IC-89: PRESBY-LASIK and Q Factor
The advantages of modifying corneal asphericity
Dr Jérôme Blondel
Nice, Genève
France, Suisse
No financial advantage.
No presentation at ESCRS congress.
Introduction: There is a considerable market corresponding to surgery for
presbyopia. The technique must be easy, reproducible, safe and if possible
reparable, because presbyopia is a progressive condition. For many years, various
corneal approaches involving the LASIK have been attempted (Dr Chaubard, Dr
Ghenassia). The mechanism of accommodation was first proposed by Helmholtz [1]
at the beginning of the 20th century. Preceded by convergence, macular blurring
relaxes the zonular muscles, increasing the curvature of the anterior lens and
modifying the refractive index of the cortical fibers. This happens earlier if there is an
existing uncorrected hyperopia, and presbyopia (the gradual loss of accommodation)
becomes symptomatic at around the age of 40 years. Like we can’t stimulate the lens
work, our aim is to change the shape of the cornea in such a way that the newly
created aberrations can interact with those of a lens that is progressively losing its
accommodation so that near sight becomes possible without losing far sight. At
present, we propose this treatment to hyperopic patients and myopic patients and to
some emmetropic patients.
Patients and methods: We propose to create a hyperprolate cornea (Dr C.
Ghenassia) that will produce negative spherical aberrations (i.e. the incident marginal
beam converges at a focus located behind that of the para-axial incident beam). The
lens that is gradually losing its accommodating potential limits the changes induced
by the cornea that has been modified. The Wavelight Allegretto* laser (Wavelight*,
Alcon*) can be used to shift the corneal asphericity as far as a value of -1. We use a
target value of -0.8 for hyperopic patients and of -1 for myopic patients. However,
shifting the corneal asphericity towards hyperprolacity leads to under-correction for
hyperopic patients and to over-correction for myopic patients.
After refraction with blurring, an orthoptic assessment (dominant eye, test for
microtropia) and a slit-lamp exam, a corneal topography using the Pentacam*
(Oculus*) and an investigation of the visual quality and residual accommodation
(OQAS*, Optical Quality Analysis System, Visiometrics*) are carried out in order to
confirm what surgery is possible. The OQAS* was developed by P. Artal and his
team [8]. It involves a double-pass aberrometer and is based on investigation of the
PSF (Point Spread Function), and reports not only the high grade aberrations, but
also very high grade ones, the diffusion, determined by anomalies in the
transparency of the media (cornea, lens, vitreous). This results in refraction, visual
quality (MTF, Modulation Transfer Function), and also in simulated accommodation,
which has in fact been artificially stimulated (defocalization curve and investigation of
the change in PSF).
The FS 60* femtosecond laser (Intralase*, AMO*) is used to produce a corneal flap.
We use a Allegretto* 200Hz laser. For a target asphericity level of -0.8, an over-
correction is applied to both eyes so that a slight predominant myopia in the
dominated eye is produced immediately after surgery. For a target asphericity level of
-1, an under-correction is applied to both eyes so the emmetropic refraction is
obtained after surgery. Conventional post-operative treatment with corticosteroidantibiotic drops and a wetting agent is administered for ten days. For hyperopic
patients, near and intermediate sight is recovered within a few hours after the usual
period of post-operative discomfort. Longer sight is recovered more slowly, and the
patients are warned, particularly if they often drive, that they may need temporary
optical correction to be able to do this, and that this will be prescribed if required.
Hyperopic patients often have imperfect binocular sight, and orthoptic rehabilitation
can often be very useful after the operation. Patients are also warned that they may
need surgical adjustment later, particularly as a result of progression of their
presbyopia. For myopic patients, the near vision is more difficult than without glasses
before surgery but they can read with light.
Discussion: The cornea has an aspherical shape (Q-value), the curvature of the
corneal meridians changes from the center out towards the edge. Prolate in most
cases, this curvature decreases from the center towards the edge of the cornea, and
the Q value ranges from -1 to 0. As a result, incident peripheral light beams converge
(in the case of those that manage to get through the pupil) slightly in front of the paraaxial beams. This discrepancy constitutes the positive spherical aberration [Gatinel,
2]. During accommodation, P. Artal [3] has shown that there is an increase in the
negative spherical aberration. By studying the changes in high grade optical
aberrations during accommodation, Ninomya et al. [4] have also shown that only
aberrations affecting spherical aberrations exhibit significant modifications, with a
trend towards negativity. The change in the curvature of the lens during
accommodation tends to produce negative spherical aberrations that offset the
positive spherical aberrations induced by the cornea. Glasser [5] has shown that
during aging there is an increase in positive spherical aberrations. Radhakrishnan et
al. [6] found a significant increase in positive spherical aberrations with age, without
any change in the other high grade aberrations.
We provide the aberrometric image of the post-operative spherical aberration after 12
months (Figures 1 and 2). This hyperopic patient has a binocular visual acuity of
10/10P2. The dominant eye displays a very slightly positive spherical aberration, and
the dominated eye a negative spherical aberration. We think that these results
explain the good visual acuity obtained for both near and far sight, and its lasting
efficacy. Furthermore, the variation of spherical aberration (Z400) modify defocus
(Z200) when pupil size decrease in accommodation-convergence reflex [7].
The OQAS* (Optical Quality Analysis System, Visiometrics*), developed by P. Artal
and his team [8], carried out post-operatively confirms that accommodation had been
restored, and that it was of corneal origin, even though we believe that the
simultaneous stimulation of sight could induce a new capacity for accommodation by
the lens. Investigation of the simulated accommodation provided by the OQAS* for
this same 62-year old patient, yielded a value of about 2 diopters for the dominant
eye, and of nearly 3 diopters for the non dominant eye (OQAS 1 and 2). The nonlinear curve reflects the effort made by the patient to accommodate in order to
follow the test during the defocalization curve. Hyperopic patients and myopic
patients could benefit from this operation from 50 years of age and the upper age
limit depends on whether there is a cataract or not. Changing the corneal asphericity
using the excimer laser after carrying out the corneal flap with the femtosecond laser
looks to us to be potentially less iatrogenic than clear corneal lens surgery.
Conclusions: Using the LASIK to modify corneal asphericity is a simple,
reproducible, and safe technique for treating presbyopia, principally in hyperopics
patients and myopics patients, but also in some emmetropics. Prolonged follow-up is
necessary in order to determine the stability over time of the protocols for modifying
corneal asphericity.
Figure 1 : dominant eye
Figure 2: Non Dominant eye
OQAS 1: Dominant eye
OQAS 2: Non Dominant eye
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définitions et conséquences sur la fonction visuelle. In Gatinel D, Hoang-Xuan T :
« Le Lasik de la théorie à la pratique », Elsevier 2003 ; 151-159.
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