The changing face of orthodontics

Most of us are familiar with orthodontics as a kind of mechanical engineering inside the mouth – all those metal braces, plates and wires. But how many of us are aware of the different sciences involved in this area of dentistry? Today’s orthodontists have to understand and apply a good deal of specialist science – everything from genetics to metallurgy.

What is orthodontics?

Orthodontics is the branch of dentistry concerned with diagnosing and correcting irregularities of the teeth and jaws. It is used for far more than achieving a perfect Hollywood smile: our jaws and teeth are used for talking as well as chewing, so orthodontics is concerned with how facial anatomy affects these functions, as well as with cosmetic improvements.

As orthodontists, we are always seeking the latest insights and techniques from relevant scientific fields and applying them to our work. Some examples are in table 1. In this article, we will look at several of these areas in more detail.

Genetics and molecular biology in orthodontics

Some of the problems that orthodontists deal with are genetic in origin (figure 2). Although most of these are minor, others result from genetic abnormalities in the way that the head and face develop before birth^w1. In the embryo, the development of facial structures begins with neural crest cells forming at the site of the brain. These cells then migrate to form a tissue that differentiates into cells called osteoblasts, chondroblasts and odontogenetic cells. These then develop to form the hard tissues of the head and neck – the bones, cartilage and teeth.

During this process, molecules called signalling factors and transcription factors play an important part. Signalling factors are a cell’s way of triggering a response in another cell, while transcription factors control which specific DNA sequences are used to produce mRNA and thus proteins. For example, we now know that if the signalling factor TGF-β is inactive, this causes cleft palates^w2 and upper jawbone malformations. Mutations in the receptor sites (where the response is triggered) for the signalling factor FGF also cause a large number of craniofacial abnormalities.

Another example is the transcription factors associated with the homeobox genes. These transcription factors are especially important in enabling the neural crest cells to develop into the skeletal structures of the head and face, so defects in the way these genes are transcribed can lead to abnormalities in facial development.

Another example of the importance of molecular biology to orthodontics is the recent discovery that dental pulp (the area of connective tissue at the centre of a tooth) contains valuable adult stem cells, which can be induced to form other types of cells. Thus when a tooth is extracted or falls out, the stem cells may be harvested and stored for future treatment. Stem cells are already being used to treat some cancers, and additional applications may be on the horizon. For example, researchers are exploring whether stem cells can be used to grow a natural replacement for a missing tooth.

Biomechanics and orthodontics

The amount of force needed to move a tooth depends on its size and the type of movement (turning or sliding). The moving force also needs an anchor, so a group of teeth and appliances are selected and used as anchorage (figures 3 and 4).

As orthodontists, our task is to decide on the best combination of forces and anchorage to achieve the right movements, without any adverse effects. We review each stage of treatment to make sure this is happening; if it is not, we need to change the treatment plan.

In traditional orthodontics, appliances such as headgear and intra-oral elastics are used to reinforce anchorage, which require a good deal of patient co-operation. Today, titanium mini-screws can be used instead in some cases (figure 4).

Metallurgy and orthodontics

The forces used in orthodontics come from the archwires (figure 5). At the beginning of the treatment, the wires need to be quite elastic to start individual teeth moving. Later on, the wires have to be more rigid to ensure stability while a whole block of teeth is moved.

Orthodontists can choose wires made from a variety of metallic materials:

• Stainless steel: this is easy to shape and has high rigidity so it provides stability.

• Nitinol alloys: these nickel-titanium alloys have very high elasticity. They produce a weak but constant force suitable for the initial alignment phases. However, they cannot be soldered.
• Shape-memory alloys^w3: these metals have variable elasticity depending on the temperature. They can be bent for insertion into the mouth; once there, they ‘try’ to recover their initial shape, exerting a force on the teeth.

A dynamic discipline

As you can see, orthodontists need to be good all-round scientists to keep up with the changing knowledge and technological innovations in their discipline. So, if a student in your class misses a science lesson because of an appointment with an orthodontist, don’t worry – it might be the perfect opportunity to learn about the latest findings in molecular biology, or provide inspiration to a budding materials scientist.