BONDANDCOLOROFCERAMOMETAL.PARTII
or Chroma varies with the porcelain brand used at a given firing temperature. REFERENCES 1. Tylman SD. Theory and practice of crown and bridge. 6th ed. St Louis:
CV Mosby Co, 19’70;562-3. 2. Barghi N, Goldberg J. Porcelain shade stability after repeated firing. J PIWTHET DENT 1977;37:173-5. 3. Barghi N, Richardson JT. A study of various factors influencing the shade of bonded porcelain. J PROSTHET DENT 1978;39:282-4. 4. Barghi N. Color and glaze: effects of repeated firings. J PROSTHET DENT 1982;47:393-5. 5. Jorgenson MW, Goodkiid RJ. Spectrophotometric study of five porce-
lain shades relative to the dimensions of color, porcelain thickness, and repeated firings. J PWSTHET DENT 1979;42:96-105. 6. Hammad IA, Stem RS. A qualitative study for the bond and color of caramometals. part I. J PR~WHET DENT 1990;63:643-53. 7. Barlow F. A point indentation loading study of two ceramometal systems relative to porcelain thickness, repeated firings and surface conditions. Master’s thesis. Minneapolis: University of Minnesota, 1976. 8. Lau CS, Yamada HN. Metal-ceramic crowns and fixed partial dentures. In: Rhoads JE, Rudd KD, Morrow RM. Dental lab procedures. 2nd ed. St Louis: CV Mosby Co, 1986;274.
Threaded incidence
endodontic dowels: of root fracture
D. A. Felton, B. E. Kanoy, University
9. Hammad IA, Goodkmd RJ, Gerberich WW. A shear test for the bond strength of ceramometals. J PROWHET DENT 1987;58:431-7. 10. Wight TA. Bauman JC, Pelleu GB Jr. An evaluation of four variables ai%cting bond strength of porcelain to nonprecious alloy. J PROSTHET DENT 1977;37:570-7. 11. Lund TW, Schwabacher WB, Goodkind R-J. Spectrophotometric study of the relationship between body porcelain color and applied metallic oxide pigments. J PR(WTHETDENT 1985;53:796-6. 12. Obregon A, Goodkind RJ. Effects of opaque and porcelain surface texture on the color of ceramometal restorations. J PRODENT 1981;46:330-40. 13. Sun AF. An investigation of two opaque modiiiers relative to their color
stability and bond strength after repeated firings with two ceramometal systems. Doctor of Science thesis. Boston: Boston University, School of Graduate Dentistry, 1985. Reprint requests to: DR. R. SHELWN STEIN SCIKWL OF GRADUATE DENTISTRY BOSTON UNIVERSITY BOSMN, MA 02118
Effect
of post design on
D.D.S., M.S.,* E. L. Webb, D.D.S., M.S.,** M.A., D.D.S.,*** and Jay Dugoni****
of North
Carolina, School of Dentistry, Chapel Hill, N.C.
The use of threaded endodontic dowels is a controversial issue. The purpose of this study was to compare the potential for root fracture resulting from the cementation of nine threaded and three nonthreaded endodontic dowel systems. The clinical crowns of 140 extracted premolar6 were removed at the cementoenamel junction. The teeth were randomly assigned to the following treatment groups of 10 teeth each: Group 1, endodontically instrumented but not obturated; group 2, instrumented and obturated, group 3, instrumented, obturated, and restored with custom-cast gold dowel and cores; groups 4 and 6, instrumented, obturated, and restored with prefabricated, nonthreaded dowels; and groups 8 through 14, instrumented, obturated, and restored with one of nine prefabricated, threaded dowels. All dowels were inserted according to manufacturer’s directions, removed, and cemented with vinyl pelysiloxane impression material. Each specimen was demineralized and cleared. Photographs at 1:l magnitlcation were taken to assess dowel fractures. Fisher’s test and chi square analysis were performed to evaluate the differences between post types, and between posted and nonposted controls (p < 0.06.). The results indicate no statistically significant differences between dowel types when compared with each other, regardless of dowel shape, taper, or presence or absence of threads, or when. compared to instrumented, nonobturated controls. The amount of remaining dentin and existing root morphology may be a determining factor for endodontically treated teeth to resist fracture during dowel placement. (J PROSTEET DENT 1991;65:179-87.)
This investigation was supported by the University of North Carolina
Faculty
Research Support Grant. *Assistant Professor,Department of Prosthodontics. **Associate Professo?, Department of Proathodontics. ***Clinical Associate Professor and Acting Chairman, ment of Prdathodontics.
****F&search Assistant, Department of Prosthodontics. 10/l/14246
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Depart-
T
hreaded endodontic dowels used in the restoration of teeth that have received root canal theranv &” are controversial. The advantages of threaded dowels have been advocatedlS4 and criticized.5-7 The presence of threads greatly enhances the retention of the cemented dowels over both smooth-sided and serrated dowels.2*s-lo The amount of retention necessary to retain a dowel and core within a 179
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Fig. 1. Typical fractures resulting from dowel cementation. a, Coronal fracture; b, midroot fracture; c, apical fracture. prepared root canal that has subsequently received a fullcoverage casting is not well documented. The long-term survival rate of teeth that had been restored with dowel and cores, few of which were threaded in design, has been documented.” The added retention effect afforded by the threaded dowel systems must be weighed against potential problems that may result from their insertion. This investigation compared the potential for root fracture resulting from the tapping and cementation process of nine threaded and three nonthreaded endodontic dowel systems.
METHODS
AND MATERIAL
The root-fracturing potential of 11 prefabricated endodontic dowel systems (nine threaded and two nonthreaded) was compared with that of custom-cast gold dowels12 and two control groups. The prefabricated dowel systems evaluated are summarized in Table I. One hundred forty recently extracted maxillary and mandibular premolars were stored in sterile saline before preparation to prevent moisture loss of the root structure. The teeth were randomly assigned to one of 14 test groups of 10 teeth each: Group 1 consisted of 10 teeth that were endodontically instrumented but not obturated (control group 1). Group 2 consisted of 10 teeth that were instrumented and obturated with gutta-percha (control group 2). Group 3 consisted of 10 teeth that were instrumented, obturated, and restored with custom-cast gold dowel and cores (19). Groups 4 and 5 consisted of 10 teeth each that were instrumented, obturated, and restored with prefabricated, nonthreaded dowels (group 5, Para-Post; group 6,’ Unimetric). Groups 6 through 14 consisted of 10 teeth each that were
180
instrumented, obturated, and restored by use of prefabricated, threaded dowels (group 6, Dentatus; group 7, obturation screws; group 8, Flexi-Post; group 9, Kurer anchors; group 10, Radix anchors; group 11, stress free; group 12, Sure-Grip; group 13, Anchorex; and group 14, Dr. Vlock). The clinical crowns were horizontally sectioned from the roots perpendicular to the long axis of the tooth with a diamond saw (Gillings-Hamco, Hamco Machines, Inc., Rochester, N.Y.) and copious water spray to prevent tooth desiccation. The roots were then stored in individually numbered vials containing 10% buffered formalin solution for 1 week prior to experimentation. Root canal therapy was initiated on each specimen by use of conventional filing procedures. Sodium hypochlorite solution (NaOcl) was used for irrigating the canals during the instrumenting procedures. Each canal was instrumented to within 1 mm of the apical opening of the canal, and each canal was enlarged to at least a size 35 file to the working length before obturation. Each canal was then rinsed copiously with NaClO, dried with paper points, rinsed with alcohol (ETOH) and dried with paper points. Ten specimens were instrumented but not obturated (group 1). All other specimens were then obturated with a gutta-percha master cone and multiple accessory points by using the lateral condensation technique13 (groups 2 through 14). A zinc oxide-eugenol (ZOE) root canal cement (Roth International, Ltd. Chicago, Ill.) was used as the sealer for this investigation. Following obturation, the prepared roots were returned to their respective vials of formalin. For each specimen of the assigned dowel system, the gutta-percha was removed from the coronal portion of the root canal, leaving 5 mm of the canal filling. The specific armamentarium for instrumentation provided by each in-
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F‘ig. 2. Scanning electron photomicrograph of prefabricated dowel systems evaluated. Dbentatus; b, Kurer Anchor; c, Sure Grip; d, Unimetric; e, Para-Post; f, Stress-Free; g, a tl lration screws; h, Anchorex; i, Dr. Vlock; j, Radix Anchor; k, Flexi-Post.
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FELTON
Table
I. Characteristics
of dowels investigated Diameter
Experimental grow 3 4
ET AL
Dowel
Custom cast Para-Post
Unimetric Dentatus Obturation screws
8
Flexi-Post
9
Kurer Anchors
10
Radix Anchors
11
Stress-Free
12 13
Sure-Grip Anchorex
14
Dr. Vlock
Manufacturer
None Whaledent International, New York, N.Y. Star Dental Mfg. Co. Vaiiey Forge, Pa. Charles B. Schwed Co., Inc. Union Broach Co., Inc. Long Island City, N.Y. Essential Dental Systems Teledyne Getz, Elk Grove Village, Ill. Caulk/Dentsply International, York, Pa. Medidenta International Woodside, N.Y. R. Chige, Inc. Svedia USA, Inc. Newport, R.I. Brasseler
(mm)*
Shape
Length (mm) (post only)
Post
Post 8; threads
T P
Variable 17.0
1.50
Variable 1.50
11.0
1.30
1.50
8.6
1.50
1.65
10.5
1.20
1.40
11.0
1.25
1.65
11.0
1.45
1.68
7.0
1.35
1.60
P
11.5
1.10
1.50
P P
12.0 8.5
1.30 1.40
1.70 1.65
P
13.0
1.30
1.56
P, Parallel; T, tapered (shape) and titanium (alloy); SS, stainless steel (Ni-Cr); GPB, gold-plated brass; A, gold. *Diameter of tapered dowels measured 3 mm from apical end.
dividual manufacturer was used for gutta-percha removal. The canals were then sequentially enlarged according to manufacturer’s instructions up to the size of reamer that was approximately 1.5 mm in diameter for each system. Custom-cast dowel and core preparations (group 3) were instrumented with a No. 2 Peeso reamer (Union Broach Co., Inc.). The canal walls were subsequently smoothed with a fine-grit diamond mounted in a contra-angle handpiece (No. 260.10 VF, ESPE-Premier Dental Products Co., Norristown, Pa.). Elastomeric impressions were made of each of the prepared post spaces with a vinyl polysiloxane impression material (Express, 3M Dental Products Division, St. Paul, Minn.) as described by Ricker et al.,14to determine whether fractures had occurred as a result of endodontic instrumentation, obturation, or from canal enlargement prior to dowel placement. The impressions were evaluated under ~200 magnification with a microscope (Bausch & Lomb Instruments Division, Rochester, N.Y.), and any specimens demonstrating evidence of root fractures were discarded and replaced with another freshly prepared specimen. Each dowel system was trial-inserted according to man-
182
ufacturer’s recommendations to the depth of the excavated gutta-percha. The dowels were removed and prepared for cementation. The exception to this procedure was the Kurer system, which required the use of a root tap to generate a dentinal “thread” into which to cement the dowel. The root tap was inserted in the canal and rotated two revolutions, removed and cleaned with sterile water, and reinserted. No more than two additional revolutions of the tap were initiated without removal of the tap and cleaning of the tap and canal. On completion of the tapping procedure, the Kurer posts were trial-inserted to ascertain complete seating. Each canal was then dried with air. Each dowel was then cemented with Express vinyl polysiloxane impression material by expressing the material onto a mixing pad, filling the canal with a lentulo spiral (No. 3, L.D. Caulk Co., Milford, Del.), coating the post with the impression material, and threading the post to place until minimal resistance was met. The material was allowed to set for 30 minutes, and the specimens were prepared for decalcification. In an effort to standardize the technique and minimize operator error, all root canals were first prepared and ob-
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made of each of the dowel systems, evaluated at X20 magnification (Fig. 2) to measure thread width and interthread distance.
Table I. Cont’d Thread width (mm)
Interthread
distance (mm)
Alloy
-
0.38
AU T
0.10
0.69
ss
0.08
0.46
GPB
0.10
0.68
ss
0.20
0.43
ss
0.11
0.40
ss
0.12
0.91
ss
0.25
1.05
ss
0.20 0.13
0.97 0.53
SS T
0.13
0.57
ss
turated by one clinician (who had 3 years’ experience in endodontic therapy). All prefabricated dowel systems were placed by one clinician (who had 10 years’ experience with prefabricated dowel systems), and all custom-cast dowel and cores were made and placed by one clinician (who had 12 years’ experience with custom cast dowel and cores). Following endodontic obturation, all dowels were trial-inserted before cementation. The specimens were demineralized in a 40 % solution of ion exchange resin and formic acid (RAF) for 15 days. Completion of demineraliiation was monitored radiographically. The teeth were washed for 12 hours in tap water to remove all traces of the formic acid, and sequentially dehydrated with ascending concentrations of ETOH, beginning with 70 % ETOH and increasing the concentration by 10 % each hour for 6 hours. The specimens were cleared and stored in cedarwood oil. Photographs of the cleared specimens were taken with color slide film at a 1:l magnification to demonstrate the incidence and degree of tooth fracture resulting from the dowel placement procedure (Fig. 1). The location of all fractures was noted, along with the approximate size of the fracture lines. Scanning electron photomicrographs were
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RESULTS The data collected are presented in Table II, and graphed in Fig. 3. Fisher’s test and chi square analysis15 were performed on the data presented in Table II to evaluate the incidence of root fractures relative to post types as compared with instrumented, nonobturated controls (group 1) (Table III). The confidence level was set at 95% @ < 0.05). The results indicate no statistically significant differences between the dowel types regardless of dowel shape, taper, presence or absence of threads, thread width, or interthread distance. In addition, when compared with the instrumented, nonobturated controls, none of the dowel systems investigated demonstrated a statistically different incidence of root fracture.
DISCUSSION Photoelastic studies show that threaded dowels produce the greatest stress within existing roots of any type of dowel design.’ Placing a “screw” in a pulpless tooth predisposes the root to fracture.5l6 Endodontically treated teeth are subject to fracture when there is an excessive loss of remaining denting* 16-lgand loss of moisture content of the dentin, which results from loss of pulpal contents and endodontic therapy.20 Several authors have investigated the potential risk of root fracture in using threaded dowel systems. Durney and Rosen,21evaluating the Dentatus, F.K.G., (Union Broach Co., Inc.) and Kurer systems, demonstrated that the torque necessary to fracture prepared roots was approximately four times the torque needed to tap the roots. Ricker et all4 compared the Kurer, Radix, and Dentatus systems with two nonthreaded controls. No fractures were produced during tapping procedures, and failure stresses for these two systems were far in excess of normal occlusal forces. Kurer and Radix systems were superior to the Dentatus system and to the nonthreaded systems. Dent&us, Medidenta, and Radix systems were compared by inserting the dowels into prepared roots until fracture of the tooth or dowel had occurred,3 demonstrating a 62% fracture rate with the Dentatus system compared with 30% for the Medidenta and 27 % for the Radix systems. Tapered threaded dowels increased the incidence of root fracture over parallel-sided threaded dowels. All three investigations evaluated fracture potentials during maximum torque or loading conditions instead of the tapping and cementation process. Factors inherent in natural tooth structure that resist root fracture include the strength (hardness) of the inorganic (enamel/dentin) component, the elasticity of tooth structure resulting from its organic component, and the
183
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Instrument Instrumrnt/Obturato Unimetric Dentatus Obturation Screws Radix Sure-grip Anchorex Custom Cast Post Pan-post Flexi-post Kurrr Dr. Vlock Stress Free 0
1
2
3
4
5
6
Number of Fractures
Fig. 3. Post-related root fractures.
amount of remaining tooth structure present. Factors inherent in the cementation of endodontic dowels that promote root fracture include a decrease in moisture content and elasticity of the remaining tooth structure after removal of pulpal contents, 21discrepancies in the amount of remaining dentin after dowel space preparation as a result of morphologic differences in root structure, and the release of residual stresses resulting from lateral condensation of gutta-percha, cementation hydraulic pressure, or discrepancies in size between the dowel and reamer system components. The strength of the remaining inorganic component of tooth structure has been shown by investigators to exceed the amount of force required to tap dowel channels20 and to exceed normal occlusal loading forces17l4 Loss of moisture content in endodontically treated teeth most likely decreased the fracture resistance of these teeth. The enlargement of the canal space during endodontic instrumentation and dowel space preparation further compromised the tooth by removing additional tooth structure that might otherwise resist fracture. Loss of moisture content is perhaps a naturally occurring phenomenon, but the drying of the canal with ETOH before obturation with gutta-percha and the generation of heat by rotary instruments during dowel space preparation might further reduce the elasticity of the remaining root structure.
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The amount of remaining dentin has a significant impact on the fracture resistance of remaining tooth structure. All of the teeth used in this study were broader in a faciolingual dimension than in a mesiodistal dimension, as were the root canal spaces within the roots. The preparation of a round dowel space with rotary instruments would, therefore, remove more tooth structure at the expense of the mesial and distal surfaces, thereby further compromising the roots in these areas. Most of the longitudinal fractures (78.7% of all fractures) occurred on the proximal root surfaces. Many of these fractures were associated with naturally occurring depressions (flutes) on the roots. Three specimens were initially rejected because the dowel space preparation resulted in perforations of the fluted root surfaces. These areas would not have been readily visible on standard periapical radiographs because the broader facial and lingual dimensions of the root would have been superimposed over the fluted central portion of the root where the perforations occurred, thereby obscuring it from view. The overwhelming majority (94%) of the fractures terminated sufficiently apical to the cementoenamel junction such that most fractures would occur below the clinical level of the alveolar crest. Subcrestal fractures occurring in vivo would not usually manifest themselves until the health
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Table
Summary of root fractures
II.
DOWELS
AND ROOT FRACTURE
Depth excavation
mm
?t SD
Dowel Instrumented, nonobturated controls Instrumented, obturated controls Custom-cast post Para-Post Unimetric Dentatus Obturation screws Flexi-Post Kurer Anchors Radix Anchors Stress-Free Sure-grip Anchorex Dr. Vlock Totals
9.5 14.6 16.5 9.5 14.4 14.8 14.5 15.7 16.0 15.7 12.9 14.4
k + f + f 2 + * f * f f
1.36 3.01 0.67 2.01 2.65 1.99 1.63 1.42 1.00 1.19 3.38 2.24
Fracture
Type of fracture
Fractures No.
%
Internal
2 4 4 3 3 3 4 4 3 6 3 3 5 47
20 40 40 30 30 30 40 40 30 60 30 30 50 36.2
2 1 1 2 1 7 (14.9%)
External
Coronal 113
Middle l/3
location Apical l/3
2 4 2 3 3 3 3 4 2 4 3 2 5 40 (85.1%)
Transapical
1 2 2 3
3 1 1 3 2
2 -
2 4 2 2
1 1 3 15 (31.9%)
2 28 (59.5%)
1 1 2
3 (6.4%)
1 (22%)
N = 10 for each dowel category. Number of vertical fractures: 1 (2.2%). Number of longitudinal fractures: 37 (78.7% ). Number of horizontal fractures: 9 (19.1%) of which 7 (77.8%) were internal.
of the investing periodontal tissues deteriorated to the point of periodontal pocket formation and tissue infection. The generation of stresses in the remaining root structure due to (1) lateral condensation during obturation of the canal, (2) the tapping/threading procedures, (3) post/ reamer size discrepancies, and (4) hydraulic pressure during cementation may be a more significant factor than previously reported. Impressions were made of the canals after dowel space preparation to eliminate the endodontic obturation and dowel space preparation procedures as variables for consideration. However, the presence of a dentinal smear layer could have obscured microfractures resulting from either procedure that the impression material should otherwise have detected. Since the smear layer was not removed in this investigation, this could not be determined. The observation that 20% of the instrumented and obturated canals exhibited fractures after decalcifying warrants further investigation into alternative means of canal obturation. The decalcification and clearing procedure removed the inorganic component of tooth structure, allowing residual stresses to be released. When the remaining organic matrix was unable to resist these stresses, root fracture occurred. The magnitude of the stresses required to fracture the remaining tooth structure after decalcification could not be
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Table III. Pearson chi-square analysis of 3 X 2 matrix comparing control groups (groups 1 and 2) with nonthreaded groups (groups 3,4, and 5) and threaded groups (groups 6 through 14) for numbers of teeth fractured or intact Fractured 1 Controls
Smooth Threaded Total
1 2 3
2 11 34 41
Intact 2 18 19 56 93
Total 20 30 so
Pearson chi-square test statistic: value, 5.826; DF, 2; probability, 0.054.
determined with the experimental design used. It is of interest to note that the Flexi-Post system, with apical stresses that remain unrelieved,22 only resulted in one fracture in the apical root structure adjacent to the flexible component of the dowel (transapical fracture); the three other specimens exhibiting fractures did so in the middle third of the root, remote from the apical dowel slot. Discrepancies in the dowel/reamer size could provide some residual stresses in root structure. This is more likely to be associated with the custom cast dowels than with the
185
FELTON
ET AL
Fig. 4. Threaded design for cutting dentin, a, Kurer Anchor (tap); b, Radix Anchor. Arrows depict sharp cutting threads.
Fig. 5. Thread design for spreading dentin. a, Obturation screws; b, Dent&us. Arrows depict blunted threads.
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matched dowel/reamer prefabricated systems evaluated. Care should be taken to avoid overexpansion of the custom dowel/core patterns during the investing stage to minimize this potential. Perhaps the greatest stresses are produced during the threading/tapping procedures and cementation of the threaded dowels. Photoelastic evidence supports the presence of stresses during dowel insertion. It has not been determined whether the cutting of threads in dentin with the sharp, angular cutting edges of taps or dowel threads (Fig. 4) results in decreased stresses in the remaining root structure as compared with the spreading of the dentin that most likely occurs with the rounded, blunted threads present on the other dowel systems (Fig. 5). The rough, irregular surfaces present on several systems (Stress-Free and Flexi-Post) could perhaps result in additional stress generated during insertion. Hydraulic stresses generated during the cementation process can also result in residual stress concentrations in the root structure. Most investigators now recognize the benefits of vent channels on dowels, and most manufacturers have placed vertical vents on prefabricated dowels. The adequacy of these cementation vents was not determined during this investigation. Although noticeably missing from the manufacturer’s instructions (except Kurer Anchors), the clinician might well be advised to insert the post, thread it into the prepared dowel space no more than two revolutions, and remove the dowel and debris before proceeding to the next two revolutions for all threaded post systems. After complete tapping of the dowel channel and insertion of cement into the channel, the dowel should be inserted, rotated counter-clockwise until a “click” is felt, then threaded into the canal until resistance is felt. This will prevent “rethreading” the canal and the generation of additional stresses during cementation. In addition, the dowel should then be rotated one-quarter to one-half turn counter-clockwise to relieve additional cemensation stress that overseating of the dowel could cause.23 The clinician’s goal in the use of any dowel and core system should be to preserve as much tooth structure as possible, maintain the apical seal of gutta-percha, provide an adequate core for retention of the final restoration, and to prepare the dowel space and cement the dowel with as little residual stress as possible. Long-term occlusal loading of endodontically treated teeth might release residual stresses, often with catastrophic results. CONCLUSIONS Within the experimental designs used in this investigation, the following conclusions can be drawn. 1. There were no statistically significant differences in the incidence of root fracture among any of the dowel systems evaluated, regardless of shape, taper, or the presence or absence of threads.
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2. Most of the root fractures resulting from dowel insertion occurred on the mesial or distal root surfaces as a result of the reduced thickness of dentin and the presence of external depressions (flutes) on these surfaces.
REFERENCES 1. Kern SB, von Fraunhofer JA, Mueninghoff LA. An in vitro comparison of two dowel and core techniques for endodontically treated molars. J PROSTHET DENT 1984;51:509-14. 2. Kurer HG. An evaluation of the retentive properties of various permanent crown posts. J PROSTHET DENT 1983;49:633-5. 3. Deutsch AS, Cavallari J, Musikant BL, Silverstein L, Lepley J, Petroni G. Root fracture and the design of prefabricated posts. J PROSTHET DENT 1985;53:637-40. 4. Sokol DJ. Effective use of current core and post concepts. J PROSTHET DENT 1984;52:231-4. 5. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design. J PROSTHET DENT 1984;52:28-35. 6. Goerig AC, Mueninghoff LA. Management of the endodontically treated tooth. Part I: Concept for restorative designs. J PROSTHET DENT 1983;49:340-5. 7. Henry PJ. Photoelastic analysis of post core restorations. Aust Dent J 1977;22:157-9. 8. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional forces. J PROSTHET DENT 1979;41:159-62. 9. Johnson JK, Sakumura JS. Dowel form and tensile force. J PROSTHET DENT 1978;40:645-9. 10. Deutsch AS, Musikant BL, Cavallari J, Bernardi S. Retentive propsrties of a new post and core system. J PROSTHET DENT 1985;53:12-4. 11. Sorensen JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J PROSTHET DENT 1984;51:780-4. 12. Shillingburg HT, Kessler JC. Restoration of endodontically treated teeth. Chicago: Quintessence Pub1 Co, 1982;45. 13. Ingle JI, Taintor JF. Endodontics. 3rd ed. Philadelphia: Lea & Febiger, 1982;252. 14. Ricker JB, Lautenschlager EP, Greener EH. Mechanical properties of post and core systems. Dent Mater 1986;2:63-6. 15. Helms RW, Christiansen DH, Hosking JD. An introduction to LINMOD. Chapel Hill: Institute for Statistical Analysis and Research, 1979;224-56. 16. Bravin R. Post reinforcement tested: the functional stress analysis of post reinforcement. J Calif Dent Assoc 1976;4:66-71. 17. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. 3 Endodont 1978;4:341-5. 18. Sapone J, Lovencki SF. An endodontic-prosthodontic approach to internal tooth reinforcement. J PROSTHET DENT 1981;45:164-74. 19. Mattison GD. Photoelaatic stress analysis of cast gold endodontic posts. J PROSTHET DENT 1982;48:407-11. 20. Heifer AR, Melnick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg 1972;34:661-9. 21. Durney EL, Rosen H. Root fracture as a complication of post design and insertion: a laboratory study. Oper Dent 1977;2:90-4. 22. Musikant BL, Deutsch AS. A new prefabricated post and core system. J PROSTHET DENT 1984;52:631-4. 23. Irvin AW, Webb EL, Holland GA, White JT. Photoelastic analysis of stress induced from insertion of self-threading retentive pins. J PROSTHET DENT 1985;53:311-6.
Reprint requests to: DR. DAVID A. FELTON SCHOOL OF DENTISTRY, RM. 302 UNIVERWY OF NORTH CAROLINA CHAPEL HILL, NC 27599-7450
187
or Chroma varies with the porcelain brand used at a given firing temperature. REFERENCES 1. Tylman SD. Theory and practice of crown and bridge. 6th ed. St Louis:
CV Mosby Co, 19’70;562-3. 2. Barghi N, Goldberg J. Porcelain shade stability after repeated firing. J PIWTHET DENT 1977;37:173-5. 3. Barghi N, Richardson JT. A study of various factors influencing the shade of bonded porcelain. J PROSTHET DENT 1978;39:282-4. 4. Barghi N. Color and glaze: effects of repeated firings. J PROSTHET DENT 1982;47:393-5. 5. Jorgenson MW, Goodkiid RJ. Spectrophotometric study of five porce-
lain shades relative to the dimensions of color, porcelain thickness, and repeated firings. J PWSTHET DENT 1979;42:96-105. 6. Hammad IA, Stem RS. A qualitative study for the bond and color of caramometals. part I. J PR~WHET DENT 1990;63:643-53. 7. Barlow F. A point indentation loading study of two ceramometal systems relative to porcelain thickness, repeated firings and surface conditions. Master’s thesis. Minneapolis: University of Minnesota, 1976. 8. Lau CS, Yamada HN. Metal-ceramic crowns and fixed partial dentures. In: Rhoads JE, Rudd KD, Morrow RM. Dental lab procedures. 2nd ed. St Louis: CV Mosby Co, 1986;274.
Threaded incidence
endodontic dowels: of root fracture
D. A. Felton, B. E. Kanoy, University
9. Hammad IA, Goodkmd RJ, Gerberich WW. A shear test for the bond strength of ceramometals. J PROWHET DENT 1987;58:431-7. 10. Wight TA. Bauman JC, Pelleu GB Jr. An evaluation of four variables ai%cting bond strength of porcelain to nonprecious alloy. J PROSTHET DENT 1977;37:570-7. 11. Lund TW, Schwabacher WB, Goodkind R-J. Spectrophotometric study of the relationship between body porcelain color and applied metallic oxide pigments. J PR(WTHETDENT 1985;53:796-6. 12. Obregon A, Goodkind RJ. Effects of opaque and porcelain surface texture on the color of ceramometal restorations. J PRODENT 1981;46:330-40. 13. Sun AF. An investigation of two opaque modiiiers relative to their color
stability and bond strength after repeated firings with two ceramometal systems. Doctor of Science thesis. Boston: Boston University, School of Graduate Dentistry, 1985. Reprint requests to: DR. R. SHELWN STEIN SCIKWL OF GRADUATE DENTISTRY BOSTON UNIVERSITY BOSMN, MA 02118
Effect
of post design on
D.D.S., M.S.,* E. L. Webb, D.D.S., M.S.,** M.A., D.D.S.,*** and Jay Dugoni****
of North
Carolina, School of Dentistry, Chapel Hill, N.C.
The use of threaded endodontic dowels is a controversial issue. The purpose of this study was to compare the potential for root fracture resulting from the cementation of nine threaded and three nonthreaded endodontic dowel systems. The clinical crowns of 140 extracted premolar6 were removed at the cementoenamel junction. The teeth were randomly assigned to the following treatment groups of 10 teeth each: Group 1, endodontically instrumented but not obturated; group 2, instrumented and obturated, group 3, instrumented, obturated, and restored with custom-cast gold dowel and cores; groups 4 and 6, instrumented, obturated, and restored with prefabricated, nonthreaded dowels; and groups 8 through 14, instrumented, obturated, and restored with one of nine prefabricated, threaded dowels. All dowels were inserted according to manufacturer’s directions, removed, and cemented with vinyl pelysiloxane impression material. Each specimen was demineralized and cleared. Photographs at 1:l magnitlcation were taken to assess dowel fractures. Fisher’s test and chi square analysis were performed to evaluate the differences between post types, and between posted and nonposted controls (p < 0.06.). The results indicate no statistically significant differences between dowel types when compared with each other, regardless of dowel shape, taper, or presence or absence of threads, or when. compared to instrumented, nonobturated controls. The amount of remaining dentin and existing root morphology may be a determining factor for endodontically treated teeth to resist fracture during dowel placement. (J PROSTEET DENT 1991;65:179-87.)
This investigation was supported by the University of North Carolina
Faculty
Research Support Grant. *Assistant Professor,Department of Prosthodontics. **Associate Professo?, Department of Proathodontics. ***Clinical Associate Professor and Acting Chairman, ment of Prdathodontics.
****F&search Assistant, Department of Prosthodontics. 10/l/14246
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Depart-
T
hreaded endodontic dowels used in the restoration of teeth that have received root canal theranv &” are controversial. The advantages of threaded dowels have been advocatedlS4 and criticized.5-7 The presence of threads greatly enhances the retention of the cemented dowels over both smooth-sided and serrated dowels.2*s-lo The amount of retention necessary to retain a dowel and core within a 179
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Fig. 1. Typical fractures resulting from dowel cementation. a, Coronal fracture; b, midroot fracture; c, apical fracture. prepared root canal that has subsequently received a fullcoverage casting is not well documented. The long-term survival rate of teeth that had been restored with dowel and cores, few of which were threaded in design, has been documented.” The added retention effect afforded by the threaded dowel systems must be weighed against potential problems that may result from their insertion. This investigation compared the potential for root fracture resulting from the tapping and cementation process of nine threaded and three nonthreaded endodontic dowel systems.
METHODS
AND MATERIAL
The root-fracturing potential of 11 prefabricated endodontic dowel systems (nine threaded and two nonthreaded) was compared with that of custom-cast gold dowels12 and two control groups. The prefabricated dowel systems evaluated are summarized in Table I. One hundred forty recently extracted maxillary and mandibular premolars were stored in sterile saline before preparation to prevent moisture loss of the root structure. The teeth were randomly assigned to one of 14 test groups of 10 teeth each: Group 1 consisted of 10 teeth that were endodontically instrumented but not obturated (control group 1). Group 2 consisted of 10 teeth that were instrumented and obturated with gutta-percha (control group 2). Group 3 consisted of 10 teeth that were instrumented, obturated, and restored with custom-cast gold dowel and cores (19). Groups 4 and 5 consisted of 10 teeth each that were instrumented, obturated, and restored with prefabricated, nonthreaded dowels (group 5, Para-Post; group 6,’ Unimetric). Groups 6 through 14 consisted of 10 teeth each that were
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instrumented, obturated, and restored by use of prefabricated, threaded dowels (group 6, Dentatus; group 7, obturation screws; group 8, Flexi-Post; group 9, Kurer anchors; group 10, Radix anchors; group 11, stress free; group 12, Sure-Grip; group 13, Anchorex; and group 14, Dr. Vlock). The clinical crowns were horizontally sectioned from the roots perpendicular to the long axis of the tooth with a diamond saw (Gillings-Hamco, Hamco Machines, Inc., Rochester, N.Y.) and copious water spray to prevent tooth desiccation. The roots were then stored in individually numbered vials containing 10% buffered formalin solution for 1 week prior to experimentation. Root canal therapy was initiated on each specimen by use of conventional filing procedures. Sodium hypochlorite solution (NaOcl) was used for irrigating the canals during the instrumenting procedures. Each canal was instrumented to within 1 mm of the apical opening of the canal, and each canal was enlarged to at least a size 35 file to the working length before obturation. Each canal was then rinsed copiously with NaClO, dried with paper points, rinsed with alcohol (ETOH) and dried with paper points. Ten specimens were instrumented but not obturated (group 1). All other specimens were then obturated with a gutta-percha master cone and multiple accessory points by using the lateral condensation technique13 (groups 2 through 14). A zinc oxide-eugenol (ZOE) root canal cement (Roth International, Ltd. Chicago, Ill.) was used as the sealer for this investigation. Following obturation, the prepared roots were returned to their respective vials of formalin. For each specimen of the assigned dowel system, the gutta-percha was removed from the coronal portion of the root canal, leaving 5 mm of the canal filling. The specific armamentarium for instrumentation provided by each in-
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F‘ig. 2. Scanning electron photomicrograph of prefabricated dowel systems evaluated. Dbentatus; b, Kurer Anchor; c, Sure Grip; d, Unimetric; e, Para-Post; f, Stress-Free; g, a tl lration screws; h, Anchorex; i, Dr. Vlock; j, Radix Anchor; k, Flexi-Post.
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Table
I. Characteristics
of dowels investigated Diameter
Experimental grow 3 4
ET AL
Dowel
Custom cast Para-Post
Unimetric Dentatus Obturation screws
8
Flexi-Post
9
Kurer Anchors
10
Radix Anchors
11
Stress-Free
12 13
Sure-Grip Anchorex
14
Dr. Vlock
Manufacturer
None Whaledent International, New York, N.Y. Star Dental Mfg. Co. Vaiiey Forge, Pa. Charles B. Schwed Co., Inc. Union Broach Co., Inc. Long Island City, N.Y. Essential Dental Systems Teledyne Getz, Elk Grove Village, Ill. Caulk/Dentsply International, York, Pa. Medidenta International Woodside, N.Y. R. Chige, Inc. Svedia USA, Inc. Newport, R.I. Brasseler
(mm)*
Shape
Length (mm) (post only)
Post
Post 8; threads
T P
Variable 17.0
1.50
Variable 1.50
11.0
1.30
1.50
8.6
1.50
1.65
10.5
1.20
1.40
11.0
1.25
1.65
11.0
1.45
1.68
7.0
1.35
1.60
P
11.5
1.10
1.50
P P
12.0 8.5
1.30 1.40
1.70 1.65
P
13.0
1.30
1.56
P, Parallel; T, tapered (shape) and titanium (alloy); SS, stainless steel (Ni-Cr); GPB, gold-plated brass; A, gold. *Diameter of tapered dowels measured 3 mm from apical end.
dividual manufacturer was used for gutta-percha removal. The canals were then sequentially enlarged according to manufacturer’s instructions up to the size of reamer that was approximately 1.5 mm in diameter for each system. Custom-cast dowel and core preparations (group 3) were instrumented with a No. 2 Peeso reamer (Union Broach Co., Inc.). The canal walls were subsequently smoothed with a fine-grit diamond mounted in a contra-angle handpiece (No. 260.10 VF, ESPE-Premier Dental Products Co., Norristown, Pa.). Elastomeric impressions were made of each of the prepared post spaces with a vinyl polysiloxane impression material (Express, 3M Dental Products Division, St. Paul, Minn.) as described by Ricker et al.,14to determine whether fractures had occurred as a result of endodontic instrumentation, obturation, or from canal enlargement prior to dowel placement. The impressions were evaluated under ~200 magnification with a microscope (Bausch & Lomb Instruments Division, Rochester, N.Y.), and any specimens demonstrating evidence of root fractures were discarded and replaced with another freshly prepared specimen. Each dowel system was trial-inserted according to man-
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ufacturer’s recommendations to the depth of the excavated gutta-percha. The dowels were removed and prepared for cementation. The exception to this procedure was the Kurer system, which required the use of a root tap to generate a dentinal “thread” into which to cement the dowel. The root tap was inserted in the canal and rotated two revolutions, removed and cleaned with sterile water, and reinserted. No more than two additional revolutions of the tap were initiated without removal of the tap and cleaning of the tap and canal. On completion of the tapping procedure, the Kurer posts were trial-inserted to ascertain complete seating. Each canal was then dried with air. Each dowel was then cemented with Express vinyl polysiloxane impression material by expressing the material onto a mixing pad, filling the canal with a lentulo spiral (No. 3, L.D. Caulk Co., Milford, Del.), coating the post with the impression material, and threading the post to place until minimal resistance was met. The material was allowed to set for 30 minutes, and the specimens were prepared for decalcification. In an effort to standardize the technique and minimize operator error, all root canals were first prepared and ob-
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made of each of the dowel systems, evaluated at X20 magnification (Fig. 2) to measure thread width and interthread distance.
Table I. Cont’d Thread width (mm)
Interthread
distance (mm)
Alloy
-
0.38
AU T
0.10
0.69
ss
0.08
0.46
GPB
0.10
0.68
ss
0.20
0.43
ss
0.11
0.40
ss
0.12
0.91
ss
0.25
1.05
ss
0.20 0.13
0.97 0.53
SS T
0.13
0.57
ss
turated by one clinician (who had 3 years’ experience in endodontic therapy). All prefabricated dowel systems were placed by one clinician (who had 10 years’ experience with prefabricated dowel systems), and all custom-cast dowel and cores were made and placed by one clinician (who had 12 years’ experience with custom cast dowel and cores). Following endodontic obturation, all dowels were trial-inserted before cementation. The specimens were demineralized in a 40 % solution of ion exchange resin and formic acid (RAF) for 15 days. Completion of demineraliiation was monitored radiographically. The teeth were washed for 12 hours in tap water to remove all traces of the formic acid, and sequentially dehydrated with ascending concentrations of ETOH, beginning with 70 % ETOH and increasing the concentration by 10 % each hour for 6 hours. The specimens were cleared and stored in cedarwood oil. Photographs of the cleared specimens were taken with color slide film at a 1:l magnification to demonstrate the incidence and degree of tooth fracture resulting from the dowel placement procedure (Fig. 1). The location of all fractures was noted, along with the approximate size of the fracture lines. Scanning electron photomicrographs were
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RESULTS The data collected are presented in Table II, and graphed in Fig. 3. Fisher’s test and chi square analysis15 were performed on the data presented in Table II to evaluate the incidence of root fractures relative to post types as compared with instrumented, nonobturated controls (group 1) (Table III). The confidence level was set at 95% @ < 0.05). The results indicate no statistically significant differences between the dowel types regardless of dowel shape, taper, presence or absence of threads, thread width, or interthread distance. In addition, when compared with the instrumented, nonobturated controls, none of the dowel systems investigated demonstrated a statistically different incidence of root fracture.
DISCUSSION Photoelastic studies show that threaded dowels produce the greatest stress within existing roots of any type of dowel design.’ Placing a “screw” in a pulpless tooth predisposes the root to fracture.5l6 Endodontically treated teeth are subject to fracture when there is an excessive loss of remaining denting* 16-lgand loss of moisture content of the dentin, which results from loss of pulpal contents and endodontic therapy.20 Several authors have investigated the potential risk of root fracture in using threaded dowel systems. Durney and Rosen,21evaluating the Dentatus, F.K.G., (Union Broach Co., Inc.) and Kurer systems, demonstrated that the torque necessary to fracture prepared roots was approximately four times the torque needed to tap the roots. Ricker et all4 compared the Kurer, Radix, and Dentatus systems with two nonthreaded controls. No fractures were produced during tapping procedures, and failure stresses for these two systems were far in excess of normal occlusal forces. Kurer and Radix systems were superior to the Dentatus system and to the nonthreaded systems. Dent&us, Medidenta, and Radix systems were compared by inserting the dowels into prepared roots until fracture of the tooth or dowel had occurred,3 demonstrating a 62% fracture rate with the Dentatus system compared with 30% for the Medidenta and 27 % for the Radix systems. Tapered threaded dowels increased the incidence of root fracture over parallel-sided threaded dowels. All three investigations evaluated fracture potentials during maximum torque or loading conditions instead of the tapping and cementation process. Factors inherent in natural tooth structure that resist root fracture include the strength (hardness) of the inorganic (enamel/dentin) component, the elasticity of tooth structure resulting from its organic component, and the
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Instrument Instrumrnt/Obturato Unimetric Dentatus Obturation Screws Radix Sure-grip Anchorex Custom Cast Post Pan-post Flexi-post Kurrr Dr. Vlock Stress Free 0
1
2
3
4
5
6
Number of Fractures
Fig. 3. Post-related root fractures.
amount of remaining tooth structure present. Factors inherent in the cementation of endodontic dowels that promote root fracture include a decrease in moisture content and elasticity of the remaining tooth structure after removal of pulpal contents, 21discrepancies in the amount of remaining dentin after dowel space preparation as a result of morphologic differences in root structure, and the release of residual stresses resulting from lateral condensation of gutta-percha, cementation hydraulic pressure, or discrepancies in size between the dowel and reamer system components. The strength of the remaining inorganic component of tooth structure has been shown by investigators to exceed the amount of force required to tap dowel channels20 and to exceed normal occlusal loading forces17l4 Loss of moisture content in endodontically treated teeth most likely decreased the fracture resistance of these teeth. The enlargement of the canal space during endodontic instrumentation and dowel space preparation further compromised the tooth by removing additional tooth structure that might otherwise resist fracture. Loss of moisture content is perhaps a naturally occurring phenomenon, but the drying of the canal with ETOH before obturation with gutta-percha and the generation of heat by rotary instruments during dowel space preparation might further reduce the elasticity of the remaining root structure.
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The amount of remaining dentin has a significant impact on the fracture resistance of remaining tooth structure. All of the teeth used in this study were broader in a faciolingual dimension than in a mesiodistal dimension, as were the root canal spaces within the roots. The preparation of a round dowel space with rotary instruments would, therefore, remove more tooth structure at the expense of the mesial and distal surfaces, thereby further compromising the roots in these areas. Most of the longitudinal fractures (78.7% of all fractures) occurred on the proximal root surfaces. Many of these fractures were associated with naturally occurring depressions (flutes) on the roots. Three specimens were initially rejected because the dowel space preparation resulted in perforations of the fluted root surfaces. These areas would not have been readily visible on standard periapical radiographs because the broader facial and lingual dimensions of the root would have been superimposed over the fluted central portion of the root where the perforations occurred, thereby obscuring it from view. The overwhelming majority (94%) of the fractures terminated sufficiently apical to the cementoenamel junction such that most fractures would occur below the clinical level of the alveolar crest. Subcrestal fractures occurring in vivo would not usually manifest themselves until the health
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Table
Summary of root fractures
II.
DOWELS
AND ROOT FRACTURE
Depth excavation
mm
?t SD
Dowel Instrumented, nonobturated controls Instrumented, obturated controls Custom-cast post Para-Post Unimetric Dentatus Obturation screws Flexi-Post Kurer Anchors Radix Anchors Stress-Free Sure-grip Anchorex Dr. Vlock Totals
9.5 14.6 16.5 9.5 14.4 14.8 14.5 15.7 16.0 15.7 12.9 14.4
k + f + f 2 + * f * f f
1.36 3.01 0.67 2.01 2.65 1.99 1.63 1.42 1.00 1.19 3.38 2.24
Fracture
Type of fracture
Fractures No.
%
Internal
2 4 4 3 3 3 4 4 3 6 3 3 5 47
20 40 40 30 30 30 40 40 30 60 30 30 50 36.2
2 1 1 2 1 7 (14.9%)
External
Coronal 113
Middle l/3
location Apical l/3
2 4 2 3 3 3 3 4 2 4 3 2 5 40 (85.1%)
Transapical
1 2 2 3
3 1 1 3 2
2 -
2 4 2 2
1 1 3 15 (31.9%)
2 28 (59.5%)
1 1 2
3 (6.4%)
1 (22%)
N = 10 for each dowel category. Number of vertical fractures: 1 (2.2%). Number of longitudinal fractures: 37 (78.7% ). Number of horizontal fractures: 9 (19.1%) of which 7 (77.8%) were internal.
of the investing periodontal tissues deteriorated to the point of periodontal pocket formation and tissue infection. The generation of stresses in the remaining root structure due to (1) lateral condensation during obturation of the canal, (2) the tapping/threading procedures, (3) post/ reamer size discrepancies, and (4) hydraulic pressure during cementation may be a more significant factor than previously reported. Impressions were made of the canals after dowel space preparation to eliminate the endodontic obturation and dowel space preparation procedures as variables for consideration. However, the presence of a dentinal smear layer could have obscured microfractures resulting from either procedure that the impression material should otherwise have detected. Since the smear layer was not removed in this investigation, this could not be determined. The observation that 20% of the instrumented and obturated canals exhibited fractures after decalcifying warrants further investigation into alternative means of canal obturation. The decalcification and clearing procedure removed the inorganic component of tooth structure, allowing residual stresses to be released. When the remaining organic matrix was unable to resist these stresses, root fracture occurred. The magnitude of the stresses required to fracture the remaining tooth structure after decalcification could not be
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Table III. Pearson chi-square analysis of 3 X 2 matrix comparing control groups (groups 1 and 2) with nonthreaded groups (groups 3,4, and 5) and threaded groups (groups 6 through 14) for numbers of teeth fractured or intact Fractured 1 Controls
Smooth Threaded Total
1 2 3
2 11 34 41
Intact 2 18 19 56 93
Total 20 30 so
Pearson chi-square test statistic: value, 5.826; DF, 2; probability, 0.054.
determined with the experimental design used. It is of interest to note that the Flexi-Post system, with apical stresses that remain unrelieved,22 only resulted in one fracture in the apical root structure adjacent to the flexible component of the dowel (transapical fracture); the three other specimens exhibiting fractures did so in the middle third of the root, remote from the apical dowel slot. Discrepancies in the dowel/reamer size could provide some residual stresses in root structure. This is more likely to be associated with the custom cast dowels than with the
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Fig. 4. Threaded design for cutting dentin, a, Kurer Anchor (tap); b, Radix Anchor. Arrows depict sharp cutting threads.
Fig. 5. Thread design for spreading dentin. a, Obturation screws; b, Dent&us. Arrows depict blunted threads.
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matched dowel/reamer prefabricated systems evaluated. Care should be taken to avoid overexpansion of the custom dowel/core patterns during the investing stage to minimize this potential. Perhaps the greatest stresses are produced during the threading/tapping procedures and cementation of the threaded dowels. Photoelastic evidence supports the presence of stresses during dowel insertion. It has not been determined whether the cutting of threads in dentin with the sharp, angular cutting edges of taps or dowel threads (Fig. 4) results in decreased stresses in the remaining root structure as compared with the spreading of the dentin that most likely occurs with the rounded, blunted threads present on the other dowel systems (Fig. 5). The rough, irregular surfaces present on several systems (Stress-Free and Flexi-Post) could perhaps result in additional stress generated during insertion. Hydraulic stresses generated during the cementation process can also result in residual stress concentrations in the root structure. Most investigators now recognize the benefits of vent channels on dowels, and most manufacturers have placed vertical vents on prefabricated dowels. The adequacy of these cementation vents was not determined during this investigation. Although noticeably missing from the manufacturer’s instructions (except Kurer Anchors), the clinician might well be advised to insert the post, thread it into the prepared dowel space no more than two revolutions, and remove the dowel and debris before proceeding to the next two revolutions for all threaded post systems. After complete tapping of the dowel channel and insertion of cement into the channel, the dowel should be inserted, rotated counter-clockwise until a “click” is felt, then threaded into the canal until resistance is felt. This will prevent “rethreading” the canal and the generation of additional stresses during cementation. In addition, the dowel should then be rotated one-quarter to one-half turn counter-clockwise to relieve additional cemensation stress that overseating of the dowel could cause.23 The clinician’s goal in the use of any dowel and core system should be to preserve as much tooth structure as possible, maintain the apical seal of gutta-percha, provide an adequate core for retention of the final restoration, and to prepare the dowel space and cement the dowel with as little residual stress as possible. Long-term occlusal loading of endodontically treated teeth might release residual stresses, often with catastrophic results. CONCLUSIONS Within the experimental designs used in this investigation, the following conclusions can be drawn. 1. There were no statistically significant differences in the incidence of root fracture among any of the dowel systems evaluated, regardless of shape, taper, or the presence or absence of threads.
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2. Most of the root fractures resulting from dowel insertion occurred on the mesial or distal root surfaces as a result of the reduced thickness of dentin and the presence of external depressions (flutes) on these surfaces.
REFERENCES 1. Kern SB, von Fraunhofer JA, Mueninghoff LA. An in vitro comparison of two dowel and core techniques for endodontically treated molars. J PROSTHET DENT 1984;51:509-14. 2. Kurer HG. An evaluation of the retentive properties of various permanent crown posts. J PROSTHET DENT 1983;49:633-5. 3. Deutsch AS, Cavallari J, Musikant BL, Silverstein L, Lepley J, Petroni G. Root fracture and the design of prefabricated posts. J PROSTHET DENT 1985;53:637-40. 4. Sokol DJ. Effective use of current core and post concepts. J PROSTHET DENT 1984;52:231-4. 5. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design. J PROSTHET DENT 1984;52:28-35. 6. Goerig AC, Mueninghoff LA. Management of the endodontically treated tooth. Part I: Concept for restorative designs. J PROSTHET DENT 1983;49:340-5. 7. Henry PJ. Photoelastic analysis of post core restorations. Aust Dent J 1977;22:157-9. 8. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional forces. J PROSTHET DENT 1979;41:159-62. 9. Johnson JK, Sakumura JS. Dowel form and tensile force. J PROSTHET DENT 1978;40:645-9. 10. Deutsch AS, Musikant BL, Cavallari J, Bernardi S. Retentive propsrties of a new post and core system. J PROSTHET DENT 1985;53:12-4. 11. Sorensen JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J PROSTHET DENT 1984;51:780-4. 12. Shillingburg HT, Kessler JC. Restoration of endodontically treated teeth. Chicago: Quintessence Pub1 Co, 1982;45. 13. Ingle JI, Taintor JF. Endodontics. 3rd ed. Philadelphia: Lea & Febiger, 1982;252. 14. Ricker JB, Lautenschlager EP, Greener EH. Mechanical properties of post and core systems. Dent Mater 1986;2:63-6. 15. Helms RW, Christiansen DH, Hosking JD. An introduction to LINMOD. Chapel Hill: Institute for Statistical Analysis and Research, 1979;224-56. 16. Bravin R. Post reinforcement tested: the functional stress analysis of post reinforcement. J Calif Dent Assoc 1976;4:66-71. 17. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. 3 Endodont 1978;4:341-5. 18. Sapone J, Lovencki SF. An endodontic-prosthodontic approach to internal tooth reinforcement. J PROSTHET DENT 1981;45:164-74. 19. Mattison GD. Photoelaatic stress analysis of cast gold endodontic posts. J PROSTHET DENT 1982;48:407-11. 20. Heifer AR, Melnick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg 1972;34:661-9. 21. Durney EL, Rosen H. Root fracture as a complication of post design and insertion: a laboratory study. Oper Dent 1977;2:90-4. 22. Musikant BL, Deutsch AS. A new prefabricated post and core system. J PROSTHET DENT 1984;52:631-4. 23. Irvin AW, Webb EL, Holland GA, White JT. Photoelastic analysis of stress induced from insertion of self-threading retentive pins. J PROSTHET DENT 1985;53:311-6.
Reprint requests to: DR. DAVID A. FELTON SCHOOL OF DENTISTRY, RM. 302 UNIVERWY OF NORTH CAROLINA CHAPEL HILL, NC 27599-7450
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