Towards the end of 2004, Joseph Wouters, decided to perform a study, looking for a solution which would enable
- a reduction in eye rotation (less tiring),
- a reduction in cervical spine flexion (less strain and less tiring) and
- a reduction in the combination of flexion of the cervical spine and a downward rotation of the eye.

Problem definition
Based on this the problem definition of this study was drawn up: What is the effect of special spectacle lenses, if possible with a prism, on the flexion of the cervical spine when undertaking dental procedures?

A number of principles were laid down for the study;
- A neck flexion of 0-25 degrees maximum.
- A relation of 60:40 between neck flexion and eye rotation.
- An eye rotation with a maximum flexion of 30 degrees.


Eye rotation

The study consisted of a number of parts:
- Pilot study
-Measurement of the effect of the prismatic refraction
- Study into head flexion and eye rotation in own preferred postures with and without special spectacles with prisms on 3 treatment areas
- Study into eye rotation when working in a standardised operating posture
- Optometric study / optometric questionnaire
- Establishing correction related to operating distance
- Listing the perceived physical discomforts – (Dutch LEO-list)
- End questionnaire
- Statistical analysis

In the pilot study, the measurement protocol was tested and a measuring instrument was tried out. For the effect measurement of the prismatic refraction on head flexion, use was made of prism lenses of various powers, which were offered in non-ascending range, and measurements were carried out on three different treatment areas.

A group of dental students and a group of office employees were measured and compared. This study showed that an incorporated prism yielded the best effect and it appeared that there was a big difference between these two groups. A cervical spine flexion of > 25 degrees was measured for all dental students.

In the research into head flexion and eye rotation, a group of 12 practising dentists was measured. Measurement took place when they were looking at three different treatment areas, with and without spectacles with prisms.
The effect of the eye rotation when they looked at these three treatment areas was also examined for a standardised operating posture with neck flexion of 15 or 30 degrees respectively.

On the basis of questionnaires, a picture was formed of possible eye-related problems. All this data led to a determination of the refraction correction on the basis of a measured operating distance.

Before and during the use of special spectacles, participants were asked to list their perceived physical discomforts.

All this data led to four design alternatives from which, on the basis of the set requirements, a choice was made in favour of alternative three.

The total refraction is achieved by adding up the inclination of spectacles and the prism refraction. Its effect results in an intuitive image shift, where a larger viewing distance enlarges this image shift. In fact, the image does not move forward, but the viewing angle changes due to prismatic refraction.

Study results
The standardised operating posture displayed exceeded the set limit of 30 degrees of eye rotation both at 15 and at a 25 degree flexion of the cervical spine.

The eye rotation exceeds the 30 degree limit for the three treatment areas.

The problem - either more flexion of the head or further rotation of the eye.

The perceived physical discomfort improved so that five out of six dentists with neck complaints showed a considerable reduction in pain within ten days of use.
(However, this must be interpreted with due caution. Not all registrations were complete and there were some user problems with the prism spectacles, which may have influenced their effectiveness).

Conclusion
In this study, a significant reduction in eye rotation was found in three treatment areas and a reduction in neck flexion in two out of three treatment areas, due to the use of prisms.
It was concluded that a further study was required to improve the points of user experience indicated.