## 1 Introduction  
In recent decades, both in adult and adolescent patients, there has been a significant increase in the demand for orthodontic treatments that could be able to combine the characteristics of aesthetics and comfort required by the patient with the biomechanical performance required by the clinician.  
In order to satisfy these requirements, various companies have started to produce and introduce in the orthodontic market the so-called clear aligners [1]. As early as 1946, Kesling introduced the possibility to consecutively use a series of thermoplastic tooth positioners to progressively reposition misaligned teeth [2, 3]; but one of the pioneers in this panorama was Align Technology Inc. (Santa Clara, CA, USA), who in the late 1990s, introduced orthodontic treatments with clear aligners using sequential application of clear alignment devices made from thermoplastic material [2, 3, 4, 5, 6]. This system is called Invisalign(r). As a result of modern technology and materials, thermoplastic appliances have evolved remarkably during the last years, so they can now be used for full orthodontic therapy [1] to treat a wide range of malocclusions.  
One of the bases of the orthodontic therapy with clear aligners are the attachments that are composite resin buttons, which can be applied to the vestibular or lingual surfaces of the teeth, required as retention elements and/or to improve the efficiency of dental movements, especially if complex [4, 5, 6, 7]. Therefore, they constitute fundamental mechanical components as they allow to transfer the determined forces from the aligner to the tooth root and crown for generating efficient tooth movement [4]. For these reasons, they have to remain intact for the entire duration of the treatment.  
In clinical practice, as material for the attachment application to the teeth, flowable composites or packable composites can be used, in accordance with the producer company's instruction [8]. To allow and facilitate the attachments realization, the clear aligners' manufacturer company provides to the clinician a specific template. This is a \"passive\" thermoplastic device, i.e., not aimed to generate teeth movements, but only to allow the correct transfer of the attachments on the teeth surface. The template is generally constituted of a less rigid plastic material compared to the material of which the \"active\" aligner (i.e., those used during the treatment with clear aligners, capable of inducing teeth movement) is made.  
Recently, Ormco Corporation (Orange, CA, USA) has introduced their Spark(tm) Clear Aligner System, of which, differently with respect to other typologies of aligners produced by other companies [5; 9; 10], scientific literature is not available. The members of this group of study, who have already used Invisalign(r) in their orthodontic practice for years, have started to used Spark(tm) too, by obtaining satisfactory clinical results.  
The primary aim of this study was to evaluate the percentage of attachments' debonding during the template's removal both in Invisalign(r) and Spark(tm) systems. Furthermore, the secondary aim was to estimate the percentage of patients who did not show attachments' debonding during the template's removal.