Dental resin composites are widely used for restoring damaged or carious teeth because of their versatility, tooth-like appearance, and satisfactory mechanical properties [1], [2]. Despite their popularity, resin-composite restorations are prone to early failure [3], necessitating a sizable portion of a dentist's effort, time, and resources to replace failed restorations. This places a considerable financial strain on the dental healthcare sector [4]. The primary cause of failure in resin-composite restorations is the breakdown of the junction between the restoration and the dental hard tissues, which is caused by the stresses from occlusion, the hydrolytic action of saliva, and the degradative products from bacteria [5]. The adhesive strength of resin composites to tooth tissues is therefore often used as an indication of the longevity of resin-composite restorations.

Clinical studies are widely acknowledged as the most accurate method for evaluating the longevity of dental restorations. This is because the restorations are affected by a range of factors beyond the properties of the restorative materials, such as the practitioners’ skills; cavity size, shape, and location; and patients’ oral conditions [6]. However, clinical studies are resource-intensive, requiring a significant number of participants. They are also time-consuming as it takes months, if not years, for restorations to fail. Consequently, laboratory testing under more controlled conditions and less time-consuming is more often performed. However, to gain insights into which part of the tooth-restoration interface is more vulnerable to the challenges posed by the oral environment, the laboratory tests must be performed under biochemical and biomechanical conditions that mimic real-world scenarios. At this time, several techniques are used to evaluate the bond between resin composites and dental hard tissues.

Static, microtensile testing is one of the most widely used methods for evaluating the quality of the tooth-restoration bond. It is considered relatively simple to perform, and the results appear to align with clinical outcomes [7]. However, materials with the same initial fracture or bond strength may not break down at the same rate when exposed to the same clinical conditions, which are often cyclic in nature [8]. While artificial aging such as long-term water storage and thermal cycling can be used to simulate the degradation seen in the oral environment, the microtensile and similar bond strength test methods only provide a measure of the static bond strength for comparative purposes; they cannot predict explicitly the lifetime of a restoration. To predict the lifetime of a restoration, cyclic, instead of static, mechanical loading must be used. Furthermore, for accurate prediction, the laboratory results need to be calibrated against clinical data [9], [10].

Clinical fatigue through occlusal forces can be simulated using cyclic loading with a constant load amplitude [11], [12]. However, even if the frequency of loading is significantly higher than that found clinically, a substantial length of time may still be required before a specimen fails. As a result, accelerated fatigue testing using, e.g., a step-stress regime, would be more desirable [13], [14]. This has the effect of accelerating the rate of degradation, thus reducing the time to failure.

Our research team has devised an accelerated test method that utilizes a dentin-composite disc in diametral compression with a continuously increasing load. The test has been found to be efficient for determining the fatigue characteristics of the composite-tooth interface [15]. We have previously investigated the combined effects of chemical and mechanical challenges on the durability of the tooth-restoration interface [10]. The study utilized the above accelerated fatigue test method combined with preconditioning, in the form of storage in a low-pH buffer, to simulate the biochemical challenge. The results showed that low-pH storage had a significant impact on the survival probability of the specimens, particularly for those that were stored for longer periods. More importantly, preconditioning was essential to provide accurate lifetime predictions for the restorations.

However, the precise effects of these chemomechanical challenges on the tooth-restoration interface remained unclear. Therefore, the current study aims to quantitatively evaluate the differences in the fracture surfaces of the dentin-composite specimens failed by the different combinations of these challenges using scanning electron microscopy (SEM). In addition, since the accelerated fatigue test with a continuously increasing load provided a unique failure load that was proportional to the number of cycles to failure, results from the previous study will be reanalyzed by converting the number of cycles to failure to an equivalent failure load. This allows the fast fracture results to be compared directly with the fatigue results, thus giving a more complete view of the degradative effects. The findings from this work are expected to help guide the selection of existing and the design of next-generation restorative materials to improve the longevity and clinical success of resin-composite restorations.