In vitro mm~rism of intact endmkMk&y teeth wfth and without endo-post reinfamnt Gary E. Guy, University
D.D.S.,*
of Washington,
and Jack I. Nicholls, School
of Dentistry,
Ph.D.**
Seattle,
Wash.
1 ulpless teeth frequently remain relatively intact after endodontic treatment with conservative access. Although it has never been adequately demonstrated that an endodontically treated tooth is more brittle than a vital tooth, fractures of pulpless teeth during mastication have occurred.’ In light of this experience, dentists have sought ways to protect pulpless teeth from fracture.2-’ Restoration and reinforcement of pulpless teeth is an important preventive measure in endodontic treatment, since failure to do so may invite future problems or “embarrassing mishaps.““. 9 Reinforcement consists of cementing a post into the root canal of a sound endodontically treated tooth.2. L * It is thought that the post will provide strength or reinforcement to prevent the tooth from fracturing.‘. 5. * Lovdahl and Nicholls,1o in an in vitro experiment on endodontically treated teeth, compared the strengths of two coronal restorative procedures with instrumented but unfilled intact pulpless teeth. Endodontically treated teeth with intact crowns demonstrated higher strengths than either of the two restorative procedures commonly used on severely mutilated teeth. Pin-retained amalgam cores were significantly stronger than cast gold dowels and cores. Two statements in the literature are questionable because they lack supportive data. One states that the importance and necessity of providing internal support for endodontically treated teeth prior to views expnxed herein are those of the authors and do not necessarily reflect the views of the United States Public Health Service. This study was partially supported by the Graduate Endodontic Fund, Univenity of Washington, School of Dentistry. *Resident in Endodontics, U.S. Public Health Service Hospital and Graduate Student. **Associate Professor. Department of Restorative Dentistry.
The
THE JOURNAL
OF PROSTHETIC
DENTISTRY
tm&ed
making crown restorations is well documented.” The second states that an endodontically treated tooth with sufficient, well-supported coronal structure for a crown preparation may be reinforced by a ParaPost.‘* Although the procedure is frequently practiced, there appears to be no published study to substantiate that cemented posts reinforce endodontically treated teeth. The purpose of this study was to compare, in vitro, the breaking loads of endodontically treated teeth, with and without cemented posts, to determine if the post reinforces the root against fracture.
METHODS
AND MATERIALS
Fifty-nine freshly extracted maxillary central incisors and maxillary and mandibular canines were selected for experimentation. All teeth were stored in physiologic saline solution from the time of extraction to the time of testing, except for those periods required to complete experimental procedures. A fiber-optic unit was used to demonstrate the absence of caries, cracks, or fractures. Tooth diameter and length were recorded for later analysis. The teeth were stored in numbered individual test tubes filled with physiologic saline solution. Radiographs were made of the teeth both in a faciolingual and in a mesiodistal direction. A 2 X 2 inch gauze sponge soaked in physiologic saline solution was used to hold the tooth during the instrumentation and filling procedures. Access for endodontic therapy was established in a conventional manner. Teeth were instrumented to W mm from the foramen to a No. 50 file and peripherally filed in the coronal two thirds of the tooth for gutta-percha filling. In each case the access opening was extended and instrumentation was completed to simulate circumstances in which large pulp chambers are present. It was presumed that such teeth would have less mineralized tissue and be potentially weaker
39
GUN
AND
NICHOLLS
Fig. 1. Cross section of a canine with a post. Alummum
,Cylinder
Fig. 3. Loading jig.
5mm Load
’ Block
Fig. t. Cross-sectional view of a mounted tooth. than more mature teeth, and thereby more in need of reinforcement. Teeth were divided into two groups, canines and central incisors. The canines were further divided in the following manner. Each tooth’s radiograph mount had the faciolingual and me&distal dimension at the facial cementoenamel junction recorded on the upper right hand corner. The radiograph mounts were arranged in increasing size of faciolin-
gual dimension. The mesiodistal dimension was used to define this ordering for teeth with the same faciolingual dimension. The mounts of the canines were then sorted into two groups beginning with the first mount to the left, next two mounts to the right, next two mounts to the left, until the last mount went to the right, yielding two piles of equal number. The mounts were sorted on the basis of dimensions only, with no reference to tooth anatomy or instrumentation. The same procedure was then performed with the mounts of the central incisors. A disinterested person decided which teeth in each category would receive posts. Instrumentation radiographs were made of each tooth from the facial and proximal aspects. All teeth were filled with gutta-percha and Grossman’s root canal sealer by a master cone and lateral condensation technique. In the nonposted teeth, a hot plugger was used to remove the gutta-percha to approximately 1 mm below the cementoenamel junction. A silicate restoration* was inserted into the access preparation. Petroleum jelly was placed on the silicate restoration. A radiograph was again made from the facial and ‘MQ,
S.S. White
Dental
Mfg.
Co.,
JULY
1~9
Philadelphia,
VOLUME
Pa.
42
NUMBER
1
ENDO-POST
REINFORCEMENT
CENTRAL I
number
with
9
post
INCISORS WIthout
II
I
1
post
2
9
1
CANINES
jfhout
number
3
16
‘c:
4
15
Fig. 4. Failure modesand distribution. Table I. Tooth movements and mean failure loads MeanfacioWe ah?
Descrtption
10
11 19 19
Centrals Centrals Canines Canines
lingual (mm)
without posts with posts without posts with posts
6.38 6.35 7.67 7.62
diameter (2 SD)
+ + f k
0.53 0.49 0.54 0.63
Mean mesiodistal diameter
5.92 6.00 5.41 5.34
+ 0.58 -t- 0.49 + 0.47 * 0.45
M-knsth of tooth
24.86 24.01 26.02 26.50
f k + f
2.18 1.32 2.06 2.11
Mean
taatwe load
86.18 -+ 22.M 91.68 22 t3.79 96.47 c 21.52
proximal aspectsand returned to the saline filled test tube. The teeth to receive posts had the gutta-percha removed to within 7 to 8 mm of the apex with a hot plugger. Reamers set 5 mm short of the tooth length were used to remove gutta-percha to that level and to prepare the canal to size No. 100 for a post. Size No. 100 Kerr Endo-Posts were test-fitted to the prepared canal. Zinc phosphate cement* was mixed according to the manufacturer’s directions and inserted into the canal with a No. 2 Lentulot spiral until the canal appeared full. Weld by a straight Kelly hemostat, the tip of the Endo-Post was dipped in the zinc phosphate cement and inserted into the canal to the prepared length. When the cement set, a high-speedNo. 34 carbide bur was used to sever the post 1 mm below the lingual access,parallel to the lingual surface of the accesspreparation (Fig. 1). The remaining cavity was restored with silicate cement and protect& with petroleum jelly. The tooth was radiographed from the facial and proximal angles and returned to the saline-filled test tube.
All teeth were mounted in an aluminum cylinder prior to load testing (Fig. 2). Each tooth was marked 2 mm below the facial cementoenamel junction, and a line wasdrawn around the root at this Sevd at right angles to the long axis of the tooth. Denture Elasticon * rubber impression material was mixed and painted on the root of the tooth with a fine came&air brush. After the Elasticon had set, a No. 11 scalpel was used to remove the Ehsticon from the apical third of the root (Fig. 2). A high-speed No. 4 carbide bur was used to notch the me&l and distal sidesof the root approximately 3 mm from the apex and 0.5 mm deep. These notches provided adequate retention for the tooth and did not appear to affect test results, since none of the teeth fractured through the notches. The teeth were mounted in the aluminum cylinders with clear orthodontic acrylic resin? to a depth identified by the circumfbrential line. Before the final set, the excess acrylic resin was removed with a No. 1 I scalpel, providing a flat surface level with the top of the aluminum cylinder and 2 mm below the facial cementoenamel junction of the
*Fleck’s tUnion
*Kerr Dental fL. D. Caulk
Extraordinary, M&y, Inc., Clifton Forge, Broach Co., Long Island City, N. Y.
THEJOURNALOFPROSTHETICDENTISTRY
Va
Mfg. Co., Romulus, Co., Milford, Del.
Mich.
41
GUZY
Load
on
Post,
Tooth
Tenson
Al
NICHOLLS
from the lingual to the labial aspect at a point beginning approximately 7 mm from the top of the socket and mounting cylinder. Displacement and load magnitude were continuously recorded by the Instron testing machine. Failure threshold was defined as the point at which a sample could no longer withstand increasing load and fracture of the root occurred. A record was kept of fracture lines on the coronal portion of the fractured tooth with a bisecting microscope at X 25 magnification.
Load
(a)
AND
RESULTS ression (b)
Stress
Dlstrlbutlon
Across
Fig. 5. Experimental Table II. Comparison
and
Post
stress distribution.
of means
of failure
t
T-testgrooping Centrals posts Caniies
Root
with
posts/Centrals
without
with
posts/Cuspids
without
loads Confidence
0.869
p < .05
0.651
p < .05
P-k
tooth. A mark was made 5 mm incisal to the aluminum cylinder on the mesial and distal sides of the tooth. A load block made of aluminum 20 X 30 X 3.5 mm with a 12 mm diameter hole (Fig. 2) was mounted on the crown of the tooth at 130 degrees and located through the mesial and distal marks. A template held the load block in position. Wax was used to seal the bottom of the load block to the template and tooth. Clear orthodontic acrylic resin was poured into the load block and around the crown of the tooth, cementing the crown into the load block (Fig. 2). When the acrylic resin had set, the template was removed and the aluminum cylinder, tooth, and load block were placed for at least 12 hours prior to testing in physiologic saline solution. An Instron testing machine* applied controlled loads to the tooth specimens. The load rate was 5.0 cm/minute for all testing. The jig shown in Fig. 3 was fabricated to load the tooth at an angle of 130 degnx-s to its long axis. This angle was chosen to simulate the average angle of contact normally found in Class I occlusions between maxillary and mandibular anterior teeth.13 The force was applied %stmn
42
Corp.,
Loa
Alamitos,
Calif.
The mean failure loads are given in Table I. The patterns of fracture are shown diagramatically in Fig. 4, with the number shown representing the number failing in that category. There was no apparent difference in fracture pattern or fracture location between teeth with and without posts. Ten central incisors without posts. Nine of this group fractured through the root with the lingual portion of the fracture more apical than the labial. One fractured approximately in a horizontal plane. The mean failure load for this group was 93.8 kg. Eleven central incisors with posts. Nine of these fractured through the root and post with the lingual portion of the fracture more apical than the labial. In one central incisor with a post the facial area of fracture was slightly more apical than the lingual. One central incisor with a post fractured through the apical portion of the lingual access of the crown, through the post area, and onto the facial surface of the root. The mean failure load for this group was 86.2 kg. Nineteen canines without posts. Fifteen fractured through the root with the lingual portion of the fracture more apical than the labial. Four fractured with the fracture line either slightly more apical on the facial surface of the root than the lingual, or they fractured approximately horizontally across the root. The mean failure load for this group was 91.7 kg. Nineteen canines with posts. Sixteen of the canines with posts fractured through the post and mot with the lingual fracture more apical than the facial. Three fractured with the fracture line either slightly more apical on the facial surface of the root than the lingual or fractured approximately horizontally across the root. The mean failure load for this group was 96.5 kg. The means of the failure loads for the two groups were compared statistically by Student’s f-test. The results are given in Table II.
JULY
1979
VOLUME
42
NUMBER
I
ENDO-POST
REINFORCEMENT
There was no statistically significant difference between the mean failure loads of the central incisors with posts and without posts. Similarly, there was no statistically significant difference between the mean failure loads of canines with posts and without posts.
DISCUSSION Several control factors were arbitrarily selected in the design of this experiment. A tapered post was used because it is machined to closely resemble the matching reamer in size.s, *, I4 A silicone rubber socket liner has been used by others for the study of tooth movement under stress.“’ The silicone rubber liner was approximately 0.25 mm thick, roughly the width of the periodontal ligament,‘j and allows freedom of movement as in the periodontal ligament. This movement appears to be small, although no measurements of actual mobility were made. Due to this movement and the higher failure loads developed than in Lovdahl and Nicholl’s”’ study, additional retention of the tooth in the silicone lined socket had to be developed. The loading angle of 130 degrees from lingual to labial was selected on the basis that it simulates the average angle of contact between maxillary and mandibular incisors in Class I occlusion. This angle was used in a past study.l” This loading angle more closely resembles a test of function than a test of external impact. Failure loads for central incisors were approximately 70% higher than similar results in the work of Lovdahl and Nicholls.“’ They loaded the test teeth further from the cementoenamel junction, thus inducing a larger moment (force X lever arm length) for the same applied load. Additionally, their failure mode was one in which the labial aspect of the crown fractured. In the present study the load was distributed over a broader area and the fracture occurred in the coronal and/or middle third of the mot (58 times out of 59). Since the purpose of this study was to test the reinforcement given to sound endodontically treated teeth, the fracture had to occur through the post area to test the effect of the post on reinforcement. This occurred in every tooth tested. The choice of post, depth, cementation procedure, and type of cement followed procedures outlined in the University of Washington Fixed Partial Denture Clinical Syllabu.~.~ A minimum of 7 days transpired between cementation of posts and testing.16 From an engineering point of view, the relation-
THE JOURNAL
OF PROSTHETIC
DENTISTRY
ship of the faciolingual diameter of the tooth to failure load is significant for the loading direction used.This significance is related directly to the stress pattern induced due to the applied loading. Fig. 5 showsthis stresspattern under the assumption that the dentin can be considered as a linear elastic material. If this linear stressvariation is invalid, the stressvariation will have the same characteristics, i.e., the maximum stress at the facial and lingual surfacesof the root. From Fig. 5 it can be deduced that with the maximum stressesgiven, failure will be initiated at one of these surfaces.The location of the post is such that it receives only minimal stressunder such external loading. Consequently, in its given position, the post does little to reinforce the root under externally applied loading. The wider the faciolingual diameter, the greater was the resistance to failure. This can be deduced from Fig. 5 where, with an increase in the stressed width, there is a corresponding decrease in the maximal stressesfor the same applied loading. The faciolingual as well as the mesiodistal diameters in this experiment were well matched, as is shown in Table I. In preparing the root canals to receive posts, some teeth had dentin removed in the apical few millimeters of the posted region (Fig. 1). This did not appear to affect test results, as the fractures occurred more coronal on the teeth and not through this enlarged area.
SUMMARY
AND CONCLUSIONS
In an in vitro experiment, the failure loads of 59 intact endodontically treated teeth with and without Kerr Endo-Post reinforcement were compared. Fifty-eight teeth fractured below the cementoenamel junction. One tooth fractured through the pulp chamber with a chisel fracture involving both the crown and root. Teeth without posts fractured through the middle or coronal one third of the root. Teeth with posts fractured through the body of the post. No statistically significant reinforcement was demonstrated by cementing a Kerr Endo-Post No. 100 into a sound endodontically treated tooth. REFERENCES 1. 2. 3.
Frank, A. I,.: Protective coronal coverage of the pulpless tooth. J Am Dent Assoc 59~895, 1959. Baraban, D. J.: The restoration of pulpless teeth. Dent Chin North Am, Nov. 1967, pp 633-635. Baraban, D. J.: Immediate restoration of pulpless teeth. J PROSTHET DENT 28:607, 1972.
43
GUZY
4.
Cc~lley, I. T., Hampson, E. L., and Lehman, M. L.: Retention of post crowns. An assessment of the relative efficiency of posts of different shapes and sizes. Br Dent J 124:63, 1968. 5. Courtade, G. L., and Timmermans, J. J.: Pins in Restorative Dentistry. St. Louis, 1971, The C. V. Mosby Co. 6. Shillingburg, H. T., Fisher, D. W., and Dewhint, R. B.: Restoration of endodontically treated posterior teeth. J PRO~MP~ DENT 24:240, 1970. 7. Silverstein, W.: The reinforcement of weakened pulpless teeth. J Paosrmrr DENT 14:372, 1964. M. E.: Fixed Partial Denture Clinical Syllabus. 8. Wamick, University of Washington, 1968. (Revised 1972, 1974.) 9. Rosen, H.: Operative proccdum on mutilated cndodontically treated teeth. J PROSTHET DENT 11:973, 1961. P. E., and Nicholls, J. I.: In vitro comparison of 10. Lovdahl, restorative modalities on pulpless teeth. University of Washington, School of Dentistry, 1976. 11. Fedcrick, D. R.: An application of the dowel and composite resin core technique. J PROSTHET DENT 32:420, 1974.
INFORMATION
12. 13.
AND
NICHOLLS
Baraban, D. J.: A simplified method for making posts and cures. J PROPHET DENT 24:287, 1970. Bjork, A.: The face in profile. Swensk Tand Tids 40:5B Supplement, 1948.
14.
Gerstein, H., and Brunell, S. C.: Prefabricated precision dowels. J Am Dent AWX 88:787, 1964.
15.
Coolidge, membrane.
16.
Hanson, E. C., and Caputo, A. A.: Cementing mediums and retentive characteristics of dowels. J PRO-ET DENT 32:55 1, 1974.
Rtpn-nf
rcqtusfJ
E. D.: Thickness of the J Am Dent Assoc 24:1260,
human 1937.
periodontal
Lo:
DR. JACK I. NICHOLLS UNIVERSITY OF WASHINGTON SCHOOL .%XTLE,
OF DEMlsTRv WASH
98195
FOR AUTHORS
Most of the provisions of the Copyright Act of 1976 became effective on January 1, 1978. Therefore, all manuscripts must be accompanied by the following written statement, signed by one author: “The undersigned author transfers all copyright ownership of the manuscript (title of article) to The C. V. Mosby Company in the event the work is published. The undersigned author warrants that the article is original, is not under consideration by another journal, and has not been previously published. I sign for and accept responsibility for releasing this material on behalf of any and all co-authors.” Authors will be consulted, when possible, regarding republication
44
of their
material.
JULY
1979
VOLUME
42
NUMBER
I
D.D.S.,*
of Washington,
and Jack I. Nicholls, School
of Dentistry,
Ph.D.**
Seattle,
Wash.
1 ulpless teeth frequently remain relatively intact after endodontic treatment with conservative access. Although it has never been adequately demonstrated that an endodontically treated tooth is more brittle than a vital tooth, fractures of pulpless teeth during mastication have occurred.’ In light of this experience, dentists have sought ways to protect pulpless teeth from fracture.2-’ Restoration and reinforcement of pulpless teeth is an important preventive measure in endodontic treatment, since failure to do so may invite future problems or “embarrassing mishaps.““. 9 Reinforcement consists of cementing a post into the root canal of a sound endodontically treated tooth.2. L * It is thought that the post will provide strength or reinforcement to prevent the tooth from fracturing.‘. 5. * Lovdahl and Nicholls,1o in an in vitro experiment on endodontically treated teeth, compared the strengths of two coronal restorative procedures with instrumented but unfilled intact pulpless teeth. Endodontically treated teeth with intact crowns demonstrated higher strengths than either of the two restorative procedures commonly used on severely mutilated teeth. Pin-retained amalgam cores were significantly stronger than cast gold dowels and cores. Two statements in the literature are questionable because they lack supportive data. One states that the importance and necessity of providing internal support for endodontically treated teeth prior to views expnxed herein are those of the authors and do not necessarily reflect the views of the United States Public Health Service. This study was partially supported by the Graduate Endodontic Fund, Univenity of Washington, School of Dentistry. *Resident in Endodontics, U.S. Public Health Service Hospital and Graduate Student. **Associate Professor. Department of Restorative Dentistry.
The
THE JOURNAL
OF PROSTHETIC
DENTISTRY
tm&ed
making crown restorations is well documented.” The second states that an endodontically treated tooth with sufficient, well-supported coronal structure for a crown preparation may be reinforced by a ParaPost.‘* Although the procedure is frequently practiced, there appears to be no published study to substantiate that cemented posts reinforce endodontically treated teeth. The purpose of this study was to compare, in vitro, the breaking loads of endodontically treated teeth, with and without cemented posts, to determine if the post reinforces the root against fracture.
METHODS
AND MATERIALS
Fifty-nine freshly extracted maxillary central incisors and maxillary and mandibular canines were selected for experimentation. All teeth were stored in physiologic saline solution from the time of extraction to the time of testing, except for those periods required to complete experimental procedures. A fiber-optic unit was used to demonstrate the absence of caries, cracks, or fractures. Tooth diameter and length were recorded for later analysis. The teeth were stored in numbered individual test tubes filled with physiologic saline solution. Radiographs were made of the teeth both in a faciolingual and in a mesiodistal direction. A 2 X 2 inch gauze sponge soaked in physiologic saline solution was used to hold the tooth during the instrumentation and filling procedures. Access for endodontic therapy was established in a conventional manner. Teeth were instrumented to W mm from the foramen to a No. 50 file and peripherally filed in the coronal two thirds of the tooth for gutta-percha filling. In each case the access opening was extended and instrumentation was completed to simulate circumstances in which large pulp chambers are present. It was presumed that such teeth would have less mineralized tissue and be potentially weaker
39
GUN
AND
NICHOLLS
Fig. 1. Cross section of a canine with a post. Alummum
,Cylinder
Fig. 3. Loading jig.
5mm Load
’ Block
Fig. t. Cross-sectional view of a mounted tooth. than more mature teeth, and thereby more in need of reinforcement. Teeth were divided into two groups, canines and central incisors. The canines were further divided in the following manner. Each tooth’s radiograph mount had the faciolingual and me&distal dimension at the facial cementoenamel junction recorded on the upper right hand corner. The radiograph mounts were arranged in increasing size of faciolin-
gual dimension. The mesiodistal dimension was used to define this ordering for teeth with the same faciolingual dimension. The mounts of the canines were then sorted into two groups beginning with the first mount to the left, next two mounts to the right, next two mounts to the left, until the last mount went to the right, yielding two piles of equal number. The mounts were sorted on the basis of dimensions only, with no reference to tooth anatomy or instrumentation. The same procedure was then performed with the mounts of the central incisors. A disinterested person decided which teeth in each category would receive posts. Instrumentation radiographs were made of each tooth from the facial and proximal aspects. All teeth were filled with gutta-percha and Grossman’s root canal sealer by a master cone and lateral condensation technique. In the nonposted teeth, a hot plugger was used to remove the gutta-percha to approximately 1 mm below the cementoenamel junction. A silicate restoration* was inserted into the access preparation. Petroleum jelly was placed on the silicate restoration. A radiograph was again made from the facial and ‘MQ,
S.S. White
Dental
Mfg.
Co.,
JULY
1~9
Philadelphia,
VOLUME
Pa.
42
NUMBER
1
ENDO-POST
REINFORCEMENT
CENTRAL I
number
with
9
post
INCISORS WIthout
II
I
1
post
2
9
1
CANINES
jfhout
number
3
16
‘c:
4
15
Fig. 4. Failure modesand distribution. Table I. Tooth movements and mean failure loads MeanfacioWe ah?
Descrtption
10
11 19 19
Centrals Centrals Canines Canines
lingual (mm)
without posts with posts without posts with posts
6.38 6.35 7.67 7.62
diameter (2 SD)
+ + f k
0.53 0.49 0.54 0.63
Mean mesiodistal diameter
5.92 6.00 5.41 5.34
+ 0.58 -t- 0.49 + 0.47 * 0.45
M-knsth of tooth
24.86 24.01 26.02 26.50
f k + f
2.18 1.32 2.06 2.11
Mean
taatwe load
86.18 -+ 22.M 91.68 22 t3.79 96.47 c 21.52
proximal aspectsand returned to the saline filled test tube. The teeth to receive posts had the gutta-percha removed to within 7 to 8 mm of the apex with a hot plugger. Reamers set 5 mm short of the tooth length were used to remove gutta-percha to that level and to prepare the canal to size No. 100 for a post. Size No. 100 Kerr Endo-Posts were test-fitted to the prepared canal. Zinc phosphate cement* was mixed according to the manufacturer’s directions and inserted into the canal with a No. 2 Lentulot spiral until the canal appeared full. Weld by a straight Kelly hemostat, the tip of the Endo-Post was dipped in the zinc phosphate cement and inserted into the canal to the prepared length. When the cement set, a high-speedNo. 34 carbide bur was used to sever the post 1 mm below the lingual access,parallel to the lingual surface of the accesspreparation (Fig. 1). The remaining cavity was restored with silicate cement and protect& with petroleum jelly. The tooth was radiographed from the facial and proximal angles and returned to the saline-filled test tube.
All teeth were mounted in an aluminum cylinder prior to load testing (Fig. 2). Each tooth was marked 2 mm below the facial cementoenamel junction, and a line wasdrawn around the root at this Sevd at right angles to the long axis of the tooth. Denture Elasticon * rubber impression material was mixed and painted on the root of the tooth with a fine came&air brush. After the Elasticon had set, a No. 11 scalpel was used to remove the Ehsticon from the apical third of the root (Fig. 2). A high-speed No. 4 carbide bur was used to notch the me&l and distal sidesof the root approximately 3 mm from the apex and 0.5 mm deep. These notches provided adequate retention for the tooth and did not appear to affect test results, since none of the teeth fractured through the notches. The teeth were mounted in the aluminum cylinders with clear orthodontic acrylic resin? to a depth identified by the circumfbrential line. Before the final set, the excess acrylic resin was removed with a No. 1 I scalpel, providing a flat surface level with the top of the aluminum cylinder and 2 mm below the facial cementoenamel junction of the
*Fleck’s tUnion
*Kerr Dental fL. D. Caulk
Extraordinary, M&y, Inc., Clifton Forge, Broach Co., Long Island City, N. Y.
THEJOURNALOFPROSTHETICDENTISTRY
Va
Mfg. Co., Romulus, Co., Milford, Del.
Mich.
41
GUZY
Load
on
Post,
Tooth
Tenson
Al
NICHOLLS
from the lingual to the labial aspect at a point beginning approximately 7 mm from the top of the socket and mounting cylinder. Displacement and load magnitude were continuously recorded by the Instron testing machine. Failure threshold was defined as the point at which a sample could no longer withstand increasing load and fracture of the root occurred. A record was kept of fracture lines on the coronal portion of the fractured tooth with a bisecting microscope at X 25 magnification.
Load
(a)
AND
RESULTS ression (b)
Stress
Dlstrlbutlon
Across
Fig. 5. Experimental Table II. Comparison
and
Post
stress distribution.
of means
of failure
t
T-testgrooping Centrals posts Caniies
Root
with
posts/Centrals
without
with
posts/Cuspids
without
loads Confidence
0.869
p < .05
0.651
p < .05
P-k
tooth. A mark was made 5 mm incisal to the aluminum cylinder on the mesial and distal sides of the tooth. A load block made of aluminum 20 X 30 X 3.5 mm with a 12 mm diameter hole (Fig. 2) was mounted on the crown of the tooth at 130 degrees and located through the mesial and distal marks. A template held the load block in position. Wax was used to seal the bottom of the load block to the template and tooth. Clear orthodontic acrylic resin was poured into the load block and around the crown of the tooth, cementing the crown into the load block (Fig. 2). When the acrylic resin had set, the template was removed and the aluminum cylinder, tooth, and load block were placed for at least 12 hours prior to testing in physiologic saline solution. An Instron testing machine* applied controlled loads to the tooth specimens. The load rate was 5.0 cm/minute for all testing. The jig shown in Fig. 3 was fabricated to load the tooth at an angle of 130 degnx-s to its long axis. This angle was chosen to simulate the average angle of contact normally found in Class I occlusions between maxillary and mandibular anterior teeth.13 The force was applied %stmn
42
Corp.,
Loa
Alamitos,
Calif.
The mean failure loads are given in Table I. The patterns of fracture are shown diagramatically in Fig. 4, with the number shown representing the number failing in that category. There was no apparent difference in fracture pattern or fracture location between teeth with and without posts. Ten central incisors without posts. Nine of this group fractured through the root with the lingual portion of the fracture more apical than the labial. One fractured approximately in a horizontal plane. The mean failure load for this group was 93.8 kg. Eleven central incisors with posts. Nine of these fractured through the root and post with the lingual portion of the fracture more apical than the labial. In one central incisor with a post the facial area of fracture was slightly more apical than the lingual. One central incisor with a post fractured through the apical portion of the lingual access of the crown, through the post area, and onto the facial surface of the root. The mean failure load for this group was 86.2 kg. Nineteen canines without posts. Fifteen fractured through the root with the lingual portion of the fracture more apical than the labial. Four fractured with the fracture line either slightly more apical on the facial surface of the root than the lingual, or they fractured approximately horizontally across the root. The mean failure load for this group was 91.7 kg. Nineteen canines with posts. Sixteen of the canines with posts fractured through the post and mot with the lingual fracture more apical than the facial. Three fractured with the fracture line either slightly more apical on the facial surface of the root than the lingual or fractured approximately horizontally across the root. The mean failure load for this group was 96.5 kg. The means of the failure loads for the two groups were compared statistically by Student’s f-test. The results are given in Table II.
JULY
1979
VOLUME
42
NUMBER
I
ENDO-POST
REINFORCEMENT
There was no statistically significant difference between the mean failure loads of the central incisors with posts and without posts. Similarly, there was no statistically significant difference between the mean failure loads of canines with posts and without posts.
DISCUSSION Several control factors were arbitrarily selected in the design of this experiment. A tapered post was used because it is machined to closely resemble the matching reamer in size.s, *, I4 A silicone rubber socket liner has been used by others for the study of tooth movement under stress.“’ The silicone rubber liner was approximately 0.25 mm thick, roughly the width of the periodontal ligament,‘j and allows freedom of movement as in the periodontal ligament. This movement appears to be small, although no measurements of actual mobility were made. Due to this movement and the higher failure loads developed than in Lovdahl and Nicholl’s”’ study, additional retention of the tooth in the silicone lined socket had to be developed. The loading angle of 130 degrees from lingual to labial was selected on the basis that it simulates the average angle of contact between maxillary and mandibular incisors in Class I occlusion. This angle was used in a past study.l” This loading angle more closely resembles a test of function than a test of external impact. Failure loads for central incisors were approximately 70% higher than similar results in the work of Lovdahl and Nicholls.“’ They loaded the test teeth further from the cementoenamel junction, thus inducing a larger moment (force X lever arm length) for the same applied load. Additionally, their failure mode was one in which the labial aspect of the crown fractured. In the present study the load was distributed over a broader area and the fracture occurred in the coronal and/or middle third of the mot (58 times out of 59). Since the purpose of this study was to test the reinforcement given to sound endodontically treated teeth, the fracture had to occur through the post area to test the effect of the post on reinforcement. This occurred in every tooth tested. The choice of post, depth, cementation procedure, and type of cement followed procedures outlined in the University of Washington Fixed Partial Denture Clinical Syllabu.~.~ A minimum of 7 days transpired between cementation of posts and testing.16 From an engineering point of view, the relation-
THE JOURNAL
OF PROSTHETIC
DENTISTRY
ship of the faciolingual diameter of the tooth to failure load is significant for the loading direction used.This significance is related directly to the stress pattern induced due to the applied loading. Fig. 5 showsthis stresspattern under the assumption that the dentin can be considered as a linear elastic material. If this linear stressvariation is invalid, the stressvariation will have the same characteristics, i.e., the maximum stress at the facial and lingual surfacesof the root. From Fig. 5 it can be deduced that with the maximum stressesgiven, failure will be initiated at one of these surfaces.The location of the post is such that it receives only minimal stressunder such external loading. Consequently, in its given position, the post does little to reinforce the root under externally applied loading. The wider the faciolingual diameter, the greater was the resistance to failure. This can be deduced from Fig. 5 where, with an increase in the stressed width, there is a corresponding decrease in the maximal stressesfor the same applied loading. The faciolingual as well as the mesiodistal diameters in this experiment were well matched, as is shown in Table I. In preparing the root canals to receive posts, some teeth had dentin removed in the apical few millimeters of the posted region (Fig. 1). This did not appear to affect test results, as the fractures occurred more coronal on the teeth and not through this enlarged area.
SUMMARY
AND CONCLUSIONS
In an in vitro experiment, the failure loads of 59 intact endodontically treated teeth with and without Kerr Endo-Post reinforcement were compared. Fifty-eight teeth fractured below the cementoenamel junction. One tooth fractured through the pulp chamber with a chisel fracture involving both the crown and root. Teeth without posts fractured through the middle or coronal one third of the root. Teeth with posts fractured through the body of the post. No statistically significant reinforcement was demonstrated by cementing a Kerr Endo-Post No. 100 into a sound endodontically treated tooth. REFERENCES 1. 2. 3.
Frank, A. I,.: Protective coronal coverage of the pulpless tooth. J Am Dent Assoc 59~895, 1959. Baraban, D. J.: The restoration of pulpless teeth. Dent Chin North Am, Nov. 1967, pp 633-635. Baraban, D. J.: Immediate restoration of pulpless teeth. J PROSTHET DENT 28:607, 1972.
43
GUZY
4.
Cc~lley, I. T., Hampson, E. L., and Lehman, M. L.: Retention of post crowns. An assessment of the relative efficiency of posts of different shapes and sizes. Br Dent J 124:63, 1968. 5. Courtade, G. L., and Timmermans, J. J.: Pins in Restorative Dentistry. St. Louis, 1971, The C. V. Mosby Co. 6. Shillingburg, H. T., Fisher, D. W., and Dewhint, R. B.: Restoration of endodontically treated posterior teeth. J PRO~MP~ DENT 24:240, 1970. 7. Silverstein, W.: The reinforcement of weakened pulpless teeth. J Paosrmrr DENT 14:372, 1964. M. E.: Fixed Partial Denture Clinical Syllabus. 8. Wamick, University of Washington, 1968. (Revised 1972, 1974.) 9. Rosen, H.: Operative proccdum on mutilated cndodontically treated teeth. J PROSTHET DENT 11:973, 1961. P. E., and Nicholls, J. I.: In vitro comparison of 10. Lovdahl, restorative modalities on pulpless teeth. University of Washington, School of Dentistry, 1976. 11. Fedcrick, D. R.: An application of the dowel and composite resin core technique. J PROSTHET DENT 32:420, 1974.
INFORMATION
12. 13.
AND
NICHOLLS
Baraban, D. J.: A simplified method for making posts and cures. J PROPHET DENT 24:287, 1970. Bjork, A.: The face in profile. Swensk Tand Tids 40:5B Supplement, 1948.
14.
Gerstein, H., and Brunell, S. C.: Prefabricated precision dowels. J Am Dent AWX 88:787, 1964.
15.
Coolidge, membrane.
16.
Hanson, E. C., and Caputo, A. A.: Cementing mediums and retentive characteristics of dowels. J PRO-ET DENT 32:55 1, 1974.
Rtpn-nf
rcqtusfJ
E. D.: Thickness of the J Am Dent Assoc 24:1260,
human 1937.
periodontal
Lo:
DR. JACK I. NICHOLLS UNIVERSITY OF WASHINGTON SCHOOL .%XTLE,
OF DEMlsTRv WASH
98195
FOR AUTHORS
Most of the provisions of the Copyright Act of 1976 became effective on January 1, 1978. Therefore, all manuscripts must be accompanied by the following written statement, signed by one author: “The undersigned author transfers all copyright ownership of the manuscript (title of article) to The C. V. Mosby Company in the event the work is published. The undersigned author warrants that the article is original, is not under consideration by another journal, and has not been previously published. I sign for and accept responsibility for releasing this material on behalf of any and all co-authors.” Authors will be consulted, when possible, regarding republication
44
of their
material.
JULY
1979
VOLUME
42
NUMBER
I
