## 2 Materials and Methods  
A prospective controlled cohort study was conducted on patients recruited at the Department of Medical and Surgical Specialties, section of Orthodontics, University of Brescia. The study protocol was approved by the Ethics Committee of the University of Brescia, with the number NP4160. The inclusion criterion was healthy patients with more than 10 years [28] in permanent dentition (with the exception of the second and third molars) who needed to undergo fixed orthodontic appliance therapy. The exclusion criteria were the presence of acute dental conditions (pulpitis, gingival or periodontal inflammation), orofacial pain and systemic diseases that could modify pain sensitivity (i.e., diabetes and peripheral neuropathy) and not self-reliant subjects, necessitating physical and psychological support for the difficulties to understand the instructions and/or to report pain perception. Conditions that could alter nociception were excluded, such as the concurrent use of anti-inflammatory and analgesic drugs, antidepressants, anticonvulsants and oral contraceptives [29]. The fulfilment of the inclusion and exclusion criteria was assessed by an expert operator with a clinical examination and the administration of an anamnestic questionnaire. In order to control the psychosocial component of pain and to verify the homogeneous distribution of subjects into groups, which represented their compatibility, the somatosensory Amplification Scale (SSAS) [4] and the State-Trait Anxiety Inventory (STAI) [30] were administered to all the subjects included. The first questionnaire (SSAS) provides an assessment of the propensity to detect and focus on normal physiological states or uncomfortable bodily sensations and automatically interpret them as signs of a serious disease [31]. This scale is a 10-item self-report questionnaire and the respondents rate the degree to which each statement is \"characteristic of you in general\" on an ordinal scale of 1 to 5; SSASscores over 30 may reflect a highly somatising condition (score range from 10 to 50) [4]. The STAI is a self-assessment questionnaire made up of 40 items, 20 on state anxiety and 20 on trait anxiety. The subjects report on a 1- to 4-point scale how much each sentence describes their own personality, providing a measurement of how much anxiety affects their character. The score can range from 20 to 80, and the higher the value, the more anxious the individual, with a cut-off point of 40 suggesting possible clinically significant symptoms [30]. The present study focused on trait anxiety, with the aim of identifying stable conditions of anxiety regardless of a particular situation, viz. anxiety expressed as habitual way of responding to external stimuli rather than as a contingent symptom. All the participants signed an informed consent. Dental impressions were taken using alginate Hydrogum 5 (Zhermack S.p.a., Rovigo, Italy) and dental casts were made of extra-hard dental stone type 4. MBT pre-informed brackets with a 0.22 slot (Mini Diagonal(r), Leone Spa, Sesto Fiorentino, Italy) were bonded on upper or lower teeth and the patients were divided into two groups: one received a 0.012 inch SS wire (AJ Wilcock Australian, PTY Ltd., Whittlesea, VIC, Australia) as a first archwire whilst the other group received a 0.014 inch Ni-Ti super-elastic wire (Memoria(r), Leone Spa, Sesto Fiorentino, Italy). The archwires were tied with elastic orthodontic ligatures (Ormco, West Collins Orange, CA, USA). The patients were blind to the type of archwire received and the same orthodontist treated all the included patients. After the placement of orthodontic appliances, an operator who was blinded to the allocation group gave the same verbal information to all the included patients. Each subject was asked to avoid analgesic drug assumption during the investigation and was instructed to report the characteristics of the pain perceived during the following 15 days by means of a questionnaire.