LANKFORDANDCHRISTENSEN
7. Make the final impression, ensuring that the pins are withdrawn in the impression, which is poured in die stone. 8. Incorporate the pins in the wax pattern and make the fixed partial denture in the usual manner. 9. Lute the final restoration in position on the abutment teeth in the usual manner (Fig. 3). REFERENCES 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth using acid-etch technique and a cast metal framework. J PROSTHET DENT 1973:30:418-23.
2. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982:47:52-K 3. Moon PC. Bond strength of the lost salt procedure: a new retention method for resin-bonded fixed prostheses. J PROSTHET DENT 1987: 57:435-39.
Fig.
3. Final restoration luted to abutment teeth.
Reprint requests to: DR. LOREN C. CHRISTENSEN 9314 HILLTOP CT. LAUREL, MD 20708-3218
6. Establish a retentive button on the plastic pin with a hot instrument. Seat the pin in the hole, and add impression adhesive to the button (Fig. 2).
Modification of the resistance coronal-radicular restorations
form of amalgam
James J. Kane, D.M.D.,* and John 0. Burgess, D.D.S., M.S.** Wilford Hall USAF Medical Center, Lackland Air Force Base, San Antonio, Texas Three groups of 10 extracted endodontically treated mandibular molars were mounted in acrylic resin. The occlusal surface was reduced until a 4 mm pulp chamber height remained. A peripheral shelf 2 mm deep and 1.4 mm wide was placed on one group, while four TMS Minim pins were placed 45 degrees to the long axis of the tooth into the facial and lingual walls of the second group. The final group served as the control and had no further treatment. All teeth were then restored with amalgam. An Instron testing machine was used to apply a controlled force to the beveled amalgam at a crosshead speed of 2 mm/mitt until fracture occurred. A peripheral shelf did not improve the fracture resistance of the coronalradicular restorations tested. The specimens with semihorizontal pins were significantly stronger than specimens from the other groups. Placement of pins into the pulp chamber is recommended when adequate dentin remains. (J PROSTHET DENT 1991;65:470-4.)
S-
The opinions expressed herein are those of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. *Major, U.S. Air Force, DC; Resident, Department of General Dentistry. **Colonel, U.S. Air Force, DC; Chief of Research and Dental Materials, Department of General Dentistry.
10/l/24047
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mce its introduction by Nayyar et al.,l the amalgam coronal-radicular restoration has been compared in vitro with its clinical alternatives: pin-retained amalgams,2-5prefabricated post-retained amalgams,5-gand cast posts and cores.3p4*6*s* lo In addition, the effect of providing a 1 to 2 mm ferrule on the tooth structure and placing a cast crown over these restorations has been studied.436*8,lo
APRIL1991
VOLUME66
NUMBER4
AMALGAM
CORONAL-RADICULAR
RESTORATIONS
Pulp chamber height 4
CEJ
Lingual
Facial
Group 1
Group 2
Group 3
Fig. 1. Diagram of teeth groups restored with amalgam. Group 1, Control; Group 2, Minim pins; GFOU~ 3, peripheral shelf.
Typically, these studies used mandibular molars with the coronal portion of the tooth removed 1 to 2 mm coronal to the cementoenamel junction (CEJ). The pulp chamber depth was measured from the highest point on the floor of the pulp chamber to the most occlusal portion of the remaining tooth structure. These molars were then mounted in either acrylic resin or stone to a level 1 to 2 mm apical to the CEJ. After the restorations were placed, they were loaded to failure with the load applied at a 45degree angle to the long axis of the tooth against the facial surface. Since the introduction of amalgam coronal-radicular restorations, little research has been done on optimizing their resistance form. One recent investigationli studied the effect of pulp chamber height and extension of amalgam into the root canal space. Pulp chamber heights of 2, 4, and 6 mm were studied both with and without extension of amalgam into the root canal space. The results showed that extension of amalgam into the root canal space increased the fracture resistance only when the pulp chamber height was less than 4 mm. Surprisingly, the 2 mm pulp chamber height groups, which had the least remaining tooth structure, had higher fracture resistance than the 4 and 6 mm groups. Two thirds of the specimens fractured through tooth structure. It was speculated that because of the longer pulp chamber walls in the 4 and 6 mm groups a longer lever arm was present to cause fracture of the tooth at lower loads than for the 2 mm group. Thus it would seem desirable to reinforce these teeth. Markley12 first suggested the use of pins cemented into dentin and splinted together by amalgam to strengthen teeth in 1958, and other investigatorsi l4 have demonstrated the reinforcing effect of horizontal threaded pins linking weakened portions of preparations to the rest of the restoration when amalgam is condensed around them. While no studies have investigated the use of horizontal
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pins with amalgam coronal-radicular restorations, several studies have used vertical pins.2-5 This investigation determined the effect of a 1.4 mm wide by 2 mm deep peripheral shelf or of four TMS Minim pins (Whaledent International, New York, N-Y.) placed at 45 degrees in the walls of the pulp chamber, on the fracture resistance of amalgam coronal-radicular restorations with a 4 mm pulp chamber height and no extension of amalgam into the root canal space. Force was applied at a 45-degree angle to the long axis of the tooth against the facial surface of the restorations to simulate a working-side contact.
MATERIAL
AND METHODS
Thirty unrestored extracted human mandibular molars free of cracks and caries were collected and were stored in water. Throughout preparation, the teeth were either stored in water or held in wet gauze to prevent dehydration. Care was taken during endodontic access to remove the entire roof of the pulp chamber and to establish a straight line accessfor each canal. However, no effort was made to remove all undercuts from the pulp chamber as in preparation for a cast post and core. After a complete pulpectomy, the coronal portion of the canals was enlarged to the size of a No. 3 Gates-Glidden drill (L.D. Caulk, Milford, Del.). Using warm gut&a-percha, the coronal portion of all canals was obturated and condensed. Following obturation of the root canal orifice, the coronal portion of the tooth was ground perpendicular to the long axis of the tooth until the height of the pulp chamber was 6 mm, as measured from the highest point on the floor of the pulp chamber to the most coronal portion of the preparation. Using a dental surveyor, the teeth were mounted in acrylic resin (L.D. Caulk Division, Dentsply International, Inc.) to a level 2 mm apical to the CEJ measured at the mesiofacial line angle. To equalize the mean facial-lingual width of the teeth in 471
KANE
AND
BURGESS
Table I. Results Sample No.
Facial-lingual
Group l-Control 1 4 5 10 13 14 15 20 25 21 Mean SM deviation
width
(mm)
10.0 10.5 12.0 10.0 11.5 10.5 11.0 11.0 10.0
10.5 10.70 0.67
(load (N)
Fracture
2090 2050 2560 1690 2120 1925 1640 2095 2595 &o 2096 312
Tooth Tooth Amalgam Amalgam Tooth Tooth Amalgam Amalgam Amalgam Amalgam
1840 3200
Tooth Amalgam Tooth Tooth Tooth Amalgam Tooth Tooth Amalgam Amalgam
Group 2-4 TMS Minim pins at 45 degrees 10.0 6 7 11.0 9 10.5 12 11.5 16 10.0 18 10.5 19 11.0 24 10.5 26 10.0 30 11.5 Mean 10.65 Std deviation 0.58
2790 2590 2605 2320 2150 2550 &5 2585 418
Group 3-Peripheral 2 3 8 11 17 21 22 23 23 29 Mean Std deviation
2205 2270 1580 2240 1450 2150 1460 1730 2040 2780 1991 427
through
shelf 10.5 11.0 10.0 10.5 11.0 10.5 10.5 10.0 11.0 11.5 10.65 0.47
each experimental group, the maximum facial-lingual width of each tooth was recorded to the nearest 0.5 mm. The teeth were then ranked from smallest to largest based on the measured facial-lingual width. The smallest tooth and the largest tooth were taken from the ranking and were placed in experimental group 1; then the second smallest tooth and the second largest tooth were placed in experimental group 2. This process was then repeated until each of the three groups had 10 teeth each. The resulting mean facial-lingual width of all three groups was recorded. A one-way analysis of variance (ANOVA) showed no statistically significant difference between the mean facial-
472
Fracture
Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined
lingual width of the three experimental groups (F = 0.025; df = 2.27; p > 0.05). Preparation of the teeth is summarized in Fig. 1. The coronal portion of all teeth was reduced until the pulp chamber height was 4 mm. The teeth in group 1 (control) required no further preparation. Two TMS Minim pins were placed in the facial walls and two in the lingual walls of the pulp chamber of the teeth in group 2 (45-degree pin group). A % round bur was used to place starting holes a minimum of 2 mm apart and a sufficient distance from the mesial and distal pulp chamber walls to allow condensation of amalgam around the pins. While pins are ideally placed
APRIL
lSS1
VOLUME
65
NUMBER
4
AMALGAM
CORONAL-RADICULAR
RESTORATIONS
5 mm aparti to reduce crazing, only 2 mm spacing was possible. Pin holes were drilled to a depth of 2 mm at approximately 45 degrees to the long axis of the tooth using a 2 mm self-limiting drill with a pumping action to clean debris. Each pin drill was used for only two teeth (eight holes), which is well within the recommended 20-hole limit.i6 The pin holes were placed 1 mm or more from the occlusal edge of the pulp chamber wall to avoid fracture of dentin. The pins were placed with a hand wrench,17 stabilized with cotton pliers, and sectioned with a ‘/4 round bur at a length of 2 mm. A peripheral shelf 2 mm deep and 1.4 mm wide was prepared on each tooth in group 3 (peripheral shelf group) using a No. 58 high-speed bur followed by a No. 59 bur (1.4 mm diameter) in a low-speed handpiece mounted on a dental surveyor. An acrylic resin stop was used to limit the shelf depth to 2 mm. Copper bands were tightly adapted and reinforced with modeling compound. Precapsulated, two-spill, regular set Dispersalloy (Johnson & Johnson, East Windsor, N.J.) was triturated according to the manufacturer’s specification using a recently calibrated Vari-Mix II amalgamator (L.D. Caulk). The restorations were condensed using a mechanical condenser (Condensaire, Teledyne Densco, Denver, Colo.) to enhance uniformity. Amalgam was condensed to the top of the copper bands, which represented an overpack of approximately 2 to 3 mm. The bands were left in place for 1 hour and were then removed. Any overhangs were carefully removed with a diamond bur. The height of the restorations was then reduced to 7.5 mm above the CEJ (the mean height of a human mandibular first molar.ls This height allowed an average of 5.5 mm of amalgam coronal to tooth structure. Using a handpiece attached to a dental surveyor and a fixture that positioned the mounted tooth at 45 degrees, a 45-degree bevel was placed along the facial-occlusal line angle. An Instron testing machine (Instron Corp., Canton, Mass.) was used to direct a controlled force against the 45degree bevel at a crosshead speed of 2 mm/min until failure occurred. The fracture load was determined by a sudden drop in load magnitude as recorded by the Instron machine.
RESULTS Fracture loads before failure are listed in Table I with mean values and standard deviations. Following verification of homogeneity of variance (Cochran C test, p > 0.05), the mean failure loads for the three groups were analyzed using a one-way analysis of variance. A highly significant difference was found between the groups (F = 6.63; df = 2.27; p < 0.005). This significant difference was then further analyzed using Duncan’s multiple range test to order the means. The results are expressed graphically in Table II. The nature of the failure, either through tooth or through amalgam, is listed in Table I for all specimens in groups 1
THE
JOURNAL
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DENTISTRY
Table II. Duncan’s multiple range test results Group
Mean (N)
2-Minim pins l-Control 3-Peripheral shelf [
2585 2096 1991
Groups connected by a bracket are not statistically significantly different at the 0.05 level.
and 2. However, for group 3 this determination could not be made, as amalgam over the peripheral shelf on the facial surface frequently fractured away at the same time the tooth fractured. For groups 1 and 2, the number of times specimens fractured through tooth versus the number of times specimens fractured through amalgam was compared using Fisher’s exact test. The result (p > 0.05) showed there was no statistically signif?cant difference in the number of fractures through tooth versus the number of fractures through amalgam between groups 1 and 2. The mean fracture load for group 1 (mean = 2096 + 312 N) was also compared with that for a similarly prepared group (group 5) (mean = 1303 + 357 N) previously publishedll using an independent Student’s t test. The results showed a highly significant difference (t = 5.28, df = + 18, p < 0.0001). Also, no statistically significant difference was found in the number of teeth fracturing through tooth versus through amalgam for these two groups (Fisher’s exact test, p > 0.05).
DISCUSSION All specimens had a pulp chamber height of 4 mm. A previous study’l found that when 4 mm or more of pulp chamber height is present, extension of amalgam into the root canal space is not beneficial. As a result, none of the specimens had amalgam extended into the root canal space. They are, however, still referred to as amalgam coronal-radicular restorations, since they are more similar to this type of restoration than to any other in spite of the fact that amalgam was not extended into the root canal space. The exact reason for the higher mean fracture load for our group 1 versus that of group 5 from a previous studyl’ is unclear. The only difference in preparation was that the teeth used in the previous study were endodontically treated, resulting in a longer time out of water during hand instrumentation. Perhaps these teeth became dehydrated in spite of being held in saline-soaked gauze, whereas teeth used in this study only had the coronal portion of the canal mechanically enlarged. In addition, this study was completed over a shorter period of time than the previous study (1 versus 9 months). Although in both instances every effort was made to keep the teeth hydrated, the time after extraction may be significant. Group 3, with a peripheral shelf, had no higher fracture load than the control group. Consistent with this finding is
473
RANE
the observation that the amalgam restoring the facial peripheral shelf frequently fractured simultaneously with the lingual tooth structure. Thus this additional resistance form did not improve the fracture resistance. In addition, placing a shelf offers several disadvantages. First, placing a peripheral shelf requires removal of tooth structure, and second, its narrow width makes condensation of amalgam more difficult. For these reasons, placing a peripheral shelf cannot be recommended. Analysis of fracture type (through tooth versus through amalgam) showed no statistically significant difference in fracture type frequencies between groups 1 and 2. Group 2, with 4 Minim pins placed at 45degree angles in the walls of the pulp chamber, did have a statistically significantly higher mean fracture resistance than the control group. Although placement of pins in the pulp chamber appears to reinforce the tooth structure, there is a danger of perforation when the remaining tooth structure is minimal. In this study, pins were placed at 45 degrees to a depth of 2 mm, which results in a 1 mm penetration of pin when a measurement is made perpendicular to the wall of the pulp chamber rather than along the path of the pin. In spite of this, in specimen No. 6 both of the pins placed in the lingual pulp chamber wall entered the enamel and nearly perforated it. This specimen had the lowest fracture strength in the group and fractured along the length of the pins in the lingual pulp chamber wall. Because of this example and the well-known crazing caused by pins,16*ls they should only be placed when sufhcient dentin remains to avoid perforation and minimize crazing. Use of smaller size pins or fewer pins, which may also minimize the deleterious effects of pins, was not investigated.
AND
BURGESS
toration tested was increased. Placement of pins in the pulp chamber is recommended when sufficient dentin remains to allow placement without perforation or significant crazing. REFERENCES 1. Nayyer A, Walton RE, Leonard LA. An amalgam coronal-radicular dowel and core technique for endodontically treated posterior teeth. J hO6THRT DENT 1980;43:611-5. 2. Michelich R, Dillard T. Navvar A. Mechanical .uronerties of amaleam buildups for endodonticaliy- treated molars [Abstract]. J Dent Res 1980;59:381. 3. Michelich R, Nayyar A. Leonard L. Mechanical properties of amalgam core buildups for endodontically treated premolars [Abstract]. J Dent Res 1982;61:186. 4, Nayyar A, McDonald TR, Turner RF, Koth DL. Strength of premolar coronal radicular buildups restored with cast crowns [Abstract]. J Dent Res 1982;61:186. 5. Merts KA, Parker MW, Pelleu GB. Shear strength of two coronal radicular amalgams and a pin-retained amalgam [Abstract]. J Dent Res 1987;66:289. 6. Gelfand M. Goldman M, Sunderman EJ. Effect of complete veneer crowns on the compressive strength of endodontically treated posterior teeth. J PROSTHRT DENT 1984,52:635-8. I. Christian GW, Button GL, Moon PC, England MC, Douglas HB. Post core restoration in endodontically treated posterior teeth. J Endo 1981;7:182-5. 8. Kern SB, von Fraunhofer JA, Mueninghoff LA. An in vitro comparison of two dowel and core techniques for endodontically treated molars. J PR~~TH~T DENT 1984;51:509-14. 9. Plasmans PJJM, Viiseren LGH, Vrijhoef MMA, Kayser AF. In vitro
10. 11. 12. 13.
comparison of dowel and core techniques for endodontically treated molars. J Endo 1986:12:382-7. Hoag EP, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHETDENT 1982;47:177-81. Kane JJ, Burgess JO, Summitt JB. Fracture resistance of amalgam coronal-radicular restoration. J PROSTHEF DENT 1990;63:807-13. Markley MR. Pin reinforcement and retention of amalgam foundations and restorations. J Am Dent Assoc 195&56:675-S. Burgess JO. Horizontal pins: study of tooth reinforcement. J PROSTHET DENT 1985;53:317-22.
SUMMARY
AND CONCLUSIONS
The effect onfracture resistance of the addition of either a peripheral shelf or of four TMS Minim pins placed at a 45-degree angle on amalgam coronal-radicular restorations with a 4 mm pulp chamber height and no amalgam extended into the root was investigated. The following conclusions were drawn. 1. A peripheral shelf2 mm deep and 1.4 mm wide did not improve the fracture resistance of the amalgam coronalradicular restorations tested, and thus cannot be recommended. 2. When four Minim pins were placed in the facial and lingual wall of the pulp chamber at 45 degrees, the mean fracture resistance of the amalgam coronal-radicular res-
474
14. Lambert RL, Robinson FB, Lindemuth JS. Coronal reinforcement with cross-splinted pin-amalgam restorations. J PROSTHETDENT 1985; 54:348-s.
15. Khera SC, Chan KC, Rittman BR. Dentinal crazing and interpin distance. J PROSTHEXDENT 1978;40:538-43. 16. Standlee JP, Collard EW, Caputo AA. Dentinal defects caused by some twist drills and retentive pins. J PROSTHEY DENT 1970;24:185-92. 17. Newitter DA, Schlissel ER. Evaluation of four instruments for inserting self threading pins. Oper Dent 1980,5:142-5. 18. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders Co, 1974268, 284. 19. Durkowski JS, Pelleu GB, Harris RK, Harper RH. Effect of diameters of self threading pins and channel locations on enamel crazing. Oper Dent 1982;7:86-91. Reprint
requests
to:
DR JOHN 0. BURGESS 3118 WHISPER BRINK SAN ANTONIO, TX 78230
APRIL
1001
VOLUME
65
NUMBER
4
7. Make the final impression, ensuring that the pins are withdrawn in the impression, which is poured in die stone. 8. Incorporate the pins in the wax pattern and make the fixed partial denture in the usual manner. 9. Lute the final restoration in position on the abutment teeth in the usual manner (Fig. 3). REFERENCES 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth using acid-etch technique and a cast metal framework. J PROSTHET DENT 1973:30:418-23.
2. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982:47:52-K 3. Moon PC. Bond strength of the lost salt procedure: a new retention method for resin-bonded fixed prostheses. J PROSTHET DENT 1987: 57:435-39.
Fig.
3. Final restoration luted to abutment teeth.
Reprint requests to: DR. LOREN C. CHRISTENSEN 9314 HILLTOP CT. LAUREL, MD 20708-3218
6. Establish a retentive button on the plastic pin with a hot instrument. Seat the pin in the hole, and add impression adhesive to the button (Fig. 2).
Modification of the resistance coronal-radicular restorations
form of amalgam
James J. Kane, D.M.D.,* and John 0. Burgess, D.D.S., M.S.** Wilford Hall USAF Medical Center, Lackland Air Force Base, San Antonio, Texas Three groups of 10 extracted endodontically treated mandibular molars were mounted in acrylic resin. The occlusal surface was reduced until a 4 mm pulp chamber height remained. A peripheral shelf 2 mm deep and 1.4 mm wide was placed on one group, while four TMS Minim pins were placed 45 degrees to the long axis of the tooth into the facial and lingual walls of the second group. The final group served as the control and had no further treatment. All teeth were then restored with amalgam. An Instron testing machine was used to apply a controlled force to the beveled amalgam at a crosshead speed of 2 mm/mitt until fracture occurred. A peripheral shelf did not improve the fracture resistance of the coronalradicular restorations tested. The specimens with semihorizontal pins were significantly stronger than specimens from the other groups. Placement of pins into the pulp chamber is recommended when adequate dentin remains. (J PROSTHET DENT 1991;65:470-4.)
S-
The opinions expressed herein are those of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. *Major, U.S. Air Force, DC; Resident, Department of General Dentistry. **Colonel, U.S. Air Force, DC; Chief of Research and Dental Materials, Department of General Dentistry.
10/l/24047
470
mce its introduction by Nayyar et al.,l the amalgam coronal-radicular restoration has been compared in vitro with its clinical alternatives: pin-retained amalgams,2-5prefabricated post-retained amalgams,5-gand cast posts and cores.3p4*6*s* lo In addition, the effect of providing a 1 to 2 mm ferrule on the tooth structure and placing a cast crown over these restorations has been studied.436*8,lo
APRIL1991
VOLUME66
NUMBER4
AMALGAM
CORONAL-RADICULAR
RESTORATIONS
Pulp chamber height 4
CEJ
Lingual
Facial
Group 1
Group 2
Group 3
Fig. 1. Diagram of teeth groups restored with amalgam. Group 1, Control; Group 2, Minim pins; GFOU~ 3, peripheral shelf.
Typically, these studies used mandibular molars with the coronal portion of the tooth removed 1 to 2 mm coronal to the cementoenamel junction (CEJ). The pulp chamber depth was measured from the highest point on the floor of the pulp chamber to the most occlusal portion of the remaining tooth structure. These molars were then mounted in either acrylic resin or stone to a level 1 to 2 mm apical to the CEJ. After the restorations were placed, they were loaded to failure with the load applied at a 45degree angle to the long axis of the tooth against the facial surface. Since the introduction of amalgam coronal-radicular restorations, little research has been done on optimizing their resistance form. One recent investigationli studied the effect of pulp chamber height and extension of amalgam into the root canal space. Pulp chamber heights of 2, 4, and 6 mm were studied both with and without extension of amalgam into the root canal space. The results showed that extension of amalgam into the root canal space increased the fracture resistance only when the pulp chamber height was less than 4 mm. Surprisingly, the 2 mm pulp chamber height groups, which had the least remaining tooth structure, had higher fracture resistance than the 4 and 6 mm groups. Two thirds of the specimens fractured through tooth structure. It was speculated that because of the longer pulp chamber walls in the 4 and 6 mm groups a longer lever arm was present to cause fracture of the tooth at lower loads than for the 2 mm group. Thus it would seem desirable to reinforce these teeth. Markley12 first suggested the use of pins cemented into dentin and splinted together by amalgam to strengthen teeth in 1958, and other investigatorsi l4 have demonstrated the reinforcing effect of horizontal threaded pins linking weakened portions of preparations to the rest of the restoration when amalgam is condensed around them. While no studies have investigated the use of horizontal
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pins with amalgam coronal-radicular restorations, several studies have used vertical pins.2-5 This investigation determined the effect of a 1.4 mm wide by 2 mm deep peripheral shelf or of four TMS Minim pins (Whaledent International, New York, N-Y.) placed at 45 degrees in the walls of the pulp chamber, on the fracture resistance of amalgam coronal-radicular restorations with a 4 mm pulp chamber height and no extension of amalgam into the root canal space. Force was applied at a 45-degree angle to the long axis of the tooth against the facial surface of the restorations to simulate a working-side contact.
MATERIAL
AND METHODS
Thirty unrestored extracted human mandibular molars free of cracks and caries were collected and were stored in water. Throughout preparation, the teeth were either stored in water or held in wet gauze to prevent dehydration. Care was taken during endodontic access to remove the entire roof of the pulp chamber and to establish a straight line accessfor each canal. However, no effort was made to remove all undercuts from the pulp chamber as in preparation for a cast post and core. After a complete pulpectomy, the coronal portion of the canals was enlarged to the size of a No. 3 Gates-Glidden drill (L.D. Caulk, Milford, Del.). Using warm gut&a-percha, the coronal portion of all canals was obturated and condensed. Following obturation of the root canal orifice, the coronal portion of the tooth was ground perpendicular to the long axis of the tooth until the height of the pulp chamber was 6 mm, as measured from the highest point on the floor of the pulp chamber to the most coronal portion of the preparation. Using a dental surveyor, the teeth were mounted in acrylic resin (L.D. Caulk Division, Dentsply International, Inc.) to a level 2 mm apical to the CEJ measured at the mesiofacial line angle. To equalize the mean facial-lingual width of the teeth in 471
KANE
AND
BURGESS
Table I. Results Sample No.
Facial-lingual
Group l-Control 1 4 5 10 13 14 15 20 25 21 Mean SM deviation
width
(mm)
10.0 10.5 12.0 10.0 11.5 10.5 11.0 11.0 10.0
10.5 10.70 0.67
(load (N)
Fracture
2090 2050 2560 1690 2120 1925 1640 2095 2595 &o 2096 312
Tooth Tooth Amalgam Amalgam Tooth Tooth Amalgam Amalgam Amalgam Amalgam
1840 3200
Tooth Amalgam Tooth Tooth Tooth Amalgam Tooth Tooth Amalgam Amalgam
Group 2-4 TMS Minim pins at 45 degrees 10.0 6 7 11.0 9 10.5 12 11.5 16 10.0 18 10.5 19 11.0 24 10.5 26 10.0 30 11.5 Mean 10.65 Std deviation 0.58
2790 2590 2605 2320 2150 2550 &5 2585 418
Group 3-Peripheral 2 3 8 11 17 21 22 23 23 29 Mean Std deviation
2205 2270 1580 2240 1450 2150 1460 1730 2040 2780 1991 427
through
shelf 10.5 11.0 10.0 10.5 11.0 10.5 10.5 10.0 11.0 11.5 10.65 0.47
each experimental group, the maximum facial-lingual width of each tooth was recorded to the nearest 0.5 mm. The teeth were then ranked from smallest to largest based on the measured facial-lingual width. The smallest tooth and the largest tooth were taken from the ranking and were placed in experimental group 1; then the second smallest tooth and the second largest tooth were placed in experimental group 2. This process was then repeated until each of the three groups had 10 teeth each. The resulting mean facial-lingual width of all three groups was recorded. A one-way analysis of variance (ANOVA) showed no statistically significant difference between the mean facial-
472
Fracture
Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined Undetermined
lingual width of the three experimental groups (F = 0.025; df = 2.27; p > 0.05). Preparation of the teeth is summarized in Fig. 1. The coronal portion of all teeth was reduced until the pulp chamber height was 4 mm. The teeth in group 1 (control) required no further preparation. Two TMS Minim pins were placed in the facial walls and two in the lingual walls of the pulp chamber of the teeth in group 2 (45-degree pin group). A % round bur was used to place starting holes a minimum of 2 mm apart and a sufficient distance from the mesial and distal pulp chamber walls to allow condensation of amalgam around the pins. While pins are ideally placed
APRIL
lSS1
VOLUME
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NUMBER
4
AMALGAM
CORONAL-RADICULAR
RESTORATIONS
5 mm aparti to reduce crazing, only 2 mm spacing was possible. Pin holes were drilled to a depth of 2 mm at approximately 45 degrees to the long axis of the tooth using a 2 mm self-limiting drill with a pumping action to clean debris. Each pin drill was used for only two teeth (eight holes), which is well within the recommended 20-hole limit.i6 The pin holes were placed 1 mm or more from the occlusal edge of the pulp chamber wall to avoid fracture of dentin. The pins were placed with a hand wrench,17 stabilized with cotton pliers, and sectioned with a ‘/4 round bur at a length of 2 mm. A peripheral shelf 2 mm deep and 1.4 mm wide was prepared on each tooth in group 3 (peripheral shelf group) using a No. 58 high-speed bur followed by a No. 59 bur (1.4 mm diameter) in a low-speed handpiece mounted on a dental surveyor. An acrylic resin stop was used to limit the shelf depth to 2 mm. Copper bands were tightly adapted and reinforced with modeling compound. Precapsulated, two-spill, regular set Dispersalloy (Johnson & Johnson, East Windsor, N.J.) was triturated according to the manufacturer’s specification using a recently calibrated Vari-Mix II amalgamator (L.D. Caulk). The restorations were condensed using a mechanical condenser (Condensaire, Teledyne Densco, Denver, Colo.) to enhance uniformity. Amalgam was condensed to the top of the copper bands, which represented an overpack of approximately 2 to 3 mm. The bands were left in place for 1 hour and were then removed. Any overhangs were carefully removed with a diamond bur. The height of the restorations was then reduced to 7.5 mm above the CEJ (the mean height of a human mandibular first molar.ls This height allowed an average of 5.5 mm of amalgam coronal to tooth structure. Using a handpiece attached to a dental surveyor and a fixture that positioned the mounted tooth at 45 degrees, a 45-degree bevel was placed along the facial-occlusal line angle. An Instron testing machine (Instron Corp., Canton, Mass.) was used to direct a controlled force against the 45degree bevel at a crosshead speed of 2 mm/min until failure occurred. The fracture load was determined by a sudden drop in load magnitude as recorded by the Instron machine.
RESULTS Fracture loads before failure are listed in Table I with mean values and standard deviations. Following verification of homogeneity of variance (Cochran C test, p > 0.05), the mean failure loads for the three groups were analyzed using a one-way analysis of variance. A highly significant difference was found between the groups (F = 6.63; df = 2.27; p < 0.005). This significant difference was then further analyzed using Duncan’s multiple range test to order the means. The results are expressed graphically in Table II. The nature of the failure, either through tooth or through amalgam, is listed in Table I for all specimens in groups 1
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DENTISTRY
Table II. Duncan’s multiple range test results Group
Mean (N)
2-Minim pins l-Control 3-Peripheral shelf [
2585 2096 1991
Groups connected by a bracket are not statistically significantly different at the 0.05 level.
and 2. However, for group 3 this determination could not be made, as amalgam over the peripheral shelf on the facial surface frequently fractured away at the same time the tooth fractured. For groups 1 and 2, the number of times specimens fractured through tooth versus the number of times specimens fractured through amalgam was compared using Fisher’s exact test. The result (p > 0.05) showed there was no statistically signif?cant difference in the number of fractures through tooth versus the number of fractures through amalgam between groups 1 and 2. The mean fracture load for group 1 (mean = 2096 + 312 N) was also compared with that for a similarly prepared group (group 5) (mean = 1303 + 357 N) previously publishedll using an independent Student’s t test. The results showed a highly significant difference (t = 5.28, df = + 18, p < 0.0001). Also, no statistically significant difference was found in the number of teeth fracturing through tooth versus through amalgam for these two groups (Fisher’s exact test, p > 0.05).
DISCUSSION All specimens had a pulp chamber height of 4 mm. A previous study’l found that when 4 mm or more of pulp chamber height is present, extension of amalgam into the root canal space is not beneficial. As a result, none of the specimens had amalgam extended into the root canal space. They are, however, still referred to as amalgam coronal-radicular restorations, since they are more similar to this type of restoration than to any other in spite of the fact that amalgam was not extended into the root canal space. The exact reason for the higher mean fracture load for our group 1 versus that of group 5 from a previous studyl’ is unclear. The only difference in preparation was that the teeth used in the previous study were endodontically treated, resulting in a longer time out of water during hand instrumentation. Perhaps these teeth became dehydrated in spite of being held in saline-soaked gauze, whereas teeth used in this study only had the coronal portion of the canal mechanically enlarged. In addition, this study was completed over a shorter period of time than the previous study (1 versus 9 months). Although in both instances every effort was made to keep the teeth hydrated, the time after extraction may be significant. Group 3, with a peripheral shelf, had no higher fracture load than the control group. Consistent with this finding is
473
RANE
the observation that the amalgam restoring the facial peripheral shelf frequently fractured simultaneously with the lingual tooth structure. Thus this additional resistance form did not improve the fracture resistance. In addition, placing a shelf offers several disadvantages. First, placing a peripheral shelf requires removal of tooth structure, and second, its narrow width makes condensation of amalgam more difficult. For these reasons, placing a peripheral shelf cannot be recommended. Analysis of fracture type (through tooth versus through amalgam) showed no statistically significant difference in fracture type frequencies between groups 1 and 2. Group 2, with 4 Minim pins placed at 45degree angles in the walls of the pulp chamber, did have a statistically significantly higher mean fracture resistance than the control group. Although placement of pins in the pulp chamber appears to reinforce the tooth structure, there is a danger of perforation when the remaining tooth structure is minimal. In this study, pins were placed at 45 degrees to a depth of 2 mm, which results in a 1 mm penetration of pin when a measurement is made perpendicular to the wall of the pulp chamber rather than along the path of the pin. In spite of this, in specimen No. 6 both of the pins placed in the lingual pulp chamber wall entered the enamel and nearly perforated it. This specimen had the lowest fracture strength in the group and fractured along the length of the pins in the lingual pulp chamber wall. Because of this example and the well-known crazing caused by pins,16*ls they should only be placed when sufhcient dentin remains to avoid perforation and minimize crazing. Use of smaller size pins or fewer pins, which may also minimize the deleterious effects of pins, was not investigated.
AND
BURGESS
toration tested was increased. Placement of pins in the pulp chamber is recommended when sufficient dentin remains to allow placement without perforation or significant crazing. REFERENCES 1. Nayyer A, Walton RE, Leonard LA. An amalgam coronal-radicular dowel and core technique for endodontically treated posterior teeth. J hO6THRT DENT 1980;43:611-5. 2. Michelich R, Dillard T. Navvar A. Mechanical .uronerties of amaleam buildups for endodonticaliy- treated molars [Abstract]. J Dent Res 1980;59:381. 3. Michelich R, Nayyar A. Leonard L. Mechanical properties of amalgam core buildups for endodontically treated premolars [Abstract]. J Dent Res 1982;61:186. 4, Nayyar A, McDonald TR, Turner RF, Koth DL. Strength of premolar coronal radicular buildups restored with cast crowns [Abstract]. J Dent Res 1982;61:186. 5. Merts KA, Parker MW, Pelleu GB. Shear strength of two coronal radicular amalgams and a pin-retained amalgam [Abstract]. J Dent Res 1987;66:289. 6. Gelfand M. Goldman M, Sunderman EJ. Effect of complete veneer crowns on the compressive strength of endodontically treated posterior teeth. J PROSTHRT DENT 1984,52:635-8. I. Christian GW, Button GL, Moon PC, England MC, Douglas HB. Post core restoration in endodontically treated posterior teeth. J Endo 1981;7:182-5. 8. Kern SB, von Fraunhofer JA, Mueninghoff LA. An in vitro comparison of two dowel and core techniques for endodontically treated molars. J PR~~TH~T DENT 1984;51:509-14. 9. Plasmans PJJM, Viiseren LGH, Vrijhoef MMA, Kayser AF. In vitro
10. 11. 12. 13.
comparison of dowel and core techniques for endodontically treated molars. J Endo 1986:12:382-7. Hoag EP, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHETDENT 1982;47:177-81. Kane JJ, Burgess JO, Summitt JB. Fracture resistance of amalgam coronal-radicular restoration. J PROSTHEF DENT 1990;63:807-13. Markley MR. Pin reinforcement and retention of amalgam foundations and restorations. J Am Dent Assoc 195&56:675-S. Burgess JO. Horizontal pins: study of tooth reinforcement. J PROSTHET DENT 1985;53:317-22.
SUMMARY
AND CONCLUSIONS
The effect onfracture resistance of the addition of either a peripheral shelf or of four TMS Minim pins placed at a 45-degree angle on amalgam coronal-radicular restorations with a 4 mm pulp chamber height and no amalgam extended into the root was investigated. The following conclusions were drawn. 1. A peripheral shelf2 mm deep and 1.4 mm wide did not improve the fracture resistance of the amalgam coronalradicular restorations tested, and thus cannot be recommended. 2. When four Minim pins were placed in the facial and lingual wall of the pulp chamber at 45 degrees, the mean fracture resistance of the amalgam coronal-radicular res-
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14. Lambert RL, Robinson FB, Lindemuth JS. Coronal reinforcement with cross-splinted pin-amalgam restorations. J PROSTHETDENT 1985; 54:348-s.
15. Khera SC, Chan KC, Rittman BR. Dentinal crazing and interpin distance. J PROSTHEXDENT 1978;40:538-43. 16. Standlee JP, Collard EW, Caputo AA. Dentinal defects caused by some twist drills and retentive pins. J PROSTHEY DENT 1970;24:185-92. 17. Newitter DA, Schlissel ER. Evaluation of four instruments for inserting self threading pins. Oper Dent 1980,5:142-5. 18. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders Co, 1974268, 284. 19. Durkowski JS, Pelleu GB, Harris RK, Harper RH. Effect of diameters of self threading pins and channel locations on enamel crazing. Oper Dent 1982;7:86-91. Reprint
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