Effect of edgewise treatment and retention on manidbular incisors

This study consisted of analyzing clinical records of fifty cases treated for correction of Class I malocclusion with varying degrees of bimaxillary protrusion and anterior mandibular crowding. Thrity of these cases required extraction of all four first premolars. Parameters characterizing changes in position of mandibular incisors during treatment with edgewise appliances and during retention and shortly afterward were studied. All the parameters indicated rather similar changes for the nonextraction and the extraction groups. The relapse of anterior mandibular crowding was significantly larger (p less than or equal to 0.05) in the nonextraction group. The IMPA and interincisal angle changed significantly more in the extraction group. Upon testing for validity of the mesiodistal to faciolingual indices, it was determined that they predicted stability of the achieved alignment of mandibular incisors. There were, however, individual cases in both groups which did not follow the central tendency. It was observed that the mesiodistal diameters of mandibular incisirs were somewhat larger in the extraction cases. There was only one pair of parameters that showed a moderately high intercorrelation (r equals 0.68); that was between correction of the anterior crowding and relapse of the intercanine expansion. It was observed that the more correction took place, the more readily expanded canines would relapse. An over-all impression is that the mandibular incisors change little in their position to each other and relative to the other structures of the mouth. The question remains: Is this due to our ability to control them well, or due to an inability to move and maintain them in their new position?