keratoconus4

I refer to article:

Intraocular lens calculations in atypical eyes by

Aazim A Siddiqui, and Uday Devgan

Intraocular Lens Calculations in Corneal Ectasia

The progressive thinning and steepening of the cornea in corneal ectasia also pose a challenge for accurate IOL power calculations. Although the most common form of corneal ectasia is keratoconus, it can also occur secondary to certain refractive procedures. Specifically, the corneal power values vary significantly in a small area of the cornea due to corneal irregularities. Further, changes to the ACD due to corneal irregularities also affect the calculation of the ELP by IOL formulae.

The suboptimal measurements of axial length and corneal power values and calculation of ELP are the basis of IOL calculation inaccuracies in ectatic corneas. Traditional keratometers and topographers are unable to accurately measure corneal power values due to the highly irregular and steeper nature of an ectatic cornea with its off-centered apex. These eyes also have longer axial lengths and deeper anterior chambers. Inaccurate measurements of these values make it harder to estimate an accurate ELP.

Standard biometers, keratometers, and topographers are unable to accurately measure the true corneal power values of ectatic corneas. In nonectatic corneas, these devices measure the true corneal power by directly measuring the anterior corneal curvature and estimating the posterior corneal curvature. To best measure the true corneal power values in ectatic eyes, direct measurement of the anterior and posterior corneal powers is needed. Corneal topographers are the gold standard and should be used for acquiring true corneal power values in ectatic corneas. These devices are capable of directly measuring the corneal curvature of both the anterior and posterior cornea. This method avoids the need to make assumptions for posterior corneal power when calculating true corneal power. The most commonly used devices are the Pentacam and Galilei.

There is a paucity of large studies that have considered which IOL formulae perform best in eyes with keratoconus. Traditional IOL formulae and keratometers are unable to account for corneal irregularities that are found in an eye with corneal ectasia. Therefore, it is recommended to use modern topographers which directly measure the anterior and posterior corneal power in combination modern IOL formulae. Newer generation formulae such as the Barrett Universal II, Ladas Super Formula, and the Hill-RBF are promising, but larger studies in ectatic corneas are needed.

The surgeon should generally use the lowest corneal power values in the central pupil zone as measured by topographers to err on the side of postoperative residual myopia. In addition, surgeons should always inform their patients about the minimal risk of corneal decompensation and worsening of corneal health in the aftermath of cataract surgery. These patients also often require a more frequent postoperative follow-up than nonkeratoconic patients.

MY COMMENTS:

Direct measurement of anterior and posterior corneal powers will not be of any use in finding a solution to this problem.

Newer generation formulae such as Barrett Universal ll, Ladas Super Formula, and the Hill - RBF will be of little use because they will always face the problem of anisometropia. What is required to solve this problem is an algorithm.

The problems that the 2 authors of this article will face with their theory are somewhat daunting .

Scientific ideas must not only be testable, but must actually be tested by many different people. This characteristic is at the heart of all science.

Performing such tests is important in science because in science only those ideas that are supported by tests are of any use. Theories that are not supported by testing are ultimately rejected.

What plans do the 2 authors have to verify their theory that the measurement of both the anterior and posterior powers of the cornea, and using either one of the newer generation formulae such as the Barrett Universal ll, Ladas Super Formula, or the Hill - RBF, will result in finding a solution to this problem?

One obvious problem that I can foresee for them is a scarcity of volunteers to take part in any of their tests. If no one is willing to act as a volunteer for their tests, then does this mean that their theory is untestable?

If their theory is incorrect, what happens to the poor, unfortunate volunteers who took part in the test? It is a very risky and virtually impossible task to replace incorrectly chosen lenses.

For more details see:

IOL Exchange: Principles and Practice, Cataract Surgery, August 2012.

So that any person volunteering for any test had better make certain that any theory that they volunteer for, is the correct one.

How will they measure a successful outcome of any test? Will the participants be expected to achieve emmetropia? If this is so, then this is an unattainable goal, because no one suffering from keratoconus is capable of achieving emmetropia. If, by some strange miracle, the volunteers do manage to achieve emmetropia, but suffer from anisometropia will the test still be regarded as a success?

Who will the first volunteer for their test be? It certainly will not be me.

Scientific ideas in the physical sciences can be tested countless times without harm to anyone. This is not so in the medical sciences. Therefore, tests in the medical sciences are usually carried out on animals, and in the case of medical tests on the eye, rabbits are usually chosen. The incidence of keratoconus in rabbits is unknown. So breeding rabbits with keratoconus may prove to be a somewhat difficult, as well as a costly enterprise. Even if such rabbits could be bred successfully, how would one be able to find out from a rabbit if a cataract operation was a success?

This means that if any tests are ever carried out, then they can only ever be carried out on real, live, human beings, not on animals. So that anyone proposing a theory, had better be very certain that their theory is correct.

It is easy for anyone to propose a theory in medical sciences. It is not so easy to prove that the theory is actually correct.

At the moment we have 2 competing theories. We have the theory as espoused by the 2 authors and supported by almost every ophthalmologist in the world. To date, I have not seen one ophthalmologist offer any criticism of this theory.

Then we have the theory as espoused by me. Only one theory can be correct. Which is the correct theory? Only time will tell.

FOOTNOTE

Prof Barrett, the creator of the Barrett Universal ll formula, is from Perth in the State of Western Australia, the same State that I am from, and he works at the Lions Eye Institute of Western Australia.

Prof Morgan, the inventor of the Xen Gel Stent device for glaucoma, is also from Perth in the State of Western Australia, and he also works at the Lions Eye Institute of Western Australia. He is also head of the Royal Perth Hospital, Department of Ophthalmology.

It is rather ironic to think that Prof Barrett, the creator of the Barrett Universal ll formula, Prof Morgan, the inventor of the Xen Gel Stent device for glaucoma, and me, the creator of the IOL algorithm for keratoconus, are all from Perth in the State of Western Australia, a backward State led by a backward Premier called Mark McGowan.

SOURCES

https://sites.google.com/view/keratoconus

https://sites.google.com/view/markmcgowan

https://sites.google.com/view/royalperthhospital

https://sites.google.com/view/lionseyeinstituteofwa

https://sites.google.com/view/cystoidmacularedema

ULIB- The keratometer index problem (by W. Haigis)

ocusoft.de>czm>texte>kprobl>kprobl

https://sites.google.com/view/dryeye

https://sites.google.com/view/cataractoperation

https://sites.google.com/view/xengelstent

https://sites.google.com/view/xen45