The relationship between ergonomics and magnification
In the past few years I have been intensely engaged in research among dentists regarding the relationship between eyeball rotation and neck flexion during work. Dentists are advised to work in an upright posture and most attempt to do so, but the majority have trouble maintaining it.
Less strain is placed on the neck muscles and shoulders when the flexion of the cervical spine decreases. Research shows that there is a balance between neck flexion and eyeball depression. A previous study among seamstresses showed a constant 60:40 ratio. So for this group to achieve the required line of sight, 60% originated from the cervical spine flexion and 40% from eyeball depression.
By studying a group of dentists in an upright position with a cervical spine forward flexion of 15º and later of 25º, it became possible to examine the effect it had on eyeball depression. In all cases, the eyeball rotated further than 30°. In addition, the ratio of 60:40 appeared to be different. Dentists use more eyeball rotation than cervical spine flexion to affect a sight line. The strong eyeball depression causes a (static) strain on the eye muscles and this will often result in headaches or eye fatigue.
On the basis of deduction, this leads to the hypothesis that it is the result of conditioning for the group of dentists examined. This conditioning is assumed to develop during training or later and perhaps as a result of complaint perception. During the study, the desire to achieve less neck flexion resulted in a greater level of eyeball depression. This is why the group examined may behave differently from the average working dental population. Here follows some background information to help you gain an improved understanding of a new spectacle design and the principles behind it.
Literature on neck strain during flexion (bending the head forward) shows that a neck flexion between 0º and 25º is recommended, a 15º flexion being the most comfortable. Greater flexions are not recommended in static working postures and the maximum endurable strain decreases immediately when the neck is flexed further. A continuous strength in the neck muscles and ligaments is required to keep the head in balance. When bending forward from the neutral posture towards the maximum flexed posture (45º, at a fixed position of C7), this force will increase by about 90%. Therefore it is important to bend the neck as little as possible.
Interaction between eyeball rotation and neck flexions
The neck and eyes are mobile and do not seem to impose many restrictions on the viewing angle. When maintaining a static head position for long periods, all the while performing repetitive actions, finding a comfortable position for the head and eyes is crucial.
While standing, the optimum viewing angle is approximately 30º below the horizontal line of sight. Eyeball depression is possible up to approximately 50º, and photographic research demonstrates that eyes usually turn to more than half this angle during work. When working while standing, there is an eyeball depression of approximately 27º compared to approximately 24º while seated. Neck and eyeball movement work together to achieve a good retinal image. At an eye rotation of 6º, a head movement will follow. There is an interaction between head flexion and eyeball depression, the total degree of which is determined by the required line of sight to observe a task adequately.
When wearing spectacle frames with inclined lenses to which prisms are fixed, eyeball depression is significantly reduced at a 15º neck flexion. The use of these prism spectacles causes a significantly lower flexion of the cervical spine, with clear vision of the treatment area.
How can the sight line be influenced to reduce neck flexion?
The less flexion of the head, the more eyeball rotation. Less flexion produces less strain on the neck and shoulder muscles, but more complaints for the visual system, due to forces on the eye muscles and so on. In essence, one problem is exchanged for another. A solution would therefore need to focus on reduction of both neck flexion and eyeball rotation.
In my first study, special spectacles with various prismatic refractions were presented to two subject groups. The result of this study determined the inclination of the frame and a prismatic refraction.
A group of 12 dentists were offered spectacles with the correct working distance and custom-made spectacle lenses. Measurements of eyeball depression and neck flexion showed significant improvements. Five of the six subjects suffering from neck and shoulder complaints reported a clear improvement after use of these spectacles over a two-week period.
However, the placement of the prism segment was perceived as too high and half of the group reported chromatic dispersion, particularly when reading black text on white paper. This resulted in the undertaking of new experiments with different inclinations, different prisms and a different prism placement height.
Testing the new trial glasses on a user group led to the second study. Dentists suffering from neck-shoulder complaints volunteered; a total of 15 participated in the study. The subjects completed so-called LEO lists (perceived physical discomfort) for a period before and after use of the special working glasses and recorded at the end of each working day any complaints/ discomfort experienced in any part of the body.
After four weeks, on the basis of these lists, 12 of the subjects showed a clear decrease in physical complaints/discomfort. One subject showed a marginal deterioration in the neck region. Five subjects reported no longer having any complaints in the neck and shoulder regions after four weeks of use.
At the beginning of the study, all subjects underwent a physical examination with a physiotherapist/manual therapist who looked for any physical restrictions within the neck and shoulder regions. This examination was repeated at the end of the study. In total, 12 of the subjects showed improvement, ranging from small to marked.
Neck flexion for the three treatment positions in the mouth were examined both in the dentist’s preferred working posture and in the standardised working posture using spectacles with prisms.
The results are shown in Table 1. The results show a clear decrease in neck flexion, far below the 25º limit set. The eyeball depression for the three treatment positions were similarly examined, the results of which are in Table 2. Here too a decrease in neck flexion is seen far below the set limit of 30º eyeball depression. A total of 12 of the subjects continued to wear the spectacles after the study period. The study results are exemplified in Figures 3 and 4.
An optical solution
It is now possible for dentists to work with this new spectacle design. Some optometrists have undertaken a specific training programme and are qualified to take the necessary eye measurements for dentists. This is carried out according to a fixed test protocol using a unique instrument designed to take into consideration the interaction between dentist and patient.
With these measurements it is then possible to manufacture spectacles for the correct working distance. It requires advanced calculations and manufacturing methods to incorporate the dentists’ spectacle prescription within the prism, as well as to place the prism segment optimally within the lens. If no vision correction is required, the working spectacles with the prism incorporated can be used as safety glasses with the same ergonomic advantages.
It is a precise combination of optometry, ergonomics and dentistry. The result is a pair of working spectacles ‘for your eyes only’.