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Reduction in tooth stiffness as a result of endodontic and restorative procedures
0099-2399/89/1511-0512/$02.00/0 JOURNAL OF E:NDODONTICS Copyright 9 1989 by The American Association of Endodontists
Printed in U.S.A.
VOL. 15, NO. 11, NOVEMBER1989
Reduction in Tooth Stiffness as a Result of Endodontic and Restorative Procedures Ernest S. Reeh, BSc, DDS, Harold H. Messer, DDS, PhD, and William H. Douglas, DMD, PhD
teeth (1-3). One of the largest difficulties with retrospective studies is that only the clinically successful cases are examined (1). The greatest failure rate in endodontically treated teeth was associated with lack of appropriate restoration (2, 3). Teeth are believed to become more brittle as a direct result of endodontic treatment leading to fracture and tooth loss (4). In reality increased brittleness has not been proven. Hardness measurements of endodontically treated teeth that were treated up to 10 yr previously indicated no difference in hardness between endodontically treated and vital teeth (5). Similiarly, punch shear testing on endodontically treated teeth showed only a small (although statistically significant) reduction in strength of 14% (6). Restorative procedures (not in relation to endodontic treatment) have been examined to determine their contribution to the resistance of the tooth to fracture. Statistical differences were difficult to demonstrate for restorative procedures due to the destructive nature of these tests. To overcome the problem of sample variation, large numbers of samples are needed. It has been shown that the resulting weakening of the tooth due to restorative procedures increases with the reduction of tooth structure (7, 8). According to Larson et al. (8) occlusal cavities significantly weaken the tooth and wider isthmus preparations result in the largest decrease of tooth resistance to fracture. Factors such as the maintenance of marginal ridge integrity and width of the isthmus region also appear to be important in reducing tooth fracture (7, 9). The conclusions that could be drawn from previous work are that the preservation of tooth structure is important in maintaining resistance to fracture, especially by preserving marginal ridges and maintaining narrow isthmus. According to Hood (10), nondestructive techniques employing linear variable differential transformers, which measure small movements of tooth structure under load, provide one method to examine the effects of sequential treatments. The relative contributions of restorative and endodontic procedures could be more accurately assessed, within physiological limits, using this technique. Hood observed that a small reduction of tooth structure, such as an occlusal cavity preparation, produced a significant decrease in tooth rigidity. Endodontic procedures also significantly decreased tooth rigidity reflecting a decreased stiffness in the tooth (10). Other sources are in agreement that the removal of hard tissue increased cuspal flexure under occlusal load (11, 12). Morin et al. (13) state that the use of strain gauges also permit the visualization of hysteresis. Hysteresis, which is the state of being behind, in strain gauge measurements would be the
Endodontically treated teeth are thought to be more susceptible to fracture as a result of the loss of tooth vitality and tooth structure. This study was designed to compare the contributions of endodontic and restorative procedures to the loss of strength by using nondestructive occlusal loading on extracted intact, maxillary, second bicuspids. An encapsulated strain gauge was bonded on enamel just above the cementoenamel junction on both the buccal and lingual surfaces, and the teeth were mounted in nylon rings leaving 2 mm of root surface exposed. Under load control, each tooth was loaded at a rate of 37 N per s for 3 s and unloaded at the same rate in a closed loop servo-hydraulic system to measure stiffness. A stress-strain curve was generated from each gauge prior to alteration of the tooth and after each procedure performed on the tooth. Cuspal stiffness, as a measure of tooth strength, was evaluated on one of two series of sequentially performed procedures: 1. (a) unaltered tooth, (b) access preparation, (c) instrumentation, (d) obturation, and (e) MOD cavity preparation; or 2. (a) unaltered tooth, (b) occlusal cavity preparation, (c) two-surface cavity preparation, (d) MOD cavity preparation, (e) access, (f) instrumentation, and (g) obturation. Results on 42 teeth indicate that endodontic procedures have only a small effect on the tooth, reducing the relative stiffness by 5%. This was less than that of an occlusal cavity preparation (20%). The largest losses in stiffness were related to the loss of marginal ridge integrity. MOD cavity preparation resulted in an average of a 63% loss in relative cuspal stiffness. The results indicate that endodontic procedures do not weaken teeth with intact marginal ridges.
Endodontic treatment is considered to weaken the tooth by increasing the brittleness, leading to a higher susceptibility to fracture. Consequently, the recommended restoration has been cuspal prbtection with a cast restoration. Clinical studies on large numbers of patients have indicated the need for special restorative considerations for endodontically treated
512
Vol. 15, No. 11, November 1989
time lag in reacting to changes in forces; this means that energy would be lost in the tooth because it did not return to its original state at the same rate during unloading as during loading. It has also been reported that stiffness decreased and hysteresis increased as the removal of tooth structure is increased (11-13). A conservative three-surface restoration has been shown to reduce tooth stiffness by almost 50% (12). Studies which use loading to fracture are undesirable, as these forces are not commonly encountered in the mouth and each tooth can be tested for only one procedure. The use of strain gauges bonded to teeth in association with nondestructive loading at physiological levels provides a method to evaluate sequentially each step in endodontic and restorative procedures in the same tooth. The purpose of this study was to examine, nondestructively, the effect of endodontic and restorative procedures on the loss of tooth strength using closed loop servo-hydraulics in conjunction with strain gauge technology. MATERIALS AND METHODS
Selection of Teeth and Preparation of Teeth Forty-two extracted, noncarious, permanent, maxillary second premolars were selected after transillumination, visual examination, and radiographic examination. The teeth were individually stored in labeled vials containing thymol in water. During preparation and testing phases of the experiment care was taken to prevent dehydration. Strain gauges (Gauge type CEA-06-032UW-120; Micro Measurements, Raleigh, NC) had lead wires soldered to them, and the backing of each gauge was trimmed to conform to the tooth contour. Gauges were then bonded to the buccal and lingual surfaces just above the cementoenamel junction using an acid-etch technique. The site for bonding was etched with 37% orthophosphoric acid for 60 s and then dried with a stream of air. The backing of the gauge was cleaned with chloroform and primed with a catalyst prior to fastening with adhesive (M Bond 2000; Micro Measurements). The gauge, the solder contacts, and the immediate tooth structure were then covered with three layers of enamel bond resin (3M Co., St. Paul, MN).
Reduction in Tooth Stiffness
reproducible. A steel sphere 6.3 m m in diameter was rigidly fastened to the upper member of the MTS machine. The upper member was brought into contact with the tooth and small occlusion points were ground into the buccal and lingual cusps with a flame-shaped bur to prevent lateral deflection of the sphere. The small contact areas were high on the cuspal inclines so as not to interfere with restorative or endodontic procedures (Fig. 1).
Closed Loop Servo-hydraulic System The MTS machine was configured for load control using a continuously increasing load at a rate of 37 N per s for 3 s and unloaded at a rate of 37 N per s for 3 s. This provided for a m a x i m u m load of 111 N. This load is within physiological limits for the human dentition (14). Load control provided compensation for movement in the tooth structure resulting in a constant rate of force delivered to the tooth. A stress strain curve was generated and recorded from the buccal and lingual gauges prior to any alteration to the tooth and after each sequential procedure. The teeth were loaded and unloaded five times prior to alteration of the tooth and then one of two procedure sequences was followed. The first sequence consisted of endodontic procedures prior to restorative procedures and the second had restorative procedures prior to endodontic procedures. The first sequence consisted of unaltered tooth, access preparation, endodontic instrumentation, obturation, and an M O D cavity preparation. The second sequence consisted of unaltered tooth, occlusal cavity preparation, twosurface cavity preparation, M O D cavity preparation, endodontic access cavity, endodontic instrumentation, and obturation. Loading and unloading was repeated five times after each step to confirm fatigue was not occurring in the sample as a result of the loading.
a
Preparation for Load Testing Two teeth with attached strain gauges were mounted in close proximity to each other in a nylon mounting ring using clear resin. The teeth were mounted leaving 2 m m of root surface exposed, approximating the support of alveolar bone to a healthy tooth. The lead wires were partially embedded in the resin to immobilize them. One tooth was used as the experimental tooth and the other tooth served as a compensator for strain which could arise from temperature fluctuations. The gauges were connected to the strain measurement system (2100 System; Micro Measurements) with the strain reading directly proportional to the deformation o f the tooth structure. The resin ring with the mounted teeth was fastened to the lower platen of the MTS 858 servo-hydraulic testing machine (MTS Systems, Eden Prairie, MN). This mounting was exactly
513
b C d ,.:...
f
9 ..
..,"
..-... -~
--~----..~
J FIG 1. Articulation of tooth with the upper member of the MTS machine, a, 6.3-mm diameter sphere; b, contact point with tooth; c, slight relief of tooth to ensure consistent contact area; d, access opening extended to pulp; e, pulp chamber; f, clear resin mounting of tooth.
514
Journal of Endodontics
Reeh et al.
Endodontic and Restorative Procedures
Statistical Analysis
Endodontic instrumentation consisted of enlarging the apical preparation to three instrument sizes larger than the first instrument to bind at working length or at least a #30. The canals were preflared, once the apical preparation was instrumented to #20, with #2 and 3 Gates Glidden burs to two thirds and one third of the canal length, respectively. The canals were obturated with laterally condensed gutta-percha for the apical one third and back filled using a modified McSpadden technique. The gutta-percha was removed to the height of the buccal cementoenamel junction and vertically condensed. Restorative procedures consisted of an occlusal cavity preparation with extensions into the major developmental grooves providing a classical butterfly preparation appearance. A twosurface preparation, either DO or MO, was then cut with the apical extension providing approximately 1 mm of enamel gingivally and a depth of 1,5 m m axially. The buccal extension was buccal to the li ne angle extending beyond where a contact point would have been. The lingual extension provided an overall width of the proximal box slightly less than the intercuspal distance. Endodontic access preparations were cut according to the standard oval outline form for upper second bicuspids. Extension of the access preparation was considered adequate when the entire coronal pulp tissue was removed and straight line access of the Gates Glidden burs was possible. When the access preparations were cut after restorative procedures had been performed, a wall of dentin between the access preparation and the axial proximal surface remained and was 0.75 to 1 m m in thickness for both proximal surfaces.
Logarithmic transformations of the data were done to make the variances homogeneous. A one-way A N O V A was done; the f value resulted in significance at <0.001. Individual t tests between group means were done using the Bonferoni method.
Derivation of Relative Stiffness The strain for each condition tested was normalized to the unaltered tooth to compensate for morphological variation between samples. This was termed the relative stiffness (RS) o f the tooth which was calculated in the following manner: RS=
stiffness of the cusp under the test condition stiffness of the same cusp in the unaltered tooth maximum stress under the test condition m a x i m u m strain under the test condition maximum stress on the unaltered tooth m a x i m u m strain in the unaltered tooth
RESULTS The strain gauge data for the two sequences are presented in Tables 1 and 2 and graphically in Figs. 2 to 4. The numbers listed are RS values with the unaltered tooth given the value of 1.00. Relative stiffness is calculated as described in Materials and Methods. Strain gauge techniques in association with nondestructive loading permit the evaluation of a sequence of procedures with each tooth serving as its own base line. This technique substantially reduced the variation between individual teeth and permitted the use of smaller sample sizes. The data in Table l were obtained from teeth in which endodontic procedures preceded cavity preparation. The data demonstrate that endodontic procedures on an intact tooth had only a small effect on the overall stiffness of the tooth. The endodontic procedures alone resulted in a decrease in stiffness of approximately 5%. The decrease occurred in association with access opening; instrumentation and obturation did not lead to a further loss of stiffness. An M O D cavity preparation following the endodontic procedures had a dramatic effect on the stiffness of the tooth, leading to a reduction to only 31% of the stiffness of the intact tooth. Despite the small reduction in stiffness accompanying endodontic procedures, the reduction was statistically significant (p < 0.05, by Bonferroni tests following one-way ANOVA). The large decrease in stiffness from cndodontically treated teeth to an MOD cavity preparation was very highly significant (p < 0.001). In a second group of teeth, cavity preparations were cut TABLE 1. Effects of endodontic procedures on the unaltered tooth followed by MOD cavity preparation Sequence of Procedures
n
Average Relative Stiffness
SD
Unaltered tooth Access Instrumentation Obturation MOD
5 5 5 5 5
1.000 0.944 0.948 0.957 0.311
0.000 0.105 0.133 0.162 0.073
Maximum stress on the unaltered tooth = maximum stress under the test condition. Therefore: Maximum strain in the unaltered tooth RS= Maximum strain under the test condition Calculation in this manner leaves the values unitless and provides a means to standardize samples. Through normalization of strain values inherent differences between teeth would be minimized and permit comparisons without the need for large sample sizes. A value of unity is then achieved for the normalized unaltered tooth. Relative stiffness values less than one (RS < 1) then reflect a decrease in the stiffness of the tooth or a greater flexure of tooth structure.
TABLE 2. Effects of restorative procedures on the unaltered tooth followed by endodontic procedures Sequence of Procedures
n
Average Relative Stiffness
SD
Unaltered tooth Occlusal preparation Two surface MOD Access Instrumentation Obturation
37 27 27 37 33 33 33
1.000 0.802 0.536 0.373 0.330 0.337 0.317
0.000 0_092 0.104 0.075 0.065 0.095 0.098
Vol. 15, No. 11, November 1989
Reduction in Tooth Stiffness
Endodontic Procedures I
Restorative Procedures UnalteredTooth
1.00
Occlusal
.80
.95
Instrumentation
2 Surface
B~
.96
Obturatlon
MOD ~
MOD 0.0
515
0.2
0.4
0.6
o.e
1.0
1.2
.0
.37 i
0 2
Relative Stiffness FIG 2. Graphic representation of the data from Table 1. Endodontic procedures had a small effect on the reduction in tooth stiffness compared with MOD cavity preparation which had a substantial effect in the reduction of tooth stiffness. (Error bar represents 1 SD.)
sequentially before endodontic procedures. An occlusal preparation that produced an isthmus width approximately one third of the intercuspal distance resulted in a 20% decrease in the stiffness of the tooth. A two-surface cavity preparation that destroyed one marginal ridge resulted in a 46% loss in tooth stiffness and an MOD preparation resulted in a 63% decrease in stiffness. The loss of each surface resulted in approximately a 20% decrease in stiffness. Endodontic procedures completed after restorative procedures resulted in a minimal further reduction (5 %) in tooth stiffness. This further reduction approximately equaled the reduction from the unaltered tooth as described in Table I. Thus, either before or after restorative procedures, endodontic procedures produced only minimal reductions in the stiffness of the tooth. Each step of the restorative procedures showed a significant loss of tooth stiffness relative to the previous step (p < 0.001 ). Despite the small additional loss of stiffness associated with endodontic access opening following M O D cavity preparation (0.37 to 0.33), the difference was statistically significant (p < 0.05); in every case there was an additional decrease in stiffness related to the access opening. DISCUSSION Closed loop servo-hydraulics is a nondestructive testing technique. This permits the sequential evaluation of a series of procedures and the use of the unaltered tooth as its own control. This technique reduces intertooth variation, permitting smaller sample sizes to achieve statistical significance9 The technique is very sensitive and reproducible. An equivalent tooth structure reduction, regardless of which sequence of procedures was used to attain the reduction, resulted in the same loss of stiffness. This was observed when the tooth structure reduction was accomplished by doing endodontic procedures followed by restorative procedures (relative stiffhess of 0.31) compared with restorative procedures followed by endodontic procedures (relative stiffness of 0.32). Endodontic procedures did not produce substantial effects on the relative stiffness of the tooth (5% reduction in tooth
i
i
o.4
0.6
|
0.8
i
1 0
i
1.2
Relative Stiffness FIG 3. Graphic representation of the restorative procedures data from Table 2. Each reduction of a tooth surface resulted in a substantial decrease in the tooth's stiffness. (Error bar represents 1 SD.)
Endodontic Procedures II .37
Access ~
.33
Instrumentation D
.34
Obturatlon
.32 .0
0.2
0.4
0 6
0.8
1 0
1.2
Relative Stiffness FiG 4. Graphic representation of the endodontic procedures data from Table 2. Endodontic procedures produced a small reduction in the tooth's stiffness after the reduction in stiffness from restorative procedures. (Error bar represents 1 SD.)
stiffness) whether endodontic access was obtained before or after cavity preparation. The study design did not permit evaluation of aging on the endodontically treated teeth. Thus, unless brittleness becomes a significant contribution to loss of tooth strength with time, endodontic procedures do not appear to be the major contributor to clinical fractures. Brittleness of endodontically treated teeth is not well documented. Of the two articles that directly address the question, A Vicker's hardness test on vital tooth dentin and root canaltreated teeth (with treatment as much as 10 yr previously) showed no statistical difference in hardness of endodontically treated teeth (5). Punch shear testing (6) indicated that endodontic treatment did not reduce toughness by more than 14%. This would suggest that the total effect of endodontic procedures is not great and is in fact comparable to an occlusal cavity preparation. The effects of cavity preparation were substantial compared with the negligible changes from endodontic procedures. Even
516
Journal of Endodontics
Reeh et al.
an occlusal cavity preparation resulted in a 4-fold greater decrease (20%) than endodontic access opening (5%). Although both procedures involve the same tooth surface, the greater encroachment of an occlusal cavity preparation on the marginal ridge area may account for this difference. Violation of marginal ridge integrity further compounded the loss of tooth strength. A two-surface cavity preparation resulted in a 46% loss in tooth stiffness. Extending the cavity preparation to an MOD resulted in a large total decrease in tooth stiffness of 63%. Endodontic procedures on the prepared tooth again resulted in a small decrease of approximately 5%. Clearly, restorative procedures are the major factor in weakening the tooth. Extrapolation from the premolars used in this study should be done with caution, but the same principles should apply to any posterior tooth. The principle of cuspal independence has been demonstrated in molars, so that a single cusp may be weakened as a result of a particular cavity outline (10, 1i, 15). The movement of cusps as measured by strain gauges or linear variable differential transformers has demonstrated the existence of cuspal independence (10). Posterior teeth would then be expected to respond in much the same manner as the bicuspids in this experiment. The exact nature and magnitude of this effect is not certain but they would be expected to reveal the same trends. Speculation with respect to molars suggests that greater movement may be experienced with tooth structure reductions. Certainly, most teeth that require endodontic treatment have lost more tooth structure than just an access preparation, but these results would suggest a need to evaluate further the restorative techniques for endodontically treated teeth. Tooth structure should be preserved where possible, consistent with good endodontic access. Marginal ridges should be preserved unless their removal is unavoidable for the purpose of restoration. CONCLUSIONS As a result of this investigation, the following conclusions can be drawn: 1. Endodontic procedures reduced tooth stiffness by only 5%, which is contributed entirely by the access opening. The 5% reduction in stiffness from endodonic procedures was insensitive to sequence as the same reduction resulted whether restorative procedures followed or preceded endodontic procedures. 2. Restorative procedures were the greatest contributor to loss of tooth stiffness. An MOD cavity preparation reduced tooth stiffness by more than 60%. 3. The loss of marginal ridge integrity was the greatest contribution to loss of tooth strength.
4. Whenever possible an intact marginal ridge should be preserved to maintain tooth strength. This research was supported in part by a Grarlt-ln-Aid of Research from the Endowment and Memorial Foundation of the American Association of Endodontists. The opinions, assertions, materials, and methodologies herein are private ones of the authors and are not to be construed as official or reflecting the views of the American Association of Endedontists or the Endowment and Memorial Foundation. This research was presented at the 45th Annual Meeting of the American Association of Endedontists, Anaheim, CA, where it won first place in the Graduate Student Presentations. This work was based on a thesis submitted to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the MS degree. This study was also supported by Grant DE 08732 from the National Institute for Dental Research, Bethesda, ME). Composite resin restorative material and dentin bonding agent were supplied by Minnesota Mines, Metals, and Manufacturing (3M). Dr. Reeh is a graduate student in endudontics, School of Dentistry, University of Minnesota, Minneapolis, MN. Dr. Messer is acting chairman of the Department of Rehabilitative Sciences, chairman of the Division of Endodontics, School of Dentistry, University of Minnesota, Minneapolis. Dr. Douglas is chairman of the Division of Biomaterials, School of Dentistry, University of Minnesota, Minneapolis.
References 1. Ross IF. Fracture susceptibility of endodontically treated teeth. J Endodon 1980;6:560-5. 2. Sorenson JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J Prosthet Dent 1984; 51:780-4. 3. Sorenson JA, Martinoff JT. Endodonticelly treated teeth as abutments. J Prosthet Dent 1985;53:631-6. 4. Heifer AR, Melnick S, Schilder H. Determination of the moisture content of vital and polpless teeth. Oral Surg 1972;34:661-70. 5. Lewinstein I, Grajower R. Root dentin hardness of endodentically treated teeth. J Endodon 1981 ;7:421-2. 6. Carter JM, Sorensen SE, Johnson RL, Teiteibaum RL, Levine MS. Punch shear testing of extracted vital and endodontically treated teeth. J Biomech 1983;16:841-8. 7. Mondelli J, Steagall L, Ishikiriama A, Navarro MFL, Soares FB. Fracture strength of human teeth with cavity preparations. J Prosthet Dent 1980; 43:419-22. 8. Larsen TD. Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent 1981 ;6:2-5. 9. Wendt SL Jr, Harris BM, Hunt TE. Resistance to cusp fracture in endodontically treated teeth. Dent Mater 1987;3:232-5. 10. Hood JAA. Methods to improve fracture resistance of teeth. In: Vanherle G, Smith DC, eds. International Symposium on posterior composite resin dental restorative materials. Symposium sponsored by 3M, St. Paul, MN, 1985; 443-50. 11. Douglas WH. Methods to improve fracture resistance of teeth. In: Vanherle G, Smith DC, eds. International Symposium on posterior composite resin dental restorative materials. Symposium sponsored by 3M, St. Paul, MN, 1985:433-41. 12. Morin D, Douglas WH, Cross M, DeLong R. Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater 1988;4:41-8. 13. Morin D, Delong R, Douglas WH. Cusp reinforcement with the acidetch technique. J Dent Res 1984;63:1075-8. 14. DeBoever JA, McCall, WD, Holden S, Ash MM Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326-33. 15. Tidmarsh BG. Restoration of endodontically treated posterior teeth. J Endodon 1976;2:374-5.
Printed in U.S.A.
VOL. 15, NO. 11, NOVEMBER1989
Reduction in Tooth Stiffness as a Result of Endodontic and Restorative Procedures Ernest S. Reeh, BSc, DDS, Harold H. Messer, DDS, PhD, and William H. Douglas, DMD, PhD
teeth (1-3). One of the largest difficulties with retrospective studies is that only the clinically successful cases are examined (1). The greatest failure rate in endodontically treated teeth was associated with lack of appropriate restoration (2, 3). Teeth are believed to become more brittle as a direct result of endodontic treatment leading to fracture and tooth loss (4). In reality increased brittleness has not been proven. Hardness measurements of endodontically treated teeth that were treated up to 10 yr previously indicated no difference in hardness between endodontically treated and vital teeth (5). Similiarly, punch shear testing on endodontically treated teeth showed only a small (although statistically significant) reduction in strength of 14% (6). Restorative procedures (not in relation to endodontic treatment) have been examined to determine their contribution to the resistance of the tooth to fracture. Statistical differences were difficult to demonstrate for restorative procedures due to the destructive nature of these tests. To overcome the problem of sample variation, large numbers of samples are needed. It has been shown that the resulting weakening of the tooth due to restorative procedures increases with the reduction of tooth structure (7, 8). According to Larson et al. (8) occlusal cavities significantly weaken the tooth and wider isthmus preparations result in the largest decrease of tooth resistance to fracture. Factors such as the maintenance of marginal ridge integrity and width of the isthmus region also appear to be important in reducing tooth fracture (7, 9). The conclusions that could be drawn from previous work are that the preservation of tooth structure is important in maintaining resistance to fracture, especially by preserving marginal ridges and maintaining narrow isthmus. According to Hood (10), nondestructive techniques employing linear variable differential transformers, which measure small movements of tooth structure under load, provide one method to examine the effects of sequential treatments. The relative contributions of restorative and endodontic procedures could be more accurately assessed, within physiological limits, using this technique. Hood observed that a small reduction of tooth structure, such as an occlusal cavity preparation, produced a significant decrease in tooth rigidity. Endodontic procedures also significantly decreased tooth rigidity reflecting a decreased stiffness in the tooth (10). Other sources are in agreement that the removal of hard tissue increased cuspal flexure under occlusal load (11, 12). Morin et al. (13) state that the use of strain gauges also permit the visualization of hysteresis. Hysteresis, which is the state of being behind, in strain gauge measurements would be the
Endodontically treated teeth are thought to be more susceptible to fracture as a result of the loss of tooth vitality and tooth structure. This study was designed to compare the contributions of endodontic and restorative procedures to the loss of strength by using nondestructive occlusal loading on extracted intact, maxillary, second bicuspids. An encapsulated strain gauge was bonded on enamel just above the cementoenamel junction on both the buccal and lingual surfaces, and the teeth were mounted in nylon rings leaving 2 mm of root surface exposed. Under load control, each tooth was loaded at a rate of 37 N per s for 3 s and unloaded at the same rate in a closed loop servo-hydraulic system to measure stiffness. A stress-strain curve was generated from each gauge prior to alteration of the tooth and after each procedure performed on the tooth. Cuspal stiffness, as a measure of tooth strength, was evaluated on one of two series of sequentially performed procedures: 1. (a) unaltered tooth, (b) access preparation, (c) instrumentation, (d) obturation, and (e) MOD cavity preparation; or 2. (a) unaltered tooth, (b) occlusal cavity preparation, (c) two-surface cavity preparation, (d) MOD cavity preparation, (e) access, (f) instrumentation, and (g) obturation. Results on 42 teeth indicate that endodontic procedures have only a small effect on the tooth, reducing the relative stiffness by 5%. This was less than that of an occlusal cavity preparation (20%). The largest losses in stiffness were related to the loss of marginal ridge integrity. MOD cavity preparation resulted in an average of a 63% loss in relative cuspal stiffness. The results indicate that endodontic procedures do not weaken teeth with intact marginal ridges.
Endodontic treatment is considered to weaken the tooth by increasing the brittleness, leading to a higher susceptibility to fracture. Consequently, the recommended restoration has been cuspal prbtection with a cast restoration. Clinical studies on large numbers of patients have indicated the need for special restorative considerations for endodontically treated
512
Vol. 15, No. 11, November 1989
time lag in reacting to changes in forces; this means that energy would be lost in the tooth because it did not return to its original state at the same rate during unloading as during loading. It has also been reported that stiffness decreased and hysteresis increased as the removal of tooth structure is increased (11-13). A conservative three-surface restoration has been shown to reduce tooth stiffness by almost 50% (12). Studies which use loading to fracture are undesirable, as these forces are not commonly encountered in the mouth and each tooth can be tested for only one procedure. The use of strain gauges bonded to teeth in association with nondestructive loading at physiological levels provides a method to evaluate sequentially each step in endodontic and restorative procedures in the same tooth. The purpose of this study was to examine, nondestructively, the effect of endodontic and restorative procedures on the loss of tooth strength using closed loop servo-hydraulics in conjunction with strain gauge technology. MATERIALS AND METHODS
Selection of Teeth and Preparation of Teeth Forty-two extracted, noncarious, permanent, maxillary second premolars were selected after transillumination, visual examination, and radiographic examination. The teeth were individually stored in labeled vials containing thymol in water. During preparation and testing phases of the experiment care was taken to prevent dehydration. Strain gauges (Gauge type CEA-06-032UW-120; Micro Measurements, Raleigh, NC) had lead wires soldered to them, and the backing of each gauge was trimmed to conform to the tooth contour. Gauges were then bonded to the buccal and lingual surfaces just above the cementoenamel junction using an acid-etch technique. The site for bonding was etched with 37% orthophosphoric acid for 60 s and then dried with a stream of air. The backing of the gauge was cleaned with chloroform and primed with a catalyst prior to fastening with adhesive (M Bond 2000; Micro Measurements). The gauge, the solder contacts, and the immediate tooth structure were then covered with three layers of enamel bond resin (3M Co., St. Paul, MN).
Reduction in Tooth Stiffness
reproducible. A steel sphere 6.3 m m in diameter was rigidly fastened to the upper member of the MTS machine. The upper member was brought into contact with the tooth and small occlusion points were ground into the buccal and lingual cusps with a flame-shaped bur to prevent lateral deflection of the sphere. The small contact areas were high on the cuspal inclines so as not to interfere with restorative or endodontic procedures (Fig. 1).
Closed Loop Servo-hydraulic System The MTS machine was configured for load control using a continuously increasing load at a rate of 37 N per s for 3 s and unloaded at a rate of 37 N per s for 3 s. This provided for a m a x i m u m load of 111 N. This load is within physiological limits for the human dentition (14). Load control provided compensation for movement in the tooth structure resulting in a constant rate of force delivered to the tooth. A stress strain curve was generated and recorded from the buccal and lingual gauges prior to any alteration to the tooth and after each sequential procedure. The teeth were loaded and unloaded five times prior to alteration of the tooth and then one of two procedure sequences was followed. The first sequence consisted of endodontic procedures prior to restorative procedures and the second had restorative procedures prior to endodontic procedures. The first sequence consisted of unaltered tooth, access preparation, endodontic instrumentation, obturation, and an M O D cavity preparation. The second sequence consisted of unaltered tooth, occlusal cavity preparation, twosurface cavity preparation, M O D cavity preparation, endodontic access cavity, endodontic instrumentation, and obturation. Loading and unloading was repeated five times after each step to confirm fatigue was not occurring in the sample as a result of the loading.
a
Preparation for Load Testing Two teeth with attached strain gauges were mounted in close proximity to each other in a nylon mounting ring using clear resin. The teeth were mounted leaving 2 m m of root surface exposed, approximating the support of alveolar bone to a healthy tooth. The lead wires were partially embedded in the resin to immobilize them. One tooth was used as the experimental tooth and the other tooth served as a compensator for strain which could arise from temperature fluctuations. The gauges were connected to the strain measurement system (2100 System; Micro Measurements) with the strain reading directly proportional to the deformation o f the tooth structure. The resin ring with the mounted teeth was fastened to the lower platen of the MTS 858 servo-hydraulic testing machine (MTS Systems, Eden Prairie, MN). This mounting was exactly
513
b C d ,.:...
f
9 ..
..,"
..-... -~
--~----..~
J FIG 1. Articulation of tooth with the upper member of the MTS machine, a, 6.3-mm diameter sphere; b, contact point with tooth; c, slight relief of tooth to ensure consistent contact area; d, access opening extended to pulp; e, pulp chamber; f, clear resin mounting of tooth.
514
Journal of Endodontics
Reeh et al.
Endodontic and Restorative Procedures
Statistical Analysis
Endodontic instrumentation consisted of enlarging the apical preparation to three instrument sizes larger than the first instrument to bind at working length or at least a #30. The canals were preflared, once the apical preparation was instrumented to #20, with #2 and 3 Gates Glidden burs to two thirds and one third of the canal length, respectively. The canals were obturated with laterally condensed gutta-percha for the apical one third and back filled using a modified McSpadden technique. The gutta-percha was removed to the height of the buccal cementoenamel junction and vertically condensed. Restorative procedures consisted of an occlusal cavity preparation with extensions into the major developmental grooves providing a classical butterfly preparation appearance. A twosurface preparation, either DO or MO, was then cut with the apical extension providing approximately 1 mm of enamel gingivally and a depth of 1,5 m m axially. The buccal extension was buccal to the li ne angle extending beyond where a contact point would have been. The lingual extension provided an overall width of the proximal box slightly less than the intercuspal distance. Endodontic access preparations were cut according to the standard oval outline form for upper second bicuspids. Extension of the access preparation was considered adequate when the entire coronal pulp tissue was removed and straight line access of the Gates Glidden burs was possible. When the access preparations were cut after restorative procedures had been performed, a wall of dentin between the access preparation and the axial proximal surface remained and was 0.75 to 1 m m in thickness for both proximal surfaces.
Logarithmic transformations of the data were done to make the variances homogeneous. A one-way A N O V A was done; the f value resulted in significance at <0.001. Individual t tests between group means were done using the Bonferoni method.
Derivation of Relative Stiffness The strain for each condition tested was normalized to the unaltered tooth to compensate for morphological variation between samples. This was termed the relative stiffness (RS) o f the tooth which was calculated in the following manner: RS=
stiffness of the cusp under the test condition stiffness of the same cusp in the unaltered tooth maximum stress under the test condition m a x i m u m strain under the test condition maximum stress on the unaltered tooth m a x i m u m strain in the unaltered tooth
RESULTS The strain gauge data for the two sequences are presented in Tables 1 and 2 and graphically in Figs. 2 to 4. The numbers listed are RS values with the unaltered tooth given the value of 1.00. Relative stiffness is calculated as described in Materials and Methods. Strain gauge techniques in association with nondestructive loading permit the evaluation of a sequence of procedures with each tooth serving as its own base line. This technique substantially reduced the variation between individual teeth and permitted the use of smaller sample sizes. The data in Table l were obtained from teeth in which endodontic procedures preceded cavity preparation. The data demonstrate that endodontic procedures on an intact tooth had only a small effect on the overall stiffness of the tooth. The endodontic procedures alone resulted in a decrease in stiffness of approximately 5%. The decrease occurred in association with access opening; instrumentation and obturation did not lead to a further loss of stiffness. An M O D cavity preparation following the endodontic procedures had a dramatic effect on the stiffness of the tooth, leading to a reduction to only 31% of the stiffness of the intact tooth. Despite the small reduction in stiffness accompanying endodontic procedures, the reduction was statistically significant (p < 0.05, by Bonferroni tests following one-way ANOVA). The large decrease in stiffness from cndodontically treated teeth to an MOD cavity preparation was very highly significant (p < 0.001). In a second group of teeth, cavity preparations were cut TABLE 1. Effects of endodontic procedures on the unaltered tooth followed by MOD cavity preparation Sequence of Procedures
n
Average Relative Stiffness
SD
Unaltered tooth Access Instrumentation Obturation MOD
5 5 5 5 5
1.000 0.944 0.948 0.957 0.311
0.000 0.105 0.133 0.162 0.073
Maximum stress on the unaltered tooth = maximum stress under the test condition. Therefore: Maximum strain in the unaltered tooth RS= Maximum strain under the test condition Calculation in this manner leaves the values unitless and provides a means to standardize samples. Through normalization of strain values inherent differences between teeth would be minimized and permit comparisons without the need for large sample sizes. A value of unity is then achieved for the normalized unaltered tooth. Relative stiffness values less than one (RS < 1) then reflect a decrease in the stiffness of the tooth or a greater flexure of tooth structure.
TABLE 2. Effects of restorative procedures on the unaltered tooth followed by endodontic procedures Sequence of Procedures
n
Average Relative Stiffness
SD
Unaltered tooth Occlusal preparation Two surface MOD Access Instrumentation Obturation
37 27 27 37 33 33 33
1.000 0.802 0.536 0.373 0.330 0.337 0.317
0.000 0_092 0.104 0.075 0.065 0.095 0.098
Vol. 15, No. 11, November 1989
Reduction in Tooth Stiffness
Endodontic Procedures I
Restorative Procedures UnalteredTooth
1.00
Occlusal
.80
.95
Instrumentation
2 Surface
B~
.96
Obturatlon
MOD ~
MOD 0.0
515
0.2
0.4
0.6
o.e
1.0
1.2
.0
.37 i
0 2
Relative Stiffness FIG 2. Graphic representation of the data from Table 1. Endodontic procedures had a small effect on the reduction in tooth stiffness compared with MOD cavity preparation which had a substantial effect in the reduction of tooth stiffness. (Error bar represents 1 SD.)
sequentially before endodontic procedures. An occlusal preparation that produced an isthmus width approximately one third of the intercuspal distance resulted in a 20% decrease in the stiffness of the tooth. A two-surface cavity preparation that destroyed one marginal ridge resulted in a 46% loss in tooth stiffness and an MOD preparation resulted in a 63% decrease in stiffness. The loss of each surface resulted in approximately a 20% decrease in stiffness. Endodontic procedures completed after restorative procedures resulted in a minimal further reduction (5 %) in tooth stiffness. This further reduction approximately equaled the reduction from the unaltered tooth as described in Table I. Thus, either before or after restorative procedures, endodontic procedures produced only minimal reductions in the stiffness of the tooth. Each step of the restorative procedures showed a significant loss of tooth stiffness relative to the previous step (p < 0.001 ). Despite the small additional loss of stiffness associated with endodontic access opening following M O D cavity preparation (0.37 to 0.33), the difference was statistically significant (p < 0.05); in every case there was an additional decrease in stiffness related to the access opening. DISCUSSION Closed loop servo-hydraulics is a nondestructive testing technique. This permits the sequential evaluation of a series of procedures and the use of the unaltered tooth as its own control. This technique reduces intertooth variation, permitting smaller sample sizes to achieve statistical significance9 The technique is very sensitive and reproducible. An equivalent tooth structure reduction, regardless of which sequence of procedures was used to attain the reduction, resulted in the same loss of stiffness. This was observed when the tooth structure reduction was accomplished by doing endodontic procedures followed by restorative procedures (relative stiffhess of 0.31) compared with restorative procedures followed by endodontic procedures (relative stiffness of 0.32). Endodontic procedures did not produce substantial effects on the relative stiffness of the tooth (5% reduction in tooth
i
i
o.4
0.6
|
0.8
i
1 0
i
1.2
Relative Stiffness FIG 3. Graphic representation of the restorative procedures data from Table 2. Each reduction of a tooth surface resulted in a substantial decrease in the tooth's stiffness. (Error bar represents 1 SD.)
Endodontic Procedures II .37
Access ~
.33
Instrumentation D
.34
Obturatlon
.32 .0
0.2
0.4
0 6
0.8
1 0
1.2
Relative Stiffness FiG 4. Graphic representation of the endodontic procedures data from Table 2. Endodontic procedures produced a small reduction in the tooth's stiffness after the reduction in stiffness from restorative procedures. (Error bar represents 1 SD.)
stiffness) whether endodontic access was obtained before or after cavity preparation. The study design did not permit evaluation of aging on the endodontically treated teeth. Thus, unless brittleness becomes a significant contribution to loss of tooth strength with time, endodontic procedures do not appear to be the major contributor to clinical fractures. Brittleness of endodontically treated teeth is not well documented. Of the two articles that directly address the question, A Vicker's hardness test on vital tooth dentin and root canaltreated teeth (with treatment as much as 10 yr previously) showed no statistical difference in hardness of endodontically treated teeth (5). Punch shear testing (6) indicated that endodontic treatment did not reduce toughness by more than 14%. This would suggest that the total effect of endodontic procedures is not great and is in fact comparable to an occlusal cavity preparation. The effects of cavity preparation were substantial compared with the negligible changes from endodontic procedures. Even
516
Journal of Endodontics
Reeh et al.
an occlusal cavity preparation resulted in a 4-fold greater decrease (20%) than endodontic access opening (5%). Although both procedures involve the same tooth surface, the greater encroachment of an occlusal cavity preparation on the marginal ridge area may account for this difference. Violation of marginal ridge integrity further compounded the loss of tooth strength. A two-surface cavity preparation resulted in a 46% loss in tooth stiffness. Extending the cavity preparation to an MOD resulted in a large total decrease in tooth stiffness of 63%. Endodontic procedures on the prepared tooth again resulted in a small decrease of approximately 5%. Clearly, restorative procedures are the major factor in weakening the tooth. Extrapolation from the premolars used in this study should be done with caution, but the same principles should apply to any posterior tooth. The principle of cuspal independence has been demonstrated in molars, so that a single cusp may be weakened as a result of a particular cavity outline (10, 1i, 15). The movement of cusps as measured by strain gauges or linear variable differential transformers has demonstrated the existence of cuspal independence (10). Posterior teeth would then be expected to respond in much the same manner as the bicuspids in this experiment. The exact nature and magnitude of this effect is not certain but they would be expected to reveal the same trends. Speculation with respect to molars suggests that greater movement may be experienced with tooth structure reductions. Certainly, most teeth that require endodontic treatment have lost more tooth structure than just an access preparation, but these results would suggest a need to evaluate further the restorative techniques for endodontically treated teeth. Tooth structure should be preserved where possible, consistent with good endodontic access. Marginal ridges should be preserved unless their removal is unavoidable for the purpose of restoration. CONCLUSIONS As a result of this investigation, the following conclusions can be drawn: 1. Endodontic procedures reduced tooth stiffness by only 5%, which is contributed entirely by the access opening. The 5% reduction in stiffness from endodonic procedures was insensitive to sequence as the same reduction resulted whether restorative procedures followed or preceded endodontic procedures. 2. Restorative procedures were the greatest contributor to loss of tooth stiffness. An MOD cavity preparation reduced tooth stiffness by more than 60%. 3. The loss of marginal ridge integrity was the greatest contribution to loss of tooth strength.
4. Whenever possible an intact marginal ridge should be preserved to maintain tooth strength. This research was supported in part by a Grarlt-ln-Aid of Research from the Endowment and Memorial Foundation of the American Association of Endodontists. The opinions, assertions, materials, and methodologies herein are private ones of the authors and are not to be construed as official or reflecting the views of the American Association of Endedontists or the Endowment and Memorial Foundation. This research was presented at the 45th Annual Meeting of the American Association of Endedontists, Anaheim, CA, where it won first place in the Graduate Student Presentations. This work was based on a thesis submitted to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the MS degree. This study was also supported by Grant DE 08732 from the National Institute for Dental Research, Bethesda, ME). Composite resin restorative material and dentin bonding agent were supplied by Minnesota Mines, Metals, and Manufacturing (3M). Dr. Reeh is a graduate student in endudontics, School of Dentistry, University of Minnesota, Minneapolis, MN. Dr. Messer is acting chairman of the Department of Rehabilitative Sciences, chairman of the Division of Endodontics, School of Dentistry, University of Minnesota, Minneapolis. Dr. Douglas is chairman of the Division of Biomaterials, School of Dentistry, University of Minnesota, Minneapolis.
References 1. Ross IF. Fracture susceptibility of endodontically treated teeth. J Endodon 1980;6:560-5. 2. Sorenson JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J Prosthet Dent 1984; 51:780-4. 3. Sorenson JA, Martinoff JT. Endodonticelly treated teeth as abutments. J Prosthet Dent 1985;53:631-6. 4. Heifer AR, Melnick S, Schilder H. Determination of the moisture content of vital and polpless teeth. Oral Surg 1972;34:661-70. 5. Lewinstein I, Grajower R. Root dentin hardness of endodentically treated teeth. J Endodon 1981 ;7:421-2. 6. Carter JM, Sorensen SE, Johnson RL, Teiteibaum RL, Levine MS. Punch shear testing of extracted vital and endodontically treated teeth. J Biomech 1983;16:841-8. 7. Mondelli J, Steagall L, Ishikiriama A, Navarro MFL, Soares FB. Fracture strength of human teeth with cavity preparations. J Prosthet Dent 1980; 43:419-22. 8. Larsen TD. Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent 1981 ;6:2-5. 9. Wendt SL Jr, Harris BM, Hunt TE. Resistance to cusp fracture in endodontically treated teeth. Dent Mater 1987;3:232-5. 10. Hood JAA. Methods to improve fracture resistance of teeth. In: Vanherle G, Smith DC, eds. International Symposium on posterior composite resin dental restorative materials. Symposium sponsored by 3M, St. Paul, MN, 1985; 443-50. 11. Douglas WH. Methods to improve fracture resistance of teeth. In: Vanherle G, Smith DC, eds. International Symposium on posterior composite resin dental restorative materials. Symposium sponsored by 3M, St. Paul, MN, 1985:433-41. 12. Morin D, Douglas WH, Cross M, DeLong R. Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater 1988;4:41-8. 13. Morin D, Delong R, Douglas WH. Cusp reinforcement with the acidetch technique. J Dent Res 1984;63:1075-8. 14. DeBoever JA, McCall, WD, Holden S, Ash MM Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326-33. 15. Tidmarsh BG. Restoration of endodontically treated posterior teeth. J Endodon 1976;2:374-5.