ured D. Waldmeier, University
for post patterns
DMD, MS,a and J. E. Grasso, DDS, MSb
Center, School of Dental Medicine,
A method of using light-cured acrylic resin as an alternative to the use of chemically-cured acrylic resins with elastomeric impressions for direct post patterns is presented. The GC Unifast LC acrylic resin is a powder/liquid type resin cured by exposure to visible light. The polymerization process has four stages before final curing: slurry, stringy, dough-like (plastic), and rubber-like (elastic). Advantages over current direct and indirect procedures include ease of manipulation of the material and no change in laboratory handling procedures. While in the dough-like state, the material can be contoured. In the rubber-like state, it is flexible to disengage from minor undercut areas, (J PROSTHET DENT 1992;68:412-5.)
any materials and techniques are currently used for the construction of cast posts and cores for endodontitally treated teeth. 1-8 These include chemically-cured acrylic resins such as Dura Lay (Reliance Manufacturing Co., Worth, Ill.) for the post fabrication, augmented with inlay wax for making the core in the direct method; and elastomeric impressions in the indirect method. Excellent reports by Hunter and Flood1 and by Perel and [email protected]
review treatment planning and restorative principles relevant to the restoration of endodontically treated teeth. A method of using a light-cured acrylic resin is discussed as an alternative to the use of chemically-cured acrylic resins for direct patterns and for elastometric impressions in an indirect method.
aAssistant Professor, Department of Restorative Endodontology. bProfessor, Department of Prosthodontics.
The light-cured acrylic resin (Fig. 1) used was G-C Unifast LC (G-C Dental Industrial Corp., Tokyo, Japan). The acrylic resin is composed of methyl methacrylate and is a powder/liquid type resin cured by exposure to visible light. The manufacturer’s suggested applications include: con-
Fig. 2. Prepared maxillary left central incisor with cord packed in the sulcus for hemostasis.
Fig. 1. G-C Unifast light-curing components: acrylic powder, resin liquid, mixing cups, spatula, and brush.
Fig. 3. Serrated plastic dowel pin fitted into the canal.
4. The acrylic resin is syringed into the tooth.
5. Pin placed into the acrylic resin.
nection and fixation of prostheses, such as temporary crowns; checkbites; stabilization of loose teeth; repair of denture bases; orthodontic and bite plates (splints); and night guards. The preparation of endodontically-treated canals has been extensively discussed elsewhere.3, 5-g Heated endodontic plu.ggers or rotary instruments can be used to remove gutta-percha from the root canal. The use of chemical solvents to remove gutta-percha has been shown to increase apical leakage, and is not recommended in post preparation procedures.rO, r1 The recommended minimum amount of gutta-percha that should be left in the root canal to maintain the apical seal varies according to different auth0rs.l’ It seems prudent, however, that after completion of post space preparation, at least 4 mm of the root filling should remain undisturbed in the apical part of the root ~anal.~~ After establishing the amount of root filling that is to remainand the post space, a diamond bur may be used to assure removal of undercuts and to finish post space preparation (Fig. 2). The selection of the bur is based on the
the coronal portion
the post portion
of the pattern.
of the pattern.
diameter of the root canal and the length of the desired post space. High speed should not be used in post space preparation. A round end tapered diamond bur should be used to shape and smooth the walls of the canal and to eliminate any undercuts. A nonround or ovoid post space preparation will eliminate the potential for rotation of the final cast restoration. There is a direct relationship between post length and retention.14 Limits are set by root anatomy, by the length of the root canal filling,12 and by the function of the restoration. The behavior of the acrylic resin in the mouth at 37’ C (98” F) must be understood. The approximate times for these stages are: slurry, % minute; stringy, l/z minute; dough-like (plastic), 2 minutes; and rubber-like (elastic), until cured. The shape of the mass of material can be changed in the first three stages. Generally, the dough-like stage is most useful because plastic deformation is permitted. A serrated, plastic dowel pin (Dura Lay Plastic Pins for Posts and Cores, Reliance Manufacturing Co.) is fitted into
Fig. 8. Addition pattern.
resin to core portion
Fig. 11. Close-up view of cemented core.
Fig. 12. View of cast post and core showing position as a bridge abutment.
the canal so it extends to the base of the canal (Fig. 3). Its purpose is to act as a carrier of the polymerizing acrylic resin and to provide support for the removal of the post. The manufacturer’s directions provide the standard powder/liquid ratio and mixing directions. The fitted pin is wetted with the monomer liquid, a C-R syringe (No. 130005, Centrix, Stratford, Conn.) is used to deposit the mixture into the canal (Fig. 4), and the pin is inserted into
the acrylic resin (Fig. 5). After the pin has been inserted into the resin, more resin is added around the coronal part of the pin to initiate a core buildup. It also assures adequate rigid bonding to the pin. After light-curing (Model VCI300, Demetron Research Corp., Danbury, Conn.) of the coronal portion (Fig. 6), the post and core pattern is gently removed, checked for undercut areas, and the post portion is light-cured (Fig. 7). The pattern is then reinserted and checked for correct fit. Further acrylic resin is added to the core portion with a bead-brush technique (Fig. 8), contoured to the desired form, and light-cured. The pattern is trimmed with fine diamond bum or with sandpaper disks to the desired contours (Fig. 9). The resulting pattern (Fig. 10) is sprued so the occlusal height is readily noted and invested for casting in a conventional manner. After completion of the post and core pattern, the root canal filling is protected from exposure to saliva and moisture by placing a cotton pellet in the post space, apical to the root canal orifice, and filling the rest of the canal with
Fig. 9. Final contouring incisal length.
10. Final acrylic resin pattern.
a temporary filling material. A minimum of 3.5 mm of the temporary material is reported to be needed to prevent leakage.16 Fig. 11 shows the polished post and core after eementation. In Fig. 12, the cast post and core will be used as a bridge abutment. DISCUSSfON The use of the G-C Unifast light-curing acrylic resin has several advantages over current direct and indirect procedures. The initial chemical cure provides sufficient rigidity for adaptation to the canal walls and the pin, yet the material remains flexible enough until final light curing to disengage from minor undercut areas. The resin does not stick to the canal walls, thus there is little concern for locking material into the canal as with chemically-cured materials. The acrylic resin can be premixed and contoured easily while in the dough-like plastic state. No change in laboratory handling procedures is apparent. A temporary post/core/crown may be made by this method similar to that constructed with the method described by Porter and Freidline.8 Acrylic resin is added to the core until a full crown contour is obtained. The disadvantages are that additional chair time is needed compared with a laboratory procedure, and some material handling experience is required for efficient and productive use of chair time. SUMMARY The use of a light-cured acrylic resin can provide an excellent alternative method to the use of chemically-cured acrylic resins and inlay wax for fabrication of direct cast post and core patterns, and to elastometric impressions in the indirect method for endodontically treated teeth.
REFERENCES 1. Hunter A, Flood A. The restoration of endodontically treated teeth. Part 1. Treatment planning and restorative principles. Aust Dent J 1988;33:481. 2. Perel ML, Muroff FI. Clinical criteria for posts and cores. J PR~STHET DENT 1972;28:405. 3. Miller AW III. Post and core systems: which one is best? J PROSTHET DENT 1982;48:27. 4. Charbeneau GT. Principles and practice of operative Dentistry. ed 3. Philadelphia: Lea & Febiger, 1988:441. 5. Kantor ME. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;33:405. 6. Weine FS, et al. The use of standardized tapered plastic pins in post and core fabrication. J PROSTHET DENT 19’73;29:542. 7. Gutman JL. Preparation of endodontically treated teeth to receive a post-core restoration. J PROSTHET DENT 1977;38:413. 8. Porter CB, Freidline CW. Interim restoration of endodontically treated anterior teeth. Gen Dent 1981;29:39. 9. Safavi KE, Grasso JE. Endodontic considerations in restoration of partial overdenture abutments. Dent Clin North Am 1990;34:645. 10. Kwan EH, Harrington GW. The effect of immediate post preparation on apical seal. J Endodont 1981;7:325. 11. Mattison GD, et al. The effect of post preparation on the apical seal. J PROSTHET DENT 19&1;51:785. 12. Zillich RM, Corcoran JF. Average maximum post lengths in endodontically treated teeth. J PROSTHET DENT 1984;52:489. 13. Neagley RL. The effect of dowel preparation on the apical seal of endodontically treated teeth. Oral Surg 1969;28:739. 14. Stokes AN. Post crowns: a review. Int Endodont J 1987;ZO:l. 15. Porte11 FR, et al. The effect of immediate versus delayed dowel space preparation on the integrity of the apical seal. J Endodont 1982;8:154. 16. Webber RT, et al. Sealing quality of a temporary filling material. Oral Surg 1971;32:928.
Reprint requests to; DR. MICHAEL D. WALDMELFR SCHOOL OF DENTAL MEDICINE DEPARTMENT OF RESTORATIVE DENTISTRY AND ENDODONTOLOGY MC-1715 UNIVERSITY OF CONNECTICUT HEALTH CENTER FARMINGTON, CT 06030