Clinical management of abutments with intracoronal attachments

Clinical management attachments

of abutments

with intracoronal

Ira D. Zinner, DDS, MSD,* Richard D. Miller, DDS,b and Francis V. Panno, DDSC New York University, Collegeof Dentistry, New York, N.Y. A method of preparing teeth to accommodate intracoronal attachments is presented. The resulting prosthesis incorporates a fixed partial denture with physiologically contoured crowns and a removable partial denture with a precisely determined path of insertion. These techniques stress the importance of surveyed diagnostic casts and coordinated tooth preparations for artificial crowns to facilitate treating complex cases. (J PROSTHET DENT 1992;67:761-7.)

T

he construction of intracoronally retained removable partial dentures (RPD) requires a detailed description of the procedures involved in the fabrication of abutment restorations. A favorable prognosis for these RPDs depends upon the precision preparation of abutment teeth in a periodontally healthy environment, a favorable crownto-root ratio, and strict adherence to the basic principles for the fabrication of RPDs. The diagnostic procedures are performed before preparation of the abutment teeth that are to receive complete crowns with intracoronal retainer systems. DIAGNOSTIC

CONSIDERATIONS

A comprehensive examination is required that includes a complete series of right-angled radiographs, caries detection, periodontal charting, an evaluation of the edentulous ridge, and diagnostic casts mounted on a semiadjustable articulator using verified maxillo-mandibular records. The mounted diagnostic casts are for (1) examination of the interrelationship of the edentulous dental arches and the remaining teeth, (2) selection of retainer systems for the proposed abutment teeth, (3) preliminary RPD design, and (4) to plan the establishment of harmonious interarch and intra-arch occlusal relationships. DIAGNOSTIC

SURVEYING

The casts are placed on a dental surveyor and the path of insertion of the RPD is determined to resist displacement of the RPD (Fig. 1). Surveying the replica casts is necessary to determine the modifications in preparation of the abutment teeth for optimal positioning of the intracoronal attachment. This preliminary survey designates the alterations to abutment preparations for controlling overcontoured cast restorations that contribute to and enhance “Clinical Professor,Department of Prosthodonticsand Occlusion. bClinical AssociateProfessor,Department of Prosthodonticsand Occlusion. CProfessorand Chairman, Department of Prosthodonticsand Occlusion. 10/l/36542 THE

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periodontal problems, while improving the technical phases of fabrication of the FPD and RPD.’ The path of insertion directly influences the contours of the artificial crowns and the position and length of the intracoronal attachments, and it enables peripheral extension of the denture base to the full anatomic tolerances. Optimal denture-base coverage will ensure stability and stress distribution of the RPD. Carious lesions, periodontal disease, and endodontics are completed before preparation of the abutment teeth and fabrication of the intracoronally retained prosthesis. Consideration should be directed to the need for splinting the abutment teeth for improved support and control periodontal deterioration associated with single abutment teeth.2, 3 Three categories of abutment teeth for attachment-retained RPDs are (1) normally aligned, (2) tilted, and (3) rotated. NORMALLY TEETH Guiding

ALIGNED

ABUTMENT

planes

After the preliminary survey on the replica cast, guiding planes are prepared before actual tooth preparation for artificial crowns (Fig. 2). This precaution is critical with bell-shaped teeth, to avoid excess shortening of the retainer, and it also serves to ensure adequate room for the attachments in the cast restoration with suitable contours.4 Guiding planes are commonly prepared on the proximal and lingual surfaces of the abutment teeth adjacent to the edentulous spaces and for planned minor connectors on the mesiolingual surfaces where indicated. Two opposing guiding planes are not prepared on the same tooth, because this will contribute to unfavorable leverage.5 HOUSING

FOR RETAINERS

A rectangular cavity “box” of adequate depth is prepared in the proximal surface of the abutment teeth to accommodate the waxing mandrel and/or the retainer seat of the selected precision or semiprecision attachment (Fig. 3). This retainer seat will permit placement of the attachment within the normal contour of the abutment.6 761

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Fig. 3. Recessed boxes prepared on proximal surface of the abutment tooth. The depth of this box houses a waxing mandrel or is 0.5 mm larger than the retainer seat of the intracoronal rest system. Left, Preparation with deep recess; right, with attachment in deep recess.

Fig. 1. Diagnostic cast. on surveyor to determine path of insertion for RPD and tooth preparation modifications for placement of its intracoronal retainers.

Fig. 2. Guiding planes are created before preparation of abutment teeth.

The box is usually prepared with a flat-ended dental diamond stone or with a carbide crosscut fissure bur. The carbide bur removes the enamel faster than the diamond stone does. The boxes for locking intracoronal attachments are centered over the crest of the edentulous ridge. However, there is a difference when a nonlocking, semiprecision Thompson dowel rest is selected. These attachment systems are not placed in relation to the crests of the ridges, but are made parallel to each other regardless of the location of the edentulous ridge (Fig. 4). This ensures rotation around a horizontal axis and avoids torquing the abutment teeth.7 Without perceptive abutment preparation, the retainer may be overcontoured because of insufficient space for placement of the intracoronal attachment.8 An autopolymerized acrylic resin guide template is used

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Fig. 4. Modifications of tooth preparation with a Thompson dowel rest. The rest seats are parallel to each other despite location of ridge crest, to create both horizontal and vertical parallelism. This requires that recessed boxes are placed to this desired parallelism (two parallel lines in the diagram).

to aid the preparation of the proximal boxes and the guide planes.‘js8*g The template is constructed on the replica cast after a preliminary survey and the development of guiding planes and boxes on the stone abutments (Fig. 5, A through G). This template is then used intraorally during preparation of the abutment teeth to ensure adequate tooth reduction for the intracoronal retainers.

FINAL

PREPARATION

The abutment is then prepared to include a beveled circumferential shoulder finish line (Fig. 6). A shoulder

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5. A, Mandibular diagnostic cast. B, Guiding planes and recessed boxes prepared on abutment teeth. The cast is sustained at the desired tilt on the surveyor table and fissure bur placed in milling machine for cutting guiding planes or boxes. C, Diagnostic cast with tinfoil on remaining teeth.

Fig.

instead of a chamfer provides additional space at the gingival seat of the preparation, to house the intracoronal retainer. Reduction is also provided on the lingual surface of the abutment for a milled, lingual bracing surface for the precision attachments. In addition, the shoulder provides a bulk of cervical metal to strengthen the casting, avoid distortion during the firing of porcelain, and diminish the chances of porcelain fracture during placement of a ceramometal restoration.10 The mandibular centric holding (buccal) cusp or the maxillary buccal cusp are reduced initially to create adequate room for esthetics in the occlusal third and facilitate optimal cusp placement by the technician.l’ The occlusal surface is adequately reduced and the circumferential shoulder is then refined around the entire tooth. The shoulder is then deepened from the midpoint on

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the buccal surface to the midpoint on the lingual surface at the proximal surface near the edentulous ridge to allow the technician to adjust the position of the attachment for optimal placement.12 This shoulder should be approximately 1.5 mm deep. Undesirable placement of an intracoronal retainer is commonly the result of insufficient preparation of this proximal half of the abutment tooth. Finally, a bevel is placed with the use of a flame-shaped plug finishing bur with a flat tip. The bevel facilitates marginal adaptation of the artificial crown and expands the gingival sulcus to accommodate a greater bulk of elastometric impression materials at the margin.13 RETAINER

HEIGHT

The semiprecision attachment should have a minimal height of 4 mm to satisfy functional requirements. With

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Fig. 5. D, Completed acrylic resin guide template on diagnostic cast covers incisal and/or occlusal third of the remaining teeth. E, Guiding planes and recessed boxes of acrylic resin are refined, after fabrication of the template. F, Verification of depth for recessed box of retainer seat with attachment mandrel on surveyor. G, Completed acrylic resin guide template on diagnostic cast. The three lines are used as reference to aid placement of recessed boxes a:nd guiding planes for a Thompson dowel rest retainer. The two ridge crest lines are perpendicular to the horizontal line to allow rotation.

precision attachments, the height of the attachment should be at least 5 mm for the desired retention, bracing, and support. The gingival seat of the retainer for either precision or semiprecision attachment should be 2 mm above the gingival crest on the proximal surface to minimize gingival inflammation. The height of the cast restoration housing a precision instrument should be 7 mm. If this height is unavailable, a different attachment system should be chosen. The minimum height for a semiprecision retainer is 4 mm; hence the height of the artificial crown should be 6 mm. When sufficient crown height is impossible, an intracoronal retainer system is contraindicated unless surgical crown lengthening is performed. Problems with retainer height

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will be evident with mounted diagnostic casts before preparation of the abutment. SUPPLEMENTARY

RETENTION

When retention appears inadequate after basic preparation because of caries, former restorations, or preparation height, grooves or boxes can be designed to augment the resistance and retention form (Fig. 7). Grooves are commonly placed at right angles to the anticipated displacement of a complete crown, so that if a mandibular molar restoration can be displaced in a mesiodistal direction, buccal and lingual grooves are beneficial. Because a maxillary molar restoration can be displaced in a buccal direc-

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buccrl

far proximal

near proximal + cl lingual

lingual

buccal

Fig. 6. Beveled shoulder preparation. Shoulder is deeper on proximal half of preparation to allow intracoronal retainer including additional extracoronal elements for bracing or retention.

tion, proximal grooving is indicated. In addition, grooves interrupt the circumference of the tooth to enhance resistance form. To augment resistance and retention on a short tooth, the grooves are prepared to increase the surface area covered by the restoration. To ensure the function of the grooves, the full diameter of the bur or diamond stone is used during the preparation to create opposing walls. Auxiliary boxes are also useful to increase the surface area on a short molar. Auxiliary boxes and grooves are common on a tooth with extensive carious lesions or restorations where there are no vertical walls on the proximal surface to house the intracoronal retainer seat. Additional resistance to rotation can be effected on a short molar abutment by creating a channel along the central and buccolingual grooves of the occlusal surface. Pin or amalgam reconstruction of these compromised teeth will not improve the retention and resistance form without additional boxes or grooves. TILTED

ABUTMENT

TEETH

The tilted abutment tooth may be managed by (1) endodontic treatment, followed by a cast post and core with an

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upright preparation incorporating adequate space for the intracoronal retainer, (2) preparation of the tooth along its long axis, followed by construction of a complete crown designed to receive a conventional clasp retainer, (3) orthodontic therapy to reposition the tilted abutment tooth, and (4) telescopic restorations. A lingually or buccally inclined abutment tooth is an esthetic problem, and the placement of intracoronal retainer is arduous unless it is aligned orthodontically before tooth preparation. After orthodontic therapy, the realigned teeth should be stabilized for several months to avoid relapse from undesirable movement. ROTATED

ABUTMENT

TEETH

Rotated abutment teeth cannot be routinely used with intracoronal retainer systems without overcontouring the abutment restoration. To avoid this predicament, a channel shoulder-pin type of retainer system may be used that provides the functions of bracing, support and retention without the liability of hypercontour. A channel-shoulderpin restoration is a retainer system with a wrought wire pin

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DuCCsl

a) compromised

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b”CCSI

b) non-compromised

Fig. 7. Grooves and/or boxes are prepared to augment retention and resistance. Groove is made by positioning bur to its full depth to create parallel walls. Box before preparation is maintained on mesial proximal surface. Additional box may be prepared on distal proximal surface if abutment has been compromised by caries or prior restorations on mesial proximal surface.

Fig. 8. A, Gingival view of complete crowns and RPD. Second premolar is rotated abutment, but RPD is retained with a channel-shoulder pm retainer. B, Matrix on splinted abutment teeth. Gutter 2mm above gingival margin is 1% mm deep and mimics gingival contour of abutments. Cast gold channels are bracing. The two retentive channels use wrough.t wire pin soldered to the patrix, and space is provided by an occlusal bevel. Retentive channels are prepared through gutter at its base so that food and debris retained in those areas can be dislodged during insertion of patrix. C, Occlusal view with patrix of RPD inserted in matrix of splinted abutment restorations. 766

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for retention, cast gold pins for bracing, and a lingual shoulder for support (Fig. 8, A through C).g This retainer can also be used with mesially tipped molars. The lingual shoulder is 2 mm above the gingival margin and is approximately 1.25 mm in depth, following the contour of the abutment tooth. The rotated tooth is not altered into an excessively contoured “normally” aligned tooth. This attachment system can be used with a metal ceramic crown incorporating a facial porcelain veneer for esthetics while concomitantly facilitating development of a harmonious occlusion. The channel-shoulder-pin retainer does not extend beyond the contours of the abutment tooth, and this is an advantage over extracoronal attachments.

3. Kratochvil FJ, Thompson WD, Caputo AA. Photoelastic analysis of stress patterns on teeth and bone with attachment retainers for removable partial dentures. J PROSTHET DENT 1981;46:21-8. 4. Dykema RW, Goodacre GJ, Philips RW. Johnston’s modem practice in fixed prosthodontics. Philadelphia: WB Saunders, 1986:403,415. GP, Castleberry DJ. McCracken’s removable 5. Henderson D, McGivney partial prosthodontics. St Louis: CV Mosby, 1985, 163. 6. Baker JL, Goodkind RJ. Theory and practice of precision attachment removable partial dentures. St Louis: CV Mosby, 1981, 90, 95. I. Zinner ID, Semiprecision rest systems for distal extension removable partial dentures. J PROSTHET DENT 1979;42:4-11. 8. Lorey RE. Abutment consideration. Dent Clin North Am 1980;24:63-79. 9. Preiskel HW. Precision attachments in prosthodontics: the application of intracoronal and extracoronal attachments. Quintessence, 1984, 1:174-7;199-200. 10. Shillingburg HT,

distortion

Hobo S, Fisher in porcelain-fused-to-metal

DW. Preparation restorations.

design and margin J PROSTHET DENT

1973;29:276-84. 11. Panno FV. Preparation

SUMMARY An approach was described for clinical management of abutment teeth using intracoronal attachments for RPDs. Specific problems were identified and resolutions suggested.

and management of full coverage restorations for combination fixed removable prostheses. Dent Clin North Am 1987; 31:505-28. 12. Blatterfein L. The use of semiprecision rest in removable partial dentures. J PROSTHET DENT 1969;22:307-32. 13. Panno FV. Crown preparation for semiprecision attachment removable partial dentures. Dent Clin North Am 1985;29:117-9. Reprint

REFERENCES 1. Preston JD. Preventing ceramic failures when integrating fixed and removable prosthesis. Dent Clin North Am 19’79;23:37-9. 2. Johnston JF. Preparation of mouths for fixed and removable partial dentures, J PROSTHET DENT 1961;11:456-62.

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