Space is the horizontal problem, and Class II and Class III are the sagittal one. Now the vertical: the deep bite, where the front teeth overlap too much, and the open bite, where they do not meet at all. They are the two hardest dimensions to control with plastic — the open bite because extrusion is the least predictable aligner movement of all, and the deep bite because intrusion is not far behind. Both are won or lost in the setup, before the first tray is ever made. This lesson is how you plan each one.
Measure the overbite
Start with the number. Overbite is the vertical overlap of the upper incisors over the lowers — normally about two millimetres, roughly a quarter of the lower crown. (Overjet is the horizontal distance — the two are often confused, and they behave differently.)
≈ 2 mm · a quarter of the lower crown
> 4–5 mm overlap · lowers may touch palate
no overlap — daylight shows
Beyond four or five millimetres of overlap, you have a deep bite; when the incisors do not overlap at all and daylight shows, an open bite. Measure it on the model, not by eye.
Deep bite: three usual causes
A deep bite has three usual causes, and the fix depends on which one you are looking at.
tipped back, trapping the lowers — Class II Div 2
grown past the occlusal plane — upper or lower
lower premolars sag below the molar–incisor line
Most deep bites are a mixture — decide the proportions before you plan, because you will need a different movement for each part.
The deep-bite plan: procline before you intrude
The plan combines movements you already know: procline the retroclined incisors (that alone often opens the bite by a millimetre or two), intrude the over-erupted ones, and level the curve of Spee — intrusion of the lower anterior with some premolar extrusion. When the incisors are both retroclined and over-erupted, sequence it: procline → intrude → retract.
Trying to intrude a retroclined incisor tips it back further and drives its root into the labial cortex — the order is not optional.
Two tools carry it: bite ramps and anchorage
Bite ramps are small ledges on the palatal of the upper anterior aligner that the lower incisors bite onto.
And anchorage: intruding the anterior lifts the back of the tray off the molars, so the premolars carry attachments to hold it down. The incisors being intruded need no attachments — the plastic pushes down on the incisal edge, and that is enough.
Open bite: three ways it closes
Now the open bite, and the three ways it closes.
retract a proclined incisor — the crown swings down · no vertical force
pull the incisors down · attachments · expect it slow
intrude the molars — the mandible auto-rotates closed · what aligners do well
Posterior intrusion is the mechanism aligners are actually good at, because the plastic sits over the occlusal surface.
Here the biomechanics work for you
This is rare: the forces are on your side. Pulling the incisors down pushes the posteriors up — the reaction to anterior extrusion is posterior intrusion, and both movements close the bite.
So the rule is: attachments on the extruding incisors, none needed on the posteriors. And over-close in the setup — plan two to three millimetres of overbite, not edge to edge.
The bite jump — a simulation, not a promise
One thing you will see in the plan and must read correctly: the bite jump. When posterior intrusion is planned, the software cannot model the biology — the mandible rotating closed around a point near the joint. So the technician simulates it: at some stage, the whole lower arch is re-seated up and forward, and the front teeth appear to meet — with no incisor movement in the table at all.
Check the arithmetic: each millimetre of posterior intrusion buys roughly two to three millimetres of closure at the front — and aligners realistically intrude half a millimetre to one. If the jump closes four millimetres off half a millimetre of intrusion, the screen is showing you a wish. Believe the intrusion you can deliver, not the closure you can see — and note that only about two-thirds of programmed open-bite closure is expressed on average.
Ask why the bite is open
The cause decides whether it stays closed. A dental open bite — from a thumb, a pen, or an anterior tongue posture — will close with mechanics and reopen the moment the habit returns. Screen for the tongue thrust, and where you find it, ask for tongue spikes on the anterior trays so the tongue is retrained while the teeth move.
A skeletal open bite — long lower face, steep mandibular plane, over-erupted posteriors — is a surgical or TAD-intrusion case, not a tray-only case. Read the profile before you promise the front teeth will meet.
Plan the retention on day one
Vertical corrections relapse more than any other, so plan retention from the first visit. For the deep bite, over-correct slightly and consider a retainer with an anterior bite plane so the bite cannot re-deepen. For the open bite, over-close in the setup, hold the posterior intrusion with retainers that cover the occlusal surfaces, and keep the tongue habit under control. Tell the patient at the start that the vertical is the dimension that most wants to go back — that conversation is easier before the relapse than after it.
The takeaway
Measure the overbite — do not guess it. For the deep bite: find the cause, procline before you intrude, ramps and premolar attachments do the work. For the open bite: three mechanisms, the reciprocal forces on your side, over-close in the setup, read the bite jump as a simulation not a promise, and treat the habit or the case will not hold. Read dental against skeletal, and plan retention from the first visit.
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