Australasian Science: Australia's authority on science since 1938

Dentists Fall for Cavity Illusion

By Stephen Luntz

An optical illusion is causing dentists to drill unnecessarily large holes in their patients' teeth.

Prof Robert O'Shea says that making this fact known could decrease the time involved in dental work and possibly the risk that teeth need to be removed.

O'Shea is an expert in perception and illusions at Southern Cross University, and was asked by A/Prof Nick Chandler of the University of Otago whether dentists drill larger holes in larger teeth even when the area that needs filling is the same size.

The result was a paper in PLoS ONE, but in the process they found, in O'Shea's words, that “it is not as simple as that – it's not that dentists made larger holes in larger teeth. The critical aspect for the illusion is the ratio between the size of the tooth and the size of the area to be drilled.”

The pair drew on the Delboeuf illusion, known since 1865 to trick people when they attempt to estimate the size of circles inside larger shapes. They, along with Dr Ranjeesh Roy, studied the holes drilled by eight endodontists into real human teeth, finding them on average 50% larger than necessary when the properties of the illusion were suitable.

In the Delboeuf illusion, people overestimate the size of circles surrounded by small circles and underestimate the size of circles surrounded by large ones. The neutral point is when the ratio of the outer to inner circles is seven. Dentists for whom the tooth as a whole acts like an outer circle with a ratio greater than seven may misjudge the size of the hole they have drilled and keep on drilling unnecessarily.

"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," says Chandler.

O'Shea says that checking whether patients are being affected is “not impossible but very, very difficult”. Potentially more useful would be studies of the way people perceive ovals, because the holes cut are seldom circular, unlike most of the research on illusions.

“Psychological research suggests it is possible to overcome this illusion if people are aware of it,” says O'Shea. “We need to incorporate this information into training and see if dentists start making more appropriate holes.”