Effect of post adaptation on fracture endodontically treated teeth John A. Sorensen,
University of California, School of Dentistry, Los Angeles, Calif. This study determined the effect of different post designs and varying amounts of post-to-canal adaptation on the fracture resistance of endodontically treated teeth. Forty freshly extracted maxillary central incisors were endodontically treated. Groups of 10 teeth were prepared according to four experimental designs. Cast post and cores and crowns were waxed, cast, and luted with zinc phosphate cement on a static loading device. The teeth were embedded in acrylic resin and the crowns were loaded on a universal testing machine at 130 degrees to the long axis of the tooth until failure. Maximum adaptation of the residual root structure with a tapered post significantly increases the fracture resistance of endodontically treated teeth, but upon failure renders the tooth nonrestorable. Tapered posts resulted in fractures that were directed more apically and lingually. Parallel-sided posts had a lower frequency of fracture upon failure, involving less tooth structure. Parallel-sided posts surrounded by large amounts of cement had no significant effect on failure loads. (J PROSTHET DENT 1990,64:419-24.)
frequent clinical dilemma occurswhen a funnelshaped canal results when restoring an endodontically treated tooth. The dentist must decide whether to use a parallel-sided post and fill the spacewith cement or usea tapered post that closely adaptsto the prepared canal. The amount of post-to-canal adaptation is an important but little researchedconsideration in the restoration of endodontically treated teeth. This in vitro study examined the effect of different post designsand the amount of post-to-canal adaptation on the fracture resistanceof endodontically treated anterior teeth.
Forty maxillary central incisorswere randomly assigned to four groupsof 10teeth and werestored in salinesolution. The teeth were alwayshandled with rubber glovesand were
kept moist with saline solution during all procedures. Endodontic treatment wascompleted on all 40 teeth and post spacewasprepared with a No.3 Peesoreamer (Union Broach Co., Long Island, N.Y.) to within 4 mm of the apex. A No.4 Para-Post (Whaledent International, New York, N.Y.) post was fitted to within 4 mm of the apex. The four groupsof teeth were prepared according to the guidelinesin Fig. 1. Simulating the clinical situation, crown preparations followed the cementoenameljunction with the facial and lingual margins 2 mm apical to the proximal Presented meeting, *Assistant **Assistant
before the Pacific Coast Society of Prosthodontists Napa, Calif. Professor, Director, Post-Graduate Prosthodontics. Professor, Section of Removable Prosthodontics.
margins. All teeth were prepared with high-speedwatercooled instrumentation and were decoronated to 15 mm length. Teeth groups 1,2, and 3 were prepared with a go-degree shoulder. Axial structure wasremoved with a cone-shaped carbide bur (Miltex Instruments Co, Lake Success,N.Y.) at slow speedunder water irrigation. Only 1 mm of axial tooth structure remained at the shoulder, creating a large funnel-shaped canal region. Group 4 teeth were prepared with a go-degreeshoulder and all remaining axial tooth structure was retained. The amount of axial tooth structure wasmeasuredat the points indicated in Fig. 2. The width of the root wasmeasured at several levels, as illustrated in Fig. 3. The length of the prepared canal was confirmed and a plastic No. 4 Para-Post burnout pattern (Whaledent International) was fitted to the canal. A preformed anterior core pattern (Parkell, Farmingdale, N..Y.) size No. 1 was adjusted to contact the proximal surfacesand was contoured to be 7 mm in length, as measuredfrom the facial margin to the incisal edge.The corepattern wasfilled with autopolymerizing acrylic resin (Duralay, Reliance Dental Mfg. Co., Chicago, Ill.) and was seated on the wet tooth maintaining the core in the long axis of the tooth from the proximal view. Group 1 teeth had the Para-Post post only, with no acrylic resin extending into the preparedcanal (Fig. 4). Group 2 had acrylic resin intimately adapted to the canal, reflecting the tapered shapeof the canal. Group 3 had a Para-Post post with 2 mm of acrylic resin extending into the canal. Group 4 had a Para-Post post only obturating the canal. After polymerization, the acrylic resin core wasprepared to 1.5 mm axial reduction using high-speedwater-
1. Experimental tooth preparation designs.
Proximal 3. Proximal view of faciolingual measurements6 through 9.
4. Facial view of four experimental post and core designs.
Fig. 2. Occlusal view of tooth structure measurements1 through 6.
cooled instrumentation. The dowel and core patterns were invested with a phosphate-bondedinvestment (Cera-Fina, Whip Mix Corp., Louisville, Ky.) and were cast in silverpalladium metal (Albacast, Jelenko, Armonk, N.Y.). The cast dowel and core patterns were refined, finished, and abraded with 50 pm aluminum oxide under 2.8 kg/cm pressure. The canalswereirrigated with water and were dried with an air syringe and paper points. Zinc phosphate cement (Fleck’s, Mizzy, Inc., Clifton Forge, Va.) was mixed according to the manufacturer’s directions and placed into the canal with a Lentulo spiral drill (Pulpdent Corp. of America, Brookline Village, Mass.) until the canal appeared full. Cement was placed on the dowel and was delicately seatedby finger pressure.During cementation, hydraulic back pressurewasallowedto be releasedand the core reseated. The teeth were placed in a static loading
device for 15 minutes with 2.72 kg of pressure.The excess cement was removed and the preparations were refined. The preparation waslubricated with salineand a crown wasdirectly waxed, usinga 3M size15polycarbonate crown (3M Co., St. Paul, Minn.) and inlay wax (Maves Co., Cleveland, Ohio). The wax crown wasremoved, the sprue wastrimmed and the crown wasinvested with a phosphatebonded investment according to the manufacturer’s instructions. The crowns were induction cast in basemetal alloy (Super 14; Dental Alloy Products, Inc., Compton, Calif.), refined under a 10 power microscope,and abraded with 50 pm aluminum oxide under 2.8 kg/cm pressure. The crown preparations were cleanedwith CPC (Cavity and Crown Preparation Cleaner, the Lavoric Corp., St. Louis, MO.) and air syringe dried. The crowns were cemented with zinc phosphatecementmixed accordingto the manufacturer’s instructions, again using the cementation
6. Proximal cross-sectionof radiographic measurements 10 and 11.
Fig. 5. Facial cross-sectionof specificationsfor endodontically treated teeth restored with post, core, and crown.
Mean failure loads group
2 3 4 1
failure load kg f SEM
49.58 29.47 28.89 22.48
f + f k
10.26 5.89 9.74 4.40
jig at 2.72 kg of pressurefor 15 minutes. After complete setting, the excesscement wasremoved and the teeth were placed in physiologic salinefor 24 hours. Fig. 5 showsthe dimensionsof the restored endodontically treated tooth. Radiographswere madeof the specimensfrom the proximal aspect and the width of the facial and lingual tooth structure wasmeasuredat the apex of the dowel (Fig. 6). The teeth were examined with a 20 power microscopeand any defects were recorded. The teeth were embedded in autopolymerizing acrylic resin usinga mold that provided a flat surface2 mm below the facial and lingual marginsof the crown. This procedure was followed to simulate the natural biologic width.l A device was made that allowed loading of the tooth at an angle of 130 degreesto its long axis (Fig. 7). This angle of loading waschosento simulate a contact angle found in classI occlusionsbetween maxillary and mandibular anterior teeth.2l3 A universal testing machine (Instron Corp., Canton, Mass.) was used to apply controlled loads to the teeth at a crossheadspeedof 2.54mm per minute. All samples were loaded until failure. The failure threshold was defined as the maximum load a sample could withstand. Failure occurred due to crown displacement,post displacement, root fracture, or post fracture. Failure loads, modes of failure, and tooth preparation designwere recorded and were statistically analyzed for significant correlations between design and failure loads.
7. Schematic of loading apparatus.
RESULTS The mean failure loads were: group 1, 22.4 kg + 4.4; group 2, 49.58 kg f 10.26; group 3, 29.47 kg k5.89; and group 4, 28.89 kg & 9.74 (Table I and Fig. 8). Analysis of variance basedon rank revealed differences amonggroups (Table II). Multiple range analysisshowedthat the mean failure load of group 2 wassignificantly different from that of groups 1, 3, and 4 (Table III). Analysis of variance wasusedto determine if there was a significant difference in the thickness of tooth structure between groups at 11 points. In general, there was not a significant difference in the thickness of tooth structure between groups and 11 points. A multiple range regressionanalysisfor the thickness of tooth structure and the failure threshold was determined for each group at each measurement. The correlation between the amount of tooth structure at the 11 measurements and the failure threshold was not statistically significant. Analysis of the modeof failure showed17 of the 20 teeth in groups 2 and 3 failed due to tooth fracture (Table IV). This high incidence of tooth fracture occurred with a post 421
Fig. 8. Mean failure loads of four preparation
Fig. 9. Proximal view of fracture distribution perimental designs.
for four ex-
Group Fig. 10. Proximal view of fracture distribution experimental designs.
design that closely adapted to the configuration of the canal. Samples with only the Para-Post post extending into the canal (groups 1 and 4) failed due to tooth fracture and cement failure. All six samples of group 1 with cement failure occurred as the Para-Post post bent. In five instances 422
Fig. 11. Facial view of fracture distribution perimental designs.
for four ex-
the tooth and the post fractured with samples drawn from two of the four groups. The failure mode and distribution were charted. When viewed from the facial aspect, group 3 (with a 2 mm extension into the canal) had multiple fractures extending 3 mm apical to the crown margin (Fig. 9). One tooth had a longitudinal fracture extending the entire length of the root. When one viewed the fracture from the proximal aspect, groups 2 (tapered posts) and 3 (the tapered extension) teeth had a higher incidence of fracture when compared with the group having the parallel-sided posts (Figs. 10 and 11). These tapered posts had fractures that originated more lingually and were directed more apically than the parallel-sided posts in groups 1 and 4. The fractures of the tapered posts involved more tooth structure than the parallel-sided post fractures.
DISCUSSION One goal in the restoration of an endodontically treated tooth is to create a design in which the tooth is preserved OCTOBER
OF POST ADAPTATION
Analysis of variance Source of variation
Sum of squares
Between groups Within groups Total (corrected)
Group 1 4 3 2 *Ninety-five
3 s 39
Multiple range analysis for rank by group count
Mode of failure Group 1
Cement failure Post fracture Tooth fracture
2 2** a**
4 3** 5**
when the restoration fails. Failure can occur because of tooth fracture, post fracture, or cement failure. Obviously, cement failure allows retreatment of the tooth. However, tooth fracture may necessitate extraction of the tooth. Post fracture may allow retreatment, but offers considerable challenge in retrieval of the post without irreversibly mutilating the tooth. Clinically, when a tapered canal occurs in an endodontitally treated tooth, some clinicians make a parallel-sided core with intimate contact of the canal. The present study questions the rationale for removing additional tooth structure to accommodate a larger prefabricated parallelsided post engaging more canal wall. Group 1, with only one third of the post in intimate contact with the tooth structure, had a mean failure threshold equal to that of group 4, which had a parallel-sided post intimately adapted to the parallel canal along its entire length. A large amount of cement encircling the incisal two thirds of the post did not affect the failure threshold. Assif and Bleicher4 examined the thickness of a composite luting agent for endodontic posts and also concluded that adaptation to the canal did not affect the retention. The present study, using zinc phosphate cement, similarly found that the cement thickness did not significantly affect the failure load. The results of this study dispute the theoretical rationale of Musikant and Deutsch,5 who suggested that a lack of close fit of the post to the canal creates a long lever arm. They hypothesized that the lever arm is long when a large amount of cement encircles the post. This theoretically increases the stresses in the root and increases the chance for failure. Their theory describes a situation that is similar to that of group 1 in the present study where cement filled the space between the post and the prepared tooth. Our results do not support this hypothesis, as groups 1 and 4 did not have statistically significant different failure loads. Groups 1 and 4 have similar modes of failure, with nearly an equal amount of cement failure and tooth fracture. The tooth JOURNAL
fractures followed a similar pattern, as groups 1 and 4 both had fractures that began facial to the midproximal surface. The thickness of the cement between the post and the canal wall did not significantly affect the fracture resistance. In this investigation, tapered posts (group 2) and posts with tapered extensions (groups 3) failed most often by root fracture. The fractures extended 3 mm past the crown margin, beyond the simulated biologic width. These fractures involved more tooth structure t.han did specimens with parallel-sided posts. Chan and Bryant’s in vitro study6 of mandibular premolars compared three different designs of posts and cores-intimately adapted cast gold tapered post and cores, and stainless steel prefabricated posts with amalgam or composite resin cores. They demonstrated that cast post and cores made using the direct technique showed a significantly higher incidence of root fracture (13 of 15 teeth), which agreed with the findings of the present study. Chan and Bryant6 further found that less than one half of the teeth restored with parallel-sided posts exhibited root fracture (12 of 40). The present study agrees with these findings, as nine fractures occurred in 20 teeth with parallel-sided posts. This in vitro study substantiates Sorensen and Martinoffs clinical study7 of dowel design which showed that 38% of the failures of tapered posts involved fractures that rendered the tooth nonrestorable, necessitating extraction. None of the teeth restored with parallelsided posts failed by tooth fracture. The thickness of the axial tooth structure at the margin of the crown preparation did not significantly affect the failure load of the teeth. The difference of failure loads of groups 1 and 3 (with 1 mm of tooth structure) and of group 4 (with a mean of 2.5 mm of tooth) was not statistically significant. These three methods of post and core design (groups 1, 3, and 4) all had significantly lower failure thresholds than did group 2 (the tapered posts) with 1 mm of axial tooth structure. Tjan and Whanga confirmed that varying thickness of axial tooth structure made no differ423
ence in failure load among teeth with 1, 2, or 3 mm thickness of facial tooth stucture. These results further confirm findings of a previous study9 in which axial wall thickness did not affect the failure load of endodontically treated teeth. That study reported that the key factor in failure threshold was coronal extension of the tooth structure above the crown margin. For post designs intimately adapted to tooth structure (groups 2 to 4), the thickness of tooth structure at the crown margin did affect the incidence of root fracture. These designs had posts that contacted the incisal one third of the root (groups 2 to 4). Groups 2 and 3, with 1 mm of dentin, had a higher incidence of root fracture than group 4 that had with several millimeters of tooth structure. Tjan and Whangs also observed a decrease in the number of fractures when there was an increase of axial tooth thickness for teeth with intimately adapted cast post and cores. Several photoelastic studies lo-l2 have shown that tapered posts produce high stress concentrations when cemented and loaded. These high stresses from tapered posts may account for the fractures that involved more tooth strut: ture than did the parallel-sided posts in this study. Mattison’sl” photoelastic study showed that a larger diameter of post increased the magnitude of stress on the tooth, and the stresses increased further as the load increased. These stresses may account for the larger fracture of the roots in group 2 and 3. Henry’s photoelastic studyll of six post and core designs found that the parallel posts designs distributed the stress more evenly, while the tapered post showed localized high stress concentrations. Standlee et all2 also found that tapered posts produced a wedging effect that resulted in high stress concentrations at the shoulder. This may account for the extensive fractures seen in groups 2 and 3 in the present study. These studies confirm that a tapered post acts as a wedge and creates hieh stress concentrations that result in root fracture. There was no statistically significant difference ( failure load between groups 1, 3, and 4. However, when group 3 teeth failed, extensive root fractures occurred (8 of 10 teeth). This failure may have occurred becauseof a stressconcentration at the junction of the submarginalcore extensionand the cement. In a clinical situation this wedging force and resultant fracture may require extraction of the tooth. In the present study, group 2 had a failure load two times ashigh as groups 1,3, and 4. Group 2 teeth had a tapered post that transferred the load along its entire length to the tooth structure, and failure occurred asthe tooth structure fractured (9 of 10 teeth). The failures in group 2 were catastrophic and involved more tooth structure, extending farther apically than fractures in groups 1 and 4. However, over onehalf of the failures in groups1and 4 occurred when the post becamedislodgedor fractured. The root fractures that occurred in groups 1 and 4 involved lesstooth structure and developedcloserto the margin. The tapered post designof group 2 appearedto be an attractive designwhen the failure load is considered, but the fractures were 424
extensive and frequent. Because of the damaging fractures, and in light of previous research demonstrating a damaging wedging force by a tapered post,7-12 this design should be used with extreme caution. When evaluating the results of this study, one must remember that this is an in vitro study and hence may limit the direct application of these results to in vivo situations. Some of the limitations include a lack of periodontal ligament and an absence of fatigue loading. Additional research should be undertaken to evaluate the importance of the thickness of tooth structure surrounding the apical portion of the post.
CONCLUSIONS 1. Upon failure, parallel-sided posts have a lower frequency of fracture and involve less tooth structure. 2. Maximum adaptation to the canal with a tapered post significantly increases the mean failure threshold. 3. However, tapered posts result in more extensive fractures involving greater tooth structure directed apically and lingually. 4. The tapered post design should be used with extreme caution. We thank J.F. Jelenko and Co. (Armonk, N.Y.) for supplying Albacast metal, and Dental Alloy Products, Inc. (Compton, Calif.) for supplying Super 14 alloy for this study. We also thank Whaledent International (New York, N.Y.) for the Para-Post posts used in this study and Ms. Irene Petriuicius for her artwork. REFERENCES Gargiulo AW, Wentz FW, Orban B. Dimensions and relations of the dentogingival junction in humans. J Perio 1961;32:261-7. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders Co, 1974:436. Moyers RE. Handbook of orthodontics. 3rd ed. Chicago: Year Book Medical Publishers Inc, 1977:411. Assif D, Bleicher S. Retention of serrated endodontic posts with a composite luting agent: effect of cement thickness. J PROSTHET DENT
1986;56:689-91. Musikant BL, Deutsch AS. Endodontic posts: part two-design of Flex&post. J Alabama Dent Assoc 1985;69:42-6. Ghan RW, Bryant RW. Post-core foundations for endodontically treated posteriorteeth.J PROSTHET DENT 1982:48:401-6. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design.J PR~.STHET DENT 1984;52:28-35. Tjan AHL, Whang SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J PROSTHET DENT
1985;53:496-500. JA, Engelman MJ. Ferrule design and fracture resistance of 9. Sorensen endodontical1ytreatedteeth.J PROSTHET DENT 1990;63:529-36. GD. Photoelastic stress analysis of cast-gold endodontic posts. 10. Mattison J PROSTHFX DENT 1982;48:407-11. PJ. Photoelastic analysis of post core restorations. Aust Dent J 11. Henry 1977;22:157-9. 12. Standlee JP, Caputo AA, Collard EW, Pollack MH. Analysis of stress distribution by endodontic posts. Oral Surg 1972;33:952-60. Reprint
SCHOOLOFDENTISTRY UNIVERSITYOFCALIFORNIA Los ANGELES,CA 90024 OCTOBER