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Monday / December 16.
HomemiequipmentGoodbye Gullstrand: Why Stop at the Vertex Sphere?

Goodbye Gullstrand: Why Stop at the Vertex Sphere?

Rodenstock has taken a decisive step forward, heralding a new era in the optimisation of lenses with DNEye PRO… Now we can finally say, “Goodbye Gullstrand!”

Figure 1. Schematic diagram of the eye with the parameters individually determined at the DNEye Scanner. The variables marked with * are derived from the measured variables.

Until recently, lenses were designed at the vertex sphere and what happened beyond that in the individual eye could not be considered. Instead Allvar Gullstrands’ schematic eye was used as the basis for these calculations.

Allvar Gullstrand received a Nobel prize for his work and, although the schematic eye was a major breakthrough at the time, advances in technology and tireless research since then means that 100 years on we can take a more advanced approach to lens design.

The drawback of using a schematic eye is that it is usually emmetropic, with fixed parameters such as axial length and spherical curves. The eyes we refract in practice do not match this model and so inaccuracies in calculation can occur. If we purely consider axial length, there is a variation of 10mm across the average of 24mm. Even with a specific amount of ammetropia, the eye length can vary enough to produce inaccuracies of design.

INDIVIDUAL ANATOMIC PARAMETERS

Rodenstock can now use the individually measured anatomic parameters of a spectacle wearer’s eye, rather than a reduced model, to place the image on the patient’s retina.

For this to happen, two challenges had to be overcome:

  • The individual data of the eye needed to be collected, and
  • A mathematical method needed to be developed to perform the complex calculations necessary.

Figure 2. When optimising and assessing the lens at the vertex sphere, the actual perception of the design on the retina can only be roughly calculated and generally deviates from the ideal design.

The first challenge was easily overcome with the Rodenstock DNEye Scanner 2. Aberrometry gives totals of lower order aberrations (LOAs) and higher order aberrations (HOAs) for the eye. Topography allows the totals to be divided between cornea and lens. Pachymetry gives anterior chamber depth. Axial length can be calculated from these parameters (Figure 1).

The second challenge required computation of light through the individual eye. To do this, Rodenstock takes advantage of its powerful, in house developed, wavefront calculation tool. This analytical approach uses closed formulas instead of complex numerical computation of innumerable individual ray traces. As a result, any number of complex wavefronts can be simulated. Additionally, their propagation through different media, and refraction at different surfaces, can be quickly and accurately calculated. This approach has found scientific recognition and use beyond ophthalmic optics, and has been used by Rodenstock to calculate lenses with the DNEye option since 2012.

In the past, this calculation tool was used to calculate the propagation of light through the lens to the vertex sphere and included compensations for cyl power and axis changes for near astigmatism and eye rotation for near. Rodenstock DNEye PRO now takes a further significant step – computation of the wavefronts no longer ends at the vertex sphere, but is continued to the eye, through its refracting surfaces to the retina. This means assessment no longer takes place at the vertex sphere, but at the retina. As a result, effects which arise due to the individual structure of the eye can be taken into consideration. Thanks to the procedure developed by Rodenstock, this calculation is not limited to sphere and cylinder – the complex shapes of the wavefront and the refracting surfaces, including higher order aberrations, are also considered.

Figure 3. With the optimisation and assessment of the lens on the retina using DNEye PRO technology, the visual fields can always be ideally mapped.

ADVANTAGE TO PATIENTS

Currently, the design of a lens dictates the positioning of aberration with respect to different areas of vision. In the past this dictated the hardness or softness of design but with digital surfacing so much more can be achieved. However, if the individual structures of the eye are not considered, then the design will not be accurately reproduced on the retina, causing loss of clarity.

If we consider a script of -3.00D then lack of clarity can be caused by axial length, crystalline lens power or a combination of both. Each scenario would require a different lens design to give optimal vision. Consideration for the refracting surfaces and individual eye dimensions mean that the design will be accurately produced for each patient.

DIAGNOSIS AND EDUCATION IN PRACTICE

The DNEye Scanner can be used as a valuable diagnostic tool with topography, pachymetry, aberrometry, tonometry, and pupil size readings. The accompanying Rodenstock Consulting program can be used to generate several reports incorporating the individual patient’s data. These can be used in discussions to help demonstrate the benefit of using DNEye PRO lenses to patients:

  • The measured and calculated data is summarised in a schematic diagram of the eye.
  • Maps of the aberrations and different scenes can be used to show how the lens is improved with consideration of the higher order aberration. The most powerful of these is the Point Spread Function (PSF) (Figure 4). This can be used to demonstrate the combined aberration effect on a point of light, the effect of purely LOAs, and purely HOAs. Using this report, the patient can be shown the effect of HOAs that have not previously been compensated for, and how compensation will improve their vision.
POSITIVE PATIENT FEEDBACK

In an internal development related study, 20 subjects were given two pairs of spectacles: Impression FreeSign 3 with DNEye (compensating solely for HOAs without using the individual eye data) and Impression FreeSign 3 with DNEye PRO. Both objective tests and subjective examinations were performed. For the summary statement “I really enjoyed wearing these spectacles”, the comparative lenses with the previous DNEye did very well with an average of 8.5 out of ten possible points. The lenses with DNEye PRO, however, outdid this excellent result. With an average of 9.6 points the lenses calculated with the new technology achieved statistically significant better values. This confirms that the new DNEye PRO technology is well received by spectacle wearers.

Figure 4. PSF shows the image of a point of light seen with total aberration and HOA.

EFFECT ON LENS DISPENSING

Practices that have embraced this technology have seen an increase in both foot traffic, patient satisfaction, and revenue. Word of mouth recommendations have been exceptional, leading to patients travelling long distances to have a scan performed. Importantly, practices have reported that price becomes far less relevant as the focus remains on delivering best visual solutions, which can now be clearly demonstrated using this device.

HOW TO ORDER LENSES

DNEye PRO is available as an order option for progressive, near comfort, road, sport, and single vision lenses in the categories Rodenstock Perfection and Rodenstock Excellence.

Once the patient’s measurement and the subsequent subjective refraction are completed, all results can be merged quickly and easily in Rodenstock Consulting for electronic ordering. There, the optimised DNEye PRO order values for distance and near vision are calculated and can be displayed before ordering.

HISTORIC ACHIEVEMENT

A lot has changed since Gullstrand made his major breakthrough, and for over 100 years we have benefited greatly from his work. Technology developments have allowed for a greater range of measurements that can be taken, and with more measurements that can be incorporated into the lens, a customised result can be achieved.

Now, using Rodenstock DNEye PRO, the image of the lens in the individual eye of the spectacle wearer can be optimised for the first time in the history of ophthalmic optics. As a result, visual fields can always be ideally mapped on the retina for guaranteed pin-sharp vision. Additionally, optical dispensers and optometrists have at their disposal, a unique tool with communicable and practical added value. This added value can strengthen their competence and distinguish them as an eye care professional with the expertise to deliver perfect vision.

Nicola Peaper BSc (Hons) Ophthalmic Optics, Cert 4 TAE, qualified as an optometrist in the UK in 1985 and practiced in private and corporate practice in the UK for 20 years. In 2001/2 she was employed as ophthalmic advisor to Kensington, Chelsea & Westminster Health Authority. 

Since moving to Australia in 2005, she has worked in fitting laboratories advising on procedures and quality. In roles as state and national training manager, she has gained extensive experience in presenting the technology behind, and the prescribing and fitting of, ophthalmic lenses. 

Nicola Peaper is currently Professional Services Manager for Rodenstock Australia. 

Sources 

Dr Stephan Trumm, Katrin Nicke, Daria Evdokimova, and Dr Wolfgang Becken. Goodbye Gullstrand! or Why stop at the vertex sphere.