Late mesial shift is due to

Late mesial shift The second transitional period is completed with the eruption of second molars at the age of 12 years. In this period, class I permanent molar occlusion is achieved by a late mesial shift of the mandibular permanent molars. The difference in the combined mesiodistal dimensions of the primary canine, first and second molar in each quadrant is greater than the combined mesio-distal dimensions of the permanent premolars and canine. This excess of space, i.e, Leeway space is greater in the mandibular arch as compared to the maxillary arch. The erupting permanent molars migrate mesially in the occlusal path, more mesial in the mandible and thereby establishing a Class I cusp to fossa relationship. These movements transform the straight terminal plane relationship of primary molars into a mesial step relationship. This phenomenon occurs mostly with closed primary dentition, where interdental spaces do not exist and primary second molars are in a straight terminal plane relationship. In this condition, an early mesial shift is not possible. The permanent maxillary, mandibular first molars emerge into a cusp-to-cusp relationship, but move to a class I relationship later. This change has been referred to as the late mesial shift. The late mesial shift is facilitated with the use of leeway space. OP Kharbanda 2nd ED