## 4 Discussion  
Appropriate light-curing is crucial for the long-term success of direct resin composite restorations because it determines the physico-mechanical and biological properties of the material [17; 18; 29]. The present study revealed that before receiving detailed light-curing instructions and hands-on training, the dental students delivered a wide range of radiant exposure between 1.9 and 12.5 J/cm\({}^{2}\) during the 10-s irradiation time, representing a more than 6-fold difference between the minimum and maximum amount of light energy applied. This suggests that clinically relevant properties of composite restorations such as wear resistance, depth of cure, and hardness may differ considerably after light-curing [11; 12; 15]. Indeed, several studies on the longevity of composite restorations showed that operator factors significantly affected the results [30; 31]. The light energy necessary to adequately polymerize a resin composite depends on the brand type, increment thickness, translucency, and shade of the material [32; 33], but a radiant exposure of 10 J/cm\({}^{2}\) is usually regarded as the critical minimum threshold [20; 27]. Before training, 15 out of 22 students  
Figure 2: Irradiance (mW/cm\({}^{2}\)) delivered by the dental students at baseline, after training with the MARC Patient Simulator, and after the 26-day preclinical course. The boxplots show the medians (black lines) with 25% and 75% percentiles (boxes); the whiskers represent \(1.5\times\) IQR (interquartile range), or minima and maxima of the distribution if below \(1.5\times\) IQR; outliers are shown as circles, and extreme outliers are shown as asterisks. Same lowercase letters indicate statistically similar values within the control group, whereas same uppercase letters indicate statistically similar values within the training group. \(p\)-values for comparisons between the control and the training group are indicated above the corresponding boxes. (68%) in the training group delivered irradiances above that threshold, while after the training, all students (100%) were able to deliver irradiances above the critical minimum. After completion of the 26-day preclinical course, 21 out of 22 students (95%) in the training group were able to surpass the stipulated threshold. These results indicate that the individualized simulator training with immediate visual feedback helped to enable a higher percentage of students to deliver clinically relevant radiant exposures. On the other hand, in the control group, the number of students able to deliver radiant exposures above the stipulated threshold remained unchanged after the 26-day preclinical course compared to the evaluation at baseline (14 out of 22; 64%). This suggests that a certain number of worse-performing students were unable to improve their light-curing skills by attending the 26-day preclinical course alone, clearly indicating the benefit of the individualized training with the patient simulator. The main benefit of the training with the patient simulator was that radiant exposures delivered by students to a simulated posterior restoration became considerably more consistent. This finding is clinically more important than a statistically significant improvement of 5% in the training group identified after the preclinical course, and an 8% significantly better performance of the training group compared to the control group after the preclinical course. Although deemed significant by the formal statistical analysis, such relatively small improvements in light-curing performance are of secondary relevance compared to the considerably narrowed distributions of irradiance values after the training with the patient simulator. Before receiving detailed instructions and training, most students chose an orange light shield to protect their eyes from the bright blue light emitted by the light-curing device. A disadvantage of using shields is that they must be handheld, which leaves no hand free to stabilize the curing light tip while light-curing.