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The influence of proximal retention grooves on the retention and resistance of Class II preparations for amalgams
The addition of proximal retention grooves to various designs of the Class II cavity preparation considerably improves the retention and resistance characteristics of amalgam restorations. The incorporation of proximal grooves should reduce the necessity fo r gross outline form as a means of gaining adequate retention and resistance in the Class I I preparation.
The influence of proximal retention grooves on the retention and resistance of Class II preparations for amalgams W. D. Crockett, DDS F. E. Shepard, ODS P. C. Moon, PhD A. F. Creal, BS, R ich m o n d, Va
Thinking has changed in recent years about the characteristics of intracoronal preparations to receive amalgam restorations, especially con cerning the Class I I type. Specifically, the trend is toward a more conservative approach than that advocated by Black1 and more recently by Simon.2 Three major considerations are involved in Class I I cavity preparations for amalgam restor ations: the achievement of adequate strength in the tooth and restoration, the placement of a res toration that affords access for cleansing pur poses, and the assurance of adequate retention and resistance form. Baum3 believes that extension onto the cusp slope to the degree advocated by Black weakens the amalgam restoration by providing acute angles at the margins and by subjecting the res toration to unnecessary occlusal stresses. Con versely, where the carious lesion allows, occlu sal restorations that are limited to 1 to l 1/^ mm in buccolingual width (except at the junction with the proximal box) will permit strong cavosurface angles and simultaneously avoid unneces sary contact with opposing cusps.
Vale4 researched the effects of the bucco lingual width of the isthmus of the cavity prep aration for amalgam restorations and found that the strength of the remaining cusps was signifi cantly influenced by the width of the preparation in this area. He found that a preparation involv ing a fourth of the intercuspal dimension did not appreciably affect the strength of the tooth struc ture, but that a preparation involving a third of the intercuspal width did weaken the tooth con siderably and this frequently resulted in cusp fracture. Apparently, Black’s advocation of broad oc clusal outline, including a wide isthmus and dovetail lock, implied that the retention and re sistance of the occlusal and proximal portions of the Class I I preparation were interdependent. Simon,2 although indicating that the proximal portion of the two-surface cavity should be in dependent of the occlusal dovetail for retention, also said that the dovetail lock was of paramount importance in all Class I I restorations. A more recent concept relating to conserva tive Class I I cavity preparations involves elim ination of the occlusal preparation if no carious JADA, Vol. 91, N ovem ber 1975 ■ 1053
a
b
c
d
Fig 1 ■ Diagram of cavity designs: a, proximal box only, no proximal retention grooves; b, proximal box only, with proximal retention grooves; c, proximal box with dovetail lock and no proximal retention grooves; and d, proximal box with dovetail lock and proximal retention grooves.
lesion exists in this area.5 If the pits and fissures are only slightly defective, the width of the oc clusal portion should be no wider than the cut made by the no. 330 or no. 56 burs. The presence or absence of the dovetail lock in the occlusal outline is determined solely by the extent o f the carious process. The blending o f the occlusal portion of the preparation with the proximal por tion is achieved by a gradual and smooth flow connecting the buccal and lingual flares in such a manner as to create a 90° cavosurface angle. T o achieve the conservative concept in the Class II preparation, the clinician must rely on separate retention and resistance form for both the occlusal and proximal portions o f the prepar ation. Simon,2 Gilmore,6 Almquist and co-work ers,5 and Baum3 believe that this is a basic prin ciple to be achieved by the use of proximal reten tion grooves in the buccoaxial and linguoaxial line angles. The retention o f the occlusal por tion o f the restoration is determined by occlusally converging buccal and lingual walls. The review o f the literature shows that internal retention is thought to be an important concept in conservative cavity design. If no occlusal car ies exists, the ideal Class II preparation for an amalgam restoration, which would maintain the integrity of the tooth most completely, is a small proximal box with no occlusal extension. T o evaluate the retention and resistance of the “ ideal” Class II cavity preparation design, the vertical and horizontal forces required to frac ture or displace the restorations were measured for four cavity design conditions (Fig 1): prox imal box only, no retention grooves; proximal box only, with retention grooves; proximal box and dovetail lock, no retention grooves; and 1054 ■ JADA, Vol. 91, Novem ber 1975
proximal box and dovetail lock, with retention grooves.
M a te r ia ls a n d m e th o d s
Five preparations o f each design were simulated in stainless steel blocks by machine milling. They were then refined to the approximate measure ments o f a Class II preparation for amalgam res toration in a molar by using a no. 56 fissure bur in the high-speed handpiece. The retention grooves were placed in the buccoaxial and linguoaxial angles with the use of a no. quarter-round bur in a high-speed handpiece. The grooves extended from the gingival floor or wall to the level o f the axiopulpal angle. Their depth was equal to the diameter o f the quarter-round bur. The final depth o f the occlusal portion was 2 mm, and the axial wall depth (gingival floor width) was 1Vi mm. The gingival wall was placed at a level o f 5 mm from the top o f the steel block. A hole was drilled through the steel blocks by entering into the side opposite the preparations and passing perpendicularly through the center of the axial wall o f the proximal preparations. The “ punch-out” channel was 0.081 inches in diameter, accommodating without friction a steel punch 0.080 inches in diameter (Fig 2 a and b). Before the amalgam was condensed into the preparation, the steel punch was inserted into the channel and locked firmly in place, flush with the axial wall, to provide an intact axial wall sur face against which to condense the amalgam. A standardized ivorine matrix* was prepared in such a way so that the preparations were slight-
Tests using both vertical and horizontal forces were conducted 24 hours after condensation. The occlusal force was applied with a flat circu lar punch, 0.080 inches in diameter (Fig 2 b). It was centered over the proximal box portion of the various restorations. The horizontal force was exerted against the inside surface of the axial wall portion of the restoration using the punch previously described (Fig 2 b). The forces required to fracture or displace the amalgam res torations were exerted and measured with an Instron Universal Testing Machine§ at a cross head speed of 0.02 inches per minute. The re sults are shown in the Table.
p i * W ¿/A
<4D is c u s s io n
b Fig 2 ■ Diagram illustrating test methods: a, proximal view of Class II preparation with dovetail lock, proximal retention grooves, and punch-out channel; b, application of vertical and horizontal forces.
ly overfilled and the restorations required slight carving to achieve margins flush with the cavity preparation. The occlusal portions were carved flat (flush with the flat surface of the steel blocks). Each preparation was filled with amalgam, tested with vertical force, repacked, and tested again, resulting in ten samples for each design. The same procedure was repeated for testing the horizontal forces to fracture or displace the restorations of each test design. A standardized technique was used during the manipulation of the amalgam. Two pellets of amalgamt were mixed in a 1:1 ratio with mercury and triturated for 20 seconds in a Wig-L-Bug amalgam ator.i This mixture was packed into the stainless dies within W 2 minutes after trituration. The occlusal surface was carved flat 5Vi minutes after tritur ation, and the matrix was removed SV2 minutes after trituration.
The forces required to fracture or displace the restorations are listed in the Table. For the preparations without retention grooves and without dovetail locks, the restorations were displaced horizontally with no fracturing. The low force measured for this preparation repre sents that required to break the fine mechanical locking of the surfaces and static friction. All other values represent the forces required to fracture the various restorations. When horizon tal force was applied to the restorations retained by a dovetail lock and no proximal grooves, the amalgam fractured in the occlusal isthmus area. When the same directional force was applied to the restorations retained by both a dovetail lock and proximal retention grooves, fracturing oc curred in the occlusal isthmus area and in the retentive groove areas, leaving amalgam in both the dovetail lock and grooves. The analysis of variance F-test7 was applied to the various samples for both the vertical and the horizontal force values listed in the Table, to determine if a statistically significant differ ence for the forces existed. The analysis of the variance test indicated no significant difference for the vertical forces; therefore, if a difference did exist, the experimental procedure was not
Table ■ Fracture force* (in pounds).
Cavity design Horizontal force Vertical force (a) Proximal box only, 347±37 no retention grooves 4.1 ±1.9 (b) Proximal box only, with retention grooves 39.9±5.5 377±30 (c) Proximal box with dovetail lock, no retention grooves 15.7±3.6 336±19 (d) Proximal box with dovetail lock, with retention grooves 50.2±6.2 356±42 'Average force and standard deviation resulting from testing of ten restorations of each cavity design. C ro ckett— others: PR O XIM A L RETEN TIO N G R O O VES ■ 1055
sensitive enough to demonstrate it. However, the analysis of variance did demonstrate a statis tical difference at the 0.01 level of confidence in the horizontal forces listed in the Table when applied to that group. Student’s t test7 was then applied to individ ual pairs of the horizontal force group, and each pair was found to exhibit a statistically sig nificant difference at the 0.01 level of confidence. For this reason, we concluded that the horizon tal force is important in evaluating the various cavity designs.
C o n c lu s io n s
Terkla, Mahler, and Van Eysden8 have demon strated clinically that a Class II amalgam restor ation with a dovetail lock and no retention grooves serves as satisfactorily as does a restor ation containing a dovetail lock and retention grooves after evaluation at one, two, and three years, if the materials are manipulated correctly. In this study any variation of the cavity design containing proximal retention grooves yielded test values for the horizontal force considerably above those measured for any other design with out proximal retention grooves. The vertical force values obtained in testing all designs did not differ significantly. The test results suggest that the proximal box preparation with retention
^
grooves will allow the clinician to place more emphasis on conserving vitally important tooth structure in the occlusal portion of Class II cav ity preparations for amalgam restorations.
Dr. Crockett is professor of restorative dentistry and director of the division of operative dentistry, Virginia Commonwealth University, Medical College of Virginia School of Dentistry, Rich mond, 23298. Dr. Shepard is associate professor of restorative dentistry; Dr. Moon is assistant professor of restorative den tistry, division of dental materials; and Mr. Creal is a third-year dental student, Virginia Commonwealth University. Address requests for reprints to Dr. Crockett. ‘ Columbia Dentoform Corp., New York, 10010. tVelvalloy, S. S. White Co., Philadelphia, 19102. tWig-L-Bug, Crescent Dental Mfg., Corp., Lyons, III 60534. Slnstron Universal Testing Machine, Instron Corp., Canton, Mass 02021. 1. Black, G.V. Operative dentistry, ed 8, revised by Blackwell, R.E. Woodstock, III, Medico-Dental Publishing Co., 1947, vol 2. 2. Simon, W.J., ed. Clinical operative dentistry. Philadelphia, W. B. Saunders Co., 1956. 3. Lambert, R.L. Amalgam restorations. In advanced restor ative dentistry, Baum, L., ed. Philadelphia, W. B. Saunders Co., 1973. 4. Vale, W.A. Cavity preparation. Irish Dent Rev 2:33 MarchApril 1956. 5. Almquist, T.C.; Cowan, R.D.; and Lambert, R.L. Conserva tive amalgam restorations. J Prosthet Dent 29:524 May 1973. 6. Gilmore, H.W. Textbook of operative dentistry. St. Louis, C. V. Mosby Co., 1967. 7. Fisher, F.E. Fundamental statistical concepts. San Fran cisco, Canfield Press, 1973. 8. Terkla, L.G.; Mahler, D.B.; Van Eysden, J. Analysis of amal gam cavity design. J Prosthet Dent 29:204 Feb 1973.
Foley’s Footnotes------------------In Mary B. Clafton's Brampton Sketches (1890), there is an unusual note of a child’s dental experience in a Massachusetts town in 1810, when the physician included a turnkey in his instrument bag. Sometimes a little girl was told if she would be very good and do her stent [stint] she should go to her grandmother’s grave on a pleasant afternoon, and sometimes as a reward of merit she was told she might go to the "good doctor” and have a tooth pulled. Dentists were quite unknown in those days, and the instruments for extracting teeth were constructed somewhat like a plough share, but the good doctor was so kindly in his ways that the little children never feared to go to him, and the plenty which he always had ready was a panacea for the pain of the tooth-pulling, or the bitter draught which he administered from his saddlebags. Gardner P. H. Foley
1056
■ JADA, Vol. 91, Novem ber 1975
The influence of proximal retention grooves on the retention and resistance of Class II preparations for amalgams W. D. Crockett, DDS F. E. Shepard, ODS P. C. Moon, PhD A. F. Creal, BS, R ich m o n d, Va
Thinking has changed in recent years about the characteristics of intracoronal preparations to receive amalgam restorations, especially con cerning the Class I I type. Specifically, the trend is toward a more conservative approach than that advocated by Black1 and more recently by Simon.2 Three major considerations are involved in Class I I cavity preparations for amalgam restor ations: the achievement of adequate strength in the tooth and restoration, the placement of a res toration that affords access for cleansing pur poses, and the assurance of adequate retention and resistance form. Baum3 believes that extension onto the cusp slope to the degree advocated by Black weakens the amalgam restoration by providing acute angles at the margins and by subjecting the res toration to unnecessary occlusal stresses. Con versely, where the carious lesion allows, occlu sal restorations that are limited to 1 to l 1/^ mm in buccolingual width (except at the junction with the proximal box) will permit strong cavosurface angles and simultaneously avoid unneces sary contact with opposing cusps.
Vale4 researched the effects of the bucco lingual width of the isthmus of the cavity prep aration for amalgam restorations and found that the strength of the remaining cusps was signifi cantly influenced by the width of the preparation in this area. He found that a preparation involv ing a fourth of the intercuspal dimension did not appreciably affect the strength of the tooth struc ture, but that a preparation involving a third of the intercuspal width did weaken the tooth con siderably and this frequently resulted in cusp fracture. Apparently, Black’s advocation of broad oc clusal outline, including a wide isthmus and dovetail lock, implied that the retention and re sistance of the occlusal and proximal portions of the Class I I preparation were interdependent. Simon,2 although indicating that the proximal portion of the two-surface cavity should be in dependent of the occlusal dovetail for retention, also said that the dovetail lock was of paramount importance in all Class I I restorations. A more recent concept relating to conserva tive Class I I cavity preparations involves elim ination of the occlusal preparation if no carious JADA, Vol. 91, N ovem ber 1975 ■ 1053
a
b
c
d
Fig 1 ■ Diagram of cavity designs: a, proximal box only, no proximal retention grooves; b, proximal box only, with proximal retention grooves; c, proximal box with dovetail lock and no proximal retention grooves; and d, proximal box with dovetail lock and proximal retention grooves.
lesion exists in this area.5 If the pits and fissures are only slightly defective, the width of the oc clusal portion should be no wider than the cut made by the no. 330 or no. 56 burs. The presence or absence of the dovetail lock in the occlusal outline is determined solely by the extent o f the carious process. The blending o f the occlusal portion of the preparation with the proximal por tion is achieved by a gradual and smooth flow connecting the buccal and lingual flares in such a manner as to create a 90° cavosurface angle. T o achieve the conservative concept in the Class II preparation, the clinician must rely on separate retention and resistance form for both the occlusal and proximal portions o f the prepar ation. Simon,2 Gilmore,6 Almquist and co-work ers,5 and Baum3 believe that this is a basic prin ciple to be achieved by the use of proximal reten tion grooves in the buccoaxial and linguoaxial line angles. The retention o f the occlusal por tion o f the restoration is determined by occlusally converging buccal and lingual walls. The review o f the literature shows that internal retention is thought to be an important concept in conservative cavity design. If no occlusal car ies exists, the ideal Class II preparation for an amalgam restoration, which would maintain the integrity of the tooth most completely, is a small proximal box with no occlusal extension. T o evaluate the retention and resistance of the “ ideal” Class II cavity preparation design, the vertical and horizontal forces required to frac ture or displace the restorations were measured for four cavity design conditions (Fig 1): prox imal box only, no retention grooves; proximal box only, with retention grooves; proximal box and dovetail lock, no retention grooves; and 1054 ■ JADA, Vol. 91, Novem ber 1975
proximal box and dovetail lock, with retention grooves.
M a te r ia ls a n d m e th o d s
Five preparations o f each design were simulated in stainless steel blocks by machine milling. They were then refined to the approximate measure ments o f a Class II preparation for amalgam res toration in a molar by using a no. 56 fissure bur in the high-speed handpiece. The retention grooves were placed in the buccoaxial and linguoaxial angles with the use of a no. quarter-round bur in a high-speed handpiece. The grooves extended from the gingival floor or wall to the level o f the axiopulpal angle. Their depth was equal to the diameter o f the quarter-round bur. The final depth o f the occlusal portion was 2 mm, and the axial wall depth (gingival floor width) was 1Vi mm. The gingival wall was placed at a level o f 5 mm from the top o f the steel block. A hole was drilled through the steel blocks by entering into the side opposite the preparations and passing perpendicularly through the center of the axial wall o f the proximal preparations. The “ punch-out” channel was 0.081 inches in diameter, accommodating without friction a steel punch 0.080 inches in diameter (Fig 2 a and b). Before the amalgam was condensed into the preparation, the steel punch was inserted into the channel and locked firmly in place, flush with the axial wall, to provide an intact axial wall sur face against which to condense the amalgam. A standardized ivorine matrix* was prepared in such a way so that the preparations were slight-
Tests using both vertical and horizontal forces were conducted 24 hours after condensation. The occlusal force was applied with a flat circu lar punch, 0.080 inches in diameter (Fig 2 b). It was centered over the proximal box portion of the various restorations. The horizontal force was exerted against the inside surface of the axial wall portion of the restoration using the punch previously described (Fig 2 b). The forces required to fracture or displace the amalgam res torations were exerted and measured with an Instron Universal Testing Machine§ at a cross head speed of 0.02 inches per minute. The re sults are shown in the Table.
p i * W ¿/A
<4D is c u s s io n
b Fig 2 ■ Diagram illustrating test methods: a, proximal view of Class II preparation with dovetail lock, proximal retention grooves, and punch-out channel; b, application of vertical and horizontal forces.
ly overfilled and the restorations required slight carving to achieve margins flush with the cavity preparation. The occlusal portions were carved flat (flush with the flat surface of the steel blocks). Each preparation was filled with amalgam, tested with vertical force, repacked, and tested again, resulting in ten samples for each design. The same procedure was repeated for testing the horizontal forces to fracture or displace the restorations of each test design. A standardized technique was used during the manipulation of the amalgam. Two pellets of amalgamt were mixed in a 1:1 ratio with mercury and triturated for 20 seconds in a Wig-L-Bug amalgam ator.i This mixture was packed into the stainless dies within W 2 minutes after trituration. The occlusal surface was carved flat 5Vi minutes after tritur ation, and the matrix was removed SV2 minutes after trituration.
The forces required to fracture or displace the restorations are listed in the Table. For the preparations without retention grooves and without dovetail locks, the restorations were displaced horizontally with no fracturing. The low force measured for this preparation repre sents that required to break the fine mechanical locking of the surfaces and static friction. All other values represent the forces required to fracture the various restorations. When horizon tal force was applied to the restorations retained by a dovetail lock and no proximal grooves, the amalgam fractured in the occlusal isthmus area. When the same directional force was applied to the restorations retained by both a dovetail lock and proximal retention grooves, fracturing oc curred in the occlusal isthmus area and in the retentive groove areas, leaving amalgam in both the dovetail lock and grooves. The analysis of variance F-test7 was applied to the various samples for both the vertical and the horizontal force values listed in the Table, to determine if a statistically significant differ ence for the forces existed. The analysis of the variance test indicated no significant difference for the vertical forces; therefore, if a difference did exist, the experimental procedure was not
Table ■ Fracture force* (in pounds).
Cavity design Horizontal force Vertical force (a) Proximal box only, 347±37 no retention grooves 4.1 ±1.9 (b) Proximal box only, with retention grooves 39.9±5.5 377±30 (c) Proximal box with dovetail lock, no retention grooves 15.7±3.6 336±19 (d) Proximal box with dovetail lock, with retention grooves 50.2±6.2 356±42 'Average force and standard deviation resulting from testing of ten restorations of each cavity design. C ro ckett— others: PR O XIM A L RETEN TIO N G R O O VES ■ 1055
sensitive enough to demonstrate it. However, the analysis of variance did demonstrate a statis tical difference at the 0.01 level of confidence in the horizontal forces listed in the Table when applied to that group. Student’s t test7 was then applied to individ ual pairs of the horizontal force group, and each pair was found to exhibit a statistically sig nificant difference at the 0.01 level of confidence. For this reason, we concluded that the horizon tal force is important in evaluating the various cavity designs.
C o n c lu s io n s
Terkla, Mahler, and Van Eysden8 have demon strated clinically that a Class II amalgam restor ation with a dovetail lock and no retention grooves serves as satisfactorily as does a restor ation containing a dovetail lock and retention grooves after evaluation at one, two, and three years, if the materials are manipulated correctly. In this study any variation of the cavity design containing proximal retention grooves yielded test values for the horizontal force considerably above those measured for any other design with out proximal retention grooves. The vertical force values obtained in testing all designs did not differ significantly. The test results suggest that the proximal box preparation with retention
^
grooves will allow the clinician to place more emphasis on conserving vitally important tooth structure in the occlusal portion of Class II cav ity preparations for amalgam restorations.
Dr. Crockett is professor of restorative dentistry and director of the division of operative dentistry, Virginia Commonwealth University, Medical College of Virginia School of Dentistry, Rich mond, 23298. Dr. Shepard is associate professor of restorative dentistry; Dr. Moon is assistant professor of restorative den tistry, division of dental materials; and Mr. Creal is a third-year dental student, Virginia Commonwealth University. Address requests for reprints to Dr. Crockett. ‘ Columbia Dentoform Corp., New York, 10010. tVelvalloy, S. S. White Co., Philadelphia, 19102. tWig-L-Bug, Crescent Dental Mfg., Corp., Lyons, III 60534. Slnstron Universal Testing Machine, Instron Corp., Canton, Mass 02021. 1. Black, G.V. Operative dentistry, ed 8, revised by Blackwell, R.E. Woodstock, III, Medico-Dental Publishing Co., 1947, vol 2. 2. Simon, W.J., ed. Clinical operative dentistry. Philadelphia, W. B. Saunders Co., 1956. 3. Lambert, R.L. Amalgam restorations. In advanced restor ative dentistry, Baum, L., ed. Philadelphia, W. B. Saunders Co., 1973. 4. Vale, W.A. Cavity preparation. Irish Dent Rev 2:33 MarchApril 1956. 5. Almquist, T.C.; Cowan, R.D.; and Lambert, R.L. Conserva tive amalgam restorations. J Prosthet Dent 29:524 May 1973. 6. Gilmore, H.W. Textbook of operative dentistry. St. Louis, C. V. Mosby Co., 1967. 7. Fisher, F.E. Fundamental statistical concepts. San Fran cisco, Canfield Press, 1973. 8. Terkla, L.G.; Mahler, D.B.; Van Eysden, J. Analysis of amal gam cavity design. J Prosthet Dent 29:204 Feb 1973.
Foley’s Footnotes------------------In Mary B. Clafton's Brampton Sketches (1890), there is an unusual note of a child’s dental experience in a Massachusetts town in 1810, when the physician included a turnkey in his instrument bag. Sometimes a little girl was told if she would be very good and do her stent [stint] she should go to her grandmother’s grave on a pleasant afternoon, and sometimes as a reward of merit she was told she might go to the "good doctor” and have a tooth pulled. Dentists were quite unknown in those days, and the instruments for extracting teeth were constructed somewhat like a plough share, but the good doctor was so kindly in his ways that the little children never feared to go to him, and the plenty which he always had ready was a panacea for the pain of the tooth-pulling, or the bitter draught which he administered from his saddlebags. Gardner P. H. Foley
1056
■ JADA, Vol. 91, Novem ber 1975