Assessing the Quality of ClinicalProcedures and Technical

Standards of Dental Laboratoriesin Fixed Partial Denture Therapy

Zakereyya S. M. Albashaireh, BDS, MSc, PblTAbdulrbman S. Ainegrisb, BDS,

Purpose: This study was conducted to assess the quality of impressions and toothpreparations serif to dental laboratories in iordan and tu determine the technical capabilitiesof these laboratoriei fo construct fixed partial dentures. Materials and Methods: A sample ofI3& impressions and slone casts were examined for clinical errors in 35 laboratories thatconstruct fixed partial dentures. They were sorted into unusable, unsatisfactory, acceptable,or satisfactory categories. The type of impression material and tray, opposing archimpressions, and occlusal records were noted, instructions to technicians were assessed forcompleteness and clarity. Information regarding laboratory slaff and equipment werecollected. Results: Half of fhe specimens inspected were categorized as unusable orunsatisfactory; these were found in commercial laboratories. They showed at least oneclinical error such as drags or indefinite finishing lines in impressions and inadequatereduction, undercuts, ot obvious taper on stone casts, Alginate impression material wasused for 65% of the cases. Only 27% of specimens were accompanied with instructions; ofthese 22% were graded poor. No occlusal records were available with 54% of thespecimens and no articuiafors were used except in dental school laboratories. The dentalschools and some commercial laboratories had the best staff and equipment and were morecapable of fabricating fixed partial dentures than Ihose of the Ministry of hHealth and theRoyal Medical Services. Conclusion: The quality of abutment preparation and impressionswere unsatisfactory or unusable in 50% of cases. Of the 37 available instructions 8 were notclear. The dental schools and some commercial laboratories were technically capable ofproducing good quality fixed partial dentures. Int J Prosfhodont 1999;! 2:236-241.

As more patients demand fixed partial dentures forthe replacement of missing teeth and endure a highcost, the quality of fixed partial denture therapy be-comes of increasing professional and public concern,^Likewise, the quality of fixed prosthodontics provided

'Assistant Professor and Consultant in Conservative Dsntistry,faculty af Dentistry, Jordan University of Science and Technology,irbid, ordan.''Specialist in Conservative Dentistry, Royal Medical Seri/ices; andClinical Supervisor in Conservative Dentistry, Faculty of Dentistry,Iordan Uniyersity of Science and Technology, Irbid, Jordan.

Reprint requests: Or Z. S. M. Albashaireh, Riyadh Dental Center,Fast Office Box ! 584, Riyadh ! 1441, Kingdom of Saudi Arabia.

to patients in Jordan is a cause of worry for professionalsin this field. Moreover, the technical standards of fixedprostheses constructed in the Ministry of Health(MOH), the Royal Medical Services (RMS), commer-cial laboratories, and, to a lesser extent, in dentalschool laboratories are thought to be disappointing.

A fixed partial denture of good quality should bewell designed and constructed. It should restore thefunction and promote the health of the masticatoryunit and provide a long service life. These criteria areinfluenced by the quality of clinical procedures, thestandards of the laboratory work, and the oral con-ditions prevailing in the patient.' Inadequate reduc-tion or imprecise preparation of abutment teeth.

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AI bahai reh/AI negri sh Quality of Laboratory- Procedures for Fixed Partial Dentures

particularly the finishing lines, may have a profoundeffect on the subsequent fit of the restoration.-However, abutments that have been prepared and fin-ished carefully require an equally careful and correctimpression technique using reliable materials andsuitable trays. Without this tbe impression stage cannullify earlier achievements in the preparation.'

Provided that the initial tooth preparation and im-pression techniques are adequate, the esthetics andstrength of a fixed partial denture are determined bythe skill of the individual technician.-" A proficienttechnician may be able to correct or mask minorfaults in a preparation and produce a reasonablerestoration. Technicians should be provided with acomplete and clear prescription of the design and de-tails ofeach restoration or component. Moreover, oc-ciusal records should be supplied when applicable.'

Despite the importance of this broad range of fac-tors, the inferior quality of dental prostheses has beenattributed largely to errors incurred in laboratories.^'"The technical quality of fixed prosthodontic workmight be compromised in laboratories where oldtypes of equipment and inexperienced staff are in-volved In the construction of fixed partial dentures.'Metal-ceramic fixed partial dentures have been stud-ied and various technical errors were reported.'Proximal contacts were overbuilt at 65% of the sitesstudied, pontic tissue contacts were excessive in themajority of cases, and poor-qualityalloys were used.

Comparatively, a few studies have been carried outto determine the quality of clinical procedures un-dertaken by dentists. Impressions for anterior crowns'and fixed partial dentures^' made in general dentalpraaice have been assessed. Over half of the crownimpressions exhibited major faults and were recordedto be unacceptable.^ Most (72%) fixed partial dentureimpressions were taken with flexible plastic traysand 36% showed defects in the recording of the pre-pared teeth."

Few investigations have been designed to examinethe quality of clinical procedures and to determine thecapability of dental laboratories that deal with fixedpartial denture technology. Therefore, a sample ofdental laboratories was surveyed to: (/) determine thequality of abutment preparation, impression tech-niques, and accompanying design prescriptions ofimpressions and/or stone casts; and (2) ascertain thetechnical capabilities and standards of these labora-tories to competently construct fixed partial dentures.

Materials and Methods

A sample of 60 (75%) dental laboratories, whichrepresented the major cities In Jordan, was randomlyselected from the Register of Dental Laboratories.

They were located as follows: 5 in dental schools, 11in the MOH, 5 in the RMS, and 39 commercial lab-oratories. A questionnaire was formed following asmall pilot study prior to this survey. It includedquestions related to the qualifications, number, andexperience of the laboratory staff, and to the equip-ment and techniques used during laboratory proce-dures. Space was provided for comments regardingfees, costs of fixed partial dentures, and training oftechnicians. The laboratories selected were visitedwithout prior appointment and the chief technicianswere inten,'iewed and asked to complete the ques-tionnaire during the meeting. They were given a listof laboratory equipment and asked to indicate if thisequipment was present and used in their laboratories.As technicians were not required to be registered, aperson was considered a technician for the purposesof this study if they were so recognized by the labo-ratory owner.

In each commercial and dental school laboratory4 specimens of impressions and stone casts were ex-amined. According to availability, 2 impressions perlaboratory were inspected in the MOH and RMS.Specimens were inspected for defects using magni-fication loupes (2x) and were categorized by thesame examiner according to the following criteria.

1. Unusable impressions: impressions that displayedobvious drags, were detached from the impressiontray, and showed indefinite and interrupted fin-ishing lines around the circumference of thepreparation.

2. Unsatisfactory casts: the prepared abutmentscaused occiusal interferences on pink wax placedbetween the maxillary and mandibular casts in in-tercuspai position, displayed visual undercuts, ordemonstrated an i ncreased degree of taper,

3. Acceptable: minor modifications of inspected im-pressions or stone casts were required, such ascleaning blood stains from impressions, blockingminor undercuts, and filling small air bubbleswith stone cast material.

4. Satisfactory: the impressions or stone casts werefree of any of the above errors.

Factors like the type of impression material and l:ray,available occiusal records, and opposing arch im-pressions of the cases selected were noted. The ex-aminer and tbe chief technician assessed details of thepractitioner's instruction to technicians and consid-ered them satisfactory if sufficient, clear instructionswere given and poor if a telephone call to the dentalpractice was required for further information (eg, theshade details) before the case could be started. Theresults were analyzed using the Chi-squared test.

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Quality of Laboratory Procedures for Fixed Partial Dentures Albasliaireh/Alnegrish

Table 1 Number (n) and Percentage ot SurveyedLaboratories that Construot Fixed Partial Dentures

Location

SchoolCommercialMOHRMSTotal

Total

539115

60

n

429

11

35

%

8074

92053

Table 2 Number (n) and Peroentage of Cases Inspected According to the FactorsStudied and Laboratory Location

Case categoryUnusableUnsatisfactoryAcceptableSatisfactoryTotal

Impression materialAlginateElastomericTotal

Impression trayRigid plasticMetalTotal

InstruGlionsAvailableMot availableTotalPoor ot available

Opposing impressionsAvailableNot availableTotal

Occlusal recordsAvailableNot availableTotal

'Institutions include school

Institutions'

n

006

U20

2IB20

164

20

173

201

164

20

164

20MOH. ar

00

3070

100

1090

100

8020

100

8515

1006

8020

100

8020

100

id RMS ia bo rato ries

Commercial

n

32361632

116

8729

116

8828

116

2096

1167

7442

116

4769

116

%

28311428

100

7525

TOO

7624

100

1783

10035

6436

100

4159

100

n

32362246

136

8947

136

10432

136

3799

1368

9046

136

6373

136

Total

%

24261634

100

6535

100

7624

100

2773

10022

6634

100

4654

100

Results

Of the 60 laboratories surveyed 35 (53%] were foundto be involved in fixed prosthodontics. The majority ofcommercial and dental school laboratories also con-structed fixed partial dentures. In contrast, one MOHlaboratory and one RMS laboratory did so Table 1 ).

Impressions and Instructions

The distribution of cases inspected according to the fac-tors studied and laboratory location is given in Table2. The data on MOH, RMS, and dental school labo-ratories were pooled and shown under "institutions."

Oniy 4 impressions were available for inspection inthe 2 MOH and RMS laboratories (2 impressions each).

A total of 132 impressions and stone casts were ex-amined in the 33 dental school and commercial lab-oratories (4 specimens each). Consequently, 136 spec-imens were examined in the 35 laboratories involvedin fixed prosthodontics. Half of these cases showedclinical errors and were categorized as unusable andunsatisfactory; the other half of the cases were distrib-uted under the acceptable and satisfactory categories.Details of the clinical errors are shown in Table 3.

All of the unusable impressions and unsatisfactorystone casts were found in commercial laboratories. Incontrast, the cases inspected in institutionalized den-tal schools, MOH, and RMS laboratories were of ac-ceptable or satisfactory quality. No statistical differ-ences were found among laboratories in relation tocase category (P > 0.05). Alginate was used for most

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AI basha i reVAl negrish Quality of Laboratory Procedures for Fixed Partial Demure

Table 3 Number (n) and Percentage of ClinicalEiTors in Impressions and Stone CastsClinical errors nImpressions

DragDetached from trayIndefinite finishing line

Stone castsOver reductionUndercutIncreased degree of taperIndefinite finishing lineTotal

10175

12987

68

15257

18131210

100

Table 4 Percentages and Locations of Laboratories Using Listed EquipmentType of equipment Schools (n = 5 Ccmmereial (n =39) f^OH(n = i i ) RMS(n =Metal work

SurveyorVacuum mixerGas fumaceElectric fumaceSand blaster

Casting machinesCentrifugal electric mufflePressure vacuum casting

Porcelain furnaceVacuum firedVacuum automatic

Other equipmentDie-locating deviceUltrasonic cleanerBench magnifying glassElectric welderDust extractor unitCompressed air to eacti unitArticLlatorsFume cupboard

6010020

100100

100100

080

601008040

10010060

100

4962675185

5139

3144

368726109295ie90

g9

1002790

0g

90

036

0272744

082

020

1002040

00

200

040

06040

1000

100

cases, while a type of elastomeric impression mater-ial was used for the rest. Plastic impression trays of arigid design (Solo disposable impression trays,Cordent) were employed for the majority of cases in-spected and metal trays were used for the remainder.

The majority of cases were not accompanied by in-struction sheets, and 22% ofthe available sheets weregraded poor and required a telephone call to the den-tal practice for clarification. A total of 90 opposingarch impressions were available; occlusal recordswere provided with 53 (70%) ofthem (Table 2).

laboratory Equipment and Techniques

Dental school laboratories were well equipped andcapable of performing the widest range of dentalwork. They had modern types of equipment such aspressure-vacuum casting machines, programmedvacuum furnaces, and die-locating devices. A num-ber of commercial laboratories possessed the basic

equipment required for the fabrication of fixed par-tial denture work. The majority of the MOH andRMS laboratories had equipment suitable for theconstruction of removable prosthodontic and ortho-dontic appliances, butwerepoody equipped to pro-duce any form of porcelain work (Tabie 4].Articulators of semiadjustable type were commonlyused in the laboratories of the dental schools andwere available, but rarely used, in some commerciallaboratories. Nonprecious alloys were used exten-sively in all iaboratoriesbecauseoftheir low cost. Thefee charged per unit varied from one laboratory to an-other and was reputation- rather than cost-dependent.

Staffing

The total number of technicians was 183; 50 of theseemployees (27%) had no dental technology qualifi-cations. Laboratory owners considered 22 of the un-qualified persons technicians and dealt with the

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Quality of laboratory Procedures for Fixed Partial Dentures Albashaireh/Alnegrish

remaining 28 persons as laboratory assistants. All tech-nicians were employed on a full-time basis, and thetechnician per laboratory ratios for commercial, MOH,dental school, and RMS laboratories were 2.44, 2.36,2.6, and 5.4, respectively. The 28 laboratory assis-tants were excluded when these ratios were calculated.

The technicians' experience varied from 1 to 25years, with most being either "juniors" (34%) with 1to 5 years of experience, or "seniors" (34%) who hadworked in the field for more than 15 years. The ex-perience of 22% of the technicians ranged from 5 to10 years, while the remaining 10% had 10 to 15years of experience.

Technician Comments

The chief technicians thought that the 2-year, full-timetraining courses at the colleges of dental technologywere insufficient. They believed that the inferior qual-ity of the products could be attributed in part to theiriow salaries, lack of incentives, and inaccurate clini-cal procedures. They gave the following suggestionsas to how the clinical standards could be raised:

Abutment teeth should be carefully prepared. Impressions should be taken using standard trays

and materials. Instructions to technicians, including design and

shade details, should be provided and writtenclearly.

Discussion

The sample was randomly selected and representedthe major cities of Jordan. The questionnaires werecompleted by the chief technician and were col-lected on the day of the visit to the laboratory. Thismethod avoided any problems related to the mailingsystem and overcame the issue of no response.Consequently, data on 60 (75%) of the 80 registereddental laboratories at the time of this study were col-lected and a 100% response rate was achieved. Thisis more than some of the response rates recorded inthe literature.^''"''^ Moreover, this procedure ensuredthat the responses ofthe chief technicians themselveswere obtained and eliminated any possible interfer-ence from laboratory owners.

Most dentists using commercial laboratories per-formed unsatisfactory tooth preparation and sent un-usable impressions. The majority of impressions weretaken with alginate; some impressions were detachedfrom the trays and showed obvious shrinkage.Moreover, in half of the specimens inspected prepa-ration features were inadequate and the finishing lineswere indistinct. These results are in agreement with

other reports^'' ' and with the comments of the tech-nicians, who thought that some of the preparation andimpression procedures were inappropriate. Fine detailsofthe preparation and surrounding soft tissues can berecorded accurately when a suitable elastomeric ma-terial is used. Clinical procedures of such low qualitycan only lead to guesswork on the part of the techni-cian and must result in a restoration that will be com-promised from the outset. Even a skillful and experi-enced technician would fail to produce a restorationof acceptable strength, biologic compatibility, and es-thetics from an impression with such errors.^ '

Many authors have reported the importance ofusing a rigid impression tray.^''^"^^ Although the rigidplastic trays that were used are better than flexibletypes, special or metal trays are recommended,^-^'The latter are significantly more rigid, can be reused,and are therefore a more satisfactory alternative interms of both cost and accuracy.

Some of the impressions were pulling away fromthe tray, a finding that is in agreement with previousstudies.^'^'^''^ Such impressions can only lead to dis-torted working dies and ill-fitting restorations,^ An ad-hesive should be applied to the tray and the manu-facturer's instructions should be adhered to whilemanipulating impression materials.'^

Dentists supplied no prescriptions of the work re-quested in the majority of cases. This is in agreementwith similar studies.''^' ' It seems that a standardizedprescription form, which is used in the dental schools,may contribute to solving this problem. Some dentistsrelied on technicians to relate casts in proper occlu-sion and sent no occiusai records. These findings arein accordance with another report." Many practi-tioners fail to understand that a "high" restoration isnot a result of error by the technician but of defectiverecording of the occlusa! surfaces of unpreparedteeth. One air bubble is sufficient to alter the articu-lation and result in a faulty restoration.^

The dental schooi and some commercial laborato-ries had modern types of equipment and were capableof producing ali kindsof dentai work. In contrast, thoseof the MOH and the RMS were mostly equipped forfabricating removable acryiic dentures, orthodonticappiiances, and some porceiain work. The 2 iabora-tories that constructed fixed partiai dentures seemed tobe incapable of providing restorations on a large scaie.They were still using oid types of furnaces for porce-lain work and gas furnaces for beating casting rings.

Laboratory owners empioyed technicians of knowninadequate training and paid them iower saiaries.Moreover, they empioyed iaboratory assistants pre-sumabiy to perform routine tasks and gnerai worksuch as cieaning, cierking, and delivering the finishedwork to ciinics. The construction of a good-quaiity

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Albashaireh/Alnegrish Quality of Laboratory Procedures for Fised Partial Dentures

fixed partial denture needs all the expertise and careof a well-trained and experienced technician. Thequality of work will certainly be jeopardized if the du-ties of the laboratory assistants are extended to in-clude coping with sophisticated types of work.^

Technicians were concerned and unhappy withtheir salaries, which varied depending on the profitof laboratory owners. The fact that no incentiveswere paid for skillful technicians may have dampenedany enthusiasm for professional development. Thismade an offer of a better-paid overseas job very at-tractive to the experienced technicians.

Conclusion

This study implies the following suggestions and rec-ommendations:

1. Clinicians should perform accurate preparationprocedures, use proper materials and trays fortaking impressions, and supply technicians withcomplete, clear instructions.

2. Owners should be required to update their labo-ratory equipment regularly, employ technicianswith adequate training and experience, and paythem reasonable salaries.

3. Regulations including uniform fee charges per unitand reasonable salaries for technicians should beapplied to control the establishment of new labo-ratories and improve the standards of existing ones.Technology courses should be revised and the train-ing period extended. Refresher courses for qualifiedtechnicians should be organized regularly.

Half of the fixed partial denture cases inspectedwere of unacceptable quality; these were found incommercial laboratories. Dentists using institution-alized dental school, MOH, and RMS laboratoriesperformed satisfactory clinical procedures and sentacceptable impressions. The dental schools and somecommercial laboratories had the staff and equipmentrequired for producing fixed partial dentures.

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