Metals sometimes fracture when hit with overwhelming force:
Metals often bend when stressed with enough force to cause permanent deformation- to yield:
When I was young, I had the notion that otherwise, metals just sat around, never-changing, nobly bearing up to the incredible stresses that we humans subject them to.
I know that one photo would make the point, but it is truly inspiring to mull once in awhile on all the things we create, and how clever we are in the design process. Especially, perhaps, when it comes to metals.
Metals do, however, deform slowly under forces that are lower than what's needed to bend or break them. This has implications in dentistry, specifically in orthodontic retention. Vital implications...
In two prior posts I reviewed the forces that constantly and relentlessly act on our teeth, the effects of these forces, and one excellent strategy to maintain the position of our teeth once orthodontics is complete:
Invisalign's Vivera retainers are made from a digital representation of the patient's teeth, derived from a physical impression, but soon we will be able to capture them from super-accurate optical scans. Since these representations of our teeth can be saved indefinitely in Align Technology's servers, new retainers that are precisely the same as all previous sets can be made at any time. This is astonishingly useful. All the patient's teeth are able to be maintained in their proper position no matter what muscle forces and genetically programmed drift sets in. The only difficulty comes into play when extensive wear, trauma or restorative dental treatment occurs. If major changes to the arch of teeth come into play new retainers will have to be made from new impressions. As long as retainers were generally worn up to the time of the major event, the overall alignment of the teeth will be maintained and the new Vivera retainers will simply capture all the changes that were made.
For years, many orthodontists have made analog retainers. These are excellent and can be made active in many cases so that minor re-alignment is possible. The one drawback to analog is that if they need replacement, the required new impressions will capture any tooth movement that has occurred since the original orthodontics was completed. This is not a good thing.
Another very popular strategy is to place a bonded wire onto the inner (lingual) sides of the lower front teeth. This is where the issue of creep comes in. There are stresses on metals that are under the level that breaks them or bends them. Over time, these lower stresses do still manage to distort them, to change their shape. From wikipedia: "In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses."
Thus, what was once a nice, evenly curved retainer bar that held the lower six front teeth in place now comes to look like this:
These retainers also do not address the fact that the muscles that close the jaw push inwards on the teeth far more forcefully than the tongue pushes outwards, causing the back teeth in the middle to crowd inward over time in most people.
Finally, these bonded retainer bars impede oral hygiene since the patient can't floss without special devices.
On balance, choosing conventional retainers or Invisalign's Vivera are highly effective, and lingual bonded retainers are ineffective and cause other damaging issues for the patient.