New Technique for the Use of a Precision Attachment in the Fabrication of a Cast Crown/Core and Post Unit: Procedures and Case Report Sheldon Nadlel-, D.M.D.," Allan S. Deutsch, D.M.D.)Bai-ry Lee Musikant, D.M.D.,tBrett I. Cohen,Ph.D.

A case report of a prefabricated cast post system, Flexi-Cast, is presented. This tech-nique provides maximium retention of the post in the root and crown to a core by incorporation of a prefabricated split-shank threaded post with an attachment sleeve. Clinicallythis appears to be a simple predictable technique for the fabrication of a crown/core as a single unit.



any approaches for the fabrication of a crown, post, and core unit are available. The traditional technique includes fabrication of a cast post and core from an impression or wax pattern, followed by a second impression of the cemented post and core in the root. The final restoration is made from this second impression. Older techniques such as the Richmond Crown, in which the post, core, and crown are fabricated as a single unit, have fallen into disuse mainly because obtaining an adequate casting with all these components is difAcult.' Currently, the most popular clinical method includes placement of a prefabricated post. The core of this post is built up with either amalgam, composite, or glass ionomer. A n impression is then taken of the core and root complex and the final restoration is fabricated from this impression.

The fxst part of this prefabricated cast post system demonstrates more than adequate retention of the post in the root. The literature shows high retention for a prefabricated split-shanked threaded post, Fled-Post (Essential Dental Systems, S . Hackensack, NJ).2" The retention of a No. 2 Fled-Post is in the range of 265.0 pounds when cemented with zinc phosphate ~ e m e n t . ~ . ~ , * The most retentive post available is preferable, since there are no clinically acceptable guidelines for post retention. This is because all posts to date, no matter how retentive, show some degree of clinical retentive failure. Crown attachment to the core is the next item of utmost concern to the dentist. The Flexi-Cast (Essential Dental Systems, S . Hackensack, NJ) assembly includes a prefabricated split-shanked threaded post, similar to the Flexi-Post, with the precision sleeve attachment made from either stainless steel or titanium (Fig. 1). The machined head portion of the post is grooved circumferentially and has a flat machined surface on one side to prevent rotation. The sleeve has grooves machined on the inside to match the circumferential grooves of the post head (seeFig. 1).When the sleeve is cemented to the post head, cement flows into the grooves of both the sleeve and the corresponding post head. In addition, the sleevehas a cover, or one closed end, to prevent crimping or loss of dimension. The sleeve is then cast into the core or as a one unit crown/core. It has been shown that the retention of the No. 2 precision sleeve to the head of the No. 2 Flexi-Cast post, when cemented zinc phosphate, is in excess of 130 pounds.gWorley et all0have shown retention of cast post crowns to cores in the range of 58 to 86 pounds. Bnrkl et al" have shown retention of cast post crowns to cores in the range of 67 pounds. Chan et al12reportedretention values of crowns for 30 degree and 7 degree tapers of 24 and 65 pounds respectively. It is theorized that the

TECHNICAL CONSIDERATIONS Any post, core, and crown technique should include the following considerations: (1)maximum retention of the post to the root and the crown to the core; (2)ease and standardization in achieving the post, core, and crown unit as a whole; (3)good distribution of insertional and masticatory stresses to the root; and (4) the time and skill needed to consistently produce a clinicallysuccessful restoration. Each of these factors is described in relation to the new technique proposed for precision attachment.

* Practicing Dentist. New York. New York Co-director of Dental Research, and *Vice-President of Dental Research. Essential Dental Laboratories. S . Hackensack. New Jersey Address reprint requests to Brett I. Cohen, Ph.D.. Essential Dental Laboratories, 89 Leuning Street. S . Hackensack. NJ 07606 0 1992 Decker Periodicals Inc.


Precision Attachment of Cast Crown/Core and Post Unit

greater skill than an impression limited to external details. A wax or acrylic pattern is the same impression using a different material. The Flexi-Cast system requires one external impression for the final restoration. Consequently,a higher rate of success should be achieved more consistently. This procedure eliminates the second impression, that of the core, in order t o fabricate the final crown restoration. This eliminates both an office visit and a second source of possible error, which may occur in taking the impression and pouring it with standard stone or epoxy materials.

CASE REPORT AND CLINICAL PROCEDURE A 74-year-old woman presented with a horizontally fractured maxillary central incisor. Endodontic therapy had been previously performed. All decay was removed as was any unsupported dentin. The appropriate post size was chosen by superimposing the depth gauge over a n accurate x-ray film. A No. 2 size Flexi-Cast post was selected.

Figure 1. Illustration of the Flexi-Cast crown/core assembly, showing beveled root preparation, seated post, and sleeve incorporated into the crown/core.

Post Hole Preparation

0 degree taper and the matching cement locks on the post and sleeve are responsible for the high retention shown by this precision a t t a ~ h m e n t . ~

The gutta percha was removed down to the depth of the post hole. Gates glidden drills were used in sequential order from a No. 3 to a No. 5 and cutting was done while the drill was wet. After 90%of the width of the post hole and 100% of the length was achieved with gates glidden drills, the primary No. 2 reamer from the FlexiCast system was used (Fig.2).This reamer is selflimiting and drills the full length of the post hole.

Standardization and Ease of Use Standardization and hence predictability of results is a recognized goal in dentistry. The Flexi-Cast crown/ core technique removes many variables of crown to core fit. The taper of the core is standardized at 0 degrees (parallel),the most retentive conflguration.12-14The same number of circumferential grooves line up in both the post head and the ~ l e e v e . ~This . ' ~ results in large surface areas created for cement locks in each tooth restored with the attachment. 13.14.16-19 The height of the attachment for each size post is constant. The net result is superior retention of the crown to the post that is predictable within a narrow range. The system is simple. The technique requires only one impression of the post head. The dentist then receives from a restorative laboratory a Anal restoration with the machined sleeve cast into it.

Distribution of Stress Upon placement, the shaft portion of the post has been shown to deliver low insertional stresses to the root, thereby allowing the post to have high retentive values coupled with low stress.20-22 The flange portion near the head of the post is countersunk into the root. This flange provides the post with greater surface area in contact with the root.23

Time and Skill The weak linkin a cast post system is an inadequate impression or pattern. Any internal impression requires

Figure 2. Preparation of the post hole using a No. 2 primary reamer.



Figure 3. Final preparation of the post hole using a countersink drill.

Figure 5. Cemented Flexi-Cast post placed in the post hole after the cement was removed from around the abutment and post head.

Next, the preparation of the countersunk dentin for the second tier and flange of the post was prepared in one operation (Fig. 3). The countersink drill is not self limiting. It can be used to drill further into the root structure. This is done in order to sink the post further into the root and gain occlusal clearance for the bulk of the crown ifnecessary (Fig. 4). If more occlusal clearance is needed, the post may be shortened from the coronal aspect and the corresponding sleeve should be shortened from the apical aspect, leaving the closed occlusal end of the post intact.

Post Insertion The post was trial seated first with no cement. This was done as per manufacturer’s instructions to both thread the root and check for fit of both the post and the occlusion. The second tier of the post is also checked to see if it is fully seated as per manufacturer’s instructions. The post was then removed, cleaned of debris and cemented into final place with zinc phosphate cement (Fig. 5). Excess cement was removed from around the abutment and the post head.

Preparationand Impression The flnish line is now prepared, a shoulder with a bevel was used. A n impression using Impregum F (ESPE-Premier, Norristown, PA) was taken of the post head and the surrounding root and tissue (Fig. 6).The impression and permanent sleeve were then sent to the laboratory.

Temporization A delrin plastic sleeve from the Flexi-Cast system kit was then placed on the post head. The temporary acrylic crown was filled with cold cure acrylic and seated over the post and delrin sleeve. The cold cure acrylic was allowed to set. The temporary crown was then removed from the post head, picking up the delrin sleeve. This delrin temporary sleeve affords the acrylic temporary approximately 3 to 5 pounds of retention. Temp Bond (Kerr Manufacturing Company, Romulus, MI) temporary cement was placed onjust the marginal areas of the acrylic temporary (Fig. 7).This was done to prevent the temporary cement from going into the grooves on the head of the post.

Figure 4. Post hole after preparation with the primary reamer and countersink drill.


Precision Attachment of Cast Crown/Core and Post Unit

Ngure 6. An Impregum impression of the post head and the surrounding root and tissue surfaces. Note that the impression of the post head is in a D shape.

Ngure 8. An epoxy model of the post head. Note that the epoxy model of the post head has a D shape with four circumferential grooves.

Laboratory Procedure


The impression (see Fig. 6) and the permanent sleevewere sent to the laboratory. The laboratory poured the impression using epoxy ( A X Epoxy,Jeneric/Pentron, Inc., Wallingford, CT')(Fig.8).Epoxy is used because the head of the post is small and would break off during separation if any other die material were used (i.e., stone). Once the die is made, the laboratory technician places the permanent stainless steel sleeve on the post head. The core and crown are then waxed up to the sleeve and cast in metal (Fig. 9). There are enough undercuts and grooves in the external surface of the sleeve to provide retention of the cast metal to the sleeve. In addition the sleeve is shaped like the letter D when viewed from the occlusal. This shape prevents the sleeve from rotating (antirotation feature) within the casting. Porcelain was added next and the crown was taken to completion (see Fig. 9).

The temporary was removed and all temporary cement was removed from the post head and root structure. The restoration was trial seated and the occlusion was checked. The crown/core was then cemented permanentlywith zinc phosphate cement (Flecks zinc phosphate cement, Valley Forge, PA) (Fig. 10).

DISCUSSl0N The heights of the post heads are standardized according to post size. A disadvantage occurs in the situation where there is little occlusal room. The tooth must be countersunkmore deeply or the post and sleeve altered. Countersinking too deeply into the root may result in the removal of excessive amounts of tooth structure. Alternatively, altering the post and the sleeve is a time-consuming operation.

Ngure 9. The final crown with the sleeve incorporated after Figure 7. An acrylic temporary placed with temporary

completion with porcelain. The internal retention grooves of the sleeve are visible.




5. Saunders RD, Lorey RE, Powers J M , Sloan KM. Comparison of fivepost-cement systems for tensile retentive capacity.J Dent Res 1988;67:304. 6. Burgess J O , SummittJB. Robbins JW.Cronin RJ Jr. Four endodontic posts tested in tension, torsion and compression. J Dent Res 1991;70:387. 7. Pameijer CH. Grasso J. In vitro retention of three self tapping post and core systems. J Dent Res 1991;70:387. 8. Cohen BI, Musikant BL, Deutsch AS. Comparison of the retentive properties of four post systems. J Prosthet Dent 1992;68:264-268. 9. Deutsch AS, Cohen BI, Musikant BL. Retentive properties ofattachment sleevesfor anewcast-post system. J Prosthet Dent 1992:67:34-36. 10. Worley JL, Hamm RC. von Fraunhofer JA. Effects of cement on crown retention. J Prosthet Dent 1982;48: 289-291. 11. Brukl CE, Nicholson J W ,Norling BK. Crown retention and seating on natural teeth with a resin cement. J Prosthet Dent 1985;53:618-622. 12. Chan KC, Boyer DB, Denehy GE, Aunan DC. Effects of metal etching on crown retention. J Prosthet Dent 1986; 55:18-21. 13. Ohkawa S,Okane H, Nagasawa T,Tsuru H. Change in retention of various telescope crown assemblies over longterm use. J Prosthet Dent 1990;64:153-158. 14. Chan KC, Hormati AA, Boyer DB. Auxiliary retention for complete crowns provided by cement keys. J Prosthet Dent 1981;45:152-155. 15. Cohen BI. Deutsch AS, Musikant BL. Retentive properties ofmisaligned Flexi-Cast post attachment sleeves. J Prosthet Dent 1992;67:191-193. 16. DeWald JP. Moody CR. Ferracane JL, Cotmore J M . Crown retention: a comparative study of core type and luting agent. Dent Mater 1987;3:71-73. 17. Maxwell AW.Blank LW, Pelleu GB. Effect of crown preparation height on the retention and resistance of gold castings. Gen Dent 1990;200-202. 18. Pllo R Cardash HS. Eng RCS, Baharav H, Helft M. Incomplete seating of cemented crowns: a literature review. J Prosthet Dent 1988:59:429-433. 19. Chan KC, Azarbal P. Kerber P. Bond strength of cements to crown bases. J Prosthet Dent 1981:46:297-299. 20. Musikant BL, Deutsch AS. A photoelastic comparison of functional stresses between two posts. J Dent Res 1990; 69:122. 21. Ross RS. Nicholls J I , Harrington GW. A comparison of strains generated during placement of five endodontic posts. J Endodont 1991;17:450-456. 22. Greenfeld RS,Roydhouse RH,Marshall FJ, Schoner B. A comparisonof twopost systemsunder appliedcompressiveshear loads. J Prosthet Dent 1989;61:17-24. 23. Cohen BI. Deutsch AS. Musikant BL. Cyclic fatigue testing of slx endodontic post systems. J Prosthodont, in press.

Figure 10. The final crown/core cemented permanentlywith zinc phosphate cement.

Another limitation is that the technique appears to be restricted to teeth with little or no coronal tooth structure remaining. To sacrifice tooth structure to place a core is contrary to current accepted procedures. Lastly, because of paralleling difficulties, this technique seems to be limited to single units. The technique could be used with multiple units but the fabrication of telescopes would be necessary.

CONCLUSION Clinically this appears to be a simple predictable technique for the fabrication of a crown/core as a single unit. The resulting crown appears to have predictably high retention to the post in all cases.

REFERENCES Kurer PF. The Kurer Anchor System; an approach to the restorative and re-use of endodontically treated teeth. Chicago: Quintessence. 1984:13-15. Deutsch AS,Musikant BL, Cavallari J, Bemardi S . Retentive properties of a new post and core system. J Prosthet Dent 1985:53:12-24. Millstein PL, Yu H, Hsu CS, Nathanson D. Effects of cementing on retention of a prefabricated screw post. J Prosthet Dent 1987;57:171-174. Brown JD. Mitchem JC. Retentive properties of dowel post systems. Oper Dent 1987: 12:15-19.


core and post unit: procedures and case report.

New Technique for the Use of a Precision Attachment in the Fabrication of a Cast Crown/Core and Post Unit: Procedures and Case Report Sheldon Nadlel-,...
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