## 1 Introduction  
Over the last quarter century, the teaching and placement of posterior resin composite restorations have steadily increased [1]. According to dental market data, more than 260 million direct composite restorations are nowadays placed each year worldwide [2]. Even though resin composites enable conservative, minimally invasive restorative therapy [3, 4, 5, 6], a median longevity of only about six years has been reported for posterior composite restorations placed in general practice [7], with secondary caries and material fractures being the primary reason for failure [7, 8].  
The success of resin composite restorations is closely related to the degree of monomer conversion of the material, which in turn is determined by the amount of radiant energy applied by the operator during light polymerization [9; 10]. Inadequate resin polymerization has been shown to deteriorate the material's physico-mechanical properties [11; 12], reduce its bond strength to the tooth [13; 14], and increase wear [15] and bacterial colonization [16] of the restoration. Furthermore, insufficient curing may compromise the biocompatibility of the composite material due to the release of unreacted monomers [17; 18; 19].  
A clinician's light-curing technique significantly affects the amount of light energy delivered to a restoration. In a previous study, [PERSON] et al. observed an up to 6-fold difference in the amount of radiant exposure applied by different operators, even when the same light-curing unit and the same irradiation time were used [20]. Even though a low radiant exposure may lead to a hard surface at the top of the composite restoration, it may simultaneously result in insufficient polymerization in deeper parts due to light scattering and absorption by the composite material [12; 21; 22; 23; 24]. Clinically, it is not possible for the practitioner to assess the depth of cure of the composite, so that insufficient polymerization of the material will remain unnoticed.  
Until recently, dental students and educators could not measure the light energy delivered to restorations. Hence, the problem of insufficient light-curing could not be addressed in practical dental education. Therefore, a patient simulator with light sensors inserted in typodont teeth has been introduced, which allows for the quantification of the applied irradiance by means of a built-in spectrophotometer and provides real-time feedback on light-curing performance [20]. Computer-assisted simulators can help identify operators who need instructional assistance, especially for technically demanding tasks [25]. Furthermore, it could be shown that training tools with digital visual feedback have great potential to optimize the learning process of students in case of complex practical operations [26]. While the relevance of hands-on training to acquire light-curing skills has been repeatedly reported in the literature [27; 28], information on the retention of light-curing skills after individualized practical training is scarce.  
The aim of this study was therefore to investigate the effectiveness of using a patient simulator to improve the ability of dental students to apply light energy from a light-curing device to a restoration, and to evaluate the effect of the simulator-based training on the retention of the light-curing skills. It was hypothesized that the students would deliver more radiant exposure after they received training with the patient simulator, including immediate visual feedback and individualized instructions.