# Orthodontic Relapse after Fixed or Removable Retention Devices: A Systematic Review
###### Abstract  
Retention constitutes a fundamental phase of orthodontic treatment, of which the patient must be made aware from the outset. Retention, which can be fixed or movable, has the task of maintaining over time and stabilising the results obtained during treatment. This study assessed the efficacy of using removable restraints versus fixed solutions for maintaining long-term outcomes. A comprehensive search across major databases--Pubmed, Web of Science, Scopus--used'relapse' and 'orthodontic' as keywords to gather articles on relapse discussions. The primary focus was relapsed cases in post-fixed orthodontic therapy. Both fixed and removable retainer systems prove effective in preserving orthodontic achievements. While fixed devices require regular wire integrity checks, mobile devices require patient compliance, proper usage, and a recommended wear time. Studies indicate that fixed retainers are generally successful, with relapse rates varying based on the retainer type. Full-time use of removable devices surpasses night-only wear. Vacuum-formed and Hawley retainers offer similar effectiveness. Fixed retainers excel in long-term alignment stability, whereas removable ones have higher failure rates yet remain beneficial. d 2023 1  
time to reorganise. The elastic fibres surrounding the teeth and the dental-gingival and interdental fibres take the longest to remodel, even up to 8 months [8]. The orthodontic movement causes remodelling and disorganisation of the collagen fibres of the extracellular matrix of the periodontal ligament, with a specific increase in immature collagen fibres, especially in the apical region of the tooth (area of greatest load) [9]. Applying orthodontic force releases inflammatory mediators and other molecules, such as interleukins, leptin, aspartate aminotransferase, etc., that support the inflammatory reaction underlying tissue remodelling [10]. Therefore, the teeth must be kept in place for enough time to allow these fibres to adapt. An alternative strategy involves using a straightforward surgical operation (perincision) to cut these gingival fibres [11]. Some studies have established that a stable occlusion is related to proper dental gearing between the arches: occlusal interference, displaced tooth contacts, and occlusal overload could result in increased tooth mobility and increased risk of relapse [12]. The orthodontic treatment should aim at positioning the teeth in the so-called \"neutral zone\". The neutral zone is an area in which the centrifugal forces of the tongue and the centripetal forces of the period soft tissues are balanced [13]. The more the teeth are outside the neutral zone due to excessive proclination or retroclination, the greater their instability and risk of recurrence [14]. Furthermore, when planning an arch form variation, the clinician should consider the increased risk of recurrence due to soft tissue forces on the teeth [15]. The dentition is in a phase of dynamic equilibrium that is constantly changing and readjusting: it has been widely demonstrated that there is a minimal residual growth of facial tissues even in adulthood, a variation in inter-arch ratios and a variation in the pressure exerted by the soft tissues [16]. These variations affect tooth balance and alignment. Therefore, a small amount of recurrence is considered physiological [17]. Removable or fixed, active, or passive retainer devices are used to achieve stability of orthodontic therapy over time. Schematically, the most used devices can be classified into  
* Passive removable: resin plates, with or without a vestibular arch, Hawley, and vacuum-formed retainers (Figure 1), such as Essix, etc. [18]. * Active removable: plates with added springs, grids, elastic hooks, spring aligners, thermofermed with set-up, and digitally customised elastomers. * Passive fixed: from classic systems (lingual arches, Maryland bridges, etc.) to all types of bonded retainers (Figure 2). * Fixed active: cemented grids in the upper jaw and lingual arches with active insertion.