ for 24 h prior to testing. Testing was performed on an Instron Universal Testing Machine (F-13). The F-10. Ticonium, Ticonium Company, P.O. Box 350, Albany, NY 12201. F-11. Ti-Lectra Polisher, Ticonium Company, P.O. Box 350, Albany, NY 12201. F-12. Toffiemire retainer, Teledyne Getz, Elk Grove Village, IL 60007. F-13. Instron Universal Testing Machine, Instron Corporation, 2500 Washington Street, Canton, MA 02021.
dies and restorations were positioned to provide a l0 ~ angle of inclination of the long axis of the tooth toward the proximal restored surface in order to generate a proximally directed force component. The load was applied to the marginal ridge of the restoration by a cylindrical carbide tip with a 1.5 mm radius of curvature. The long axis of the cylindrical tip was positioned perpendicular to the direction of loading and located relative to the restoration to be tested so that it extended across the restored marginal ridge in a mesiodistal direction (Fig. 2). The headspeed of the testing machine was adjusted to 0.1 ram/rain to produce an axial strain rate in the box portion of the restoration equivalent to the strain rate specified for compressive strength testing in the A D A specition (19). Loading was continued until the specimen failed by fracture or displacement. The load required, as well as the mechanism of failure was recorded. A total of 15 restorations were tested for each combination of material and preparation design (300 total). The results of the physical properties testing and the preparation testing were described with means and standard errors for each material. Pairwise comparisons between materials were made with the Wilcoxon rank-sum test. The effects of copper and particle
Fig. 2. The cast metal replicas of teeth were inclined 10~ toward the restored proximal sur-
face. The restorations were loaded by a 3.0 mm diameter carbide rod oriented perpendicular to the marginal ridge.
Conservative Class H amalgam restorations Table 1. 24-h physical properties testing Tensile strength Velvalloy Spheraloy Sybraloy Dispersalloy
Significant groupings*
55.7_+1.4 MPa 52.8+1.7 MPa 46.3_+2.5 MPa 46.9+1.8 MPa
] ] ] ]
Compressive strength
Significant groupings*
367.6_+ 1.2 MPa 335.6_+ 2.0 MPa 492.2_+11.4 MPa 448.1_+ 3.2 MPa
] ] ] ]
* Pairwise comparisons between each pair of materials were based on the Wilcoxon ranksum test. Value(s) within an individual bracket showed no significant differences at p = 0.05. Values within an individual bracket differ significantly from those in other brackets at p = 0.05. Table 2. Significance of copper content and particle shapet Tension High copper vs. low copper Filing vs. spherical particles
p chi-square p chi-square
Compression
< 0.01" (1 d.f.) = 2 8 . 5 7 > 0.10 (1 d.f.) = 0 . 1 0
p chi-square p chi-square
< 0.01" (1 d.f.) = 14.69 > 0.10 (i d.f.) = 1.80
* Chi-square statistics with 1 d.f. from stratified rank tests and their p-value status. * Significant with p < 0.01.
shape on physical properties were analyzed with the van Elteran stratified rank tests (22). The effects of amalgam and preparation on failure load were evaluated through a multiple linear regression model. This model simultaneously accounted for preparation and amalgam effects through a set of indicator variables. Comparisons for amalgams, preparations, copper content, particle shape, and mode of failure were undertaken through t-statistics for the corresponding estimated effects from the multiple linear regression model (23). Computations were made with the GLM procedure in SAS (24). In addition, chi-square tests were used for further evaluations of mode of failure.
Results The results of the 24-h physical properties tests are shown in Table 1. The mean compressive strength values ranged from a low of 335.6 MPa for
Spheraloy to a high of 492.2 MPa for Sybraloy. The Wilcoxon rank-sum tests indicated significant differences between each of the 4 amalgams in compressive strengths at p - 0 . 0 5 . In tension, the 2 low-copper amalgams, Velvalloy and Spheraloy, had mean values of 55.7 MPa and 52.8 MPa respectively. The 2 high-copper amalgams had lower mean tensile strengths of 46.9 MPa for Dispersalloy and 46.3 MPa for Sybraloy. The Wilcoxon ranksum tests indicated the tensile strengths of both the low-copper amalgams were significantly greater than those of the high-copper amalgams. Evaluations were made of the effects of copper content and particle shape on the strength of amalgam, both in tension and compression, by means of stratified rank tests (Table 2). These tests indicate that high-copper amalgams are significantly stronger than low-copper amalgams in compression, but are significantly weaker than lowcopper amalgams in tension. Particle shape of the alloys had no significant
effect on tensile or compressive strengths. Two modes of failure were observed in the metal die preparation tests. The predominant failure (79%) was a cohesive failure of the amalgam evidenced as a chipping or crushing of the marginal ridge with no evidence of dislodgement of the restoration. The second mode of failure was a proximal displacement or dislodgement of the box portion of the restoration which accounted for 21% of the failures. A significantly higher frequency of dislodgement failures was noted in Preparation 4 (chi-square (4 d.f.) = 37.937, p = 0.0001) (Fig. 3). No significant difference in failure mode was noted that could be related to amalgam used (chisquare (3 d.f.) = 0.808, p = 0.848). The regression analysis indicated a significant difference (p = 0.002) between the mean failure loads (averaged across preparations and amalgams) of those restorations that were dislodged and those that failed by marginal ridge fracture with mean values of 32.7 kg and 28.8 kg, respectively. The mean failure loads and standard errors of the metal die preparation tests are shown in Table 3.
Discussion As expected, the high-copper amalgams were significantly stronger than the low-copper amalgams when tested for compressive strength. Conversely, the low-copper amalgams were stronger than the high-copper amalgams when tested for diametral tensile strength. Since one might expect significant tensile stresses to be induced across the occlusal isthmus and in the proximal retention grooves of a restoration loaded toward the proximal, the hypothesis would be that the low-copper amalgams might have a mechanical advantage over the high-copper amalgams when isthmus or retention groove failures occurred. However the majority (79%) of the restorations in this
Table 3. Failure loads as a function of alloy and preparation design*
Velvalloy Spheraloy Dispersalloy Sybraloy Preparation Means
Preparation 1
Preparation 2
Preparation 3
Preparation 4
Preparation 5
Material Means
26.7_+2.6 27.2_+2.7 24.6_+i .4 20.7-+1.5 24.8_+1.2
33.8_+3.6 29.6+2.6 25.3_+2.8 23.3+2.7 28.0_+1.5
28.5_+3.0 27.5+2.4 26.0_+3.7 25.7+2.8 26.9_+1.5
39.0_+3.4 31.8_+2.5 33.7+_2.9 30.8-+3.5 33.8-+1.6
31.5-+2.8 37.7_+4.1 38.7+3.9 30.0-+3.6 34.5_+1.8
31.9-+1.4 30.8_ + 1.4 29.7_+1.5 26.1-+1.4
Failure loads expressed in kilograms _ standard error of the mean. 10"
147
148
Stur'devant et al.
The results indicating that box-only restorations tend to fail by displacement more often than restorations that have an occlusal dovetail seem to be in contradiction to the fact that the boxonly restorations accept higher loads prior to failure. One possible explanation could be that the occlusat morphology of the marginal ridges carved on the box-only restorations may have been slightly different than those carved on the box and dovetail restorations. Another possibility is that without the occlusal dovetail the box-only restorations are less firmly constrained within their preparations and are able to shift slightly during loading, thus are able to avoid early stress concentrations that might lead to a marginal ridge fracture. This hypothesis is compatible with the higher frequency of displacement of the restorations in the box-only preparations.
INCIDENCE OF DISPLACEMENT BY PREPARATION
PROXIMAL BOX & OCCLUSAL DOVETAIL & NO RETENTION LOCKS (PREPARATION 1)
i.S fi!iii i
ii]
PROXIMAL BOX & OCCLUSAL DOVETAIL & STANDARD RETENTION LOCKS (PREPARATION2) PROXIMAL BOX & OCCLUSAL DOVETAIL & FULL LENGTH RETENTION LOCKS (PREPARATION3) PROXIMAL BOX ONLY & STANDARD RETENTION LOCKS (PREPARATION4) PROXIMAL BOX ONLY & FULL LENGTH RETENTION LOCKS (PREPARATION5) 0
l
l
I
1
10
20
30
40
!
50
PERCENT DISPLACED
Fig. 3. Incidence of displacement of restorations during simulated occlusal loading.
study did not fail in the anticipated mode, but failed cohesively via marginal ridge fractures. These results are similar to those obtained by Crockett (9) when testing Class II restorations in metal dies. Averaged across preparations, there was no significant difference between the failure load values for Dispersalloy, Velvalloy, and Spheraloy. Sybraloy was found to give significantly lower failure load values than Spheraloy and Velvalloy (Table 4). Neither copper content nor particle shape of the alloys tested had a consistent effect on failure loads in tbe metal die testing. It must be noted that the relative order of the amalgams is the same both in the diametral tensile tests and in the metal die testing. Averaged across amalgams, the boxonly restorations (Preparations 4 and 5) required significantly higher loads for failure than restorations that included an occlusal dovetail (Preparations 1, 2, and 3) (Table 5). These relationships also applied when controlling for mode of failure. Within the boxonly preparations (Preparations 4 and 5), the length of the retention grooves had no significant effect on failure load. Within the 3 preparations that included occlusal dovetails (Preparations 1, 2, and 3), the presence, length, or absence of proximal retentive grooves did not have a significant effect on failure load. Even though there was no difference between the failure loads between the 2 box-only preparations, the difference in retentive groove length had an effect on mode of failure (Fig. 3). The boxonly preparation with shorter length retentive grooves (Preparation 4) had the
highest incidence of failures by displacement (47%). The box-only preparation with full-length retention grooves (Preparation 5) had half as many displacement failures as did Preparation 4 (23%). Preparations that included an occlusal dovetail and retention grooves (Preparations 2 and 3) had fewer displacement failures (7%) than the box-only preparations and had no difference in frequency of displacement which could be related to groove length. The complete absence of retention grooves in the preparation with an occlusal dovetail (Preparation 1) increased the frequency of failure by displacement (18%).
Conclusions
The physical property tests of the amalgams selected in this study indicate that high-copper amalgams have significantly higher compressive strengths than low-copper amalgams. In tension these same high-copper amalgams were weaker than the low-copper amalgams. Particle shape of the alloys did not have an effect on compressive or tensile strength. In the metal die tests, neither copper
Table 4. Differences between alloys in metal die tests * Spheraloy
Dispersalloy
Sybraloy
0.392 -
0.126 0.506 -
0.0006" 0.010" 0.055
Velvalloy Spheraloy Dispersalloy
* Comparisons between pairs of alloys (p-values) which were based on the multiple linear regression model. * Significant with p < 0.01. Table 5. Differences between preparations in metal die tests t Preparation 2
Preparation 3
Preparation 4
Preparation 5
Preparation
0.110
0.280
0.0001"
0.000l*
Preparation 2
_
0.602
0.003"
0.00l*
Preparation 3
_
-
0.0004"
0.0001"
Preparation 4
_
_
-
0.766
1
Comparisons between pairs of preparations (p-values) which were based on the multiple linear regression model. * Significant with p < 0.0l.
C o n s e r v a t i v e Class H a m a l g a m r e s t o r a t i o n s
content nor particle shape of the alloys had a discernible effect on the load required for failure of the restoration. O f the 2 physical properties tested prior to the metal die tests, tensile strength was m o r e predictive of load required for restoration failure than compressive strength. The metal die tests indicate that the box-only restorations require significantly higher loads for failure than restorations that include an occlusal dovetail. Length of retention grooves did not have a significant effect on failure loads of the box-only restorations, but it did have an effect on m o d e of failure, with more displacement failures occurring in preparations with shorter grooves. Within the 3 preparations that included occlusal dovetails the presence, length, or absence of proximal retentive grooves did not have a significant effect on failure load. The complete absence of retentive grooves did increase the n u m b e r of failures which occurred by displacement. I n v i v o failure mechanisms are very complex and difficult to predict in vitro. W h e t h e r the patterns of failure observed in this laboratory study would be observed in a carefully controlled clinical study remains to be seen. It may be that the forces required for the marginal ridge failures and the restoration displacements in the laboratory exceed the loads generated within the oral cavity. This study is intended to be a preliminary screening program for testing conservative preparation designs prior to clinical trials. If failures are seen later clinically, it will be possible to relate back to the physical properties testing and preparation screening to determine possible cause and effect relationships and the predictive ability of this laboratory test. The results from this study indicate that the box-only approach to Class II lesions is a viable, m o r e conservative m e t h o d to treat proximal caries when occlusal grooves are not faulty or when an existing occlusal restoration is not defective. The box-only approach also has applications in the repair of existing Class II amalgam restorations when only the proximal por-
tion is in need of replacement. These results indicate that the box-only restorations are equally as strong as restorations in conventional preparations, and should be tested clinically for their suitability in routine operative dentistry.
Acknowledgements - This study was sup-
ported by NIDR Grant No. 5-P50-DE02668-19. The authors wish to express appreciation to Dr. Gary Koch and his staff for their guidance and assistance in the statistical analysis of the results of this study, and to Dr. Henry Zaytoun for his assistance in the fabrication of the metal dies.
References 1. Black GV. Operative dentistry. Medico-Dental Publishing Company, 1890. 160-166. 2. Strickland WD, Wilder AD. Amalgam restorations for Class II cavity preparations. In: Sturdevant CM, ed. The art and science of operative dentistry. St. Louis: Mosby, 1985: 213-246. 3. Ward ML. Modern tests for the strength of amalgam. J A m Dent Assoc 1924: 11: 487-496. 4. Taylor NO. A survey of amalgam alloys. J A m Dent Assoc 1929: 16: 590-601. 5. Mahler DB, Terkla LG, Johnson LN. Evaluation of techniques for analyzing cavity design for amalgam restoration. J Dent Res 1961: 40(3): 497-503. 6. Mondelli J, Ishikiriama A, Navarro MF, Galon J, Coradazzi JL. Fracture strength of amalgam restorations in modern Class II preparations with proximal retentive grooves. J Prosthet Dent 1974: 32: 567-571. 7. Yates JL, Hembree JH, McKnight JP. A comparison of the effect of cavity design on the fracture strength of three dental alloy systems. J Dent Child 1976: 43(3): 187-191. 8. Amarim W, Navarro MF, Mondelli J, Samipo E. Influence of axiopulpal line angle and proximal retention on fracture strength of amalgam restorations. J Prosthet Dent 1978: 40: 169-173. 9. Crocket WD, Shepart FE, Moon PC, Creal A E The influence of proximal retention grooves on the retention and resistance of Class II preparations for amalgams. J A m Dent Assoc 1975: 91: 1053-1056.
149
10. Almquist TC, Cowan RD, Lambert RL. Conservative amalgam restorations. J Prosthet Dent 1973: 29: 524-528. 11. Terkla LG, Mahler DB. Clinical evaluation of interproximal retention grooves in Class II amalgam cavity design. J Prosthet Dent 1967: 17: 592-602. 12. Terkla LG, Mahler DB, Eysden JV. Analysis of amalgam cavity design. J Prosthet Dent 1973: 29: 204-209. 13. Leinfelder KF. Clinical evaluation of high-copper amalgams. J Gen Dent 1983: 31: 105-109. 14. Vrijhoef MMA, Vermeersch AG, Spanauf AJ. Diametral tensile strength of twenty-three hardened commercial amalgams. J Dent Res 1979: 6: 153-157. 15. Eames WB, MacNamara JF. Eight highcopper amalgam alloys and six conventional alloys compared. Oper Dent 1976: 1: 98-107. 16. Osborne JW, Gale EN, Chew CL, Rhodes BF, Phillips RW. Clinical performance and physical properties of twelve amalgam alloys. J Dent Res 1978: 57: 983-988. 17. Phillips RW. Dental amalgam: dimensional change, strength, and creep. In: Phillips RW, ed. Skinner's science of dental materials. Philadelphia: Saunders, 1982: 320-328. 18. Vrijhoef MMA, Vermeersch AG, Spanauf AJ. Is creep of dental amalgam a relevant property? Quintessence Int 1980: 11(5): 15-18. 19. American Dental Association Council on Dental Materials, Instruments and Devices. Revised Specification No. 1 for Alloy for Dental Amalgam. J A m Dent Assoc 1977: 95: 614-617. 20. American Dental Association Council on Dental Materials, Instruments and Devices. Addendum to American National Standards Institute/American Dental Association Specification No. 1 for Alloy for Dental Amalgam. J A m Dent Assoc 1980: 100: 246. 21. American Dental Association Council on Dental Materials, Instruments and Equipment. Guide to Dental Materials and Devices 7th Edition. A m Dent Assoc, 1974. 170-173. 22. Lehmann EL. Nonparametrics: statistical methods based on ranks. HoldenDay, 1975. 23. Neter J, Wasserman W, Kutner MH. Applied linear regression models. Homewood: Irwin, 1983. 24. SAS Institute, Inc. The GLM procedure. In: SAS users guide. SAS Institute 1985: 433-506.