Open Journal of Stomatology, 2011, 1, 140-149
doi:10.4236/ojst.2011.14021 Published Online December 2011 (http://www.SciRP.org/journal/ojst/
OJST
).
Published Online December 2011 in SciRes. http://www.scirp.org/journal/OJST
Vertical marginal gap & retention of ceramic full coverage &
inlay retained ceramic fixed partial dentures
Cherif Mohsen
Faculty of Dentistry, Minia University, Minia, Egypt.
Received 14 July 2011; revised 20 August 2011; accepted 6 September 2011.
ABSTRACT
Objectives: A comparison study between ceramic full
coverage FPDs & 3 designs of ceramic inlay retained
FPDs regarding vertical marginal gap & retention.
Materials & Methods: Twenty samples were construc-
ted and divided into 4 groups according to the type of
restorations: full coverage, inlay-shaped (occluso-pro-
ximal inlay + proximal box), tub-shaped (occluso-pro-
ximal inlay), and proximal box-shaped FPDs. All
samples were subjected to a vertical marginal gap
measurements followed by a retention test. Results:
The vertical marginal gap data showed no significant
difference between full coverage FPDs, the tub-shap-
ed inlay retained FPDs and the proximal box-shaped
inlay retained FPDs. While there was a difference
between these three designs and the inlay retained
FPDs. Regarding retention, the full coverage FPDs
recorded higher retentive strengths and was signifi-
cant difference than all inlay retained FPDs designs
tested. The inlay-shaped design was significant dif-
ference than the other two inlay retained FPDs de-
signs. Conclusions: There was no significant differ-
ence between full coverage FPDs, tub-shaped & pro-
ximal box shaped inlay retained FPDs as regard ver-
tical marginal discrepancies. While, the inlay-haped
design showed the highest vertical marginal discrep-
ancies. The premolar & molar retainers for the same
type of restorations showed no difference in vertical
marginal discrepancies. All measured vertical mar-
ginal discrepancies were in the range of clinical ac-
ceptance. The full coverage FPDs recorded higher
retentive strengths than all inlay retained FPDs de-
signs tested. The inlay-shaped design recorded the
highest retentive strengths among the three inlay re-
tained FPDs designs. There was no difference as re-
gard retentive strengths between tub-shaped & pro-
ximal box shaped inlay retained FPDs.
Keywords: Inlay Retained FPDs; Full Coverage FPDs;
Retentive Strengths; Vertical Marginal Discrepancies
1. INTRODUCTION
All-ceramic crowns are popular for the restoration of sing-
le teeth due to esthetic appearance and metal-free struc-
ture [1]. Zirconium dioxide all-ceramic material demon-
strates optimal material properties such as high fracture
toughness, enabling its use with posterior fixed partial
dentures (FPDs) [2-4]. In recent years, yttria-stabilized
tetragonal zirconia polycrystals (Y-TZP) ceramic has
been made available to dentistry through the computer-
aided design/computer-aided manufacturing (CAD/CAM)
technique [5]. Using this core material for all-ceramic
frameworks resulted in excellent mechanical performan-
ce and superior strength and fracture resistance [6].
Inlay-retained fixed partial dentures can be construct-
ed using dental alloys, ceramic materials, and fiber-re-
inforced composite. Clinical results for metal inlay re-
tained FPDs are favorable. However, visibility of the
metal retainer and the change in natural tooth translucen-
cy are esthetically unfavorable. This encouraged resear-
ches on metal-free, tooth-colored materials for inlay-re-
tained FPDs [7-12]. Researchers suggested various de-
signs for inlay retained FPDs such as grooves, tub, box-
shaped proximal preparations, occluso-proximal prepa-
rations of inlay design, use of a rest seat on the occlusal
surface, lingual tooth reduction and retentive-slot prepa-
rations [11,13-16]. The size of these preparation features
depends on the size of the tooth, and for molar proximo-
occlusal inlay preparation are suggested [17].
The Cerec inLab 3D system is the latest addition to
Sirona’s CAD/CAM product line, introduced in (2005).
Its advanced software allows for broad range of indica-
tions: crown copings, multi-unit bridge frameworks, in-
lays, onlays and fully contoured crowns out of single,
solid blocks. It also allows anatomically perfect results
due to the biogeneric occlusal surface design of inlays
and onlays. The biogeneric modeling function is based
on data acquired from thousands of natural teeth. The
preparation margin is marked with just a few mouse cli-

C. Mohsen / Open Journal of Stomatology 1 (2011) 140-149 141
cks—and the software does all the rest. In order to en-
sure the accuracy of the restoration, the lab technician
sees what will be milled on the screen before it is sent to
the milling machine. Milling performance and precision
has been optimized to +/−25 microns. The tandem burs
now spin at 60,000 RPM resulting in considerably faster
milling time—approximately 6 minutes for a full-con-
tour crown [18].
The marginal adaptation of the fit of a fixed restora-
tion to the prepared tooth is important to minimize plaq-
ue accumulation and therefore the risk of gingivitis, pe-
riodontitis, secondary caries, pulpitis and prosthesis fail-
ure. The margin adaptation is influenced by many factors
among them the prosthesis type, the tooth preparation
geometry, use of die spacers, the physical properties of
the luting cement and the prosthesis seating forces dur-
ing the setting reaction [19,20]. The maximum accept-
able clinical marginal gap varied in dental literature; Chri-
stensen (1966) [21] and McLean & Fraunhofer (1971)
[22] reported that a marginal opening of 120 μm must be
the limit of the clinical acceptability. Other author de-
fined clinical acceptable marginal opening after cemen-
tation to be smaller than 150 μm [23]. For CAD/CAM
generated restorations, the generally acceptable marginal
gap discrepancies are between 50 and 100 μm [24-27].
Tinschert et al. (2001) [28] reported mean marginal dis-
crepancies between 61 μm and 74 μm for ZrO2 ceramic
FDP frameworks. Wolfart et al. (2003) [29] investigated
the in vivo marginal discrepancy of experimental all-
ceramic FDPs and reported values between 89 and 130
μm. Reich et al. ( 2005) [30] reported a median marginal
discrepancy of 65 μm for 3-unit ZrO2 ceramic FDP fra-
meworks. Goëhring et al. (2001) [31] in an in-vitro study
about the marginal adaptation of box shaped inlay re-
tained FPDs constructed from Targis-Vectris, showed
that inlay-retained fixed partial dentures resulted in a ma-
rginal gap less than 100 μm.
The longevity of fixed prostheses depends on the re-
tention and marginal integrity of restorations [32]. Many
factors have been demonstrated to influence the reten-
tion of fixed prosthetic appliances, such as the size and
shape of prepared teeth [33], manipulation of cement
[34-36], retentive properties of cement [37-39], cement
film thickness [40,41], relieving space or venting for
cement [42,43] , cement application [44], roughness of
dentinal surface[45], the convergence and the prepara-
tion height [46-49]. Dental literature is poor on studying
the retentive strength of the inlay retained ceramic FPDs.
Ohlmann et al. (2008) [50] in a preliminary clinical study
on all-ceramic inlay-retained fixed partial dentures, re-
ported that 6 out of 13 inlay retained FPDs were subject-
ed to debonding. .Harder et al. (2010) [51] in an eight-
year outcome study of posterior inlay-retained all-ce-
ramic fixed dental prostheses, reported that the percent-
age of debonding of this type of FPDs was 15%.
The purpose of this study is to investigate and com-
pare between the full coverage ceramic FPDs and the
three designs of ceramic inlay retained FPDs as regard the
vertical marginal gap as well as their retention.
2. MATERIALS AND METHODS
2.1. Grouping
Twenty zirconia ceramic bridges (In Coris Zi, Sirona, Ger-
many) were constructed in this study. Specimens were
divided into four groups, five specimens each, according
to abutment preparation. These four groups were, full
coverage, inlay-shaped, tub-shaped, and proximal box-
shaped preparations.
2.2. Model Construction
A prefabricated hard plastic master model (Elbana, Alex-
andria, Egypt) of adult dimensions and features with
interchangeable hard acrylic resin teeth was chosen to be
used. The artificial lower left first molar was removed
from the model and its socket was blocked using pink
acrylic resin (Acrostone, Egypt) to simulate a healed
ridge. Eight artificial teeth representative of mandibular
second premolars and second molars (four teeth each)
were used and received the different preparation designs
tested (full coverage, inlay-shaped, tub-shaped, and pro-
ximal box-shaped FPDs). Each artificial tooth received
one preparation.
Before the beginning of any preparation, a putty im-
pression for the unprepared teeth was taken using an ad-
ditional silicon rubber base material (Virtual, Ivoclar-
Vivadent, Liechtenstein) in a sectional stock tray. The
silicon impression was removed from the tray and cut
longitudinally at the retainers’ site. This was done in ord-
er to get a silicon index to act as a guide during veneer-
ing of the frameworks.
In order to assure that abutments used for different te-
sted designs have the same relation to each other, the same
master model was used for the four different prepara-
tions.
2.3. Full Coverage Design Preparation
The artificial acrylic second premolar and molar teeth
were first prepared by free hand by one operator to re-
ceive full coverage ceramic restorations using a tapered
diamond stone with flat end. The preparation designs
had a shoulder finish line of 1 mm thick and occlusal
clearance of 2 mm. Opposing walls were prepared with
minimal occlusal convergence. Then by using a surveyor
the convergence angle was adjusted to be nearly 5˚ Fig-
ure 1.
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Figure 1. Preparation of Full coverage FPDs.
2.4. Inlay Design Preparation
The intra-coronal preparation procedures were perform-
ed in accordance with general principles for ceramic intra-
coronal ceramic restorations [52]. Intra-coronal prepara-
tions of the abutments (inlay, tub shaped and proximal
box-shaped) had the following dimensions: The inlay
preparation consisted of an occluso-proximal box and was
designed with rounded internal edges, smooth rounded
corners, and rectangular floor without bevels at the oc-
clusal or gingival margins. The occlusal inlay had a pre-
paration depth that allowed a thickness of 2.0 mm for the
ceramic. The occlusal preparation was 4 mm wide and
extended 4 or 6 mm mesio-distally for the premolar or
molar models, respectively. The proximal box was 1 mm
wide and had approximately 5˚ divergence, extending 2
mm apical to the isthmus floor [53]. The preparations
corresponded to a proximal connector area of 3 mm × 3
mm for molars and premolars. The tub-shaped prepara-
tion consisted of an occluso-proximal inlay and was pre-
pared with the same dimensions as the inlay-shaped pre-
paration, except for the proximal box preparation. The
proximal box featured the same dimensions as the pro-
ximal box of the inlay-shaped preparation. Dimensions
were measured with a digital caliper ruler. The premolar
was prepared with an occluso-distal and the molar with a
occluso-mesial intra-coronal preparation (Figures 2-4).
2.5. Duplication into Cobalt-Chromium Models
Five impressions were made with addition silicone im-
pression material (Virtual, Ivoclar Vivadent, Schaan, Lie-
chtenstein) for the master model with each abutments of
one of the tested designs inserted into it. Then, blue inlay
wax (Crown & bridge, Bego, Germany) was molten and
poured into the impressions, then sprued and cast using
Co-Cr alloy (Wironit, Bego, Bremen, Germany). The
twenty metallic models were carefully checked then fin-
ished and polished.
Figure 2. Schematic figure of inlay design.
Figure 3. Schematic figure of tub design.
Figure 4. Schematic figure of proximal
box design.
2.6. Duplication of Co-Cr Models into Stone
Models
In order to be able to scan the prepared models using the
Enios scanner to the computer, and consequently being
able to design and fabricate the zirconia frameworks us-
ing the cerec inlab software. An impression was taken
using an additional silicon rubber base impression mate-
rial in a sectional tray for the Co-Cr models. Then a spe-
cial stone (Esthetic gold stone material, Bensheim, Ger-
many) for cerec system was mixed according to the
manufacturer instructions. The stone was then poured in-
to the impressions on a vibrator to be sure that the stone
casts are free from any voids.
2.7. Fabrication of Restorations
The duplicated models were then scanned using the En-
ios scanner. Data were transferred to the computer con-
nected to the cerec inlab milling machine to analyze the
four tested models and start designing their correspond-
ing zirconia frameworks in order to fabricate their resto-
rations. Frameworks were designed according to the ma-
nufacturer directions and cerec inlab software 3.65 recom-
mendations. The pontic ridge lap area was adjusted to be
a bullet pontic, the connector size was adjusted to have
the dimension of 3 mm × 3 mm. for the full coverage
FPDs, The occlusal surface was designed to have thick-
ness of 0.7 mm while the walls were designed to be 0.5
mm thick after firing. While for the 3 inlay retained de-
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signs tested, The inlay base was designed to have the
thickness of 0.7 mm while the walls were designed to
have the thickness of 0.5 mm after firing.
The frameworks designs were manipulated by the soft-
ware and sent to the cerec inlab milling machine, in an
enlarged state in order to compensate the shrinkage that
will occur during sintering. Five samples were milled to
each design.
The twenty enlarged green-state partially sintered frame-
works were then fired using a special furnace (Sirona-
InFire HTC, Bensheim, Germany). The frameworks were
put on the furnace tray containing sinter balls, for better
heat distribution. They were subjected to a preprogram-
med firing cycle according to the type of the ceramic
block. This firing cycle extended for seven hours and half.
Then the sintered frameworks marginal fit was checked
visually with a silicone indicator paste (Fit Checker
black, GC, Bensheim, Germany) and an explorer.
The frameworks were then veneered using a high mel-
ting fine-structure feldspar ceramic (Vita VM9, Vita, Ger-
many) by the aid of the previously constructed rubber
indices before any preparation of the artificial teeth. Then
the samples were glazed according to manufacture instruc-
tions.
The fitting surfaces of all samples were airborne-par-
ticle abraded (110 μm Al2O3) for 10 seconds with 2.5 bar
pressure, which is an essential step for reliable bonding
of zirconia ceramic [54]. The other ceramic surfaces of
the FPDs were protected with silicone during airborne-
particle abrasion.
2.8. Cementation of the FPDs
Samples of all groups were cleaned with distilled water
in an ultrasonic unit for approximately 1 minute. They
were then rinsed thoroughly with water spray and dried
with oil-free air. Then all FPDs were cemented adhesi-
vely to their respective Co-Cr models using, a dual cured
adhesive resin cement (Multilink Ivoclar, Schaan, Lie-
chtenstein). In order to sandblast the Co-Cr models, the
preparation margins were covered with a tap, then this
tap was removed after sandblasting. According to the
manufacturer instructions, a thin layer of zirconia primer
was applied to the pre-treated fitting surface of each sam-
ple using a micro-brush supplied with the cement. The
primer was left to react for 3 minutes, and then dried
with water and oil free air jet. The mixed primer is self
cured within 10 minutes. The mixed primer A/B was
applied with a micro-brush on the whole die surfaces of
each model. The applied primer was subsequently, dried
with oil free air jet. This will lead to considerable accel-
eration of the curing process; i.e. the curing time is
shortened. Meanwhile, the adhesive resin was mixed
with ratio 1:1 over a mixing paper pad using the supplied
double-push syringe and was then applied on the inner
surfaces of the coping. Each coping was seated in place
first with adequate finger pressure and the excess cement
was removed immediately using a scaler. Then immedi-
ately placed under a loading device of 5 Kg for 1 minute
then removed from the loading device and light curing
was performed.
2.9. Vertical Marginal Gap Measurements
Marginal gap was measured at 20 predetermined points
for each tested groups with USB digital microscope (Sco-
pe Capture Digital Microscope, Guangdong, China) at
100× magnification and photographed using image an-
alysis software (Scope Capture 1.1.1.1. Ltd Co.)
2.10. Retention Test
Each model with its own bridge was secured with tigh-
tening screws into the lower fixed compartment of a ma-
terials testing machine (Model LRX-plus; Lloyd Instru-
ments Ltd., Fareham, UK). Two orthodontic wires [(0.9-
mm (0.036-inch)] engaging the mesial and the distal con-
nectors were held in grips attached to the upper movable
compartment of the machine by a metal hook as shown in
(Figur es 5 and 6). A tensile force was applied in pull mode
Figure 5. Assembly for retention test for inlay retain-
ed FPDs.
Figure 6. Assembly for retention test for full coverage
FPDs.
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via materials testing machine at a crosshead speed of 5
mm/min. The load required to dislodge the restoration
was recorded in Newton.
2.11. Statistical Tests
As regard the vertical marginal gap data were collected,
calculated, tabulated, and statistically analyzed then a
Two-way ANOVA and Fisher’s LSD tests was used. As
regard retention, data were collected, calculated, tabula-
ted, and statistically analyzed. A one-way ANOVA test
followed by a Tukey test was performed to determine
significant differences between the tested groups using a
confidence level of 0.05 (p < 0.05)
3. RESULTS
3.1. Vertical Marginal Gap
The results of the vertical marginal gap data obtained
from the tested groups are shown in Figure 7. A compa-
rison between the means of the tested groups is shown in
Figure 6. A two way ANOVA Test was used to determi-
ne significant differences between the tested groups, (p
< 0.05) (Table 1). Fisher’s LCD method for multiple
comparisons of means at (p < 0.05) was done following
the two-way analysis of variance.
Vertical
Margina
l
Gap in
Microns
Figure 7. Comparison between vertical marginal gap of the
tested groups.
Table 1. Analysis of variance for the vertical marginal gap.
Source SS df MS F P
Total 2642 39 - - -
Columns
Factor 90 1 90 1.383 <0.05
Rows Factor 465 3 155 2.382 <0.05
Between Cells 560 - - - -
Interaction 5 3 1.667 0.026 <0.05
Error 2082 32 65.0625 - -
*Critical value within columns: 5; *Critical value within rows: 7.
The results showed that full coverage FPDs recorded
the least vertical marginal gap while the inlay retained
FPDs recorded the highest values among all the tested
FPDs designs. Statistically, there was no significant dif-
ference between full coverage ceramic FPDs, the tub-
shaped ceramic inlay retained FPDs and the proximal box
shaped ceramic inlay retained FPDs. While there was a
difference between these three designs and the inlay re-
tained ceramic FPDs (IRFPDs). The results also showed
no difference between the premolar and the molar retain-
er for the same FPDs design. The range of the obtained
vertical marginal gap varied from 47 to 59 μm, which is
the range of the clinical acceptability [24-27].
3.2. Retenti on
The means and standard deviations of the retentive streng-
th for the tested groups are presented in Table 2. A com-
parison between the means of the tested groups is shown
in Figure 8. A one way ANOVA Test was used to deter-
mine significant differences between the tested groups (p
< 0.05) (Table 2). The Tukey test for multiple com-
parisons of means at (p < 0.05) was done following the
one-way analysis of variance. The results showed that
Table 2. Analysis of variance between and within different
groups for retentive strength test.
Source of
variation
Sum of
squares
SS
Degree of
freedom
DF
Mean
squares
MS
F. ratio
F
Significance
P
Total 41239.519 - - -
Between
groups 40440.563 13480.19 269.96 <0.05
Within
groups 798.9416 49.93 - -
*Critical value: 14.31.
Retentive
strength in
Newton
Figure 8. Comparison between retentive strength of the tested
groups.
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the full coverage FPDs recorded higher retentive streng-
ths while the proximal box-shaped inlay retained FPDs
recorded the least strengths. Statistically, the full cover-
age FPDs was significant difference than all inlay re-
tained FPDs designs tested. The inlay-shaped design was
significant difference than the other two inlay retained
FPDs designs
4. DISCUSSION
The important factors for zirconia selecting all-ceramic
fixed restorations are marginal gap, retentivity and me-
chanical strength [55]. Mechanical strength was reported
in several researches [12,56-58]. Several researchers ha-
ve reported marginal gaps both in vitro and in vivo [59-
61]. The large gap may cause cement solubility and re-
sult in plaque accumulation, marginal leakage, second
caries, and eventually crown failure [62,63]. The clini-
cally acceptable marginal gap for CAD/CAM generated
restorations are within 100 μm [24-27]. Debonding of
all-ceramic fixed restorations was common in-vivo stu-
dies [50,51]. This study was performed to compare the
vertical marginal gap and retention of full coverage ce-
ramic FPDs and 3 most common designs of ceramic in-
lay retained FPDs.
Minimal or no tooth preparation of the abutment teeth
is desirable for the replacement of missing teeth. Inlay-
retained FPDs require less amount of tooth reduction and
maintain the integrity of the periodontal tissues. There-
fore they are a conservative option for the restoration of
damaged teeth [58]. In this study, the vertical marginal
gap and the retentivity of ceramic inlay FPDs and all ce-
ramic FPDs were compared.
The preparation designs for partial-coverage restora-
tions are not standardized, in contrast to those for com-
plete coverage restorations [64]. Researchers suggested
various inlay designs such as grooves, tub, box-shaped
proximal preparations and occluso-proximal prepara-
tions. They also suggested the use of a rest seat on the
occlusal surface, lingual tooth reduction and retentive-
slot preparations [11,13-16]. The size of these prepara-
tion features depends on the size of the tooth. The tested
design is the most used in CIRPFDs [10-12,50,53,58].
The dimension of the full coverage all ceramic FPDs
were constructed according to Rosenstiel et al. (2006)
[65].
Behr et al. (1999) [11] suggested that the taper of the
preparation is an important factor, affecting the fracture
resistance of the restoration, and should not exceed 5
degrees. Shillinburg et al. (1997) [52] reported that in
case of short preparation, its walls must have as little
degree of tapering as possible to increase its retentivity.
In this study, a surveyor was used to adjust the degree of
tapering for the four tested preparation designs.
In this research, twenty FPDs were constructed and in
order to standardize the restorations and to fabricate
identical restorations for each type of tested restorations;
Cerec in Lab was used. This CAD/CAM system allows
for broad range of indications as well as anatomically
perfect results due to the biogeneric occlusal surface de-
sign of inlays and onlays. The biogeneric modelling func-
tion is based on data acquired from thousands of natural
teeth. The preparation margin is marked with just a few
mouse clicks—and the software does all the rest [66].
Another reason for the use of Cerec in Lab system is that
Att et al. (2009) [67] reported that the VITA YZ-Cerec
restorations showed significantly smaller marginal gap
values than the DCS and Procera restorations.
For standardization, one type of zirconia ceramic bloc-
ks was used for both types of restorations (In Coris Zi).
A zirconia ceramic was used due to the ability of yttria-
tetragonal zirconia polycrystal to prevent crack propaga-
tion and thus yield excellent mechanical performance
and superior strength and fracture resistance, compared
to other ceramics [11,14,15] and the production of strong
inlay-anchored FPDs [31,50,68,69]. Zirconia based ce-
ramic IRFPDs demonstrates higher fracture resistance
than glass ceramic [12,70].
In order to standardize the veneering layer, a rubber
index was fabricated for the acrylic abutment teeth be-
fore any preparations. The thickness of the framework
was adjusted using the Cerec in Lab software. After con-
struction of the ceramic framework, veneering material
(Vita VM9) was used and its thickness adjusted by the
aid of the rubber index.
In this research, the vertical marginal gap measure-
ment was performed as it is the most frequent method to
quantify the accuracy of fit of a restoration. Although,
many testing methods and measuring tools are available
in the dental literature, the direct view method using the
optical measuring microscope is considered more con-
vient, accurate, easy and rapid for determining the mar-
ginal gap distance [71]. This method was used in this
study. The number of the measurement points per crown
used in previous studies has varied considerably. Groten
et al. (2000) [72] suggested that ideally 50 points, or 20 -
25 measurements should be made for each crown. Al-
though, in many studies measurements ranged from 4 -
12 [73-75]. In this current study, 20 measurements per
retainer were done so that the mean discrepancy value
obtained from measurement points of each retainer can
provide a reasonable presentative quantity.
The retention tests was performed using two ortho-
dontic wires engaging the mesial and the distal connectors
and held in grips attached to the upper movable com-
partment of the machine by metal hook. This configu-
ration allowed all forces to be directed parallel to the long
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axis of the retainers during testing.
As regard the results of the vertical marginal gap
measurements, there was no significant difference be-
tween full coverage FPDs, tub-shaped & proximal box
shaped inlay retained FPDs as regard vertical marginal
discrepancies. This may be due to the fact that the full
coverage FPDs has a longer perimeter but different mar-
ginal configurations than these 2 designs of inlay re-
tained FPDs, these 3 designs were measured at 20 pre-
determined points each, this means that the distance be-
tween each pre-determined point for the full coverage
FPDs was much a part than that of the 2 designs of inlay
retained FPDs. Chan et al. (1989) [76] reported that the
marginal discrepancy of each restoration may vary great-
ly at different location. The number of pre-determined
points for measuring the vertical marginal gap should be
promotional to the perimeter of the finish line, so that
the distance between each 2 pre-determined points be
equal. At the same time, the results showed a difference
in vertical marginal gap between the full coverage FPDs
and the inlay-shaped design, this may be due to the dif-
ference in the preparation geometry, marginal configure-
tions [55] and the location and orientation of tubules [77]
between these 2 designs. Also, the results showed a sig-
nificant difference between tub-shaped & proximal box
shaped inlay retained FPDs from one side and the
inlay-shaped FPDs from the other side, this may be attri-
buted to the fact that the latter design possess a much
longer perimeter and has the geometry of the former 2
designs together.
The results also showed that the premolar & molar re-
tainers for the same type of restorations showed no dif-
ference in vertical marginal discrepancies. These results
were in agreement with Ebeid (2006) [78]. This may be
attributed to the small difference between premolar and
molar regarding dimensions.
The obtained vertical marginal gap data in the present
study ranged from 47 to 59 μm are within the clinical
acceptable values, since the criteria of 100 μm was con-
sidered by some authors as the maximum clinical ac-
ceptable marginal gap [24-27].
As regard the forces needed to dislodge the all cera-
mic as well as the inlay retained FPDs, the results show-
ed that, a significant difference was found between the
tested FPDs. The all ceramic FPDs recorded the highest
forces to be dislodged and were significant difference
than all inlay retained FPDs designs tested. The inlay-
shaped design was significant difference than the other
two inlay retained FPDs designs. These results could not
be compared with results of other researchers as this
study is the pioneer in studying in-vitro the retentivity of
inlays retained FPDs. In two clinical evaluations, Edel-
hoff et al. (2001) [13] and Edelhoff & Sorensen (2002)
[79], they reported that compared to crown retained
FPDs, de-bonding of inlay retained FPDs appear to be
much too high. These difference in retention between the
two tested FPDs may be attributed to the fact that longer
preparations (full coverage FPDs) have more surface
area and therefore are more retentive than the shorter
preparations (inlay retained FPDs) [52] . Another reason
for these difference may be due to the conclusion of Er-
nst et al. (2005) [80] who reported that the frictional re-
sistance between a prepared tooth and ceramic crown is
as important as the adhesive bond strength.
5. CONCLUSIONS
1) There was no significant difference between full
coverage FPDs, tub-shaped & proximal box shaped
IRFPDs as regard vertical marginal discrepancies.
2) The inlay-shaped design showed the highest ver-
tical marginal discrepancies.
3) The premolar & molar retainers for the same
type of restorations showed no difference in vertical
marginal discrepancies.
4) All measured vertical marginal discrepancies
were in the range of clinical acceptance.
5) The full coverage FPDs recorded higher reten-
tive strengths than all inlay retained FPDs designs
tested.
6) The inlay-shaped design recorded the highest
retentive strengths among the three inlay retained
FPDs designs.
7) There was no difference as regard retentive str-
engths between tub-shaped & proximal box shaped
IRFPDs.
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