Final answer:
The ideal time to place a distal extension space maintainer to prevent the mesial drift of a permanent first molar is immediately after the extraction of the primary second molar, in order to preserve the space for the permanent tooth's eruption.
Step-by-step explanation:
To prevent mesial drift of a permanent first molar, the ideal time to place a distal extension space maintainer is immediately after extraction of the primary second molar. This is essential because once the primary tooth is removed, the space becomes vulnerable to closure due to the mesial drift of the adjacent teeth. Placing a space maintainer preserves the gap for the eventual eruption of the permanent second molar.
Mesial drift can lead to malocclusion and potential orthodontic problems if not properly managed. The timing for placement of a space maintainer is critical and is usually not delayed until the extraction site has completely healed, as this provides an opportunity for the teeth to start shifting.