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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 13  |  Issue : 1  |  Page : 41-49

Laboratory comparative study of three different types of clasp materials


Prosthodontic Department, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission02-Mar-2016
Date of Acceptance17-Mar-2016
Date of Web Publication26-Jul-2016

Correspondence Address:
Nahla Y Abdel-Rahim
Prosthodontic Department, Faculty of Dentistry, Tanta University, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1687-8574.186936

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  Abstract 

Purpose
The objective of this in vitro study was to evaluate the retentive strength of 3 clasp materials Acetal resin (Ac), Cobalt- Chrome (Co-Cr) alloy and Nickel-Chrome (Ni-Cr) alloy before and after cyclic loading.
Materials and methods
A total of 72 clasps (24 clasps for each material) were fabricated on standardized metallic molar models. The clasps of each material were divided into 3 groups 8 clasps each according to degree of undercuts (0.01, 0.02 & 0.03 inch). Each clasp assembly was subjected to 730, 1460, 2190 and 2920 cycles of insertion and removal of a removable partial denture. Retentive strength was measured before and after each cyclic period. Retentive strength values were analyzed using ANOVA and Fisher's PLSD multiple comparison test at the 0.05 level of significance.
Results
Ac clasps had significantly lower retentive strength than Co-Cr and Ni-Cr clasps. Clasps with 0.01 inch undercut showed significantly less retention than clasps with 0.02 inch undercut and the latter showed significantly less retention than clasps with 0.03 inch undercut. All clasps exhibited continuous significant decrease in retentive strength from the first period of cyclic loading till the end of the cycling except for Acetal resin at 0.01 inch undercut which show no significant difference after 730 cyclic loading.
Conclusion
There was significant decrease in retentive strength of the 3 clasp materials.

Keywords: acetal, esthetic clasp, retention


How to cite this article:
Abdel-Rahim NY, Abd El-Fattah FE, El-Sheikh MM. Laboratory comparative study of three different types of clasp materials. Tanta Dent J 2016;13:41-9

How to cite this URL:
Abdel-Rahim NY, Abd El-Fattah FE, El-Sheikh MM. Laboratory comparative study of three different types of clasp materials. Tanta Dent J [serial online] 2016 [cited 2017 Oct 22];13:41-9. Available from: http://www.tmj.eg.net/text.asp?2016/13/1/41/186936


  Introduction Top


Comprehensive treatment plans for partially edentulous patients are usually more complicated than treatment plans formulated for edentulous patients or for patients who do not require the replacement of the missing teeth[1]. The clasp-retained partial denture, with extra coronal direct retainers, is probably used a 100 times more than the precision attachment partial denture[2]. There are two basic types of direct retainers. One is the intracoronal retainer, which is cast or attached totally within the restored natural contours of an abutment tooth. The other type of retainer is the extra coronal one, which uses mechanical resistance to displacement from components placed on or attached to the external surface of an abutment tooth. The extra coronal or clasp direct retainer is used more frequently than is attachment[3].

Clasps are used as direct retainers for the removable partial denture (RPD). The flexible clasp tip engages the undercut of the abutment to provide retention. The components of any clasp assembly must satisfy six requirements: retention, stability, support, reciprocation, encirclement, and passivity[2],[4],[5].

The suprabulge clasp approaches the undercut from an occlusal direction and is more visible. The infrabulge clasp, approaching the undercut from a gingival direction, also referred to as the gingivally approaching clasp, has more potential for being hidden in the distobuccal aspect of a tooth[6].

Cast circumferential clasps are frequently used in the RPD technology for their remarkable simplicity, easy construction, and excellent retention[7]. Retentive force control of clasp retainers is one of the most essential factors for the successful function of RPD[8].

Flexibility of the clasp is affected by the clasp dimensions and the mechanical properties of its constituent alloy; thus, for the flexibility to be assessed, the relationship between the clasp dimensions and mechanical properties of the alloy and the resulting flexibility must be obtained;

flexibility of the retentive clasp arm plays an important role in determining the amount of its retention[9]. Base metal alloys make up a majority of the dental metal alloys. The rising cost of gold and precious metals has been instrumental in the development of nonprecious alloys, which have a lower cost for prosthodontic castings. Two main classes of base metal alloys are the nickel–chrome (Ni–Cr) system and the cobalt–chrome (Co–Cr) system[10].

Acetal (Ac) was first proposed as an unbreakable thermoplastic resin-RPD material in 1971. Acetal resin clasps were flexible, did not need periodic adjusting to keep them tight, and the tooth-colored esthetics were appreciated by the patients[11]. Acetal resins are characterized by fracture strength, wear resistance, and flexibility. These characteristics make them suitable for carrying out clasps, partial denture frameworks, provisional fixed partial dentures, artificial teeth for removable dentures, resin-bonded fixed partial dentures, and orthodontic appliances[12].

The traditional use of the metal clasp like cobalt–chromium (Co–Cr), gold, stainless steel, and titanium, hampers esthetics, as its obvious display conflicts with the 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 the abutment tooth[13]. RPD retentive clasp arms must be capable of flexing and returning to original form and should satisfactorily retain prosthesis. In addition, clasps should not unduly stress abutment teeth or be permanently distorted during service. In this respect, gold alloys have been favored as clasps compared with base metal alloys because of their high-yield strengths and low moduli of elasticity. However, gold alloys have not been regarded as the ideal metal for clasp construction because of the relatively high cost and unavoidable permanent deformation after long-term service[2].

Several reports investigated the retention properties of various clasp designs[8],[14],[15], and the effect of clasp fatigue on the retention properties of denture clasps[16]. Permanent deformation and fatigue fracture result in the loss of retention and reduced stability of the prosthesis, which can compromise patient comfort and necessitate time-consuming and expensive repairs or reconstructions [17],[18],[19],[20]. Fatigue test is believed to simulate the clinical situation. Studies have shown that some materials and RPD designs possess greater fatigue resistance than do others. Thus, information on the fatigue behavior of materials and structures would guide dentists and dental technicians during RPD design, material selection, and fabrication[21]. The null hypothesis was that no difference exists in the retentive strength of the three different clasp materials before and after cyclic loading.


  Materials and Methods Top


Extracted maxillary first molar were selected for this study. all patients are informed about the purpose of the study and using of their extracted teeth according to ethics committee of Faculty of Dentistry b, Tanta University. Impression of acrylic maxillary first molar was made with condensation silicon impression material (C-silicon impression material; Zhermack, Milan, Italy), and a wax model representing the first molar was poured. A wax plate with dimensions 25×10×2 mm was prepared. The wax molar tooth was fixed in the wax plate with dimensions 1.2×4×3 mm. Acrylic block was made of self-cured acrylic resin 5 cm in length, 2 cm in height, and 4 cm in width. The base of the acrylic block and its superior surface were made parallel to the surveyor table (Ney Surveyor; Dentsply, New York, New York, USA). A cavity was created at the center of the superior surface to receive the testing model (the waxed tooth that was fixed in a base)[22][Figure 1].
Figure 1: Testing model with acrylic block on a surveyor.

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The selected model was inserted in the cavity in a position perpendicular to the superior surface of the acrylic block. The acrylic block with the corresponding selected model was transferred to the surveyor table without tilting. The analyzing rod of the surveyor was made parallel to the long axis of the tooth, and the carbon marker was used to determine the height of the contour of the selected tooth.

The selected tooth was trimmed on a surveyor with a trimming knife to provide mesial and palatal guide planes, which were evaluated by using the analyzing rod. A 0.01 inch (0.25 mm)[23] undercut area on the distobuccal surface was created with a wax carver and was measured by means of the undercut gauge on the dental surveyor. The rest seat preparation was triangular in shape with the base of the triangle resting on the marginal ridge; the rounded apex of the triangle was directed toward the center of the occlusal surface of the tooth. The width of the rest seat preparation was one third of the distance between the buccal and the palatal cusp tips. The prepared depth of the occlusal rest seat was 2 mm. The floor of the rest seat preparation was spoon-shaped and directed toward the center of the occlusal surface of the tooth.

A ledge was placed on the buccal surface to standardize the locations and lengths of the retentive arms [Figure 2]. On the palatal surface, a piece of wax, rectangular in shape, was placed as a reference to standardize the locations and lengths of the reciprocal arms[24]. The same procedure was carried out to make two molar models with 0.02 and 0.03 inch undercuts. The three wax models, with 0.01, 0.02, and 0.03 inch undercuts, were flasked with heat-cure acrylic resin to facilitate the duplication of each model.
Figure 2: The prepared waxed model with a ledge.

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After finishing, these acrylic models were surveyed again. Each acrylic model was duplicated with condensation silicon impression material and poured with molten casting wax to make three identical sets of wax models for each undercut. Nine wax models were ready to be cast to obtain nine metal models made from Co–Cr[25],[26]. From the previously prepared nine metallic models, three metallic models for 0.01, 0.02, and 0.03 inch undercuts were used to produce the clasp samples for acetal resin material as follow:

  1. A condensation silicon impression material in a stock tray was made for each model.
  2. Each of the three impressions was poured eight times to obtain 24 stone (Expando-Rock; Bredent, Germany) models.


Preformed half round standard Aker clasp patterns (1.2 mm) (readymade Aker clasp pattern; Bego, Italy) with occlusal rest, retentive arm, reciprocal arm and minor connector were adapted on the refractory casts, with one clasp pattern for each cast. A wax plate with dimensions 4×3×7 mm was prepared and attached to the minor connector parallel to the path of insertion. A rounded vertical plastic sprue[27], 20 mm in length and 3 mm in diameter, was attached to the prepared wax plate, which was used latter on for maintaining the clasp in the testing machine[25] [Figure 3] as follow:
Figure 3: Refractory cast with clasp pattern, wax plate, and plastic sprue.

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  1. The wax pattern of the Aker clasp assembly was sprued at the thickest part of the clasp wax pattern with 20 mm sprue length and 3 mm diameter. Five wax patterns per flask were invested and all the small sprues were attached to a large sprue with 50 mm length and 10 mm diameter. After the surface-tension reducing solution was applied to the wax patterns, they were invested in a vaseline-insulated flask (aluminum flask; Bredent) [Figure 4]. Hard stone was used as investment. Gypsum paste was poured into one of the two halves of the flask and the duplicated casts containing the spruing of the clasp patterns were dipped. When the investment finally set, the gypsum surface was insulated and the second half of the flask was assembled. The same hard stone was prepared and poured into the upper chamber of the flask, covering thoroughly the wax pattern and sprues.
  2. After the gypsum set, the flask was submerged in warm water in a thermostatic container. The two halves of the flask were then disassembled and the wax was boiled out using clean hot water. The mold was then insulated using a special agent, which was applied in a single layer on the gypsum surface.
  3. Preheating temperature and time were checked (15 min at 220°C). The corresponding cartridge of injecting acetal resin was selected. The cartridge was introduced into the heating cylinder.
  4. When the programmed preheating time elapsed, an audible signal was heard. The two halves of the flask were assembled and fastened with screws. The flask was inserted and secured in the corresponding place of the injecting unit (Thermopress 400 unit; Bredent) [Figure 5]. The injecting procedure was initiated and the flask was left to slowly cool down for 8 h.
  5. Before investment removal, screws were loosened and the flask was gently disassembled. The stone blocking the vents in the upper side of the flask was removed using the hook and a mallet. The sprues were cut off using carbide and diamond burs using low pressure to avoid overheating the material. Acetal resin clasps were ready to be tested [Figure 6].
Figure 4: Acetal resin flask.

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Figure 5: Acetal resin flask secured in the corresponding place in the injecting unit.

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Figure 6: Acetal resin clasp on metallic model.

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From the previously prepared nine metallic models, three metallic models for 0.01, 0.02, and 0.03 inch undercuts were used to produce the clasp samples for Co–Cr clasps and three metallic models for 0.01, 0.02, and 0.03 inch undercuts were used to produce the clasp samples for Ni–Cr clasps as follow:

  1. A condensation silicon impression material in a stock tray was made for each
  2. Each of the three impressions was poured eight times with silicophosphate investment to obtain 24 refractory models for each material.


The obtained wax patterns of clasps were prepared and casted. Each metallic clasp was evaluated for casting defects and porosity. The porosity of all specimens was examined radiographically by using an intraoral X-ray machine[28]. Each testing model with the tooth was attached from its base to the fixed compartment of the testing machine with a load cell of 5 kN. The occlusal rest of the Aker clasp was fully seated in its rest seat. The vertical sprue was attached to the movable compartment of the universal testing machine[22]. Each clasp was initially activated by withdrawing the clasp over the maximum convexity of the tooth until the complete separation of the clasp from the tooth. To perform the retention test, an insertion/removal test set up was used.

This test allowed the placement (insertion) of the clasp to its predetermined terminal position and its subsequent removal from this position, thus simulating the placement and removal of a RPD[22],[26]. The clasps were subjected to a cyclic insertion removal test, performed with a cross-head speed of 5 mm/min for all clasp specimens. The retentive strength values were captured at the first insertion and removal, and then after 730, 1460, 2190, and 2920 continuous cycles corresponding to 6, 12, 18, and 24 months, respectively, in service were recorded by the computer software [Figure 7].
Figure 7: Retention test of clasp on the universal testing machine.

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  Results Top


The mean and SD values of retentive strength in Newton (N) of Ac, Co–Cr and Ni–Cr clasps used with three different degree of undercuts (inch) before and after cyclic loading are presented in [Table 1],[Table 2],[Table 3] and [Figure 8],[Figure 9],[Figure 10]. Statistical analysis using a three-way analysis of variance followed by Fisher's PLSD for comparison of the effect of clasp materials, cyclic loading, and degree of undercuts are presented in [Table 4],[Table 5],[Table 6],[Table 7].
Table 1: Mean values and SD of retentive force (N) in 0.01 inch undercut before and after cyclic loading

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Table 2: Mean values and SD of retentive force (N) in 0.02 inch undercut before and after cyclic loading

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Table 3: Mean values and SD of retentive force (N) in 0.03 inch undercut before and after cyclic loading

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Table 4: Three-way analysis of variance for retentive force (N)

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Table 5: The Fisher's PLSD for retentive force (N), significant level: 5%, effect of clasp materials

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Table 6: The Fisher's PLSD for retentive force (N), significant level: 5%, effect of cyclic loading

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Table 7: The Fisher's PLSD for retentive force (N), significant level: 5%, effect of degree of undercut

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Figure 8: Retentive force of three clasp materials in 0.01 inch undercut before and after cyclic loading.

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Figure 9: Retentive force of the three clasp materials in 0.02 inch undercut before and after cyclic loading.

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Figure 10: Retentive force of the three clasp materials in 0.03 inch undercut before and after cyclic loading.

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For comparison of the effect of clasp materials on retentive force, if mean difference was greater than the Fisher's PLSD value critical difference (0.24) ([Table 5]), the difference was considered significant. For comparison of the effect of cyclic loading on retentive force, if mean difference was greater than Fisher's PLSD value (0.31) ([Table 6]), the difference was considered significant. For comparison of the effect of degree of undercut on retentive force of three tested clasp materials, if the difference was greater than Fisher's PLSD value (0.24) ([Table 7]), the difference was considered significant.

Data for retentive force of Ac, Co–Cr, and Ni–Cr clasps with 0.01 inch undercut before and after cyclic loading are presented in [Table 1] and [Figure 8]. Before loading, the highest mean value of retentive force was found with a Co–Cr clasp (9.41 ± 1.00 N), followed by a Ni–Cr clasp (9.14 ± 0.88 N), whereas the lowest mean value of retentive force was found with an Ac clasp (1.72 ± 0.37 N). During cyclic loading the retentive force gradually decreased and the lowest mean value of retentive force was recorded with an Ac clasp (0.90 ± 0.23 N) after 2920 cycles.

Data for retentive force of the three tested clasp materials with 0.02 inch undercut before and after cyclic loading are presented in [Table 2] and [Figure 9]. Before cyclic loading, the highest mean value of retentive force (12.96 ± 1.57 N) was recorded with a Co–Cr clasp, and with a Ni–Cr clasp it was 12.00 ± 1.01 N, whereas with an Ac clasp it was 3.11 ± 0.32 N. During cyclic loading, the retentive force gradually decreased and the lowest retentive force (2.01 ± 0.26 N) was recorded with an Ac clasp after 2920 cyclic loading.

Data for retentive force of Ac, Co–Cr, and Ni–Cr clasps with 0.03 inch undercut before and after cyclic loading are presented in [Table 3] and [Figure 10]. The mean initial retentive force of Co–Cr clasps (15.58 ± 1.73 N) was greater than that of a Ni–Cr clasp (14.21 ± 1.38 N), and the mean initial retentive force of Ni–Cr clasp was greater than that of an Ac clasp (4.57 ± 0.44 N), and the decrease in retentive force of Co–Cr clasp (3.7 3 ± 0.77 N) was also greater after the cyclic fatigue test. The retentive force of Co–Cr clasp of different undercuts were greater after the 2920 cyclic test.


  Discussion Top


There are several types of RPDs. All of them use standard denture teeth as replacements for the missing natural teeth. The differences between them are the materials used to support the denture teeth and retain the RPD in the mouth[29]. Many materials, polymers and metal alloys, have been used in denture construction. None of the materials fulfils all the requirements for the components of the ideal denture[11]. The clasp of RPD is often made from the same cast metal alloy as the metal framework of the denture[28].

The base metal alloy systems most commonly used today include stainless steels, nickel–chromium, cobalt–chromium, titanium, and nickel–titanium alloys[30]. The base metal alloys offer high values for modulus of elasticity and strength, excellent corrosion resistance, and very low metal cost compared with the alternative type IV gold casting alloys[31]. An increased awareness of esthetics in dentistry has resulted in the need for RPDs, which reveal little or none of the metal supporting structures or retentive elements[32]. Wear of the tooth surface may result in a reduction in the circumference of the tooth, and a further shortcoming of the metal clasps is their poor esthetic appearance. Tooth-colored clasps made of polymer, such as acetal resin (polyoxymethylene)[33] or metal clasps veneered with composite were used[34].

Acetal resin has been used as an alternative denture base and denture clasp material since 1986 and was promoted, primarily, for superior esthetics[11],[22],[33].

The sample tooth used in this study was readymade upper first molar to standardize the study according to the studies by Arda and Arikan[22], Chenget al.[25], and De Torreset al.[26] who recommended using a metal model representing the upper first molar made in Co–Cr alloy to facilitate the testing of clasps on the universal testing machine. Kimet al.[35] used metallic crown on lower second molar to evaluate retentive force of a Co–Cr clasp. In their study, Tannouset al.[36] recommended the use of metallic crown on premolar for testing retentive forces and fatigue resistance of acetal resin and Co–Cr clasps by the insertion/removal test.

The preformed wax pattern for Aker was used to facilitate the standardization of the shape and thickness of clasps and to eliminate the factors that may affect flexibility and also to eliminate the manual variations[35],[36]. This study was designed to compare the retentive forces of clasps in three different amounts of undercuts. The 0.01 inch (0.25 mm) undercut was chosen because it represented the undercut commonly used for Co–Cr clasps[37], whereas the 0.02 inch (0.50 mm) and 0.03 (0.75 mm) inch undercuts were selected to simulate the cases where clasps should be placed closer to the gingival margin, and where undercut tends to be deeper, thereby producing a more esthetic result[35],[36].

The clinical experience for the retention loss of RPD after some time of denture wearing raised the question of whether constant deflection of the clasp during insertion and removal of the denture affect the clasp retention by causing metal fatigue[28]. Fatigue is responsible for 90% of all service failure[38]. The retentive clasp arms are the parts of RPD most frequently damaged[39], as clasps in clinical use are subjected to cyclic bending during insertion, removal of partial dentures, and also during mastication[40].

On the basis of a previous study that determined the number of clasp failures in RPDs, a decision to carry out fatigue tests was made

[41]. On the basis of the data obtained in this investigation, the Co–Cr and Ni–Cr clasps showed significantly higher retention force than did Ac clasps in the different degree of undercuts before and after loading. This lower retentive force of Ac clasps was due to greater flexibility of the Ac clasp and because of using the same diameter as metal clasps not larger in cross-sectional area, as reported in a study by Turneret al.[33], who stated that to obtain stiffness similar to that of a cast Co–Cr clasp measuring 15 mm in length and 1 mm in diameter, a suitable Ac clasp must be shorter (∼5 mm) and have a larger cross-sectional diameter (∼1.4 mm). Fittonet al.[11], in their study, stated that the Ac clasps must have greater cross-section area than metal clasps to provide adequate retention.

The lower retentive force of acetal resin clasps compared with other clasp types is in agreement with the results obtained in the respective studies by Arda and Arikan[22] and Satoet al.[42]. The results obtained in the study made to evaluate the retention qualities of an acetal resin clasp by Wuet al.

[32] were in agreement with the results of this study, which compared deformation of acetal resin and metal alloy RPD direct retainers after repeated dislodgments over a test die for a simulated 3-year period and found a statistically significant difference in the retentive force after cyclic loading. The present study confirms these findings, because the retentive force of acetal resin clasps in 0.01, 0.02, and 0.03 inch undercuts show statistically significant difference over the five periods tested (2 years of simulated use ) except that acetal resin clasp in 0.01 inch undercut showed no significant difference in retentive force after 730 cycles; this may be attributed to the greater flexibility of the material; moreover, the degree of undercut engaged by the clasp was 0.01 inch, which may have only a low effect on the retentive properties of the clasp.

On the other hand, the results of this study disagreed with that of a study by Arda and Arikan[22], who indicated that acetal resin clasps with 1.2 mm thickness and with 2.0 mm thickness were resistant to deformation. The retentive forces of both types of acetal resin clasps did not decrease over the 3-year cycling period and showed no permanent deformation and no significant difference. The results of this study showed statistically significant difference for Co–Cr and Ni–Cr clasps from the first to the final period of cyclic loading. Co–Cr and Ni–Cr clasps lost retentive force within 730 cycles of placement and removal and continued to lose retentive force during the remaining testing period. Past studies have indicated that there was a loss of retention because of permanent deformation of the Co–Cr clasps[14].

In the current study, the retentive force of the Co–Cr, Ni–Cr, and Ac clasps decreased in the test cycles, but that of Co–Cr and Ni–Cr was still greater than that of Ac clasps at the end of the test period. The mean retentive force for the 1.2 mm-thick Ac clasps at the end of the cycling test in the three undercuts ranged from 0.90 to 2.96 N; this was in agreement with the study conducted by Arda and Arikan[22], who used a 1.2-mm-thick clasp and found that the retentive force after cyclic loading was 1.08 N, but these results disagreed with that of a study conducted by Satoet al.[42], who concluded that the mean retentive force for the 1.0-mm-thick acetal resin clasps at the end of the cycling test ranged from 1.7 to 3.7 N, and for the 1.5-mm-thick clasps it ranged from 5.4 to 10.8 N.

The retentive force of Co–Cr and Ni–Cr alloy cast clasps decreased significantly after cyclic loading in the 0.01, 0.02, and 0.03 inch undercuts, and yet the residual retentive forces were maintained at 2.19 ± 0.54 to 3.96 ± 0.85 N; this was in agreement with Arda and Arikan[22], who found the retentive force of Co–Cr clasps after 4380 cycles to be 3.3 N. According to a study by Satoet al.[42], a retentive force of 5N is required for adequate function of RPD; and Frank and Nicolls[43] in their study showed that 3–7.5N represented an acceptable amount of retention for a bilateral distal-extension RPD; these results indicate that Ac could be used in the fabrication of clasps for RPD but with further modification of the clasp as they do not provide a suitable retention for RPD even after 2 years of stimulated use, and that for metal clasps, the clasp's retentive capacity was still adequate to satisfy the need for retentive force after 2 years of simulated use.

In the present study, retentive force value in 0.01 inch undercut had less value compared with cycling specimens made for the 0.02 inch undercut and 0.03 inch undercut; this result was supported by a study conducted by Powers and Sakaguchi[44], who stated that cycling clasps on a die with a 0.01 inch undercut had less effect on the retention forces compared with cycling specimens made for the 0.02 inch undercut, and 0.03 inch undercut can be attributed to the difference in the amount of strain the metal underwent as it was removed from the undercut.


  Conclusion Top


Considering the limitations of this study, the following conclusions were drawn:

  1. The mean retentive strength required to remove Ac clasps was found to be significantly lower than that required for the removal of Co–Cr clasps and Ni–Cr clasps.
  2. The retentive strength required for Ac, Co–Cr, and Ni–Cr clasps demonstrated significant change over the five periods tested.
  3. The mean retentive strength of Co–Cr and Ni–Cr clasps showed marked decrease after 2920 cycles and was nearly equal to that of acetal resin, but still a significant difference was found.
  4. Acetal resin, Co–Cr, and Ni–Cr clasps showed significant deformation after 24 months of simulated clinical use.
  5. Retentive strength of clasps with 0.03 inch undercut was greater than that of 0.02 and 0.01 inch undercuts in the three clasp materials.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
 
 
    Tables

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



 

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