A simple understanding of aberration of the eye is that it can be classified into lower and higher order aberrations. While lower order aberrations are correctable with glasses, higher order aberrations (HOA) cannot be corrected with simple patient prescription glasses. Nevertheless, HOA has not been studied or researched adequately in the history of Ophthalmic science until recently as the bulk of the ametropia is a cause of the lower order aberrations. With growing patient expectations, the need for correcting HOA, has resonated more deeply in Ophthalmic fraternity, as the patient lifestyles have been more demanding and they are no longer happy with quantity, but also look forward to quality of vision.
One such HOA is the spherical aberration of the eye. Frits Zernike’s contribution to the aberration of the eye shows us that the spherical aberration ( together with Chromatic aberration ) can be the causative factor for most image degrading botherations in optics. Both spherical and chromatic aberration are on-axis aberrations of the eye, unlike coma, trefoil, or other HOA of the eye. What it means in effect is that, if the human eye was rotationally symmetric on its axis, that is, a perfect eye with zero human lens tilt or a cornea perfectly aligned to the visual axis ( angle alpha = Zero), the eye may still have spherical aberration. In this rare eyes, though coma, trefoil and other forms of HOA may be negligible or nil, some amount of spherical aberration is bound to happen . Hence, spherical aberration and chromatic aberration in optics are on- axis aberration.
We all understand that spherical aberration of the cornea happens when the marginal rays of the eye are over refracted than the para-axial rays or the rays that pass relatively through the center of the eye. The measured value of an average human cornea is said to be around positive .275 microns. This however is balanced by the human lens which has largely negative spherical aberration in its relaxed state. However the sign and amount of spherical aberration for the natural lens with accomodation is debatable and has a large variation in eyes.
The average human cornea is prolate in shape, that is, it is steeper in the middle and flatter in the periphery. However there are some who may have a more spherical cornea, or worse, more oblate cornea. In an oblate cornea, the cornea is flatter in the center and steeper in the periphery. An oblate cornea would have more spherical aberration than a spherical or a prolate cornea. A perfectly prolate cornea would infact have no spherical aberration as the curvature towards the periphery of the cornea would balance any over refraction. Though the average cornea is prolate, yet some amount of positive spherical aberration is observed in the eye with an average value of +.275 microns.
Hence while spherical aberration is measured in microns or RMS, the causative factor is an underlying shape of the cornea, that is, the prolate, spherical, or oblate shape of the cornea. This shape of the cornea can be defined in definite numbers, usually called as Q value. Other forms of expression are the Shape Factor or SF, or eccentricity of eye. An average prolate cornea with a spherical aberration of positive .275 microns may have a Q value of -.26. More the Q value turns towards zero or positive, the spherical aberration of the cornea increases. A highly oblate cornea may have a Q value of more than +.1.
Since multifocal IOLs can cause a degrading image quality due to more than one focal point, a high Q value ( that is a very high spherical aberration of cornea ) is a contraindication for multifocal IOLs. It is strongly advised, that clinics assess the Q factor/ Shape Factor/ or eccentricity of the eye to determine if the patient is a candidate for multifocal IOLs.
Where can we find the Q value/or shape factor or eccentricity value of the eye ? Corneal Topography. While not all gives, but there are many which gives this value. Specially tomography or scheimflug based instruments.
Lastly, we all know the image degrading effect of spherical aberration as the pupil dilates. But to what extent. This again leaves a scope of debate. But in one such paper, Koch showed the spherical aberration in an average eye may be increasing from .15 diopter at 3 mm pupil to .35 diopter over a 4.5mm pupil. With younger patients who have a better dilation of pupil than older patients, the effect of spherical aberration may be challenging.
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