Abstract: Abstract: Objective To compare the pretreatment and posttreatment changes of arch widths and buccal corridor widths between premolar extraction and nonextraction treatment,and to analyze the correlation between arch width changes and buccal corridor width changes.Methods Pretreatment and posttreatment casts,frontal smiling photographs of 50 extraction and 50 nonextraction patients were collected to obtain their corresponding pretreatment and posttreatment changes of arch widths and buccal corridor widths.The amount of changes between pretreatment and posttreatment archwidths and buccal corridor widths were measured in order to explore the extraction effects on them，and the correlations between the changing amounts of arch widths and that of buccal corridor widths were measured as to compare the extraction effects on arch widths and buccal corridor widths.Results ①There was a significant increase inthe posttreatment maxillary intercanine width in the extraction group but not in the nonextraction group.②Pretreatment and posttreatment arch widths between the maxillary premolars in both groups increased,and the amounts were greater in the extraction group than in the nonextraction group.③Pretreatment and posttreatment arch widths between the maxillary first molars in both groups increased,and the amount was greater in the nonextraction group than in the extraction group.④There were no significant differencesin any buccal corridor widths measured in both groups.⑤There were correlations between the pretreatment first molar and posttreatment canine arch widths and the buccal corridor widths.Conclusions The pretreatment and posttreatment changes of arch widths are related to extraction，while the buccal corridor widths are not.Besides,these arch width differences are not directly related to changes of buccal corridor widths in most cases.

Key words: Arch width, Buccal corridor width, Buccal corridor index