Fixed prosthodontics: The dentist and the dental laboratory technician

Fixed prosthodontics: The dentist dental laboratory technician Zvi 1. Abramowsky, Tel-Aviv University,

and

the

D.D.S.* Tel-Aviv, Israel

-Lh e proper utilization of auxiliary personnel is essential if dentists are to provide optimal dental services for an ever-increasing number of patients. Most dentists use the services of commercial dental laboratories, and frequently the laboratory product does not conform to the quality standard the dentist envisages. Several years of private and institutional practice have acquainted me with the reasons for the apparent gap between the dentist’s expectations and the dental technician’s products. Essentially the reasons are the following: (1) The dental technician does not see the actual condition of the patient’s mouth, and is not qualified to recognize and treat oral disease; (2) the emotional state of a patient may require deviation from routine procedures; (3) the impression or cast delivered to the laboratory may be defective; (4) the dentist’s prescription may be inadequatel-“; and (5) the dental technician may not recognize the biologic shifts and changes which occur during the interval between making the impression and delivering the prosthesis. The dental technician must work on an unyielding hard cast when in the mouth; everything moves, even a fixed prosthesis. DISCREPANCIES

SEEN BEFORE

PROSTHESIS

IS TRIED

IN MOUTH

A prosthesis should be carefully examined before trying in the mouth. Lack of time should never be an excuse for not inspecting it completely. The abutments must cover all ground surfaces of the prepared tooth. This can be seen on the dies (Fig. 1). The contour of the abutment must be compatible with periodontal health. All components of the prosthesis must have a shape that will enable food to flow easily and make the prosthesis “self cleansing” rather than impacting. The flow of food should stimulate the gingiva, and food should not be trapped in undercuts. The occlusal table must not be too wide or too narrow. The supporting cusps should go into the corresponding fossae of the opposing teeth without interference. The *Clinical Education.

Senior Lecturer

and Head, Fixed Prosthodontics,

Faculty of Continuing

Medical 537

538

.I. Prosthet. November.

Abranzowsky

Fig. 1. The casting faces of the abutment

of the prosthrsis teeth arc covered

adjusted on the dies by the castinp.

Fig. 2. Two posterior fixed prostheses. The aliqmrnt is correct. The alignment on the left is incorrrct

and

master

of the components

cast.

All

of the

prepared right

Dent. 1971

su-

prosthcsi>

inclined planes of the RuidinS cusps should guide the mandible in all functional alignment in normal dental arches must follow thus escursions. 7.1~ buccolingual “contact line.” The line connecting all contact points (areas) should be continuous with the line formed by the central groovrs of the posterior teeth (Fig. 2) . This position of pontics in a prosthesis is important; the horizontal forces generated b\ the muscles of the tongue. cheeks, and lips should be balanced and neutralized b!, each other at the dental arch Enc. ‘l’his \vill ensure adequate tongue space and tongue comfort. Contact areas with adjacent teeth in the arch should be full and properly shaped (Fig. 3) . No form conducive to food impaction should be allowed. Forces should be transmitted through contact areas in a favorable direction. IJnfavorabl! directed forces transmitted through contact areas I\-ill tend to extrude and.ior tilt natural teeth. The gingival papillae immediately adjoining the edcntulous ridge arca, as ~~11 as other papillae areas, should have a crudbe, so they will not be compressed (Fig. 4). Over-all, full-arch impression techniques for all prepared abutments require cutting the individual dies from the master cast, with unavoidable destruction of the interdental areas on the cast. Pontics as they come in contact with the master cast must be checked for accuracy. The form of the contacts with the tissue as prescribed by the dentist must be established. In spite of strict instructions not t.o scrape the cast? we find that many dental laboratory technicians do it on the assumption that the “fit” is better with the soft and yielding tissues (Fig. 5) .” The abutment casting must be checked for occlusal thickness with an appropriate instrument (Fig. 6) . The prosthesis should be polished to a high luster which is essential for cleanliness. There should be no Yeftovers” of veneering material or polishing pastes

Volume Number

26 5

Fixed

Fig. 3. (A)

All contact areas are acceptable molar on the right. (B) The contact area between the molars, it is not acceptable. The by a defective contact area.

except

the

one between

between the bicuspid bone

destruction

firosthodontics

the

second

and molar is

between

the molars

539

and

third but is caused

correct,

in the inside of the abutments. The inside of the abutment should be inspected for bubbles which must be removed. The color of the restoration should conform to the prescribed shade guide color and the drawing supplied to the dental technician. DISCREPANCIES

DISCOVERED

DURING

“TRY

IN”

OF PROSTHESIS

The prosthesis should go into place with a slight amount of friction. Moderate finger pressure or occlusal muscular force should be sufficient to seat the prosthesis firmly and permanently. Hammering the prosthesis into place is dangerous to both the prosthesis and the abutment teeth and should not be used. Blanching or ischemia of soft tissues on the edentulous ridge or of the gingival margins around abutments should not occur. Ischemia may cause atrophy or necrosis of overly displaced tissues. The occlusion must be checked in the maximal intercuspation contact position Deflective or interceptive contacts and in the protrusive and lateral excursions. must be eliminated on the working and nonworking segments of the arch in posterior restorations. The accuracy of the interocclusal record, the exactness of the opposing cast, and the reliability of the articulating instrument are important factors. Cheek biting can be detected at this stage before cementation. Cheek biting,

540

Abramowsky

Fig.

4. The

Fig.

5. Displacement

space

Fig.

6. The

thickness

(arrow)

is provided

of mucosal

for the gingival

tissues

of cast abutments

caused must

by

papilla. IOO much

he c,hecked

before

pressure

by

pontir-<.

cementation.

especially with new prostheses may be due to an inadequate horizontal overlap oi the posterior teeth (or buccal placement of mandibular teeth) so that an edgc-toedge occlusion results. Speech difficulties in phonation may be caused by bulky and incorrect contours of the prosthesis. Openings between pontics and close to the mucosa of the edentulous ridge, especially in upper incisor pontics, may cause lisping and whistle sounds. Some dentists believe that speech impediments will disappear with time and us( of the prosthesis. Others (including the author) have shown that this is not true for the majority of elderly persons. At best, the patient may learn to live with them. The correction of discrepancies which interfere with speech make for happier patients-and better functioning prostheses. Contact areas between the restoration and the natural teeth must be carefull) checked at this stage because of possible inaccuracies in the master cast. Contacts

Volume Number

26 5

Fixed prosthodontics

541

should not be too tight or too loose. Careful attention should be given to the interproximal gingival papilla, the shape and direction of the contact area of the restoration, and occlusal marginal ridges. To establish these areas accurately and suitably for the individual patient, the morphology and contents of the interproximal region should be carefully assessed in the dentinon, if present. The esthetic aspects, such as the shade and shape of the individual teeth and their positions, should be given the necessary consideration. The patient should be consulted for acceptance of the cosmetic result. DISCREPANCIES DISCOVERED DURING USE OF PROSTHESIS

AFTER

FINAL

CEMENTATION

AND

These defects may be grouped as follows: ( 1) breaking off of the veneering material; (2) changes in color of the veneering material and/or skeletal material; (3) breakage of the metal frame; (4) perforation of occlusal surfaces of abutments; (5) loosening of an abutment; (6) oxidation of the alloy; and (7) soft-tissue damage due to tissue displacement. DISCUSSION The dentist will not fulfill his responsibility toward his patients if his treatment does not ensure adequate planning for proper functioning and longevity of the entire stomatognathic system. The dentist must first establish objective criteria by which both he and the laboratory technician can determine the quality of the fabrication. Therefore, he must explain directly, not via vague notations, to the dental technician why he elects to follow certain clinical procedures and to use specific materials. The prescription must be complete and detailed1 and should accompany (1) impressions or casts which are acceptable to both the dentist and technician, (2) accurate interocclusal records, and (3) a plan of treatment written out in sequence and recorded on diagnostic casts. The discrepancies enumerated earlier may all be avoided if the dentist develops an excellent working rapport with his technician and realizes that to correct many existing deficiencies technologic cooperation requires considerable thought and education.2 The dentist should not impose unreasonable time deadlines on his technician and should ensure his adequate remuneration. When the prosthetic problem can be best solved by the technician seeing the patient, it is much better to consult the dental technician in the operatory and to show him the condition in the mouth and to explain the difficulties of the particular prosthesis than to send the patient to the commercial laboratory. SUMMARY

AND

CONCLUSIONS

Difficulties arise because the dental laboratory and the technician who fabricates the prosthesis are not under the dentist’s direct supervision. The dental technician works on an unyielding cast, a condition which is totally different than that existing in the patient’s mouth. Impressions and interocclusal records must not only be

542

J.

Abramowsky

Prosthet. November,

Dent. 1971

perfect, but they must be acceptable to the dental laboratory technician. ‘T’his is the only information that the dental laboratory technician has in order to produce a prosthesis that must be compatible with the esthetic and functional requisites of the paient undergoing fixed prosthodontic treatment. I’o avoid remakes and to minimize adjustments, the dentist must give the tcchnician full and detailed instructions along with his work authorization. The dental technician has to know and feel that he is part of a health team. Good personal relations with the dental technician will yield better laboratory and good patient-dentist r&lproducts. ‘I’his will ensure better service to patients, tions will be established. References 1. Gehl, D. H.: Investment in the Future, J. PROSTHET. DEST. 18: 190-201, 1967. 2. Nuttall, E. B.: Coordination of Fractionalized Restorative Service, J. PROSTHET. DENT. 289-293,

3. Cavazos, 143-153,

1969. E.: Tissue 1968.

22. HUHERMAS STK. TEL-AVIV, ISRAEL

Response to Fixed Partial

Denture

22:

Pontics, J. PROSTHET. DEST. 20: