Laser-Sintered versus Conventional Cobalt-Chromium Bars in Implant-Supported Complete Mandibular Overdentures: An In Vitro Strain Analysis Comparative Study

Abstract

Objective The aim of this study was to compare the effect of laser-sintered and conventional cobalt-chromium bars on strain distribution around implants in mandibular implant-supported overdentures.

Materials and Methods A three-dimensional (3D) epoxy resin model for completely edentulous mandible was used for this study. Two implants were placed in the canine region, and the 3D model was scanned by a laboratory scanner. Twelve cobalt-chromium bars were divided into two groups. Group 1 includes six bars that were designed by software and created using the laser sintering method. Group 2 includes six bars that were made using computer-aided design and computer-aided manufacturing to make a wax model, fabricated by a conventional lost wax-casting technique. Each implant was surrounded by four channels in the epoxy resin model, and four strain gauges were cemented. A universal testing machine was used to apply the load, and a strain measuring analysis was performed.

Results Null hypothesis was rejected as there was significant difference in strain distribution around implants between both groups for the microstrains around the implants in bar-retained implant-supported prostheses, in both bilateral and unilateral results.

Conclusion The laser sintering method showed better results in comparison with conventional methods in fabrication of bar-retained implant-supported prostheses due to less trauma to the implant and supporting structures.

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:

• Group 1: Six bars made by laser sintering method.

• Group 2: Six bars made by wax lost method.

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.

Conflict of Interest

None declared.

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Publication History

Article published online:
27 January 2025