Clear aligners: attachments are not an accessory, they are what makes movement possible

The clear aligner has become the commonest request in adult orthodontics, and commercial communication has created the idea that it can do everything a fixed appliance does. The differences are real and should be known before accepting a case.

The principle is that a tray slightly different from the current position applies force as it tries to return to its own shape. The limitation is that a smooth surface on a smooth crown can push, but struggles to pull and to rotate.

Attachments solve precisely this. They are composite prominences bonded to the tooth that give the aligner something to grip, and their shape is not decorative: it determines the type of movement obtainable.

An attachment optimised for extrusion has its active surface facing occlusally; one for rotation has surfaces generating a force couple. Without them, extrusion and rotation are the least predictable movements.

Loss of tracking is the principal clinical problem, and it means the tooth has not followed the programmed position. It is recognised by a visible gap between aligner and incisal edge.

There are three causes, in order of frequency. Insufficient wear by the patient, movement programmed beyond the system's capability, or a detached or misplaced attachment.

Distinguishing them matters because the remedy differs: in the first case one steps back a few trays, in the second a new scan is required, in the third rebonding the attachment suffices.

The movements with the best documented predictability are buccolingual tipping and closure of small spaces. The difficult ones are extrusion of incisors, rotation of canines and premolars, and bodily root movement.

Extrusion is difficult for a physical reason: the aligner can push a tooth into the bone, but to draw it out it must grip it, and without an attachment it has nothing to hold.

Closing extraction spaces is where aligners show their greatest limitations. The risk is tipping of the crowns into the space with the roots left behind, and correcting that situation takes time.

Interproximal reduction is often part of the plan and must be performed judiciously. Removing enamel is irreversible, and the amount planned by the software should be verified clinically before being carried out in full.

Retention after aligners follows the same rules as after fixed appliances, and must be planned from the outset. A patient who has worn trays for a year tends to regard treatment as over when the last one is delivered.

In summary: attachments determine which movements are possible, loss of tracking has three causes to distinguish, extrusion and rotation remain the difficult movements, and extraction cases warrant caution.