Two- and three-body wear of BPA-free resin composites

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Abstract

Objectives
This study evaluated wear resistance, surface hardness, and wear-induced roughness and morphology of nine BPA-free resin composites and one conventional Bis-GMA-based composite with high wear resistance (Clearfil AP-X, control).

Methods
Two- and three-body wear tests were performed using the ACTA wear machine (ISO/TS 14569–2). Wear depth and surface roughness (Ra) were measured via profilometry, and surface morphology was analyzed using scanning electron microscopy (SEM) after each wear experiment. Vickers hardness was assessed on separate specimens at the same time points as the wear measurements. Data were analyzed using ANOVA with post-hoc tests and linear regression to explore relationships between wear, hardness, and filler content.

Results
Clearfil AP-X showed the lowest three-body wear (28.2–36.0 µm) and highest hardness (109 HV). Among BPA-free composites, Clearfil Majesty Flow (37.9–42.2 µm) and Venus Diamond (36.9–42.9 µm) demonstrated the highest wear resistance. Three-body wear moderately correlated with filler volume (R² = 0.51) and hardness (R² = 0.50), while two-body wear showed weak correlation. Surface roughness increased significantly after three-body wear, especially in micro-hybrid composites. SEM revealed matrix degradation, filler exposure, and crack formation.

Significance
BPA-free composites showed variable wear performance. Clearfil Majesty Flow and Venus Diamond performed similarly to the Bis-GMA-based control. While filler volume ratio and hardness are among key determinants of wear behavior, it is evident that other factors also play a role. Further in vivo studies are needed to confirm long-term clinical performance and safety of BPA-free materials.

Keywords
BPA-free resin composites
Dental restorative materials
Wear resistance
Surface hardness
Surface roughness

1. Introduction
Resin composites are the most commonly used restorative materials in dentistry. Resin composites have increasingly replaced dental amalgam as the standard restorative material in many countries, particularly for newly placed restorations, driven by growing aesthetic demands and regulatory restrictions on mercury‑containing materials. Since the adoption of the Minamata Convention on Mercury in 2013, a global phase-out of mercury-containing products, including dental amalgams, has been underway. As a result, the number of countries fully or partially banning the use of dental amalgam continues to grow each year, making resin composites the most viable and widely preferred alternative in contemporary restorative dentistry.
However, the biocompatibility of dental resin composites has also become a subject of debate and extensive investigation over the past few decades [1]. The incomplete polymerization of resin-based dental materials, including resin composites, leads to the release of various unreacted methacrylate monomers, many of which have been shown to exhibit cytotoxic or allergenic effects [1], [2]. Among the substances that may be released from composites, one of particular concern is bisphenol A (BPA)- a compound used in the synthesis of several methacrylate monomers, known as BPA derivatives. These include BPA glycidyl dimethacrylate (Bis-GMA), BPA dimethacrylate (Bis-DMA), BPA ethoxylated dimethacrylate (Bis-EMA), and ethoxylated bisphenol dimethacrylate (EBDMA), which are often major components in many resin composite formulations [3], [4]. In eluates from composites based on BPA derivatives, BPA is typically detected in trace amounts [2], [5]. These traces mainly originate from the manufacturing process or may form as a breakdown product—particularly from bis-DMA in the presence of salivary or bacterial enzymes [6], [7].
BPA is an endocrine disruptor with estrogenic activity and has been associated with adverse effects on the reproductive, metabolic, immune, and neurological systems, as well as with developmental abnormalities [8], [9], [10]. Although BPA is present only in trace amounts in resin composites, mainly as an impurity in BPA-derived methacrylate monomers, it can still be detected in patient’s saliva and urine following the placement of BPA-based composite restorations [11], [12], [13]. Furthermore, a growing body of evidence shows that BPA can exert harmful effects on various organs even at very low concentrations [3]. This has led the European Food Safety Authority (EFSA) to significantly reduce the tolerable daily intake (TDI) for BPA over the past decade - from 50 µg/kg body weight/day before 2015, to 4 µg/kg bw/day between 2015 and 2023, and most recently to just 0.2 nanograms/kg bw/day as of April 2023.
Growing evidence and public awareness about the negative health effects of BPA have also impacted the dental materials manufacturers. About a decade ago, new BPA-free dental resin composites were introduced. In these composites, Bis-GMA and other BPA-derivatives have been replaced with alternative methacrylate monomers, such as urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGDMA), TCD-DI-HEA monomer, or entirely different polymer systems, such as ORMOCER®-based materials [14]. Although creating a BPA-free composite might seem straightforward, replacing Bis-GMA (and its derivatives) in resin composites presents a significant challenge [15]. Bis-GMA has been widely used in dental composites since its development in 1963 due to its favorable mechanical properties, including low polymerization shrinkage and high wear resistance [16]. Since these properties are critical for the longevity and clinical performance of composite restorations, any alternative BPA-free resin matrix must exhibit at least comparable characteristics to conventional BPA-based composites. Nevertheless, the literature on the wear resistance of BPA-free composites remains limited.
Therefore, the objectives of this study were: 1) to compare the two-body and three-body wear resistance, surface hardness, and post-wear surface roughness and morphology of nine BPA-free and one BPA-containing (control) resin composites; 2) to evaluate the influence of filler content (weight and volume percentage) and surface hardness of resin composites on two- and three-body wear resistance. The null hypotheses tested were: 1) that there is no difference in these properties among tested BPA-free and BPA-containing resin composites, and 2) that filler content and surface hardness have no influence on two-and three-body wear resistance of resin composites.


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