Retentive forces and fatigue resistance of thermoplastic resin clasps

Abstract

Objectives

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.

Methods

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).

Results

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).

Significance

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.

Introduction

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 [1]. 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 [2], using lingual retention design [3], or proximal undercuts (also known as rotational path insertion) [4], [5], [6].

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 [7], [8], [9]. 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 [10]. In medicine PAEK has been demonstrated to be excellent substitute for titanium in orthopedic applications [10], [11], and it has been used in dentistry as provisional implant abutment [12].

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 [7].

Various metallic materials have been used to fabricate the clasps of PRDPs and the physical properties of these materials have been examined [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The most common alloys used for clasps are cobalt–chromium (CoCr) alloys [21]. 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 [16], while retention decrease was reported by both Bridgeman et al. [20] and Kim et al. [24].

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 [25]. Previous studies indicated that PRDP clasps made of more elastic materials demonstrated a higher resistance to retention loss [24], [25].

Due to the low modulus of elasticity (2–4 GPa) (Table 3) [8], [10], 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 [7], [8]. Arda and Arikan found that POM clasps are resistant to deformation and may offer a clinical advantage over conventional metal clasps [9]. 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.