What happens when you start to move a tooth has been the focus of scientific study since appliances first came into use more than a century ago. There are countless articles describing the physical displacement of a tooth, the effects of compression and decompression of the periodontal tissues, the biochemistry which accompanies the process, etc.
It comes as a shock to discover that the widely accepted ideas about tooth movement we use every day in the clinic are seriously out of date. Not only are they out of date but their application may worsen an existing condition or create an unsustainable end result which will collapse on the removal of the retainers. To demonstrate why we have to go back to first principles.
Diagrams A and B show two quite different responses to a force. Diagram A represents a linear reaction. The vertical axis records the increasing amount of force being applied (the load). The horizontal axis records the amount of movement created by that force (the effect). The more the load increases, the more there is an effect, as the graph demonstrates. This is the familiar reaction of common sense mechanics. The principles are widely used in engineering and architecture as well as in everyday activities. As the graph goes upwards the structure eventually weakens under the increasing load and breaks down. An engineer has to take this into account and indicate the maximum allowable load the structure will bear before collapsing.
Diagram B represents what happens when a force is applied to a live organism. It describes a non-linear reaction. The vertical axis still represents the load being applied and the horizontal axis shows the resulting movement. However, the graph line does not start from the zero point at the junction of the two axes. A live organism is already under load by the nature of its being alive. It is prestressed. Its reaction to a force will be probable only, not predictable. Secondly, it takes only a slight increase in load (the vertical axis) to stimulate considerable movement along the horizontal axis. Eventually, the force required to move the organism does become greater and the graph rises steeply. As it does so the organism stiffens, strengthens and becomes much more resistant, a reaction quite different to that of a linear structure.
We think of treatment intervention in linear terms but the body responds with non-linear reactions. We have to reassess what we are doing if we are to take advantage of the biological response. The goal is to harness the body’s intrinsic reactions and its mechanisms for healing as best we can. Fortunately, other health disciplines such as orthopedic surgery are moving towards the same goals.