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Clear Aligner Case Selection: The Predictability Numbers

Case selection decides whether a case tracks before you place a single attachment. A movement-by-movement guide, in millimetres and degrees, to what clear aligners predictably do — the green, amber, and red zones — plus how to grade difficulty and the red flags to screen out.

Case selection is your single highest-leverage decision — it decides whether a case tracks before you place a single attachment. And it rests on one thing: knowing, in millimetres and degrees, what an aligner can predictably do.

Every movement sits on a predictability scale

Some movements are highly reliable; others under-express and need attachments, auxiliaries, and overcorrection. For every movement there’s a green zone you can plan with confidence, an amber zone that’s harder and needs close monitoring, and a red zone — where you’ll need auxiliaries, or a different plan entirely. Read every tooth against this scale.

The linear movements

predictable harder — monitor needs help
Crowding & Spacingper arch
0 3 6 8
mm
Overjet
0 2 4 8
mm
Midline discrepancy
0 2 4 8
mm
Translationmesial / distal
0 2 4 8
mm
Crowding & spacing per arch; overjet and midline; bodily translation. Up to green is comfortable; amber needs IPR/attachments and planning.

Rotations depend on tooth shape

Flat teeth rotate well; round teeth are the problem — there’s little surface to grip. Plan an attachment and overcorrection for anything round.

Incisorsflat — easiest
0 30 50 60
°
Canines & Premolarsround — the challenge
0 25 40 60
°
Molars
0 20 30 60
°

Vertical — the least predictable

The four are not equal. Anterior intrusion is the aligner’s strength; extrusion is the hardest of all, because the plastic has nothing to grip and pull.

Anterior Intrusionaligners' strength
0 1.5 2.5 4
mm
Posterior Intrusion
0 1 2 4
mm
Anterior Extrusion
0 0.5 1.5 4
mm
Posterior Extrusionhardest of all
0 0.5 1 4
mm
Anterior intrusion comes almost naturally; anterior extrusion needs buttons and elastics past half a millimetre; posterior extrusion barely moves.

Tipping, torque & overbite

Crown tipping is predictable to about ten degrees. Root torque reads the same on paper but is harder to express — plan the couple. And deep-bite correction is confident to about two millimetres.

Tippingcrown angulation
0 10 15 20
°
Torqueroot movement
0 10 15 20
°
Overbitedeep-bite correction
0 2 3 4
mm

Grade every case before you accept it

Simple
  • Mild crowding
  • Tipping
  • Minor rotations
Moderate
  • Rotations of round teeth
  • Deep bites
  • Movements creeping into amber
Complex
  • Large discrepancies
  • Extractions
  • Anything skeletal or surgical
Grade against the numbers, then match the grade honestly to your own experience.

The red flags

Some cases should give you pause regardless of the numbers — screen these out early:

  • A significant skeletal discrepancy, where the jaws, not the teeth, are the real problem
  • Active periodontal disease
  • A patient unlikely to wear the aligners the full 22 hours
  • Pain, or a joint (TMJ) problem
  • Severe rotations, impactions, or ankylosis

The numbers are ceilings, not walls

You can push past these ceilings — but only with three things in place:

  • Real experience
  • Auxiliaries planned before you start, not added in a panic later
  • Regular monitoring, to catch drift while it’s small

Plan before you start, and you are never surprised.

Key takeaways

Know the numbers, grade the case, respect the red flags, and match it to your experience. Choose well, and the mechanics that follow have a real chance of working. The green/amber/red zones are ceilings for a confident plan — push past them only with experience, auxiliaries planned up front, and monitoring.

Shafaf partners with trained dentists across Iraq and the region. Talk to us about partnering or find a Shafaf provider.

Frequently Asked Questions

What movements are most predictable with clear aligners?

Linear movements — mild crowding and spacing (up to ~3 mm/arch), small overjet and midline corrections (~2 mm) — and anterior intrusion, which is the aligner's strength (~1.5 mm). Crown tipping (~10°) and rotations of flat incisors (up to ~30°) are also reliable. These sit in the green zone you can plan with confidence.

How much crowding can clear aligners fix?

About 3 mm per arch is comfortable and predictable; 3–6 mm is workable with IPR and careful planning; beyond 6 mm you're usually into extractions or a different plan. Read every arch against the scale rather than treating a single number as a hard wall.

Which tooth movements are hardest for clear aligners?

Extrusion — the plastic has nothing to grip and pull, so trust only ~0.5 mm even at the front, and posterior extrusion barely moves. Rotating round teeth (canines, premolars) is hard because there's little surface to grip. Posterior intrusion and true root torque also under-express. These need attachments, auxiliaries, and overcorrection planned from the start.

How do you grade clear aligner case difficulty?

Simple: mild crowding, tipping, minor rotations — all in the green. Moderate: rotations of round teeth, deeper bites, movements creeping into amber. Complex: large discrepancies, extractions, anything skeletal or surgical. Grade every case against the predictability numbers, then match the grade honestly to your own experience.