A customized acrylic resin shell for fabricating an

Prosthodontics

~e A customized acrylic resin shell for fabricating an amalgam core on the coronally debilitated, endodontically treated posterior tooth Esteban D. Bonilla*jTomas Anderkvist*jGary D. Miller**

_ENeE This article describes a technique forfabricating a custom acrylic resin shell that will enslIfe a properly designed amalgam core f or the coronally debilitated. endodontically treated posterior tooth. A few simple procedures result in a core foundation f or root protection, an ideal cro wn preparation, and a m ore predictable final restoration. (Quintessence lnt 1995;26:317-324.)

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Introduction Numerous Imterials and techniques in dentistry are used for making a core for the endodontically treated posterior tooth. The materials include cast gold, cast palladium-silver, glass-ionomer cements, amalgam, and resin composite with or without pins and prefabri cated pOSt. I - IO It is incumbent on the clinical dentist to restore the tooth to function after the root canal therapy has been performed. No simple technique has been described for restoring the missing coronal tooth structure with amalgam as a core. Techniques presently used for labricating amalgam cores may impinge on the peri odont iu m and even lead to an improper final-prepara, tion design. The amalgam core should allow the final restoration to have proper contours, emergence pro file , contact area, and esthetics. The customized acrylic resin shell technique will ensure a mo re successful restoration with biologic form, function , and esthetics. End odontically treated teeth are susceptible to fracture during fu nctio n because of the decreased

" C linical Assistant Pro fessor o f Re storative Dc,ntistry. Fi xed Prost ho· donti cs, University of SOllthern California. School of Dentist ry. Los Angeles. Cali forn ia.

** A5Sociate

P ro fe~s o r of Restorative D enti stry. C ha irman of th e Int ernati onal Stu dent Program. University of So uthern Ca li fo rn ia. School of Dentistry. Los Angeles. CalifoJ:llja.

Reprint requests: Dr Gary D. Miller. Associate Professor of R estorati ve DeU listry, University of Southern C alifo rnja. Schoo l of D entistry. R oom 4372 . Unive rsi ty Pork-MC 064 1, Los Angeles. Ca lifo rnia 9008 9·0 64 1.

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Volume 26, Num ber 5/ 1995

amount of dentin, the disruption of anatomic form , and, possibly, the loss of intrinsic str ength . 11- 16 The amalgam core increases the resistance and retention form by restoring the lost coronal tooth structure necessary for placement of an artificial crown. TIle strength of an endodontically treated tooth is directly related to the amount of the remaining tooth struc ture i7 and, therefore, is not reinforced by the post. 1& Another way to reduce the potential for fracture is to have the margins "anchored " ill sound tooth structure apical to the core and not in the buildup material. The 2 to 3 mm of prepared natural tooth structure api cal to the core constitutes the ferrule effect. 19.20 Standard criteria for the reduction of axial walls and the type and position of the finish lines can be followed. There should be enough thickness for the base of these axial walls to prevent fracture. Incorrect circumferen tial design for the preparation of the remaining natural tooth structure may trigger a crack21 or even a fracture . Amalgam has been advoca ted as a suitable material for cores in coronally debilitated , endodontically treated posterior teeth .22 Amalgam has been proven clinically predictable through its long-term use in vivo. 23 The decision to use so me form of amalgam as a dowel core depend s on several clinical factors, such as the size and shape of the pulp chamber; the thickness of the remaining circumferential tooth structure; the length, width, and number of remaining supragingival axial walls; the root mo rphology; the availability of periodontal suppo rt; and the tooth's overall value in

317

Prosthodontics

Fig 1 Radiographic appearance of tooth 30 following endodontic therap y

the treatment plan. The purpose of this article is to describe a specific technique for fabricating an amal gam core for the coronally debil itated tooth from a customized acrylic resin shelL

Technique Following successful endodontic therapy (Fig 1), and prior to tooth preparation, irreversible hyd rocolloid impressions of the patient's dental arches are made, including the endodontically treated tooth and the opposing tooth. A ll defects on the crown to be restored are waxed to fulllb nn on the study cast (Figs 2 and 3). A new cast is made of the corrected tooth (Fig 4). A vacuum-formed mat rix is made on this cast from a 0.02-inch-thick sheet of polypropylene mouthguard materiaL The mat rix is trimmed so that it may be comfortably placed in the patient's mouth (Fig 5). A conventional complete-coverage restoration is prepared (Figs 6 to 8). It is recommended that aU fragile axial walls or walls with crack lines be reduced in height un til the axial waUs have a proper structural design. Molar teet h with pulp chambers that are too shallow (less than 3 mm in beight) and less than 2 mrn of remaining coronal tooth st ructure or fewer than two cusps or walls require a dowel 10 augment retention of an amalgam core. The margin of the preparation should be kept a minimum of I mm past the core-tooth junction. Premolar teeth with no cusps or minimal support from one cusp should receive cast dowel cores. 24 If a post space must be prepared, it should have walls that are as parallel as possible to di.stribute forces

318

along the long axis of the tooth. A warm endodontic plugger is user to remove a maximum of 3 mm of gutta-percha from the canals. Undercuts may be prepared with a small inverted cone bur, but conserva tion of tooth structure is a most important principle to follow. The chamber and remairling waJls should be inspected for cracks before the acrylic resin shell is fabricated. Large cotton pellets are placed in the pulp chamber and the vacuum-formed matrix is placed over the tooth. During this sequence, the preparation can be evaluated for proper reduction. Self-curing acrylic resin is placed in the matrix, and the acrylic resin shell is fabricated. The shell should fit well. Sometimes relining is necessary. The shell is contoured and polished (Fig 9). The adaptation should be good and stable with properly gauged bulk, contours , and profile. The acrylic resin shell should be out of occlusion by a minimum of 0.5 mm. The occlusal surface is removed to allow direct access to the pulp chamber (Figs 10 and 11). It is im portant to be generous with the occlusal access so that the axial walls that are fabricated in amalgam will be aligned adequately to contribute to good retention and resistance form, as found in conventional preparations. A lack of relief in the shell for amalgam alignment will move the axial wall inward and create a step. The shell's walls are I mm thick (Fig 12). The custom acrylic resin shell becomes a blueprint for the final restoration, assuring the clinician of attaining proper biologic contours tor the artificial crown. The rubber dam is placed and the acrylic resin shell is seated (Figs 13 and 14). The acrylic resin shell can be stabilized with a small amount of provisional cement or zinc phosphate cement mixed with petro leumjelly. This is best accomplished by adding a small amount of cement at the cervical margins with an explorer. The amalgam is inserted and condensed with moisture control. The aUoy is condensed from the occlusal direction until it is even with the top of the acrylic resin shell (Figs 15 and 16). Occlusally, the amalgam core can be carved to the desired occlusal pattern so that there is no contact in centric and lateral excursions. The rubber dam is removed (Fig 17). It is advisable to allow 24 hours for the amalgam co re to set before the final preparation and impres sions are made, to optimize its physical properties. The optimal material would be a high-copper-containing spherical alloy, because such alloys are stable, readily condensed into small secondary retentive features, and achieve high early and ultimate strength. Additionally,


Volume 26 , Number 5/1995

Fig 2a

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Quintessence

Prosthodontics

m endodontic I of 3 mm of cuts may be but conserva 1t principle to Ills should be resin shell is

pulp chamber ced over the ration can be :uring acrylic rlic resin shell I. Sometimes mtoured and be good and :mtours, and d be out of

Fig 2a

Buccal view of diagnos tic waxup

allow direct II). It is im access so that algam will be retention and preparations. iignment will a step. The

:s a blueprint clinician of the artificial : acrylic resin lic resin shell If provisional j with petro dding a small ~ins with an 1densed with .ed from the le top of the ;clusally, the ired occlusal ic and latera l Fig 17). :he amalgam and impres )perties. The [-containing .able, readily features, and \dditionally,

Fig 5

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Po lypropylene matrix replicating diagnostic waxup.

Volume 26, Number 5/1995

Fig 2b

Occlu sal view of di agnostic waxu p

Fig 3

(left) Occlusal and buccal views of debilitated tooth.

Fig 4

(below) Occlusobuccal view of stone cas t.

Prosthodontics Fi g 6

(le ft) Oc clusal view of tooth preparation

Fig 7

(be/ow) Buccal view tooth prepara tion.

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Buccal view of acrylic re sin shell.

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Occ lusal view of ac rylic resin shell.

Quintessence Intern ational

Volum e 26, Number 5/ 1995

Aer'

Fi g 14 (rig ce ment is rei

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Prosthodontics

lion.

Fig 10a

Occlusal surface of shell is remo ved generously.

Fig 10b

Intraoral view o f finalized shell.

Fig 11 (left) Occlusal access provides for axial wall alignment between she ll and tooth preparation

Fig 12 (below) Gauging of shell's walls will secllre proper contours for the final restoration .

oth prepara ti on.

Fig 13 place

Acrylic resin shell is se ated with rubber dam in

Fig 14 (right) Occlusal view of seated shell. Excess cement is removed wi th explorer

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321

Prosthodontics

Fig 15 Amalgam is condensed in small increments until even with the shell's top.

Fig 19

Suce

Fig 16 (right) The amalgam is carved and eventual centric or lateral contacts are removed.

Fig 17 (left) Occlusal view of the shell with rubber dam removed.

Fig 18 (be/ow) The shell's line angles are cut with a small fissure bur to facil itate its removal.

Fig 2 1 Radii bu ildup follow

optional usage of a bonded amalgam technique may increase retention and optimize seal and has some potential for reinforcement. 25 The acrylic resin shell is easily removed; the shell is cut at the four line angles with a smaH fissure bur and the resin parts are removed with an explorer (Fig 18).

322

The final preparation is made according to the type of restoration planned (gold veneer or metal-ceramic crown), impressions are made and the provisional restoration is cemented (Figs 19 to 24).

Fig 23 Occl biologic conto

Fig 24 (Tight resin shell in ~

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Volume 26, NumberS/1995

Quintessence"

Prosthodontics

Buccal view of finalized tooth preparation.

Fig 20

Occlusal view o f the fin al amalgam core .

Fig 21 Radiographic appearance of the amalgam core buildup following pu lp chamber prepara tion

Fig 22

Buccal view of fin al ceramometal restoration.

Fig 19

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ut with a small

) the type of etal-ceramic provisional

Fig 23 Occlusa l view of final restora ti on wi th proper biologic contours.

Fig 24 (right) Outlin e of the amalgam core with the acrylic resin shell in place.

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323

Prosthodontics Discussion Amalgam is a viable option for use as core material for the coronally debilitated, endodontically treated pos terior tooth. A build-up technique with amalgam and an acrylic resin shell has been presented . The method has several advantages:

1. There is no impingement on the periodontium, as may happen with o ther methods (eg copper band, Toffelmire band , orthodontic band) . Therefore, the risk of injury to and recession of the surrounding periodontal tissues is red uced . 2. It is a simple and inexpensive procedure. 3. A conservative restorative preparation approach can follow evaluating the quality and quantity of remaining tooth structure. 4. The acrylic resin shell confines the amalgam material. It also acts as a provisional restoration , thus preventing root fracture' and saving time. 5. Amalgam has good physical properties as core material.

3. Dewhirst RB. Fisher DW. Shillingburg liT. Dowel core fabrication. J South Ca lif Dent Assoc 1977 :37 :444-449. 4. Gutmaun JL. Preparation o f endodontically treated teeth to receive a post-core restoration . J Prosth et Dent 1977 :.18:413-419. 5. Doo ley BS. Preparation and conStriction of post-retention crowns for anterior teeth . Aust Dent J 1967: 12:554-550. 6. Hinsc hfc1d Z. Stern N . Post-core the biomechanical aspect. Aust Dent J 1972;17:467-46 8. 7. Roscn H . Operative procedure-s on multilated end odo ntically treated teeth . J Prosthet Dent 1961: 11 :973 -986.

M o ha mmed

S. Sh.illinghurg HT, Fisher OW, Dewhirst RB. Restoration of end odontically treated posterior teeth. J Pro stbet Dent 1970 :2 4:401 409.

9. Markley M. Pin reinforcement and retention of amalgam founda

tions and restoration s. J Am Dent Assoc 19 58;56:675.

10 . Lovdahl PE , Nicholls JJ. Pin retained ama lgam cores vs. cast-gold dowel corcs. J Prosthet Dent 1977:38 :5 07 -514. 11. Helfer AR. Melnick S. Schilder H . Determination of the mois ture

content of vital and pulpless teeth. J Oral Surg 1972: 3 4:661-670.

12. Fra nk AL. Protective. coronal coverage of the pulpless tooth. J Am Dent Assoc 195 9:59:89 5-900.

13. Baraban OJ . Thc restoration of pulp less teeth. Dent C lin North Am

1967 :Nov:63 3-653.

14 . Nayyar 1\, Waiton RE. Leonard LA. An amalgam coronal radicular dowel and core technique for endodontically trea ted postcri o.r teeth . J Prosthet Dent 1980;43:511-515 .

6. The final preparation and impression procedure is more predictable and less time consuming because the periodontium is healthy.

15. Micbelich R . Dillar T, Nayyar 1\. Mechanical properties ofamalgum build up for endodontically trcated molars [abstract 455]. .J Dent

Res 1980:59:321.

The customized acrylic resin shell allows the clinician control over contou es, contact areas, emer gence profiles, and esthetics. A well-sealed amalgam core is attainable, and the remaining root is protected. The endodontist tends to save the roots ofteeth, and the custom acrylic resin shell with the amalgam core allows the clinician to restore such teeth with greater predictability and !ongevity.

17. Hock D. Impact Resista nce of Post and Co res I thesis]. Ann 1\ rbor: University of Michigan . 1976 .

16 . Wcinc FS. Endodontic Therapy. St Lo uis : Mosby. 1972 :385-410.

IS. Nayyar A. Coronal radicula r build-up for endodontically treated teeth. ("Jin Dent J 19 83 ;4: 1-2 8. 19. Sorenson JA, Engelman MJ. Mit o WT. Effect of ferrule design 011 fracture resistance of pulpless teeth I abstract 1421 . J Dent R es 19 88; 67: 130. 20. Nayyar A. Amalgam coro nal radicular build-Up for molars and premolar. C lin Dent J 19 88: I :4 1. 2 1. I1 0ag EP, Dwycr TG . A comparative evaluation of three post and core techniques. J Prosthet Dent 1982:47 : 177 -181. 22. Prothe ro JH . Prosthetic 191 6:736 - 744.

References

A visual Part I. ~

Denti stry.

Chicago:

Medio-Dental ,

23. Rosen H . C racked toot h syndromc. J Prosthet Dent 1982;47: 36 -4 3.

J. Eissmann Hr, Radke RA . Post-endodontic restoration. In: C ohen S. Burns RC (eds). Pathways of the Pulp. St Louis . Mosby, 1976 :537 575 .

24. Abou-Rass M , Donovan TE . The resto ration of endodontically treated teeth. Calif Dent J 1993: Dec:61-67.

2. Barker Be W. Restoration on non-vital teeth with crowns. Aust Dent J 1963:8: 191-200.

25 . Donovan TE , Chee WL. Endodontically treated teeth : A summary of restomtivc concerns. Calif D en t J 1993;Dec:54. 0

Introduction

Whether to upragingiva many author ve neer croWl are consider margins of t i.nclude clinil tions, cervic proxi mal co margins of freque n Uy p ex tensi ve aI

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