The precision impression: where the micrometres are lost that later go missing at the margin

A crown that fails to seat at the margin is almost always blamed on the laboratory. In most cases the error arose earlier, in the impression, and the technician faithfully reproduced something that was already wrong.

Polyvinyl siloxanes are today the reference for fixed prosthodontics. They have excellent dimensional stability, elastic recovery above 99 per cent, and can be poured days later without distortion.

Polyethers offer comparable accuracy and one distinguishing characteristic: they are hydrophilic even in the unset state, so they copy a slightly moist surface better. The limitation is high rigidity, which makes removal difficult in the presence of undercuts or compromised periodontium.

Reversible hydrocolloids retain an indication where maximum hydrophilicity is required, but demand dedicated equipment and immediate pouring. Alginates have no place in fixed prosthodontics: their permanent deformation exceeds that of addition silicones by an order of magnitude.

The two-step technique with a putty matrix relined in light body is the most widespread, and also where the commonest error occurs: failing to create sufficient space for the light body material.

If the putty rests on the prepared tooth, the light body has no thickness in which to flow and the impression records the putty, not the detail. Space is created with a spacer during the first stage, or by removing material before relining.

The one-step technique with two viscosities simultaneously eliminates this risk but requires an experienced assistant and strict timing: the setting phases of the two materials must overlap, otherwise a separation interface forms.

Soft tissue management determines whether the margin will be readable. The double cord is the technique with the most robust documentation: a thin cord left in place during the impression and a thicker one removed immediately before.

The lower cord controls crevicular fluid, the upper one opens the space. Removing only the second maintains the opening without blood invading the sulcus at the moment the material enters.

Aluminium chloride paste systems are a less invasive alternative on thin periodontal biotypes, where cord can cause recession. They are less effective when the margin is deep or the sulcus particularly narrow.

Moisture is the most frequent and most underestimated enemy. Blood or crevicular fluid produce bubbles precisely at the margin — the one point that truly matters — and the operator sees them only after removal.

An impression with a bubble at the margin is not corrected with wax in the laboratory: it is retaken. The time for a retake is always less than that of a crown to be remade.

In summary, impression accuracy turns on three points: adequate space for the light body material, soft tissue open and dry, and a material chosen for the case rather than from habit. The laboratory can only reproduce what it receives.