Microapplicators: the wrong diameter deposits three times the adhesive required

The microapplicator is an item costing a few cents that determines the quantity of adhesive deposited in a cavity, and therefore the thickness of the layer after curing.

The relationship is direct: the size of the head determines how much fluid is held by capillarity, and how much is released on contact with the surface.

An oversized applicator deposits an excess that pools in the cavity angles. A thick adhesive layer does not cure uniformly: the deep portion remains partly unconverted, creating a weak zone precisely at the interface with dentine.

The consequences appear over time — post-operative sensitivity, marginal debonding, margin discolouration — and none is attributable by eye to the quantity of adhesive deposited months earlier.

Available sizes typically cover three grades. Fine, around 1.5 millimetres, suits conservative cavities and fissures. Regular, around 2 millimetres, medium-sized cavities. Ultrafine, below a millimetre, root canals and areas of difficult access.

The commonest error is using a single size for everything, typically the regular, because that is what is bought in the large pack.

Capillarity depends on fibre structure, not on diameter alone. Heads of density-controlled nonwoven fibre hold the fluid and release it gradually on contact; foam heads release it all at the first touch.

The difference shows when applying adhesive to a vertical wall: a head releasing gradually allows distribution, one releasing all at once produces a drip.

Fibre shedding is the defect rendering an otherwise sound product unusable. A detached fibre left in the adhesive is a permanent defect in the interface, and is not removable once cured.

Quality heads are heat-welded to the stem, not glued: the weld resists the solvents contained in adhesives, whereas glue dissolves on contact with the alcohol or acetone of the primer.

Stem flexibility should be chosen by use. A rigid stem allows controlled pressure on accessible surfaces; a flexible or bendable one reaches distal zones and undercuts, where rigidity prevents the correct angle.

Applicators with a memory-bend stem hold the set angle instead of springing straight: a detail that matters when working on an upper third molar.

In summary, the microapplicator should be chosen by size according to the cavity, with a heat-welded head and controlled capillarity. It costs a few cents more than the generic version, and determines the thickness of a layer that cannot be corrected afterwards.