Why is zinc phosphate cement deemed unsuitable for deep restorations?

Zinc phosphate cement is considered unsuitable for deep restorations primarily because it can cause pulp irritation. This cement has a relatively low pH upon mixing, which can be harmful if it comes into close contact with the pulp chamber or the dentin beneath deep restorations. If the cement is used in deep cavities where the dentin is thin, the acidic nature can lead to pulpitis or irritation of the pulp tissue.

In comparison to other dental materials designed for deep restorations, which typically have a more neutral or alkaline pH and provide additional therapeutic benefits, zinc phosphate does not offer the same level of protection for the pulp. Options related to difficulty of removal, lack of adhesion, and sealing do not address the specific concern of pulp health, which is critical when considering the placement of materials in deeper cavities. Therefore, the risk of pulp irritation is a significant factor making zinc phosphate cement less favorable for such procedures.