Acetal Resin Clasp Research Articles

Clinical Evaluation of Retention of Metallic Versus Thermoplastic Resin Frameworks in Maxillary Distal Extension Cases

Background and Objective: Acetal resin has been introduced as an esthetic partial denture material. However, the retention of this material when compared to cobalt-chromium (Co-cr) frame works still unclear when applied clinically. Purpose: To evaluate the retention of (acetal resin) as compared to (Co-cr)frame works and clasps. Materials and Method: The present study is a randomized controlled trial that included 24 healthy patients with Kennedy class I partially edentulous maxilla and dentulous mandibles. The patients were selected from the out-patient clinic (Dep. of Prosthetic Dentistry, Minia University) and divided randomly into two equal groups according to the used denture base material; Group 1 received maxillary partial denture with (Co-cr) clasps and frameworks. Group 2 received maxillary partial denture with acetal resin clasps and frameworks. Retention of the prosthesis was evaluated at partial denture insertion, after 3, 6 and 12 months. Paired t-test was used at p ≤ 0.05 to assess the changes in the above parameter in each group. Student t-test was used to compare between the two groups. Results: Retentive force mean values of thermoplastic group were significantly lower than metallic group at denture insertion and at all follow up intervals. Conclusion: Acetal resin frameworks and clasps provide lesser retention values when compared to chrome cobalt. Clinical Implications: Although there was lesser retention power of acetal resin than Co-cr, further study has to be done regarding various thicknesses and designs of clasps and framework for its successful dental application.

Restoring the harmony of function and aesthetics in partially edentulous arches: A case report

Restoring the harmony of function and aesthetics in partially edentulous arches: A case report - APRD- Print ISSN No: - 2581-4796 Online ISSN No:- 2581-480X Article DOI No:- 10.18231/j.aprd.2019.029, IP Annals of Prosthodontics and Restorative Dentistry-IP Ann Prosthodont Restor Dent

Different Materials Used as Denture Retainers and Their Colour Stability.

BACKGROUND:Retainers are of great importance for the longevity of the prosthetic removable partial denture during various functions especially the esthetic one. The key of successful clasp selection is to select a direct retainer that will control tipping and torquing forces on the abutment teeth, provide retention against reasonable dislodging forces and are compatible with both tooth and tissue contour together with the aesthetic desire of the patient.AIM:This study aims to compare different clasp material to enhance the choice of the clasp based on the aesthetic point of view.METHODS:The colour evaluation of the tested materials had been evaluated by computer aided technique with digital camera with 3 Mega Pixels of resolution.RESULTS:In the current research, the technique of colour evaluation was carried out to compare different clasp materials to enhance the choice of the clasp based on the aesthetic point of view. Most commonly, Removable Partial Denture (RPD) retainers are fabricated identically from the metal framework’s alloy as Cobalt Chromium (CoCr) alloy although it is unaesthetic. This esthetic problem has been overcome by other methods and by utilising different materials, these included covering the retainers with tooth-coloured acrylic resin, as well as the introduction of esthetic materials as; Thermoplastic Acetal, Versacryl, and Thermopress.CONCLUSION:It has been concluded that the non-metallic Acetal resin clasp shows superior physical properties regarding colour stability.

EVALUATION OF RETENTION OF TOOTH-COLORED VERSUS CAST METAL DENTAL CLASPS FOR A MANDIBULAR DISTAL EXTENSION PROSTHESIS

The aim of this study was to compare the retention of two different forms of clasps in mandibular distal extension prosthesis, which were; the Conventional casted metal clasps & the Tooth-colored acrylic resin clasps “Acetal resin”.Materials & Methods: For the rationale of this clinical investigation, fifteen patients were selected from the Outpatient Clinic of the Prosthodontic Department, Faculty of Oral and Dental medicine, Cairo University in accordance with certain criteria. Every patient got a skeleton parial denture that was constructed in a conventional manner. A hole was made in the middle of the lingual plate during fabrication of the framework; A Retention Measuring-Aided Device was made to allow attachment of the digital Forcemeter to this hole for measuring retention. Then, the metal clasp was removed and replaced by acetal resin clasp and the same procedures of measuring retention were performed in the same manner. Retention of the two clasp types was measured four times (at time of insertion, one month, two months and three months, respectively), by the aid of a prefabricated device (The Retention Measuring-Aided Device) for precise reproducible measurement.Results: There was a statistically significant difference (P ≤ 0.05) in the mean Forcemeter measurements from base line to one month. Later, from one month to 2 months and from 2 months to 3 months; there was a statistically insignificant difference (P ≥ 0.05) in the mean Forcemeter measurements. Conclusions and recommendations: Using Tooth-colored dental clasps would not compromise the retention of removable partial denture and can help to enhance the esthetics of removable partial dentures.

EVALUATION OF ACETAL RESIN AND COBALT–CHROMIUM CLASP DEFORMATION AND FATIGUE RESISTANCE IN REMOVABLE PARTIAL DENTURE CLASPS AN IN-VITRO STUDY

Purpose: this study was aimed to evaluate the fatigue resistance (amount of clasp deformation) of acetal resin clasps and Cobalt-Chrome (Co-Cr) clasps after attachment/detachment cycles on abutment teeth with two different undercuts. Materials and Method: Twenty models were constructed by placing either an upper 1st premolar or upper 1st molar inside an acrylic rectangular block. Models were divided according to the abutment teeth into two groups (GI for 1st premolars group and GII for 1st molars group), 10 each. Each group was divided into two subgroups according the framework material, SGA for acetal resin clasp and SGC for Co-Cr clasp. Each testing models and its framework were mounted inside universal testing machine (Lloyd instruments Ltd, England). Cycling was carried out for each specimen till 2920 cycles. The data of the amount of clasp deformation after cycling were collected and tabulated. The data were subjected to statistical analysis using student’s t and paired t tests. Results: After 2920 cycles, the mean values and standard deviations of the clasp deformation for SGIC, SGIA, SGIIC and SGIIA were 0.0532±0.006, 0.007±0.003, 0.04323±0.0048 and 0.0275±0.004 mm, respectively. Conclusions: Co-Cr clasps had significant clasp deformation more than acetal resin clasps. Increase the thickness of cross section of the acetal resin clasp more than 1 mm was recommended in case of engaging undercut more than 0.25 mm.

Effect of cast Co-Cr and acetal resin removable clasp on the surface of enamel

Objective: Objetive of the study was to evaluate the effect of cast Co-Cr and acetal resin clasp on the surface of tooth. Methodology: Ten extracted human mandibular molars were selected for the study. Clasp holding surfaces of the teeth were demarcated and examined by stereomicroscope and optical profilometer to evaluate the surface roughness qualitatively and quantitatively. The molars were then mounted on a cast and on which cast Co-Cr and acetal resin clasps were fabricated with an attached vertical strut. The assembly was placed on a masticatory simulator. Three thousand cycles of insertion and removal were carried out to simulate 2 years of usage. After the experiment was completed, the molars were evaluated for the surface changes by stereomicroscope and optical profilometer. Results: Qualitative analysis by stereomicroscope showed linear scratches and grooves on the surface of enamel caused by the use of cast Co-Cr and acetal resin clasp. Minimal scratches were seen on the enamel when acetal resin clasp was used. Optical profilometry showed statistically significant results (P < 0.0001) with the Co-Cr clasp. Acetal resin clasps showed insignificant results. The retentive force of cast Co-Cr clasp showed a decrease from 12.4 N to 8.1 N. Whereas, the retentive force of acetal resin clasp reduced from 5.2 N to 4.03 N at the completion of the experiment. Conclusions: Acetal resin clasps do not abrade the surface of tooth and maintain retention. Acetal resin clasps are esthetic and are available in 16 different shades.

Evaluation of Fatigue Resistance of Acetal Resin and Cobalt–Chromium Removable Partial Denture Clasps. An In-vitro Study: Part 1

This study was aimed to evaluate the retentive forces of acetal resin clasps and Cobalt-Chrome (Co-Cr) clasps during attachment/detachment cycles on abutment teeth with two different undercuts and at different intervals.

An Evaluation of Retentive Ability and Deformation of Acetal Resin and Cobalt-Chromium Clasps.

To compare the retentive ability and deformation of Acetal resin with Cobalt-Chromium clasps via Insertion Removal apparatus after subjecting them to stimulate clinical use. Materials used for this study are commercially available Cobalt-Chromium alloy namely Wironit, Bego, Germany and Acetal resin namely Biodentaplast, Bredent, Germany. The test samples were divided into two major groups based on the type of materials used in the study. Each major group is further subdivided into two sub groups based on the retentive undercut depths used to engage the clasps. So a total of 20 specimens were prepared, comprising of 5 specimens in each sub group. Then the specimens were tested for retention force and deformation. The results of this study indicate that acetal resin clasps are resistant to deformation and may offer a clinical advantage over the conventional metal clasps. The retentive force of acetal resin clasps did not decrease over the cycling periods. This would be attributed to the resilient nature of acetal resin. Under the conditions of the present study cobalt chromium clasps lost retentive force within 730 cycles of placement and removal and continued to lose retentive force during the remaining test period. This invitro study demonstrated that retentive force of cobalt chromium clasp is superior to that of Acetal resin for removable partial dentures. As acetal resin clasps exhibits greater flexibility and long term retentive resiliency, it can be used for removable partial dentures where aesthetics or periodontal health is a primary concern.

MARGINAL BONE LEVEL EVALUATION OF IMPLANT SUPPORTED KENNEDY CLASS I PARTIAL OVERDENTURE USING TELESCOPIC CROWNS VERSUS ACETAL RESIN CLASPS DIRECT RETAINERS

Objectives : This study was conducted to evaluate and compare marginal bone level for abutments of implant supported Kennedy class I partial overdenture using Telescopic Crowns versus acetal resin clasps direct retainers after 18 month of clinical use. Materials & methods : Ten male partially edentulous patients (Kennedy class I) with the premolars are the last standing abutments were selected and all patients were rehabilitated with metallic partial denture supported by two osseointegrated implants one on each side of the lower arch positioned in the area of second molar and were randomly divided into two equal groups according to type of abutments retainers ,Group I; Patients were rehabilitated with Tooth Implant Supported partial overdenture with two telescopic crowns ,Group II; Patients were rehabilitated with Tooth Implant Supported partial overdenture with two Acetal Resin clasps direct retainer on each side. Evaluation by measuring marginal bone level for main abutments was made at the time of insertion, after 6 month ,after 12 month and last after 18 month using radiographic evaluation. Results: partial overdentures retained by telescopic crowns showed significant increase in marginal bone loss as compared with Acetal resin clasps retainers. Conclusion: using tooth implant partial overdentures retained by Acetal resin clasps shows better effect on supporting structure as compared by partial dentures with telescopic crowns retainers.

Clinical and radiographic comparative study between two different types of clasps materials used in unilateral maxillary Kennedy class II removable partial denture cases

Objective: The objective of the present study was to compare between two different clasp materials for the treatment of maxillary class II Kennedy classification clinically, radiographically.Materials and methods: Twenty male patients were selected for this study. The patients were divided into two groups. Each group consists of 10 patients. For group I each patient have maxillary removable partial denture that was constructed and retained by cobalt–chromium RPI clasp on the maxillary first premolar with cross arch stabilization (using double Aker clasp) on the other side. For group II each patient has the same except that the retentive I bar of RPI clasp was made of acetal resin. Patient satisfaction, clinical, and radiographic evaluations were carried out for every patient at insertion, 3, 6, and 9 months, the data were collected, tabulated, and statistically analyzed using t-test.Results: Satisfaction for the treatment. Group II patients were more satisfied of the treatment regarding aspects of the patients' comfort and esthetics. No significant difference in the aspects of phonetics, the ability to eat, prosthesis loosening, the gingival health, general satisfaction and food impaction between the two groups were found, also there were insignificant increase in the gingival recession, pocket depth, and plaque index around the abutments tooth in both groups. Radiographically, there was significant decrease in the bone height around the abutments in both groups.Conclusion: Removable partial denture with acetal resin clasp is better for esthetic and patient comfort. Good periodontal condition of the abutment tooth of both groups is maintained. Acetal resin clasp is the clasp of choice when esthetic is of primary concern to the patient.

Comparison of Retention of Clasps Made of Different Materials Using Three-Dimensional Finite Element Analysis.

Retention and esthetics are believed to play a crucial role in deciding the success of removable partial dentures. To compare retention of acetal resin and cobalt-chromium clasps. A finite element model was designed with an edentulous space between mandibular right second premolar and second molar. Occlusal rests were placed on distal fossa of the second premolar and mesial fossa of second molar. An undercut depth of 0.01inch was created on the mesiobuccal surface of the premolar and distobuccal surface of second molar. Three dimensional finite element model of clasp assembly was designed and assigned with the properties of two different materials namely acetal resin and cobalt-chromium in successive steps. A horizontal bar was constructed between the occlusal rests of the prosthesis. Later, variable amount of dislodging force, in increasing order, was applied at the centre of the horizontal bar and the force at which the clasp arm gets dislodged was noted with respect to each of the material. The obtained values were noted and then subsequently analyzed. The amount of force required to dislodge acetal resin and cobalt-chromium clasps was found to be 0.02N and 2N respectively. The results obtained suggested that acetal resin clasp exhibited less retentive force than cobalt-chromium clasps.

Fatigue resistance and flexural behavior of acetal resin and chrome cobalt removable partial denture clasp: An in vitro study

Statement of Problem: The denture clasp undergoes fatigue on the repeated deflection of the clasp during insertion and removal of the removable partial denture over the undercuts of the teeth. This study was conducted to know how acetal resin functions in this application. Aims: The purpose of the study was to evaluate fatigue resistance of acetal resin clasps subjected to repeated constant deflections and compare it with cobalt.chromium clasp. Materials and Methods: Acetal resin and Co.Cr clasp specimens were subdivided. (n = 10) specimens to perform deflection of 0.25 mm and 0.50 mm. Constant load for deflection of either 0.25 or 0.50. mm was applied to 2. mm from the point of the clasp at the appropriate loading frequency assuming that the clasps were inserted and removed from the abutment with undercut of either 0.25. mm or 0.50. mm. The fatigue test was carried out until 10 6 cycles or until permanent deformation detected. The data obtained were subjected to unpaired t -test. Results: It was observed that the deformation of chrome cobalt clasp specimen during the loading cycle of 106 under deflection of 0.5. mm was 0.1530 ± 0.0177 mm Chrome cobalt clasp under 0.25 mm deflection and an acetal resin specimen under 0.25 and 0.5 mm deflection did not show any significant deformation. Conclusions: The clasp with acetal resin may provide less retention than cobalt chromium.based alloys. For acetal resin clasp, no permanent deformation was detected after 106 loading cycles when deflected to 0.25. mm and 0.5. mm and has mean flexural strength of 72.436 MPa and modulus of elasticity of 2.245 GPa.

Retentive forces, tensile strength and deflection fatigue of Acetal thermoplastic clasp material in comparison with cobalt-chromium alloy

Nowadays there is an increasing of the emphasis on aesthetic, dentist have been concerned about providing aesthetics and functional removable partial dentures to their patients and this was make the mission more difficult because of the goal now is achieving optimal aesthetic of the denture - while maintaining retentive, stable, and conservative to the health of supporting tooth and supporting tissue. The traditional use of metal clasp like cobalt-chromium, gold, stainless-steel and titanium hampers esthetics because of its obvious display conflicts with patient's prosthetic confidentiality. Acetal resin (poly oxy methylene) may be used as alternative denture clasp material. This material was promoted primarily on the basis of its superior esthetic. Material and method: In this study, Acetal resin of Flexite and co-cr alloy of Wirocast companies were used. Four metal models of two premolars and two molars were surveyed to have 0.25mm and 0.5mm undercut depth for each one of the materials. eighty clasps were prepared and tested by testing machine to measure the load required to dislodge Acetal and co-cr clasps of 2mm. thickness (for premolar and molar). Tensile test were utilized for both Acetal resin and co-cr. tensile values will be used in special formula to calculate the amount of deflection. Results: The results of this study revealed that Acetal resin clasp of molar with 0.5mm undercut depth have the higher retentive force, and premolar with 0.25 mm undercut depth have the lowest retentive force as compared to co-cr. Also Acetal had lower values of the tensile strength as compared to co-cr alloy but it had higher deflection value than co-cr and it can withstand higher deflections than those of co-cr before having permanent deformation or fracture Conclusions: Acetal resin can be used with limitation as an alternative to co-cr alloy clasps in removable partial dentures.

Comparison between Acetal Resin and Cobalt-Chromium Removable Partial Denture Clasp Retention: An in vitro Study

ABSTRACT Statement of problem The retention qualities of acetal resin clasps are questionable. Purpose To asses the initial retention and after cycling for 1,200 cycles of acetal resin clasps as compared to cobalt-chromium clasps. Materials and methods Extracted maxillary first premolars and molars were collected. Each tooth was embedded in acrylic block up to the cementoenamel junction. On each block, partial denture model consisted of Akers clasp, minor connector, cobalt-chromium horizontal plate representing the denture base, and a vertical arm on the horizontal plate was constructed. Models were divided into four groups (five each). Group I had acetal resin clasps on premolars. Group II had cobalt-chromium clasps on premolars. Group III had acetal resin clasps on molars. Group IV had cobalt-chromium clasps on molars. Akers clasps were designed to utilize 0.01 inch undercut. Each model was connected to the universal testing machine through the vertical arm. The model was placed in artificial saliva and a tensile load was applied to measure the initial retention. Then retention was measured after cycling for 200, 400, 600, 800, 1,000 and 1,200 cycles. One-way analysis of variance followed by Duncan's multiple range test was used to test the effect of groups and cycling on tensile load required for clasp dislodgment. Results Groups I and III showed no change in retention on cycling up to 1,200 cycles. Group I showed significantly higher retention than group III. Groups II and IV showed significant reduction in retention upon cycling more than 800 cycles. In spite of this reduction in retention, it is still higher than that in groups I and III. Conclusion Cobalt-chromium clasps showed higher retention than acetal resin clasps despite the deformation encountered in the former. Acetal resin clasps showed more retention qualities on premolar teeth than that on molar teeth. Clinical implications Acetal resin clasps can be used to retain Kennedy's class III partial denture. However, when used on molar teeth, it is recommended to increase the thickness of the retentive clasp arm or to use a deeper undercut. How to cite this article Mohamed T, Baraka OA, Badawy MM. Comparison between Acetal Resin and Cobalt-Chromium Removable Partial Denture Clasp Retention: An in vitro Study. Int J Prosthodont Restor Dent 2013;3(2):50-56.

Acetal resin as an esthetic clasp material

Removable partial dentures (RPDs) are an effective and affordable treatment option for partial edentulism. Restoration of esthetics is an important function of RPD and it determines the success of the treatment. The goal of achieving optimal esthetics - while maintaining retentive integrity, stability, and protecting the health of the tooth - is the most difficult task. The traditional use of the metal clasp like cobalt chromium (Co-Cr), gold, stainless steel, and titanium hampers esthetics, since its obvious display conflicts with patient's prosthetic confidentiality. Acetal resin (polyoxymethylene or POM), a thermoplastic resin, may be used as an alternative denture clasp material. This material was promoted primarily on the basis of superior esthetics, which allowed the clasps to better match the color of abutment tooth. PubMed and Google Scholar were used to search original research articles, case reports, and other reviews about acetal resin. The keywords used were acetal resin, esthetic, clasp, removable partial denture, thermoplastic, metal free framework. This article is the result of a literature study on acetal resin clasp material for conventional RPD.

Comparison between Acetal Resin and Cobalt-chromium Removable Partial Denture Clasps: Effect on Abutment Teeth Supporting Structures

ABSTRACTStatement of problemAcetal resin has been introduced as an esthetic partial denture clasp material. However, the effects of these clasps on the abutment teeth supporting structures were not clear.PurposeTo evaluate the effects of acetal resin clasps on the abutment teeth supporting structures as compared to cobalt-chromium clasps.Materials and methodsTwenty patients, 12 males and 8 females, with Kennedy class III modification I partially edentulous maxilla and dentulous mandibles were selected for this study. Patients were divided into two equal groups; group 1 received maxillary cobalt-chromium partial denture frameworks with acetal resin Akers clasps. Group 2 received maxillary partial denture with cobalt-chromium frameworks and Akers cobalt-chromium clasps. Crevicular fluid, epithelial attachment loss, and bone height and density of the abutment teeth were evaluated at partial denture insertion and after 6 and 12 months. Paired t-test was used at p ≤ 0.05 to assess the changes in the above parameters in each group. Student t-test was used to compare between the two groups.ResultsCrevicular fluid measurements were significantly higher in the first group than that in the second group. There were no differences in epithelial attachment loss between the two groups. There were significantly higher reductions in the bone height and in the bone density in the second group than that in the first group.ConclusionAcetal resin clasps were superior to cobalt-chromium clasps as produced fewer reductions in bone height and in bone density around the abutment teeth inspite of produced increase in the crevicular fluid.Clinical implicationsSince there were lesser reductions in bone height and in bone density around acetal resin clasp abutments, it could be used successfully to retain partial dentures. However, meticulous oral hygiene and proper insertion and removal of partial denture with acetal resin clasps were required to decrease gingival inflammation and crevicular fluid amount.

An in vitro comparison of retentive force and deformation of acetal resin and cobalt-chromium clasps

The use of metal clasps on anterior teeth may cause esthetic problems. Recently, acetal resins have been used as an alternative tooth-colored denture clasp material to improve esthetics. However, there are few studies to support acetal resin use. The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin and cobalt-chromium clasps after 36 months of simulated clinical use. Forty clasps each of acetal resin (1.2 or 2.0 mm thick) and cobalt-chromium (Dentorium)(1.2 mm thick) were fabricated using half-round standard prefabricated clasp patterns. The groups were further subdivided (n=10) into the type of tooth (premolar or molar metal model) and undercut (0.25 mm or 0.50 mm). The retentive force of the clasps was measured in distilled water by a specially designed insertion-removal testing apparatus with intervals corresponding to 0, 6, 12, 18, 24, 30, and 36 months of simulated clinical use of a removable partial denture. The distance between the clasp tips (mm) was measured with a microscope before and after the insertion-removal testing procedure. Comparison of the mean values of the retentive force (gram force) of the clasps and the distance (mm) between the clasp tips was conducted with 3-way analysis of variance and a Least Significant Difference (LSD) multiple range test (alpha=.05). The mean values of tensile load required to dislodge acetal resin clasps with 1.2-mm thickness (111.6 g or 0.11 N) and with 2.0-mm thickness (178.4 g or 1.75 N) was significantly lower than that to dislodge Co-Cr clasps (694.1 g or 6.81 N) (P<.001). The retentive force needed to dislodge all 3 types of clasps was significantly lower for the molar than premolar models and also lower for the models with 0.25-mm undercuts than for those with 0.50-mm undercuts (P<.001). After 36 months of simulated clinical use, there was evidence of deformation in the cobalt-chromium clasps but no deformation noted for the acetal resin clasps. The retentive force of cobalt-chromium clasps (297.4 g or 2.91 N) after deformation remained significantly higher (P<.001) than the retentive force of acetal resin clasps that were 1.2 mm (110.7 g or 1.08 N) and 2.0 mm thick (177.5 g or 1.74 N), respectively. Within the limitations of this study, the results suggest that both thicknesses of acetal resin clasps evaluated required less force for insertion and removal than Co-Cr clasps over a simulated 36-month period.

In vitro calcium release from four calcium hydroxide liners

The use of metal clasps on anterior teeth may cause esthetic problems. Recently, acetal resins have been used as an alternative tooth-colored denture clasp material to improve esthetics. However, there are few studies to support acetal resin use.The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin and cobalt-chromium clasps after 36 months of simulated clinical use.Forty clasps each of acetal resin (1.2 or 2.0 mm thick) and cobalt-chromium (Dentorium)(1.2 mm thick) were fabricated using half-round standard prefabricated clasp patterns. The groups were further subdivided (n = 10) into the type of tooth (premolar or molar metal model) and undercut (0.25 mm or 0.50 mm). The retentive force of the clasps was measured in distilled water by a specially designed insertion-removal testing apparatus with intervals corresponding to 0, 6, 12, 18, 24, 30, and 36 months of simulated clinical use of a removable partial denture. The distance between the clasp tips (mm) was measured with a microscope before and after the insertion-removal testing procedure. Comparison of the mean values of the retentive force (gram force) of the clasps and the distance (mm) between the clasp tips was conducted with 3-way analysis of variance and a Least Significant Difference (LSD) multiple range test (α=.05).The mean values of tensile load required to dislodge acetal resin clasps with 1.2-mm thickness (111.6 g or 0.11 N) and with 2.0-mm thickness (178.4 g or 1.75 N) was significantly lower than that to dislodge Co-Cr clasps (694.1 g or 6.81 N) (P<.001). The retentive force needed to dislodge all 3 types of clasps was significantly lower for the molar than premolar models and also lower for the models with 0.25-mm undercuts than for those with 0.50-mm undercuts (P<.001). After 36 months of simulated clinical use, there was evidence of deformation in the cobalt-chromium clasps but no deformation noted for the acetal resin clasps. The retentive force of cobalt-chromium clasps (297.4 g or 2.91 N) after deformation remained significantly higher (P<.001) than the retentive force of acetal resin clasps that were 1.2 mm (110.7 g or 1.08 N) and 2.0 mm thick (177.5 g or 1.74 N), respectively.Within the limitations of this study, the results suggest that both thicknesses of acetal resin clasps evaluated required less force for insertion and removal than Co-Cr clasps over a simulated 36-month period.