Unilateral posterior crossbite (UPC) with functional shift is one ofthe most common malocclusions in mixed dentition. Left untreated,it may cause long-term effects on the growth and development of thejaws and teeth. Early orthodontic intervention is usually undertakento correct the condition at the primary or mixed dentition stage.Quad-helix (QH) or removable expansion plates (EP) are commontreatment alternatives to correct crossbites and treatment ofcrossbite may consume a relatively large part of the total resourcesin orthodontic care. In Sweden the treatment can be performed eitherin specialist orthodontic clinics or in general dentistry. The role ofthe orthodontist in general dental care is essential in diagnostics andtreatment planning, whereas general dentists may often provide apart of the treatment after consultation with, or under the supervisionof, an orthodontist. Economic evaluations have become an integral component of healthservices. The main reason is that resources within the health sector(personnel, time, facilities, equipment, and knowledge) are limited.Nevertheless, studies comparing the costs of orthodontic treatmentsperformed in general versus specialist dentistry are virtuallynon-existent.To date there are no studies evaluating the oral health related qualityof life (OHRQoL) in children with unilateral posterior crossbite. Itis thus important to analyse and compare the OHRQoL betweenchildren with and without malocclusions (children with normalocclusion). When treatment effects of unilateral crossbite correction have beenassessed, most studies have used two-dimensional evaluations suchas linear measurements. However, orthodontic treatment effectsincluding crossbite correction may, if possible, be described in allthree planes. There are no studies in the literature that have explicitlyinvestigated the three-dimensional treatment effects of unilateralcrossbite correction between different appliances and, in this context,related the treatment changes to growth changes associated withuntreated subjects with unilateral posterior crossbite as well as insubjects with normal occlusion and with no or mild orthodontictreatment need. The research questions addressed in this thesis originate fromidentified knowledge gaps and clinical needs in orthodontic care,and to provide as high clinical evidence as possible a multi-centrerandomised control trial (RCT) has been performed as well as acontrolled trial regarding comparisons and the impact differentmalocclusions may have on OHRQoL.The results are expected to be beneficial for the patients who willbe offered the most widely accepted and effective treatment, which isof importance for the dentists for decisions as to which treatment willgive the best outcome, and beneficial for both dental care providersand society in care planning and the allocation of resources.Therefore, this thesis was based on four studies: Paper I: a systematic literature review was undertaken to answer thefollowing questions:• Are there any articles regarding health economics in orthodonticsand is it possible to make any conclusions from thearticles?The literature search spanned from January 1966 to September2014 and was later supplemented and extended to April 2019.Paper II: the aims were to investigate:• The OHRQoL using the Child Perceptions Questionnaire(CPQ8-10) in 93 children with unilateral posterior crossbite,71 children with excessive overjet and 65 children with normalocclusion with no or mild orthodontic treatment need. Paper III and IV: these two papers originated from a multi-centreRCT. The aims were to investigate:• Clinical effectiveness and cost-analysis in specialist and generaldentistry (Paper III)• Three-dimensional evaluations of crossbite correction (PaperIV)Key findings in Paper I and the supplementary search:• Few orthodontic studies have presented both economic andclinical outcomes. There is currently insufficient evidence availableabout the health economics of orthodontic interventions.• Further studies are still warranted and preferably using thesame clinical outcomes. Key findings in Paper II• Children with excessive overjet reported significantly lowerOHRQoL compared to children with unilateral posteriorcrossbite or normal occlusion.• The children generally reported low CPQ scores that imply anoverall fairly good OHRQoL.Key findings in Paper III• Treatment of unilateral posterior crossbite in mixed dentitionis recommended to be performed by a specialist orthodontistusing the quad-helix appliance since the quad-helix treatmentperformed in specialist orthodontic clinics had the highest costeffectiveness. Key findings in Paper IV• Crossbite children had, before treatment, significantly smallerpalatal surface and volume than normal control children.• After treatment, there were no significant differences betweenthe treatment groups and the normal group, which impliesthat the palatal surface and projection area together with thepalatal shell volume for the treatment groups and the normalgroup were equivalent. Conclusions and clinical implications:With a superior success rate and cost-effectiveness, it is concludedthat treatment of unilateral posterior crossbite in mixed dentition isrecommended to be performed by specialist orthodontists using thequad-helix appliance.The unilateral posterior crossbite correction resulted in a normalisationof the occlusion, palatal area and volume.