0099-2399/92/1807-0332/$03.00/0 JOURNAL OF ENDODONTICS Copyright © 1992 by The American Association of Endodontists
Printed in U.S.A.
VOL. 18, NO. 7, JULY 1992
Are Endodontically Treated Teeth more Brittle? Christine M. Sedgley, BDS, MDSc, FRACDS, and Harold H. Messer, MDSc, PhD
extracted vital teeth and 6 teeth endodontically treated at least 1 yr previously. These conclusions should perhaps be viewed with caution, as Smith and Cooper (9) reported a 3-fold range in punch shear strength of dentin within a single tooth depending on site of sampling. In contrast to the study of Carter et al. (8), it has recently been suggested that the compressive and tensile strengths of dentin from pulpless teeth are not significantly different from those of normal dentin (10). Also, Lewinstein and Grajower (11) found no difference in the Vickers microhardness of dentin of 16 extracted vital teeth and 32 teeth extracted at various intervals after endodontic treatment (0.2 to 10 yr). Furthermore, recent research indicated that no statistically significant differences existed in the collagen cross-link content of dentin of age- and site-matched root-filled and normal teeth (12). Substantiated studies showing a reduction in elasticity and an increase in brittleness in dentin of endodonticaUy treated teeth are thus sparse, leaving relevant clinical questions unanswered. This study was planned to answer the question of whet~er loss of pulp vitality results in changes in tooth structure" Biomechanical properties (punch shear strength, toughness, load to fracture, and microhardness) of the dentin of extracted teeth with a known history of endodontic treatment were compared with those of the contralateral vital tooth from the same patient. In addition, because the effects of time after extraction may produce inconsistent data (13), matched-pair vital teeth were used to compare punch shear strength, toughness, and load to fracture measurements obtained immediately following extraction and 3 months later.
This study compared biomechanical properties (punch shear strength, toughness, hardness, and load to fracture) of 23 endodontically treated teeth (mean time since endodontic treatment: 10.1 yr) and their contralateral vital pairs. Analyses using paired t tests revealed no significant differences in punch shear strength, toughness, and load to fracture between the two groups. Vital dentin was 3.5% harder than dentin from contralateral endodontically treated teeth (p = 0.002). The similarity between the biomechanical properties of endodontically treated teeth and their contralateral vital pairs indicates that teeth do not become more brittle following endodontic treatment. Other factors may be more critical to failure of endodontically treated teeth.
The endodontic and prosthodontic literature contains repeated references to the widely held clinical perception that endodontic treatment weakens teeth, resulting in increased brittleness (i-4). While Rosen (1) described the dentin of endodontically treated teeth as "desiccated and inelastic," Johnson et at. (2) additionally speculated that the elasticity of dentin decreased with time following endodontic treatment. Alternatively, it has been suggested that, rather than endodontic treatment, loss of tooth structure associated with restorative procedures was the major factor in weakening teeth. Recently, it was shown that endodontic procedures reduced the relative cuspal stiffness of premolar teeth by only 5%, in contrast to an occlusal cavity preparation (20%) and an MOD cavity preparation (63%) (5). The purported brittleness of endodontically treated teeth has been attributed to decreased moisture content. The supporting evidence for this is primarily a study by Heifer et al. (6) which showed a 9% lower moisture content of pulpless versus vital dog teeth. Very few studies have actually compared physical properties of endodontically treated versus vital teeth. Stanford et al. (7) found no significant differences in certain properties of dentin (modulus of elasticity, proportional limit, and strength) from three pairs of patient-matched vital and pulpless incisors. However, details of previous endodontic treatment were not provided. Carter et al. (8) reported that the dentin of endodontically treated teeth had a 14% lower punch shear strength and toughness than vital teeth. This was based upon a comparison between 21 freshly
MATERIALS AND METHODS Biomechanical Properties of Endodontically Treated versus Vital Teeth
SELECTION OF TEETH Teeth were obtained from the Casualty and Oral Surgery Departments of the Royal Dental Hospital of Melbourne and affiliated hospitals. Ethics approval was obtained from the Human Ethics Committee. Twenty-three matched pair teeth with relatively large, regularly shaped straight roots were selected following clinical and radiographic examination of patients scheduled for multiple extractions for prosthetic reasons. One tooth of each pair had a history of endodontic
332
Vol. 18, No. 7, July 1992
treatment, and vitality of the contralateral tooth was confirmed by thermal testing or by examining the tooth for the presence of pulp tissue when the tooth was sectioned. Those teeth excluded had a history of periapical surgery, extensive periodontal disease, and endodontic treatment performed less than 1 yr previously. A history of patient age and sex, and time since endodontic treatment was recorded. Each pair was extracted by the same operator at the same appointment. Following extraction, teeth were stored in sterile-buffered saline plus 0.05% sodium azide. During subsequent preparation and testing phases of the experiment, care was taken to prevent dehydration. Seventeen of the matched pairs were prepared and tested for punch shear strength, toughness, and load to fracture immediately after extraction (within 3 to 5 h). One pair was tested 3 days after extraction, two pairs 2 months after extraction, and three pairs 3 months after extraction. Microhardness testing was performed within 5 days of the other tests. All testing was done at room temperature. PREPARATION OF TEETH The root tip was flattened using a diamond bur in a highspeed handpiece. The tooth was then mounted vertically in an aluminum ring by embedding the apical 3 mm in coldcure acrylic. The ring was then mounted in an Isomet lowspeed saw (Buehler, Lake Bluff, IL) and the crown of the tooth was removed just below the cementoenamel junction. Two slices 0.3 to 0.4 mm in thickness were then cut from the cervical root dentin perpendicular to the long axis of the tooth. Each prepared dentin slice was smoothed on 600 grit silicon carbide abrasive paper. Both cross-sections were subsequently used for punch shear testing, with one section additionally used for microhardness testing. The remaining root segment was trimmed, where necessary, to leave a segment 8 mm in length, for loading to fracture (Fig. I). Using a cone-shaped stone bur in a slow-speed handpiece, a l-ram deep seat was then cut in the coronal opening of the root canal for seating of the loading device. PUNCH SHEAR TESTING Punch shear strength (MPa) is the maximum stress that a material can withstand before failure in a punch shear mode of loading. Toughness (MJ/m -3) is defined as the energy required to fracture a material, and is measured as the area under the elastic and plastic portions of a stress-strain curve. A punch shear apparatus similar to that described by Carter et al. (8) (Fig. 2) mounted on a Shimadzu universal testing machine (Autograph IS-5000; Shimadzu, Seisakusho Ltd., Kyoto, Japan) was used. The apparatus consisted of two cylindrical steel dies aligned together with four dowels and secured with two screws. A tungsten carbide bush was mounted within the central axis of the upper and lower dies, and a cylindrical tungsten carbide rod was used as a punch (type 522; Pr~isionswerkzeuge GmhH, Ravensburg, West Germany). The internal diameter of the upper bush was 1.000 mm and of the lower bush 1.050 mm. Punch diameter was 0.993 mm. Concentricity between the punch and the bush of the lower die was confirmed by making an impression of the punch and die on polyester film which was then viewed under x6 magnification.
Brittleness of Teeth
i••
333
8 mm
FtG 1. Diagrammatic representation of preparation of root segment for load to fracture testing. The tooth was mounted vertically in 3mm of cold-cure acrylic resin and the crown and two cervical slices (for punch shear strength, toughness, and microhardness testing) were removed. A 1-mm-deep seat was then cut in the coronal opening of the 8-ram root segment (R) for seating of the loading device (L). Using a constant crosshead speed of 0.1 mm min-1 the root segment was loaded to fracture.
c,-,
|
,~
A
FiG 2. Diagrammatic representation of punch shear device for punch shear strength and toughness testing. The cervical dentin sample (C) 0.3- to 0.4-ram thick was constrained between upper and lower dies secured with two screws. A tungsten carbide rod (A) 0.993 mm in diameter was mounted in a tungsten carbide bush (B) located in the center of the die. A load was applied to the rod at a crosshead speed of 0.1 mm min -1 until punching occurred.
A comparator employing a micrometer reading to 0.01 mm was used to measure the thickness of each section buccally and lingually at a distance mid-way between the root canal and the periphery of the section. The dentin slice to be punched was positioned over the bush of the lower die. Care was taken to ensure accurate positioning of the test site over the punch hole. All tests were performed at least 1.0 mm from the specimen periphery and root canal edge on buccal and lingual aspects. The upper die was then placed over the lower die and secured so that the dentin section was constrained. A load was applied to the punch at a constant crosshead speed of 0.1 mm min -~ until punching had occurred, as determined from the chart recording. Two tests were performed per section. Punch shear strength and toughness values were calculated as described by Carter et at. (8), with correction for machine compliance. The mean punch shear strength and toughness for each tooth were then calculated. LOAD TESTING The root segment was positioned vertically on the lower platen of the Shimadzu testing machine with the coronal face upward. A cylindrical hardened steel rod attached to the upper crosshead was lowered until the cone-shaped point of the rod
334
Sedgley and Messer
rested in the prepared coronal root face seat (Fig. 1). Using a constant crosshead speed of 0.1 m m min-~ each root segment was loaded to fracture.
Journal of EndodonUcs
One dentin slice per tooth was glued to a glass slide (Petrographic Slides; Buehler) with epoxy resin (Araldite Super Strength Epoxy Resin; Selleys Chemical Co. Pry Ltd., Padstow, NSW, Australia) and polished (Minimet Polisher; Buehler) until all scratches created by the saw were removed. The specimen was wet polished on 600 grade silicon carbide paper, followed by Texmet Cloth (Buehler) and Microcloth (Buehler) with diamond polishing paste. Microhardness of dentin was measured using a microhardness tester (Leitz MINILOADHardness Tester; Ernst Leitz G m b H Wetzlar, FRG) under a 100-g load, and the Vickers Hardness Number was obtained. Each pair was tested at identical sites. Three indentations were made mid-way between the root canal and the periphery of the specimen on both mesial and distal aspects, and the mean value was calculated. The thickness of all slices exceeded 1.5 times the length of the diagonal of the indentation produced.
maining 20 pairs, 11 were from females and 9 from males. Patient ages ranged from 21 to 72 yr with a mean of 47.2 yr. Time since endodontic treatment ranged from 1 to 25 yr with a mean of 10.1 yr; only four teeth had been treated less than 5 yr previously. The teeth tested were maxillary central incisor (n = 6), maxillary lateral incisor (n = 5), maxillary canine (n = 4), maxillary first premolar (n = 2), maxillary second premolar (n = 3), maxillary second molar (palatal root) (n = 1), mandibular canine (n = 1), and mandibular third molar (single rooted) (n = 1). All teeth were extracted for prosthetic reasons. None of the endodontically treated teeth were removed because of failure of endodontic treatment. The mean punch shear strength, toughness, hardness, and load at fracture are shown in Table 2. There were no statistically significant differences between endodontically treated and contralateral vital teeth in punch shear strength (p = 0.710) and toughness (p = 0.089). Despite a difference in hardness of only 3.5 %, vital teeth were highly significantly (p = 0.002) harder than contralateral endodontically treated teeth. Two samples for load testing were lost during processing and the data for their matched pairs were excluded from further analyses. The load to fracture values were not significantly different between the two groups (p = 0.149).
Effect of Storage on Physical Properties
DISCUSSION
Eight matched pairs of vital teeth were obtained as before, but with crowns unrestored and intact. Patient age and sex were recorded. One tooth of each pair was tested immediately after extraction for punch shear strength, toughness, and load to fracture as described above. The contralateral tooth was tested 3 months later following storage in sterile-buffered saline plus 0.05% sodium azide. Microhardness testing was not performed.
The clinical concept of "brittle" or weakened endodontically treated teeth has been attributed to loss of tooth structure following trauma, caries, endodontic access, instrumentation and irrigation procedures, and/or to changes in properties of teeth following endodontic treatment (3, 6, 8). Although Heifer et al. (6) reported a 9% lower water content of pulpless teeth versus vital teeth, few studies have compared the biomechanical properties of dentin of endodonticaUy treated teeth with vital teeth (7, 8, 11). The only report of a difference
MICROHARDNESS TESTING
Statistical Analysis The data for each property tested were subjected to paired t test analysis. For punch shear strength, toughness, and hardness, paired t tests were performed on the mean values obtained for each tooth. RESULTS
Effect of Storage on Physical Properties Eight matched pairs were obtained from four males and three females. Patient ages ranged from 21 to 60 yr with a mean of 40.5 yr. The teeth tested were maxillary central incisor (n = 4), maxillary lateral incisor (n = 2), maxillary canine (n = 1), and mandibular canine (n = 1). There were no significant differences between the two groups in punch shear strength, toughness, and load to fracture (Table 1). These results indicated that data from teeth stored for up to 3 months after extraction could be included in the subsequent analyses.
Endodontically Treated versus Vital Teeth Data on patient age, sex, and time since endodontic treatment were unavailable for three matched pairs. Of the re-
TABLE 1. Effect of storage on biomechanical properties of teeth* Immediate Punch shear strength (MPa) Punch shear toughness (MJ/m -3) Load to fracture (N)
Testing
Three-Month Storage
P
64.97 __+7.91
63.63 __ 9.99
= 0.733
35.16 +_ 3.49
33.09 +_ 5.72
= 0.267
724 _+ 206
642 -+ 187
= 0.215
• All values represent mean -+ SD for
eight matched.pairs.
TABLE 2. Comparison of properties of endodontically treated versus contralateral vital teeth* Endodontically Treated Teeth
Vital Teeth
p
Punch shear strength (MPa)
70.42 _+ 12.39 69.76 ___11.69 = 0.710
Punch shear toughness (MJ/m -3)
42.51 + 10.38 40.08_+ 8.91
= 0.089
Microhardness(Vickers hardness no.) Load to fracture (N)
66.79__ 4.83
69.15 + 4.89
= 0.002
611 ___148
574 ___153
= 0.149
• All values represent mean _+ SD for 23 matched pairs.
Brittleness of Teeth
Vol. 18, No. 7, July 1992
in properties between endodontically treated and vital teeth (8) was based on six endodontically treated teeth. Stanford et al. (7) reported no difference in compressive properties (modulus of elasticity, proportional limit, and strength) of three matched pairs, hut details of previous endodontic treatment were unavailable. Although not eliminating all dissimilarities, the use of matched pairs in our study allowed a reduction in certain variables usually associated with testing extracted human teeth. Additionally, the larger number of matched pairs and the range of tooth type compared with previous studies provided the opportunity to make a broader assessment. The mean time since endodontic treatment (10.1 yr) should have been sufficient for any structural changes in dentin to have occurred. Punch shear strength and toughness values were in the lower range of what has been previously reported (8, 9, 14). Differences between our study and previous work with respect to sample thickness, punch diameter, and the use of constrained samples could account for these differences, although Carter et al. (8) reported an increase in values of samples constrained during testing. The microhardness measurements obtained in this study agree well with those of previous investigators (11, 15, 16). Although we found that microhardness values for vital teeth were statistically significantly higher than those for contralateral endodontically treated teeth (p = 0.002), this small difference (3.5%) is unlikely to be clinically significant. A comparison of vertical load to fracture of endodonticatly treated teeth and contralateral vital teeth has not been previously reported. Clinically, vertical root fractures are a common means of failure of endodontically treated teeth and are thought to result from the trauma associated with lateral condensation procedures and pin/post placement in endodontically treated teeth (17). An in vitro study by Pitts et al. (18) found that vertical root fractures occurred using large spreaders under minimum loads of 7.2 kg in maxillary central incisors. The testing procedure of our study attempted to simulate vertical root fracture by applying a wedging load to the coronal face of the root segment until longitudinal fracture occurred. There are several problems associated with the mechanical testing of dentin. These include obtaining sound developmentally normal teeth that have not been damaged during extraction, cutting small pieces of dentin from the tooth without creating thermal or mechanical damage, and the unknown effects of storage media and time. The effects of storage on biomechanical testing of dentin have not been extensively reported. Carter et al. (8) found that different storage media (tap water or mineral oil) did not affect shear strength and toughness results but that dry storage significantly increased values for mandibular incisors. In our study it was apparent that 3-month storage in sterile saline did not affect the values obtained. Other testing procedures may be more sensitive to potential postextraction changes. Causton et al. (13) found that storage affected the bond shear strength between polycarboxylate cement and human dentin and subsequently suggested that testing of dentin should take place within 20 min of tooth extraction. However, the preparation procedures necessary for testing of dental tissues may require longer working times. We have attempted to answer the question of whether the loss of a vital pulp, resulting in subsequent loss of the nutrient
335
supply to dentin, could lead to progressive changes in the biomechanical properties of dentin. The results of this and other investigations suggest that this is not the case (10, 12). The similarity between the biomechanical properties of endodontically treated teeth and their contralateral vital pairs suggests that other factors may be more critical to failure of endodontically treated teeth. Reeh et al. (5) showed that the endodontic access cavity produced only a 5% decrease in stiffness, in contrast to an MOD preparation which decreased tooth stiffness by 63%. We suggest that it is rather the cumulative loss of tooth structure from caries, trauma, and restorative and endodontic procedures that leads to susceptibility to fracture. Another possibility that has been suggested but never extensively explored is that loss of pressoreception (19), or an elevated pain threshold (20) allows larger loads on endodontically treated teeth without triggering a protective response. This project was supported by a grant from the National Health and Medical Research Council of Australia and was based on a thesis submitted to the University of Melbourne in partial fulfillment of the requirements for the MDSc degree. The authors gratefully acknowledge the technical assistance of David Cheadie and Koren Mitchell of the School of Dental Science, University of Melbourne, Melbourne, Australia. Dr. Sedgley is a graduate student in endodontics, School of Dental Science, University of Melbourne, Melbourne, Australia. Dr. Messer is professor of Restorative Dentistry, School of Dental Science, University of Melbourne.
References
1. Rosen H. Operative procedures on mutilated endodontically treated teeth. J Prosthet Dent 1961;11:972-86. 2. Johnson JK, Schwartz NL, Blackwell RT. Evaluation and restoration of endodontically treated posterior teeth_ J Am Dent Assoc 1976;93:597-605. 3. Greenfeld RS, Marshall FJ. Factors affecting dowel (post) selection and use in endodontically treated teeth. J Can Dent Assoc 1983;49:777-83. 4. Radke RA Jr, Eissmann HF. Postendodontic restoration. In: Cohen S, Burns RC, eds. Pathways of the pulp. 5th ed. St. Louis: CV Mosby, 1991. 5. Reeh ES, Messer HH, Douglas WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endodon 1989;15:512-6. 6. Heifer AR, Me/nick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Meal Oral Patho11972;34:661-70. 7. Stanford JW, Weigal KV, Paffenbarger GC, Sweeney WT. Compressive properties of hard tooth tissue. J Am Dent Assoc 1960;60:746-56. 8. Carter JM, Sorenson SE, Johnson RR, Teitelbaum RL, Levine MS. Punch shear testing of extracted vital and endodontically treated teeth. J Biomech 1983; 16:841-8. 9. Smith DC, Cooper WEG. The determination of shear strength. A method using a micro-punch apparatus. Br Dent J 1971 ;130:333-7. 10. Huang TG, Schilder H, Nathanson D. Effects of moisture content and endodontic treatment on some mechanical properties of human dentin [Abstract]. J Endodon 1991 ;17:194. 11. Lewinstein I, Grajower R. Root dentin hardness of endodontically treated teeth. J Endodon 1981;7:421-2. 12. Rivera E, Yamauchi M. Dentin collagen cross-links of root-filled and normal teeth. [Abstract]. J Endodon 1990;16:190. 13. Causton BE, Johnson NW. Changes in the dentin of human teeth following extraction and their implication for in vivo studies of adhesion to tooth substance. Arch Oral Bio11979;24:229-32. 14. Roydhouse RH. Punch shear test for dental purposes. J Dent Res 1970;49:131-6. 15. Craig RG, Peyton FA. The microhardness of enamel and dentin. J Dent Res 1958;37:661-8. 16. Renson CE, Braden M. The experimental deformation of human dentin by indenters. Arch Oral Bio) 1971 ;16:563-72. 17. Tamse A. latrogenic vertical root fractures in endodontically treated teeth. Ended Dent Traumato11988;4:190-6. 18. Pitts DL, Matheny HE, Nicholls Jl. An in vitro study of spreader loads required to cause vertical root fracture during lateral condensation. J Endodon 1983;9:544-50. 19. Lowenstein NR, Rathkamp R. A study on the pressoreceptive sensibility of the tooth. J Dent Res 1955;34:287-94. 20. Randow K, Glantz P-O. On cantilever loading of vital and non-vital teeth. Acta Odontol Scand 1986;44:271-7.
Printed in U.S.A.
VOL. 18, NO. 7, JULY 1992
Are Endodontically Treated Teeth more Brittle? Christine M. Sedgley, BDS, MDSc, FRACDS, and Harold H. Messer, MDSc, PhD
extracted vital teeth and 6 teeth endodontically treated at least 1 yr previously. These conclusions should perhaps be viewed with caution, as Smith and Cooper (9) reported a 3-fold range in punch shear strength of dentin within a single tooth depending on site of sampling. In contrast to the study of Carter et al. (8), it has recently been suggested that the compressive and tensile strengths of dentin from pulpless teeth are not significantly different from those of normal dentin (10). Also, Lewinstein and Grajower (11) found no difference in the Vickers microhardness of dentin of 16 extracted vital teeth and 32 teeth extracted at various intervals after endodontic treatment (0.2 to 10 yr). Furthermore, recent research indicated that no statistically significant differences existed in the collagen cross-link content of dentin of age- and site-matched root-filled and normal teeth (12). Substantiated studies showing a reduction in elasticity and an increase in brittleness in dentin of endodonticaUy treated teeth are thus sparse, leaving relevant clinical questions unanswered. This study was planned to answer the question of whet~er loss of pulp vitality results in changes in tooth structure" Biomechanical properties (punch shear strength, toughness, load to fracture, and microhardness) of the dentin of extracted teeth with a known history of endodontic treatment were compared with those of the contralateral vital tooth from the same patient. In addition, because the effects of time after extraction may produce inconsistent data (13), matched-pair vital teeth were used to compare punch shear strength, toughness, and load to fracture measurements obtained immediately following extraction and 3 months later.
This study compared biomechanical properties (punch shear strength, toughness, hardness, and load to fracture) of 23 endodontically treated teeth (mean time since endodontic treatment: 10.1 yr) and their contralateral vital pairs. Analyses using paired t tests revealed no significant differences in punch shear strength, toughness, and load to fracture between the two groups. Vital dentin was 3.5% harder than dentin from contralateral endodontically treated teeth (p = 0.002). The similarity between the biomechanical properties of endodontically treated teeth and their contralateral vital pairs indicates that teeth do not become more brittle following endodontic treatment. Other factors may be more critical to failure of endodontically treated teeth.
The endodontic and prosthodontic literature contains repeated references to the widely held clinical perception that endodontic treatment weakens teeth, resulting in increased brittleness (i-4). While Rosen (1) described the dentin of endodontically treated teeth as "desiccated and inelastic," Johnson et at. (2) additionally speculated that the elasticity of dentin decreased with time following endodontic treatment. Alternatively, it has been suggested that, rather than endodontic treatment, loss of tooth structure associated with restorative procedures was the major factor in weakening teeth. Recently, it was shown that endodontic procedures reduced the relative cuspal stiffness of premolar teeth by only 5%, in contrast to an occlusal cavity preparation (20%) and an MOD cavity preparation (63%) (5). The purported brittleness of endodontically treated teeth has been attributed to decreased moisture content. The supporting evidence for this is primarily a study by Heifer et al. (6) which showed a 9% lower moisture content of pulpless versus vital dog teeth. Very few studies have actually compared physical properties of endodontically treated versus vital teeth. Stanford et al. (7) found no significant differences in certain properties of dentin (modulus of elasticity, proportional limit, and strength) from three pairs of patient-matched vital and pulpless incisors. However, details of previous endodontic treatment were not provided. Carter et al. (8) reported that the dentin of endodontically treated teeth had a 14% lower punch shear strength and toughness than vital teeth. This was based upon a comparison between 21 freshly
MATERIALS AND METHODS Biomechanical Properties of Endodontically Treated versus Vital Teeth
SELECTION OF TEETH Teeth were obtained from the Casualty and Oral Surgery Departments of the Royal Dental Hospital of Melbourne and affiliated hospitals. Ethics approval was obtained from the Human Ethics Committee. Twenty-three matched pair teeth with relatively large, regularly shaped straight roots were selected following clinical and radiographic examination of patients scheduled for multiple extractions for prosthetic reasons. One tooth of each pair had a history of endodontic
332
Vol. 18, No. 7, July 1992
treatment, and vitality of the contralateral tooth was confirmed by thermal testing or by examining the tooth for the presence of pulp tissue when the tooth was sectioned. Those teeth excluded had a history of periapical surgery, extensive periodontal disease, and endodontic treatment performed less than 1 yr previously. A history of patient age and sex, and time since endodontic treatment was recorded. Each pair was extracted by the same operator at the same appointment. Following extraction, teeth were stored in sterile-buffered saline plus 0.05% sodium azide. During subsequent preparation and testing phases of the experiment, care was taken to prevent dehydration. Seventeen of the matched pairs were prepared and tested for punch shear strength, toughness, and load to fracture immediately after extraction (within 3 to 5 h). One pair was tested 3 days after extraction, two pairs 2 months after extraction, and three pairs 3 months after extraction. Microhardness testing was performed within 5 days of the other tests. All testing was done at room temperature. PREPARATION OF TEETH The root tip was flattened using a diamond bur in a highspeed handpiece. The tooth was then mounted vertically in an aluminum ring by embedding the apical 3 mm in coldcure acrylic. The ring was then mounted in an Isomet lowspeed saw (Buehler, Lake Bluff, IL) and the crown of the tooth was removed just below the cementoenamel junction. Two slices 0.3 to 0.4 mm in thickness were then cut from the cervical root dentin perpendicular to the long axis of the tooth. Each prepared dentin slice was smoothed on 600 grit silicon carbide abrasive paper. Both cross-sections were subsequently used for punch shear testing, with one section additionally used for microhardness testing. The remaining root segment was trimmed, where necessary, to leave a segment 8 mm in length, for loading to fracture (Fig. I). Using a cone-shaped stone bur in a slow-speed handpiece, a l-ram deep seat was then cut in the coronal opening of the root canal for seating of the loading device. PUNCH SHEAR TESTING Punch shear strength (MPa) is the maximum stress that a material can withstand before failure in a punch shear mode of loading. Toughness (MJ/m -3) is defined as the energy required to fracture a material, and is measured as the area under the elastic and plastic portions of a stress-strain curve. A punch shear apparatus similar to that described by Carter et al. (8) (Fig. 2) mounted on a Shimadzu universal testing machine (Autograph IS-5000; Shimadzu, Seisakusho Ltd., Kyoto, Japan) was used. The apparatus consisted of two cylindrical steel dies aligned together with four dowels and secured with two screws. A tungsten carbide bush was mounted within the central axis of the upper and lower dies, and a cylindrical tungsten carbide rod was used as a punch (type 522; Pr~isionswerkzeuge GmhH, Ravensburg, West Germany). The internal diameter of the upper bush was 1.000 mm and of the lower bush 1.050 mm. Punch diameter was 0.993 mm. Concentricity between the punch and the bush of the lower die was confirmed by making an impression of the punch and die on polyester film which was then viewed under x6 magnification.
Brittleness of Teeth
i••
333
8 mm
FtG 1. Diagrammatic representation of preparation of root segment for load to fracture testing. The tooth was mounted vertically in 3mm of cold-cure acrylic resin and the crown and two cervical slices (for punch shear strength, toughness, and microhardness testing) were removed. A 1-mm-deep seat was then cut in the coronal opening of the 8-ram root segment (R) for seating of the loading device (L). Using a constant crosshead speed of 0.1 mm min-1 the root segment was loaded to fracture.
c,-,
|
,~
A
FiG 2. Diagrammatic representation of punch shear device for punch shear strength and toughness testing. The cervical dentin sample (C) 0.3- to 0.4-ram thick was constrained between upper and lower dies secured with two screws. A tungsten carbide rod (A) 0.993 mm in diameter was mounted in a tungsten carbide bush (B) located in the center of the die. A load was applied to the rod at a crosshead speed of 0.1 mm min -1 until punching occurred.
A comparator employing a micrometer reading to 0.01 mm was used to measure the thickness of each section buccally and lingually at a distance mid-way between the root canal and the periphery of the section. The dentin slice to be punched was positioned over the bush of the lower die. Care was taken to ensure accurate positioning of the test site over the punch hole. All tests were performed at least 1.0 mm from the specimen periphery and root canal edge on buccal and lingual aspects. The upper die was then placed over the lower die and secured so that the dentin section was constrained. A load was applied to the punch at a constant crosshead speed of 0.1 mm min -~ until punching had occurred, as determined from the chart recording. Two tests were performed per section. Punch shear strength and toughness values were calculated as described by Carter et at. (8), with correction for machine compliance. The mean punch shear strength and toughness for each tooth were then calculated. LOAD TESTING The root segment was positioned vertically on the lower platen of the Shimadzu testing machine with the coronal face upward. A cylindrical hardened steel rod attached to the upper crosshead was lowered until the cone-shaped point of the rod
334
Sedgley and Messer
rested in the prepared coronal root face seat (Fig. 1). Using a constant crosshead speed of 0.1 m m min-~ each root segment was loaded to fracture.
Journal of EndodonUcs
One dentin slice per tooth was glued to a glass slide (Petrographic Slides; Buehler) with epoxy resin (Araldite Super Strength Epoxy Resin; Selleys Chemical Co. Pry Ltd., Padstow, NSW, Australia) and polished (Minimet Polisher; Buehler) until all scratches created by the saw were removed. The specimen was wet polished on 600 grade silicon carbide paper, followed by Texmet Cloth (Buehler) and Microcloth (Buehler) with diamond polishing paste. Microhardness of dentin was measured using a microhardness tester (Leitz MINILOADHardness Tester; Ernst Leitz G m b H Wetzlar, FRG) under a 100-g load, and the Vickers Hardness Number was obtained. Each pair was tested at identical sites. Three indentations were made mid-way between the root canal and the periphery of the specimen on both mesial and distal aspects, and the mean value was calculated. The thickness of all slices exceeded 1.5 times the length of the diagonal of the indentation produced.
maining 20 pairs, 11 were from females and 9 from males. Patient ages ranged from 21 to 72 yr with a mean of 47.2 yr. Time since endodontic treatment ranged from 1 to 25 yr with a mean of 10.1 yr; only four teeth had been treated less than 5 yr previously. The teeth tested were maxillary central incisor (n = 6), maxillary lateral incisor (n = 5), maxillary canine (n = 4), maxillary first premolar (n = 2), maxillary second premolar (n = 3), maxillary second molar (palatal root) (n = 1), mandibular canine (n = 1), and mandibular third molar (single rooted) (n = 1). All teeth were extracted for prosthetic reasons. None of the endodontically treated teeth were removed because of failure of endodontic treatment. The mean punch shear strength, toughness, hardness, and load at fracture are shown in Table 2. There were no statistically significant differences between endodontically treated and contralateral vital teeth in punch shear strength (p = 0.710) and toughness (p = 0.089). Despite a difference in hardness of only 3.5 %, vital teeth were highly significantly (p = 0.002) harder than contralateral endodontically treated teeth. Two samples for load testing were lost during processing and the data for their matched pairs were excluded from further analyses. The load to fracture values were not significantly different between the two groups (p = 0.149).
Effect of Storage on Physical Properties
DISCUSSION
Eight matched pairs of vital teeth were obtained as before, but with crowns unrestored and intact. Patient age and sex were recorded. One tooth of each pair was tested immediately after extraction for punch shear strength, toughness, and load to fracture as described above. The contralateral tooth was tested 3 months later following storage in sterile-buffered saline plus 0.05% sodium azide. Microhardness testing was not performed.
The clinical concept of "brittle" or weakened endodontically treated teeth has been attributed to loss of tooth structure following trauma, caries, endodontic access, instrumentation and irrigation procedures, and/or to changes in properties of teeth following endodontic treatment (3, 6, 8). Although Heifer et al. (6) reported a 9% lower water content of pulpless teeth versus vital teeth, few studies have compared the biomechanical properties of dentin of endodonticaUy treated teeth with vital teeth (7, 8, 11). The only report of a difference
MICROHARDNESS TESTING
Statistical Analysis The data for each property tested were subjected to paired t test analysis. For punch shear strength, toughness, and hardness, paired t tests were performed on the mean values obtained for each tooth. RESULTS
Effect of Storage on Physical Properties Eight matched pairs were obtained from four males and three females. Patient ages ranged from 21 to 60 yr with a mean of 40.5 yr. The teeth tested were maxillary central incisor (n = 4), maxillary lateral incisor (n = 2), maxillary canine (n = 1), and mandibular canine (n = 1). There were no significant differences between the two groups in punch shear strength, toughness, and load to fracture (Table 1). These results indicated that data from teeth stored for up to 3 months after extraction could be included in the subsequent analyses.
Endodontically Treated versus Vital Teeth Data on patient age, sex, and time since endodontic treatment were unavailable for three matched pairs. Of the re-
TABLE 1. Effect of storage on biomechanical properties of teeth* Immediate Punch shear strength (MPa) Punch shear toughness (MJ/m -3) Load to fracture (N)
Testing
Three-Month Storage
P
64.97 __+7.91
63.63 __ 9.99
= 0.733
35.16 +_ 3.49
33.09 +_ 5.72
= 0.267
724 _+ 206
642 -+ 187
= 0.215
• All values represent mean -+ SD for
eight matched.pairs.
TABLE 2. Comparison of properties of endodontically treated versus contralateral vital teeth* Endodontically Treated Teeth
Vital Teeth
p
Punch shear strength (MPa)
70.42 _+ 12.39 69.76 ___11.69 = 0.710
Punch shear toughness (MJ/m -3)
42.51 + 10.38 40.08_+ 8.91
= 0.089
Microhardness(Vickers hardness no.) Load to fracture (N)
66.79__ 4.83
69.15 + 4.89
= 0.002
611 ___148
574 ___153
= 0.149
• All values represent mean _+ SD for 23 matched pairs.
Brittleness of Teeth
Vol. 18, No. 7, July 1992
in properties between endodontically treated and vital teeth (8) was based on six endodontically treated teeth. Stanford et al. (7) reported no difference in compressive properties (modulus of elasticity, proportional limit, and strength) of three matched pairs, hut details of previous endodontic treatment were unavailable. Although not eliminating all dissimilarities, the use of matched pairs in our study allowed a reduction in certain variables usually associated with testing extracted human teeth. Additionally, the larger number of matched pairs and the range of tooth type compared with previous studies provided the opportunity to make a broader assessment. The mean time since endodontic treatment (10.1 yr) should have been sufficient for any structural changes in dentin to have occurred. Punch shear strength and toughness values were in the lower range of what has been previously reported (8, 9, 14). Differences between our study and previous work with respect to sample thickness, punch diameter, and the use of constrained samples could account for these differences, although Carter et al. (8) reported an increase in values of samples constrained during testing. The microhardness measurements obtained in this study agree well with those of previous investigators (11, 15, 16). Although we found that microhardness values for vital teeth were statistically significantly higher than those for contralateral endodontically treated teeth (p = 0.002), this small difference (3.5%) is unlikely to be clinically significant. A comparison of vertical load to fracture of endodonticatly treated teeth and contralateral vital teeth has not been previously reported. Clinically, vertical root fractures are a common means of failure of endodontically treated teeth and are thought to result from the trauma associated with lateral condensation procedures and pin/post placement in endodontically treated teeth (17). An in vitro study by Pitts et al. (18) found that vertical root fractures occurred using large spreaders under minimum loads of 7.2 kg in maxillary central incisors. The testing procedure of our study attempted to simulate vertical root fracture by applying a wedging load to the coronal face of the root segment until longitudinal fracture occurred. There are several problems associated with the mechanical testing of dentin. These include obtaining sound developmentally normal teeth that have not been damaged during extraction, cutting small pieces of dentin from the tooth without creating thermal or mechanical damage, and the unknown effects of storage media and time. The effects of storage on biomechanical testing of dentin have not been extensively reported. Carter et al. (8) found that different storage media (tap water or mineral oil) did not affect shear strength and toughness results but that dry storage significantly increased values for mandibular incisors. In our study it was apparent that 3-month storage in sterile saline did not affect the values obtained. Other testing procedures may be more sensitive to potential postextraction changes. Causton et al. (13) found that storage affected the bond shear strength between polycarboxylate cement and human dentin and subsequently suggested that testing of dentin should take place within 20 min of tooth extraction. However, the preparation procedures necessary for testing of dental tissues may require longer working times. We have attempted to answer the question of whether the loss of a vital pulp, resulting in subsequent loss of the nutrient
335
supply to dentin, could lead to progressive changes in the biomechanical properties of dentin. The results of this and other investigations suggest that this is not the case (10, 12). The similarity between the biomechanical properties of endodontically treated teeth and their contralateral vital pairs suggests that other factors may be more critical to failure of endodontically treated teeth. Reeh et al. (5) showed that the endodontic access cavity produced only a 5% decrease in stiffness, in contrast to an MOD preparation which decreased tooth stiffness by 63%. We suggest that it is rather the cumulative loss of tooth structure from caries, trauma, and restorative and endodontic procedures that leads to susceptibility to fracture. Another possibility that has been suggested but never extensively explored is that loss of pressoreception (19), or an elevated pain threshold (20) allows larger loads on endodontically treated teeth without triggering a protective response. This project was supported by a grant from the National Health and Medical Research Council of Australia and was based on a thesis submitted to the University of Melbourne in partial fulfillment of the requirements for the MDSc degree. The authors gratefully acknowledge the technical assistance of David Cheadie and Koren Mitchell of the School of Dental Science, University of Melbourne, Melbourne, Australia. Dr. Sedgley is a graduate student in endodontics, School of Dental Science, University of Melbourne, Melbourne, Australia. Dr. Messer is professor of Restorative Dentistry, School of Dental Science, University of Melbourne.
References
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