Root fr selection Patrice?
ure in endodontically and crown design M&t,
Boston University School of Graduate Montreal, Quebec, Canada
and R. Sheldon Dentistry,
Boston, Mass., and University
Standardized plastic analogues simulating an endodontically treated maxillary central incisor root were used to investigate the resistance to root fracture in endodontically treated teeth. Three different post and core systems were used: (1) cast post and core, (2) Para-Post Plus post, and (3) Flexi-Post post. The core build-up material selected in this study was Ketac Silver material, after which a crown preparation was made on each analogue. Two types of preparations were use& a wide chamfer margin (butt-joint) and a wide chamfer margin with a 1.0 mm circumferential bevel. An olvercasting was fabricated and was permanently cemented on each preparation. Assemblies were subjected to an increasing lingual oblique force until fracture occurred. Results were tabulated using the analysis of variance (F test) and Student’s t test. Beveled preparations with a concomitant final restoration provided a significant increased resistance to root fracture. Furthermore, vertical fracture occurred twice as often with nonbeveled preparations. (J PROSTHET DENT 1992;68:428-35.)
Presented in partial fulfillment of the degree of Master’s of Science in Dentistry at Boston University School of Graduate Dentistry, Boston, Mass. aFormer Graduate Student in Prosthodontics, Boston University School of Graduate Dentistry; presently Assistant Professor in Fixed Prosthodontics, University of Montreal. bAssistant Dean of Clinical Affairs, Research Professor of Pros-
thodontics, Boston University School of Graduate Dentistry. 1011137741
of the analog used in this study.
he restoration of the endodontically treated tooth is a challenging aspect of restorative dentistry. Because of the present state of the art and science of endodontics, there has been a notable increase in the retention of endodontically treated teeth, especially for restorative purposes. Such teeth are often considered brittle1 and weakened by endodontic treatment and thus the need for supportive restorations has long challenged the ingenuity and skill of dentists. Numerous prosthetic advances spawn many controversial reports of a standardized approach for the restoration of an endodontically treated tooth. However, since the inception of the Richmond crown,2 it is generally accepted that the technique should consider two distinct phases: (1) the post and coronal section and (2) the individual final restoration.4 * Generally, a cast post and core is designed with a slightly tapered canal as a primary consideration. Investigators have reported that the paramount factor determining the resistance to forces and s&sequent root fracture of an endodontically treated tooth is the amount of sound tooth structure remaining after preparation of the post space.5-7 Although the cast post and core is frequently used, there is an increasing tendency for the use of prefabricated systems.8-12 Generally when a prefabricated post is used, the coronal section is concomitantly fabricated intraorally. Build-up materials for the coronal section are usually amalgam,r3-r5 composite resin,i6-rg glass ionomer cement, or a combination of glass ionomer cement with amalgam.20~ 21 Composite resin has been shown to be dimensionally unstable when exposed to moisture.22v 23 Retention of posts has always been a major concern for
diagram of the standardized
Fig. 4. Device used for a static loading (5 lb).
3. Cast post and. core in place.
dentists. Length and design are two important variables of retention of posts. To achieve maximum retention, most reports advocate that the post should be as long as possible while still retaining a positive apical seal of 5 mm of gutta-percha. 24-28 Investigators 28-3o have demonstrated that the parallel-sided post offers superior retentive properties when compared with tapered posts. Surface texture
is another important consideration for retention. Threaded posts are the most retentive. 31,32Serration on the post surface will increase retention compared with a smooth surface.28 The diameter of the posts has a proportional retentive effect30,33 that is a self-limiting factor because of the danger of perforation and the generation of adverse stresses.34, 35 The cementation technique also has an important effect upon the eventual retention and stress distribution of the post and coronal section. According to several studies,36-38 the introduction of cement into the root canal during cementation procedures is essential to achieve a uniform bubble-free layer of cement that distributes the stresses evenly throughout the entire root canal. No cement
Plus post No. 5 in place.
post No. 1 in place.
a clear advantage over another cehas demonstrated mer ]t 29,39,40 7slo system would be successful in restoring an endodontica lly treated tooth unless an adequate final restoration is fabl ricated. Many authors have stressed the importance of regarding its possible the final restoration, particularly brat zing or casing action.41-47 Rosen@’ in 1961 stated: “The
final restoration should have an extracoronal brace or subgingival collar, which by its hugging action will prevent vertical shattering of the tooth.” Eissmann and Radke4g also advised the addition of a “ferrule” via a 2.0 mm gold collar design for the final restoration, thereby preventing vertical fracture. Rosen and Partida Riveras tested the benefit of a gold collar for iatrogenically induced fractures
diagram of the standardized
by means of screw posts. The results were highly favorable for the use of the gingival apron, Although several author& 52 have stated that a gold ferrule (bevel) encasing the root beyond a chamfer margin decreases the likelihood of root fracture of endodontically treated teeth, there is little scientific evidence to substantiate these claims. The purpose of this study was to clarify the role of such a bevel on the tooth preparation and subsequent crown restoration with respect to root fracture with simulated clinical forces.
Forty-eight clear plastic standardized analogues (Viade Products Inc., Camarillo, Calif.) simulating an endodontically treated maxillary central incisor were used (Fig. 1). They were divided into three groups of 16 specimens each. Group 1 remained intact. The depth of the prepared canal was 12 mm or four fifths of th.e total length of the analogue. A post and core wax pattern was carefully adapted to the specific dimensions of the preparation (Fig. 2). The diameter of the post ranged from 0.8 mm at the apical portion to 2.8 mm at the cervical portion. A No. 14 gauge Williams plastic sprue (Williams Dental Co. Inc., Buffalo, N.Y.) was used to ensure proper standardized spruing. The sprued patterns were attached to a No. 4081 sprue former (Whip-Mix Corp., Louisville, Ky.) and were painted with a thin coat of Debubblizer varnish (Kerr Mfg. Co., Emeryville, Calif.). The investment used in this study was vacuum mixed Cristobalite material (Whip Mix Corp.) at a water/powder ratio of 15 ml for 50 gm, according to the manufacturer’s instructions. The vacuum was broken after mixing. The investment was carefully painted on the pat-
tern before it was poured into the ring. A casting ring (No. 4086, Whip-Mix Corp.) was used with one layer of wet lining cellulose (Whip-Mix Corp.). The investment was bench set for 1 hour before insertion in a cold oven. The temperature was raised 15’ F/min until a temperature of 900° F was attained and was held at that level for an additional hour. The casting was made with a centrifugal casting machine using the Elektra alloy (Williams Dental Co., Inc.). The alloy was melted with a gas/oxygen torch. The casting was allowed to bench cool for 5 minutes before it was quenched in water. The casting was retrieved, divested, cleaned, and fitted to its respective analogue (Fig. 3). Before final cementation, the internal surface of the casting was air-abraded with aluminum oxide. The cementation was accomplished with zinc-phosphate cement (Mizzy Inc., Cherry Hill, N.J.) according to the manufacturer’s instructions. The cement was painted on the surfaces of the casting and was introduced into the simulated root canal with a Lentullo paste filler (Brasseler U.S.A., Savannah, Ga.). Finally the inserted casting was placed under a static load of 5 lb for 10 minutes (Fig. 4). Group 2 had the root canal enlarged with a No. 5 ParaPost drill measuring 1.15 mm in diameter (Whaledent International Co., New York, N.Y.) to a depth of 12 mm or four fifths of the total length of the analogue. A Para-Post Plus post (Whaledent International Co.) of corresponding diameter was then fitted and trimmed (Fig. 5). If there was any misfit, another analogue was prepared. The Para-Post Plus post was cemented with zinc-phosphate cement using the same technique described earlier for the cast post and core. The cemented post was placed under a static load of 5 pounds for 10 minutes. Finally a core was fabricated with Ketac Silver material (ESPE, Seefeld/Oberbay, Germany)
Fig. 1% Metallic area.
block with a relief zone at the apical ~
++*+++++ . . . . . . . . .._.
!3 c 0
Fig. 12. Typical
I. Force (in kilograms)
curve for each specimen.
for fracture of
specimens Specimen NO.
Fig. 111. Secured embedded testing machine table.
on the Instron
and was shaped with diamond bum (Brasseler U.S.A.) according to the specific dimensions of the preparation. Group 3 had the root canal enlarged with a No. 1 FlexiPost reamer mesuring 1.10 mm in diameter (Essential Dental Systems, New York, N.Y.) to a depth of 12 mm or four fifths of the total length of the analogue. A Flexi-Post post (Essential Dental Systems) of corresponding diameter was then test fitted in the analogue (Fig. 6). If there was any misfit, another analogue was prepared. The Flexi-Post post was cemented with zinc-phosphate cement using the technique described earlier for the cast post and core. Finally a core was fabricated with Ketac Silver material and was shaped according to the specific dimensions of the preparation. The three groups were further divided into two subgroups (A and B) of eight specimens each. Group A remained intact (Fig. 7). Group B received a 1.0 mm concave bevel circumferentially (Fig. 8) with a V&fluted carbide bur No. 7104 (Brasseler U.S.A.). An overcasting crown was waxed and adapted directly on the analogue to specific dimensions (Fig. 9). The wax pattern was then invested and cast using the same metal and technique described earlier for the cast post and core. The
I 2 3 4 5 6 7 8
30 35 27 40 59 37 43 31
47 53 49 39 43 40 35 39
57 40 34 36 29 28 24 30
51 38 37 42 33 45 29 55
32 43 32 33 48 32 28 32
49 56 27 35 51 46 46 28
P/C, Cast post and core; P/P, Pam-Post post and Ketac Silver build-up; F/P, Flexi-Post post and Ketac Silver build-up; nb, nonbeveled preparation; b,
Sample p/&b p/c,, p/P, p/P,
data of samples
Standard deviation (kg)
37.750 43.125 34.750 41.250 35.000 42.250
36.0 41.5 32.0 40.0 32.0 46.0
32 18 33 26 20 29
i 10.096 f 6.058 + 10.292 + 8.828 rt 6.782 + 10.873
Abbreviations as in Table I.
overcastings were vented at the incisal area with a No. 4 round bur (Brasseler U.S.A.) and were cemented with zinc-phosphate cement under a static load of 5 lb for 10 minutes. A specifically designed block was fabricated in Duralay material (Reliance Dental Mfg. Co., Worth, Ill.) and was
Fig. 13. Cracking of the core buildup material after preparation. (Original magnification X20.)
Fig. 14. Cracking of the core buildup insertion. (Original magnification X20.)
F test summary for the nonbeveled group DF
Between-group 2 Within-group 21