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Psychology research drills down into dentists’ operations


Brigid Veale
4 November 2013
Going to the dentist could soon become a little less intrusive, with new research linking the fields of psychology and dentistry helping to reduce the size of holes that need to be made in teeth.

Professor Robert O’Shea, a psychology expert in visual perception from Southern Cross University, has teamed up with two researchers in dentistry from New Zealand’s University of Otago, Associate Professor Nicholas P Chandler and Dr Rajneesh Roy.

They have recently published a paper in PLoS ONE titled 'Dentists make larger holes in teeth than they need to if the teeth present a visual illusion of size'.

“Our finding was quite a surprise,” Professor O’Shea said.

“It has shown that under some circumstances, dentists remove more of a tooth than necessary, prolonging the operation, and, if nothing else, exposing patients to the sound of the cutting instrument for longer than necessary.

“Importantly, removing more tooth structure than necessary exposes patients to increased risk of complications after the procedure, such as cracking or splitting of the tooth.”

Associate Professor Chandler added: “These complications may require the tooth to be removed, leading to more expense for the patient.”

Professor O’Shea, whose research is focused on visual perception, said the visual illusion (or optical illusion) that led dentists to make larger-than-necessary holes in teeth, is one in which a large context makes a small, enclosed area appear smaller than it really is — the Delboeuf illusion, known since 1865.

In the case of teeth, the context is the size of the tooth, and the affected area is the small part of the tooth that the dentist drills out.

“It is not as simple as that — it’s not that dentists made larger holes in larger teeth,” Professor O’Shea said. “The critical aspect for the illusion is the ratio between the size of the tooth and the size of the small area.

“We found that dentists make larger holes in small teeth too, if the ratio was in the range that produces the illusion.

“We do not know if dentists are aware of this. We think that dentists think, either consciously or unconsciously, after they have made a hole of a particular size: ‘That looks rather small — I need to make that hole bigger’”.

To conduct the research, eight practising dentists who were experts in the particular sort of operation required — endodontists — each operated on at least 21 real human teeth.

“At the time of the study this represented about 50 per cent of all the practicing endodontists in New Zealand, where the research was carried out,” said Associate Professor Chandler.

The operations were conducted on teeth that had already been extracted for other purposes.

“Although the operations were not conducted on patients, we nevertheless asked the endodontists to operate on each tooth as they normally would, using their usual instruments. We reminded them to remove as little tooth as possible,” Associate Professor Chandler said.

“Over the past few decades, dental treatment has changed as practitioners adopt the concepts of ‘minimal intervention dentistry’. Often teeth do not need to be drilled and filled, but when they do, modern instruments and materials allow cavities to be made much smaller than they were previously.

“Trainee dentists around the world learn their initial skills on plastic model teeth or extracted natural teeth in a simulation environment. This new knowledge is something that could be included at a very early stage of clinical training, before they meet patients.”

Professor O’Shea said: “The outcome of our research could be relevant not only to dentists, but to all providers of health care. When operating, health-care providers generally try to save as much healthy tissue as possible. It is important for them to know that their eyes can deceive them into removing more healthy tissue than necessary.”

The paper is available at

Photo: Professor Robert O'Shea.