Keywords cobalt-chromium bars - implant supported overdenture - direct 3D printing - strain
gauge analysis
Introduction
Edentulism or complete tooth loss can be due to periodontal disease, abscess formation,
trauma, and vertical tooth fracture. It is a common oral health problem that affects
people all around the world, especially the elderly. It affects the person's physiological,
aesthetic, social, and psychological health.[1 ]
[2 ]
Edentulism rehabilitation increases quality of life and lowers morbidity. Most of
the studies included in a recent systematic review on the rehabilitation of edentulism
and mortality found a higher proportion of deceased edentulous patients not wearing
dentures than denture users.[3 ]
Many alternative treatment plans have been used for the treatment of edentulism. The
most common method is removable complete denture; also, conventional complete denture
prostheses face several problems, such as poor retention, insufficient stability,
denture sores, severe pain, and discomfort, resulting in mastication, speech, nutritional,
and functional defects over a period of time. The other method is dental implants,
which are an excellent choice for retention and support. As a result, implant-supported
overdentures are preferable to traditional complete dentures use.[4 ]
Implants are connected to overdentures by a variety of attachment systems. The original
purpose of attachment systems was to improve the retention and stability of tooth-supported
overdentures. The systems are either nonsplinted attachments that are connected directly
to the implant (magnet, ball, locator, telescopic) or splinted attachments that use
a bar and a bar-clip attachment to splint the implant together.[5 ]
Attachment systems have traditionally been made of base metal alloys with straight
forward castability, low cost, and excellent corrosion resistance, particularly cobalt-chromium
(Co-Cr).[6 ] Bar attachments can be made using computer-aided design/computer-aided manufacturing
(CAD/CAM) or the traditional method of creating a wax pattern on the master cast,
investing, and casting (lost wax technique).[7 ]
Dental alloys built on cobalt and chromium elements are one of the most often used
for manufacturing dental construction metal frameworks because they are relatively
inexpensive and have high levels of biocompatibility, strength, corrosion, and wear
resistance. Co-Cr dental alloys have the following chemical composition: small quantities
of W, Si, Al, and others; 2 to 6% of Mo; 25 to 32% of Cr; and 53–to 67% of Co. To
reinforce the solid solution, Mo, W, and Cr are added. Due to the relatively high
amount of Cr, carbides are generated in the microstructure of the details, as well
as a dense passive layer of Cr2O3 with a thickness of 1 to 4 nm on the surface, determining
the high corrosion, wear resistance, and hardness.[8 ]
Laser-sintering is an additive construction technology used in prosthetic dentistry
and is one of the quickest prototyping systems. The working idea is based on the layer-by-layer
consolidation of microscopic, powdered particles utilizing a high-power laser source
to produce complicated parts from three-dimensional (3D) data.[9 ]
In dental research, strain gauges are a well-established tool for measuring strain
distribution around implant-supported overdentures.[10 ]
Strain gauges (strain gauge amplifier [SGA]) as a numerical technique are sensitive
to minor differences in restorative materials and the surrounding environment. Alternative
techniques, such as photoelasticity, augment the findings from SGA, confirming that
the gauges accurately measure high voltages in places of elevated stress. A multitude
of recommendations have been disseminated due to the requirement for standardized
laboratory experiments, facilitating the comparison of findings with another research.
On the other hand, finite element analysis is simply a simulation and a simplification,
it is challenging to apply it to actual biomechanical processes in the human body.[11 ]
Strain gauges come in a variety of forms, including those made of metal foil, semiconductors,
and optical fibers. Optical fiber strain gauges are less sensitive to electromagnetic
noise and have great linearity and reproducibility.[10 ]
The objective of this study was to compare laser-sintered and conventional Co-Cr bars
in mandibular implant-supported overdenture through measuring the resulted strain
distribution around implants using strain gauge analysis.
Null Hypothesis
The null hypothesis was that there was no significant difference in strain distribution
around implants between laser-sintered and conventional Co-Cr bars in mandibular implant-supported
overdenture.
Materials and Methods
This study is an in vitro comparative study. It was carried out on 12 Co-Cr bars according
to the calculated sample size.
Groups (n = 6/group) according to the method of fabrication are as follows:
Model Preparation
This research used a 3d printer was used to fabricate 3D model of mandible that resemble
a human mandible. To replicate resilient edentulous ridge mucosa, a 2-mm thick layer
of resilient silicone soft lining material was used.[10 ]
Two implant holes are made in the lower canine areas on both sides, parallel to each
other. Each implant is surrounded by four channels, one on each side. These channels
run parallel to the implant's long axis and contain the strain gauge rosettes (Kyowa
strain gauges [KFGS-1-120-C1-11-L1M2R]) ([Fig. 1 ]).
Fig. 1 Three-dimensional (3D)-printed cast with two implant holes.
Provide 2 mm of epoxy resin between the strain gauge and the implant based on the
predicted depth.
Implant Insertion
Frontier titanium implants of sizes 3.75*13 mm were inserted into their locations.
The spacer was constructed with molding wax, and then the special tray was constructed
with cold-cured acrylic material.
Then the spacer was removed, the surface area under the spacer was scratched, and
the implant sites were covered with wax. Then, chairside soft relining—Long-Term (Mollosil)—was
applied to create artificial soft tissue that mimics the oral mucosa's soft tissue
([Fig. 2 ]).
Fig. 2 Application of chairside soft relining.
Frontier Titanium Base was placed into implant sites, and Frontier Scan Body was inserted
in their sites. Then, the dental bar coupled to two abutments was designed using a
specialized software (EXOCAD Dental DB V2.4-7341 Plovdiv) after a 3D model was scanned
with a laboratory scanner (desktop Ceramill Map400 lab scanner) ([Fig. 3 ]).
Fig. 3 Design of dental bar on cast.
Then, 12 bars were fabricated from Co-Cr alloy by two different methods in two groups.
Group 1 bars were fabricated by a laser sintering machine (VULCANTEC VM120); group
2 used a CAD/CAM milling machine (Roland-DGSHAPE DWX-52D) to make a wax model; and
then bars were fabricated by the wax lost casting technique ([Figs. 4 ] and [5 ]).
Fig. 4 Wax bars attached to a sprue.
Fig. 5 Group 2 bars after casting.
Overdenture Fabrication
An acrylic complete mandibular overdenture was fabricated according to a conventional
technique (impression, pouring the model, fabricating the bite rim, setting up the
teeth, waxing, flasking, curing, finishing, and polishing) on a 3D-printed cast model
to mimic the clinical state of an edentulous patient with a complete mandibular overdenture.
After the bar is installed, to create the denture, the cast is reproduced using a
dental stone ([Fig. 6 ]). The special tray was built using the CAVIX shellac baseplate, and a final overdenture
was built. In the center of the bar, a single sleeve (Rhein83) was inserted. A separate
medium was applied to the entire bar, except for the middle sleeve, and then self-acrylic
resin was applied to the relieved area of the denture's fitting surface and allowed
to set completely.
Fig. 6 Final overdenture.
Strain Gauge Installation
At the different surfaces of each implant, four strain gauges (Kyowa strain gauges
[KFGS-1-120-C1-11-L1M2R]) were cemented ([Fig. 7 ]).[12 ] The microstrain in the area around the implants was measured. Four strain gauges
around the implant are used to monitor the effect of the applied loads, vertical and
oblique, and one gauge for each channel at the epoxy model over the flat surface of
the buccal, lingual, mesial, and distal channel around each implant inserted in the
epoxy model.[13 ]
Fig. 7 Frontal view after installations of strain gauge rosettes.
The denture was positioned on the cast, then the right side was coded with the letter
(E), and the left side was coded with the letter (W). Each strain gauge's wire was
coded so that it could be recognized during measurements. The wires were coded according
to cast coding and the initial letter of each direction around the implant, for example,
E B means right side of the cast in the buccal direction ([Fig. 8 ]). Then, all the wires were linked to the strain gauge material, KYOWA (Japan), Type
PCD-300A.
Fig. 8 Wire coding.
A universal testing machine (LLOYD LR 5K) was used to provide compressive static force
to the denture (vertical and oblique) using a specific rounded rod applicator attached
to the universal testing machine's upper head.
At the central fossa of the right first molar, a 100 N (as a similar load magnitude
has been measured during normal chewing) load with a 0.5-mm^min constant rate was
delivered unilaterally ([Fig. 9 ]). Following that, the right and left first molars had metal rods positioned at their
occlusal surfaces, and the metal rods received the same load ([Fig. 10 ]). Then, the model was obliqued 30 degrees by using the dental surveyor. At the central
fossa of the first right molar, 65 N with a 0.5-mm^min constant rate was delivered
unilaterally. The metal rod positioned at the right and left first molars at their
occlusal surfaces was loaded. A load was applied to each testing bar, and strain gauges
were used to record microstrains. The strain distribution around the implant was analyzed
statistically.[12 ]
[13 ]
[14 ]
Fig. 9 Horizontal, unilateral load.
Fig. 10 Horizontal, bilateral load.
Results
All data were calculated, tabulated, and statistically analyzed.
The results confirmed that there was a significant difference between the laser sintering
method and the wax loss method for the microstrains around the implants in bar-retained
implant-supported prostheses.
Laser Sintering Method–Horizontal
[Table 1 ] shows the comparison between the microstrains around the implants in the laser sintering
method group under horizontal loading on the right and left sides in bilateral and
unilateral.
Table 1
Mean and SD of the microstrains around the implants in group 1 under horizontal loading
on the right and left sides in bilateral and unilateral
Bilateral
Unilateral
Independent t -test
p -Value
< 0.05
Right
83.86 ± 18.85
114.10 ± 14.71
7.39
< 0.001[a ]
Left
36.42 ± 22.07
12.63 ± 8.23
3.79
0.019[a ]
Independent t -test
3.63
5.65
p -Value
< 0.05
0.001[a ]
0.005[a ]
Abbreviation: SD, standard deviation.
a Means significant difference at p < 0.05.
The statistical analysis showed a significant difference between bilateral and unilateral
on the right and left sides, However, the microstrains around the implants recorded
a higher value in unilateral than bilateral on the right side, while the microstrains
around the implants on the left side gave a higher value than unilateral using an
independent t -test at a p -value of < 0.05 ([Table 1 ] and [Fig. 11 ]).
Fig. 11 Microstrains around the implants in the laser sintering method group under horizontal
loading on the right and left sides in bilateral and unilateral.
Laser Sintering Method–Oblique
[Table 2 ] shows the comparison between the microstrains around the implants in the laser sintering
method group under oblique loading on the right and left sides in bilateral and unilateral.
Statistical analysis showed a significant difference between bilateral and unilateral
in the right and left sides, the microstrains around the implants recorded high values
in unilateral than bilateral (21.26 ± 7.81) on the right side, while the microstrains
around the implants on the left side in bilateral gave a higher value than unilateral
using an independent t -test at a p -value of < 0.05 ([Table 2 ] and [Fig. 12 ]).
Table 2
Mean and SD of the microstrains around the implants in group 1 under oblique loading
on the right and left sides in bilateral and unilateral
Bilateral
Unilateral
Independent t -test
p -Value
< 0.05
Right
21.26 ± 7.81
37.93 ± 6.27
2.88
0.04
Left
16.0 ± 4.62
8.09 ± 1.07
2.89
0.03[a ]
Independent t -test
1.003
8.12
p -Value
< 0.05
0.373 ns
0.001[a ]
Abbreviations: ns, not significant; SD, standard deviation.
a Means significant difference at p < 0.05.
Fig. 12 Microstrains around the implants in the laser sintering method group under oblique
loading on the right and left sides in bilateral and unilateral.
Wax Lost Method Group–Horizontal
[Table 3 ] shows the comparison between the microstrains around the implants in the wax loss
method group under horizontal loading on the right and left sides, both bilateral
and unilateral. The statistical analysis showed a significant difference between bilateral
and unilateral on the right and left sides. the microstrains around the implants recorded
a higher value in unilateral than bilateral on the right side, while the microstrains
around the implants on the left side gave a higher value than unilateral using an
independent t -test at a p -value of < 0.05 ([Table 3 ] and [Fig. 13 ]).
Table 3
Mean and SD of the microstrains around the implants in group 2 under horizontal loading
on the right and left sides in bilateral and unilateral
Bilateral
Unilateral
Independent t -test
p -Value
< 0.05
Right
117.42 ± 8.13
178.63 ± 18.18
3.27
0.024[a ]
Left
63.05 ± 3.01
43.75 ± 9.12
2.19
0.039[a ]
Independent t -test
6.47
5.35
p -Value
< 0.05
< 0.001[a ]
< 0.001[a ]
Abbreviation: SD, standard deviation.
a Means significant difference at p < 0.05.
Fig. 13 Microstrains around the implants in the wax lost method group under horizontal loading
on the right and left sides in bilateral and unilateral.
Wax Lost Method Group–Oblique
[Table 4 ] shows the comparison between the microstrains around the implants in the wax lost
method group under oblique loading on the right and left sides in bilateral and unilateral.
Statistical analysis showed a significant difference between bilateral and unilateral
on the right side and no significant difference on the left sides; however, the microstrains
around the implants recorded a higher value in unilateral than bilateral on the right
side, while the microstrains around the implants on the left side, the bilateral,
gave a higher value than unilateral using an independent t -test at ([Table 4 ] and [Fig. 14 ]).
Table 4
Mean and SD of the microstrains around the implants in group 2 under oblique loading
on the right and left sides in bilateral and unilateral
Bilateral
Unilateral
Independent t -test
p -Value
< 0.05
Right
32.53 ± 10.94
57.37 ± 6.41
3.392
0.027[a ]
Left
25.24 ± 8.71
13.21 ± 7.86
1.77
0.151
Independent t -test
0.902
7.536
p -Value
< 0.05
0.418 ns
< 0.001*
Abbreviations: ns, not significant; SD, standard deviation.
a Means significant difference at p < 0.05.
Fig. 14 Microstrains around the implants in the wax lost method group under oblique loading
on the right and left sides in bilateral and unilateral.
Discussion
In comparison with traditional complete dentures, the stability of the mandibular
denture with two interforaminal implants has produced consistent and more predictable
prosthodontic results.[12 ]
The stress on the bone surrounding the implant is one of the main causes of implant
treatment failure, so it is important to identify the variables that have an impact
on the strain and stress values. According to Misch, implant splinting with the bar
attachment reduces loading pressures, particularly when the implants are positioned
at the same occlusal height, an equal distance from the midline, and with a similar
angulation.[13 ]
Co-Cr was chosen as the bar's material because of its exceptional strength. Additionally,
the long component life of the ball components made possible by this alloy's low tribological
wear rate eliminates the requirement for routine component replacement.[14 ]
Instead of using traditional wax pattern production techniques, milling wax was used
in this study to mill the framework of the wax design. As a result, the fitness was
improved and technician errors decreased.[15 ]
In this study, an ascending load was delivered using a digital loading machine. It
is simple to use and digital and it provides quick data capture, precise position
measurement, and full computer integration. Five minutes had to pass between readings
to allow for the strain gauge sensors' heat to dissipate to ensure the accuracy of
the results.[16 ]
The first molar was selected as the location of load application since most of the
forces are centered here. The load magnitude was set at 100 N because it was determined
that this was the region's typical biting force in earlier studies.[17 ] To allow bilateral force distribution a metal bar was used.[18 ]
Stress analysis in complex geometries like bone and dental implant systems presents
challenges, as overstressing the surrounding bone could lead to bone resorption and
implant failure. Therefore, it is crucial to investigate the strain resulting field
in the surrounding bone.[19 ]
Following Yoo et al's assessment of stress distribution, a vertical load was applied
since the implants in mandibular implant overdentures appear to transfer stress through
vertical stress forces.[20 ]
Strains were measured using strain gauges because they are small, linear, and cause
less interference during testing. Another study was conducted using the strain gauge
technology mainly because it was said to be a reliable system with few issues.[21 ] The strain gauges measure strain into a loaded structure by translating the change
in electrical wire resistance into strain measurement. Although a disadvantage of
using strain gauges is the high technique sensitivity of the method.[10 ]
In the present study, unilateral force was applied to simulate each patient's favorite
chewing side, and bilateral stress was applied to simulate bilateral chewing.[22 ]
In agreement with El-Abd et al, the study examined bilateral loading at the occlusal
surface of first molars and unilateral force on the central fossa of the first molar
of a mandibular overdenture retained by the implant.[17 ]
[23 ]
The closer the implant is to the load application, the more strain is transmitted
to it, providing an explanation for the statistically significant increase in microstrain
records on the loaded side for both the study groups. According to the study findings,
when posterior loads are applied, the overdenture tends to rotate anteriorly around
a fulcrum line. The denture disengaged from the unloaded side because of this rotation,
which reduced the microstrain communicated to the implant on the unloaded side.[23 ]
[24 ]
Another study stated that for both bar groups, the loaded side experienced greater
strain than the unloaded side, which is agreeable with the results of earlier studies.[23 ]
[25 ]
[26 ] When comparing the microstrain mean values of both groups while applying strain
bilaterally, the study has shown that there was a statistically significant difference
in the mean values of microstrain between the conventional (lost wax technique) group
and the laser-sintered group on the nonloading side, where the highest mean value
of microstrain was found in the conventional bar group (lost wax technique), while
the least mean value of microstrain was found in the laser-sintered group.
Additionally, statistical analysis of the microstrain mean values of both groups while
applying strain bilaterally showed a statistically significant difference in the mean
of microstrain between the loaded-side conventional bars (lost wax technique) group
and the laser-sintered group, where the highest mean value of microstrain was found
in the conventional bars (lost wax technique) group and the least mean value of microstrain
was found in the laser-sintered group.
In this study, the conventional bars (lost wax technique) group showed higher values
of microstrains in comparison with the laser-sintered group in both unilateral and
bilateral directions. Zhou et al evaluated the mechanical properties and microstructures
of cast, machined, and selective laser melting additive manufacturing (AM) Co-Cr dental
alloys, revealing enhanced mechanical properties and microstructure in the AM group.[27 ]
[28 ]
[29 ]
The tested Co-Cr alloys' mechanical characteristics were affected by the manufacturing
process. Compared with casting and milling, laser sintering demonstrated superior
mechanical characteristics.[30 ]
One of the microstructural elements that influence the mechanical properties of Co-Cr
alloys is grain size. The laser-sintered Co-Cr specimens had smaller grain sizes,
a more uniform and solid structure, and fewer porosities and flaws than the cast specimens
did. Smaller grain size (< 35 μm) means additional grain barriers that can stop the
movement of grains during dislocation, resulting in grain boundary strengthening,
which increases strength. Consequently, the cast Co-Cr specimens' large grain size,
which ranges from many hundreds of micrometers to millimeters, leads to the low hardness
ratings of these materials. In contrast, the laser-sintered Co-Cr specimens' tiny
grain size and high presence of minute precipitates may explain their greater strength,
either by grain boundary strengthening or precipitation hardening.[31 ]
[32 ]
[33 ]
[34 ]
When compared with the as-cast state, the direct metal laser sintering (DMLS) samples'
improved strength can be due to tiny hills with a finer microstructure formed because
of fast solidification and local melting and the varying ratio of the cubic (γ) and
hexagonal (ε) phases of the Co-Cr alloy. Compared with traditional casting of Co-Cr
alloys, laser sintering of dental Co-Cr alloys yields removable partial denture (RPD)
frameworks with good mechanical properties.[28 ]
[35 ] According to studies published in the literature, laser sintering had a positive
impact on mechanical characteristics and microstructure evolution.[35 ]
[36 ]
The study compared patient satisfaction with traditional and laser sintering RPDs,
finding that the selective laser sintering (SLS) method resulted in a higher level
of satisfaction. The study found that SLS-based RPD significantly improved their mastication
and speaking skills, making it more effective, retentive, stable, and comfortable.
The enhanced mechanical properties of laser-sintered alloys, which are tougher, denser,
and demonstrate superior microstructural organization with higher yield strength and
ultimate tensile strength than cast Co-Cr alloys, may be the cause of this large difference.[19 ]
[37 ]
Limitations
- Strain measurement limitations: It does not resemble accurate stresses and strains
that occur in patients wearing implants overdenture. Strain gauges are sensitive to
placement and calibration.
- This short-term study results require further long-term study.
- Laser sintering techniques clinically are still cost-effective more than lost wax
techniques.
Conclusion
Within the limitations of this study, it could be concluded that:
Compared with bilateral loading, unilateral loading was more traumatic to the implants
regarding the bar fabrication method, as bilateral loading offers a wide distribution
of stain.
In conventional Co-Cr bars, unilateral loading was more traumatic to the implants
than in laser-sintered Co-Cr bars.
Conventional wax loss methods can be considered the last choice in bar fabrication,
especially those used in implant-supported overdentures, due to the high strains they
produce to implants and implant-supporting structures.
The laser sintering method is better than conventional wax loss methods when used
to fabricate bars in bar-retained implant-supported prostheses because it is less
traumatic to the implant and supporting structures. So, this leads to more conservation
and durability of the implants.
