Cast Clasps Research Articles

Influence of Different Undercut Depths of Clasp Fabricated by Selective Laser Melting on Retentive Force.

The purpose of this study was to investigate the influence of undercut depths on abutment teeth regarding the retentive force of clasps fabricated through selective laser melting (SLM), and to compare them with conventional cast clasps. Akers clasps made of cobalt chromium alloy were fabricated using the SLM method (SLM), and the retentive forces were compared with clasps made with the conventional cast method (Cast). Three undercut amounts (0.25 mm, 0.15 mm, and 0 mm) were applied on the abutment tooth. The specimens were subjected to 10,000 repetitive insertion/removal cycles. SLM-0.15 showed slightly lower initial retentive force than the Cast specimens, it remained within an acceptable range. During insertion/removal test, the SLM-0.15 specimen showed a significant difference between the initial retentive force and the retentive force after 5,000 cycles, indicating that SLM-0.15 was the least likely to change in retentive force within the parameters established in this study. The inner clasp surface on the SLM groups had higher surface roughness before testing compared to the Cast specimen. Akers clasps fabricated by SLM demonstrated optimal initial retentive forces with smaller undercuts than conventional Cast clasps, and the retentive forces changed less with repetitive insertion/removal.

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

PurposeThe 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. MethodsRandomly 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. ResultsA 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. ConclusionsThe 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.

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.

Examination of inter-rater and intra-rater reliability during retentive force measurement of different clasps using the developed small-sized retentive force measurement device

BackgroundThe design process of a removable partial denture (RPD) consists of rests, major connectors, minor connectors, denture base, and retainer. The abutment tooth contour determines the location of the retention portion of the clasp at the retention areas. The load capacity of the clasp depends on various factors such as type, the position of a clasp, tooth position, clasp length, and pullout location. As a general rule, the amount of retention required to dislodge the RPD from the supporting structure should always be the minimum necessary to resist reasonable dislodging forces. Excessive force from the clasps may cause many problems. Although there are many reports on the retentive force of cast clasps using large devices, it has not been possible to measure it using simple devices until now.MethodThe purpose of the present study is to develop a small-sized retentive force measurement device that can easily measure the retentive force of a clasp used for an RPD. It is to examine inter-rater and intra-rater reliability. A small-sized retentive force measurement device that can be easily measured in the laboratory has been developed. Using commercially available hard plaster cast, a skilled dental technician has made 10 types of cast clasps used in clinical practice using conventional techniques. Three assessors measured the retentive force of 10 types of cast clasps. To confirm the reliability, the intra-class correlation coefficients ICC (1,1) and ICC (1,3) of the 3 assessors were calculated, and the reliability within the assessor was examined. The inter-class correlation coefficients ICC (3,1) and ICC (3,3) were calculated, and the reliability of the assessors was examined.ResultThe intra-class correlation coefficients of 3 assessors are as follows: assessor 1 has ICC (1,1) = 0.971, ICC (1,3) = 0.990, assessor 2 has ICC (1,1) = 0.967, ICC (1,3) = 0.989, assessor 3 has ICC (1,1) = 0.962, ICC (1,3) = 0.987. The inter-class correlation coefficients of 3 assessors are as follows: ICC (3,1) = 0.993, ICC (3,3) = 0.998. From the evaluation standard of the intraclass correlation coefficients of reliability value by ICC, it was evaluated as almost perfect and high reproducibility was confirmed.ConclusionThe developed small-sized retentive force measurement device has reproducibility within and between the assessors.

Fitness accuracy and retentive forces of additive manufactured titanium clasp

PurposeLaboratory and clinical assessment of cast titanium dentures were reported, little is known about additive manufacturing (AM) frameworks. This study evaluated in vitro AM titanium alloy clasps for clinical use. MethodsAfter scanning the stainless steel dies to simulate the first molar, an Akers clasp was designed by CAD. AM clasp specimens were manufactured using laser sintering and AM machines from CP titanium grade 2 (CPTi), Ti-6Al-4V (Ti64), and Ti-6Al-7Nb (Ti67) powders. As controls, cast titanium clasps were conventionally made with same three titanium alloys. After nondestructive inspection and surface element analysis, surface roughness, fitness accuracy, initial retentive forces, and changes in retentive forces were measured. ResultsAlthough few porosities and little contamination of AM clasps were confirmed, the AM clasp arm and tip indicated slightly worse fitness accuracy as compared to cast clasps. The initial retentive forces of all titanium clasps showed appropriate retentive forces within the acceptable ranges, AM clasps tended to have lower decreases in retentive forces with up to 10,000 insertion/removal cycles as compared to those of the cast clasps. ConclusionsAM titanium clasps would be acceptable for clinical use similar to that of cast titanium clasps.

Influence of retainer and major connector designs of removable partial dentures on the stabilization of mobile teeth: A preliminary study.

The purpose of this study was to investigate the influence of retainers and major connector designs on the stabilization of remaining mobile teeth using removable partial dentures (RPDs). We prepared experimental RPDs with several retainer designs and major connectors for lower Kennedy class I models. The simulated RPD insertion and removal test was conducted and retentive force and mobility of mobile remaining teeth with and without RPD placement were measured throughout a simulation test. Regardless of reduction of retentive force, the placement of RPDs using cast clasps and/or lingual plates resulted in reduced mobility of the remaining teeth than use of wrought wire clasps and/or lingual bars. The results suggested that cast clasps and lingual plates are effective for the stabilization of mobile, remaining teeth. Additionally, the stabilizing effect of RPD on abutment teeth was not diminished, despite decreases in retentive force.

Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering

PurposeThe purpose of this study was to evaluate internal porosities, retentive force values and survival of cobalt–chromium (Co–Cr) alloy clasps fabricated by direct metal laser-sintering (DMLS) and compare them to conventionally cast clasps. MethodsEmbrasure clasps were digitally designed fitting teeth 35 and 36 on identical metal models (N = 32). Sixteen clasps were fabricated using DMLS (group DMLS) and another sixteen clasps were additively manufactured from wax and then cast from a Co–Cr alloy (group CAST). Internal porosities were examined using micro-focus X-ray (micro-CT) and analyzed applying Kolmogorov–Smirnov test, Mann–Whitney test, and T test (significance level: p < 0.050). A universal testing machine was used to determine the retentive force values at baseline and after 1095, 5475, 10,950 and 65,000 cycles of simulated aging. Data were analyzed employing Kolmogorov–Smirnov test, one-way ANOVA, and Scheffé’s post-hoc test (significance level: p < 0.050). Survival was estimated for 65,000 cycles of artificial aging using Kaplan–Meier analysis. ResultsMicro-CT analysis revealed a higher prevalence (p < 0.001), but a more homogeneous size and a significantly smaller mean (p = 0.009) and total volume (p < 0.001) of internal porosities for group DMLS. The groups showed mean initial retentive force values of 13.57 N (CAST) and 15.74 N (DMLS), which significantly declined over aging for group CAST (p = 0.003), but not for group DMLS (p = 0.107). Survival was considerably higher for group DMLS (93.8%) than for group CAST (43.8%) after 65,000 cycles of aging. ConclusionsClasps made by laser-sintering could be an alternative to conventional cast clasps for the fabrication of removable partial denture frameworks.

The effect of build orientation on the microstructure and properties of selective laser melting Ti-6Al-4V for removable partial denture clasps

Statement of problemSelective laser melting (SLM) Ti-6Al-4V has been used for removable partial dentures, but the impact of SLM Ti-6Al-4V build orientation is not evident. PurposeThe purpose of this in vitro study was to investigate the microstructure and properties of SLM Ti-6Al-4V clasps with different build orientations compared with cast Ti-6Al-4V clasps. Material and methodsForty-eight clasps were made from Ti-6Al-4V alloys—by SLM with 3 different build orientations (SLM0, SLM45, and SLM90) and cast (CAST) as a control. The microstructure was investigated by using a metallographic microscope and a confocal laser scanning microscope. The fit and surface roughness of the clasps were measured, and the physical properties were evaluated. In addition, the von Mises stresses in the clasps were calculated by finite element analysis. All specimens were then subjected to insertion and removal tests in artificial saliva to model 5 years of clinical use. After these tests, 3-point bend tests were used to analyze the fracture surface of the clasp arms, which were observed by using a scanning electron microscope. All data were statistically analyzed (α=.05). ResultsThe microstructure of the Ti-6Al-4V specimens was a comixture of α+β phases. In addition, growth directions of β grains were approximately parallel to the build orientation, with acicular α grains present between β grains. SLM0 and SLM45 had significantly higher roughness than SLM90. Even though the fit was inferior to that of SLM90, SLM0 and SLM45 still performed better than cast specimens (P<.05). The finite element analysis showed that the maximum von Mises stress was located on the middle part of the retainer arms and that the values of the 0.50-mm undercut clasps were much lower than the elastic limit. In addition, the decrease of retentive force in SLM90 clasps was less than that of the CAST group (P<.05). CAST clasps showed brittle fracture, whereas all SLM clasps showed ductile fracture. ConclusionsThe microstructure of SLM Ti-6Al-4V affected the properties of clasps by changing the anisotropy of specimens. Among the tested groups, SLM90 clasps had the best fit, the lowest surface roughness, and the best fatigue resistance. Furthermore, SLM Ti-6Al-4V clasps could be engaged into 0.50-mm undercuts.

Titanium clasp fabricated by selective laser melting, CNC milling, and conventional casting: a comparative in vitro study

PurposeThis study aimed to investigate the effects of selective laser melting (SLM), milling methods, and casting on the behavior of titanium clasp. MethodsThe clasp and its die simulating the molar were designed using 3D software. Clasp specimens were fabricated using SLM approaches (SLM Ti) and computerized numerical control (CNC) milling technology (Milling CPTi). Cast clasps of the same forms were also prepared as controls using titanium alloy powder (Cast Ti) and commercial pure titanium (Cast CPTi), following the conventional casting methods. The surface roughness and accuracy of clasps were analyzed. The changes in retentive force and permanent deformation were measured up to 10,000 insertion/removal cycles. One-way analysis of variance and Tukey’s test or Kruskal–Wallis H test were performed for data analysis and comparisons. ResultsThe Milling CPTi clasps had a smoother inner surface than the other groups (p<0.05). The accuracy of the inner surface showed no significant difference among the groups, whereas that of the outer surface showed significant differences (p<0.05). The SLM Ti clasp had significantly higher retentive forces than the other groups (p<0.05), but it rapidly reduced after 2000 insertion/removal cycles until the fracture of all specimens was at 4000 cycles. The Milling CPTi clasps had more permanent deformation, but the rate of reduction of retentive force was only 9.5% (at 10,000 cycles). ConclusionsMilling has the potential to replace casting for fabricating removable partial denture (RPD) titanium clasps. However, SLM should be further improved for fabricating RPD titanium clasps before clinical application.

Effect of cyclic bend loading on a cobalt-chromium clasp fabricated by direct metal laser sintering

Statement of problemLimited information is available regarding the mechanical properties of a partial removable dental prosthesis clasp fabricated by direct metal laser sintering (DMLS). PurposeThe purpose of this in vitro study was to measure the mechanical properties of cast clasps, DMLS clasps, and annealed DMLS clasps and to determine the correlation between cyclic bend loading, annealing, and mechanical properties. Material and methodsSixty half round clasp specimens were fabricated, with a length of 20 mm and a diameter of 1.5 mm at 10 mm from the tip. The specimens were divided into 6 groups (n=10). Twenty specimens were fabricated by casting (CS, CSAC), and 40 specimens were fabricated by DMLS (DS, DSAC, ADS, ADSAC). Of the DMLS specimens, 20 underwent annealing (ADS, ADSAC). Equivalent to 5 years of clinical use, 0.25 mm of bending was applied to group CSAC, DSAC, and ADSAC 14 600 times. After cyclic bending loads, a cantilever bend test was conducted on all groups, and the loads were measured. To compare the load values, 1-way ANOVA and the Tukey HSD test were performed and statistical significance among the groups was evaluated (α=.05). Scanning electron microscopy and energy-dispersive spectroscopy analyses were also used for surface morphology evaluation and element analysis. ResultsBending loads of the DMLS specimens were significantly lower than those of the cast specimens (P<.05). The influence of cyclic bending loads was significant for cast specimens (P<.05), but not for DMLS specimens (P>.05), in terms of variation of loads. Annealed DMLS specimens had significantly higher bending loads than DMLS specimens (P<.05). ConclusionsThe DMLS specimens had lower load values at 0.25 mm bending than cast specimens. The DMLS specimens were less influenced by cyclic bending loads than the cast specimens. Annealing increased load values of the DMLS specimens at 0.25 mm bending.

Fitness and retentive force of cobalt-chromium alloy clasps fabricated with repeated laser sintering and milling

PurposeWith computer-aided design and computer-aided manufacturing (CAD/CAM), the study was conducted to create a removable partial denture (RPD) framework using repeated laser sintering rather than milling and casting techniques. This study experimentally evaluated the CAM clasp and compared it to a conventional cast clasp. MethodsAfter the tooth die was scanned, an Akers clasp was designed using CAD with and without 50μm of digital relief on the occlusal surface of the tooth die. Cobalt-chromium (Co-Cr) alloy clasps were fabricated using repeated laser sintering (RLS) and milling as one process simultaneously (hybrid manufacturing; HM). The surface roughness of the rest region, gap distances between clasp and tooth die, initial retentive forces, and changes of retentive forces up to 10,000 insertion/removal cycles were measured before and after heat treatment. The HM clasp was compared to the cast clasp and the clasp made by repeated laser sintering only without a milling process. ResultsThe HM clasp surface was smoother than those of cast and RLS clasps. With the digital relief, the fitness accuracy of the HM clasp improved. The retentive forces of the HM clasps with relief and after heat treatment were significantly greater than for the cast clasp. HM clasps demonstrated a constant or slight decrease of retention up to 10,000 cycles. ConclusionsHM clasp exhibited better fitness accuracy and retentive forces. The possibility of clinically using HM clasps as well as conventional cast clasps can be suggested.

Clasp fabrication using one-process molding by repeated laser sintering and high-speed milling

PurposeA single machine platform that integrates repeated laser sintering and high-speed milling for one-process molding has been developed. MethodsThe Akers clasp was designed using the CAD system (DWOS Partial Frameworks, Dental Wings) and fabricated using repeated laser sintering and a high-speed milling machine (LUMEX Advance-25, Matsuura) with 50-μm Co–Cr particles (CAM clasp). As controls, cast clasps of the same forms were also prepared using conventional casting methods with a Co–Cr alloy and CP titanium Grade 3. After the surface roughness was measured, the gap distance between the clasps and the tooth die was assessed using the silicone film method. The initial retentive force and changes in retention up to 10,000 cycles were also measured. The data were analyzed using two-way ANOVA and Tukey’s multiple comparison test (α=0.05). ResultsCAM clasps exhibited significantly smoother surfaces than those of cast Co–Cr and CP Ti clasps (p<0.05). However, the gap distances of the CAM clasps were significantly greater than those of the cast clasps (p<0.05). The retentive forces of both CAM and cast Co–Cr clasps were significantly higher than those of CP Ti clasps. (p<0.05). The retention of CAM clasps demonstrated a constant or slight decrease from 1000 up to 10,000 cycles. ConclusionsThe CAM clasp made by repeated laser sintering and high-speed milling can be used effectively as an RPD component.

上顎前歯部を含む中間欠損に対して，インプラント支持オーバーデンチャーを 用いた1 症例：バーアタッチメントと鋳造鉤を用いたIOD

上顎前歯部を含む中間欠損に対して，インプラント支持オーバーデンチャーを 用いた1 症例：バーアタッチメントと鋳造鉤を用いたIOD

Fatigue resistance and retentive force of cast clasps treated by shot peening

PurposeThe fatigue failure of denture clasps has often been observed in removable partial denture rehabilitation. To increase their fatigue strength, shot peening was evaluated as a surface treatment. In this study, we evaluated the fatigue resistance and retention of cast clasps by using a shot peening treatment. MethodsA cobalt–chromium (Co–Cr) alloy, commercial pure titanium (CP Ti), silver–palladium–gold (Ag–Pd–Au), and a gold–platinum (Au–Pt) alloy were cast and then treated with shot peening. The retentive forces of the clasps were measured up to a repetition of 10,000insertion/removal cycles in distilled water at 37°C. A fatigue test was also performed using a 15-mm cantilever. Specimens were loaded with a constant deflection of 2.0mm with 20Hz. A shot peening treatment indicated a better stability of retentive forces than that without shot peening. The retentive force of Co–Cr clasps without shot peening was remarkably decreased at 500 cycles of insertion/removal repetition. ResultsThe clasps with a shot peening treatment provided approximately 1.4–3.6 times higher fatigue strengths than those without a shot peening treatment. ConclusionTo prevent the fatigue failure of the denture clasps and use the dentures for long term, a shot peening treatment would be recommended.

Retention Strength of Cobalt-Chromium vs Nickel-Chromium Titanium vs CP Titanium in a Cast Framework Association of Removable Partial Overdenture

There is little information considering the framework association between cast clasps and attachments. The aim of this study was to evaluate the retention strength of frameworks match circumferential clasps and extra resilient attachment cast in three different alloys (cobalt-chromium, nickel-chromium titanium and commercially pure titanium), using two undercut (0.25 and 0.75 mm) and considering different period of time (0, 1/2, 1, 2, 3, 4 and 5 years). Using two metallic matrices, representing a partially edentulous mandibular right hemiarch with the first molar crown, canine root and without premolars, 60 frameworks were fabricated. Three groups (n = 20) of each metal were cast and each group was divided into two subgroups (n = 10), corresponding the molar undercut of 0.25 mm and 0.75 mm. The nylon male was positioned at the matrix and attached to the acrylic resin of the prosthetic base. The samples were subjected to an insertion and removal test under artificial saliva environment. The data were analyzed and compared with ANOVAs and Tukey's test at 95% of probability. The groups cast in cobaltchromium and nickel-chromium-titanium had the highest mean retention strength (5.58 N and 6.36 N respectively) without significant difference between them, but statistically different from the group cast in commercially pure titanium, which had the lowest mean retention strength in all the periods (3.46 N). The association frameworks using nickel-chromium- titanium and cobalt-chromium could be used with 0.25 mm and 0.75 mm of undercut, but the titanium samples seems to decrease the retention strength, mainly in the 0.75 mm undercut. The circumferential clasps cast in commercially pure titanium used in 0.75 mm undercuts have a potential risk of fractures, especially after the 2nd year of use. This in vitro study showed that the framework association between cast clasp and an extra resilient attachment are suitable to the three metals evaluated, but strongly suggest extra care with commercially pure titanium in undercut of 0.75 mm. Frameworks fabricated in Cp Ti tend to decrease in retentive strength over time and have a potential risk of fracture in less than 0.75 mm of undercut.

The accuracy of fit of cast clasps designed with conventional wax pattern and light cured patterns - A comparative in vitro study

The accuracy of fit of cast clasps designed with conventional wax pattern and light cured patterns - A comparative in vitro study

Systematic Clinical Evaluation and Correction Procedures for Support of Removable Partial Dentures

Support, retention, and bracing are the three main functions of a direct retainer in removable partial dentures (RPDs). RPDs must have sufficient supporting ability for proper occlusal rehabilitation. Support ability depends on the fit, size, shape, and location of the occlusal rest. Support cannot be adjusted chairside in most cases. The purpose of this article is to present systematic evaluation criteria for support in an RPD and to describe methods for adjusting cast clasps with improper support. Appropriate design of RPDs and preparation of abutments are also described.

Finite Element Analysis of Thermal Stresses in Circumferential Cast Clasps of Removable Partial Dentures

Several studies evaluated the removable partial dentures by the finite element analysis, but none of them evaluated thermal stresses. The purpose of the study was to explore the influence of thermal oral changes induced by hot/cold liquids and food on the circumferential cast clasps of removable partial dentures. A 3-dimensional finite element method was used to explore the temperature distribution, thermal stress and the influence of thermal changes on stresses and displacements of circumferential clasps during functions. Thermal variations induce stresses in dental clasps, high temperatures having a more aggressive effect than lower one. Cold liquids and food induce high stresses in the retentive clasp arms while hot ones in the occlusal rests of the clasps and for the back action clasp also in the minor connector. The study suggests the importance of consFigureidering thermal variations for stress analyses of the cast clasps.

Pre-overloading to Extend Fatigue Life of Cast Clasps

Permanent deformation in bending is associated with the development of residual stresses. The objective of this study was to characterize those residual stresses and test whether they can be manipulated to extend the fatigue lives of cast clasps. Simulations with validated non-linear finite element models were used to characterize the residual stresses in clasps cast from Ti-6Al-7Nb, Co-Cr, and Type IV gold alloys. In addition, two groups of as-cast and pre-overloaded (subjected to a load that produced 20 microm of permanent deformation) Ti-6Al-7Nb clasps (10 specimens each) were subjected to cyclic 0.5-mm deflections at 5 Hz until fatigue. Pre-overloaded specimens demonstrated significantly longer fatigue lives (32,200 +/- 17,300 cycles) than did those tested in the as-cast condition (17,900 +/- 7600 cycles), consistent with the maximum tensile stress values revealed by finite element analysis.

Design and Finite Element Analysis of Circumferential Cast Clasps

Design and Finite Element Analysis of Circumferential Cast Clasps