|Year : 2012 | Volume
| Issue : 1 | Page : 11-14
Acetal resin as an esthetic clasp material
K Lekha, NP Savitha, Meshramkar Roseline, Ramesh K Nadiger
Department of Prosthodontics, Shri Dharmasthala Manjunatheswara College of Dental Sciences and Hospital, Dharwad, Karnataka, India
|Date of Web Publication||22-Mar-2012|
Department of Prosthodontics, Shri Dharmasthala Manjunatheswara College of Dental Sciences and Hospital, Dharwad, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
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.
Keywords: Acetal resin, clasp, esthetics, metal free framework, removable partial denture, thermoplastic
|How to cite this article:|
Lekha K, Savitha N P, Roseline M, Nadiger RK. Acetal resin as an esthetic clasp material. J Interdiscip Dentistry 2012;2:11-4
| Introduction|| |
Patient demands a removable partial denture (RPD) for health, anatomic, psychological, or financial reasons. Fabricating an esthetically pleasing RPD while avoiding the unsightly display associated with conventional clasp assemblies often presents a challenge to the dentist. The increased emphasis on physical appearance in contemporary society has increased the demand for esthetic restoration. Use of metal clasps on anterior teeth may cause esthetic problems. Methods to overcome this esthetic dilemma include the painting of clasps with tooth-colored resin, , use of lingually positioned clasps, , engagement of mesial rather than distal undercuts,  and use of gingival approaching clasps. Unless clasps can be avoided by using precision attachments,  some of the RPD framework will be invariably visible. The possibility of injecting the plasticizing resin into the mould has opened a new perspective to full denture and RPD technology.  Acetal (Bio Dentaplast, Bredent, Senden, Germany), a thermoplastic resin, may be used as an alternative denture clasp material.
| Methods for Data Collection|| |
PubMed and Google Scholar were used to search original research articles, case reports, and other reviews about acetal resin material, from 1960 to 2011, from peer-reviewed journals. The keywords used were acetal resin, aesthetic, clasp, removable partial denture, thermoplastic, metal free framework. For further refinement, the following exclusion criteria were defined: Publications were limited to those of English language and from the scientific, peer-reviewed literature. Furthermore, publications possessing a questionable peer-review process (e.g., manufacturer-supported) were excluded for consideration. From these searches, a total of 21 publications were selected for this review.
| Review of Literature|| |
Acetal was first proposed as an unbreakable thermoplastic resin RPD material in 1971. It was during this period that Rapid Injection Systems developed the first tooth-colored clasps with a thermoplastic fluoropolymer. In 1986, Dental "D" reintroduced tooth-colored clasps using acetal resin. The clasps were flexible, did not need periodic adjustment to keep them tight, and the tooth-colored esthetics were appreciated by the patients. Pressing Dental followed in the early 1990s with an acetal resin (marketed in the US by DENTSPLY Austenal) which, in addition to tooth-colored clasps, has been used for an entire partial denture framework as well as other appliances. 
In 1992, The Flexite Company developed and patented the first pre-formed tooth-color clasps known as Clasp-Eze. This product, made of a nylon material, is available in pink and clear color shades and currently sold worldwide. DENTSPLY recently introduced the Success FRS "flexible resin system" for their Success denture press. The FRS system utilizes a flexible tissue-colored thermoplastic resin for flexible partial dentures. Currently, Cosmetic Dental Materials has introduced Aesthetic Perfection T (patent pending), a new line of thermoplastic acetal, acrylic, and polycarbonate materials that can be used in most thermoplastic presses. These materials offer excellent esthetics combined with favorable physical properties and easy processing characteristics. ,
Polyoxymethylene (POM), which is formed by polymerizing formaldehyde, may be used as an alternative denture clasp material. The homopolymer POM is a chain of alternating methyl groups linked by an oxygen molecule. Acetal as a homopolymer has good short-term mechanical properties, but as a co-polymer has better long-term stability.  Acetal resin is very strong, resists wear and fracture, and is quite flexible. It also exhibits high creep resistance and high fatigue endurance as well as is hydrophobic, which means that the material does not absorb water or saliva. It is monomer free and offers an innovative and safe treatment alternative for patients who are allergic to conventional resins. It has little or no porosity, which reduces the accumulation of biological material like plaque, which in turn resists odor and stains. These characteristics make it an ideal material for pre-formed clasps for partial dentures, single-pressed unilateral partial dentures, partial denture frameworks, provisional bridges, occlusal splints, and even implant abutments. Acetal resins resist occlusal wear and are well suited for maintaining vertical dimension during provisional restorative therapy.  Because of its biocompatibility, it was considered as an RPD framework material for patients with allergic reactions to Co-Cr framework.  The material was promoted primarily on the basis of superior esthetics, which allowed the clasp to better match the color of the abutment tooth.  This material has fostered its use in total hip replacement  and as artificial heart valve occluders.  It has been used to form a stress-absorbing component in a dental implant system (IMZ). 
| Discussion|| |
The possible use of polyacetal resin as a denture base material was considered by Smith over 40 years ago.  A material "Dental D" is the first dental product involving these resins to come into the market. These injection molded resins have been used as an alternative denture base and direct retainer material since 1986, and were promoted primarily on the basis of superior esthetics, which allowed the clasp to better match the color of the abutment tooth. 
Fitton tested some physical characteristics of POM (acetal resin) for dental use like the modulus of elasticity in compression, extension, and flexure, stress relaxation, force displacement behavior of clasp forms, impact strength, and glass transition temperature. Results showed that resin clasp may be resilient enough to engage the undercuts for the retention of RPD. But the low flexural modulus requires the resin to be used in greater cross-sectional area than the metal alloys in order to gain useful retention. This greater bulk has implication for plaque accumulation and maintenance of periodontal health. 
The flexural properties of acetal resin clasps were also investigated by Turner et al. The conclusion was to have stiffness similar to a 15-mm-long and 1-mm-diameter cast Co-Cr clasp, an acetal resin clasp must be about 5 mm shorter but with greater cross-sectional diameter (1.4 mm). 
Physical properties of acetal resin were also tested by Martinez-Gonzalez et al. Metal, porcelain, and acetal post and core were used to restore extracted maxillary canine teeth and loaded at an angulation of 45° and a forward speed of 0.5 mm/min to fracture teeth. The sample restored with acetal resin posts and cores showed the greatest resistance, statistically comparable to metallic post and core, and presented no fractures, while metallic post and core exhibited different angulations and the ceramic restoration showed cracks. 
A particular advantage of RPD made of acetal resin applies to the patients with large oral defect as a result of a maxillectomy procedure where postoperative radiotherapy was planned. In these patients, the density of the defect has to be restored to ensure standardized radiation distribution. This is achieved with various types of boluses that often require tissue surface positioning stents to help support them. Traditional metal-clasp retained stents were discarded as the clasps caused backscatter of the radiation beams. A radiolucent material was needed to retain these prostheses. Dental D, an acetal resin, was used in the fabrication of a positioning stent. It was assessed in terms of ease of manufacture, cost, fit, retention, and radiolucency. The material was found to be more costly and time-consuming to manufacture than conventional metal-retained acrylic resin prostheses, but its radiolucency made it ideal for use in patients during radiotherapy treatment. 
Arda and Arikan simulated a 36-month clinical use of RPD clasps made of acetal resin and assessed their retentive force and deformation by comparison with similar clasps cast of Co-Cr. The result showed no deformation for the acetal resin clasp after 36 months of simulated clinical use unlike the Co-Cr clasp which presented an increase in the distance between the tips. However, the acetal resin clasps require less force for insertion and removal than Co-Cr clasps even after the simulated period. 
Ozkan et al. compared the color stability of pigments in acetal resin with conventional polymethyl methacrylate PMMA. The results showed a slight color change for both the materials after 4000 thermal cycles. The discoloration of both the materials was significant after 12,000 thermal cycles. However, discoloration values were clinically acceptable. Regarding the color of metal-free RPD made of acetal resin, Chu and Clow showed that acetal resin clasps are simple and effective means of improving RPD esthetics. 
Water absorption and water solubility of pink and white acetal resins were also investigated. Pink acetal resin showed significantly lower water absorption than the polymerized acrylic resin and white acetal resin. Water absorption and solubility of acetal resin were within ISO specification limits. 
Yota Takabayashi studied the characteristics of denture thermoplastic resins for nonmetal clasp dentures. The following conclusions were drawn after conducting water absorption solubility, flexural strength, modulus of elasticity, tensile strength, and color stability tests to reveal the mechanical and physical properties of thermoplastic and conventional acrylic resins.  It was concluded that though the flexural strength and modulus of elasticity were relatively low in the thermoplastic resins, they demonstrated great toughness and resistance to fracture; thermoplastic resins could withstand stress through a considerable degree of deflection, indicating that they have sufficient longevity for repeated insertion and removal from the oral cavity. The water absorption values of all the tested materials met the ISO standards for Type 3 denture base materials, indicating that the thermoplastic resins are stable and hygienic materials.
| Merits and Demerits of Acetal Resin Claps against Conventional Clasps|| |
- Clasps fabricated with acetal resin are esthetically pleasing because the color matches with that of tooth color.
- Because of their low modulus of elasticity, they can be used in larger undercuts than recommended for chromium-cobalt alloy and also exert less stresses on abutment teeth. This may be advantageous in clinical situation where esthetics and periodontal health are priorities.
- Acetal resin provides less retention compared to chrome cobalt. So, further study has to be done regarding various thicknesses and designs of clasps and framework for its successful dental application.
- In patients with metal allergy, acetal resin can be used as a clasp material.
| Conclusion|| |
It is always a challenge to obtain optimal esthetics while maintaining retention, stability, and healthy tooth structure with cast partial dentures. Acetal resins are highly versatile engineering polymers that bridge the gap between metals and ordinary plastics. Because they offer the strength of metal and the flexibility and comfort of plastic, they make an ideal material for the fabrication of dental prostheses, particularly clasps. They are monomer free and offer an innovative and safe treatment alternative for patients who are allergic to conventional resins. For patients who do not wish to have metal in their mouth, for cases where no preparation of teeth is desired, or in periodontally compromised cases where minimum stresses onto the abutments are desired, acetal resin partial and removable bridges offer a vastly expanded range of applications.
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