Luting cements: why the choice changes according to how much the preparation retains

Cement selection is often made from habit, with the same product used for every case. It is an error paid for in two opposite ways: decementation on short preparations, and difficulty of removal on restorations that will need replacing.

The criterion governing the choice is the retention of the preparation. A tall abutment with slightly convergent walls retains by its own form: here the cement seals rather than holds.

A short or overly tapered preparation does not retain by form, and the cement must compensate. In that case true adhesion is required, not simply filling the gap.

Conventional cements — zinc phosphate, polycarboxylate — have a century of documentation and no chemical adhesion. They hold by friction and by filling irregularities, and work well on retentive preparations.

Zinc phosphate has a marked initial acidity that can cause post-cementation sensitivity on vital abutments. Polycarboxylate is better tolerated but has lower mechanical strength.

Glass ionomers release fluoride and bond weakly to dentine. Resin-modified versions improve mechanical properties but absorb water and may expand, risking fracture in thin all-ceramic restorations.

Resin cements have the highest strength and are mandatory on glass ceramics, where retention depends almost entirely on adhesion. They do, however, require rigorous isolation and a conditioning sequence that permits no shortcuts.

Self-adhesive cements represent the most used compromise today: they require neither separate etching nor primer, and offer intermediate strength. They suit zirconia and metal-ceramic on reasonably retentive preparations.

Conditioning of the restoration surface differs by material, and confusing it defeats adhesion. Glass ceramics are etched with hydrofluoric acid and silanated; zirconia does not respond to hydrofluoric acid and requires air abrasion and a phosphate monomer primer.

Treating zirconia with hydrofluoric acid remains a common error: it produces no micromechanical retention and wastes time while giving the impression the protocol has been followed.

Excess cement deserves attention it rarely receives. Studies on peri-implantitis series have identified cement remnants in over half the cases examined: cement forced into the sulcus is neither visible nor removable with an interdental brush.

Prevention is simple and concerns design: screw-retained restorations where possible, and in cemented ones supragingival or at most equigingival margins. A deep margin on an implant is a choice paid for years later.

In summary, cement is chosen according to how much the preparation retains by its own form, and conditioning follows the restoration material. The most expensive cement does not compensate for a non-retentive preparation, and the cheapest does not ruin a well-prepared case.