For an animal to survive it is essential that every tooth meets perfectly. However the genes that control our growth are not able to achieve that degree of precision. So during our evolution our teeth evolved the ability to be able to move in various directions to ensure good occlusion and the Tooth Eruption Mechanism (TEM) evolved to enable this to be achieved.
Lee and Proffit (AJO & DO.107: 38-47 1995) showed that each tooth erupts until it touches an opposite tooth (or thumb, finger or tongue) and if the mouth is left open too much the teeth will continue to erupt (figure 1). They also found that teeth intrude during a meal and erupt overnight and suggested that they should naturally be in contact for between 4 and 8 hours a day depending on the bite force.
Teeth will also move sideways as can be shown by adding plastic to one side of an unopposed molar and watch the cheek or tongue move them away. They will respond to the push of a feather (one to two grams) while fixed orthodontic fixed braces regularly apply forces fifty or 100 grams which are associated with both pain and damage.
Teeth naturally tilt when being moved especially if they are single rooted and for many years this was standard orthodontic practice but in 1972 Laurance Andrews studied a number of nicely aligned cases and introduced his ‘Six Keys’ for ideal occlusion describing the exact angle and position of each of 28 teeth. He then recommended rectangular wire and prescription brackets to place each tooth in its exact position.
Many clinicians view the teeth and bone as rigid because of their ability to crush tough food. However in the long-term they are influenced by very light forces. Not surprisingly most research has concentrated on the ‘ideal’ or ‘fastest’ way of moving teeth and there has been little research on the minimum force required to move a tooth. Sam Weinstein’s classical research6 concluded that teeth would move with a force of under 2 gm. He thought that contact from the cheek was able to move teeth, suggesting a ‘stiffness’ of the cheek of’ 0.8 gm and a mean ‘resting force’ of 4.89 gm. He also thought that there might be a genetic factor in the stiffness.
More recent research7 suggests that the lowest force that will produce ‘optimal’ bodily tooth movement is between 50 and 100gm. However the optimal force for tooth movement need not equate with the minimal force. Also bodily movement is very different from tilting, so in reality we have little idea of the minimum force required to move a tooth.
My own rather primitive research, using blobs of composite resin on the lingual or buccal surfaces of unopposed teeth, confirmed my belief that one or two grams from the tongue or even a feather could move a tooth if applied for long enough.
Forces of more than a few grams obstruct the fine blood vessels that supply oxygen to the periodontal membrane. Small intermittent movements cause little harm but larger movements over longer periods will block the oxygen hours/days these cells start dying. Spring forces may reduce the number of visits but they increase the period of occlusion and continuous arch wires inevitably cause much damage. It has been estimated that fixed arch wires shorten the life of the teeth by five to ten years and occasionally much more.
Orthotropics aims to use very light forces “no more force than a feather nor movement of 1/16th mm per day”. At this rate a Stage 1 Biobloc will widen a maxilla 10mm in less than 3 months with little tilting of the teeth.