## 1 Introduction  
Orthodontic treatment aims to reach a full alignment of teeth and the optimal coordination of upper and lower maxillary bones in order to obtain an aesthetic harmony of a smile and face within the anatomical and functional limits of the patient. Clinical effective alignment should aim to aid the optimal speed of tooth movement, inducing the minimum level of discomfort for the patient [1].  
The International Association for the Study of Pain reported an exhaustive definition of pain that was described as \"an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage\" [2]. Therefore, pain sensation is the result of sensory, affective and cognitive components, involving several psychological processes. Pain stimulates both behavioural and emotional reactions that can in turn influence its perception to a variable extent, depending on character traits. In this regard, the literature reports that the presence of somatosensory amplification and trait anxiety entail increased pain perception, both quantitative and qualitative [3, 4, 5]. A recent study investigating the effect of clinical, demographic, psychological and genetic factors on pain levels experienced during fixed orthodontic treatment reported that pain in teeth is mainly related to the introduction of a fixed appliance rather than to the force applied, and patients with high catastrophising scores report higher values of discomfort [6]. Painrepresents a common side effect during fixed orthodontic treatment, conditioning the treatment acceptance more than the social discomfort caused by the braces [7]; furthermore, treatment discontinuation and poor compliance have been attributed to pain experienced in the early stages of orthodontic therapy [8]. Consequently, pain prevention and reduction should be the main concerns of an orthodontist. According to the literature, fixed appliances cause higher pain intensity and teeth sensitivity than removable appliances [9]; the discomfort starts 2 h after the application of the forces [10], raises over the next 24-48 h and tends to disappear within 6-7 days [11]. Despite non-steroidal anti-inflammatory drugs (NSAIDs) being widely used to reduce pain perception during orthodontic treatment [12], it has been shown that this class of drugs can reduce the rate of tooth movement [13; 14] through the inhibition of prostaglandandins. Considering also systemic side effects such as gastrointestinal, renal and hepatic toxicity [15; 16] brought about by NSAIDs, their use in orthodontics should be reconsidered. As far as archwires are concerned, several studies investigated which archive was more effective in terms of alignment, pain provoked and consequent root resorption, achieving contrasting results [17; 18; 19; 20]. A few studies have made comparisons between different archwires viz. super-elastic nickel-titanium (Ni-Ti) and multi-stranded stainless steel (SS) archives [19; 21; 22; 23] and conventional Ni-Ti and super-elastic Ni-Ti archives [17; 24], but no conclusions can be drawn concerning the differences in the rate of alignment or pain [25]. No data are available on the possible differences between single-strand SS and different types of Ni-Ti archwires. There is a lack of well-designed and conducted investigations to determine if the performance of the initial arch material makes a clinically relevant difference to the alignment of teeth and pain in the initial stage of orthodontic treatment [25; 26]. Moreover, clinical observations have suggested the existence of a direct relationship between the degree of crowding and the intensity of pain perception after the application of orthodontic forces [27], but other studies have reported no statistically significant correlation among these parameters [18]. The aim of this prospective controlled cohort study was to compare the perception of pain experienced using a 0.014 inch super-elastic Ni-Ti and a 0.012 inch SS wire during the first 15 days of orthodontic treatment.