Retentive forces and fatigue resistance of thermoplastic resin clasps



The objective of this study was to evaluate the retentive force of clasps made from three thermoplastic resins and cobalt–chromium (CoCr) alloy by the insertion/removal test simulating 10 years use.


On standardized premolar metal crowns 112 clasps were fabricated, including 16 CoCr (1.0 mm thick) clasps and 32 clasps (1.0 or 1.5 mm thick) from each of the following thermoplastic resins: polyetheretherketon (PEEK), polyetherketonketon (PEKK) and polyoxymethylene (POM). Specimens were divided in subgroups with clasp undercuts of 0.25 mm and 0.5 mm, respectively. Each clasp assembly was subjected to an insertion/removal test on its abutment crown for 15,000 cycles. To analyze the retention over the course of insertion/removal test, retention was measured every 1500 cycles. Data were statistically analyzed using 3-way ANOVA ( α = 0.05).


Resin clasps with 1.5 mm thickness showed higher retention (4.9–9.1 N) than clasps with 1.0 mm thickness (1.2–3.1 N; P ≤ 0.001). Resin clasps of both dimensions had significantly lower retentive force than CoCr clasps (11.3–16.3 N; P ≤ 0.001). Clasps with 0.25 mm undercut showed significantly less retention than clasps with 0.50 mm ( P ≤ 0.001). All clasps exhibited an increase in retentive force during the first period of cycling followed by continuous decrease till the end of the cycling but it was still significantly not different compared to the initial retentive force ( P = 0.970).


Thermoplastic resin clasps maintained retention over 15,000 joining and separating cycles with lower retention than CoCr clasps. However, the retention of adequately designed resin clasps might be sufficient for clinical use.


The emphasis on physical appearance in contemporary society has increased the demand for esthetic dental restorations. Although the success of implant dentistry has expanded the scope of esthetic fixed prostheses, there are still many patients who for health, anatomic, psychological, or financial reasons are not candidates for implants . These patients have the option of receiving partial removable dental prostheses (PRDPs) to replace missing teeth.

A major esthetical problem with PRDPs is the display of the clasp assemblies. Many methods have been used to overcome the esthetic problem such as etching the clasp arm and coat it with a layer of tooth-color resin , using lingual retention design , or proximal undercuts (also known as rotational path insertion) .

Direct retainers fabricated in a tooth-colored material and made from thermoplastic resin have been used to improve the appearance of metal clasp assemblies and are promoted for superior esthetics . However, little information on the long-term performance of such clasps regarding retention is available in the literature.

Polyetheretherketon (PEEK) and polyetherketonketon (PEKK) are polymers from the group polyaryletherketone (PAEK) which is a relatively new family of high-temperature thermoplastic polymers, consisting of an aromatic backbone molecular chain, interconnected by ketone and ether functional groups . In medicine PAEK has been demonstrated to be excellent substitute for titanium in orthopedic applications , and it has been used in dentistry as provisional implant abutment .

Polyoxymethylene (POM) also known as acetal resin, an injection-molded resin has been introduced as an alternative to conventional PMMA. POM is formed by the polymerization of formaldehyde. The homopolymer, polyoxymethylene is a chain of alternating methyl groups linked by an oxygen molecule. It has a relatively high proportional limit with little viscous flow enabling it to behave elastically over a great enough range to be used as a material for clasp construction .

Various metallic materials have been used to fabricate the clasps of PRDPs and the physical properties of these materials have been examined . The most common alloys used for clasps are cobalt–chromium (CoCr) alloys . There have been studies that investigated the retention properties of CoCr alloys using repeated insertion/removal tests. Rodrigues et al. indicated an increase in retentive force during the simulating test , while retention decrease was reported by both Bridgeman et al. and Kim et al. .

Retentive clasp arms must be flexible and should retain the PRDP satisfactorily. In addition, clasps should not unduly stress abutment teeth or be permanently distorted during service . Previous studies indicated that PRDP clasps made of more elastic materials demonstrated a higher resistance to retention loss .

Due to the low modulus of elasticity (2–4 GPa) ( Table 3 ) , thermoplastic resin has superior flexibility compared to the conventional CoCr alloys. Because of the reduced possibility of traumatic overloading, clasps made from thermoplastic resin can be designed to engage deeper undercuts on abutment teeth.

There are few studies that examined flexural properties of POM to determine the appropriate design for PRDP clasp . Arda and Arikan found that POM clasps are resistant to deformation and may offer a clinical advantage over conventional metal clasps . However, to our best knowledge there are no studies evaluating the use of PEEK and PEKK as clasp materials.

Therefore, this in vitro study investigated the retentive force of different thermoplastic resin clasps during repetitive placement and removal on abutment teeth with two different thicknesses and two amounts of undercut. Conventional CoCr clasps were included as control group. The null hypothesis was that there would be no difference in the retentive force between resin clasps and cast CoCr alloy clasps.

Materials and methods

Three thermoplastic resins (POM, PEEK and PEKK) and a conventional CoCr alloy were evaluated in this study.

All used materials are presented in Table 1 .

Table 1
Materials used for clasp fabrication.
Brand name Composition Manufacturer Batch number
Acetal Dental Polyoxymethylene (POM) Dental Srl, San Marino, Italy 01065
Bio XS Polyetheretherketon (PEEK) Bredent, Senden, Germany 540XS016901
PEKKtone A Polyetherketonketon (PEKK) Cendres Metaux, Bienne, Switzerland X1074M
Wironit Co 64%; Cr 28.6%; Mo 5% (CoCr) Bego, Bremen, Germany 12769

Abutment fabrication

An artificial maxillary first premolar (KaVo, Biberach, Germany) was embedded in auto-polymerizing acrylic resin (Technovit 4000; Heraeus-Kulzer, Wehrheim, Germany) using custom-made copper holders with a diameter of 15 mm.

The maxillary first premolar was prepared for a surveyed complete metal crown. The prepared premolar was dublicated twice using a polyether impression material (Impregum Penta H and L; 3M Espe, Seefeld, Germany). The impressions were poured in Type IV stone (GC Fujirock EP; GC, Leuven, Belgium), and a complete crown was waxed on each preparation (Crowax; Renfert GmbH, Hilzingen, Germany).

The waxed crowns were surveyed to provide an undercut of 0.25 mm on one crown and 0.50 mm on the other. Occlusal rests, 2.5 mm long, 2.5 mm wide, and 2 mm deep, were placed mesially. Mesial and lingual guide planes, two thirds the length of the crown, were prepared with a surveyor blade to standardize the path of insertion. The waxed 2 crowns were duplicated with silicon material (Speedy Wax Transpaduplisil 101; Zahntechnik Norbert Wichnalek, Augsburg, Germany), and then 16 crowns with 0.25 mm undercut and 16 with 0.50 mm undercut were made by inserting heated liquid wax (Speedy Wax Injektionswachs 70; Zahntechnik Norbert Wichnalek) into the silicone mold. Then the crowns were cast in CoCr alloy (Wironit 99; Bego, Bremen, Germany).

After fitting and finishing, the crowns were cemented in place on the abutments with zinc phosphate cement (Hoffmann quick setting, Hoffmann, Berlin, Germany). The guide planes were evaluated for parallelism.

Clasp fabrication

To standardize the position of clasp arm undesirable undercut areas were blocked out with the sculpturing wax (Crowax, Renfert, Hilzingen, Germany) with approximately 2 mm surrounding thickness. Impressions of each model were made in polyether impression material (Impregum Penta H and L) with custom impression trays. Each impression was poured with die-investment material (Obtivest, DeguDent, Hanau, Germany) to make a refractory cast for the CoCr clasps and, with Type IV dental stone (GC Fujirock EP, GC), to make refractory casts for the thermoplastic resin clasps.

For CoCr clasps, preformed half-round tapered clasp patterns (1 mm × 1.4 mm tip) with occlusal rests, and retentive and reciprocal arms (Wachsprofile, Bego) were adapted along the ledges formed with block-out material prior to making impressions. A round wax sprue was connected to the residual ridge base parallel to the path of insertion using a surveyor. This sprue was later used to maintain clasp test specimens in the masticatory simulator. Each assembly (die and pattern) was invested (Obtivest, DeguDent) according to the manufacturer’s instructions and cast in CoCr alloy. Finally the clasps were trimmed, airborne-particle abraded with 50 μm alumina at 0.25 MPa pressure.

For fabrication of the 1.0 mm and 1.5 mm thick thermoplastic resin clasps straight semicircular clasp patterns (Wax patterns, Omnident, Rodgau Nieder-Roden, Germany) (1 mm × 2 mm and 1.5 mm × 3 mm) were used. The previously described wax sprue was connected to the residual ridge base parallel to the path of insertion using a surveyor. The parameters for the injection procedures have been set according to the manufacturers ( Table 2 ).

Table 2
Injection parameters for thermoplastic resin clasps. a
Pre-heating temperature/time 100/30 min 200/20 min 150/30 min
Melting temperature 220 °C 380 °C 325 °C
Pre-injection time 20 min 25 min 20 min
Injection pressure 4 bar 4 bar 7 bar
Post-injection time 5 min 2 min 1 min
Cooling time 60 min 60 min 60 min
Injection device J-100; Pressing Dental Srl, San Marino, Italy Thermopress 400 injection molding system; Bredent Thermopress 400 injection molding system; Bredent

a According to the manufacturers.

Eight clasps were fabricated for each material, clasp size, and retentive undercut combination. A total of 112 clasps were made, including 16 CoCr clasps as control group .

Table 3
Mechanical properties of the materials used for clasp fabrication. a
Modulus of elasticity Tensile strength
POM 2.4 GPa 55 MPa
PEEK 4 GPa 97 MPa
PEKK 4 GPa 89 MPa
CoCr 211 GPa 880 MPa

a According to the manufacturers.

Testing conditions

To perform the retention test, a masticatory simulator (Willytec, Munich, Germany) was used. The machine allows the placement of the clasp to its predetermined terminal position and its subsequent removal from the abutment crown, thus simulating the placement and removal of a PRDP. The models with the crowns were mounted in the masticatory simulator. Each clasp specimen was then placed on the corresponding abutment crown and fixed to the upper part of machine with auto-polymerizing acrylic resin (Technovit 4000, Heraeus-Kulzer) ( Figs. 1 and 2 ). The test conditions were maintained at room temperature (20 ± 2 °C) and wet condition (deionized water). To analyze the data obtained during the simulation test, intervals every 1500 cycles were established. A total of 15,000 cycles were performed, representing the simulated insertion and removal of the PRDP over 10 years, estimating that the patient would perform four complete cycles per day. The test was performed at a constant speed of 8 mm/s. The value established for each time interval corresponded to the arithmetic average of 10 consecutive insertion/removal cycles. The force required for each specimen removal was captured and stored using data acquisition software (LabView, National Instrument, Munich, Germany). Statistical analysis was done with three-way analysis of variance (ANOVA). The significance level was set at 5% ( α = 0.05).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Retentive forces and fatigue resistance of thermoplastic resin clasps
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