Stress distribution on different bar materials in implant-retained palatal obturator.

Implant-retained custom-milled framework enhances the stability of palatal obturator prostheses. Therefore, to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars, in three different materials under two load incidences were simulated. A maxilla model which Type IIb maxillary defect received five external hexagon implants (4.1 x 10 mm). An implant-supported palatal obturator prosthesis was simulated in three different materials: polyetheretherketone (PEEK), titanium (Ti:90%, Al:6%, V:4%) and Co-Cr (Co:60.6%, Cr:31.5%, Mo:6%) alloys. The model was imported into the analysis software and divided into a mesh composed of nodes and tetrahedral elements. Each material was assumed isotropic, elastic and homogeneous and all contacts were considered ideal. The bone was fixed and the load was applied in two different regions for each material: at the palatal face (cingulum area) of the central incisors (100 N magnitude at 45°); and at the occlusal surface of the first left molar (150 N magnitude normal to the surface). The microstrain and von-Mises stress were selected as criteria for analysis. The posterior load showed a higher strain concentration in the posterior peri-implant tissue, near the load application side for cortical and cancellous bone, regardless the simulated material. The anterior load showed a lower strain concentration with reduced magnitude and more implants involving in the load dissipation. The stress peak was calculated during posterior loading, which 77.7 MPa in the prosthetic screws and 2,686 με microstrain in the cortical bone. For bone tissue and bar, the material stiffness was inversely proportional to the calculated microstrain and stress. However, for the prosthetic screws and implants the PEEK showed higher stress concentration than the other materials. PEEK showed a promising behavior for the bone tissue and for the integrity of the bar and bar-clip attachments. However, the stress concentration in the prosthetic screws may represent an increase in failure risk. The use of Co-Cr alloy can reduce the stress in the prosthetic screw; however, it increases the bone strain; while the Titanium showed an intermediate behavior.

Introduction

Palatal obturator prostheses and microvascular reconstructive techniques are common treatment options for rehabilitation of those patients undergoing maxillectomy during surgical resection of tumors [1, 2]. Nevertheless, prosthetic rehabilitation remains the most widely used approach, especially for large maxillary defects [3, 4], with improvements in oral functions [5] and significant increase in the quality of life of patients [6]. The design of the obturator prosthesis and retention mechanisms depend basically on the size and location of the defect, clinical conditions of bone, remaining teeth, soft tissue, muscle control, and the physical and mental health conditions of the patient [7]. However, the retention of these prostheses for edentulous patients is often insufficient. In a systematic review on functional outcomes in oncologic patients, the authors observed that rehabilitation with implants significantly improved prosthesis retention, presenting a beneficial effect for masticatory efficiency and greater satisfaction of patients [8].

The possibility of using computer-aided design/computer-aided-manufacturing (CAD/CAM) technology to machine reliable prostheses in different materials (metal, ceramics and polymers) has diversified the standard designs of implant-supported prostheses and their clinical performance [9, 10]. Furthermore, CAD/CAM frameworks retained by implants improved the palatal obturator stability and functional results for patients with partial maxillectomy [11]. Titanium and cobalt-chromium alloys represent standard materials for CAD/CAM frameworks, with good performance and similar fit [12], while polyetheretherketone (PEEK) is an inert, non-allergenic polymeric biomaterial, indicated as a substitute for metal alloys in assorted types of prostheses and orthoses, including craniofacial prostheses [13, 14].

In vitro studies and short-term clinical reports evaluated the use of PEEK in dentistry for partial/total; fixed/removable; tooth supported/implant-supported [15-21], and maxillofacial prosthesis, including palatal obturators [22]. PEEK has shown some advantages such us the fact of it has an elastic modulus similar to the native bone, is easily obtained in personalized three-dimensional (3D) forms, propitiates the manufacture of radiolucent prostheses, with good biomechanical properties, and less accumulation of biofilm than ceramics and metallic alloys, which are usual materials in restorative dentistry [15-21]. Despite that, the biomechanical behavior of PEEK obturator prostheses retained by implants remains unknown, as well the mechanical response during chewing loads in the implants, prosthetics screws, cortical and cancellous bone for the patients rehabilitated with implant-retained obturator prostheses.

The assessment of biomechanical behavior can be performed through simulations to obtain pre-clinical data with bioengineering tools such as strain gauge, digital image correlation, photoelasticity and finite element analysis (FEA). The latter allows us to understand how the distribution of strain in bone tissue and stress in implants can be influenced by the restorative material [23], prosthesis and framework design [24, 25], manufacturing technique [26], number and distribution of implants [27-29] and attachment systems [30, 31]. Computer-assisted implant planning has become an important diagnostic and therapeutic tool in modern dentistry. The ideal implant positions can be planned virtually with the help of guided surgery software allowing for three-dimensional visualization before treatment. The combination of planning and case study by FEA can help to choose and predict the most suitable mechanical results.

Thus, the objective of this study was to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars in three different materials: PEEK, titanium and cobalt-chromium alloys in different load incidences. The null hypothesis is that different materials for the framework will not modify the mechanical response in the analyzed structures regardless of the applied load.

Methods

Pre-processing

A computer tomography (CT) from São Paulo State University database, without maxillofacial abnormalities, were saved in DICOM (Digital Imaging and Communications in Medicine) format. This file comes from the University database, the authors have no access to any identifying information or taking the CT scan. The DICOM file was converted to STL (stereolithography) file in a 3D slicer software. Using CAD (computer-aided design) software (Rhinoceros Version 4.0 SR8, McNeel North America, Seattle, WA, USA), a model of an edentulous maxilla was constructed following the main anatomical characteristics of the patient’s bone: size, shape and absence of lesion. The next step was to reconstruct the NURBS (non-uniform rational B-spline) surfaces from mesh with precision. For that, the BioCAD method [32] was applied and the anatomical lines of the surface were created. The pre-processing phase is summarized in a flowchart (Fig 1). The 3D volumetric model of the bone was then finished based on the surface created by the curve network manually generated (Fig 2A). The cortical bone (Fig 2E) contained 1 mm thickness in juxtaposition with cancellous bone (Fig 2D) [33]. The command offset surface was used to create the soft tissue with 2 mm thickness [34] (Fig 2F).

Fig 1

Process flowchart.

Diagrammatic representation of the steps and the correlated software applied in the pre-processing and processing phases.

Fig 2

3D model and geometries.

Edentulous maxilla (A); Type IIb maxillary defect (B); Implants distribution on maxillary crestal bone (C); Cancellous bone (D); Cortical bone (E); Soft tissue (F); Five external hexagon implants (G); Bar and bar-clip attachments (H); Five prosthetic screws (I); Full-arch total prosthesis (J).

External hexagon implants (10 × 4.1 mm), previously modeled [25] were selected. The platform had a diameter of 4.1 mm, similar to a regular conventional implant. The external hexagon was extruded (0.7 mm high) and attached to the previously created cylindrical body [33] (Fig 2H). The minimum distance between the implants was 4 mm (Fig 2G). The prosthetic screw was modeled for each implant (Fig 2I). The total number of implants and their position were based in a previous report with the similar prosthetic approach [11] (Fig 2B). After that Type IIb defect [35] was simulated following the length and width from a clinical report [36] (Fig 2B).

The bar was modeled following the maxilla shape and the implants position. It presents 3 mm maximum thickness and 4 mm width, rounded corners and flat surfaces. An anteroposterior structure was used crossing the hemimaxillectomy defects similar to [11] report (Fig 3).

Fig 3

3D model.

Milled implant-retained bar with 3 clip-bar attachments in different materials. PEEK (A); Titanium alloy (B); Cobalt-Chromium alloy (C).

After that, the full-arch total prosthesis was modeled containing artificial teeth [33] and palatal coverage (Fig 2J). The clip connector for the fixture system was modeled in the same position as the bar from the framework.

Processing

For the FEA, each solid geometry was imported to the computer aided engineering (CAE) software (ANSYS 19.2, ANSYS Inc., Houston, TX, USA) in STEP format. A 3D mesh was generated, and tetrahedral elements were considered for the models. A convergence test (maximum of 10% of difference in results between each calculation) determined the total number of elements (169,546) and nodes (306,914) for the model (Fig 4A). The Elastic modulus and Poisson ratio of each material were assigned to each solid component with isotropic and homogeneous behavior (Table 1). The contacts were considered perfectly bonded between the structures.

Fig 4

Boundary conditions and loadings configuration in FEA models.

Mesh (A); Boundary conditions (B); Anterior load was applied at cingulum area of the central incisor (C); Posterior load was applied at occlusal surface of the first molar (D).

Table 1

Mechanical properties of the materials/solid geometry used in the current study.

Material/solid geometry	Young’s Modulus (GPa)	Poisson Ratio
Cancellous bone [37]	1.37	0.3
Cortical bone [38]	14.7	0.3
Soft tissue [39]	0.68	0.45
PEEK [40]	3.7	0.4
Titanium alloy [41]	110	0.3
Cobalt-Chromium alloy [42]	200	0.3
Acrylic resin [43]	2.83	0.45

For the boundary conditions, the bottom surface of cancellous bone was restricted in all directions Fig 4B). Wedel et al. [44] established 120N as the occlusal force in patients with congenital and acquired maxillofacial defects while another studies reported the load range of 48–300 N for overdenture patients [45-47]. In this study, both simulated loads are inside this range.

The load was applied in two different moments for each material: at the palatal face (cingulum area) of the central incisors with 100 N magnitude applied at 45° [48] (Fig 4C); and at the occlusal surface of the first left molar with 150 N magnitude applied normal to the surface [29] (Fig 4D).

Results were reported in von Mises stress [25] distribution for the framework, implants and screw; and in microstrains (με) for bone tissue [49].

Results

The calculated microstrain distribution in the maxilla as a function of the framework’s material and load incidence were plotted in colorimetric graphs in the Figs 5 and 6 for cortical and cancellous bone, respectively. It was possible to observe that the posterior load showed a higher strain concentration in the posterior peri-implant tissue, near the load application side for cortical and cancellous bone. The anterior load showed a lower strain concentration with reduced magnitude and more implants involving in the load dissipation.

Fig 5

Microstrain distribution in the maxillary cancellous bone.

Upper line: anterior loading; Bottom line: posterior loading. Framework’s material: PEEK (A and D); Titanium alloy (B and E); Cobalt-Chromium alloy (C and F).

Fig 6

Microstrain distribution on the maxillary cortical bone.

Upper line: anterior loading; Bottom line: posterior loading. Framework’s material: PEEK (A and D); Titanium alloy (B and E); Cobalt-Chromium alloy (C and F).

Moreover, the higher the framework elastic modulus, the higher the bone strain regardless the evaluated bone tissue and load incidence. The peak value of each group was exported from the analysis software to quantify the strain (Table 2). According to Wolff's law, strain values below 50 mm/mm are able to promote bone remodeling by disuse, and those values above 3000 mm/mm are able to promote bone remodeling by micro-damage. Thus, the three types of framework’s material showed no values able to induce an unwanted bone remodeling.

Table 2

Results in terms of bone microstrain (με) and stress peak values (MPa) according to the framework’s material and load incidence.

	PEEK		Titanium		CoCr
	Anterior	Posterior	Anterior	Posterior	Anterior	Posterior
Cortical Bone (με)	411	1,460	460	2,360	492	2,686
Cancellous Bone (με)	588	1,258	567	1,896	587	1,987
Framework (MPa)	4.6	9.5	66.3	51.4	71.7	62.5
Implant (MPa)	25.4	86.4	20.8	79.2	19.8	74.1
Prosthetic Screw (MPa)	53.2	77.7	22.6	40.3	22.1	25.3

The stress distribution in the framework for all groups is displayed in the Fig 7. Similar to the bone tissue mechanical behavior, the higher the framework elastic modulus, the higher the stress concentration regardless the load incidence. For posterior load, the stress concentration occurred in the lateral side; and for the anterior load, the stress concentration occurred between the anterior implants, near the screw access holes.

Fig 7

The stress distribution in the framework.

Upper line: anterior loading; Bottom line: posterior loading. Framework’s material: PEEK (A and D); Titanium alloy (B and E); Cobalt-Chromium alloy (C and F).

A higher stress concentration in the framework promoted a lower stress concentration in the implant (Figs 8 and 9) and prosthetic screw (Figs 10 and 11). Observing the results displayed in the Fig 9 it is possible to see that the higher the framework elastic modulus, the lower the stress concentration in the implants. The posterior load showed a higher stress magnitude with more red fringes in the colorimetric stress map in comparison with the anterior load, with the most posterior implant being the most affected. For the prosthetic screw, the same stress pattern observed in the implants occurred (Fig 11). The highest stress concentration calculated for the screw is the combination of PEEK framework and posterior load.

Fig 8

Maps of von Mises stress distribution results for implants according to framework’s material, under anterior loading.

PEEK (A); Titanium (B); Cobalt-Chromium alloy (C). On the bottom line, an enlarged view of the implants that presented the highest stress concentration.

Fig 9

Maps of von Mises stress distribution results for implants according to framework’s material, under posterior loading.

PEEK (A); Titanium (B); Cobalt-Chromium alloy (C). On the bottom line, an enlarged view of the implants that presented the highest stress concentration.

Fig 10

Maps of von Mises stress distribution results for prosthetic screw according to framework’s material, under anterior loading.

PEEK (A); Titanium alloy (B); Cobalt-Chromium alloy (C). On the bottom line, an enlarged view of the prosthetic screws that presented the highest stress concentration.

Fig 11

Maps of von Mises stress distribution results for prosthetic screw according to framework’s material, under posterior loading.

PEEK (A); Titanium alloy (B); Cobalt-Chromium alloy (C). On the bottom line, an enlarged view of the prosthetic screws that presented the highest stress concentration.

The results in terms of stress peak values (MPa) in the framework, prosthetic screw, and implant are summarized in Table 2.

Discussion

This study aimed to evaluate the mechanical response of implant-retained obturator prostheses with clip-bar system, in different materials. The hypothesis was rejected due to the differences of stress concentration observed among materials.

Technical and biological complications or even failures are usual outcomes in dental prosthesis and it should be considered, in order to prevent them. Biomechanical tests are consolidated methods which have been contributing to the comprehension of the behavior of restorative materials and this study compared the mechanical response of CoCr and titanium alloys (materials commonly used for CAD/CAM frameworks) with the PEEK, an alternative to metal alloys in dentistry. The elastic modulus of Co-Cr alloy (200 GPa) [42] is almost double of the titanium's modulus (110 GPa) [41] and both have the same Poisson coefficient, while PEEK presents both, elastic modulus and Poisson coefficient, closer to those of acrylic resin [43] (material used in synthetic teeth). This difference makes the metal alloys behave differently from the PEEK when the anterior and posterior loads are applied. Therefore, the higher stress concentration was calculated in the CoCr bar, followed by titanium and finally the PEEK bar.

According to Bhering et al. [50], rigid frameworks transmit lower load to the implant and prosthetic components, when compared to the less rigid ones. Nonetheless, variations of framework material rigidity did not demonstrate a significant effect on the stress values in the marginal bone around the implants [51] and Medeiros et al. [26] observed that the occlusal coating had a greater influence on the load dissipations than the framework. In consonance with Erkmen et al. [52] the current study demonstrated that by using less rigid material for milled bar in implant-retained prostheses the stresses within both, the framework and the veneering parts, decreased due to the flexible nature of the material that absorbs stresses. The results from a longitudinal study on the use of PEEK milled bar as framework for implant-supported full-arch fixed prostheses suggest that this material may become an appropriate treatment option [21].

Apart from the material, the custom-milled framework design also influences the stress distribution [25]. In conventional maxillary overdentures that do not require palatal coverage, the stresses tend to be concentrated in the distal of the last implant, in the cantilever region [30]. However, obturator prostheses aims both coverage and adequate sealing of the oroantral communication, and thus the cantilever ceases to exist if there is residual bone on the affected side, or if zygomatic implants are installed. The maximum displacement of the obturator prosthesis increases as less residual bone is present, as well as less implants and clips [53]. Therefore, the unaffected side by maxillectomy should receive a larger number of implants to reduce the stress concentration in the framework [27]. The present study evaluated the performance of a milled bar for obturator prosthesis with polygonal geometry, without cantilever and with total palatal coverage and three clips in the region of the maxillary defect, supported by five implants. This model was based on a clinical case [36] and it was observed that despite the number of implants at unaffected side, the unilateral posterior loading promoted higher stress concentration in the mesial buccal and distal palatal portions of the cervical bone (cancellous and cortical) on the most distal implant on the affected side.

Regarding to the anchorage system, bar-clips possess a more favorable design to distribute the loads than O-rings [54] and presents lower strain values when submitted to compressive occlusal loads [55]. Furthermore, this system had a better biomechanical performance with the lowest strain values around the dental implants when subjected to forces simulating prosthesis removal [56]. In the current study, three the bar-clips were simulated in different materials and it was observed that PEEK attachments concentrated less stress than the metallic ones. This could be consider a possible advantage of using PEEK, taking into account that according to Tanoue et al. [57] clips can prevent the fracture of the prosthesis base more than metal clips, regardless of the number, due the lower concentration of stress observed around plastic clips. However, it is important to emphasize that the most frequent complication in implant-supported overdentures with bar-clip system are associated with the retention clips, requiring its replacement in 33% of cases [58].

For implant-supported rehabilitation, the marginal bone loss with exposure of the implant threads can be considered one of the biological complications. Frost [49] suggests that unrepaired bone resorption starts when strains exceed 3000μ strain. In the present study, the use of PEEK milled bar suggests a better mechanical performance for bone tissue with less possibility of unwanted bone resorption due to less peri-implant deformation independent on the simulated masticatory load.

Stress concentration on prosthetic screws is influenced by implant connection and the material selected for abutments and frameworks. Less rigid abutments like those manufactured in reinforced fiber composite and PEEK are not absolutely relevant for morse-taper implant [59]. In contrast, for external hexagon implants, flexible prosthetic frameworks increases the stress generated in the prosthetic screw threads [23] and may decrease the survival of restorations under cyclic fatigue [60]. This is relevant because abutment screw loosening, the fixing screw fractures, screw retightening and loosening of the abutment are usual technical complications in implant-retained prosthesis. Thus, the results of the present study suggest that the use of PEEK might facilitate the emergence of mechanical complications in prosthetic screws compared with metal frameworks.

Inherent limitations of the finite element analysis studies and biologic simulations were observed in the present investigation and two assumptions figured as the principals. The first one was the simulated materials presented a homogeneous structure and linearly isotropic behavior that do not represent the defects population incorporated during the prosthesis manufacturing. The second was that the implants were assumed 100% osseointegrated, although histomorphometric studies indicated that there is not a 100% bone–implant interface. Nonetheless, these assumptions are consistent with other FEA studies [29, 50, 52] and are consequences of the challenges in establishing the properties of living tissues and the osseointegration level in bone-implant surfaces. Furthermore, it would not interfere with the qualitative and comparative results because they were present for all models. Future fatigue life studies and clinical evaluations may complement the results or evaluating the different framework materials described for this technique.

Conclusion

For this treatment modality, regardless the loading region, PEEK can be suggested as framework material to reduce the bone strain around the implants and the stress concentration in the bar structure. However, the use of PEEK increase the risk of prosthetic screws loosening and even fracture in comparison with metallic alloys.

30 Sep 2020

PONE-D-20-26196

Stress distribution on different bar materials in implant-retained palatal obturator

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Reviewer #1: This paper presents the results of the numerical investigations on the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars. The research topic is quite important and well suited in the journal scope. The results obtained are well interpreted. Moreover, structure of this manuscript and graphic quality of figures are acceptable.

There are some more comments from this reviewer that can help the Authors to improve the quality of the manuscript. Minor revisions are suggested. The detailed comments in the order of appearance in the text are summarized as follows.

The novelty of this manuscript has not been clearly specified. I found many similar numerical analyses in the literature. The elastic properties of materials are assumed basen on the literature. External hexagon implants (10 × 4.1 mm), previously modeled [25] were selected. An anteroposterior structure was used crossing the hemimaxillectomy defects similar to [11]. report.The value of simulated loads is based also on the literature. etc., etc., So, what is the novelty of this manuscript???

2. Definition of the materials is too general. In the Abstract the authors say that "PEEK, titanium and Co-Cr alloys" were considered. The grades of titanium and Co-Cr alloys and/or chemical composition must be clearly specified.

3. Line 30: "PEEK". Please define all abbreviations the first time they appear in the abstract, and the main text.

4. The method of determination of optimal mesh size is very mysterious. Line 122: "A convergence test of 10% determined [...]". 10% of what?

5. Conclusions are to general. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Review comment

This manuscript entitled “Stress distribution on different bar materials in implant-retained palatal obturator” primarily aimed to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars in three different materials: PEEK, titanium and cobalt-chromium alloys in different load incidences. The authors bring an interesting study. However, all the issues that listed below must be revised thoroughly before this manuscript being accepted for publication. I give a major revision for this manuscript.

Specific comments

Abstract

1. Abbreviations that exist in manuscript should be explained when they first appear, such as “PEEK” (Line 30). Please add the official explanation.

2. The detailed modeling method can be presented in the method session, the authors should give more information regarding the results, conclusion and application (if possible) of this study in this part.

Introduction

1. “PEEK has shown some advantages such as…, which are usual materials in restorative dentistry” (Line 65-68), some references should be added.

2. The novelty of this study should be further highlighted in the introduction part.

Methods

1. It is suggested that a flow chart for the pre-processing is needed in order to help readers better understand the methods.

2. Did this study apply any statistical methods to compare the differences between materials? If did, please specify it, and if not, please explain how the significant results came out without any statistical analysis.

Results

1. The results session should be rewritten based on the statistical analysis.

Discussion

1. “FEA” (Line 271), although I know the meaning of this abbreviation, the official explanations should be added.

2. “Computer-assisted implant planning has become …the most suitable mechanical results” (Line 282-285), this paragraph should be moved to the introduction part, as it seems to be the novelty of this study.

Conclusion

1. Are there any implications based on the results of this study? Please specify.

While written English is reasonably good, but please do check the language and grammar mistakes throughout the whole article to improve clarity.

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Reviewer #1: No

Reviewer #2: Yes: Yaodong Gu

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1 Oct 2020

Reviewer #1: This paper presents the results of the numerical investigations on the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars. The research topic is quite important and well suited in the journal scope. The results obtained are well interpreted. Moreover, structure of this manuscript and graphic quality of figures are acceptable.

There are some more comments from this reviewer that can help the Authors to improve the quality of the manuscript. Minor revisions are suggested. The detailed comments in the order of appearance in the text are summarized as follows.

The novelty of this manuscript has not been clearly specified. I found many similar numerical analyses in the literature. The elastic properties of materials are assumed basen on the literature. External hexagon implants (10 × 4.1 mm), previously modeled [25] were selected. An anteroposterior structure was used crossing the hemimaxillectomy defects similar to [11]. report. The value of simulated loads is based also on the literature. etc., etc., So, what is the novelty of this manuscript???

R: First and foremost, we would like to thank the referee for the constructive criticisms, which have contributed to improve the content and the style of the manuscript. The reported parameters are used to ensure a well-controlled modelling process and it is common in studies like that. However, the finite element analysis of a hemimaxillectomy defect with this prosthesis design and different materials were never reported before in literature. Also, it is important to note that the implant model was previously reported but not in this kind of oral rehabilitation as you can see in the reference 25. In the same way, the reference 11 that was used as hemimaxillectomy defect reference is a case series, which have not evaluated the mechanical response.

2. Definition of the materials is too general. In the Abstract the authors say that "PEEK, titanium and Co-Cr alloys" were considered. The grades of titanium and Co-Cr alloys and/or chemical composition must be clearly specified.

R: This information has been inserted in the text.

3. Line 30: "PEEK". Please define all abbreviations the first time they appear in the abstract, and the main text.

R: The PEEK definition has been inserted in the text.

4. The method of determination of optimal mesh size is very mysterious. Line 122: "A convergence test of 10% determined [...]". 10% of what?

R: 10% of difference in results between each calculation. This information has been inserted in the methods section.

5. Conclusions are to general. The conclusions must be drawn appropriately based on the data presented.

R: The conclusion section has been improved.

Reviewer #2: Review comment

This manuscript entitled “Stress distribution on different bar materials in implant-retained palatal obturator” primarily aimed to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars in three different materials: PEEK, titanium and cobalt-chromium alloys in different load incidences. The authors bring an interesting study. However, all the issues that listed below must be revised thoroughly before this manuscript being accepted for publication. I give a major revision for this manuscript.

Specific comments

Abstract

1. Abbreviations that exist in manuscript should be explained when they first appear, such as “PEEK” (Line 30). Please add the official explanation.

R: The abbreviation has been explained in the text.

2. The detailed modeling method can be presented in the method session, the authors should give more information regarding the results, conclusion and application (if possible) of this study in this part.’

R: Thank you. The abstract section has been improved which more emphasis in the results and shorten modeling process.

Introduction

1. “PEEK has shown some advantages such as…, which are usual materials in restorative dentistry” (Line 65-68), some references should be added.

R: References were inserted in this statement.

2. The novelty of this study should be further highlighted in the introduction part.

R: An improvement in the study’s novelty has been performed in the line 73.

Methods

1. It is suggested that a flow chart for the pre-processing is needed in order to help readers better understand the methods.

R: A new figure 1 was inserted with a flow chart for the pre-processing step.

2. Did this study apply any statistical methods to compare the differences between materials? If did, please specify it, and if not, please explain how the significant results came out without any statistical analysis.

R: As the finite element method already provide a calculated results, there is no need to apply a posterior statistic test. In order to predict difference between the groups, the stress maps can be compared qualitatively and the stress peaks (Table 2) assumed as “significant different”, instead statically signiant difference, if this difference was higher than 10% (The mesh convergence, line 129. This result evaluation and interpretation is well defined and accepted in literature, as you observe in studies with similar methodology.

Results

1. The results session should be rewritten based on the statistical analysis.

R: Thank you for your suggestion, however it is not applicable for the present study.

Discussion

1. “FEA” (Line 271), although I know the meaning of this abbreviation, the official explanations should be added.

R: Thank you for your observation. The abbreviation has been explained in the text.

2. “Computer-assisted implant planning has become …the most suitable mechanical results” (Line 282-285), this paragraph should be moved to the introduction part, as it seems to be the novelty of this study.

R:This paragraph has been moved to the introduction section.

Conclusion

1. Are there any implications based on the results of this study? Please specify.

R: The conclusion has been modified to: “For this treatment modality, regardless the loading region, PEEK can be suggested as framework material to reduce the bone strain around the implants and the stress concentration in the bar structure. However, the use of PEEK increases the risk of prosthetic screws loosening and even fracture in comparison with metallic alloys”.

While written English is reasonably good, but please do check the language and grammar mistakes throughout the whole article to improve clarity.

R: The language has been reviewed.

Submitted filename: Answer to reviewers.docx

Click here for additional data file.

12 Oct 2020

PONE-D-20-26196R1

Stress distribution on different bar materials in implant-retained palatal obturator

PLOS ONE

Dear Dr. Tribst,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please, address the changes suggested by reviewer 2.

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We look forward to receiving your revised manuscript.

Kind regards,

Antonio Riveiro Rodríguez, PhD

Academic Editor

PLOS ONE

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: N/A

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

Reviewer #2: Review comment

The authors have addressed the comments carefully. However, there are still some questions that must be revised before this manuscript being accepted for publication. I give a minor revision for this manuscript.

Specific comments

1. Please merge the fourth and fifth paragraphs of the Introduction session further.

2. The purpose of this study was to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars in three different materials under two load incidences, however, from the conclusion (Abstract and Conclusions part), it seems that the authors only indicated some advantages and disadvantages of PEEK material, what about the other two materials? Any differences? Please specify.

3. Abbreviations that exist in main content of this manuscript only need to be explained when they first appear, Line 251 polyetheretherketone (PEEK), Line 292 finite element analysis (FEA), please modify.

**********

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Reviewer #1: No

Reviewer #2: Yes: Yaodong Gu

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14 Oct 2020

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: The authors have addressed the comments carefully. However, there are still some questions that must be revised before this manuscript being accepted for publication. I give a minor revision for this manuscript.

Specific comments

1. Please merge the fourth and fifth paragraphs of the Introduction session further.

R: These paragraphs were merged as suggested.

2. The purpose of this study was to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars in three different materials under two load incidences, however, from the conclusion (Abstract and Conclusions part), it seems that the authors only indicated some advantages and disadvantages of PEEK material, what about the other two materials? Any differences? Please specify.

R: Yes. As we can observe in the results section. Since PEEK is the novelty, our conclusion was based on its mechanical response. However, the conclusion section has been improved.

3. Abbreviations that exist in main content of this manuscript only need to be explained when they first appear, Line 251 polyetheretherketone (PEEK), Line 292 finite element analysis (FEA), please modify.

R: The abbreviation explaining has been already presented in the first apparition of PEEK, in line 65. The first apparition of FEA was in line 79 and the abbreviation explaining has been already presented too.

Submitted filename: Answer to reviewers.docx

Click here for additional data file.

19 Oct 2020

Stress distribution on different bar materials in implant-retained palatal obturator

PONE-D-20-26196R2

Dear Dr. Tribst,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Antonio Riveiro Rodríguez, PhD

Academic Editor

PLOS ONE

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: (No Response)

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: (No Response)

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: (No Response)

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: (No Response)

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

**********

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Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: Yes: Yaodong Gu

21 Oct 2020

PONE-D-20-26196R2

Stress distribution on different bar materials in implant-retained palatal obturator

Dear Dr. Tribst:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

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on behalf of

Dr. Antonio Riveiro Rodríguez

Academic Editor

PLOS ONE