Download Analysis of the misfit of dental implant

Analysis of the misfit of dental
implant-supported prostheses made
with three manufacturing processes
Marc Fernández, MSc,a Luis Delgado, MSc,a
Meritxell Molmeneu, MSc,a David García, CDT,b and
Daniel Rodríguez, PhDc
Technical University of Catalonia, Barcelona, Spain
Statement of problem. The microgap between implant components has been associated with complications such as screw
loosening or adverse biologic responses.
Purpose. The purpose of this study was to quantify the microroughness of the mating surfaces of implant components
manufactured with different processes, to quantify the microgap between implant components, and to determine whether a
correlation exists between microroughness and the microgap.
Material and methods. Nine dental implants with a standard external connection were paired with 3 milled, 3 cast, and 3
sintered compatible cobalt-chromium alloy abutments. The abutment surface was examined, and the roughness parameter Sz
was measured by using a white-light interferometric microscope at 10 to 100 magnification. The abutment surface and
the microgap of the implant-abutment connection were observed with scanning electron microscopy, and the microgap width
was quantified from micrographs made of each implant-abutment pair. The mean and standard deviation of roughness and
microgap were evaluated. A 1-way ANOVA (a¼.05) was used to assess the influence of the manufacturing process on
roughness and microgap. The Pearson correlation was used to check dependence between roughness and microgap.
Results. The milled abutments possessed a connection geometry with defined edges and a mean roughness of 29 mm,
sintered abutments showed a blurred but functional connection with a roughness of 115 mm, and cast abutments showed a
connection with a loss of axial symmetry and a roughness of 98 mm. A strong correlation was found between the roughness
values on the mating surfaces and the microgap width.
Conclusions. The milled components were smoother than the cast or sintered components. A correlation was found between
surface roughness and microgap width. (J Prosthet Dent 2014;111:116-123)
Clinical Implications
The microgap of implant-abutment connections could be reduced with
smoother mating surfaces.
The long-term success of dental implants has been well established and is
extensively documented in relation to
biologic factors,1,2 surgical procedures,
and restorative principles that influence
the effectiveness of oral implants.3,4 In
spite of their success, dental implants
may present problems associated with
the loosening and fracture of the prosthetic screw that clamps the dental
prosthesis to the implant. The screw
tightens the prosthetic abutment to the
dental implant with controlled torque,
usually through an antirotation geometry, either protruded (external connection) or sunken (internal connection).5-7
The torque generates a force (preload)
in the screw equal in magnitude to
the clamping force of the abutment
to implant minus friction and local
deformation forces on the mating surfaces.8 Loosening or fracture of the screw
occurs when the joint that separates the
forces that act on the screw joint are
greater than the clamping forces that
hold the screw joint or greater than the
mechanical resistance of the screw.5,9-11
Screw-related failures can be associated with implant-abutment connection
misalignments.7,12,13 Different variables
a
Research Technician, Biomaterials, Biomechanics and Tissue Engineering Group, Department of Materials Science, Technical
University of Catalonia.
b
Private practice, Barcelona, Spain.
c
Assistant Professor, Biomaterials, Biomechanics and Tissue Engineering group, Department of Materials Science, Technical
University of Catalonia.
The Journal of Prosthetic Dentistry
Fernández et al
February 2014
have been studied as primary causes of
screw loosening, such as the errors accumulated in the multiple fabrication steps
required for an implant-supported fixed
prosthesis,14-16 the characteristics of the
materials used in the reconstructions,17-19
or the surface irregularities among the
mating surfaces of implant, abutment,
and screw.12,13,17,20 Surface irregularities
are closely related to the presence of a
microgap between implant components.
The microgap has been linked to periimplant inflammation and bacteria
infiltration.4,21,22
The fabrication and surface finishing
processes of the connection components are major factors of surface
roughness.23 Most implant components
are precision milled or cast, but new
manufacturing techniques, for example,
laser sintering, are becoming available.
The control of roughness on the mating
surfaces of implant components before
their use could reduce screw loosening
as well as the microgap between implant
components.24
Previous studies have analyzed the
microgap of the implant-abutment
connection by using an optical microscope and by scanning electron microscopy
(SEM).25-27 However, these methodologies
are not well suited for a fast and accurate
quantification of the misalignment between components or the microroughness
on mating surfaces with different fabrication processes. One of the techniques used
in quality-control processes that measures
surface roughness with adequate accuracy
and speed is coherence scanning interferometry (also known as white-light interferometry). This technique can produce
high-quality 3-dimensional surface maps
of a macroscopic surface with vertical
resolutions up to 0.1 nm.28
The purpose of this study was to
quantify the microroughness of the mating
surfaces of implant components manufactured with 3 different processes (milled,
laser sintered, and cast) with coherence
scanning interferometry, to quantify the
microgap between implant components
with SEM, and to determine whether a
correlation exists between microroughness
and microgap. The null hypotheses were
that no difference would be found in the
Fernández et al
117
surface roughness or microgap among
surfaces manufactured with different processes and no correlation would be found
between the surface roughness and
microgap of implant components.
MATERIAL AND METHODS
Nine dental implants with a standard
(4.1-mm diameter) external hexagonal
connection (Avinent Implant System;
Avinent) were paired with 9 compatible
cobalt-chromium alloy (CoCr) abutments. Three abutments were chosen
within the same production batch from 3
different manufacturing processes. Abutment groups were named according to
the abutment manufacturing process.
Milled abutments were manufactured
with a high-speed milling process (Core3dcentres; Core3d Protech SL) with
cutting tools that gradually shaped the
component by removing excess material.
Sintered abutments were processed from
raw material in powder form deposited in
the working tray (EOSINT; EOS GmbH
Electro Optical System). A temperature
increase of the powder in the working tray
Table I.
Quantitative abutment surface
analysis
The abutment surface was examined by
using a white-light microscope equipped
with Michelson/Mirau interferometric objectives (Wyko NT9300 Optical Profilometer; Veeco Instruments) in vertical
scanning interferometry mode at 10
to 100 magnification. Three measurements were made for each abutment specimen of each type of manufacturing
process. A matrix of 811 stitched images
Characteristics of specimens
Material
Manufacturing
Method
Applied
Torque,
Ncm
Milled
CoCr
High-speed milling
35
Sintered
CoCr
Laser sintering
35
CoCr
Casting
35
Process
Description
MIL
SIN
CAS
Cast
Group
in a given spot with a laser beam caused
the particles of the material to bind
locally, and, through repetition of the
process, the desired structure was created
layer by layer. Cast abutments were
fabricated with the lost wax casting technique (Laboratorio de Prótesis Dental
Garbident) by preparing the structure
with different entry channels and adding
reservoirs to improve solidification of the
material. The 9 abutments were each fixed
to an implant with a new screw and
tightened with a torque wrench at the
prescribed load (Table I).
MIL, milled abutment; CoCr, cobalt-chromium alloy; SIN, sintered abutment; CAS, cast abutment.
1 Scanning electron microscopy image of abutment
mating surface with area of analysis (10).
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Volume 111 Issue 2
of 635479 mm each, with 20% of overlapping, was performed, which defined an
analysis area of 4.54.5 mm. The surface
analyzed is shown as the area between the
dashed circles (Fig. 1). Data filtering and
analysis were performed with specific image
analysis software (Wyko Vision 32; Veeco
Instruments). A Gaussian filter was used to
eliminate tilt from every surface analysis.
The Sz roughness parameter (average
difference between the 5 highest peaks
and 5 lowest valley s) was evaluated for all
specimens28:
P5
Sz ¼
1 absðPeak
HeightsÞ þ
5
P5
1 absðValley
DepthsÞ
Qualitative abutment surface
evaluation
The abutment surface and the
microgap of the implant-abutment
connection for the 3 groups were
observed with SEM (6400 Scanning
Microscope; Jeol Ltd). The microgap
was observed at different magnifications, depending on the type of abutment (Fig. 2).
Microgap measurements
The microgap was measured by
using 5 SEM images made from
each implant-abutment pair. A given
implant-abutment pair was introduced
in the SEM with a position predefined by
the SEM holder. Once the microgap region was brought into the field of view, a
picture was made, and a predefined
axial rotation was applied to the unit.
The process was repeated until 5 regions
of interest were documented. Seventytwo measurements per image of the
microgap were evaluated with the aid of
image analysis software (OmniMet;
Buehler).
2 Scanning electron microscopy image of implant-abutment
gap. A, General view (15). B, Detailed view (500).
check the dependence between the Sz
parameter and the microgap. The P
value was calculated (a¼.05) for the
null hypotheses that no correlation
would be found between the surface
roughness and microgap of implant
components. All statistical analyses
were done with a statistical software
package (Minitab; Minitab Inc).
Statistical analysis
RESULTS
Means and standard deviations of
the Sz parameter and the microgap of
the 3 groups were determined. A 1-way
ANOVA (a¼.05) was used to assess the
influence of the manufacturing process
on the Sz parameter and the microgap.
The Pearson correlation was used to
of the components produced by
laser sintering and casting, with a mean
Sz of 29 mm (Fig. 3). The sintered
components had the highest Sz
(115 mm), and the cast components
had an Sz of 98 mm. Statistically
significant differences were found
among the 3 groups (milled, laser
sintered, and cast) (P¼.002). However,
no statistically significant differences
were found between the sintered and
cast components (P¼.269) (Fig. 4).
Quantitative surface analysis
Qualitative surface evaluation
The measurements made with the
interferometric equipment indicated
that the mating surface of the milled
components was smoother than that
The Journal of Prosthetic Dentistry
The milled abutment surface showed
accurate connection geometry with
defined edges (Fig. 5A). The sintered
Fernández et al
February 2014
119
3 Reconstructed surfaces with interferometric microscopy of abutment mating surfaces. A, Milled. B, Sintered. C, Cast.
abutment showed a blurred but functional connection (Fig. 5B). The cast
abutment, however, showed a connection that lacked axial symmetry
(Figs. 3C, 5C).
160
140
Sz (µm)
120
Microgap measurements
100
80
60
40
20
0
MIL
SIN
CAS
4 Roughness parameter Sz values of abutment mating
surfaces.
Fernández et al
The mean value of the microgap of
the implant-milled abutment system
measured 0.73 mm, which was smaller
than the gap of sintered (11.30 mm)
and cast (9.09 mm) abutments
(Fig. 6). SEM micrographs agreed with
the measured values (Fig. 7). A statistically significant difference of the factor “manufacturing technique” was
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Volume 111 Issue 2
5 Scanning electron microscopy images of abutment mating surfaces (10). A, Milled. B, Sintered. C, Cast.
25
was 0.96, which implied a strong correlation between microroughness on the
mating surfaces and the width of the
microgap. The P value for the null hypothesis was P<.001. Thus, the null
hypothesis was rejected, and a positive
significant relationship was identified
between microroughness and microgap
width.
Gap (µm)
20
15
10
5
0
DISCUSSION
MIL
SIN
CAS
6 Implant-abutment microgap values for each study
group.
found by 1-way ANOVA (P¼.01).
However, a post hoc test revealed no
statistically significant difference between the sintered and cast abutments
(P¼.26).
Correlation between roughness and
microgap
The Pearson correlation between
the Sz parameter and the microgap
The Journal of Prosthetic Dentistry
The null hypotheses were rejected
based on the results, which implied
that coherence scanning interferometry
can be used to measure surface irregularities on the mating surfaces of
implant components. Surface irregularities and connection misalignments
between implant components have
Fernández et al
February 2014
121
7 SEM images of implant (left)-abutment (right) microgap. A, Milled (2250). B, Sintered (750). C, Cast (750).
been considered as a possible cause
of mechanical complications such as
screw loosening and/or fractures.5,8
Achieving a passive fit (no shear
stresses at the connection level) is
important for implant success and is
generally considered acceptable for a
microgap of less than 10 mm.12,18,19,27
Microroughness on the mating surfaces of implant components plays
an important role on insertion forces
and the geometry of the connection
because the energy required to flatten
these irregularities affects the clamping
forces generated by the retaining
screw and the irregularities interfere
with a perfect contact between mating
surfaces.10,18,27
Fernández et al
White-light interferometry is a fast and
accurate technique for measuring the
surface microroughness of implant components and is suitable for use in the
quality control of implant components.
The analysis of the surface roughness of
abutments made with 3 manufacturing
techniques (milled, laser sintered, and
cast) with this method allowed the
differentiation of the surfaces based on
the measured roughness and their correlation with the microgap implantabutment measured by SEM.25
Some studies have reported different
variables that affect the implant-abutment
interface. Prosthetic frameworks, for
example, can present distortions due
to inaccuracies compounded by the
multiple fabrication steps that compromise the implant-abutment interface
fitting.14-16 The results of the present
study indicate that the manufacturing
technique is also a variable that influences the presence of a microgap,
probably because of the different
surface roughness produced by each
manufacturing method. A rough mating
surface inevitably produces a microgap
between implant and abutment and
hinders the achievement of a passive
fit.12,13,17,20 Milled surfaces have a better fit and a larger number of contacts
with the implant mating surface than
cast and sintered surfaces, which allows
a better closure of the microgap between
implant components.10-12
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The presence of a microgap seems to
allow bacterial infiltration, which leads to
clinical alterations in the periimplant
tissues.21 Although the mean values
measured in this study are less than12 mm,
reported values in other studies of
microgap sizes range from 0 to 135 mm,
with bacterial leakage reduced for the
minor microgap sizes.22 Therefore, a minor effect of bacteria on surrounding tissues is more likely for milled components
than for cast or sintered ones.
Statistically significant results were
obtained from this study. A possible
limitation of the results, however, is
related to the number of specimens
included. No power analysis was performed to determine adequate sample
size because the inclusion of more
specimens would have posed significant
practical difficulties. The sample size was
similar to those of other studies of
roughness and microgap,23,24 and
the compelling correlation between surface roughness and microgap is an
applicable result of this study.
Another possible limitation of this
study is that the results were established
with external connection implants, in
which the contact geometry of the
microgap is evident. The results, however,
may be extended to dental implants with
internal connection, because the closure
of the microgap is closely related to the
flattening of the surface irregularities of
the mating surfaces of implant, screw,
and abutment.10
The results of the present study were
measured on cobalt-chromium specimens but are presumably related to the
manufacturing process, not only to
the characteristics of the material used in
the prostheses.16,17 The effect of the
processing techniques on other materials
could also affect the surface roughness
and connection misfit, thereby affecting
the microgap.18,19
Further investigations could look for a
possible correlation among the materials,
roughness of mating surfaces, microgap
presence, and torque forces applied to
implant components. Because existing
studies are not conclusive, future studies
could also focus on the possible clinical
effects of abutments fabricated with the
3 methods to see if the differences found
in the present study have clinical
relevance.21,22
CONCLUSIONS
Within the limitations of this study,
the following conclusions were drawn:
1. Coherence scanning interferometry can measure the surface roughness
of components manufactured with
different processes with sufficient accuracy (P¼.002) to differentiate them.
2. Milled implant components have
smoother surfaces than cast or sintered
components.
3. Surface roughness correlates
(P<.001) with the microgap width between external connection implants and
abutments.
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123
Corresponding author:
Dr Daniel Rodríguez
Department of Materials Science e Pavelló E
ETSEIB e UPC
Av. Diagonal 647
08028 Barcelona
SPAIN
Acknowledgments
The authors thank Avinent Implant System S.L.
and Core3d Centres for providing the implants
and abutments used for this study and
Laboratorio de Prótesis Dental Garbident
for providing the prosthetic definitive
restorations.
Copyright ª 2014 by the Editorial Council for
The Journal of Prosthetic Dentistry.
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