# Orthodontic Relapse after Fixed or Removable Retention Devices: A Systematic Review
## 4 Discussion  
### Bonded Retainers Compared  
[PERSON] et al. [25] conducted a trial comparing flexible spiral wire and ceramic interlocking retainers, finding that both retainers effectively maintained treatment results with slight relapse. [PERSON] et al. [28] compared CAD/CAM (Memoatin(r)) and wire-based retainers (Ortho-Flextech(tm)), observing a high failure rate with the CAD/CAM retainer (50%) on the upper arch compared to the other [57, 58]. Although not statistically significant, the failure of the former is three times higher than the latter. [PERSON] et al. [29] evaluated twisted stainless-steel wire (G&H orthodontics) and titanium ribbon retainers (Retainium(r)), reporting similar clinical effects but lower failure rates with the titanium ribbon retainer. [PERSON] et al. [30] compared stainless-steel CAD/CAM (manually and machine-bent wires) and conventional retainers, highlighting better stability with the CAD/CAM machine-bent group. [PERSON] et al. [31] found no significant differences between magnetic and cemented canine-to-canine retainers. [PERSON] and [PERSON] [37] demonstrated that fibre-reinforced composite retainers exhibited less relapse tendency than stainless steel wire retainers due to their increased rigidity, which allows limited physiological tooth movement. [PERSON] et al. [38] investigated the stability of lingual retainers and reported no clinically significant worsening of periodontal health or relapse in any group. [PERSON] et al. [39] compared two upper-bonded retainer systems (Splinx and Ortho-FlexTech) and found a lower relapse rate with the Ortho-FlexTech retainer. [PERSON] et al. [44] evaluated different lingual retainers and one upper retainer type. They determined that the retainer bonded to the lower six teeth was more effective in maintaining lower alignment over time than the retainer bonded to the canines alone. Additionally, movement in the lower arch was associated with active forces generated during retainer application. Both inferior retainers were effective in maintaining inter-canine distance. The upper retainer was effective in maintaining upper alignment. The upper inter-canine length remains stable even though the retainer was not extended to the upper canines. [PERSON] et al. [55] investigated the movement of lower incisors in relation to orthodontic therapy recurrence or natural growth. One group had a retainer bonded to all lower anterior teeth, another had a wire bonded only to the two canines, and a third group was the control. Incisor movement occurred in 25% of the patients due to natural growth, but the measured LII of 1.5 mm at the 12-month follow-up was considered clinically irrelevant and showed different distribution patterns. The superimposition of plaster models revealed rotational movement, tipping, and slight extrusion of the canines. The canines experienced the most changes and recurrences. A subset of patients (13.32%) experienced severe recurrence characterised by significant expansion of the inter-canine diameter and notable overfeul reduction during treatment phases [44]. [PERSON]'s long-term study [48] showed that using retainers prevented more significant increases in the Little Irregularity Index (LII) compared to the untreated group. However, long-term recurrence was still observed, indicating the need for lifelong retention to limit alterations from normal development. [PERSON] et al. [26] showed that most changes occur within the first year, mainly caused by the \"memory\" of fibres.