Orthodontic expansion is a broad topic and one that can be considered in numerous ways.  Expansion can either be Skeletal or Dental (but is often a combination of both).  Skeletal expansion involves manipulating the upper jawbone (the Maxilla) to encourage greater development in the width dimension.  As the upper jaw consists of two "halves" and these two parts don't fully grow together or fuse until a later date, an orthodontist treating a younger patient can gently guide these two "halves" of the upper jaw away from each other in such a way that they continue to "reach out" and fill in their slight separation with bone.  Once the patient has finished their growth in the width direction, they will have achieved a much greater lateral growth than mother nature could provide herself.  Dental expansion is much less dramatic and typically only involves moving teeth outward but within the confines of their bony surroundings (upper or lower jaw).  This can sometimes be referred to as "Arch Development".  The lower jaw (the Mandible) is really just associated with dental expansion as the two "halves" of the lower jaw grow together (fuse) at about 1 year old and can be considered a single bone thereafter.  

An "expander" can be any orthodontic appliance that provides lateral movements.  Usually this is referring to skeletal expansion but some expanders can be modified or used specifically for dental expansion only.  The upper jaw is anchored near the "back" where it connects primarily to the base of the skull.  Because it is so well anchored in the back, expanders will tend to open a space (or diastema) between the upper front teeth.  This space may be considered unwanted by orthodontic patients or parents but it's actually a good sign that skeletal changes are happening.  The space can be closed shortly after during a later stage in treatment.

Dental expansion can be achieved in various degrees via clear aligners, fixed appliances (aka "braces"), and light and heavyweight skeletal expanders.  Clear Aligners and Braces can usually only provide 2-4mm of dental expansion.  Lighter expanders may provide intermediate amounts of expansion (3-5mm).  Anything over the 4mm expansion mark will usually require a more "robust" type of expansion appliance and are more in the domain of skeletal expansion.  These values are constrained by the appliance itself and the growth potential of the patient in question (more on that below). 

Certain types of expansion appliances will be designed to focus their efforts to the front or the back of the mouth depending on where the help is needed most.  Patients who have undergone cleft lip and palate treatment may find that their primary lip closure or secondary revision has created scar tissue which has resisted the natural lateral (as well as forward) development of the upper jaw.  In this case a "spider expander" may assist in focusing the expansion efforts towards the front where mother nature was hindered by prior scar tissue. The image below shows a patient with the "spider expander" who is not a cleft lip and palate patient.


The upper jaw has nearly reached full side-to-side development by about age 15-16 years old.  Most expansion treatment protocols typically take about 1 year or more to complete (say 6 months of expansion followed by 6 months of stabilization) so you would expect that we could begin to treat a patient who needed skeletal expansion up until 15 years old.  The truth is a bit more complex than that, however.  The border between the two upper jaw halves begins to "fuse" or grow together the further along a patient is in development.  Some people will develop a bit earlier and thus the two halves will meet and create a fairly rigorous interlocking (imagine the teeth of a zipper).  If this happens earlier than most children of the same age, less clinical expansion will be achieved than if the same expansion treatment protocol had been started earlier.

Because age and individual development play such a role, the general rule of thumb is that "earlier is better" in terms of expansion.  The earlier we start with expansion the better chance the upper jaw will have to reach its maximum lateral growth potential.  The earlier we start expansion, the lighter the forces that are needed to help separate the two halves and (all things being equal) the more comfortable the process is for the patient.

Is a patient older than 15-16 years old out of luck if they need expansion?  No - expansion can still be accomplished at ages 16 and older however this expansion will typically involve more force applied to the border between the two halves.  At ages 18-28 years old we will typically need significant force to overcome the almost completely fused border.  This force can be provided via a microimplant-anchored expansion appliance (see image below).  The patient expands the appliance at significantly higher frequencies (3-4 times per day) than a child who is 10-20 years younger.  Once the two halves of the upper jaw bone are separated enough, normal expansion can take over and treatment proceeds as before.

Finally, when other surgical treatment is considered or when the expansion requirements are much greater than be accomplished with a standard expander, or when a patient is older than 28-30 years of age, surgical expansion may be considered.  In this scenario, the upper jaw is surgically modified to loosen its connections with surrounding bones and a skeletal expansion appliance is used to guide the upper jaw (post-operatively) into a better harmony with its lower jaw counterpart.  For patients already undergoing other surgical treatments or those that have exceeded the age-limitations of TAD-based expander, this may be the best (or only) course of action.

Generally speaking, however, the earlier expansion can be attempted the better the results and comfort during treatment.