Retention strategies, cost-effectiveness and long-term stability oftreatment outcome are essential aspects of orthodontic treatmentplanning.The overall aim of this thesis was to compare and evaluate threedifferent retention strategies, with special reference to short- andlong-term clinical stability and cost-effectiveness. The approach wasevidence-based, hence randomized controlled methodology was usedin order to generate high levels of evidence.This thesis is based on four studies:Papers I and II are based on randomized controlled trials, evaluatingthe stability of treatment outcome after one and two years of retention,using three different retention strategies: a maxillary vacuum-formedretainer combined with a mandibular canine-to-canine retainer; amaxillary vacuum-formed retainer combined with stripping of themandibular anterior teeth and a prefabricated positioner.Paper III presents a cost-minimization analysis of two years ofretention treatment.Paper IV is based on a randomized controlled trial documentingthe results five years post-retention.The following conclusions were drawn:Papers I and II• From a clinical perspective, asssessment after one year ofretention disclosed that the three retention methods weresuccessful in retaining the orthodontic treatment results.• After two years of retention, all three retention methods wereequally effective in controlling relapse at a clinically acceptablelevel.• Most of the relapse occurred during the first year of retention;only minor or negligible changes were found during the secondyear.• The subjects were grouped according to the level ofcompliance (excellent or good). After two years of retentionthere was a negative correlation between growth in bodyheight and relapse of mandibular LII in the group of subjectswith excellent compliance. The group with good complianceshowed a positive correlation (Paper II, Figure 3).• After two years of retention, growth in body height, initialcrowding and gender had no significant influence onmandibular LII (Paper II, Figure 4 and Table 4).Paper III• The cost minimization analysis disclosed that although thethree retention methods achieved clinically similar results, theassociated societal costs differed.• After two years of retention, the vacuum-formed retainer(VFR) in combination with a canine-to-canine retainer (CTC)was the least cost-effective retention appliance.Paper IV• After five years or more out of retention, the three retentionmethods had achieved equally favourable clinical results.Key conclusionsand clinical implicationsThis study compared the short- and long-term outcomes of orthodonticretention by three different methods: a maxillary vacuum-formedretainer combined with a mandibular canine-to-canine retainer;a maxillary vacuum-formed retainer combined with stripping ofthe mandibular anterior teeth and a prefabricated positioner. Allmethods gave equally positive clinical results in both the short-term,i.e. after one and two years of retention, and in the long-term, fiveyears or more post-retention. After two years of retention, the level of compliance affected theretention treatment result. However, no such effect was shown forbody height, the severity of initial crowding or gender.Today, there is increasing emphasis on the importance of economicaspects of healthcare. Of the three methods evaluated in this study,the least cost-effective, after two years of retention, was a vacuumformedretainer combined with a bonded canine-to-canine retainer.The clinical implication of this finding is that in patients meetingthe inclusion criteria, interproximal stripping of the mandibularanterior teeth, or the use of a prefabricated positioner, are highlyappropriate alternatives to a mandibular bonded canine-to-canineretainer.The overall conclusions are that there are a number of effectiveretention methods available and the clinician is not limited to routineuse of a bonded mandibular canine-to-canine retainer. The mostappropriate retention method should be selected on an individual,case to case basis, taking into account such variables as orthodonticdiagnosis, the expected level of patient compliance, patient preferencesand financial considerations.