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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.
:._. ~ = L - . .-..:,.....'2, Number 3,1999 237 Ttie
International journal of Prosthodontic
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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
Tfie internationai Journal of Pro5thoclonti< 238 Volume 12,
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AI basha i reVAl negrish Quality of Laboratory Procedures for
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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
^ - I r - - ; -.X Number 3,1999 239 The Inlernatiorial louml of
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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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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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