Polishing: adjusted zirconia left unpolished wears the opposing tooth

Polishing is the final step of every restoration and the first to be sacrificed when running late. It is also the step that determines how much the restoration will damage the tooth opposing it.

The finding that governs everything is this: correctly polished zirconia causes wear of the antagonist comparable to natural enamel, while the same zirconia left rough is markedly abrasive.

It is not the hardness of the material that wears the antagonist, it is surface roughness. A smooth surface glides; a rough one acts as abrasive paper at every masticatory cycle.

From this follows the rule most often broken: every occlusal adjustment requires repolishing. The adjustment is made with a diamond bur and leaves a rough surface, precisely at the point contacting the antagonist with every swallow.

A high contact reduced in thirty seconds and left unpolished creates a permanent abrasive area. The patient notices nothing, and the wear appears years later on the opposing tooth.

Polishing systems for zirconia use diamond abrasives embedded in silicone rubber, in three successive grits. The first removes bur grooves, the second evens out, the third brings up the shine.

Skipping the intermediate grit is the commonest error: the fine grit cannot remove the deep grooves left by the first, and produces a surface that looks polished but remains striated.

Furnace glazing is an alternative to mechanical polishing, not a complement. A layer of fused glass at the surface gives superior lustre, but it is thin and wears away: after some years of function the underlying surface is exposed again.

For this reason many prefer mechanical polishing on loaded occlusal surfaces: it lasts as long as the material. Glazing remains ideal on non-functional aesthetic surfaces.

Composite requires a different sequence because it is a heterogeneous material. Filler particles and resin matrix wear at different rates, and aggressive polishing removes the matrix leaving the filler exposed.

For composite one uses flexible discs in decreasing grits on accessible surfaces, and silicone rubber points for anatomical areas, always at low speed with light pressure.

Speed is the parameter common to all materials. Polishing is performed at low revolutions: high speed generates heat that degrades the composite matrix and can induce phase transformation in zirconia.

In summary: roughness determines antagonist wear more than hardness, every occlusal adjustment must be repolished without exception, the grit sequence permits no skipping, and polishing is done at low speed with light pressure.