When a tooth moves through the bone the bone in front of the tooth dissolves and after a period reforms behind it (this time delay leaves the teeth loose for a period- and possibly very susceptible to gum disease), thus allowing the tooth to move. To a smaller degree the tooth also dissolves, but the tooth generally has less ability to repair and does not normal repair completely. The loose of parts of the root is called root resorbtion, the root is literally resorbing.
Almost all fixed appliances cause root resorption. Dr Kurol (et al 1996) using only light forces found that “93% of teeth showed some root resorption but none of this could be seen on peri-apical radiographs”, these are the fine detail x-rays taken by dentists (such problems are unlikely to be shown on OPG views- the big one take by a machine that moves around you). In one tooth after 3 weeks he saw “resorption had reached the pulp in the apical third of the root” (this means that 1/3 of the whole tooth, nearly ½ the root, had been lost) and “In eight teeth after 3 to 7 weeks resorption had extended half way to the pulp” (in as little as 3 weeks of orthodontics these teeth were effectively destroyed). As a result Kurol who is a widely respected and very logical scientist, concluded “root resorption is an early and frequent iatrogenic consequence of orthodontic treatment, even after application of a force below what is often used in other clinical situations”.
Weiland (2003) compared super- elastic NITI wires (these are the very bendy wires usually used at the beginning of a treatment) with standard stainless steel and found “The amount of root resorption was significantly larger in the super-elastic group”. In addition root damage during treatment may result in iatrogenic external resorption many years later (sometime the effect can be delayed).
Mirabella and Artun (1995) reported “40% of patients show resorption of more than 2.5mm”. Nearly all patients have some root resorbtion. Unfortunately we have no idea if this root resorption shortens the life of the teeth by months, years, or decades. Resobtion is an unfortunate complication of fixed appliances; it is thought that the worst resorbtion occurs when there is a jiggling effect. If springy wire is pushing the teeth in one direction most of the day but at night elastic is pulling in another direction, the tooth is being pushed one way and then another, a peg in the ground, over time it will become loose. Also if the mechanical forces of the fixed appliance work against the forces of the soft tissues, the lips and the tongue, then teeth may be in perpetual jiggling, which possibly explains the most rapid root destruction.
What are the long term problems?
Teeth with sever root resorbtion are most likely lost within a few years of the problem, without any root there is little that can be done. Without a root the tooth has little or no support and the forces transmitted through the rooth are then concentrated in a small area of root attachment that is easily overloaded and damaged. Like a tree with no roots these teeth rarely last more than a year or two. Teeth with very little damage usually repair almost completely and are unaffected by orthodontics. The others lie between these two.
Gum disease and root resorbtion; During the life of most people their gums recede, or them become long in the tooth. This usually happen between 30 and 40 years old with the gums moving down the teeth progressively. As the gums recede the effective length of the root becomes shorter and eventually teeth can become loose and are then lost. Obviously the longer the root the more natural resistance to this problem you have. Root resorbtion can obviously affect this.
Endodontic treatment and root resorbtion; not infrequently the space inside a tooth dies and becomes infected. The treatment for this is root canal therapy- filling this space, as without a blood flow it is a potential “safe” harbour for pathogenic bacteria. This is a specialised and complex procedure best undertaken with a microscope and very thin instruments that can extend right down the inside of the tooth. It is well understood that the normal narrowing of the tip of the tooth is a great natural advantage that helps to block the far end of these fillings. Teeth without a narrowing at the end, either from only partial construction (often when a tooth is damaged before it is fully formed) or from root resorbtion, have a far lower success following root therapy.
So root resorbtion is a normal consequence of root movement that is unseen and underappreciated and since mass orthodontic treatment is still a relatively new phenomenon what we are seeing now may well only be the tip of the iceberg. It is only now that the generations that had mass orthodontics are reaching their mid-30’s, expect to hear more about this.