Monday, April 22,  2013

            I know.  You’re wondering if Invisalign is all that they say it is.  And the answer is…..it is getting closer and closer.  Moving teeth where you want them to go is a complex business because it involves controlling the pressure applied to the root of the tooth while only being able to influence the crown.  The pressure  causes a stimulus in bone remodeling at the surface of the ligament that holds the root to the bone.  Putting pressure on this ligament causes remodeling of the bone along the length of the root.  Imagine your finger buried in sand and trying to move the end of your finger through the sand when you can only apply force through the base of the finger.  The end gets much less force, but to move the end at the same rate as the rest of the finger, you have to push the end of the finger ahead faster so that it keeps up with the rest of the finger.  The same principle applies in moving teeth.  If you want to move the root of the tooth as fast or sometimes faster than the rest of the tooth, you have to increase the force felt at the end of the root while only being able to attach to the crown of the tooth.  We call that managing the “moment-to-force ratio” and with traditional braces (brackets on each tooth and a wire running between them, springs and elastic chains or rubber bands to create force on teeth or groups of teeth) you can accomplish this in an approximate kind of way.  With the original Invisalign product, the attachment of the plastic aligner to the tooth was pretty sloppy, so it was possible to tip a tooth through the bone, but root movement was difficult.  Through the generations of the the product, attachments (shaped tooth-colored bumps of various sizes and shapes) have been added to selected teeth so that the aligner “grips” the tooth better and can apply a moment-to-force ratio to the tooth via the attachments.  The materials have also changed so that they are more elastic than the original and the deformation of the material helps with application of force.    With traditional braces, the progress is assessed at each visit so that adjustments of the bracket, wire, or auxillary can be made to direct and maximize tooth movement.  With Invisalign, before treatment begins,  a very accurate impression of the teeth is digitally scanned and prepared by the lab, and then forwarded to me over the Internet so I can direct the tooth movement.  An aligner (clear plastic tray that engages the teeth) fits imperfectly when treatment begins, but puts force on the teeth so that within 2 weeks the teeth move to fit the tray.  At that interval, another programmed tray is worn for the next 2 weeks, and so on until the teeth are where you want them.  All of that movement is planned by the orthodontist before treatment is begun and, while some changes can be made during the course of treatment, the computer model has to accurately predict how the teeth will move, as all the aligners are manufactured at the outset.  Clever orthodontists have figured out how to apply forces between the upper and lower teeth with rubber bands through slits and hooks on the aligners so shifting teeth forward and backward relative to the teeth in the opposite arch is possible.  And it works.  It is esthetic, hygiene is easier than with conventional braces, and they are removed to eat which is more pleasant.  That said, teeth still get sore, it is still a hassle to keep up with the trays and to wear them 22 hours a day.  For some, braces that are semi-permanently attached are less bother, but many who have always wanted straight teeth but would not go for braces now have another option, and I have many satisfied patients.  They say put your money where your mouth is…..well I am…. sort of.  My wife is starting treatment within a few weeks, and I will get to live with the process second-hand.  If I can just keep her happy………