Expansion
CALCULATOR
“Let’s do four weeks of expansion”
I don’t know about you, but this drives me absolutely crazy. How can all of our patients—of different shapes, sizes, genders, and ethnicities—need 4 weeks of expansion? I’m exaggerating of course, but I think you get my point.
The Expansion Calculator is a tool to mathematically determine the ideal amount of expansion that can safely be performed to coordinate the arches based on each patient’s unique anatomy. It allows us to avoid using cookie cutter expansion prescriptions for all our patients.
The calculator is based on the idea that the mandible is the immutable arch; expansion cannot be achieved, only uprighting of the buccal segments. Therefore, the width of the mandible is the limiting factor for maxillary expansion.
The goal is to have the mandibular posterior teeth upright and centred in the mandibular trough. We then need the palatal cusps of the maxillary posterior teeth to fit in the occlusal groove of the mandibular posterior teeth.
We need 3 initial measurements in order to plan for and achieve this harmony
measure the widths of:
Centres of mandibular troughs (BLACK)
Central grooves of mandibular molars (PURPLE)
Mesiopalatal cusps of maxillary molars (BLUE)
Punch these three values in, and the Expansion Calculator will automatically find the discrepancy between the dental measurements and bony measurement, determining how much uprighting / expansion is needed in the mandible / maxilla respectively.
You can also punch in the amount of expansion per turn (usually 0.2mm/turn, but ask your lab), your preferred number of turns / day, and the level overcorrection you want (in anticipation of relapse), to get an EXACT number of days that your patient needs to expand for.
It should also be mentioned that a narrow maxillary dental arch does not necessarily need an expander to resolve the issue. It all depends on the etiology—are the teeth tipped in or is it a skeletal problem. In other words, is the palatal width is adequate or inadequate.
A proper diagnosis is critical to determine if expansion should be carried out through the use of an appliance (Rapid Palatal Expander) or via archwire expansion. The key is to look at the inclination of the Mx posterior segments. One trick is to look at the models from the posterior.
ADEQUATE PALATAL WIDTH
If the upper dentition is tipped palatally, arch wire expansion can be used to tip the maxillary dentition buccally
INADEQUATE PALATAL WIDTH
If the upper dentition is already tipped buccally, the maxillary dentition probably shouldn’t be tipped any further.
At this point, sutural expansion may be needed.
For growing patients, consider an RPE
For adults, consider a SARPE
Contemporary Orthodontics. Sixth Edition. Proffit.
In the end, you should be able to achieve:
Coordinated maxillary and mandibular dentitions
Coordinated maxillary and mandibular alveolar bases
An appropriate curve of Wilson
The Expansion Calculator can also be used in conjunction with the Space Calculators for borderline extraction cases when you want to determine the maximum amount of space you can create from expansion to try and avoid extractions.
The goal:
ARCH COORDINATION
… the palatal cusps of the maxillary molars should intercuspate with the fossae and marginal ridges of mandibular molars, the buccal cusp of the mandibular premolars should intercuspate with the marginal ridges of the maxillary premolars, and the mandibular canines and incisors should intercuspate with marginal ridges of the maxillary canines and incisors.
The maxillary teeth should be upright and centered in the alveolar/basal bone and coordinated with the mandibular teeth, which should also be upright and centered in the alveolar/basal bone to obtain a proper intercuspation.
Orthodontics, Current Principles and Techniques. Sixth Edition. Graber et al.
If a crossbite is present
consider this flowchart proposed by Ngan and Fields (1995) to determine how to best resolve the issue
