Retainers are provided after nearly every type of orthodontic treatment except the simplest type in very young patients. This is because the bone and gums need time to adjust to the new positions of the teeth. Failure to provide (or wear) retainers may mean that the teeth will relapse, that is, move either partly or wholly back to their original position.
How have they evolved?
In the late 1960’s a scientist called Reitan found that the reorganisation process takes roughly a year. The first few months of the retention phase is the time that the teeth are most prone to rapid relapse. Some movements of teeth are more prone to relapse than others. For example, rotated or twisted teeth are very prone to relapse and the gaps between previously spaced teeth are even more likely to re-open unless retained. This is because the elastic fibres in the gums take a long time to adjust. Even after the first year is over, your teeth may still move. This is because your teeth respond to pressure (otherwise your brace wouldn’t have worked). Once the brace is off, your teeth come under the influence of your lips, which apply inward pressure on your teeth and your tongue, which pushes the other way. The stable position of your teeth is the balance zone between the two pressures and this is where your orthodontist will have aimed to put them. However this balance zone moves throughout your life as your face matures and changes with age.
Some of the more recent findingsScientists in the UK and the USA have found that the front part of your mouth actually narrows by a millimetre or two after the age of thirteen. A Swedish scientist named Bjork looked at serial xrays of children and adolescents and found that the lower jaw keeps growing for longer than the top, possibly leading to movement of the teeth as they try to keep contact. Furthermore, your face keeps lengthening, albeit again by a millimetre or two, into middle age. This does not sound like much but millmetres make quite a difference to the appearance of teeth.
Why do teeth move?
The most common time for teeth to move after having had braces is when you are in your late teens or early twenties. Many people notice that their lower front teeth have crowded. Traditionally the wisdom teeth have been blamed for this, but an Irish researcher, Margaret Richardson, looked into this subject and found that wisdom teeth do not make a significant contribution to crowding of the lower incisors, so please do not have your wisdom teeth out to prevent or treat this problem. In summary, nobody knows exactly why the teeth tend to move at this age, however the most likely explanation is that your face is always changing and your teeth change also. Therefore, even though you have had a course of bracework and your teeth are perfectly aligned, the only way to guarantee that your teeth will stay straight is to keep wearing a retainer, at least on a part-time basis – for as long as you want to keep them straight! I will discuss the different types of retainer in a subsequent post.
What types of retainer are there?
There are two basic types of retainers, removable or fixed and both have their pros and cons. Removable retainers are usually worn full-time for the first three to four crucial months after removal of the brace but are removed for eating and cleaning. Afterwards they may be worn at night only. They are very effective and reliable, but obviously this depends on their being worn as instructed.
There are two categories of removable retainers:
Vacuum formed retainers are the most common. These are made of polyethylene or polypropylene sheets which are vaccum formed to a mould of your teeth. They are light and transparent, virtually invisible and as they only cover the teeth, there is rarely and interference with speech apart from the first day or so. However they may be damaged if the instructions for wear and cleaning are not followed and being almost invisible, are easily lost!
The other type are made of acrylic and wire, the commonest being known as a Hawley retainer. These retainers are more robust but heavier and cover at least part of the palate, therefore are more prone to affect speech. There is often a wire crossing the front teeth and so are more noticeable. These retainers are used in certain cases where a vacuum formed retainer is not suitable, e.g. when the upper jaw has been expanded or after major jaw surgery.
Advantages of removable retainers
The major advantage of a removable retainer is that the wearer can control and supervise the retention of the teeth. Removable retainers need only occasional checks by the orthodontist as damage is easily spotted by the wearer. Furthermore, if the retainer is not worn for a time and the teeth move slightly, the situation may often be saved by pushing the retainer in until the teeth move back into place. Being removable, there is nothing to hinder cleaning of the teeth. Your local dentist will be able to tell you more about this.
Fixed retainers are fine wires glued to the backs of the teeth and are usually preferred by patients as they eliminate the need to remember to wear a retainer. They also do not interfere with speech. However they do have several disadvantages. They cannot be placed in every situation as they may interfere with the way the teeth meet. This will either cause discomfort or will break under the force of the bite. Being fixed they tend to accumulate debris. This may be difficult to remove, even by a dentist and may cause disease. They may also become detached and need urgent repair. Should a fixed retainer become partly detached and the wearer does not notice, the detached teeth may drift. These may be very difficult to bring back into line without a further course of bracework. Occasionally, if the fine wire is damaged or bent, it may act as a spring and move the teeth itself. Again, the wearer may not notice until significant movement has occurred and considerable re-treatment will be necessary to straighten the teeth once again. So, while fixed retainers are useful in certain cases, they are certainly not ‘fit and forget’ as most people would like them to be. A fixed retainer needs regular, lifelong supervision and maintenance, either by a general dentist who has some extra training and knows what to look for, or by an orthodontist.