In order for ophthalmic surgeons to communicate their findings clearly and unambiguously, care must be taken to use words in a way that they have one precise meaning in given context. There are several terms that are frequently misused in published research at present, said Emanuel Rosen MD, FRCS, Manchester UK.

A case in point is the use of the word axis with reference to corneal astigmatism, he said. When testing an eye’s refraction with a phoropter lens, the term “axis” is correctly used in the two-dimensional sense of the axes of, for example, an ellipse, which has a steep meridian and a flat meridian. In the case of astigmatism with a steep meridian, for example, at 90 degrees the phoropter astigmatic correcting lens would be said to have an axis @180 degrees ie, its flat meridian would be at 90 degrees to neutralise the cornea’s steep meridian.

However, when referring to the astigmatic component of the cornea, the anatomical term meridian is correct, he noted. The cornea’s astigmatic component is defined in terms, therefore, of steep and flat meridia for the cornea in that sense has no axes. Astigmatic incisions on the cornea, therefore, need to be so defined for their purpose is to flatten the steep meridian and by corresponding coupling steepening the flat meridian.

“The cornea has no axes, only meridians ranging from zero to 180 degrees. The axes of the astigmatic testing lenses put in front of the eye to correct cylindrical refractive error are perpendicular to the steep meridian. Therefore, the use of the term axis to define where action should be taken on the cornea, for example, has to be in meridional terms not in terms of axes,” Dr Rosen said.


Axes and angles

The meridians of the eye correspond to the lines of longitude on a globe. The basis of the meridians is the measurements taken of the cornea’s front surface’s radius of curvature by means of any of a number of keratometry devices. One thing the instruments all have in common is that they require the patient to fixate on a target. Therefore, the meridians intersect at the line of sight, although there again the terminology can vary.

“To be honest most of us are not really quite clear about that. There is some confusion of terminology and also many of the terms have been used interchangeably,” said Oliver Findl MD, Hanusch Hospital, Vienna, Austria.

The line of sight is one of the eye’s many axes. Others include the optical axis, the pupillary axis, the visual axis and the fixation axis. However, some of those definitions are used differently in a purely theoretical sense than they are in a clinical setting.

“Actually it's not complicated per se, but it is confusing because different communities and different authors and different researchers and different clinicians have been using different names and different definitions, so it's really a bit of a mess. I recommend that people include a paragraph explaining their definitions when they are writing these papers and articles,” said Pablo Artal PhD, University of Murcia, Murcia, Spain.

He noted that the pupillary axis is a line perpendicular to the cornea that passes through the centre of the pupil. The line of sight is the line passing from the centre of the pupil to the object of regard. The visual axis is the line passing from the fovea through the nodal point, near the back of the crystalline lens, to the object of regard. The optical axis is defined as a line extending from the vertex of the cornea through the nodal point of the eye to the posterior pole of the eye, which is, in turn, defined by the geometric centres of the cornea and the lens.

The angle of the pupillary axis to the visual axis is the angle kappa, the angle from the pupillary axis to the line-of-sight is the angle lambda. However, because the line of sight is nearly identical to the visual axis, and because the eye’s nodal point cannot be determined with current technology, angle lambda tends to be referred to clinically as angle kappa.

For similar reasons, the optical axis is more of a theoretical concept which applies best to eye models where the refractive surfaces are centred with respect to each other, Dr Artal noted.

“The optical axis in an optical system is easy to understand as a line that is going through the centre of the curvature of all the refractive surfaces. However, in the case of the human eye, the surfaces of the cornea and the lens are not actually aligned. They are kind of decentred with a bit of tilt. Therefore, in terms of classical optics the eye doesn't really have an optical axis.

Since all of these different axes have their own set of meridians, the question arises as to which axis should be used to gauge IOL centration, tilt and in the case of toric IOLs, rotation. Dr Artal recommended using the pupillary axis for that purpose.

“In my opinion IOLs should be centred with relation to natural undilated pupil because I think that is how the best visual results can be obtained,” he said.


Emanuel Rosen:

Oliver Findl:

Pablo Artal:

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