Which cement always requires the use of a liner?

The cement that always requires the use of a liner is commonly recognized as calcium hydroxide. This type of cement is primarily used for its pulp-capping and cavity-lining properties. Calcium hydroxide has a unique formulation that provides several benefits, including the stimulation of reparative dentin formation and a high pH that can neutralize acids and promote healing in the pulp.

When utilizing this cement, it is important to apply a liner underneath it, especially in deeper cavities where there is a risk of pulp exposure or where the dentin is significantly thin. The liner acts as a protective barrier that mitigates thermal shock and chemical irritation to the pulp, providing a safe environment for healing.

In contrast, zinc phosphate, polycarboxylate, and glass ionomer cements serve different roles and do not inherently require a liner because they don't have the same sensitivity to pulp exposure or the same protective properties. Each of these cements has its own indications where a liner may not be necessary, and they can be used in a wider variety of situations without the consistent need for a protective liner beneath them.

Thus, calcium hydroxide's specific application as a liner requires it for safeguarding the dental pulp, making it the correct choice in this context.