Retention and fatigue performance of modified polyetheretherketone clasps for removable prosthesis.

PURPOSE: Polyetheretherketone (PEEK) is considered as an alternative to metal material for removable partial denture (RPD). However, the retentive force is not strong as a metal RPD. This study investigated the retention and fatigue performance of PEEK clasps with different proportions of clasp arm engaging the undercut to verify a new strategy to improve their clinical performance. METHODS: Three groups (n = 10/group) of PEEK clasps with their terminal 1/3, 2/3 and the whole of retentive arms engaging the undercut were fabricated along with a group (n = 10) of conventional cobalt-chrome (CoCr) clasps as control group. Retentive forces were measured by universal testing machine initially and at an interval of 1500 cycles for a total of 15,000 fatigue cycles. The fatigue cycles were conducted by repeated insertion and removal of the clasp using fatigue testing machine. Each clasp was scanned by Trios3 scanner before and after fatigue test to obtain digital models. The deformation of the clasp was evaluated by root mean square (RMS) through aligning the two models in Geomagic wrap (2021). Scanning electron microscopy (SEM) and finite element analysis were carried out to observe the abrasion and the von Mises stress of the clasp arm. Kruskal-Wallis H test was used to compare the retentive forces and the RMSs of the studied groups followed by Bonferroni multiple comparisons. RESULTS: The whole of PEEK clasp arm engaging the undercut provided higher mean retentive forces (7.99 ± 2.02 N) than other PEEK clasp groups (P < 0.001) and was closer to CoCr clasps (11.88 ± 2.05 N). The RMSs of PEEK clasps were lower than CoCr clasps (P < 0.05) while the differences among PEEK clasps were of no statistical significance (P > 0.05). SEM showed that evidences of surface abrasion were observed on the section that engaged the undercut for all groups of clasps. The stress concentration mainly occurred on the initial part of the retentive arm. The maximum von Mises stress of each group was below the compressive strength of PEEK. CONCLUSIONS: Proportions of PEEK clasp arm engaging the undercut positively influenced the retentive force and the fatigue resistance of PEEK clasps was superior than CoCr clasps. It is a feasible method to improve the retention of PEEK clasps by increasing the proportion of clasp arm engaging the undercut. Clinical trials are needed to further verify this innovation.

Comparative analysis of the retention force and deformation of PEEK and PEKK removable partial denture clasps with different thicknesses and undercut depths.

STATEMENT OF PROBLEM: The retentive force and deformation of milled polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) removable partial denture (RPD) frameworks are not well understood. PURPOSE: The purpose of this in vitro study was to assess the retentive force and deformation of PEEK and PEKK Akers clasps with different designs and undercut depths. MATERIAL AND METHODS: A master model containing the first and second molar abutments was used to design Akers clasps of different cross-sectional dimensions, undercut depths (0.5 and 0.75 mm), and materials. The components of the removable partial denture framework also included an occlusal rest and were manufactured using a milling machine (n=5). The fatigue resistance of the Akers clasps was measured before and after deformation regarding the retentive forces. RESULTS: The PEEK2-U50 clasp had the largest retentive force with no significant difference among all groups before and after the insertion and removal cycle. In addition, the increased cross-sectional dimensions of the design resulted in significant differences in retentive forces between the PEEK1 and PEEK2 groups and between the PEEK and PEKK materials. CONCLUSIONS: Increasing the clasp's cross-sectional dimensions significantly impacted retentive forces, especially between different PEEK groups and between PEEK and PEKK materials.

Assessment the retentive force and XRD analysis on the recycling esthetic thermoplastic acetal clasps / Avaliação da força de retenção e análise de DRX dos grampos de resina estética termoplástica acetal reciclada

Objective: To reduce environmental biohazards, technicians should be educated with safe biological waste disposal procedures and dental material recycling. The present study assesses the retentive force, deformation test and XRD analysis of re-injection acetal resin clasps. Material and Methods: specimens were prepared for clasp retention test injection according to the manufacture's instruction for acetal resin. The first group represent control group with (100% New) and other five groups as experimental groups (group II (25% old +75% new), group III (50% old + 50%) new, and group IV (75% old + 25% new) with different percentages between new acetal materials and old acetal materials, while two last groups with several times (group V (100% old) and group VI (re-injection two times)) only used old acetal materials. The X-ray diffraction (XRD) device used to give structural information for the materials to be tested. The study data were analyzed via One-way ANOVA (LSD) at a significant P-value of (p≤0.05) and a confidence level of 95%. Results: After comparing the results, a significant improvement in the retentive force of acetal clasp specimens after re-injection two times there was found high mean value in group re-injection two times and lower mean value recorded in control group. In XRD patterns of there was no diffraction peak refers to crystal structure of acetal resin. Conclusion: the present study findings concluded that the recycling of acetal resin enhanced the retentive force of acetal clasp and not affected on the crystal structures of material(AU)

Objetivo: Para reduzir os riscos biológicos ambientais, os técnicos devem ser educados com procedimentos seguros de eliminação de resíduos biológicos e reciclagem de material dentário. O presente estudo avalia a força de retenção, teste de deformação e análise de DRX de grampos de resina acetal reinjetada. Material e Métodos: as amostras foram preparadas para o teste de retenção dos grampos e a injeção realizada de acordo com as instruções do fabricante para resina de acetal. O primeiro grupo representa o grupo controle com (100% novo) e os outros cinco grupos como grupos experimentais (grupo II (25% antigo + 75% novo), grupo III (50% antigo + 50% novo) e grupo IV (75% antigo + 25% novo) com porcentagens diferentes entre materiais de acetal novos e antigos, enquanto os dois últimos grupos (grupo V (100% antigo) e grupo VI (reinjeção duas vezes)) utilizaram apenas materiais de acetal antigos. O dispositivo de difração de raios X (DRX) foi utilizado para fornecer informações estruturais dos materiais a testados. Os dados do estudo foram analisados por ANOVA um fator (LSD) com um valor P significativo de (p≤0,05) e um nível de confiança de 95%. Resultados: Após a comparação dos resultados, foi encontrada uma melhora significativa na força de retenção das amostras de grampo de acetal após duas reinjeções, além disso, foi registrado um maior valor médio no grupo reinjeção duas vezes e menor valor médio no grupo controle. Nos padrões de DRX não houve pico de difração referente à estrutura cristalina da resina acetal. Conclusão: os resultados do presente estudo concluíram que a reciclagem da resina de acetal aumentou a força retentiva do grampo e não afetou as estruturas cristalinas do material(AU)

Polyetheretherketone materials for removable partial denture frameworks: an integrative review.

Polyetheretherketone (PEEK) resin is a high-performance thermoplastic polymer that has been introduced as a possible candidate to replace metallic components in dental prostheses. The objective of this integrative review was to compare, through analysis of studies in the literature, the mechanical performance of removable partial denture frameworks and clasps manufactured with PEEK with the performance of cobalt-chromium (Co-Cr) frameworks and clasps. The guiding question was, "Does the use of PEEK as a substitute for Co-Cr alloys for the construction of removable partial denture frameworks result in better mechanical properties?" The PubMed/ MEDLINE, Embase, Web of Science, Scopus, and SciELO databases were searched for articles published through October 2021. The JBI Critical Appraisal Checklist for Quasi-Experimental Studies was used to assess the methodologic quality of the selected in vitro studies. A total of 208 articles were identified. After the exclusion of duplicates and articles that did not meet the inclusion criteria, 7 studies- -4 in vitro and 3 three-dimensional finite element analyses- -published between 2012 and 2021 were included in the integrative review. The appraisal checklist revealed that the reviewed studies had a low risk of bias and high methodologic quality. The results of the review showed that PEEK alloys have adequate mechanical properties for use in clasps and removable partial denture frameworks, but Co-Cr alloys exhibit better mechanical properties and are more suitable in most circumstances.

Comparison of the fatigue life of pure titanium and titanium alloy clasps manufactured by laser powder bed fusion and its prediction before manufacturing.

PURPOSE: In this study, the fatigue properties of additively manufactured titanium clasps were compared with those of commercially pure titanium (CPTi) and Ti-6Al-4V (Ti64), manufactured using laser powder-bed fusion. METHODS: Fourteen specimens of each material were tested under the cyclic condition at 1 Hz with applied maximum strokes ranging from 0.2 to 0.5 mm, using a small stroke fatigue testing machine. A numerical approach using finite element analysis (FEA) was also developed to predict the fatigue life of the clasps. RESULTS: The results showed that although no significant differences were observed between the two materials when a stroke larger than 0.35 mm was applied, CPTi had a better fatigue life under a stroke smaller than 0.33 mm. The distributions of the maximum principal stress in the FEA and the fractured position in the experiment were in good agreement. CONCLUSIONS: Using a design of the clasp of the present study, the advantage of the CPTi clasp in its fatigue life under a stroke smaller than 0.33 mm was revealed experimentally. Furthermore, the numerical approach using FEA employing calibrated parameters for the Smith-Watson-Topper method are presented. Under the limitations of the aforementioned clasp design, the establishment of a numerical method enabled us to predict the fatigue life and ensure the quality of the design phase before manufacturing.

Feasibility study and material selection for powder-bed fusion process in printing of denture clasps.

BACKGROUND AND OBJECTIVE: The retention of selective laser melting (SLM)-built denture clasps is inferior to that of cast cobalt-chromium (Co-Cr) clasps engaging 0.01-in undercuts, which are commonly used in clinical practice. Either the clasps engage in excessively deep undercuts or inappropriate printing process parameters are applied. With appropriate undercut engagement and levels of process parameters, the retention of SLM-built clasps (including Co-Cr, commercially pure titanium [CP Ti], and Ti alloy [Ti-6Al-4V] ones) may be comparable to that of cast Co-Cr clasps. Therefore, this feasibility study aimed to evaluate their retention to guide dentists during material selection for the powder-bed fusion process during the printing of denture clasps. METHODS: We engaged the clasp arm at an appropriate undercut depth (0.01 or 0.02 in), built clasps at the orientation of their longitudinal axes approximately parallel to the build platform, generated square prism support structures at a critical overhang angle of 30°, applied optimized laser parameters (laser power, scan speed, and hatch space), and adopted annealing treatment for Co-Cr, CP Ti, and Ti-6Al-4V clasps. After postprocessing and accuracy measurement, an insertion/removal test of the clasps for 15,000 cycles was performed to simulate 10 years of clinical use, and the retentive force was recorded every 1500 cycles. Permanent deformation of the retentive arms of the clasps was measured. Cast Co-Cr clasps engaging 0.01-in undercuts were designated the control group. RESULTS: The initial retentive forces of the SLM-built Co-Cr clasps engaging 0.01-in undercuts and CP Ti and Ti-6Al-4V clasps engaging 0.02-in undercuts were comparable to those of the control group. SLM-built Co-Cr clasps engaging 0.01-in undercuts and Ti-6Al-4V clasps engaging 0.02-in undercuts had similar final retentive force and less permanent deformation compared with those of the control group; SLM-built CP Ti clasps engaging 0.02-in undercuts had lower final retentive force and greater permanent deformation. CONCLUSIONS: Considering the long-term retention and permanent deformation of the retentive arms, Co-Cr and Ti-6Al-4V alloys, except CP Ti, are recommended for printing denture clasps. SLM-built Co-Cr clasps should engage 0.01-in undercuts, and Ti-6Al-4V clasps should engage 0.02-in undercuts.

Different Undercut Depths Influence on Fatigue Behavior and Retentive Force of Removable Partial Denture Clasp Materials: A Systematic Review.

PURPOSE: To perform a systematic review that provides an overview of the current literature on fatigue behavior of removable partial denture (RPD) clasp materials based on different retentive areas. MATERIALS AND METHODS: The review followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines. Electronic searches were done via PubMed, Scopus, and OVID for studies reporting on RPD clasps and the fatigue failure of clasp materials. Inclusion criteria were English language with full text and in vitro studies only. Exclusion criteria were studies that did not assess the fatigue of RPD clasp materials. A quality assessment and selection of full-text articles were performed according to Consolidated Standards of Reporting Trials criteria. RESULTS: A total of 182 articles were initially identified and screened. After applying inclusion and exclusion criteria, 15 articles were selected for the final analysis. Seven of the included studies utilized a vertical insertion/removal testing approach. Ten studies used the constant deflection test. Three studies used untapered specimens, and 12 studies used tapered specimens. Ten studies performed post-test analysis. CONCLUSIONS: Cobalt-chromium (CoCr) is the strongest material in terms of fatigue in relation to the undercut depth and the modern, digitally manufactured RPD clasp materials also exhibit comparable fatigue behavior. Recent RPD clasp materials such as polyetheretherketone (PEEK) or laser sintered CoCr, however, require further study in terms of their fatigue behavior. In order to improve the quality of future studies, a standardized and calibrated fatigue testing method is needed with standardized specimen size and shape, which will reduce the risk of bias and enable meta-analysis for bulk comparison between studies.

The impact of an additional implant under the saddle of removable partial dentures in Kennedy Class II edentulism on oral health-related quality of life and oral function: a case series report.

BACKGROUND: Implant-supported removable partial dentures (ISRPDs) provide effective prosthodontic treatment for partially edentulous patients. ISRPDs offer greater patient satisfaction and better oral function compared with removable partial dentures (RPDs) by enhancing denture stability and support. However, few clinical studies have focused on RPD design in patients with mandibular Kennedy Class II edentulism. The aim of this case reports was to investigate the oral function, oral health-related quality of life, and satisfaction of four patients with unilateral distal-extension mandibular RPDs with the same design which were replaced with ISRPDs. In addition, we investigated how each patient's evaluation varied with the change from RPD to ISRPD. CASE PRESENTATION: Four patients had unilateral distal-extension mandibular edentulism and were missing the first and second molars and the first and second premolars. They received one implant (4.0 mm in diameter, 8.0 mm in length; IAT EXA PLUS Bone level; Nippon Piston Ring Co. Ltd, Saitama, Japan) at the position equivalent to the first molar in the edentulous residual ridge perpendicular to the occlusal plane. Implant position was determined by surgical guide plate. RPDs were fabricated after the residual mucosal membrane had healed. The basic design of the RPD was as follows: a cobalt-chromium alloy cast metal framework denture with a lingual bar as the major connector, a double Akers clasp on the molars and an auxiliary retainer on the premolar as indirect retainers, and a wrought wire clasp and a cast cingulum rest (combination clasp) as direct retainers. Masticatory performance, occlusal force, oral health-related quality of life, and satisfaction were estimated at baseline, and at time points after insertion of the RPD and after insertion of the adapted ISRPD. Each evaluation item showed a tendency for improvement on insertion of the new RPD. Masticatory performance and satisfaction tended to be better after insertion of the ISRPD than after insertion of the RPD. CONCLUSION: Our findings suggest that ISRPDs provided better patient satisfaction and masticatory performance than RPDs in patients with mandibular Kennedy Class II edentulism. Trial registration UMIN Clinical Trials Registry and Japan Registry of Clinical Trials, UMIN000025283 and jRCTs012180003. Registered 19 February 2016 and 17 December 2018, https://www.umin.ac.jp/ and https://jrct.niph.go.jp/.

[Finite element analyses of retention of removable partial denture circumferential clasps manufactured by selective laser melting].

OBJECTIVE: To compare the retentions of different designs of cobalt-chromium (Co-Cr), pure titanium (CP Ti), and titanium alloy (Ti-6Al-4V) removable partial denture (RPD) circumferential clasps manufactured by selective laser melting (SLM) and to analyze the stress distribution of these clasps during the removal from abutment teeth. METHODS: Clasps with clasp arm size A (1.9 mm width/1.1 mm thickness at the body and 0.8-taper) or B (1.2 times A) and 0.25 mm or 0.50 mm undercut engagement were modeled on a prepared first premolar die, named as designs A1, A2, A3, and A4, respectively. The density and elastic modulus of SLM-built Co-Cr, CP Ti, and Ti-6Al-4V were measured and given to different groups of clasps. The density, elastic modulus, and Poisson ' s ratio of enamel were given to the die. The control group was the cast Co-Cr clasp with design A1, to which the density and elastic modulus of cast Co-Cr alloy were given. The Poisson's ratio of all metals was 0.33. The initial 5 N dislodging force was applied, and the maximum displacement of the clasp along the insertion path was computed. The load was reapplied with an increment of 5 N than in the last simulation until the clasp was completely dislodged. The retentive force range of different groups of clasps was obtained. The retentive forces of the SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with equivalent computed retentive force range to the control group were validated through the insertion/removal experiment. The von Mises stress distributions of these three groups of SLM-built clasps under 15 N loads were analyzed. RESULTS: SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with designs B1 or B2, and Co-Cr clasps with design A2 had higher retentive forces than those of the control group. SLM-built CP Ti and Ti-6Al-4V clasps with design A1 had lower retentive forces than those of the control group. SLM-built Co-Cr clasp with design A1 and SLM-built CP Ti and Ti-6Al-4V clasps with design A2 had equivalent retentive forces to those of the control group. The insertion/removal experiment showed that the measured retentive forces of these three groups of SLM-built clasps were (21.57±5.41) N, (19.75±4.47) N, and (19.32±2.04) N, respectively. No statistically significant measured retentive force difference was found among these three groups of SLM-built clasps (P>0.05). The maximum von Mises stress of these three groups of SLM-built clasps exceeded their responding yield strength except for the Ti-6Al-4V one. CONCLUSION: SLM-built Co-Cr circumferential clasps had higher retention than CP Ti and Ti-6Al-4V ones with the same clasp arm size and undercut engagement. The retention of SLM-built circumferential clasps could be adjusted by changing the undercut engagement and clasp arm size. If SLM-built circumferential clasps are used in clinical practice, the Ti-6Al-4V clasp with clasp arm size A and 0.50 mm undercut engagement is recommended considering the long-term use of RPD in the patient's mouth.

Fitness accuracy and retentive forces of milled titanium clasp.

Since cast titanium prostheses have many drawbacks, multi-directionally forged titanium grade 2 (MDF) was developed, and the application of the milling process was proposed for improving the titanium clasp. This in vitro study evaluated milled titanium clasps, including MDF titanium. Milling clasps were manufactured with commercially pure (CP) titanium grade 2 (CP 2), grade 4 (CP 4), Ti-6-Al-4V, and MDF. As a control, a CP 2 cast titanium clasp was fabricated in the conventional manner. No porosities and catastrophic failures were observed in the four milled titanium clasps. Fitness accuracy and retentive forces of milled CP 2 and CP 4 tended to be worse, and the milled MDF showed the higher retentive forces (12.45 N) than did cast and milled CP 2 clasps (9.32 N and 4.42 N). Milled titanium clasps can be recommended for longer-term clinical use as compared to cast clasps.

Retentive Forces of Removable Partial Denture Clasp Assemblies Made from Polyaryletherketone and Cobalt-Chromium: A Comparative Study.

PURPOSE: To compare retentive forces of removable partial denture clasps traditionally fabricated with cobalt-chromium (CoCr) material and two computer-aided design and computer-aided manufactured (CAD/CAM) thermoplastic polymers. MATERIALS AND METHODS: Forty-eight clasp assemblies (16 CoCr, 16 polyetheretherketone (PEEK) and 16 polyetherketoneketone (PEKK) thermoplastic polymer) were fabricated for 48 mandibular tooth analogs. Individual clasps were inserted and removed on the tooth analogs utilizing a chewing simulator for 15,000 cycles to simulate 10 years of use. Retentive forces were measured utilizing a mechanical load tester at baseline and intervals of 1500 cycles. Data were analyzed with one-way Analysis of Variance, Tukey post-hoc, and paired T tests. RESULTS: Mean retentive forces between all groups were significantly different (p < 0.001). Retentive forces of CoCr clasps were significantly higher than both polymers (p < 0.001). The mean retentive forces for PEEK were not significantly different from PEKK (p = 0.23). A significant increase in retentive forces was observed for all three clasps after the first period of cycling, followed by continual decrease for the remaining cycles. At the endpoint of 15,000 cycles, no clasp assemblies showed lower retentive forces than at initial baseline. CONCLUSION: Thermoplastic polymer clasps demonstrated lower retentive forces compared to CoCr clasps. All three groups displayed a similar pattern of initial increase, followed by a gradual decrease, of retentive force. Despite this observation, the clasps maintained similar or higher retentive forces than measured at baseline. This resistance to fatigue and ability to fabricate with CAD/CAM technologies provides support for clinical use of these high-performance polymer (HPP) materials.

Guidelines for the choice of circumferential wrought wire and cast clasp arms for removable partial dentures.

PURPOSE: The aim of this study was to establish guidelines for the selection of cast and wrought-wire clasps for removable partial dentures (RPDs) that would be appropriate for clinically encountered undercuts and facial curvatures. METHODS: Randomly selected discarded casts were collected and 30 premolars and 30 molars were surveyed, sectioned to a line representing the clasp and scanned using a flatbed scanner. The average clasp curvature and length for each group was determined and a three-dimensional model printed, to which wrought wire clasps of 0.9- and 1.0-mm diameter were adapted. Standard wax clasp patterns were adapted and cast in a stellite alloy. Each clasp was deformed beyond its proportional limit; and the forces exerted at that limit and at deflections of 0.25 mm, 0.5 mm, and 0.75 mm were measured, and a safety limit was calculated that would ensure elastic deformation at the required undercuts. RESULTS: A table was produced with guidelines for those clasps that would provide the highest retentive force within the proposed safety limit. The highest forces were provided by cast clasps in a 0.25-mm undercut. Wrought round wire of 1-mm diameter provided the next highest retentive forces, in a 0.25-mm undercut for premolar clasps arms and 0.5-mm for molar clasps. CONCLUSIONS: The results provide valid guidelines for the use of combinations of clasp material and undercut that would exert the maximum retentive force without deformation for both short (premolar) as well as long (molar) clasps, for wrought and cast clasps.

Retentive force and fitness accuracy of cobalt-chrome alloy clasps for removable partial denture fabricated with SLM technique.

PURPOSE: Evaluating the fitness accuracy and retentive force of cobalt-chrome (Co-Cr) alloy clasps fabricated using the selective laser melting (SLM) technique. METHODS: Premolar and molar abutment models with a 0.5-mm undercut depth, 1.5-mm-thick occlusal rest seats, and guiding planes were designed and fabricated using a milling machine. On these models, Akers clasps with 0.25- and 0.5-mm undercut depths were designed and fabricated with SLM and a traditional lost wax casting method. Based on the manufacturing methods, abutment types, and undercut depths, the clasps were divided into eight groups (10 per group). The fitness accuracy of the clasps was evaluated by measuring the gap distance between the clasps and abutments using a silicone film method. The initial retentive force and changes in retention up to 7,200 insertion/removal cycles of the clasps were also measured. The data were analyzed using multiple linear regression, paired t-tests, and one-way ANOVA (α=0.05). RESULTS: For both the SLM and cast clasps, the fitness accuracy of the rest was greater than that of the clasp tip and shoulder. No significant difference was found in the fitness accuracy between the SLM and cast clasps, regardless of the abutment type and undercut depth before or after insertion/removal cycles (p>0.05). There was also no significant difference in the initial retentive force between the SLM and cast clasps (p>0.05). After 7,200 insertion/removal cycles, the SLM clasp exhibited a greater residual retentive force (p<0.05). CONCLUSION: The SLM technique for manufacturing the clasps of removable partial dentures has promising clinical applications.

Finite element analyses of retention of removable partial denture circumferential clasps manufactured by selective laser melting / 北京大学学报(医学版)

OBJECTIVE@#To compare the retentions of different designs of cobalt-chromium (Co-Cr), pure titanium (CP Ti), and titanium alloy (Ti-6Al-4V) removable partial denture (RPD) circumferential clasps manufactured by selective laser melting (SLM) and to analyze the stress distribution of these clasps during the removal from abutment teeth.@*METHODS@#Clasps with clasp arm size A (1.9 mm width/1.1 mm thickness at the body and 0.8-taper) or B (1.2 times A) and 0.25 mm or 0.50 mm undercut engagement were modeled on a prepared first premolar die, named as designs A1, A2, A3, and A4, respectively. The density and elastic modulus of SLM-built Co-Cr, CP Ti, and Ti-6Al-4V were measured and given to different groups of clasps. The density, elastic modulus, and Poisson ' s ratio of enamel were given to the die. The control group was the cast Co-Cr clasp with design A1, to which the density and elastic modulus of cast Co-Cr alloy were given. The Poisson's ratio of all metals was 0.33. The initial 5 N dislodging force was applied, and the maximum displacement of the clasp along the insertion path was computed. The load was reapplied with an increment of 5 N than in the last simulation until the clasp was completely dislodged. The retentive force range of different groups of clasps was obtained. The retentive forces of the SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with equivalent computed retentive force range to the control group were validated through the insertion/removal experiment. The von Mises stress distributions of these three groups of SLM-built clasps under 15 N loads were analyzed.@*RESULTS@#SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with designs B1 or B2, and Co-Cr clasps with design A2 had higher retentive forces than those of the control group. SLM-built CP Ti and Ti-6Al-4V clasps with design A1 had lower retentive forces than those of the control group. SLM-built Co-Cr clasp with design A1 and SLM-built CP Ti and Ti-6Al-4V clasps with design A2 had equivalent retentive forces to those of the control group. The insertion/removal experiment showed that the measured retentive forces of these three groups of SLM-built clasps were (21.57±5.41) N, (19.75±4.47) N, and (19.32±2.04) N, respectively. No statistically significant measured retentive force difference was found among these three groups of SLM-built clasps (P>0.05). The maximum von Mises stress of these three groups of SLM-built clasps exceeded their responding yield strength except for the Ti-6Al-4V one.@*CONCLUSION@#SLM-built Co-Cr circumferential clasps had higher retention than CP Ti and Ti-6Al-4V ones with the same clasp arm size and undercut engagement. The retention of SLM-built circumferential clasps could be adjusted by changing the undercut engagement and clasp arm size. If SLM-built circumferential clasps are used in clinical practice, the Ti-6Al-4V clasp with clasp arm size A and 0.50 mm undercut engagement is recommended considering the long-term use of RPD in the patient's mouth.

Analytical model of I-bar clasps for removable partial dentures.

OBJECTIVE: Clasps of removable partial dentures (RPDs) often suffer from fatigue stress that leads to plastic deformation, loss of retention, and RPD failure. Recently, computer-based technologies were proposed to optimize clasp geometry design. The objective of this study was to create an analytic model of I-bar clasps for computer-aided design (CAD)-RPD. METHODS: The analytical model based on mechanical laws was established to simulate I-bar clasp retention, and optimize its design. The model considered the lengths of the vertical (L1) and horizontal (L2) arms of the I-bar as well as the radius (r) of its half-round cross-section. The analytical model was validated with mechanical experiments evaluating the retention of cobalt-chromium (Co-Cr) clasps in vitro and compared with finite element analysis (FEA). RESULTS: The analytical model was in good agreement with the mechanical experiments and FEA, and showed that I-bar clasp design could provide optimal mechanical performance as long as the length of arms (L1 and L2) do not exceed 6 mm. Clasps with L1 > 8 mm and L2 > 9 mm presented stress values exceeding the fatigue limit of Co-Cr. The proposed solution was to increase the radius of I-bar to conserve the initial mechanical performance of Co-Cr. SIGNIFICANCE: Co-Cr I-bar clasps perform best on teeth with reduced mesiodistal dimensions (canine and premolar), and their designs could be optimized to prevent stress from reaching the yield strength and the fatigue failure limit.

Effects of repetitive insertion/removal cycles and simulated occlusal loads on retention of denture retainers.

The purpose of this study was to investigate the effects of repetitive insertion/removal cycle tests on denture retainers with simulated occlusal loads on the retentive force and deformation of clasp. Abutment teeth in the form of mandibular secondary premolars and clasp in the form of Akers clasps were prepared. The retentive force of the clasp on the abutment teeth were evaluated before and after undergoing repetitive insertion/removal cycle tests with or without cyclic loading. Changes in the clasp shape were monitored using a 3D scanner and scanning electron microscope. The initial retentive force was approximately 10 N and this value later decreased due to deformation of the clasp tips. In contrast to the non-load group, the load group exhibited a reduction in retentive force during earlier stages. Therefore, cyclic loading was related to a decrease in retentive forces, specifically in the early stages of repetitive insertion/removal cycles.

Laboratory efficiency of additive manufacturing for removable denture frameworks: A literature-based review.

The purpose of this literature review was to verify the laboratory efficiency of additive manufacturing (AM) systems for removable partial denture (RPD) frameworks. All available relevant articles in English published from 1990 to 2020 were found by searching online databases and by hand research. A total of 17 articles dealt with the surface roughness, fitness accuracy, and retentive forces of AM frameworks. The surface roughness of AM was inferior to that of casting and milling. Whether conventional cast or AM RPD frameworks had superior fitness accuracy could not be clarified. As compared with casting and AM, milling enabled the fabrication of RPD clasps with comparable or better fitness accuracy. Over time, AM clasps had retentive force values of superior consistency as compared with those of conventional cast clasps. Clasps fabricated by repeated laser sintering and high-speed milling could obtain smoother surfaces and more suitable retention than those of AM clasps.

Retention force of polyetheretherketone and cobalt-chrome-molybdenum removable dental prosthesis clasps after artificial aging.

OBJECTIVES: To examine the retention force of removable dental prosthesis (RDP) clasps made from polyetheretherketone (PEEK) and cobalt-chrome-molybdenum (CoCrMo, control group) after storage in water and artificial aging. MATERIALS AND METHODS: For each material, 15 Bonwill clasps with retentive buccal and reciprocal lingual arms situated between the second pre- and first molar were manufactured by milling (Dentokeep [PEEKmilled1], NT digital implant technology; breCAM BioHPP Blank [PEEKmilled2], bredent), pressing (BioHPP Granulat for 2 press [PEEKpressed], bredent), or casting (remanium GM 800+ [CoCrMo], Dentaurum); N = 60, n = 15/subgroup. A total of 50 retention force measurements were performed for each specimen per aging level (initial; after storage [30 days, 37 °C] and 10,000 thermal cycles; after storage [60 days, 37 °C] and 20,000 thermal cycles) in a pull-off test. Data were statistically analyzed using one-way ANOVA, post hoc Scheffé and mixed models (p < 0.05). RESULTS: Initial, PEEKpressed (80.2 ± 35.2) and PEEKmilled1 (98.9 ± 40.3) presented the lowest results, while PEEKmilled2 (170.2 ± 51.8) showed the highest values. After artificial aging, the highest retention force was observed for the control group (131.4 ± 56.8). The influence of artificial aging was significantly higher for PEEK-based materials. While PEEKmilled2 and PEEKpressed showed an initial decline in retention force, all other groups presented no impact or an increase in retention force over a repetitive insertion and removal of the clasps. CONCLUSIONS: Within the tested PEEK materials, PEEKmilled2 presented superior results than PEEKpressed. Although CoCrMo showed higher values after artificial aging, all materials exhibited sufficient retention to recommend usage under clinical conditions. CLINICAL RELEVANCE: As RDPs are still employed for a wide range of indications, esthetic alternatives to conventional CoCrMo clasps are sought.

Is the high-performance thermoplastic polyetheretherketone indicated as a clasp material for removable dental prostheses?

OBJECTIVES: To investigate the retention force of polyetheretherketone (PEEK) removable dental prosthesis clasps in comparison with a cobalt-chrome-molybdenum control group after storage in artificial saliva. MATERIALS AND METHODS: Clasps were milled (Dentokeep (PEEKmilled1), NT digital implant technology; breCAM BioHPP Blank (PEEKmilled2), bredent), pressed (BioHPP Granulat for 2 press (PEEKpressed), bredent), or cast (remanium GM 800+ (cobalt-chrome-molybdenum), Dentaurum); N = 60, n = 15/subgroup. Retention force was examined 50 times/specimen in a pull-off test using the universal testing machine (Zwick 1445), where pull-off force was applied with a crosshead speed of 5 mm/minute until the maximum force dropped by 10%, at different aging levels: (1) initial, after storage in artificial saliva for (2) 90 and (3) 180 days. Statistical analysis was performed using one-way ANOVA followed by post hoc Scheffé-test and mixed models (p < 0.05). RESULTS: Cobalt-chrome-molybdenum presented the highest retention force. No differences were observed between polyetheretherketone materials. Cobalt-chrome-molybdenum showed a significant decrease of its values after artificial aging, while polyetheretherketone materials presented similar results over the course of aging. Regarding a repetitive insertion and removal, even though PEEKmilled2 and cobalt-chrome-molybdenum showed an initial increase, ultimately, a decrease in retention force was observed for all tested groups. CONCLUSIONS: Although the control group showed significantly higher results, the retention force of polyetheretherketone materials indicate a potential clinical application. Neither the manufacturing process nor artificial aging showed an impact on the retention force of polyetheretherketone clasps. CLINICAL RELEVANCE: Mechanical properties of novel removable dental prosthesis clasp materials devised to meet the growing esthetic demands of patients need to be investigated to ensure a successful long-term clinical application.