Refer this article as: Bleshoy, H., Does the eye rotation center play a role in the choice of lens type?, Points de Vue, International Review of Ophthalmic Optics, N69, Autumn, 2013
Does the eye rotation center play a role in the choice of lens type?
The position of the optical centre and progression zones in spectacle lenses has been discussed for many decades. The effect of head- and eye movements has been investigated in situations involving everyday tasks such as distance vision, computer work and reading in combination with static and dynamic performance.
The inter-person variability is big when looking at such parameters as PD, vertex distance, head shape as well as more general body related aspects such as movement, body position, head tilt etc. It is not uncommon that we observe a person with a slight head tilt to one side or the other (fig. 1) or demonstrating a slight head turn right or left. Very frequently the muscles of the neck and upper tarsus are seen as important elements in variations in head posture, and problems in those muscles will almost certainly be a causative factor in some form for corrective counter measure in head and eye positioning. In addition to this we have to allow for the visual needs of each individual.Optometrists in the clinics are all familiar with persons who function without visual problems when using a standard pair of reading glasses which has not been fitted in any particularly way. Despite not taking account of variations between right and left eye, different reading distances or general visual needs, these persons do not complain of vision related problems. On the other hand we are aware of people with high visual demands in varied situations, in which settings even small inadequate corrections will cause significant problems.
Research into the structure and design of progressive lenses has been ongoing for many decades. All serious glass manufacturers devote significant resources to research aimed at achieving a better understanding of visual function in different behavioral patterns. The very understanding of the visual drawbacks we experience with by age, has led to a remarkable transformation in the design of spectacle lenses. The need to optimize the visual function has changed dramatically over the last 10 years and we now have work related tasks which put the visual system to the limit. This involves vision based decision making and not least effectiveness in our busy business lives today. We know that the demand for energy, by the visual function, is very high. It is estimated to demand between 25-50% (Jensen 2008) of the total energy available. By optimizing the visual function, we may be able to limit the "wrong" or inappropriate waste of energy, which then may be made available for other and more useful purposes.
Content
Purpose and background for the study
Anatomic as well as physiological circumstances are very variable from person to person. Head position plays a significant role and accordingly the centre of eye rotation (ERC) is of interest. Every optometrist has come across Listing's law during their education, which describes the eye position during saccades. Many studies have followed and Crawford & Vilis (1991) showed that during slow movements the eye position will often deviate from Listings area despite being relatively small variations. During fast eye movements these are likely to be compensated for by a head movement. In this manner a continuous communication exists between eye- neck- & shoulder muscles groups and the visual input which is dictated by the level of concentration and awareness of the person as well as the demand of the visual task in which that person is engaged. Controlling these elements is highly associated with the physical and mental status of the person, and in turn puts a high demand on the energy available at the time.
In the literature we may find numerous and quite individual variations on eye behavior regarding position, and it may seem impossible to take all of these into account at all times. This, however, should not stop us being aware of possible problems, and in the individual circumstance deal with possible solutions which may solve or at least reduce the visual discomfort and inefficiency which may be encountered during everyday life.
Over the last few years, Essilor has been very interested in the center of rotation of the eye (ERC). This research has led to a better understanding of what the ERC actually is, the position of the ERC and which effect there is on the use of spectacles with single vision and multifocal lenses. With this research in mind, Essilor has developed a production technique which may compensate for the individual deviations in ERC.
Definition of the center of rotation of the eye (ERC):
Centre of rotation of the eye
When the eye rotates in its orbit, there is a point within the eyeball that is more or less fixed relative to the orbit. This is the centre of rotation of the eye. In reality, the centre of rotation is constantly shifting but by a small amount. It is considered, for convenience, that the centre of rotation of an emmetropic eye lies on the line of sight of the eye 13.5 mm behind the anterior pole of the cornea when the eye is in the straight ahead position (straightforward position), that is when the line of sight is perpendicular to both the base line and the frontal plane.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
In the present study we define the normal vertex distance as 12 mm, which provides us with a total distance from the center of eye rotation (ERC) to the back surface of the spectacle lens to be 13,5 mm + 12 mm = 25,5 mm. This standard will be used in the following analysis of the clinical data.
Study design
Hypothesis: Eyecode™ will provide an improvement in visual function and visual comfort for the spectacle user.
Study design:
1. 12 patients were selected amongst existing users of Essilor lenses in the following categories:
a. Varilux Comfort New Ed 4 patients Lens type A
b. Physio 2,0 F360 4 patients Lens type B
c. Physio 2,0 4 patients Lens type C
2. The patients were contacted from a list of patients generated from a patient database (Optik-IT - practice based database). Patients had to be using one of the lens types mentioned above, and were approached in a numerical order as generated by the Optic-IT database. In the event a patient did not which to participate, the patient next on the list was contacted. All patients were contacted by the same investigator (HB).
3. Inclusion criteria:
a. Must have been issued new spectacle lenses in one of the 3 categories within the past 6 months
b. Must participate on a voluntary basis
c. Must be able to attend the necessary visits to the clinic
d. Start of the study during week 26 where ERC measurements must be performed for lenses with Eyecode™
e. Fitting of new lenses in existing frame starting week 30
4. Measurements for Eyecode™ and ordering of lenses with Eyecode™. All measurements were performed by the same investigator (JJ Essilor)
5. Test of Eyecode™ design during approx. 2 weeks
6. Filling in questionnaire 1 (see appendix 5)
7. Exchange of Eyecode™ lenses back to the original lenses without Eyecode™
8. Filling in questionnaire 2 (see appendix 6)
9. Forced choice of preferred lenses between the two lens types with/without Eyecode™. Preferred lenses to be fitted and issued
10. Conclusive report
Results
Patients were recruited into the following three categories:
| a. Varilux Comfort New Ed | 3 patients |
| b. Physio 2,0 F360 | 4 patients |
| c. Physio 2,0 | 4 patients |
It was not possible to recruit all 4 patients in group a within the time limit.
All participants accepted the inclusion criteria.
Raw data for ERC are presented in table 1 & 2. Individual data are presented for right and left eyes. Most patients were hyperopes, which is not unusual for a presbyopic population. The group consisted of 7 hyperopes, 2 emmetropes and 2 myopes.
Most patients showed good harmony in ERC between right and left eyes, and only patient No 6, 7 and 10 deviated in their ERC between right and left eye of up to 0.9 mm. For analysis purposes the mean between right and left ERC was used.
Tab. 1: Clinical data for ERC.
Tab.2: Patient data for ERC
When switching from lenses without Eyecode™ to lenses manufactured on basis of Eyecode™, all participants answered questionnaire 1. It is noteworthy that all participants experienced the shift to something positive or unchanged compared to the original lens. None of them experienced a negative effect.
Tab. 3: Assessment of the change in lenses without Eyecode to the lenses with Eyecode design (Questionnaire 1)
In such clinical trials, there may be a relative high risk that the test persons automatically will believe that something new means an improvement. In order counter such an effect in the best possible way (if not blind study is used) is to ask the test persons to wait several days before completing the questionnaire, but answer it within 10-14 days. This reduces the immediate favorable effect of something being new, and helps in making the optical function focus of the assessment. In addition a cross-over test is applied, by switching the test lenses back to the original lenses. The results of this second phase are presented in the answers to the second questionnaire and are illustrated in Table 4.
Tab. 4: Assessment of the change in lenses from Eyecode design to lenses without Eyecode (Questionnaire 2)
Conclusion
The response given when changing back to lenses without Eyecode™ are almost unanimous in all areas. None of the test person experienced any advantage by changing back to the original lenses. All of them decided on using the lenses with Eyecode™ when asked which lenses they would prefer. Further all had the offer to keep the original lenses and have them fitted into a similar frame at only the cost of the frame, but none of them accepted this offer.
It is noteworthy that only two test persons experienced difficulties in converting back to the original lenses, where 8 of the test persons didn't experience ant difficulties. This may appear somewhat misleading when each question is analyzed separately. The answers in general gives the impression that the original lenses performed worse or indifferent to the Eyecode™ lenses.
It might be expected that the test persons demonstrating the largest deviation from the norm value of ERC would be those who showed the biggest advantage. However, this advantage was not seen to be exclusive to this group. Table 5 show the level of positive responses in all questions of questionnaire 1. We may observe a correlation between the level of deviation from norm ERC (25.5 mm) and the level of positive responses when changing to lenses with Eyecode™ technology.
Tab. 5: The noted advantage in relation to the level of deviation from norm ERC value.
At the cross over back to lenses without Eyecode™ design none of the test persons demonstrated a positive response. Table 6 shows that all test persons experienced a poorer visual function when returning to the original lenses. However, we cannot conclude any correlation between the test persons with the largest deviation also show the largest response.
Tab. 6: Negative response when changing back to the original lenses without Eyecode™
When analyzing all questions in table 3, and weighing each question equally, it may be observed how each test person judge the advantages when changing from the original lens design to lens design with EyeCode™ (table 7). Only 3 test persons valued the advantages to be less than 50%.
Tab. 7: Percentile improvement for each individual person when changing to lenses with Eyecode™
In the same way we may analyze all questions in questionnaire 2, when changing from EyeCode™ lenses back to the original lenses. This compiles all data in table 4, and the result may be seen in table 8. Likewise we observe that only 3 test persons judge the disadvantages to be less than 50% when changing back to the original lenses. The conclusion is that the vast majority of the test persons judge the EyeCode™ design to be the most advantageous (table 6).
Tab. 8: Percentile worsening for each individual person when changing back to lenses without Eyecode™
The ultimate choice between lens designs with or without EyeCode™ was decided in various ways. All test persons who answered this question declared that it was easy to change to the new design. It is of interest, though that a large group (8 out of 10) also mentioned that it was easy to revert back to the original design. When the test persons were asked to make at choice of which lens design they wanted to continue with after the test period, all of the test persons decided to use EyeCode™ designed lenses (table 9 & 10).
Table 9 & 10. The test persons final choice between lenses with or without Eyecode™.
Persons who already were used to more advanced lens designs (F-360) were those who appreciated the advantages of EyeCode™ the most. The less advanced designs like Varilux Comfort, and to a certain degree also Physio 2, also appreciated the EyeCode™ design although to a slightly lesser degree. Furthermore we may see that deviations of more than 1 mm in ERC from the norm of 25.5 mm, tend to make it even more appreciated that EyeCode™ design will improve the visual performance and comfort.
This pilot study may only provide an indication on the effect of EyeCode™ design. The small number of test persons limits the possibility of statistical analysis. It may, however, give an indication on the effect of using individual designs for persons who deviate more than 1 mm from the standard centre of rotation of the eye (ERC). This may be even more important and relevant to persons who have a high visual demand, and who already are very much aware of their choice of individually designed lenses such as F-360.
There is in this study exclusively focused on the clinical assessment available from randomly selected test persons. There is no attempt to explain how a sophisticated lens design as Eyecode™ is designed to compensate for individual variations in the eye's rotation center. The study and the findings must be assessed based on those practical clinical conditions which optometrists encounter in their everyday lives.
The demands for an optimal visual function are greater than ever. We are measured by the effectiveness and productivity in our workplaces, and the visual function is that of our senses which delivers by far the most information in our daily lives. Although there is a risk of too detailed conclusions, this study gives us a feeling that more individualized lens designs, such as EyeCode™, may be able to satisfy our visual needs to a higher degree. It is therefore recommendable that information on these newer individualized designs are given at least to those persons who may be considered the target group (+/- 1.0 mm deviation from std ERC). As a minimum future spectacle wearers should be informed about the new designs, in order that they may be able to make an informed decision.
References : [1, 2, 3]
Statement of independence The author of this report has a natural curiosity in trying to combine theoretical issues with clinical practice in order to provide the most advantageous vision correction to those who need them. With this in mind, Essilor Denmark asked the author to conduct a clinical evaluation on how spectacle lenses designed to compensate for deviating centers of rotation of the eye, are received by the end user.The author has no financial interest in the product, and the investigation was undertaken without any specific demands from Essilor. The conclusion and interpretation is that of the author alone.
References
Refer this article as: Bleshoy, H., Does the eye rotation center play a role in the choice of lens type?, Points de Vue, International Review of Ophthalmic Optics, N69, Autumn, 2013
