Late mesial shift of permanent first molars

# The closure of which of the following spaces results in the late mesial shift of permanent 1st molars?
A. Canine
B. Interocclusal
C. Extraction
D. Leeway
E. Primate

The correct answer is D. Leeway space. 

Leeway space is closed as the 1st permanent molars move mesially after the

exfoliation of the primary 2nd molar and is

usually seen among children from 11-12

years of age.

Leeway space is created from the difference of sizes between the primary canine and molars compared to the erupting premolars and permanent canines.

The discrepancy in size of primary molars and premolars creates a free space that allows the late mesial migration of permanent 1st molar after the exfoliation of the primary 2nd molar.