Dentoskeletal changes after Rapid Maxillary Expansion: influence of the activation protocol

Rapid maxillary expansion (RME) is a common clinical orthodontic procedure[1,2] used to increase the maxillary arch dimension by opening the mid-palatal suture. The current trend in orthodontics has shifted towards the principles of dentofacial orthopedics and non-extraction treatment modalities, for this reason over the 90% of orthodontists offer this procedure as a treatment option[3] in primary, mixed or permanent dentition[4]. The main clinical use of RME appliance is to treat the maxillary arch deficiency associated with posterior crossbites, (prevalence ranging from 7.1 to 23.3% [5,6], Class II malocclusion[7,8], Class III malocclusion[8] and crowding. The Hyrax appliance is the most common type of RME appliance (figure 2), with an expansion screw that is attached to 2 or 4 teeth that is usually activated once or twice daily for about 2-4 weeks[4]. RME generates large forces to exceed the limits of orthodontic tooth movement producing maximum orthopedic repositioning, affecting the circummaxillary suture system and more specifically the mid-palatal suture, but also compressing the periodontal ligament, bending of the alveolar processes and tipping of the anchoring teeth and inducing other skeletal and dental effects as confirmed by numerous studies[9–11]. Other authors[12,13] have reported that RME with conventional appliances has some skeletal side effects as the anterior and inferior displacement of the maxilla with a consequent posterior-inferior rotation of the mandible. The expansion force depends on the activation protocol; the screw, for example, can be activated once or twice daily for about 2-4 weeks and a single activation produces approximately 3 to 10 pounds of force[14]. Some studies[15–18] compared the different dento-skeletal effects obtained in patients treated with RME and slower expansion procedures. The dentoskeletal effects of the most common activation protocol two-quarters turn per day (2QD) have been widely studied in literature; otherwise few studies analyzed the changes of one-quarter turn per day (1QD) activation protocol on trasversal diameters of superior and inferior arch[17]. The aim of this doctoral thesis is, after having assessed the effects of the RME on the craniofacial structures, to study the influence of different activation protocols on these changes.