Fracture resistance restorations
James J. Kane, D.M.D.,* John 0. Burgess, James B. Summitt, D.D.S.***
Wilford Hall USAF Medical Center, Lackland Air Force Base, San Antonio,
The effect of pulp chamber depth and extension into the root canal space on fracture resistance was examined on endodontically treated teeth with coronalradicular amalgam restorations. Six groups of 10 mandibular molars were mounted in acrylic resin, and crowns were ground apically until the wall height of the pulp chamber was 2,4, or 6 mm. Three millimeters of gutta-percha was removed from the three canals of one-half the teeth and amalgam was condensed into the canal space to a height 7.5 mm above the cementoenamel junction (CEJ). The remaining teeth had amalgam condensed from the floor of the chamber to 7.6 mm above the CEJ. The amalgam restorations were loaded with an lnstron instrument (Instron Corp., Canton, Mass.) until failure. Amalgam extension into the root canal space contributed minimally to the fracture resistance of the amalgam coronal-radicular restoration with four or more millimeters of chamber wall. If less than 4 mm of chamber wall height remained, however, the fracture load was substantially increased. Amalgam extension into the root canal space should be confined to teeth with limited remaining pulp chambers. (J PROSTHET DENT 1990;63:607-13.)
oronal-radicular stabilization of endodontically treated posterior teeth can be accomplished by condensing amalgam into the pulp chamber and 2 to 4 mm into the root canal space. Using this technique, Nayyar et al.’ reported 400 successful restorations when cast crowns were placed as the final restoration for posterior pulpless molars. Since the introduction of amalgam coronal-radicular restorations, they have been compared in vitro to clinical alternatives: pin-retained arr~algams,~-~ prefabricated postretained amalgams,5-g and cast posts and cores.3p4*61g*lo The effect of placing a 1 to 2 mm ferrule on tooth structure gingival to the amalgam core and placing a casting over these restorations has been reviewed.4* 6 8*lo These studies commonly used mandibular molars with the coronal portion removed 1 to 2 mm from the cementoenamel junction (CEJ). The molars were mounted in
Opinions expressed herein, unless otherwise specifically indicated, are those of the authors. They do not purport to express views of the Department of the Air Force or any other Department or Agency of the United States Government. *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. ***Colonel, U.S. Air Force (DC); Chairman, Department of General Dentistry. 10/l/19474
TEE JOURNAL OFPROSTHETIC DENTISTRY
acrylic resin or stone 1 to 2 mm apical to the CEX After placing restorations, a load to failure was applied to the facial surface at a 45-degree angle to the long axis of the tooth. Christian et al7 stated that the average remaining pulp chamber height was 3 to 4 mm when the coronal portion of mandibular molars was removed 1 to 2 mm coronal to the CEJ. The average pulp chamber height in these studies was 3 to 4 mm, with approximately 2 mm of pulp chamber height apical to the CEJ. Amalgam was usually condensed 2 to 4 mm into the root canals. In similar in vitro studies with the use of molars or premolars,293 fracture strengths of amalgam coronal-radicular restorations were compared with restorations with four-threaded pins. In both studies the pulp chamber was filled with a cement base for the pin-retained restorations. The results disclosed no statistically significant difference between the restorations2 and also disclosed that the amalgam coronal-radicular restorations required a substantially larger force to cause failure.3 The investigation of Mertz et al5 compared amalgam coronal-radicular restorations with threaded pins, but amalgam was condensed into the pulp chamber for both restorations. There was no statistically significant difference between the amalgam coronal-radicular restorations and pin-retained restorations. These three studies supported the use of amalgam coronal-radicular restorations as a substitute for pinretained amalgam restorations. Conflicting results were obtained when comparing amal-
CROWN HEIGHT 7.5mm CEJ X4 CC
DISTAL ROOT - 2mm
CC - 2mm
Cc - 2mm
1. Tooth preparation.
gam coronal-radicular restorations with post-retained amalgam restorations. Three investigation& 6pg revealed no statistically significant difference between amalgam coronal-radicular restorations and post-retained amalgam restorations. Christian et al.’ directed forces at 90 degrees to the long axis of the tooth and recorded a mean fracture force for post-retained amalgam restorations of 142 pounds compared with 123 pounds for amalgam coronal-radicular restorations. Kern et al.* directed forces at 60 degrees to the long axis of the tooth and noted a statistically significantly higher mean fracture load for post-retained amalgam restorations compared with amalgam coronal-radicular restorations. Hoag and DwyerlO and Gelfand et al.6 reported that cast post and cores had greater resistance to fracture than amalgam coronal-radicular restorations in molars. Conversely, Michelich et ale3 demonstrated that amalgam coronal-radicular restorations were statistically significantly stronger than cast posts and cores in premolars. When used as cores for cast crowns, as suggested by Nayyar et al.> amalgam coronal-radicular restorations compared favorably with cast posts and cores. Nayyar et al.,4 with premolars, and Gelfand et al.6 and Hoag and Dwyer,‘O with molars, demonstrated no statistically significant difference in fracture force between cast posts and cores and amalgam coronal-radicular restorations treated with cast crowns. These studies supported the use of amalgam coronal-radicular restorations as cores for cast restorations. Kern et al.,a however, illustrated that post-retained amalgam cores had significantly higher mean fracture loads than amalgam coronal-radicular restorations used as a final restoration or as a core for a cast crown. With the exception of the study by Kern et al.,8 the amalgam coronal-radicular restoration appears to be a suitable final restoration or a core for a cast crown. Controlled clinical verification was lacking, however. 608
I. Experimental groups
Group I Pulp chamber depth: 2 mm Amalgam into root canals:Yes Group II Pulp chamber depth: 4 mm Amalgam into root canals:Yes Group III Pulp chamber depth: 6 mm Amalgam into root canals:Yes
Group IV Pulp chamber depth: 2 mm Amalgam into root canals:No Group V Pulp chamber depth: 4 mm Amalgam into root canals:No Group VI Pulp chamber depth: 6 mm Amalgam into root canals:No
Although Nayyar et al.l recommended extending amalgam into the root canal space, there is little room for error with this technique. For example, Hoag and DwyerlO prepared the root canal space for mandibular molars with a No. 4 Gates-Glidden drill (L. D. Caulk, Milford, Del.) to 4 mm or approximately 6 mm apical to the Cl&J. Since the root length of a mandibular first molar was approximately 4 mm from the CEJ,” the Gates-Glidden drill would be expected to enter the cervical one third of the root. According to Tilk et al., l2 the preparation left less than 1 mm of remaining wall thickness in the mesiolingual root in some mandibular first molars. Since amalgam was condensed against minimal wall thickness, caution in extending amalgam into the root canal was obvious, yet studies have not shown the advantages of this extension. Although a pulp chamber height of 3 to 4 mm was normal in the studies reviewed, the effect of pulp chamber height on force required for failure has not been studied. This investigation determined the effect that pulp chamber height and extension of coronal-radicular amalgam restorations into the root canal space have on the fracture resistance of endodontically treated teeth.
Sixty unrestored, intact extracted human mandibular molars were collected and stored in water. During preparation the teeth were also held in wet gauze to prevent dehydration. Care was exercised during endodontic accessto remove the entire roof of the pulp chamber and establish straight line accessfor each canal, but no effort was made to remove all undercuts from the pulp chamber as in preparation for a cast post and core. Mesial canals were prepared to the apex with a No. 35 file while distal canals were prepared to the apex with a No. 50 file. After obturation, the coronal portion of the tooth was ground perpendicular to the long axis until the height of the pulp chamber was 6 mm. This measurement wa8 recorded from the highest point on the floor of the pulp chamber to the highest coronal portion of the preparation. The teeth were mounted in acrylic resin (L. D. Caulk) with the use of a dental surveyor to a level 2 mm apical to the CEJ measured at the mesiofacial line angle. To equalize the mean facial-lingual width in each experimental group, the maximum facial-lingual width was JUNE
GROUP II GROUP Ill GROUP IV GROUP V GROUP VI Fig. 2. Mean fracture loads in newtons.
recorded to the nearest 0.5 mm. The teeth were then ranked from smallest to largest based on facial-lingual width. The smallest and the largest teeth were identified from the ranking and placed in experimental group I; then the second smallest and second largest teeth were placed in group II. This process was repeated until each of the six experimental groups had 10 teeth. The mean facial-lingual width of the six groups ranged from 10.40 to 10.45 mm. A one-way analysis of variance (ANOVA) revealed no statistically significant difference between the mean facial-lingual width of the six experimental groups (F = 0.008; df 5 5, 54; p > > 0.05). The experimental design is summarized in Table I. Preparation of the teeth is summarized in Fig. 1. The coronal portion of the teeth was reduced until the pulp chamber height was as described in Table I. The root canal space of groups I through III was prepared using the method of Christian et al.7: 1. Distal root No. 4 Gates-Glidden drill to No. 5 Gates-Glidden drill to 2. Mesiofacial and mesiolingual No. 3 Gates-Glidden drill to No. 4 Gates-Glidden drill to
4 mm 2 mm roots 4 mm 2 mm
Tilk et al.‘* believed this preparation left at least 1 mm of root wall thickness in all three canals of the first mandibular molars with 95% confidence. TEE
Copper bands were tightly adapted and reinforced with compound. Precapsulated, 2-spill, regular set Dispersalloy (Johnson & Johnson, East Windsor, N.J.) was triturated according to the manufacturer’s specification with the use of a Caulk Vari-Mix II amalgamator (Caulk Dentsply, Milford, Del.). Condensation into the root canal space (groups I through III) was performed manually with gutta-percha condensers. The remainder of the restoration was condensed with the use of a mechanical condenser (Condensaire, Teledyne Densco, Denver, Colo.) for uniformity. Amalgam was condensed to the top of the copper bands, representing an overpack of approximately 2 to 3 mm. The bands remained in place for 1 hour, and overhangs were removed with a diamond bur. The height of the restorations was then reduced to 7.5 mm above the CEJ to simulate the mean height of a human mandibular first molari This height allowed a minimum of 3.5 mm of amalgam coronal to the tooth-amalgam margin. With the use of a handpiece attached to a dental surveyor and a fixture positioning the tooth at 45 degrees, a 45-degree bevel was developed along the facial-occlusal line angle. An Instron testing machine (Instron Corp.) was used to direct a controlled force against the 45-degree bevel at a crosshead speed of 2 mm/mm until failure. The fracture load was determined by a sudden descent in load magnitude registered by the Instron machine. There was a l-month delay between condensing the restorations and application of force. 609
Facial-lingual width (mm)
Fracture load (N)
Group I 5 6
1770 2030 1085 2030 1655 2025 1200 2000 1920 1390 G-ii 364
Tooth Tooth Amalgam Tooth Tooth Amalgam Tooth Tooth Tooth Amalgam
1165 1350 1325 845 1125 1140 1310 1080 1090 1080 1151
Tooth Tooth Amalgam Tooth Amalgam Tooth Tooth Tooth Tooth Tooth
10.0 10.5 11.0 10.0 10.5 10.0 10.0 12.0 9.0 11.0 lo.40 150
9 12 19 20 28 30 37 44 46 57
11.0 10.0 10.0 9.5 12.0 10.0 10.5 10.5 10.0 11.0
865 720 820 805 1150 720 950 1195 1285 945 946 201
Tooth Tooth Tooth Tooth Amalgam Amalgam Tooth Tooth Tooth Tooth
24 25 32 35 39 48 SD Group II 11 13 14 21 36 41 42 47 58 59
RESULTS Peak fracture loads for each group are presented in Table II $th mean values and standard deviations. Fig. 2 graphically illustrates the means. Following verification of homogeneity of variance (Co&ran C test, p > 0.05), the data were subjected to a two-way analysis of variance (ANOVA). A highly significant difference with variation in pulp chamber height from 2 to 6 mm (F = 12.44; df = 2,54; p < 0.001) was determined. However, no significant difference was found with extension of amalgam into the root canal space (F = 0.05; df = 154; p = 0.83). More importantly, a highly significant interaction between the two factors (F = 5.61; df = 2,54; p = 0.006) was revealed, indi-
Mean SD Group III
10.0 12.5 10.5 11.0 10.5 lb.0 10.0 11.0 10.0
cating the interaction of one variable on the other (Fig. 3). Because of this relationship, further investigation of the means was initiated using Duncan’s multiple range test (Pig. 4). A statistically significant difference (p < 0.05) between groups I and IV was documented but not between groups II and V or between groups III and VI. This implied that extension of amalgam into the root canal space produced an elevated fracture strength if the pulp chamber height was 2 mm. The type of fracture is also presented in Table II for each restoration. For 41 of 60 restorations, failure resulted in tooth fracture while 19 restorations failed without tooth fracture.
Facial-lingual width (mm)
7 16 26 31 40 43 49 50 51 52
11.0 10.0 9.5 10.0 10.5 11.5 10.0 10.5 10.5 11.0 10.45 0.57
1800 1050 1550 1030 1465 1160 1215 1125 1470 1345 1321 249
Tooth Tooth Amalgam Tooth Tooth Arhalgam Amalgam Amalgam Amalgam Tooth
1 2 3 15 18 22 33 38 53 54
11.5 10.5 11.0 9.5 10.5 10.0 10.0 11.0 10.0 10.5 lo.45 0.57
1620 1510 1540 885 1230 595 1480 1660 1485 1025 1303 357
Amalgam Tooth Amalgam Amalgam Tooth Amalgam Tooth Tooth Tooth Amalgam
4 8 21 23 29 34 45 55 56 60
10.5 10.5 9.5 10.0 11.0 10.0 11.0 10.0 11.0 11.0 lo.45 0.52
885 815 1015 1170 1085 1090 2040 485 1270 1460 1132 415
Tooth Tooth Amalgam Amalgam Tooth Tooth Tooth Tooth Tooth Tooth
Mean SD Group VI
DISCUSSION The mean fracture load reported for group II of 1151 +- 150 newtons agreed with the similarly prepared group B of Plasmans et al9 of 1180 f 240 newtons. While these results were well above the mean maximal clenching force of healthy patients with natural teeth reported as 162 pounds14 (721 N), a maximal biting force of 975 pounds15 (4337 N) was recorded in one bruxer; clearly, the type of restoration for a specific patient should consider the higher functional demand of that individual Slightly more than two thirds of the failures were attributed to tooth fracture. The strength of the amalgam was comparable to the strength of the remaining tooth struc-
Mean SD Group V
Fracture load (N)
ture. Since force was applied to the facial surface to simulate a working side contact, the tooth fractures occurred through the lingual one half of the tooth. Extension of amalgam into the root canal space was beneficial only when the pulp chamber height was 2 mm. This is not surprising because extension of amalgam into the root canal space may offer additional resistance form to pulp chamber heights as shallow as 2 mm. However, when the pulp chamber height was 4 or 6 mm, no advantage was realized from extending amalgam into the root canal space. Although it can be performed uneventfully, extension of amalgam into the root canal space is accompanied by some risk of perforation. Clinically, eliminating the extension of
FRACTURE STRENGTH 1800 *
PULP CHAMBER HEIGHT (mm) Fig. 3. Effect of extending amalgam into root canal space (mm) at each pulp chamber height.
Fig. 4. Duncan rankings of group means in newtons.
amalgam into the root canal space simplified the preparation by limiting the number of instruments used. Thus, this approach is not recommended when a pulp chamber height of 4 mm or more is present. The range of 2 to 6 mm of pulp chamber heights was studied because it represents the dimensions commonly encountered clinically. The 2 mm height required the removal of the coronal portion of the tooth approximately to the CEJ. The 6 mm pulp chamber height left only 3.5 mm of occlusal amalgam, and when this thickness is further reduced as the occlusal surface is carved, less than the recommended 2 mm of amalgam would cover the CUSPS.‘~ Despite only a 2 mm pulp chamber height with minimal
resistance to displacement, the restorations in the 2 mm group failed by the amalgam dislodgment without fracture. These results were recorded without intentional undercuts in the pulp chamber, and amalgam was extended into the root canal space in only one half of the patients. The mean fracture load decreased as the pulp chamber height increased from 2 to 6 mm (Fig. 4). While the exact reason remains elusive, the longer lingual pulp chamber wall present as pulp chamber height increased created a protracted lever arm for the amalgam core to push against, causing fracture at lower loads despite more tooth structure. Nevertheless, it is difficult to dismiss preserving tooth
structure, since problems increase with tooth reduction. Clinically, it is more arduous to isolate a tooth and adapt a matrix band to a tooth reduced to approximately the CEJ. In addition, the matrix band may be retained longer in vitro than in vivo, decreasing the incidence of fracturing the restoration during matrix band removal. Other limitations of this in vitro investigation are that the acrylic resin supporting the teeth and the force of the Instron instrument do not fully simulate the clinical situations. However, these results reinforce the complete removal of unsupported tooth structure while providing an adequate bulk of amalgam occlusally without diminishing the fracture resistance of the restoration. In addition, dentists should consider amalgam coronal-radicular restorations with only 2 mm of pulp chamber height remaining and compensate for reduced chamber height by extension of amalgam into the root canal space.
The effect on fracture resistance of pulp chamber heights of 2, 4, and 6 mm and the extension of amalgam into the root canal space were investigated. The conclusions drawn were: 1. Amalgam extension into the root canal space occasionally presented a risk of perforation and was inconvenient. This procedure should be restricted to teeth with less than 4 mm of pulp chamber height. 2. The mean fracture load increased with greater tooth reduction and decreasing pulp chamber heights. However, the restorative difficulties encountered with teeth of short clinical height must be considered before reducing the crown height.
REFERENCES 1. Nayyar A, Walton RE, Leonard LA. An amalgam dowel and core technique for endodontically treated PROSTHET DENT
2. Michelich R, Dillard T, Nayyar A. Mechanical properties of amalgam buildups for endodontically 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 1981;60:630. 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. Mertz KA, Parker MW, Pelleu GB. Shear strength of two coronalradicular 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 PROSTHET DENT 1984;52:635-8. 7. 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, Mueninghotf LA. An in vitro comparison of two dowel and core techniques for endodontically treated molars. J PROSTHET DENT
9. Plasmans PJJM, Visseren LGH, Vrijhoef MMA, Kayser AF. In vitro comparison of dowel and core techniques for endodontically treated molars. J Endo 1986;12:382-7. 10. Hoag EP, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHET DENT 1982;47:177-81. 11. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders, 1974;271, 273. 12. Tilk MA, Lommel TJ, Gerstein H. A study of mandibular and maxillary root widths to determine dowel size. J Endo 1979;5:79-82. 13. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders, 1974268, 284. 14. Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal force during chewing and swallowing aa measured by sound transmission. J PROSTHET DENT 1981;46:443-9. 15. Gibbs CH, Mahan PE, Mauderli A, Lundeen HC. Limits of human bite strength. J PROSTHET DENT 1986;56:226-9. 16. P&mans PJJM, Kusters ST, Thissen AMG, Van’t Hof MA, Vrijhoef MMA. Effects of preparation design on the resistance for extensive amalgam restorations. Oper Dent. Reprint
DR. JOHN 0. BURGESS 3118 WHISPER BROOK SAN ANTONIO, TX 78230
coronal-radicular posterior teeth. J
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