Final answer:
The crown margin for a lower molar with an extensive MOD amalgam restoration should ideally be placed 1mm apical to the amalgam margin, which provides a more reliable foundation and promotes periodontal health.
Step-by-step explanation:
A lower molar requiring a crown with an extensive MOD amalgam restoration often raises the question of where to place the crown margin. It is generally recommended by dental professionals that the crown margin is most appropriately placed 1mm apical to the amalgam margin. This placement ensures that the crown margin rests on sound tooth structure rather than on the existing amalgam, which may not provide a reliable foundation for the crown over time. By placing the margin apically, it also aids in the preservation of the periodontal health of the tooth and optimizes the longevity of the crown restoration.