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 corrected with glass, higher order aberrations (HOA) cannot be corrected with simple patient prescription glasses. Nevertheless, HOA like spherical aberration, coma, trefoil, chromatic aberration, have not been studied or researched adequately until recently as the bulk of the ametropia is a cause of the lower order aberrations. With growing patient expectation, 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 can be the causative factor for most image degrading botherations in optics. Both spherical like chromatic aberration are on-axis aberrations of the eye, unlike coma aberration 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, 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.
Spherical aberration of the cornea happens when the marginal rays of the eye are over refracted than the paraxial rays or the rays that pass close to the optical axis. The measured value of spherical aberration for 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 positive spherical aberration than a spherical or a prolate cornea. This is because an oblate cornea will have the marginal rays over refracted while passing through a steeper peripheral 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.
Spherical aberration is measured in microns or RMS. The causative factor of spherical aberration is an underlying shape of the cornea or corneal asphericity, that is, whether the shape of cornea is prolate, spherical, or oblate. This shape of the cornea or corneal asphericity 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 +.27 microns may have a Q value of -.26. More the Q value turns towards zero or positive, the spherical aberration of the cornea increases and becomes positive. A highly oblate cornea may have a Q value of more than +1, thus resulting in a spherical aberration of more than +4.34 diopters(1).
On the other hand, a cornea can be highly prolate, that is the corneal asphericity or Q value is increasingly towards negative side. Thus a corneal asphericity or Q value of -1 may result in a negative spherical aberration of the cornea to the tune of -.48 diopters for a 5 mm pupil (1). The Q value and the corresponding average spherical aberration of cornea is provided in the table below:
Ideally, the patient's corneal asphericity should be measured before choosing an aspheric Intra Ocular Lens (IOL). IOLs could be classified into spherical IOLs ( like the AcrySof SP or SA60AT, or the Sensar Ar40e), negative spherical aberration lens that compensate for the corneal positive spherical aberration ( AcrySof IQ, Tecnis or Vivinex XY1 ), and truly aspheric lenses that do not add or negate to total ocular aberration of the eye. Since most patients would have a spherical aberration of approximately +.27 on the cornea, implanting a purly spherical IOL will add to the positive spherical aberratin of the cornea, thus increasing total ocular positive spherical aberration of eye. On the other hand, implanting an AcrySof IQ or Vivinex lens that has a negative spherical aberration in the lens, would result in negation of the positive spherical aberration of the cornea, thus reducing the total ocular positive spherical aberration of eye. Implanting a truly aspheric lens ( optic is spherical aberration free ) like the B&L enVista would neither add or negate the corneal positive spherical aberration, thus leaving the eye with the positive aberration of the cornea.
The choice of lens however should depend on the corneal asphericity, that is the Q factor or Shape Factor of the patient's cornea. Not every patient has corneal asphericity -.26 (Q factor) that would result in a classic +.27 micron of spherical aberratin of cornea. Some patients may have an hyper prolate cornea, that is, choosing a lens like AcrySof IQ, Vivinex, or Tecnis may add to the already very high negative spherical aberration of cornea. Patient's with Keratoconus are know to have a hyper prolate cornea. Another group of patients who may not benefit from a negative spherical aberration providing lens like AcrySof IQ, Tecnis, or Vivinex are patients who have gone for a hyperopic lasik correction. Such patients typically land up with a flattened corneal periphery, resulting in negative spherical aberration of cornea. To choose a negative spherical aberration IOL for such patients would mean we would add to the already negative spherical aberration of the cornea. Therefore such patients should benefit from a spherical IOL like the AcrySof SP (SA60AT), or atleast an aberration free IOL like the enVista which would not add to the already negative spherical aberration of the cornea.
Since multifocal IOLs can cause a degrading image quality due to more than one focal point, a high positive or negative Q value, that would cause a significant 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.
Corneal Asphericity and Spherical Aberration, Antonio Calossi, June 2007 Journal of refractive surgery (Thorofare, N.J.: 1995) 23(5):505-14