Background: Crowding is one of the most prevalent types of malocclusion as well as a major problem in clinical orthodontics. It is essential to know the cause of crowding in order to achieve a better treatment. Objectives: A longitudinal analysis of arch dimension changes in late PRIMARY AND early mixed DENTITION periods in children was designed AND investigated the eff ect of eruption of precedence on the arch size in the samples under study. Methods: This longitudinal study was carried out on 32 children aged 5. 5-6. 5 years who had no erupted PERMANENT DENTITION. Impression was taken AND the arch perimeter, depth AND width as well as tooth size were measured by a digital calliper AND a brass wire on casts. After eruption of the fi rst PERMANENT tooth, the second impression was taken; the above-mentioned parameters were measured on the second casts. Data were analysed by ANOVA test. Results: Four pattern of eruption of precedence were as follows: maxillary 1st molar, mANDibular incisors, MANDibular 1st molars AND simultaneous eruption of maxillary AND mANDibular 1st molars. With eruption of mANDibular central incisors, inter canine width AND overjet increased by 0. 81 AND 0. 27 mm, respectively. The mANDibular arch perimeter increased by 2. 25 mm with simultaneous eruption of maxillary AND mANDibular PERMANENT fi rst molars. The maxillary arch perimeter increased by 1. 25 mm AND 1. 50 mm with eruption of maxillary fi rst molars AND simultaneous eruption of maxillary PERMANENT fi rst molars, respectively. Conclusions: Eruption precedence had a signifi cant eff ect on changing mANDibular inter canine width, maxillary arch perimeter, mANDibular arch perimeter AND overjet.

Background: The aim of this study was systematic review AND meta-analysis of prevalence in current AND rele-vant literature about this developmental disorder to present the profile of malocclusion in Iran. Methods: This review study was carried out with systematically identified AND critically assessed studies report-ing malocclusion prevalence among Iranian population in PERMANENT DENTITION. National AND international data-bases were searched for articles about prevalence of malocclusion by Angle classification in different regions of Iran from 1994 to 2015. After applying inclusion AND exclusion criteria, the quality of articles was checked by professional checklist. Data extraction AND meta-analysis was performed. A rANDom-effect model was employed. Publication bias was checked. Results: Of 2768 articles, 21 cases were included. The pooled prevalence of malocclusion was about 87% (95% CI: 78. 3– 92. 2) in Iranian population; however, the prevalence of malocclusion across individual studies varied considerably (ranging from 23. 7% to 99. 7%). Prevalence of normal occlusion, class I, II AND III malocclusion were reported as 13. 3% (CI 95%: 7. 8– 21. 7), 50. 7% (CI 95%: 42. 9-58. 4), 21% (CI 95%: 17. 5-25. 1), 5. 5% (CI 95%: 3-10); respectively. Maximum prevalence of malocclusion was in the East of Iran. Conclusion: The results showed a high prevalence of malocclusion in Iranian population. The baseline infor-mation could be appropriately utilized for the future planning to meet the orthodontic treatment need among the Iranian population.