Problem areas encountered by dental laboratories

Miscellaneous

Problem

areas encountered

Alexander A. Calomeni, lieutenant USAF Hospital, APO San Francisco,

by dental Colonel, Calif.

laboratories USAF (DC)*

M

any difficulties encountered by dental laboratories in the fabrication of prosthetic appliances are due to inadequate treatment planning, improper mouth preparation, and defective casts. The dentist is totally responsible for the diagnosis, treatment planning, survey and design, and the supervision needed to obtain accurate casts. He must provide an easy-to-follow work authorization to the laboratory in the form of diagrams and written instructions, and diagnostic casts that have been surveyed with a clear design drawn on them. The master casts must be made after the necessary preparations and alterations have been made in the mouth. The dentist must supply accurate jaw relation records and specify the materials to be used in the construction of the prosthesis. Both the dentist and the laboratory must recognize their respective responsibilities in providing prosthodontic service. It is imperative that the dentist assume the responsibility and initiative in designing and planning oral prostheses for his patient. The laboratory technicians are skilled craftsmen who are responsible for handling the materials and mechanical details. Dentists are negligent if the laboratory technicians are entrusted with the responsibility for the biologic and physiologic needs of patients. Each prosthodontic patient should have a complete oral examination, including complete oral roentgenograms and diagnostic casts. Photographs and other diagnostic aids should be used when possible. With this information and a tentative design on the diagnostic casts, corollary treatment should be instituted to include surgical, periodontal, restorative, and other corrective procedures as indicated. Diagnostic casts are indispensable prerequisites for designing of both fixed and removable prostheses. They will reveal essential information such as: ( 1) Irregularities in the occlusal plane; (2) loss of vertical dimension of occlusion; (3) inadequate mesiodistal space for tooth replacement; (4) tipped or rotated teeth; (5) inRead before the 38th Parallel Dental Society Meeting in Seoul, Korea. The views expressed herein are those of the author and do not necessarily of the United States Air Force or the Department of Defense. *OIC

Area Dental

reflect the views

Laboratory.

523

adequate cross-occlusal space for embrasure type of clasps; 16 11inadequate space for occlusal or incisal rests; and ! 7 i improper contours of abutment teeth, lacking rctention areas, which require recontour, or crowns or inlays. REMOVABLE

PARTIAL

DENTURES

There are certain recurring problems in orders sent to dental laboratories, The followiry list represents the essential information that must be supplied for preparing removable partial dentures. These items involve thr most frequently recurring discrepancies or difficulties when they are not properly carried out: 1 Thr work authorization should have complete instrurtions and be easy to follolv. Unusual terms, nomenclature. or \.ague abbreviations should not be used. To he valid. the work authorization rnllst be signed by the dentist and carry certain other illformation as may be required by laws in various states. 2. The design on the dia,gnostic. casts should clearly show (a) the major and minor connector design. ibi the outiirws of the clasps in proper relation to the sur\Try lines. ;c i thr outlines of the rest seats? id j the edentulous region design, and \ c I the type of replacement desired. Tripod marks should be placed on the cast to inform the laborator)- of the path of insertion and of the tilt of the cast to be used.

Bracing

Fig. 1. (A) Thr correct correct clasp relationship.

relationship of a circumferential clasp to the survey line. The bracing part of the clasp is an undercut area.

iR)

An in-

Fig. 2. (A) An inadequate glide path for the clasp at a. The clasp will “hang-up” on the occlusal surface of the tooth. If changing the tilt of the cast does not prevent this, then the tooth must be recontoured (dotted line) to lower the survey line. (B) .4n adequate glide path (ni is established for the clasp to enter the undCrcut area on the tooth.

Volume 19 Numbw

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Problems

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This enables the technician to duplicate the survey lines of the diagnostic cast on the master cast with his surveyor in his laboratory.* 3. Master casts should be-clean and free of debris, baseplate material, separating medium, wax, grease, and other contaminants. 4. There must be sufficient occlusal clearance for occlusal rests, clasps, and minor connectors. 5. The design of clasps must be made in relation to survey line and with consideration for esthetics (Fig. 1) . 6. Adequate glide paths must be established for clasps (Fig. 2). When the survey line is too high, the retentive end of the clasp will “hang-up” on the occlusal surface. 7. The survey lines must not be located too high t,oward the occlusal surface, preventing the clasp from being properly positioned close to the gingival third of the crown (Fig. 2, A). A clasp placed too near the occlusal surface increases harmful leverage and is annoying to the tongue. 8. Retentive areas must exist on teeth to be clasped. The retentive end of the clasp must be designed to conform to the undercut area on the tooth. Also, the bracing or rigid part of the clasp must be designed so that it is located entirely occlusal to the survey line (Fig. 1). The rigid part of the clasp cannot flex into an undercut area. 9. Rest seat preparations must be properly prepared without sharp fragile margins. Undercut rest seat preparations, or bubbles, or rest seat areas scraped to remove defects will prevent the appliance from going into place in the mouth (Fig. 3). 10. Lingual bars (major connectors) must not be positioned in deep undercut regions caused by tilted teeth or undercuts in other tissue (Fig. 4) . Il. Facings should not be requested when they are not indicated, e.g., after extreme resorption of the ridge, between tilted proximal teeth, with inadequate mesiodistal space, where there are problems of horizontal or vertical overlaps, or with a reduced occlusal vertical dimension (Fig. 5) .

*Some military central dental laboratories have dentists transfer the design of the appliance onto the master cast, and these dentists want the cast to be tripoded with the design drawn only on the diagnostic cast.

Fig. 3. An undercut

rest seat preparation

on a tooth as viewed

from the proximal

surface.

526

Calomeni

.I. Pros. Dent. May, 196X

Fig. 4. A lingual inclination 01 3 tooth can pr~~durr do excessivei) deep undercut. The lingual bar or major connector placed in this region would require an excessive hlockout and create a food trap and an annoyance to thr tongue.

Fig. 5. Inclined

teeth have reduced the spaw for replacrment by an artificial tooth. Thr proximal surfaces require recontouring for an esthetic replacement. The lower anterior terth require a reduction of their incisal edges to provide space for the rrplaccmrnt.

12. ‘I‘be type uE materials or restoration to be used rnnst be specified in the work authorization, e.g., the kind d acrylic resin or porcelain teeth? the kind 01 pontic, and the type of metal to be usctl in the metal skeleton. 13. The color or shade of the teeth must be included when facings, tube teeth. or finished appliance are requested. 14. ‘4ccurate interocclusal records must be furnished. “Sandwich” or “mush bite” records are inadequate. 15. The casts must be sharp in detail, particularly around gingival cre\-ices and interproximal surfaces of the teeth. If the detail of the casts is not sharp, it indicates that the patient was not given a prophylaxis prior to making the impressiorr. 16. The casts should include an accurate recording of the shape of the soft tissues in all edentulous regions, particularly the tissue reflection regions, the hamular notches, and the retromolar regions. 17. The tongue space of the lower cast should be free of excess stone. ‘I’his can be done by blocking out the tongue space before pouring the cast or by trimming it before the final set of the stone 18. The packaging must protect the casts against breakage.

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Problems

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CLASP TYPE OF PARTIAL DENTURES McCracken1 has summarized the reasons for the failure of the clasp type of partial dentures. He includes in his list: (1) Inadequate diagnosis and treatment planning, (2) failure to use the surveyor, (3) inadequate mouth preparation, (4) failure to provide the technician with a specific design and information necessary to follow this design, (5) failure of the technician to follow the design and instructions given by the dentist, (6) incorrect use of clasp design and improper use of cast clasps, (7) failure to provide adequate tissue support for distal extension, and (8) failure to provide occlusal forms of posterior teeth in harmony with the occlusal forms of the remaining natural teeth.

FIXED PARTIAL DENTURES AND CROWNS Another group of situations will cause problems in dental laboratories in the preparation of fixed partial dentures and crowns. These include: 1. Dies are inadequately prepared, or lack detail. Their margins are indistinct and irregular, and are not marked with crayon pencil to indicate the termination of metal. They may have bubbles or voids. The preparations may include undercuts, and undercut areas in the preparation may be too close to the margins so they cannot be blocked out without altering the margin, 2. An insufficient number of teeth may be included in the impressions to assure the proper occlusal relationship of the casts and teeth. The casts are difficult to occlude unless an occlusal index or interocclusal relation record is supplied. 3. The preparations of the teeth are inadequate because the teeth have not been cut down enough to provide for an adequate thickness of gold, gold and acrylic resin, gold and porcelain, or porcelain on their facial surfaces. A sufficient amount of tooth structure must be removed to provide for an adequate thickness of veneer material in order to obtain the proper tooth contour and tooth color. 4. The preparations are made without sufficient clearance between the lingual surface and the opposing teeth. 5. Two different dies are supplied to the laboratory, and the technician does not know which is the accurate one. Metallic copings or crowns are not interchangeable on different dies. The two dies received by the laboratory may be a single die and one in a full-arch cast. The metallic restoration will fit accurately only on the die on which it is constructed. 6. Removable dies are not accurately keyed into the cast, which allows the dies to move and prevents the completion of the fixed partial denture. 7. The work authorizations fail to specify the type of pontic to be used. 8. A porcelain shade guide is not used when the shade is selected for a ceramic veneer or pontic. The acrylic resin or other plastic shade guides are not the same as the shades on porcelain shade guides. Porcelain colors must be selected from porcelain guides. 9. Porcelain jacket crown dies often are not tapered properly for the adaptation of the platinum matrix. These dies should not be “ditched” under the shoulder margin because this complicates the removal of the platinum matrix from the die. The die should have a parallel surface of 3 to 4 mm. under the shoulder on the root part of the die before it tapers toward the end (Fig. 6).

528

CaZommi

.I. l”ros. Dent. May. 191%

Fig. 6. (A i h correctly shaped root form for a die. Dies for porcelain jacket crowns have parallel sides of 3 mm. to + mm. below the shoulder to facilitate the removal platinum matrix. (R) An incorrect taper of the root part of the die.

should of the

10. Dowel pins are often improperly placed. They are either misaligned (II tilted. Dowel pins should be placed parallel to the lonp axis of the preparation and waxed in place, using straight pins as their support. 11. Individual preparations may have undercuts, or two or more fixed partial denture abutment preparations are not parallel to each other. The laboratory technicians cannot correct these errors made by the dentist. COMPLETE

DENTURES

1. Incomplete and inadequate work authorizations cause problems for laboratory technicians in the fabrication of complete dentures. Special instructions for the staining or characterization of the teeth or denture base should be included in the prescription. During the “try-in of” the wax denture. the dentist should arrange, move, or recontour teeth for each individual patient. Rather than rely solely on written instructions, the dentist himself must determine and satisfy the esthetic and functional requirements. 2. The mold or shade of tooth to bc used must be determined by the dentist and notecl on the work authorization alon,q with the patient’s age. 3. The complete namr of thr shade guide from which the tooth was selected must be supplied. rather than abbreviations which may refer to mart‘ than ones shade guide. 4, The occlusion rims must br properly marked or contoured to indicate the midline. the cuspid-to-cuspid width of thp six anterior treth, the in&al length, the occlusal plane, and the labial contour. 5. Occlusion rims must be properly keyed or indexed in the wax to indicate the centric relation. 6. Maxillary casts must havcb the postprior palatal seal marked and carved b?. the dentist. SUMMARY ‘I’he failurr of nlany prosthetic appliances is due primarily to inadequate treatIncnt planning, improper mouth preparation, and a failure to provide the dental laboratory technician with adequate and necessary information.

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Problems

encountered

by dental

laboratories

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The most frequently recurring deficiencies and discrepancies in work authorizations sent to dental laboratories have been listed for the fields of removable partial dentures, fixed partial dentures and crowns, and complete dentures. CONCLUSIONS The dentist must assume total responsibility for laboratory procedures, and he must supervise and provide direction to the laboratory for the combined successful effort of both the dentist and dental laboratory technician. Reference 1. McCracken, W. L.: Partial Denture Construction, Principles Louis, 1964, The C. V. Mosby Company, pp. 33 and 34. USAF HOSPITAL, Box 7113 APO SAN FRANCISCO, CALIF. 96323

and Techniques,

ed. 2, St.