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Objectives: Study I: To evaluate the reliability and validity of different superimposition methods and to increase the precision with which craniofacial growth and treatment can be quantified. Study II: To explore the craniofacial changes that occur from early adolescence to late adulthood. Study III: To assess the impact of premolar extractions on dentoskeletal and facial morphologies up to late adulthood. Study IV: In a 50-year follow-up, to study how early extraction of four premolars affects the development of age-related lower incisor crowding. 

Materials and Methods: Study I: Forty pairs of cephalograms were analysed at mean ages of 9.9 (T1) and 15.0 (T2) years. Three superimposition methods were assessed: the Sella-Nasion (SN); the Tuberculum Sella-Wing (TW); and Björk’s structural. Björk’s structural method was performed using three techniques: direct, tracing template, and subtraction. Study II: Thirty subjects with a Class I normal occlusion and harmonious facial profile were investigated. Study data were obtained from cephalograms performed at 12 (T1), 15 (T2), 30 (T3), and 62 (T4) years of age. The craniofacial changes were assessed using superimposition-based and conventional cephalometric methods. Study III: Two groups were included. The Extraction group (N=30 with Class I crowding malocclusion) had their first premolars extracted at a mean age of 11.5 years, without subsequent orthodontic treatment. The Control group included 30 untreated subjects with Class I normal occlusion. Study data were obtained from cephalograms performed at 12 (T1), 15 (T2), 30 (T3) and 62 (T4) years of age. The dentoskeletal and soft tissue changes were assessed using superimposition-based and conventional cephalometric methods. Study IV: Two groups were included. The Extraction group (N=24 with Class I crowding malocclusion) that had their first premolars extracted at mean age of 11.5 years, without subsequent orthodontic treatment. The Control group included 21 untreated subjects with Class I normal occlusion. Study data were obtained from dental casts and cephalograms performed at mean ages of 11.4 and 13.0 years, for the two groups, respectively (T1), and at mean ages of 30.9 years (T2) and 61.7 years (T3).

Results: Study I: The numerical data from the superimposition-based cephalometrics reflected a graphical illustration of superimposition and differed significantly from the data acquired using conventional cephalometrics. While there were no significant differences between the TW method and Björk’s three techniques, significant differences were found between the SN method and the other methods. Study II: The maxilla and mandible showed significant anterior growth from T1 to T2, and significant retrognathism from T3 to T4. The anterior facial height and jaw dimensions increased significantly until T3. From T3 to T4, significant posterior rotation of the mandible and opening of the vertical jaw relation were observed, in addition to significant retroclination of the upper incisors, decrease in lip prominence, and straightening of the facial profile. Study III: There were no significant differences between the Extraction and Control groups in terms of the skeletal sagittal relation, incisor inclination and protrusion (or for most of the soft tissue parameters) during the observation period. Study IV: The Extraction group showed significant improvement in the space deficiency of the lower teeth and no changes in the irregularity of the lower incisors up to late adulthood. In contrast, both the space deficiency of the lower teeth and irregularity of the lower incisors were significantly exacerbated in the Control group, up to late adulthood. 

Conclusions: The superimposition-based cephalometric method accurately generates numerical data for the craniofacial changes. Superimposition using the TW method is valid, reliable, and feasible, and is recommended to be used for superimposition-based cephalometrics. Moreover, craniofacial changes and development of lower incisor irregularity and crowding continue up to late adulthood in untreated subjects who were originally classified as having normal occlusion. For successful long-term outcomes, clinicians should therefore consider age-related changes in patients when planning for orthodontic, orthognathic, and prosthodontic treatments. Treatment with the extraction of four premolars alone in patients with Class I malocclusion with severe crowding does not impact the long-term dentoskeletal and soft tissue profile, and results in unchanged lower incisor alignment.