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					                     SAFIR 2007

    MONOFOCAL ASPHERIC IOLs




      Study of the Visual Performance
                Of Aspheric IOLs

            Jean-Pierre Rozenbaum
Paris XV XX (Prof. Nordmann’s department) / Centre Eiffel

                  Etienne Hachet
                         Nancy
      Assessment of the benefits offered by
          aspheric IOL (IOL A) requires:
• Evaluation of their performance and comparison of
   this performance with that of spherical implants.
     • An understanding of the various designs.
The search for vision quality improvements is a shared
  objective in both refractive and cataract surgery.


  The first photoablative profiles provided good VA
        under conditions of maximum contrast,
but often created halos in dim light and occasionally a
       significant drop in VA for mesopic vision.
The problem identified for these profiles is that they
 should have incorporated Zernicke polynomials
                  and the HOA.


   Accordingly, aspheric profiles have now been
 developed that eliminate these visual disturbances
                   in most cases.
In cataract surgery the approach is the same, but
               there is a difference:
   patients are always pleased to recover good
   VA and the main concern is to obtain good
             post-operative refraction.
  However, with implant calculation becoming
 increasingly accurate, and with the intervention
   itself causing less and less astigmatism, the
problem is how to determine which patient, out of
two who have the same refraction, will ultimately
           have the best visual envelope.
                           Visual envelope.


    • This concept defines how the eye perceive shapes and
       details under the various conditions of contrast and
                           luminance.


•      The underlying principles are the analysis of contrast
                       vision and MTF.


        (The measurement of the standard VA at high contrast forms part of the
                             determination of this envelope)
•The MTF is studied using an aberrometer.
It does not consider retinal and cerebral integration that can
distort the analysis.
•It thus offers an objective means of evaluating IOL.
            How does MTF measurement work?


• The aberrometric measurement determines the PSF.
                                 Spread function

• The PSF is used to produce a convoluted image.

        E
The MTF is used to define the visual envelope.
The perception threshold will be studied for each spatial
frequency.
Linking the various thresholds produces a curve.




Low contrast          Not seen NS           NS


                     Seen Seen      Not seen


 High contrast        Seen      Seen Seen         Not seen


               Large optotype               Small optotype
                                                             JP Rozenbaum
        Spatial frequency (cpd)
To summarise:

- if the image projected onto the retina is kept sharp,
- then the eye is better able to identify it when its size is
reduced (by distance) or when the contrast conditions
are less favourable.
Results
               Requirements specification.
•The SA induced by the cornea are on average +0.28 µm.
•The overall SA for the eye of a young subject is +0.10 µm.
•An excess of +SA has an adverse effect.
•A certain degree of +SA improves the depth of field.
                      RESULTS:


• During SAFIR 2005/2006 and ESCRS:
We presented aberrometric studies on the Tecnis AMO and
then on the Sofport AO B&L.
They suggested that the IOL A produced better results
compared with their spherical equivalents.
•Today, we shall present the results of a multi-centre
trial of the very latest IOL A:
                XL Stabi ZO Zeiss Ioltech
                 XL Stabi ZO Zeiss Ioltech:
                    aspheric posterior profile.
    What differentiates this IOL A from other IOL A is its
  geometry - calculated from a physiological ocular globe
      model and not simply from a corneal model.
        This has made it possible to validate its design
                    using MTF curves.


A specific calculation is performed for each implant power.
          Study of the first 200 cases of
           XL Stabi ZO Zeiss Ioltech

 No case of dysphotopsia
 No case of a significant error in implant power
 No astigmatism that could not be linked to corneal
  curvature.
        MFT results for XL Stabi ZO
              Zeiss Ioltech
                                                                         4 centres
                 Spherical aberration MTF

       1
                                         Average SA - Control group
                                         Average SA - All ZO lenses
                                         Diffraction limit
                                                                         47 eyes
      0,8
                                                                       Control group:
      0,6
                                                                        young phakic
MTF




      0,4
                                                                        subjects
      0,2


       0
                                                                       Mesopic with no
            0   20          40            60
                 Spacial frequency (cycles/degree)
                                                            80
                                                                        pharmacological
                                                                        dilation
DISCUSSION
Do the aspheric IOLs offer an improvement?
The various studies revealed an improvement in vision
quality for the various IOL A compared with their IOL S
equivalents. Moreover, the IOL S are highly sensitive to tilt.
Can they cause dysphotopsia?
No dysphotopsia was found for the 500 IOLs of different
types fitted.
Only the Tecnis lens, which is extremely prolate, shows a
certain sensitivity to decentration and tilt according to one
of the designers, Holladay, and confirmed by Altmann.
What effects do the various aspheric implants induce?
 1) PROLATE
Generate negative SA.
AMO Tecnis: 1st generation IOL A generates -SA of –0.28 µm
Alcon IQ: generates -SA evaluated at –0.20 µm
e.g.: If corneal SA = + 0.15 µm: Tecnis: -0.13 µm;   IQ: -0.05 µm


 2) NON PROLATE
B&L Ab Free: cancels out the +SA that a comparable IOL S might
  have generated. The resulting SA corresponds to the SA of the
  cornea.
In comparison with the IOL A based on a corneal model, the
design of the XL Stabi ZO Zeiss Ioltech appears to be very
interesting. It is based on a physiological ocular model, and
its design integrates pupil offset and the offset of the visual
axis.
Its geometry is based on MTF curves that provide an
objective means of measuring the visual envelope.
The clinical results are good and have been confirmed by
MTF curves, measured using the WASCA aberrometer
(Zeiss), in a study that is on-going.
Are there any particular indications?
The first generations of IOL A , based on a corneal model
with an average SA = +0.27µm, made it possible to define
the indications and contraindications as a function of the
patient’s cornea.
The design of the XL Stabi ZO implant lens is based on a
physiological ocular model and on the MTF. The objective
is to produce a multi-purpose implant, that is less sensitive
to offset and tilt than a spherical implant.
                         Conclusion
The MTF provides an objective means of evaluating IOL and
is used to evaluate the visual envelope.
The aspheric IOL is an evolution that has been developed by
all the laboratories. It is the product of the shared desire to
deliver improved vision quality to match the achievements in
refractive surgery.
The XL Stabi ZO implant lens is an interesting design whose
good initial results have been confirmed by these first studies.
                         Bibliography
1.   G.E Altmann: Optical perform.of 3 IOL designs in the
     presence of decentration: JCRS 31, March 2005
2.   J.T Holladay: a new IOL design to reduce spherical
     aberration of pseudophakic eyes JRS, Nov. 2002.
3.   R.M. Kershner: Retinal image contrast and functional
     visual performance with aspheric, silicone, and acrylic
     IOL. JCRS 29, Sept. 2003
4.   R.Bellucci: comparison of W.front ab.and optical quality
     of eyes implanted with 5 different IOL: JRS 20 Jul. 2004
5.   S. Marcos: Aberrations and visual performance
     following standard laser vision correction. J Refract Surg
     2001;17:596-601
6.   D. Gatinel: Visions. Surgical Insights W/S 2007:31-38
7.   C. Corbe, Fajnkuchen …
8.   L. Wang: High-order aberrations from the internal optics
     of the eye: JCRS 31, August 2005

				
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posted:8/14/2011
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