gms | German Medical Science

"Emmetropia" is not exactly defined. It is mostly understood as an optical situation in which the distant visual acuity cannot be improved by standard spectacles with sphero-cylindrical glasses. But this situation may be neither optimal nor stable already during the day, because rerraction depends on pupil width. In the presence of spherical aberration the best focus position is shifted with changing pupil diameter.

"Best focus" graphically means that the intensity of light has the highest value at that focal distance. The best focus cannot be calculated with Gaussian optics but only with raytracing. In the human eye the best focus can be approximately found in the following way. A ray has to be constructed that enters the eye parallel to the optical axis and passes through the pupil aperture at a distance of d=p/4 . y'2 rrom the optical axis, with p pupil diameter, provided that all optical surfaces are rotationally symmetrie and centered to the optical axis. This ray then intersects the optical axis in the best focus distance rrom the reference surface. G raphically, the form ula giyen above can be understood as a condition for a ray that divides the pupil area into an inner and an outer part of equal size.

In the presence of an astigmatism the best focus ray has to be calculated for both of the principal meridians separately. The arithmetic mean of the distances of the intersection points of these two rays with the optical axis from the reference surface for the focus position is a good approximation of the overall best focus distance. This best focus position changes with an increasing or decreasing of the entrance aperture, i.e. the pupil width in the human eye.

In most cases larger pupils cause a shift of the best focus towards myopia, a phenomenon which is commonly known as "night myopia". It is present also in most phakic eyes, but in pseudophakic eyes the myopie shift is mostly higher. But pupil diameter is not the only influence responsible for this focus shift in pseudophakic eyes. It additionally depends on many other optical parameters. Instead of separately describing the functional dependence rrom the IOL radii, a more comprehensive description is possible with the socalled "shape factor" of the IOL S = (Rl + R2)/(Rl–R2) with Rl, R2 :

IOL vertex radi!. The shape factor of the IOL has a high impact on the eye's total spherical aberration. Other parameters influencing spherical aberration are the asphericities of anterior and posterior corneal surface and of the IOL surfaces. In practice, the focus shift can be more than one diopter in a "quite normal" pseudophakic eye when the pupil width is only changed rrom 2mm to 4mm.

Another problem with "emmetropia" is that the optical situation may be not improvable with spectacle glasses, but nevertheless not optimal. Once again spherical aberration, but also a lot of other higher-order optical errors decrease the optical quality. This can objectively be quantified by the MTF (modulation transfer function) or measures ofthe local contrast, subjectively by the subject's viusal acuity or contrast sensitivity.

These higher-order optical errors decrease the optical in-focus quality, but on the other hand, they increase the so-called "depth of focus". In practice this means that visual objects can be recognized also out of focus, and then, they are better recognizable in the presence of higher-order errors than without these errors. This phenomenon can cause an important improvement of a subject's visual quality in many situations.

If distant emmetropia is required from a cataract patient, this requirement is often based on - in terms of evolution time scales - very old subjective feelings. Humans have developed as hunters, sometimes also as possible victims of other hunters. Optimum distant visual acuity and optimum stereopsis were selection criteria over millennia. However, is this condition nowadays really still the optimum for our visual requirements during most of the day? Most things we are interacting with during the day are much closer than 5m. This starts in the morning with chores of personal hygiene or with having breakfast, and continues with actions like reading wristwatches or using mobile phones. If we have discussions with other people in 1–3m distance or work on a computer in 40–80cm distance, different distances of sharp vision are desirable, and on average, distant emmetropia without accommodation may be disadvantageous compared to other accommodation-free visual situations.

If bifocal IOLs are not included into the discussion here because of their unavoidable contrast loss and halos, monovision is an alternative that allows a high degree of spectacle independence. If a refraction of -O.5D in one and -1.5D in the other eye is achieved, spectacles are only needed for car driving with higher velocities or for reading during a longer time period. Most activities during the day can be performed without spectacles, however, at the expense of slightly reduced visual acuity and stereoscopie vision. This reduction of stereoscopie vision, however, is in fact only relevant under very specific conditions, e.g. when playing tennis.

The intermediate visual acuity between the both foci of monovision can additionally be improved by an appropriate choice of spherical aberration. An eye implanted with an aspheric IOL correcting the eye's spherical aberration has best in-focus optical quality, but the intermediate range between the both foci of monovision is worsened. A standard spherical IOL can improve intermediate vision (however, at the cost of worse in-focus quality), but has additionally another negative effect: larger pupil width causes a myopie shift, in contrary to the physiological near-miosis.

A physiological behavior, however, can be achieved with an hyperprolate IOL. It changes the sign of the focus shift from spherical aberration, i.e. a myopie shift is achieved with miosis, and a hyperopic shift with mydriasis, just as physiologically required. However, one disadvantage of such IOLs should not be overlooked: compared to spherical IOLs or aspherical IOLs compensating the eye's spherical aberration, such hyperprolate IOLs have a higher sensitivity against misalignments like decentration and tilt. Therefore, high surgical precision of IOL implantation technique is required, particularly a weil centered capsulorhexis.