NZSM Online

Get TurboNote+ desktop sticky notes

Interclue makes your browsing smarter, faster, more informative

SciTech Daily Review

Webcentre Ltd: Web solutions, Smart software, Quality graphics

Feature

Fighting Tooth and Tooth

If you grind your teeth, you may not only be sharpening them, but unconsciously preparing to use them for attack or defense.

Fiona Rendle

Most humans grind their teeth. It is innate behaviour which today most people perform in their sleep, says Professor George Knox, retired head of Zoology at Canterbury University. Knox is a supporter of the theory that this serves to sharpen teeth, with perhaps the ultimate aim of using them as a weapon.

Tooth grinding creates an angled plane known as a facet, on the top of teeth. This gives the teeth planes with a sharp edge like a chisel head. When a person chews, the top and bottom teeth slide across one another working in a similar way to scissors. Knox illustrates this effect with his own porcelain dentures that are no longer rounded on the top but have angled planes worn on them by teeth-grinding action.

Thegosis in humans involves a movement of the teeth of the lower jaw right across the teeth of the upper jaw, in a direction diametrically opposite to chewing. This movement serves to sharpen the teeth by grinding them against one another. To get the ground surfaces to match up, the lower jaw has to be moved sideways. This movement is not easy to accomplish consciously.

The concept of thegosis was developed by Dr Ronald Every, who held an honorary research fellowship at Canterbury University some years ago. Every has gained a reputation for his ability to treat facial pain and other complex tooth problems. Through research he concluded that tooth grinding involved tooth sharpening, and he coined the term thegosis, from the Greek word thego, meaning "to sharpen".

Wide Evidence

The evidence for thegosis, argue its supporters, is very wide and can be seen in photographs of teeth which have parallel marks that are too even to be caused by abrasion. The obvious difference can be seen when photographs of teeth worn by abrasion and teeth worn by sharpening are compared.

Other evidence includes the function of a muscle in the jaw. The external pterygoid muscle in the human jaw is a powerful muscle that functions minimally when people are chewing or opening their jaw. The muscle is set up in such a way that it is not in the correct orientation for its currently accepted function but is precisely in the right place for assisting powerful grinding.

Evidence can also be seen in one's own mouth. If the bottom jaw is pulled right over to one side, tooth surfaces match up, and it can be seen where the teeth grind against one another. The sharpened teeth edges can most easily be felt on the front teeth where only one blade is present. By running a finger around the inside of the front teeth of the lower jaw, one can feel the sharpened blade, known as the leading edge. This blade can also be felt on the outside of the upper front teeth.

Every contends that most mammals sharpen their teeth to ensure that the teeth are effective tools and weapons. He has extended this to see it as "a phylogenetically derived behaviour that sharpens a tooth by grinding it violently against another; the action occurs concomitantly with the sharpening of the physiological determinants that not only prepare the tooth, but prepare the animal to grasp, attack, defend, incise or masticate".

Every says that he has yet to find anything to contradict his ideas, but has found much to support them. Further research is being carried out by Christchurch dental surgeon, Dr Kevin Scally, who has designed a unique machine that can analyse jaw movements, including thegotic movements.

Scally, who has spent 26 years working in this area, looks on Every's work as comparable to Einstein's relativity theory in physics, but in the area of biology, in that it encompasses many fields including dentistry, medicine, psychology, palaeontology, ethology and anthropology. Scally says that thegosis provides the background to bring together the many aspects of teeth and jaw science in a way that makes sense.

To date, the theory has not been widely accepted, but Scally says dentists only have to change their view by 90 degrees to see sharp edges where previously they have seen flat surfaces. When one sees the sharp edges, teeth become an exquisite biological machine rather than separate blunt units, he maintains.

Kaiapoi dentist, Dr Ian Short, supports Every's theory. He uses the concept to design dentures that are correctly shaped to withstand thegotic wear. He says to work out a patient's thegotic pattern takes about half an hour. While this time may not always be available, he tries to take these patterns into account even when designing fillings so that they do not fall out when teeth grinding occurs.

Functionless or Lost Function?

Many scientists think thegosis is irrelevant to dentistry and lacks scientific credibility. Tooth grinding is taught as part of a group of learned behaviours called bruxism, which cause damage and are said to have no known function.

Knox says teeth grinding can cause problems. One problem stems from the change in the human diet to less abrasive food, which means that teeth are not getting as worn by abrasion. This has led to an imbalance in the sharpening and abrasive mechanisms.

Now when the jaw moves the distance for teeth sharpening, the teeth do not slide smoothly over one another, as the teeth have not been worn enough abrasively. This leads to problems with pressure being exerted on a variety of anatomical structures and injury being inflicted.

It can take a long time for new ideas to enter mainstream awareness, but supporters say that observational and experimental evidence is helping Every's theory to gain piecemeal acceptance. At the moment Every is bringing together over 50 years of research in this area in a book called Right Under Your Nose -- Toothprints of Man's Violence, which will probably fuel further discussion.

Toothy Tools and Weapons

If you don't have any scissors handy to cut your sellotape, what do you do? You use your teeth. Teeth are humanity's oldest tools, and possibly oldest weapons. Young children use their teeth as weapons and have to be taught not to bite people. Aggressive biting can be seen on the sports field or in sexual attacks.

It is widely known that human canines became reduced during human evolution, but no one knows why. Long canines were replaced by rows of teeth in which all were sharpened, with the front teeth able to give a lethal chunk-removing bite. This bite is known as the segmentative bite.

It is commonly believed that humans were defenceless after the canines were reduced in length, but evolution by its very nature is a process that selects the most advantageous adaptations. Every lists 24 advantages of the short canine over the long, including its reduced vulnerability (the long canine can easily be broken off) and its spread throughout the social group (large canines tend to be limited to a few members).

Increased grinding also occurs when people are under particular kinds of stress, which the thegotic theory explains as providing a mechanism to enhance the usefulness of teeth as a weapon -- the person feels under attack, so unconsciously sharpens their teeth to defend themselves. Part of the theory also explains basic facial expressions based on displaying teeth in various positions. One example of this can be seen when people snarl in anger, displaying teeth in the sharpening position.

Fiona Rendle is a student in Canterbury University's Journalism Department.