Keywords dental implants - dental marginal adaptation - dental materials - computer-aided design
- metal ceramic alloys
Introduction
Marginal adaptation is one of the most important factors in the long-term success
of implant restorations. An accurate adaptation between implant abutment and restoration
is necessary for clinical success and prosthesis durability.[1 ] Lack of marginal adaptation may result in several biological and mechanical problems
such as pain, marginal bone loss, plaque accumulation, increase in gingival index
and periodontal pocket depth, abutment loosening, osseointegration loss, and even
implant failure.[2 ]
[3 ]
[4 ] The marginal adaptation of cement-retained implant restorations can be affected
by different factors including impression materials and techniques, restoration type,
fabrication procedure, material used, technician expertise, cement type, and cementation
process.[5 ]
[6 ]
[7 ]
[8 ]
[9 ] Various techniques have been suggested for measuring marginal discrepancy, and one
of the most common non-aggressive techniques is direct view;[10 ]
[11 ]
[12 ]
[13 ] other methods include impression replica technique,[11 ] cross-sectioning technique,[14 ] contact scanner technique,[15 ] and laser videography.[16 ] One of the acceptable processes is application of video measuring machine (VMM),
which relies on non-contact video measurement of high resolution images. This system
provides an inexpensive, accurate, and fast procedure to monitor critical dimensions
of object without scarifying the specimen.[17 ]
The framework pattern of a restoration can be fabricated by either conventional, computerized,
or a combination techniques using a variety of materials such as wax, composite, acrylic
resin, and even directly by metal.[18 ]
[19 ] After the introduction of computerized systems—for example, computer assisted design/milling
(CAD/milling) and computer assisted design/rapid prototyping (CAD/RP)—fabrication
of higher quality restorations became promising without the limitations of conventional
methods.[19 ]
[20 ]
[21 ] Computer-assisted procedures omitted several steps in fabrication flow,[22 ] improved procedural reliability,[23 ] facilitated using new materials not applicable in conventional methods, reduced
labor and cost, improved quality control, and increased production rate.[19 ]
[24 ] However, transformation of point angles to smooth surfaces, and the limitation of
finite resolution, leading to round edges are reported as disadvantages.[22 ]
Recent studies have reported contradictory results for the marginal discrepancy of
restorations made by different methods.[23 ]
[25 ]
[26 ]
[27 ]
[28 ]
[29 ]
[30 ]
[31 ]
[32 ]
[33 ]
[34 ]
[35 ]
[36 ] Several studies reported greater marginal discrepancies in restorations fabricated
by the CAD/CAM systems,[23 ]
[25 ]
[26 ]
[27 ]
[28 ]
[29 ] while others showed greater values in restorations made by conventional methods,[30 ]
[31 ]
[32 ]
[33 ] or even reported no significant differences[34 ] ([Table 1 ]). The present study aimed to compare the marginal discrepancy of single implant-supported
frameworks fabricated by different materials, using additive conventional/computerized
and subtractive computerized methods. The null hypothesis was that there will be no
significant differences between marginal adaptation of specimens made by different
methods.
Table 1
A summary of related studies
Study
Measurement method
Material
Fabrication method
Marginal discrepancy (µm)
Tan et al[23 ]
Direct view technique
Titanium blocks
CAD/CAM
79.43 ± 25.46
High noble
Conventional wax-up/CAM
73.12 ± 24.15
Conventional wax-up, casting
23.91 ± 9.80
Farjood et al[25 ]
Cross-sectioning, digital microscope
Wax
CAD/RP wax/casting
89.8 ± 8.3
Conventional wax-up, casting
69.5 ± 15.6
Han et al[26 ]
Cross-sectional
Wax
titanium blocks
Conventional wax-up, casting
Shoulder: 55.2 (20.0)
chamfer: 52.2 (14.2)
Knife edge: 76.1(9.4)
CAD/CAM hard metal
Shoulder: 67.0 (14.1)
chamfer: 59.8 (14.9)
knife edge: 80.7(10.4)
Vojdani et al
Cross-sectioning, digital microscope
Wax
CAD/CAM wax, casting
157.37 ± 20.63
Conventional wax-up, casting
69.54 ± 15.60
Kim et al[29 ]
Micro CT imaging
Cr-Co
Conventional wax-up, casting
70.4 ± 12.0
CAD/CAM milling
123.5 ± 32.1
Selective laser melting
98.7 ± 26.9
Nejatidanesh et al[30 ]
Replica technique
IPS e.max CAD
CAD/CAM
32.02 ± 10.38
Zirconia
34.26 ± 11.41
IPS e.max press
Conventional wax-up, press
74.99 ± 24.51
Base metal
Conventional wax-up, casting
59.19 ± 17.81
Ghodsi et al[31 ]
Replica technique
Wax
CAD/CAM
18.0 ± 1.0
Cr-Co blocks
176.07 ± 53.54
Ng et al[32 ]
Direct view technique
Lithium disilicate
Conventional wax-up, pressing
74 ± 47
CAD/CAM
48 ± 25
Xu et al[33 ]
Replica technique
Co-Cr
Conventional wax-up, casting
170.19
Selective laser melting
102.86
Lalande et al[34 ]
Sectioning
Complete gold crown
Conventional wax-up, casting
52 ± 31
CAD/CAM acrylic, casting
45 ± 27
Nesse et al[35 ]
Replica technique
(3-unit)
Co-Cr
Conventional wax-up, casting
Good marginal fit
CAD/CAM milling
Best marginal fit
Selective laser melting
Poor marginal fit
Afify et al[36 ]
Direct view technique
(3-unit)
Wax
CAD/CAM milling + casting
35.5 ± 18.5
Noble alloy
CAD CAM milling
18.7 ± 20.4
Noble alloy
Direct laser sintering
22.8 ± 13.5
Materials and Methods
The sample size of 12 for each group was determined using a power analysis to provide
statistical significance (a = 0.05) at 80% power. Seventy-two implant analogues (Fixture
Laboratory analogue, Ufit Dental implant system, South Korea) were mounted vertically
in acrylic resin (Acrylic resin for patterns, GC America INC, Alsip, IL, USA). Impression
coping was used on a dental surveyor (Ney Dental International, Bloomfield, CT, United
State) as a guide to ensure the parallel mounting of each specimen. One-piece abutments
(Solid abutment; Ufit Dental implant system, South Korea) of 5.5 mm length and 6 degrees
of convergence were secured in the fixture analogues. Experimental groups (n = 12) were prepared as follows by the same expert technician to prevent inter-operator
bias (descriptive chart of prepared groups has been shown in [Fig. 1 ]). The conventional wax group was considered as the control group.
Fig. 1 Descriptive chart of specimens.
CAD/Milling Specimens (3 Groups)
Thirty-six abutments were sprayed (Scanspray; Renfertp GmbH, Hilzingen, Germany) and
scanned by a laser scanner (3Shape D810, 3Shape, Copenhagen, Denmark). Data was transmitted
to a software program (3Shape’s CAD Design software, 3Shape, Copenhagen, Denmark).
The cement space was set at 30 μ starting 0.5 mm from the margin; the anatomic patterns
were designed and milled using three different materials: wax, soft, and hard Cr-Co
metal. Wax patterns (Yeti,; Dentalproduct GmbH, Engen, Germany) were milled by a milling
machine (CORiTEC 350i; Imes-icore, Eiterfeld, Germany) using a T35-drill with a 2
mm diameter, invested in phosphate-bonded investments (Z4-C&B investment; Neirynck
& Vogt, Schelle, Belgium), and cast by Ni-Cr alloy in a casting machine (Nautilus
CC plus; Bego, Bremen, Germany). Soft metal Cr-Co patterns (Ceramill Sintron; Amann
Girrbach AG, Austria) were milled by Amann Girrbach CAM system (Ceramill motion 2;
Amann Girrbach AG, Austria) using drill no. 760605 with 2.5 mm diameter, and sintered
at 1300°C in vacuum oven (Argovent; Amann Girrbach AG, Austria). Hard metal Cr-Co
blocks were milled in a milling machine (CORiTEC 450i; Imes-icore GmbH, Eiterfeld,
Germany) using a T40-drill with a 2.5 mm diameter. A silicone index was made from
the first pattern to be used for standardization of the thickness/contour of conventional
wax and acrylic resin patterns.
CAD/RP Specimens (1 Group)
After scanning the abutments and designing the patterns in the same way as CAD/milling
models, wax patterns (n = 12) were prepared using a 3D printer (R66PLUS, Solidscape Inc, Merrimack, NH) by
an Inkjet base system. The copings were invested in phosphate-bonded investments (Z4-C&B
investment [and casted in Ni-Cr alloy (Nautilus CC plus]).
Conventional Specimens (2 Groups)
For wax patterns (control group), two layers of spacer (PICO-FIT; Renfert GmbH, Hilzingen,
Germany) were applied to the abutments starting 0.5 mm from the margin, with a total
thickness of approximately 30 μm. After drying, a layer of separating medium (Picosep;
Renfert GmbH, Hilzingen, Germany) was applied. The wax patterns were formed by inlay
wax (GEO classic; Renfert GmbH, Hilzingen, Germany), and based on silicone index obtained
from the first CAD/milling wax pattern. The marginal wax was reflowed before investing.
For acrylic patterns, two layers of spacer (Bredent; XPdent, Miami, United States)
were applied on abutments starting 0.5 mm from the margin for an approximate total
thickness of 30 µm. Acrylic resin patterns (GC Corp; Tokyo, Japan) were formed based
on the same silicone index. The wax and acrylic copings were invested in phosphate-bonded
investments (Z4-C&B investment) and casted in Ni-Cr alloy (Nautilus CC plus). Casting
sprues were separated from the models and the internal surfaces of the copings were
sandblasted (Basic master; Renfert GmbH, Hilzingen, Germany) by Al2 O3 particles (50 μm) under 0.3 MPa pressure.
The internal surface of each coping was evaluated by a binocular loop (HEINE HR-C
2.5x, HEINE, Herrsching, Germany) and visible macro nodules were removed with a tungsten
carbide bur (H71EF; Brasseler GmbH.KG, Komet, Siegel, Germany). Invisible nodules,
irregularities, or pressure points were determined using a disclosing agent (Occlude
indicator spray, Pascal International Inc, Seattle, Washington), and adjusted by round
bur (Teezkavan; Tehran, Iran) up to the point that complete siting was confirmed by
two prosthodontists blinded about materials/methods used for fabrication of the specimen.
The copings were stabilized on abutments by pressure-indicating paste (GC fit checker,
GC Corp, Tokyo, Japan), and the marginal discrepancy was measured in 12 points (middle
of buccal, mesial, distal, and lingual surfaces, and two points between each adjacent
pair) marked on acrylic base. Marginal discrepancy was measured by a noncontact video
measuring machine (AV350 + CNC; Starrett, Galileo Vision System, Birmingham, England)
with Heidenhain 0.1 micron resolution scale and 3-axis stage with 350 × 350 × 200
mm XYZ travel ([Fig. 2 ]). (SPSS Inc; Chicago, IL, United States) was used for statistical analysis. The
discrepancy values were reported in millimeter scale and analyzed by one-way ANOVA
and Tukey tests (p < 0.05).
Fig. 2 VMM measurement of casted specimen. VMM, video measuring machine.
Results
The (mean ± SD) for the marginal discrepancy of implant-supported frameworks fabricated
from CAD/milling hard metal, CAD/milling soft metal, CAD/milling wax patterns, CAD/RP
wax patterns, conventional wax patterns, and conventional acrylic pattern were 0.12
± 0.07 mm, 0.09 ± 0.06 mm, 0.11 ± 0.06 mm, 0.11 ± 0.04 mm, 0.20 ± 0.12 mm, and 0.12
± 0.07 mm respectively ([Fig. 3 ] [Table 2 ]). According to the one-way ANOVA test, there was a statistically significant difference
among the marginal discrepancy in six groups (p = 0.018). The Tukey test indicated a significant difference between CAD/milling soft
metal and control group (conventional wax patterns) (p = 0.011); a significant difference was also reported between CAD/milling wax patterns
and control group (p = 0.046).
Table 2
Descriptive data of different evaluated groups
Specimen
n
Minimum (mm)
Maximum (mm)
Mean (mm)
Std. deviation
Conventional wax
12
0.040
0.471
0.2035
0.1204
Conventional acrylic
12
0.050
0.273
0.1246
0.0690
CAD/milling hard metal
12
0.000
0.225
0.1234
0.0698
CAD/milling soft metal
12
0.000
0.205
0.0957
0.0673
CAD/milling wax
12
0.000
0.215
0.1116
0.0605
Rapid prototyped wax
12
0.020
0.204
0.1176
0.0476
Fig. 3 Mean marginal discrepancy (A) in different groups; (B ) separated by different measured points.
Discussion
The present study was conducted to compare the marginal discrepancies in single-unit,
implant-supported frameworks prepared by different methods/materials. The investigated
groups were CAD/milling hard metal, CAD/milling soft metal, and conventional casting
of patterns fabricated by CAD/milling wax, CAD/RP wax, and conventional hand-formed
wax (control) or acrylic resin. The null hypothesis was rejected as there was significant
difference between the specimens formed by different methods. Marginal discrepancy
was significantly less in CAD/milling soft metal and CAD/milling wax compared with
the control group. The sintering shrinkage of pre-sintered metal has been reported
in approximately 11% of cases.[37 ] However, the milling system compensates for dimensional change by milling a larger
pre-sintered coping; according to the result of the present study, it seems the compensation
worked well and the soft metal group showed the least marginal gap (95.7 µm ± 0.0673).
Wax pattern fabrication is a time-consuming and labor-intensive step which is highly
dependent on technician’s skill. It is also claimed that removing wax pattern from
a die with shoulder margin can lead to a margin opening of approximately 35 µm. Moreover,
wax color usually makes it difficult to detect small defects in wax patterns.[38 ] CAD/CAM restorations, on the other hand, reduce the effect of technician’s expertise;
however, their accuracy still depends on the computer software design, milling material,
and sintering shrinkage.[39 ]
[40 ] According to the present study, CAD/milling wax caused significantly less marginal
gap (111.6 µm ± 0.0605) compared with hand-formed wax pattern group (203.5 µm ± 0.1204).
All the fabrication methods were made directly on the abutments to eliminate the effect
of impression and pouring materials on the obtained results. Therefore, using the
same material (wax) and process (conventional casting), the result confirms the significant
effect of procedure (CAD/CAM vs. hand forming).
The present study result is inconsistent with the Vojdani[27 ] and Kim [29 ] studies. Furthermore, in a study by Farjood, the marginal discrepancy in the conventional
wax group was significantly less than that of the CAD/RP group.[25 ] On the other hand, Nejatidanesh[30 ] and Xu[33 ] reported smaller marginal discrepancies in the CAD/CAM compared with conventional
group which this study agrees with. Han[26 ] reported a significant difference between marginal adaptation in CAD/milling hard
metal and conventional wax-up group, while there was no significant difference between
these two groups in the present study. Conversely, Ghodsi reported that CAD/CAM technique
for wax milling led to better marginal adaptation rather than milling metal blocks,[31 ] while this study found no significant difference between these groups.
The controversial results could be explained by the effects of different factors on
the accuracy and adaptation of computer- or hand-made models. Several studies confirm
that different prostheses length,[41 ] materials,[31 ]
[42 ] finishing line configuration,[26 ]
[43 ] and even framework design,[44 ] and measurement method[31 ] could affect the accuracy obtained by different fabrication methods.
McLean and von Fraunhofer suggested that the clinically accepted marginal discrepancy
is 120 µm,[45 ] which means that the conventional wax patterns’ marginal adaptation in the present
study was not clinically acceptable; however, the marginal discrepancies in other
groups were within the acceptable range.
Evaluating the accuracy of different methods will help the clinician in finding the
best method according to the related situation in this growing world of science. The
present study measured vertical marginal discrepancies. However, not cementing the
specimens, not subjecting the specimens to thermal cycling or aging, and not performing
layering stage could be mentioned as study limitations. It is suggested to consider
horizontal marginal discrepancy and measure the adaptation both before and after cementation
to compare the difference and assess the effect of cementation on marginal gap.
Conclusion
Keeping in mind the limitations of this study, it can be concluded that the framework
fabricated by the conventional wax-up technique had, by far, the highest marginal
gap compared with the other methods. We also found that the marginal fit of framework
made by the CAD/CAM soft metal method was better than the other techniques. In addition,
frameworks fabricated by the RP method showed acceptable adaptation on the abutment
analogs.
