Definition: A conventional water-based dental cement formed by an acid–base reaction between zinc oxide powder and phosphoric acid liquid, producing a rigid zinc phosphate salt matrix that binds unreacted zinc oxide particles.
Clinical Case
A 54-year-old patient attends for cementation of a full metal crown on an upper first molar. The preparation is well-tapered, with adequate axial and occlusal reduction. You need a luting agent that has a long clinical track record, predictable handling, good compressive strength and a thin film. The restoration margins are subgingival on the palatal but supragingival buccally, and aesthetics are not critical. You decide to use zinc phosphate cement.
Uses
Luting medium for:
Cast metal and metal–ceramic crowns
Bridges
Inlays
Onlays
Posts and cores
Cavity base/lining beneath metallic restorations (especially amalgam), where sufficient dentine thickness exists or where supplemented by a sublining.
How to use them in clinical practice?
Assess the indication
Indirect restoration requiring mechanical retention, with no need for chemical adhesion.
Pulpal distance adequate (or base/sublining present).
Prepare tooth and restoration
Clean and dry tooth (avoid over-desiccation).
Clean internal surface of restoration; check fit.
Dispense
Place a measured volume of liquid on a cool glass slab.
Divide powder into small increments.
Mix
Incorporate powder incrementally into the liquid over a wide area of the slab.
For luting: stop when a smooth, creamy, stringing mix is obtained.
For lining/base: incorporate more powder until a thicker, putty-like mix is achieved.
Avoid over-mixing once the mix visibly thickens.
Load
For luting: load cement into/onto the restoration (cooler than tooth) just before seating.
For bases: place into cavity floor and adapt gently.
Seat / place
Seat restoration with firm, continuous pressure.
Maintain pressure until initial set; avoid movement.
Clean-up and check
Remove excess cement at the gel stage with an instrument and floss interproximally.
After set, check occlusion and margins.
Constituents
Powder
Zinc oxide (main reactive component, ~90%)
Small amounts of other oxides (e.g. MgO, etc.) to modify setting and handling
Liquid
Aqueous phosphoric acid (typically 50–60% concentration)
Dissolved zinc and/or aluminium phosphates as buffers to moderate pH and reaction rate
Chemistry
On mixing:
Phosphoric acid attacks the surface of the zinc oxide particles.
Zinc ions go into solution and react with phosphate ions to form zinc phosphate, which precipitates as a matrix.
Unreacted zinc oxide cores become embedded in this matrix.
The reaction is acid–base, rapid and exothermic, but moderated by:
The presence of phosphate buffers in the liquid
Heat-treatment (sintering) of the powder
Cooling the glass slab
The pH starts low (around 2–4) and rises gradually as the reaction progresses.