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 Table of Contents  
RESEARCH
Year : 2022  |  Volume : 22  |  Issue : 3  |  Page : 240-248

Evaluation and comparison of vertical marginal fit of three different types of multiunit screw-retained framework fabricated for an implant-supported prosthesis – An in vitro study


1 Department of Prosthodontics, Faculty of Dental Sciences, SGT University, Gurgaon, Haryana, India
2 Department of Oral Pathology, Faculty of Dental Sciences, SGT University, Gurgaon, Haryana, India

Date of Submission20-Jan-2022
Date of Decision24-Apr-2022
Date of Acceptance26-Apr-2022
Date of Web Publication18-Jul-2022

Correspondence Address:
Bhupender Kumar Yadav
House No. 1358, Sector 10 A, Gurgaon - 122 001, Haryana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jips.jips_28_22

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  Abstract 


Aim: The present study aimed to evaluate on a comparative basis the vertical marginal fit between conventionally casted, direct metal laser sintered (DMLS), and milled computer-aided design/computer-aided manufacturing (CAD-CAM) one-piece metal framework supported by five implants using one-screw test and screw resistance test.
Settings and Design: This is an in vitro study.
Materials and Methods: Five implants were placed parallel to one other in a Styrofoam master model. A total of 30 implant-supported screw-retained superstructures were manufactured using three techniques, i.e., conventionally casted, milled, and sintered. To evaluate the vertical marginal discrepancy, screw resistance test, and one-screw test were used, and measurements were made using a stereomicroscope.
Statistical Analysis Used: The data was analysed using two statistical tests, i.e., ANOVA and the post hoc Bonferroni test.
Results: On evaluating the frameworks using one-screw test, the mean vertical misfit value at the terminal implant for the control group was 292.58 ± 15.46μm, for conventionally casted framework 398.41 ± 21.13 μm, for DMLS 343.44 ± 24.73 μm, and for CAD-CAM was 304.03 ± 14.23 μm, whereas the average misfit values at four implants on applying screw resistance test were 1268.65 ± 84.24 (control), 1774.88 ± 67.70 (casted), 1508.02 ± 62.19 (DMLS), and 1367.29 ± 81.87 (CAD-CAM). The average misfit values on two implants using screw resistance test were 635.02 ± 57.33 for the control group; for conventionally casted, it was 879.75 ± 35.93; for (DMLS) framework, it was 761.51 ± 32.85; and for milled CAD-CAM framework, it was 687.07 ± 42.17 μm.
Conclusion: The mean vertical marginal discrepancy, when compared with control, was least in milled CAD-CAM frameworks, followed by sintered DMLS and conventionally casted frameworks. Hence, according to the present study, CAD/CAM technique is recommended to achieve maximum marginal fit in full mouth screw-retained implant-supported FDPs.

Keywords: Computer-aided design/computer-aided manufacturing, casting, DMLS, implants, screw-retained


How to cite this article:
Singh M, Yadav BK, Phukela SS, Ritwal P, Nagpal A, Saluja P. Evaluation and comparison of vertical marginal fit of three different types of multiunit screw-retained framework fabricated for an implant-supported prosthesis – An in vitro study. J Indian Prosthodont Soc 2022;22:240-8

How to cite this URL:
Singh M, Yadav BK, Phukela SS, Ritwal P, Nagpal A, Saluja P. Evaluation and comparison of vertical marginal fit of three different types of multiunit screw-retained framework fabricated for an implant-supported prosthesis – An in vitro study. J Indian Prosthodont Soc [serial online] 2022 [cited 2022 Aug 10];22:240-8. Available from: https://www.j-ips.org/text.asp?2022/22/3/240/351277




  Introduction Top


The marginal gap present at the interface of frameworks and the underlying implant body is critical for a successful long-term osseointegration. Inaccurate implant superstructures can result in various mechanical complications such as abutment or prosthesis screw loosening or breakage of different components in the system. It can also result in biological complications of the surrounding tissue such as loss of osseointegration, swelling, pain, inflammation, and crestal bone loss.[1] A poor marginal seal at the implant framework junction may further propagate compressive, tensile, and shear load, resulting in failure of the restoration or failure of the underlying implants.

An ill-fitted framework can create a marginal gap between the superstructure and the implant or abutment, leading to plaque accumulation and biological complications.[2] In multiunit implant-supported prostheses, achieving marginal fit is more complex and arduous. On the other hand, it is easier to attain passive fit in cement-retained implant prostheses because of 40 μm cement space provided by the die spacer.[3] For overlying implant frameworks, the passive fit is relatively important. Passive frameworks are assumed to transfer less pathological forces on the supporting structures. Establishing a passive fit among the superstructures and implants screw-retained multiunit implant prosthesis will prevent stress transformation from the framework to the implant body and the underlying and surrounding bone. It is achieved with the precise adaptation between the superstructures and the implants, without any marginal gap formation. Although the influence of passive fit on bone response has not yet been demonstrated in experimental Vivo studies, there seems to be a consensus on the importance of passive fit between dental implant components and the superstructure. The osseointegrated implants have no resilience in the bone, and consequently, bony tissues do not adapt to a misfitting framework without generating stress in the bone and the metal framework.[4]

The primary approach for metal framework fabrication was the “lost wax technique.” This technique entails many inevitable procedures, armamentariums, and materials that can cause inaccuracies in the final framework. Many methods have been described in the literature to improve implant framework fit, which can be divided into two main categories, first is the refinement of fit through adding certain steps such as the use of cement-retained restorations, sectioning and soldering the framework, horizontal sectioning, and laser welding or vertical welding with use of the CrescoTi Precision TM technique.

The second is eliminating specific fabrication steps by utilizing modern technology such as computer-aided design/computer-aided manufacturing (CAD-CAM). The ability of this technique to improve implant-supported FDP's accuracy is achieved by skipping conventional manufacturing steps, such as the impression of the prepared tooth, wax frameworks, investing, and finally casting in metal alloys. Specific steps such as intraoral or laboratory scanning, designing through advanced software, multiple axis milling, and material processing make the CAD/CAM technique imprecise.[5],[6],[7] In a comprehensive review by Abduo et al.[7] in 2014, on the vertical marginal fit of CAD/CAM implant superstructures, the authors concluded that the accuracy of CAD/CAM implant superstructure was significantly better than that of the conventionally casted one-piece frameworks and the sectioned and laser-welded frameworks.

Direct metal laser sintering (DMLS), also known as “Three dimensional (3D) printing,” is a comparatively recent technology for the fabrication of dental prostheses. DMLS technique utilizes a process in which metal dental frameworks are built using a high-powered laser beam focused onto a bed of the Co-Cr alloy powder and subsequently welding it together into thin solid layers of around 0.020 mm, followed by cooling. In DMLS, it is easier to manufacture complicated angular designs and structures, which are otherwise difficult or impossible in subtractive (machining) technologies. Hence, it is expected that this method is superior to milling. Many studies have suggested that the DMLS technique has a promising future and can be a possible alternative to the conventional casting technique.[8]

Even though the concept of conventional casting, additive (DMLS), and subtractive manufacturing (CAD-CAM) technologies for implant and biomaterial manufacturing is well accepted, there are still limited data available on the comparison of these three techniques in the current scientific literature. Hence, the present study aims to evaluate on a comparative basis the marginal fit and flexural strength between conventionally casted screw-retained, direct metal laser sintered (DMLS) screw-retained and milled CAD-CAM one-piece metal framework supported by five implants using one-screw test and screw resistance test.


  Methodology Top


Three manufacturing techniques for the fabrication of screw-retained implant prosthesis, i.e., conventional casting, CAD-CAM, and DMLS, were compared in the present study. A total of 30 frameworks were fabricated, 10 each from 3 techniques. The study was approved by the Institutional Review Board (SGTU/FDS/MDS/24/1/547).

A Styrofoam edentulous mandibular model was used as a master model. Five conical hex regular platform implants (4 mm × 10 mm, super line, Dentium implants, Buk-su, Daegu, Korea) were inserted in the model [Figure 1]. A surveyor was used to place the implants parallel to one another and perpendicular to the horizontal crestal plane. Implants were placed supra crestal and marked as 1, 2, 3, 4, and 5 from right to left. Two posterior implants, i.e., 1 and 5 were placed in the mandibular first molar region. Two implants, i.e., 2 and 4, were placed in 1st premolar region on both sides, and one implant, i.e., 3 was placed in the midline. Osteotomy was drilled in 1, 2, 3, 4, and 5 regions in Styrofoam up to 4.0 mm drill conventionally using Dentium Implant kit. 4 mm × 10 mm implants (Dentium Dental Implants, Korea) were placed in the drilled site on the master model. A tiny dot was engraved on the facial edge of all five implants using a laser (20W, Fiber Laser) which will act as a common reference point to help measure the marginal discrepancy under a stereomicroscope.
Figure 1: Implants placed in model

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Five abutments with Ti bases (conical hex, UCLA Abutment; Dentium) were attached to implants on the master model and were splinted with pattern resin and ligature wires. After setting, the framework was resectioned and splinted again with pattern resin to compensate for the polymerization shrinkage. The framework formed on the master model will act as the standard control framework to be compared with the framework created from the three different techniques.

A special tray with double spacer and 3 stoppers was fabricated over the master model; holes were made in the special tray for open tray impression coping corresponding to implant positions. Five direct transfer copings (4 mm hex, open tray; Dentium) were used to make the impression. The open tray impression copings were splinted with ligature wire and red auto polymerized pattern resin. Splinting was done to stabilize the open tray transfer copings and prevent displacement while making the impression. Impression was made using polyether impression material (Impregum F; 3M ESPE) with medium body consistency. The special tray which was used to take the impression was coated with tray adhesive (Polyether Adhesive; 3M ESPE), and the material was manipulated in a pentamix auto mixer. Then, some material was loaded into the syringe and transferred to the abutment implant areas, and the rest was put on the custom tray to make the final impression. Following the setting of the impression, the screws of the transfer copings were loosened, and the model was separated from the impression. Implant analogs were attached to the open tray transfer copings [Figure 2], and Type IV die stone was mixed and poured into the impression to obtain a working cast.
Figure 2: Polyether impression with analogues

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Five UCLA abutments with Ti base (4 mm, conical hex, Dentium) were secured to the implant analogs for wax-up of the superstructure. The wax framework, along with the working cast, was dispatched to a milling center (Dentcare Kerala). The framework was initially scanned using a 3D laser scanner (3Shape E 3). After obtaining the images following the scanning, the framework was designed using 3D software [Figure 3]. In this way, a CAD file of the framework was obtained, following which milling was done to manufacture the frameworks from Co-Cr metal blocks using a high-speed five-axis machine (3Shape Dental System 2012). Ten frameworks were milled in a similar fashion using the same machine with the same settings to minimize the bias in the manufacturing process.
Figure 3: Computer-aided design designing for the framework

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DMLS sintered frameworks were manufactured using an AM250 laser melting machine (Reni Shaw plc.) using ASTM75 Co-Cr powder. During the sintering process, the powdered metal without binder and flux was sintered by scanning with a high-power laser beam at 20 or 40 μm per layer. Following the sintering of the first layer, the recoater arm of the machine swept over a new layer of powder and thus forming a fresh layer to be sintered on the already built layer. Ten frameworks were sintered in a similar fashion simultaneously [Figure 4].
Figure 4: Framework on master cast

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UCLA abutments were secured to implant analogs on the master model and tightened with hex for conventional casted frameworks. Wax patterns were fabricated using blue inlay wax (BEGO USA). The wax pattern dimensions were standardized using an index obtained from the CAD/CAM milled and sintered frameworks. Before fabricating wax patterns, UCLA abutments were attached to each other with pattern resin. An electrically controlled wax bath was used to melt the wax and applied to the UCLA abutments and pattern resin. The wax patterns, along with UCLAs, were removed carefully from the cast so as to minimize distortion and were sprued. Before investing, a surface tension reducing agent (Silikon- and Wach Entspanner, DFS) was carefully applied to the patterns, and then the investment was done using a phosphate bonded investment (Vesto-Fix, DFS, and Germany). The wax patterns were then casted in Co-Cr alloy and finished. The castings were examined for gross defects before placing them on the master Styrofoam model. The finished frameworks were then tried on the master model [Figure 5].
Figure 5: Framework on master model

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Evaluation of vertical marginal fit

The vertical marginal fit between the superstructure and implant was assessed using a stereomicroscope [Figure 6] by employing one-screw test and screw resistance test. Laser dots were marked on the framework at the base of each abutment of all the frameworks and the other on the implant. The same trained investigator measured the distance between the two points to give the readings for the vertical misfit between the frameworks in micrometers using pictures obtained from a million instructions per second (MIPS) mounted on a stereomicroscope.
Figure 6: Framework under stereomicroscope

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The model and the framework were placed on the microscope's stage horizontally and stabilized using the stage clips. Stage clips helped in maintaining a continuous seating force between the framework and the model during the microscopic measurement. The stereomicroscope was focused at ×100 magnification, and the image obtained from the MIPS mounted on the stereomicroscope was captured in the computer attached to it. The image analysis software was then calibrated to a microns scale, and the image obtained was analyzed for the vertical misfit by calibrating the distance from the laser engraving on a framework to the engraving on the implant.

One-screw test

One-screw test was implemented to quantitatively evaluate the discrepancy measurement. In one-screw test, screw that corresponds to implant number 5 and 3 was first tightened by hand. After this, screw of implant number 5 was tightened to 15 N cm torque, and the screw at implant number 3 was removed. The vertical marginal fit was measured at implant number 1 at the interface of the implant and the abutment superstructure using a stereomicroscope.

Screw resistance test

The assessment of the marginal fit between the framework and implants was also performed using screw resistance test. The screw resistance test is done in two parts; in the first part, the screw corresponding to implant number 3 was tightened with a torque of 15 N cm, and the readings of vertical misfit were measured on implants number 1, 2, 4, and 5 using a stereomicroscope. In the second step, the screw corresponding to implants number 2, 3, and 4 was tightened with a torque of 15 N cm, and the vertical marginal fit was simultaneously measured at implants number 1 and 5 using a stereomicroscope.

A master chart with all the readings was prepared, and the data were analyzed using two statistical tests, i.e., ANOVA and the post hoc Bonferroni test. The software used for the statistical analysis was SPSS (the statistical package for the social sciences) version 21.0 and Epi-info version 3.0 SPSS Inc., IBM Corp., Armonk (N.Y., USA).


  Results Top


This in vitro study compared the fit accuracy of full arch screw retained frameworks fabricated from three different techniques. Marginal fit was measured by employing one screw test and screw resistance test using stereomicroscope.

Evaluation of the marginal fit using one screw test

On evaluating the frameworks using one-screw test, the mean vertical misfit value at the terminal implant for the control group was 292.58 ± 15.46μm, for conventionally casted framework 398.41 ± 21.13μm, for DMLS 343.44 ± 24.73 μm, and for CAD-CAM it was 304.03 ± 14.23 μm.

Evaluation of the marginal fit using screw resistance test

The average misfit values at four implants on applying screw resistance test were 1268.65 ± 84.24 (control), 1774.88 ± 67.70 (casted), 1508.02 ± 62.19 (DMLS), and 1367.29 ± 81.87 for CAD-CAM respectively. Whereas, the average misfit values on two implants using screw resistance test were 635.02 ± 57.33 for the control group; for conventionally casted, it was 879.75± 35.93; for (DMLS) framework, it was 761.51 ± 32.85; and for milled CAD-CAM framework, it was 687.07 ± 42.17 μm.


  Discussion Top


The present in vitro research compared the marginal fit and accuracy of full arch screw retained frameworks fabricated from three different manufacturing systems using the same material, i.e., CAD/CAM milling or subtractive manufacturing system, DMLS sintering or additive technique, and conventional casting. The hypothesis was rejected that the vertical marginal fit of a multiunit screw-retained prosthesis would not be influenced by fabrication technique. The frameworks fabricated using CAD/CAM technique depicted the best marginal fit. The conventionally casted frameworks had the highest mean marginal discrepancy with higher variability of results. The results are in accordance with Gema Arroyo-Cruz,[9] who stated that the CAD-CAM technique has been extensively used in the designing and manufacturing of implant prosthesis and is believed to produce high-quality restorations with very few inaccuracies. Furthermore, CAD/CAM simplifies the process, eliminates various steps such as investment, burnout, casting, finishing, and polishing, and reduces the time required for manufacturing implant restorations.[10],[11],[12],[13]

Brånemark[14] was the first person who stated that the misfit of the implant framework should be not more than 10 μm. Whereas Zeroas et al.[15] concluded that a 30 μm discrepancy at the implant-abutment junction would be admissible if it does not exceed 10% of the perimeter. Recently, Jemt and Book[4] in their research stated that a vertical misfit of around 150 μm would also be acceptable. However, with the existing techniques and materials available for manufacturing implant frameworks, a certain degree of inaccuracy and vertical misfit is inescapable, as demonstrated by various in vitro investigations in the past and comparable to those presented in this study.[16],[17],[18] Therefore, the vertical marginal misfit values obtained in the present study may reasonably represent the situation clinically.

Begoña Ormaechea et al.[19] proposed the 1-screw test for long span/full arch implant superstructures, and it states that the vertical marginal discrepancies tend to be more magnified at the opposite terminal abutment in long-span frameworks. The mean vertical misfit value at the terminal implant for the control group was 292.58 ± 15.46 μm, for the conventionally casted framework was 398.41 ± 21.13 μm, direct metal laser sintered (DMLS) screw-retained framework was 343.44 ± 24.73 μm, and milled CAD-CAM one-piece framework was 304.03 ± 14.23 μm when observed under a stereomicroscope [Table 1].
Table 1: Descriptive statistics of the different groups according to one-screw test

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The mean marginal difference when the control group was compared with the CAD-CAM framework was 11.45 μm which was statistically nonsignificant. In contrast, the mean marginal discrepancy was statistically significant when the control group was compared with the DMLS framework (50.86 μm), and marginal discrepancy further increased when compared with conventionally casted frameworks, i.e., 105.82 μ [Table 2] and [Table 3]. The present study results agree with the studies conducted by Klineberg and Murray,[20] Helldén and Dérand,[21] and Lencioni et al.,[22] who compared Au and Ti FDP frameworks fabricated by CAD CAM and conventional methods and found similar results. These results obtained were probably because CAD/CAM technique is very accurate and reproduce the same results repeatedly as it eliminates almost all the errors related to conventional casting procedure such as investing, dewaxing, casting, finishing, and polishing.[22],[23],[24]
Table 2: Comparison of one-screw test within the group using ANOVA test

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Table 3: Post hoc analysis for one-screw test by using the Bonferroni test

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Buzayan and Yunus[25] reported that the fabrication method is an essential criterion influencing the marginal fit, probably because each manufacturing technique produces different surface roughness. The authors noted that CAD-CAM milled restorations had a superior marginal fit and better contact with the implant body than casted restorations, eliminating the micro gaps between implant components.

The fit and passivity of CAD-CAM fabricated prosthesis might be influenced by the scanning procedure, which is used to transfer the position of implants. This procedure is of two types, i.e., direct and indirect. The direct technique is scanning directly inside the mouth, and the indirect means the cast is scanned in the laboratory with labotatory scanners. In the present study, the indirect technique was performed because the indirect method has been reported to provide more precise values when compared with the direct technique.[26] Ortorp and Jemt[27] reported that CAD-CAM frameworks fabricated from direct technique had increased vertical misfit values at the interface than for conventionally manufactured frameworks. Although the literature quoted above suggests that the indirect scanning technique may provide the desired accuracy of CAD-CAM implant superstructures, still more research is required to compare the two methods.

The screw resistance test was introduced by Jemt and Book[4] in 1991. The test was based on his clinical experience, and the clinically acceptable level of marginal misfit was set at 150 μm. A 5-year clinical study demonstrated the absence of mechanical fatigue fractures in fixed prostheses provided to a group of edentulous patients, and the authors recommended that the screw resistance test was clinically adequate for fit assessment. Therefore, a screw resistance test was used to assess the marginal fit in the present study.

The screw resistance test has two parts – in the first part, only the screw in the middle corresponding to implant number 3 was torqued, and marginal discrepancy at the remaining 4 implants was measured under a stereomicroscope. The mean vertical marginal discrepancy value for the control group was 1268.65 ± 84.24 μm, the non-hexed screw-retained conventionally casted framework was 1774.88 ± 67.70 μm, the DMLS framework was 1508.02 ± 62.19 μm, and milled CAD-CAM framework was 1367.29 ± 81.87 μm. On comparing the four groups statistically, the mean vertical marginal discrepancy value in CAD-CAM milled framework (98.64 μm) was found to be nonsignificant on compared with the control group, whereas it was statistically significant when the control was compared with the other two frameworks, i.e., conventionally casted (506.23 μm) and DMLS (239.37 μm) [Table 4], [Table 5], [Table 6].
Table 4: Descriptive statistics of the different groups according to screw resistance test part 1 (μm) (i.e., screw secured on implant number 3)

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Table 5: Comparison of marginal fit at 4 implants (1, 2, 4 and 5) using screw resistance test part 1 (μm) (i.e., screw secured on implant number 3) within the group using ANOVA test

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Table 6: Post hoc analysis for screw resistance test part 1 using Bonferroni test

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Further, in the screw resistance test, the screws correspond to implants number 2, 3, and 4 were torqued, and the vertical misfit of the frameworks at implants number 1 and 5 was measured using a stereomicroscope. The mean vertical misfit value for the control group was 635.02 ± 57.33 μm, screw-retained conventionally casted framework was 879.75 ± 35.93 μm, DMLS framework was 761.51 ± 32.85 μm, and Ti-milled CAD-CAM one-piece framework was 687.07 ± 42.17 μm. The mean values of vertical misfit of conventionally casted, DMLS, and CAD-CAM were statistically significant compared to the control group. This study concluded that the CAD-CAM framework showed a statistically insignificant difference with control and a better marginal fit than DMLS and conventionally casted frameworks [Table 7], [Table 8], [Table 9].
Table 7: Descriptive statistics of different groups according to screw resistance test part 2 (i.e., screw secured on implant number 2, 3, and 4)

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Table 8: Comparison of marginal fit on 2 implants (1 and 5) screw resistance test part 2 (i.e., screw secured on implant number 2, 3, and 4) within the group using ANOVA test

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Table 9: Post hoc analysis for screw resistance test part 2 using Bonferroni test

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The milled and sintered frameworks were associated with lower marginal discrepancy values when compared with conventionally casted frameworks. Both CAD/CAM and DMLS are advanced CAM systems for implant-supported restorations; one is an additive manufacturing (sintering), and another is subtractive (milling). The fit produced in the CAD-CAM technique can also be influenced by factors related to the accuracy of the units, such as the software version used, calibration of the machine, condition of the tools, and the overall working condition of the milling or additive unit. In the present study, the machines used to mill and sinter Co-Cr were high-speed five-axis with simultaneous motion. The recommended pressure and temperature conditions were used according to the manufacturer's instructions. However, since both the machines were different, differences in the precision of the fit achieved in CAD-CAM and DMLS may have occurred in the present study. However, the results of the present study were in contradiction with the research of Ortorp and Jemt.[27] and Jemt and Book[4] who reported a superior marginal fit for sintered structures, whereas the investigations done by Patterson[8] and Buzayan and Yunus[25] did not reported any statistically significant differences in the marginal accuracies in the framework made by the three methods which are being compared in the present study.

Vertical marginal fit of milled superstructures was better than that of sintered and conventionally casted ones. This result of the present study indicates a promising future for CAD-CAM manufactured implant-supported prosthesis as the materials used are more homogenous, and during the manufacturing phase, the physical and mechanical properties of the materials are less affected when compared to conventional casting.[28] As the CAD-CAM technology is getting more advanced and developing at a fast pace, it will become even more accurate and precise in the near future. Furthermore, with increased usage, it might become more cost-effective and flexible, as, presently, the cost is a potential limitation of this computerized technique.

While it is difficult to determine if the selected parameters in the present study are clinically relevant or reflect vital information to predict the clinical problem, every clinician should aim to maximize the passivity of the fit of the prostheses. Nonetheless, technology that delivers high precision and decreases variability should be obligatory for implant-supported restorations.

In screw-retained implant prostheses, higher preload forces are indicated compared to other prosthesis types resulting in higher stress and tension generation within the peri-implant tissues and the adjacent bone. Furthermore, newer materials introduced for the fabrication of implant superstructures such as Co-Cr, Titanium, and zirconia are less yielding than previously used gold alloys, which might introduce even higher stress levels.[29] Thus, to reduce the stress produced by these modern restorative materials, achieving passivity in the frameworks becomes even more essential for implant-supported prosthesis success and longevity.

A stereomicroscope was used to assess the marginal fit in the current study. Examination of the interface of abutment/superstructure and implant fixture with stereomicroscope is a justifiable method as it allows for direct measurement of any discrepancies on photomicrographs by using the provided scale. Stereomicroscope produces high contrast images with low magnification, with a minimum amount of flare and geometrical distortion. It uses a fiber-optic light source to illuminate the small specimens, making it ideal when dealing with thick or opaque samples.

The results obtained in the present study were statistically significant. However, a possible limitation of the study may be related to the number of samples included and the number of assessment points in each framework. Evaluation points could not be increased due to technical and practical difficulties with a measurement under the stereomicroscope. The sample size and the number of points for measuring marginal discrepancy in the present study were similar to the previous studies on the fit and micro gap in implant restorations.[30],[31],[32],[33] The current in vitro study results can be correlated with the clinical trials, which would provide meaningful results and help assess crestal bone loss, peri-implant soft tissue health, screw loosening, screw fracture, and framework fracture. This would help in increasing the longevity of full-arch screw-retained implant-supported restorations.


  Conclusion Top


Within the scope of this in vitro study, it can be concluded that frameworks manufactured by CAD/CAM technique had better vertical fit values when compared with conventionally casted or DMLS fabricated frameworks. Compared with control, the mean vertical marginal discrepancy was least in milled CAD-CAM frameworks followed by sintered DMLS and conventionally casted frameworks when tested using one screw and a screw resistance test. Hence, according to the present study, CAD/CAM technique is recommended to achieve maximum marginal fit accuracy in full mouth screw-retained implant-supported FDPs.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9]



 

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