Orthodontic digital setups built to your prescription

Before a single tooth is moved, the setup needs a frame. Arch form comes first, because it decides how much space the arch offers and therefore how much of the crowding has to be met by IPR, expansion or proclination. The safest default is the patient's own pre-treatment form, since it reflects the soft tissue envelope the dentition has settled into; a different form is used when you prescribe one, and the plan says which was applied.

The midline is the second decision, and it is two decisions rather than one. The upper dental midline is related to the facial midline, judged from your smiling frontal photograph, and the lower is related to the upper. When both cannot be corrected without unwanted movement elsewhere, the upper to face relationship is usually the one worth protecting, but that is a clinical call and the setup follows what you prescribe.

Both choices are recorded on the plan rather than left implicit. A setup that quietly widens an arch to resolve crowding, or shifts a midline to make the numbers work, is a setup that will surprise someone at the chair. If either has to move to reach your objectives, that appears as a note for you to accept or reject.

Andrews' six keys to normal occlusion remain a useful checklist for a finished setup, not because every case can meet them but because they name what is being traded away when a case cannot. The keys are molar relationship, crown angulation, crown inclination, absence of rotations, absence of spaces, and a flat to slightly curved occlusal plane.

In a digital setup each key is something you can look at directly. Molar relationship is read on the articulated models. Angulation and inclination are read against the long axis assigned to each tooth during segmentation. Rotations are read at the contact points. Spaces are read as residual gaps in the final arch. The curve of Spee is read in the sagittal view and levelled to the depth you specify, since levelling consumes arch length and interacts with the space analysis.

Where the setup departs from a key, the departure is stated. A Class II finish held on the molars by prescription, a slightly retained curve of Spee in a deep bite case, a rotation left in a peg lateral awaiting restorative work: these are legitimate outcomes when you have chosen them, and unhelpful surprises when nobody has.

Overjet and overbite are the two figures most often left vague in a prescription and most often disputed at review, so the setup states both at the finish. Conventional finishing targets described in orthodontic teaching sit around 2 to 3 mm of overjet and an incisal overlap of roughly 1 to 2 mm, or about one fifth of the lower incisor crown height. These are the usual reference values, not a commitment about a particular case.

Reaching them is a vertical problem as much as a sagittal one. A deep bite can be reduced by intruding incisors, by extruding posterior teeth, or by proclining the lower labial segment, and the three have very different demands on aligner mechanics and very different stability. An open bite has the mirror set of choices. The setup makes the chosen route explicit so you can accept or change it, rather than presenting only the end position.

Anterior tooth size discrepancy belongs in the same discussion. Where the Bolton ratios are outside their published means, ideal overjet, overbite and coincident midlines cannot all be achieved at once by moving teeth alone, and the setup identifies the discrepancy so you can decide between IPR, residual spacing for restorative work, or accepting a compromise.

A setup shows a target, and it is honest about the fact that a target is not an outcome. Torque, the labiolingual inclination of a crown and its root, is the movement removable appliances control least reliably, because delivering it requires the appliance to generate a moment on a smooth crown surface rather than simply push it. Root movement in general expresses less readily than crown tipping, and the difference grows with the amount asked for.

That has consequences for how a setup should be read. A final position showing corrected upper incisor torque is a statement of intent, and the staging that follows will need attachments, a slower per-stage figure and possibly planned overcorrection to pursue it. Where a torque change is large, the setup will say so rather than presenting it as equivalent to a simple alignment.

The same applies to root paralleling next to an extraction site, uprighting a mesially tipped molar, and derotating a round-rooted premolar. None of them are impossible; all of them are slower than the crown movement they are drawn beside, and a setup that ignores that produces a stage count that will not survive contact with the patient.