The effect of slot preparation length strength of slot-retained restorations
on the transverse
Dana R. McMaster, DMD,a Ronald C. House, DDS, MS,b Maxwell H. Anderson, DDS, MS, MEd,c and George B. Pelleu, Jr., PhDd Naval Dental School, National Naval Dental Center, Bethesda, Md. An in vitro study was conducted (1) to compare fracture strength of amalgam restorations retained with retentive slots of dillerent lengths when stressed with a transverse force, (2) to determine if beveling the slot preparation resulted in an increase in fracture resistance to a transverse force, and (3) to evaluate the incidence of unrestorable tooth fracture as it relates to slot preparation length. Six groups of 10 specimens were prepared with slots of increasing length, with each specimen receiving four slots of equal length. Slot preparations in group 6 were beveled. Results showed that shorter slot preparations provided statistically equal amounts of resistance to a transverse force as did the longer preparations. Beveling the slot preparation did not significantly increase fracture strength. Specimens restored with longer slot preparations failed unrestorably more often than restorations retained with shorter slot preparations. (J PROSTHET DENT 1992;67:472-7.)
Re
storing the decimated tooth presents a challenge to the clinician in providing maximum retention and resistance form while minimizing trauma to the tooth. The most appropriate forms of resistance and retention, however., continue to be debated in the literature. Although self-threading pins are a popular form of resistance and retention in the placement of amalgam and resin restorations, they have some disadvantages. These include (1) the considerable chair time and expertise necessary to properly place the pin~,‘-~ (2) the cost involved, (3) the possible perforation of the pulp or periodontal ligament during pin placement,2* 4-6and (4) pulpal inflammation and sensitivity if pins are placed into vital dentin49 7p8or if forces of mastication are concentrated in the region of the ~in.~ Enamel cracks or crazing may result if pins are placed close to the dentinoenamel junction (DW).g-12 Decreased
The opinions or assertions contained in this article are the private views of the authors and are not to be construedss official or ss reflecting the views of the Department of the Navy, the Department. of Defense, or the U.S. Government. This project was supported under the Naval Medical Research and Development Command Research Task No. MOO95-06-3014. Presented at the 18th Session of the American Association for Dental Research, San Francisco, Calif. BLieutenant Commander (DC) USN; Formerly Resident, Comprehensive Dentistry Department. Currently, Assistant Dental Officer, USS Fulton (AS-11). bCaptain (DC) USN, Executive Officer, National Naval Dental Center. CCommander (DC) USN, Formerly Chairman, Operative Dentistry Department. Currently, Assistant Professor, University of Washington School of Dentistry, Department of Restorative Dentistry, Seattle, Wash. dFormerly Chairman, Research Department. 10/l/32709
472
Fig. 1. Al’uminum ring and mounting jig. Note depression on ring to orient specimen.
compressive, tensile, and crushing strength of amalgam has been shown to occur in the presence of pins.13-15 Aware of the drawbacks associated with the threaded pin, dentists have sought alternative forms of retention and resistance.5 In 1980, Shavell’ proposed the “amalgapin”, and Seng et aL2 advocated a method of supplemental retention with retentive amalgam inserts. More recently, numerous authors4*16-20 have proposed the use of retentive slots. The pinless forms of retention, such as the slot preparation and the amalgapin, offer numerous advantages when compared with the threaded pin. Encroachment on pulpal or periodontal tissues is reduced because of the minimal depth necessary for placement. The pinless forms are quickly placed with minimum trauma to the remaining tooth. No special equipment is necessary. Slots do not decrease the compressive and transverse strength of amalgam restorations,
and they can be placed in any location where
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dentin is present, possibly reinforcing fragile remaining cusps by way of a “staple” effect. The use of slots conserves remaining tooth structure by minimizing the cuspal reduction required to gain sufficient amalgam bulk over the top of retentive pins.3 Finally, slots and amalgapins are passively retentive and do not subject the dentin to elastic loading as threaded pins do. Studies suggest that the slot preparation provides retention and resistance equal to that of threaded pins. OuthWaite et aLla showed that there was no significant difference between the transverse force necessary to cause failure in circumferential slot-retained amalgam restorations and in restorations retained by four threaded pins. Garman et al3 concluded that “dentinal slot retention was as effective as self-threading pin retention in extensive amalgam restorations.” Plasmans et a1.20found little difference in retention between an extensive amalgam restoration retained with four self-threaded pins and a restoration retained with a circumferential slot. They concluded that a circumferential slot could withstand in excess of 10 times the highest reported tensile load generated in the mouth. Investigators continue to debate whether beveling the entrance to the amalgapin or slot preparation results in an increase in resistance to a transverse force. Shavell’ beveled the chamber orifice of amalgapins to provide additional amalgam bulk. Outhwaite et aLla felt that a cavosurface bevel was necessary for increased strength. Davis et all7 employed a 0.5 mm bevel to the amalgapin entrance to provide for sufficient amounts of amalgam. These studies, however, did not investigate the specific benefits of beveling. A recent study by Roddy et a1.21found that increasing the size of the amalgam insert by beveling the channel entrance did not increase the fracture strength of the amalgam restoration to forces applied at a 45-degree angle. It has yet to be determined if a similar effect exists with slot retention. Historically, investigators studying slot retention involving the lengthy circumferential form of slot preparation have theorized that longer slot preparations must provide for maximum retention and resistance. The longer slot preparation, however, requires the removal of more dentin, resulting in a weakened tooth that may not withstand normal occlusal forces. Plasmans et al.lg concluded from in vitro testing on extensive amalgam restorations that “when oblique forces were applied, all extensive amalgam restorations retained with circumferential amalgam slots showed fracture without the possibility of rerestoration.” If such restorations fail in this manner consistently, the use of longer slots should be questioned. The possibility that shorter slots provide for equal retention and resistance has not been examined. The purpose of this study was (1) to compare the transverse strength of amalgam restorations retained with retentive slots of different lengths when stressed at a 45degree angle, (2) to determine if an increased resistance to a transverse force is realized by beveling the cavosurface
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2. Precision drill press (2 0.001 inch tolerance) with aluminum mounting platform containing mounted specimen.
Fig.
margin of the slot preparation, and (3) to evaluate the incidence of unrestorable tooth fracture as it relates to slot preparation length.
MATERIALS
AND METHODS
Sixty extracted human molars were randomly assigned to one of six groups, each containing 10 specimens. The specimens were stored in normal saline with 0.2 % sodium azide and were then placed in a humidor providing 95 % to 100 % humidity at room temperature. Specimens were reduced occlusally on a model trimmer (Handler Mfg. Co., Westfield, N.J.) under water coolant to within 3 mm of the cementoenamel junction (CEJ) perpendicular to the long axis of the tooth. The occlusal surface was finished with a 280-grit sandpaper disk, and two layers of cavity varnish were applied. The roots of each tooth were notched lightly for retention, and the specimens were mounted in a cylindrical aluminum mounting ring containing a uniform mix of polymethyl methacrylate. The teeth were centered perpendicular to their long axis with an aluminum mounting jig (Fig. 1). A depression located on the exterior surface of each mounting ring allowed the uniform placement of the ring in a machined holding block. The mesial surface of each specimen was oriented toward the depression for a consistent force vector. 473
McMASTER
Fig.
3. Slot preparations placed to burhead depth with a No. 33% inverted cone carbide bur.
Fig.
Table
4. Circumferential slot preparation (5 mm length per
side).
I. Test sample grouping* Group
*N, 10 samples
Slot preparation
Four 1 mm Four 2 mm Four 3 mm Four 4 mm Circumferential Four 2 mm beveled
1 2 3 4 5 6
pergroup. Fig.
The retentive slot preparations were placed with a No. 33 ‘/z inverted cone bur (S.S. White Dental Products International, Philadelphia, Pa.) in a precision drill press (Servo Product Co., Altadena, Calif.) with an 0.001 inch tolerance (Fig. 2). The drill press was used to ensure a consistent go-degree bur angle to the occlusal plane and to ensure consistent preparation length and depth. An aluminum mounting platform was manufactured to secure the aluminum mounting ring containing the specimen to the precision drill press. Each slot preparation was prepared to the depth of the bur head (Fig. 3). Preparation length was determined to be that undercut portion of the slot prepared by the inverted cone bur moving through dentin. A new bur was used for each tooth. Specimens in groups 1 through 4 received four slot preparations of equal length (Table I). Specimens in group 5 received a single continuous circumferential slot preparation measuring 5 mm per side (Fig. 4). All preparations were placed as near to 0.5 mm inside the DEJ as possible and at 90-degree angles to one another. Fig. 5 shows a sample from group 4 containing four 4 mm slot preparations. Samples in group 6 were prepared with four 2 mm slots in the manner described above, and then each slot was modified with a No. 2 round bur (S.S. White) mounted in the drill press (Fig. 6). The bur was placed into the slot prep474
ET AL
5. Group 4 specimen with four 4 mm slot prepara-
tions.
aration to a depth of 0.2 mm, creating a bevel 0.5 mm wide, as advocated by Davis et al. I7 The width and depth of slot preparations in all groups were constant. A copper band was secured below the cavosurface margin with red modeling plastic (Fig. 7). Silver amalgam (Tytin, Kerr Mfg. Co., Romulus, Mich.) was triturated in a calibrated amalgamator (Vari-Mix II, L. D. Caulk Co., Milford, Del.) according to the manufacturer’s instructions and hand-condensed5720~22into the slot preparations with a No. 1 condenser (Markley, American Dental Mfg. Co., Missoula, Mont.). The bulk of the amalgam was mechanically condensed (Condensaire, Teledyne Densco, Denver, Colo.) to assure complete condensation. Seventy-two hours after condensation, the copper bands and any marginal overhangs were removed with a carbide finishing bur mounted in a high-speed handpiece. The restorations were reduced to a height of 4 mm occlusal to the cavosurface margin (Fig. 8). A 45-degree 1 mm bevel was placed at the occlusoaxial line angle of the restoration to create a flat plane for the application of the transverse force. All test samples were prepared by the same operator. Each test sample was positioned in an aluminum mount-
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Fig. ‘7. Copper band secured below cavosurface margin with modeling plastic. Fig. 6. Slot preparation entrance beveled with a No. 2 round carbide bur.
ing jig (Fig. 9), mounted in a testing instrument (United Calibration Corp., Garden Grove, Calif.), and subjected to a constant 45degree transverse force. The force (measured in kilograms per square centimeter) was applied at a constant crosshead speed (0.5 mm/min) until the sample fractured. Fracture was understood to occur when a sudden decrease in kilogram force was noted on the recording chart. An unrestorable fracture was determined to be a clinical crown or root fracture occurring more than 2 mm apical to the CEJ. A one-way analysis of variance (ANOVA) was performed to compare the mean fracture force among the different groups. Scheffe’s multiple comparison test was used to differentiate between test groups. A chi square analysis was made to determine significance among groups with respect to the number of unrestorable root or crown fractures. The Fisher 2 X 2 exact test was used to differentiate among groups.
RESULTS Fracture forces for the six slot preparation groups are shown in Table II. No significant difference existed between the unbeveled groups. A significant difference was found between mean fracture forces of the unbeveled circumferential slot group and those of the 2 mm beveled slot group. Beveling of the slot preparation, however, did not significantly increase resistance to a 45degree force compared with the noncircumferential group. The number of unrestorable fractures within each group is shown in Table II. The 1 mm slot group had significantly fewer unrestorable tooth fractures than the 4 mm slot group. Beveling of the slot preparation did not sig-
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Fig. 8. Completed restoration with 1 mm bevel at occlusoaxial line angle for application of transverse force.
nificantly reduce unrestorable tooth fractures in the 2 mm groups.
DISCUSSION The concept of the slot preparation has not been clearly defined in the literature. Outhwaite et al.l* prepared slots with a No. 33 ‘/z inverted-cone friction-grip bur placed to a bur head depth of 0.75 mm. The bur was moved laterally through dentin, paralleling the DEJ at a distance of 0.5 mm until the preparation was completed. The nature of the in-
475
McMASTER
ET AL
Table II. Slot preparation related to fracture force and unrestorable tooth fractures
Transverse Slot preparation*
lmm 2mm
(fracture) force (kg/cm2)
Unrestorable tooth fractures
3mm
166 + 32 178 + 32 184 t 34
1.4 4 6
4mm Circumferential 2 mm bevel
177 ?I 39 145 k 537 205 k 30-f
7 4
f3$
*N, 10 samples per group. @ignificant difference (one-way ANOVA and Scheffb’s multiple comparison test, p < 0.05). SSignificant difference (chi square analysis and Fisher’s 2 x 2 exact test p < 0.05).
Fig. 9. Mounting ring containing specimen mounted in holding block on 45-degree inclined plane. Specimen in position to receive transverse load by a testing instrument.
10. Unrestorable fracture occurring with circumferential slot preparation.
Fig.
verted-cone bus suggests that the preparation is undercut along its length.lg Other researchers have defined the slot preparation in different terms. Plasmans et al.lg prepared “amalgam slots” by placing an inverted-cone diamond stone vertically to the depth of the burhead, hut without the lateral element described by Outhwaite et al.ls This vertical placement of the burhead into dentin without the lateral component may not provide the undercut necessary for maximum retention. The preparation by Plasmans et all9 appeared to more closely resemble the amalgapin preparation. It was recommended by Outhwaite et al’s that the slot preparation be undercut and continuous while paralleling the DEJ. Although a number of studies evaluate the resistance provided by the pinless forms of retention, research exam476
ining the resistance provided by slot preparations of increasing lengths has not been reported. Our findings demonstrate that resistance to a transverse force is not significantly improved by lengthening the slot preparation. In fact, shorter slot preparations provided statistically equivalent amounts of resistance when compared with the longer preparations, and a greater incidence of unrestorable tooth fractures occurred as slot length was increased. This high number of unrestorable tooth fractures occurring with the longer slot lengths is corroborated in a study by Plasmans et al.,lg who reported that all circumferential slot-retained samples failed unrestorably. Examination of the unrestorable specimens in our study revealed that most fractures occurred within the slot preparation itself. This was especially true with the longer slots (Fig. 10). Longer preparations require the removal of more dentin, possibly providing an avenue along which cracks may initiate and propagate under stress. However, this procedure may result in a weakened tooth. Our results showed that shorter slot lengths provided statistically equal amounts of resistance to a transverse force, with significantly less chance of unrestorable fracture. Davis et all7 and Shavelll felt that the addition of a cavosurface bevel to the slot preparation provided additional bulk and strength when restoring large amalgam restorations. In our study, no significant difference was demonstrated by beveling the slot preparation. This result is consistent with that reported by Roddy et a1.21Also, beveling of the slot preparation did not decrease the number of unrestorable tooth fractures. Any of the slot preparation lengths tested in this study would provide sufficient resistance to withstand the maximum biting forces (82 kgf) measured in a study by Helkimo et a1.22Their values were well below the minimum resistance provided by any slot preparation length measured in our study. Gibbs et al.,23 however, have reported biting forces (449 kgf) in bruxing clenching patients that far exceed the maximum resistance measured in our study. This should emphasize the potential for failure of restoraAPRIL
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tions retained with any form of resistance and retention when stressed under a large transverse force. The sensitivity to matrix movement when condensing amalgam restorations retained by slot retention was observed by Outhwaite et al. la This sensitivity was also evident when we prepared our samples. Clinically, the pinless forms of retention and resistance require that a stable matrix be placed and that special care be exercised when removing the matrix band. The careful and complete hand condensation of amalgam into the slot preparation is essential to the success of the restoration. Roddy et aL21 and Leach et a1.24reported that hand-condensed preparations were densely compacted and well adapted to the walls of the amalgapin, whereas mechanically condensed preparations varied in density and were not well adapted to the apical portions of the channel. For this reason, all slot preparations in our study were hand-condensed. Spherical alloys make complete adaption to the slot preparation simpler because of their high plasticity. They also possess a high early strength, allowing for earlier matrix removal. The unbeveled circumferential slot preparation group provided significantly less resistance to a 45-degree force than the beveled 2 mm slot group, resulting in a greater number of unrestorable tooth fractures. Future studies examining resistance and retention form should employ the shorter 2 mm beveled preparation in lieu of the more extensive circumferential slot preparation. SUMMARY Sixty extracted molars were randomly assigned to six groups and were prepared with standardized length slot preparations. The samples were subjected to a 45-degree force until failure occurred. The results were: 1. No significant difference was found in resistance to a transverse force provided by four slot preparations of 1,2, 3, and 4 mm, or by a single circumferential slot. Shorter slot preparations provide as much resistance to a transverse force as do longer, more extensive preparations. 2. Beveling of the 2 mm slot preparation did not provide a significant increase in resistance to a transverse force when compared with 1, 2, 3, or 4 mm unbeveled slots. However, the beveled 2 mm slot preparation did provide significantly more resistance to transverse force than the unbeveled circumferential slot preparation. 3. Unrestorable tooth fractures occurred significantly less often with the 1 mm slot group than with the 4 mm slot group, suggesting that the shorter slot lengths, which provide statistically equal amounts of resistance to a transverse force, are less likely to fail catastrophically than the 4 mm slots. 4. Beveling the cavosurface margin of the slot preparation did not decrease the incidence of unrestorable tooth fracture. We thank Dr. F. Eichmiller at the National Institutes of Standards and Technology, Gaithersburg, Md. for the use of the United testing machine; LCDR W. Roddy for the use of mounting rings, THE
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DENTISTRY
mounting jig, and holding block; Mr. F. Sharpnack and Mr. D. Gotthardt at the Armed Forces Radiobiological Research Institute, Bethesda, Md., for construction and use of essential equipment; Shou-Hua Li, PhD, Statistician, Biometrics Unit, National Institutes of Dental Research, Bethesda, Md., for the statistical analysis in this study; and Phyllis Reidy and Sally Hobson for the artwork.
REFERENCES 1. Shave11HM. The amalgapin technique for complex amalgam restorations. Cal Dent Assoc J 1980;8:48-55. 2. Seng GF, Rupell OL, Nance GL, Pompura JP. Placement of retentive amalgam inserts in tooth structure for supplemental retention. Gen Dent 1980,28:62-6. 3. Garman TA, Outhwaite WC, Hawkins IK, et al. A clinical comparison of dentinal slot retention with metallic pin retention. J Am Dent Assoc 1983;107:762-3. 4. Schuchard A, Reed OM. Pulpal response to pin placement. J PROSTHET DENT 1973;29:292-300.
5. Birtcil RF Jr, Venton EA. Extracoronal amalgam restorations utilizing available tooth structure for retention. J PROSTHET DENT 1976;35:171-8. 6. Birtcil RF. Utilizing available tooth structure to retain large amalgam restorations. Restorative techniques for individual teeth. New York: Masson Publishing USA, Inc, 1981:153-72. 7. Suzuki M, Goto G, Jordan RE. Pulpal response to pin placement. J Am Dent Assoc 1973;87:636-40. 8. Dolph RW. Intentional implanting of pins into dental pulp. Dent Clin North Am 1970;14:73-86. 9. Trabert KC, Caputo AA, Collard EW, Standlee JP. Stress transfer to the dental pulp by retentive pins. J PROSTHET DENT 1973;30:808-15. 10. Standlee JP, Collard EW, Caputo AA. Dentinal defects caused by some twist drills and retentive pins. J PROSTHET DENT 1970;24:185-92. 11. Dilts WE, Welk DA, Laswell HR, et al. Crazing of tooth structure associated with placement of pins for amalgam restorations. J Am Dent Assoc 1970;81:387-91. 12. Collard EW, Caputo AA, Standlee JP. Rationale for pin-retained amalgam restorations. Dent Clin North Am 1970;14:43-51. 13. Welk DA, Dilts WE. Influence of pins on the compressive and transverse strength of dental amalgam and retention of pins in amalgam. J Am Dent Assoc 1969;78:101-4. 14. Going RE, Moffa JP, Nostrant GW, et al. The strength of dental amalgam as influenced by pins. 3 Am Dent Assoc 1968;77:1331-4. 15. Galindo Y. Stress-induced effects of retentive pins. A review of the literature. J PROSTHEX DENT 1980;44%3-6. 16. Barney JI, Croll TP, Castaldi CR. The slot-retained complex amalgam restoration. J Dent Child 1984;51:184-9. 17. Davis SP, Summitt JB, Mayhew RB, et al. Self-threading pins and amalgapins compared in resistance form for complex amalgam restorations. Oper Dent X%3$%8-93. 18. Outhwaite WC, Garman TA, Pashley DH. Pin vs slot retention in extensive amalgam restorations. J PROSTHET DENT 1979;41:396-400, 19. Plasmans PJ, Kusters ST, de Jonge BA, et al. In vitro resistance of extensive amalgam restorations using various retention methods. J PROSTHET DENT 1987;57:16-20.
20. Plasmans PJJM, Welle PR, Vrijhoef MMA. The tensile resistance of extensive amalgam restorations with auxiliary retention. Quintessence Int 1986;17:411-4. 21. Roddy WC, Blank LW, Rupp NW, et al. Channel depth and diameter: effects on transverse strength of amalgapin-retained restorations. Oper Dent 1987;12:2-9. 22. Helkimo E, Ingervall B. Bite force and functional state of the masticatory system in young men. Swed Dent J 1978;2:167-75. 23. Gibbs CH, Mahan PE, Mauderli A, et al. Limits of human bite strength. J PROSTHET DENT 1986,56:226-S. 24. Leach CD, Martmoff JT, Lee CV. A second look at the amalgapin technique. Cal Dent Assoc J 1983;11:43-9. Reprint requests to: CAPT STEVE ARTHUR, DC, USN CHAIRMAN, RESEARCH DEPARTMENT NAVAL DENTAL SCHOOL NATIONAL NAVAL DENTAL CENTER BETHESDA, MD 20889-5077
477
on the transverse
Dana R. McMaster, DMD,a Ronald C. House, DDS, MS,b Maxwell H. Anderson, DDS, MS, MEd,c and George B. Pelleu, Jr., PhDd Naval Dental School, National Naval Dental Center, Bethesda, Md. An in vitro study was conducted (1) to compare fracture strength of amalgam restorations retained with retentive slots of dillerent lengths when stressed with a transverse force, (2) to determine if beveling the slot preparation resulted in an increase in fracture resistance to a transverse force, and (3) to evaluate the incidence of unrestorable tooth fracture as it relates to slot preparation length. Six groups of 10 specimens were prepared with slots of increasing length, with each specimen receiving four slots of equal length. Slot preparations in group 6 were beveled. Results showed that shorter slot preparations provided statistically equal amounts of resistance to a transverse force as did the longer preparations. Beveling the slot preparation did not significantly increase fracture strength. Specimens restored with longer slot preparations failed unrestorably more often than restorations retained with shorter slot preparations. (J PROSTHET DENT 1992;67:472-7.)
Re
storing the decimated tooth presents a challenge to the clinician in providing maximum retention and resistance form while minimizing trauma to the tooth. The most appropriate forms of resistance and retention, however., continue to be debated in the literature. Although self-threading pins are a popular form of resistance and retention in the placement of amalgam and resin restorations, they have some disadvantages. These include (1) the considerable chair time and expertise necessary to properly place the pin~,‘-~ (2) the cost involved, (3) the possible perforation of the pulp or periodontal ligament during pin placement,2* 4-6and (4) pulpal inflammation and sensitivity if pins are placed into vital dentin49 7p8or if forces of mastication are concentrated in the region of the ~in.~ Enamel cracks or crazing may result if pins are placed close to the dentinoenamel junction (DW).g-12 Decreased
The opinions or assertions contained in this article are the private views of the authors and are not to be construedss official or ss reflecting the views of the Department of the Navy, the Department. of Defense, or the U.S. Government. This project was supported under the Naval Medical Research and Development Command Research Task No. MOO95-06-3014. Presented at the 18th Session of the American Association for Dental Research, San Francisco, Calif. BLieutenant Commander (DC) USN; Formerly Resident, Comprehensive Dentistry Department. Currently, Assistant Dental Officer, USS Fulton (AS-11). bCaptain (DC) USN, Executive Officer, National Naval Dental Center. CCommander (DC) USN, Formerly Chairman, Operative Dentistry Department. Currently, Assistant Professor, University of Washington School of Dentistry, Department of Restorative Dentistry, Seattle, Wash. dFormerly Chairman, Research Department. 10/l/32709
472
Fig. 1. Al’uminum ring and mounting jig. Note depression on ring to orient specimen.
compressive, tensile, and crushing strength of amalgam has been shown to occur in the presence of pins.13-15 Aware of the drawbacks associated with the threaded pin, dentists have sought alternative forms of retention and resistance.5 In 1980, Shavell’ proposed the “amalgapin”, and Seng et aL2 advocated a method of supplemental retention with retentive amalgam inserts. More recently, numerous authors4*16-20 have proposed the use of retentive slots. The pinless forms of retention, such as the slot preparation and the amalgapin, offer numerous advantages when compared with the threaded pin. Encroachment on pulpal or periodontal tissues is reduced because of the minimal depth necessary for placement. The pinless forms are quickly placed with minimum trauma to the remaining tooth. No special equipment is necessary. Slots do not decrease the compressive and transverse strength of amalgam restorations,
and they can be placed in any location where
APRIL1992
VOLUME67
NUMBER4
SLOT
PREPARATION
LENGTH
RESISTANCE
dentin is present, possibly reinforcing fragile remaining cusps by way of a “staple” effect. The use of slots conserves remaining tooth structure by minimizing the cuspal reduction required to gain sufficient amalgam bulk over the top of retentive pins.3 Finally, slots and amalgapins are passively retentive and do not subject the dentin to elastic loading as threaded pins do. Studies suggest that the slot preparation provides retention and resistance equal to that of threaded pins. OuthWaite et aLla showed that there was no significant difference between the transverse force necessary to cause failure in circumferential slot-retained amalgam restorations and in restorations retained by four threaded pins. Garman et al3 concluded that “dentinal slot retention was as effective as self-threading pin retention in extensive amalgam restorations.” Plasmans et a1.20found little difference in retention between an extensive amalgam restoration retained with four self-threaded pins and a restoration retained with a circumferential slot. They concluded that a circumferential slot could withstand in excess of 10 times the highest reported tensile load generated in the mouth. Investigators continue to debate whether beveling the entrance to the amalgapin or slot preparation results in an increase in resistance to a transverse force. Shavell’ beveled the chamber orifice of amalgapins to provide additional amalgam bulk. Outhwaite et aLla felt that a cavosurface bevel was necessary for increased strength. Davis et all7 employed a 0.5 mm bevel to the amalgapin entrance to provide for sufficient amounts of amalgam. These studies, however, did not investigate the specific benefits of beveling. A recent study by Roddy et a1.21found that increasing the size of the amalgam insert by beveling the channel entrance did not increase the fracture strength of the amalgam restoration to forces applied at a 45-degree angle. It has yet to be determined if a similar effect exists with slot retention. Historically, investigators studying slot retention involving the lengthy circumferential form of slot preparation have theorized that longer slot preparations must provide for maximum retention and resistance. The longer slot preparation, however, requires the removal of more dentin, resulting in a weakened tooth that may not withstand normal occlusal forces. Plasmans et al.lg concluded from in vitro testing on extensive amalgam restorations that “when oblique forces were applied, all extensive amalgam restorations retained with circumferential amalgam slots showed fracture without the possibility of rerestoration.” If such restorations fail in this manner consistently, the use of longer slots should be questioned. The possibility that shorter slots provide for equal retention and resistance has not been examined. The purpose of this study was (1) to compare the transverse strength of amalgam restorations retained with retentive slots of different lengths when stressed at a 45degree angle, (2) to determine if an increased resistance to a transverse force is realized by beveling the cavosurface
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
2. Precision drill press (2 0.001 inch tolerance) with aluminum mounting platform containing mounted specimen.
Fig.
margin of the slot preparation, and (3) to evaluate the incidence of unrestorable tooth fracture as it relates to slot preparation length.
MATERIALS
AND METHODS
Sixty extracted human molars were randomly assigned to one of six groups, each containing 10 specimens. The specimens were stored in normal saline with 0.2 % sodium azide and were then placed in a humidor providing 95 % to 100 % humidity at room temperature. Specimens were reduced occlusally on a model trimmer (Handler Mfg. Co., Westfield, N.J.) under water coolant to within 3 mm of the cementoenamel junction (CEJ) perpendicular to the long axis of the tooth. The occlusal surface was finished with a 280-grit sandpaper disk, and two layers of cavity varnish were applied. The roots of each tooth were notched lightly for retention, and the specimens were mounted in a cylindrical aluminum mounting ring containing a uniform mix of polymethyl methacrylate. The teeth were centered perpendicular to their long axis with an aluminum mounting jig (Fig. 1). A depression located on the exterior surface of each mounting ring allowed the uniform placement of the ring in a machined holding block. The mesial surface of each specimen was oriented toward the depression for a consistent force vector. 473
McMASTER
Fig.
3. Slot preparations placed to burhead depth with a No. 33% inverted cone carbide bur.
Fig.
Table
4. Circumferential slot preparation (5 mm length per
side).
I. Test sample grouping* Group
*N, 10 samples
Slot preparation
Four 1 mm Four 2 mm Four 3 mm Four 4 mm Circumferential Four 2 mm beveled
1 2 3 4 5 6
pergroup. Fig.
The retentive slot preparations were placed with a No. 33 ‘/z inverted cone bur (S.S. White Dental Products International, Philadelphia, Pa.) in a precision drill press (Servo Product Co., Altadena, Calif.) with an 0.001 inch tolerance (Fig. 2). The drill press was used to ensure a consistent go-degree bur angle to the occlusal plane and to ensure consistent preparation length and depth. An aluminum mounting platform was manufactured to secure the aluminum mounting ring containing the specimen to the precision drill press. Each slot preparation was prepared to the depth of the bur head (Fig. 3). Preparation length was determined to be that undercut portion of the slot prepared by the inverted cone bur moving through dentin. A new bur was used for each tooth. Specimens in groups 1 through 4 received four slot preparations of equal length (Table I). Specimens in group 5 received a single continuous circumferential slot preparation measuring 5 mm per side (Fig. 4). All preparations were placed as near to 0.5 mm inside the DEJ as possible and at 90-degree angles to one another. Fig. 5 shows a sample from group 4 containing four 4 mm slot preparations. Samples in group 6 were prepared with four 2 mm slots in the manner described above, and then each slot was modified with a No. 2 round bur (S.S. White) mounted in the drill press (Fig. 6). The bur was placed into the slot prep474
ET AL
5. Group 4 specimen with four 4 mm slot prepara-
tions.
aration to a depth of 0.2 mm, creating a bevel 0.5 mm wide, as advocated by Davis et al. I7 The width and depth of slot preparations in all groups were constant. A copper band was secured below the cavosurface margin with red modeling plastic (Fig. 7). Silver amalgam (Tytin, Kerr Mfg. Co., Romulus, Mich.) was triturated in a calibrated amalgamator (Vari-Mix II, L. D. Caulk Co., Milford, Del.) according to the manufacturer’s instructions and hand-condensed5720~22into the slot preparations with a No. 1 condenser (Markley, American Dental Mfg. Co., Missoula, Mont.). The bulk of the amalgam was mechanically condensed (Condensaire, Teledyne Densco, Denver, Colo.) to assure complete condensation. Seventy-two hours after condensation, the copper bands and any marginal overhangs were removed with a carbide finishing bur mounted in a high-speed handpiece. The restorations were reduced to a height of 4 mm occlusal to the cavosurface margin (Fig. 8). A 45-degree 1 mm bevel was placed at the occlusoaxial line angle of the restoration to create a flat plane for the application of the transverse force. All test samples were prepared by the same operator. Each test sample was positioned in an aluminum mount-
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Fig. ‘7. Copper band secured below cavosurface margin with modeling plastic. Fig. 6. Slot preparation entrance beveled with a No. 2 round carbide bur.
ing jig (Fig. 9), mounted in a testing instrument (United Calibration Corp., Garden Grove, Calif.), and subjected to a constant 45degree transverse force. The force (measured in kilograms per square centimeter) was applied at a constant crosshead speed (0.5 mm/min) until the sample fractured. Fracture was understood to occur when a sudden decrease in kilogram force was noted on the recording chart. An unrestorable fracture was determined to be a clinical crown or root fracture occurring more than 2 mm apical to the CEJ. A one-way analysis of variance (ANOVA) was performed to compare the mean fracture force among the different groups. Scheffe’s multiple comparison test was used to differentiate between test groups. A chi square analysis was made to determine significance among groups with respect to the number of unrestorable root or crown fractures. The Fisher 2 X 2 exact test was used to differentiate among groups.
RESULTS Fracture forces for the six slot preparation groups are shown in Table II. No significant difference existed between the unbeveled groups. A significant difference was found between mean fracture forces of the unbeveled circumferential slot group and those of the 2 mm beveled slot group. Beveling of the slot preparation, however, did not significantly increase resistance to a 45degree force compared with the noncircumferential group. The number of unrestorable fractures within each group is shown in Table II. The 1 mm slot group had significantly fewer unrestorable tooth fractures than the 4 mm slot group. Beveling of the slot preparation did not sig-
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Fig. 8. Completed restoration with 1 mm bevel at occlusoaxial line angle for application of transverse force.
nificantly reduce unrestorable tooth fractures in the 2 mm groups.
DISCUSSION The concept of the slot preparation has not been clearly defined in the literature. Outhwaite et al.l* prepared slots with a No. 33 ‘/z inverted-cone friction-grip bur placed to a bur head depth of 0.75 mm. The bur was moved laterally through dentin, paralleling the DEJ at a distance of 0.5 mm until the preparation was completed. The nature of the in-
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Table II. Slot preparation related to fracture force and unrestorable tooth fractures
Transverse Slot preparation*
lmm 2mm
(fracture) force (kg/cm2)
Unrestorable tooth fractures
3mm
166 + 32 178 + 32 184 t 34
1.4 4 6
4mm Circumferential 2 mm bevel
177 ?I 39 145 k 537 205 k 30-f
7 4
f3$
*N, 10 samples per group. @ignificant difference (one-way ANOVA and Scheffb’s multiple comparison test, p < 0.05). SSignificant difference (chi square analysis and Fisher’s 2 x 2 exact test p < 0.05).
Fig. 9. Mounting ring containing specimen mounted in holding block on 45-degree inclined plane. Specimen in position to receive transverse load by a testing instrument.
10. Unrestorable fracture occurring with circumferential slot preparation.
Fig.
verted-cone bus suggests that the preparation is undercut along its length.lg Other researchers have defined the slot preparation in different terms. Plasmans et al.lg prepared “amalgam slots” by placing an inverted-cone diamond stone vertically to the depth of the burhead, hut without the lateral element described by Outhwaite et al.ls This vertical placement of the burhead into dentin without the lateral component may not provide the undercut necessary for maximum retention. The preparation by Plasmans et all9 appeared to more closely resemble the amalgapin preparation. It was recommended by Outhwaite et al’s that the slot preparation be undercut and continuous while paralleling the DEJ. Although a number of studies evaluate the resistance provided by the pinless forms of retention, research exam476
ining the resistance provided by slot preparations of increasing lengths has not been reported. Our findings demonstrate that resistance to a transverse force is not significantly improved by lengthening the slot preparation. In fact, shorter slot preparations provided statistically equivalent amounts of resistance when compared with the longer preparations, and a greater incidence of unrestorable tooth fractures occurred as slot length was increased. This high number of unrestorable tooth fractures occurring with the longer slot lengths is corroborated in a study by Plasmans et al.,lg who reported that all circumferential slot-retained samples failed unrestorably. Examination of the unrestorable specimens in our study revealed that most fractures occurred within the slot preparation itself. This was especially true with the longer slots (Fig. 10). Longer preparations require the removal of more dentin, possibly providing an avenue along which cracks may initiate and propagate under stress. However, this procedure may result in a weakened tooth. Our results showed that shorter slot lengths provided statistically equal amounts of resistance to a transverse force, with significantly less chance of unrestorable fracture. Davis et all7 and Shavelll felt that the addition of a cavosurface bevel to the slot preparation provided additional bulk and strength when restoring large amalgam restorations. In our study, no significant difference was demonstrated by beveling the slot preparation. This result is consistent with that reported by Roddy et a1.21Also, beveling of the slot preparation did not decrease the number of unrestorable tooth fractures. Any of the slot preparation lengths tested in this study would provide sufficient resistance to withstand the maximum biting forces (82 kgf) measured in a study by Helkimo et a1.22Their values were well below the minimum resistance provided by any slot preparation length measured in our study. Gibbs et al.,23 however, have reported biting forces (449 kgf) in bruxing clenching patients that far exceed the maximum resistance measured in our study. This should emphasize the potential for failure of restoraAPRIL
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tions retained with any form of resistance and retention when stressed under a large transverse force. The sensitivity to matrix movement when condensing amalgam restorations retained by slot retention was observed by Outhwaite et al. la This sensitivity was also evident when we prepared our samples. Clinically, the pinless forms of retention and resistance require that a stable matrix be placed and that special care be exercised when removing the matrix band. The careful and complete hand condensation of amalgam into the slot preparation is essential to the success of the restoration. Roddy et aL21 and Leach et a1.24reported that hand-condensed preparations were densely compacted and well adapted to the walls of the amalgapin, whereas mechanically condensed preparations varied in density and were not well adapted to the apical portions of the channel. For this reason, all slot preparations in our study were hand-condensed. Spherical alloys make complete adaption to the slot preparation simpler because of their high plasticity. They also possess a high early strength, allowing for earlier matrix removal. The unbeveled circumferential slot preparation group provided significantly less resistance to a 45-degree force than the beveled 2 mm slot group, resulting in a greater number of unrestorable tooth fractures. Future studies examining resistance and retention form should employ the shorter 2 mm beveled preparation in lieu of the more extensive circumferential slot preparation. SUMMARY Sixty extracted molars were randomly assigned to six groups and were prepared with standardized length slot preparations. The samples were subjected to a 45-degree force until failure occurred. The results were: 1. No significant difference was found in resistance to a transverse force provided by four slot preparations of 1,2, 3, and 4 mm, or by a single circumferential slot. Shorter slot preparations provide as much resistance to a transverse force as do longer, more extensive preparations. 2. Beveling of the 2 mm slot preparation did not provide a significant increase in resistance to a transverse force when compared with 1, 2, 3, or 4 mm unbeveled slots. However, the beveled 2 mm slot preparation did provide significantly more resistance to transverse force than the unbeveled circumferential slot preparation. 3. Unrestorable tooth fractures occurred significantly less often with the 1 mm slot group than with the 4 mm slot group, suggesting that the shorter slot lengths, which provide statistically equal amounts of resistance to a transverse force, are less likely to fail catastrophically than the 4 mm slots. 4. Beveling the cavosurface margin of the slot preparation did not decrease the incidence of unrestorable tooth fracture. We thank Dr. F. Eichmiller at the National Institutes of Standards and Technology, Gaithersburg, Md. for the use of the United testing machine; LCDR W. Roddy for the use of mounting rings, THE
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mounting jig, and holding block; Mr. F. Sharpnack and Mr. D. Gotthardt at the Armed Forces Radiobiological Research Institute, Bethesda, Md., for construction and use of essential equipment; Shou-Hua Li, PhD, Statistician, Biometrics Unit, National Institutes of Dental Research, Bethesda, Md., for the statistical analysis in this study; and Phyllis Reidy and Sally Hobson for the artwork.
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5. Birtcil RF Jr, Venton EA. Extracoronal amalgam restorations utilizing available tooth structure for retention. J PROSTHET DENT 1976;35:171-8. 6. Birtcil RF. Utilizing available tooth structure to retain large amalgam restorations. Restorative techniques for individual teeth. New York: Masson Publishing USA, Inc, 1981:153-72. 7. Suzuki M, Goto G, Jordan RE. Pulpal response to pin placement. J Am Dent Assoc 1973;87:636-40. 8. Dolph RW. Intentional implanting of pins into dental pulp. Dent Clin North Am 1970;14:73-86. 9. Trabert KC, Caputo AA, Collard EW, Standlee JP. Stress transfer to the dental pulp by retentive pins. J PROSTHET DENT 1973;30:808-15. 10. Standlee JP, Collard EW, Caputo AA. Dentinal defects caused by some twist drills and retentive pins. J PROSTHET DENT 1970;24:185-92. 11. Dilts WE, Welk DA, Laswell HR, et al. Crazing of tooth structure associated with placement of pins for amalgam restorations. J Am Dent Assoc 1970;81:387-91. 12. Collard EW, Caputo AA, Standlee JP. Rationale for pin-retained amalgam restorations. Dent Clin North Am 1970;14:43-51. 13. Welk DA, Dilts WE. Influence of pins on the compressive and transverse strength of dental amalgam and retention of pins in amalgam. J Am Dent Assoc 1969;78:101-4. 14. Going RE, Moffa JP, Nostrant GW, et al. The strength of dental amalgam as influenced by pins. 3 Am Dent Assoc 1968;77:1331-4. 15. Galindo Y. Stress-induced effects of retentive pins. A review of the literature. J PROSTHEX DENT 1980;44%3-6. 16. Barney JI, Croll TP, Castaldi CR. The slot-retained complex amalgam restoration. J Dent Child 1984;51:184-9. 17. Davis SP, Summitt JB, Mayhew RB, et al. Self-threading pins and amalgapins compared in resistance form for complex amalgam restorations. Oper Dent X%3$%8-93. 18. Outhwaite WC, Garman TA, Pashley DH. Pin vs slot retention in extensive amalgam restorations. J PROSTHET DENT 1979;41:396-400, 19. Plasmans PJ, Kusters ST, de Jonge BA, et al. In vitro resistance of extensive amalgam restorations using various retention methods. J PROSTHET DENT 1987;57:16-20.
20. Plasmans PJJM, Welle PR, Vrijhoef MMA. The tensile resistance of extensive amalgam restorations with auxiliary retention. Quintessence Int 1986;17:411-4. 21. Roddy WC, Blank LW, Rupp NW, et al. Channel depth and diameter: effects on transverse strength of amalgapin-retained restorations. Oper Dent 1987;12:2-9. 22. Helkimo E, Ingervall B. Bite force and functional state of the masticatory system in young men. Swed Dent J 1978;2:167-75. 23. Gibbs CH, Mahan PE, Mauderli A, et al. Limits of human bite strength. J PROSTHET DENT 1986,56:226-S. 24. Leach CD, Martmoff JT, Lee CV. A second look at the amalgapin technique. Cal Dent Assoc J 1983;11:43-9. Reprint requests to: CAPT STEVE ARTHUR, DC, USN CHAIRMAN, RESEARCH DEPARTMENT NAVAL DENTAL SCHOOL NATIONAL NAVAL DENTAL CENTER BETHESDA, MD 20889-5077
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