A common concern for optometrists and doctors across is that what would be the IOL formula of choice for a patient turning up for cataract surgery with a lasik surgery done years ago, and with no history of the laser treatment. This is one of the most common question I have come across while conducting Biometry workshops in many a place.
The concern here is well justified since none of the traditional IOL formula would work for a patient whose anterior posterior corneal curvature values are altered and no longer represent the normal range of 82.5 % ratio. Without taking into account this change, the traditional formula would not get the Effective Lens Position (ELP ) right, post surgery.
However, is this the only concern with such patients ? Are there any other considerations that we may have to be aware of with regard to the biometry of the post refractive surgery patient. Add to this, it is not just a concern with post refractive surgery patients only. Patients with Keratoconus, or with patients having bio mechanical changes within the central 3 to 4 mm of the cornea, would have a significant amount of anterior to posterior corneal ratio altered. That is, all these category of patients will have the principal planes of refraction of the cornea changed. To address this issue, it is no longer a question of the right biometry formula, but the right Keratometry reading, one that reduce the simplified assumptions of post refractive, Keratoconus, or biomechanically compromised cornea patients. Since modern Keratometers ( auto refraction based Keratometers or optical biometers, or placido based simulated Keratometers ) only work on the anterior corneal measurements, the corneal readings may itself be far from satisfactory to be fed in any post Lasik IOL calculation formula.
What would be the right process to measure Keratometry reading. To answer this we need to have a clear understanding of Keratometers and Topographers. While a detailed discussion is beyond the scope of this short article, an elementary understanding of corneal measurement is important to understand the option at hand for such patients.
Corneal Readings from Auto Keratometers or Optical Biometry or Topography based Simulated K(simK) - The readings here is taken of the anterior corneal curvature only and a ring ( not a zone ) of approximately 3 mm is measured. No information is derived of the cornea in the center of the ring, something which is of crucial importance for myopic Lasik surgery as the flattest portion of these corneas may be within the central 1 to 1.5 mm of the cornea. Without this understanding, the IOL power derived may have too low a power resulting in hyperopic surprise. Add to this, these Keratometry readings are derived from a Refractive Index of 1.3375, which is not the true refractive index of the cornea, but at best a fudge factor to compensate the weakness of not measuring the posterior cornea. That is, since these Keratometers do not measure the posterior cornea, they assume that RI of 1.3375 would be appropriate to compensate for the posterior corneal curvature. The true corneal index is 1.336. The index of 1.3375 works as long as the anterior to posterior cornea ration is within the normal bounds. For patients with Lasik, or Keratoconus,etc, this may not work.
The simulated Keratometry reading from the topographer, is a measurement of the anterior cornea with many topographers taking the central pupil diameter as a reference for the zone of measurement. However, as described above with auto Keratometers or optical biometry, no information is derived from the posterior cornea. The IOL Master 700 has however an option of measuring posterior cornea, however the challenge of measuring a ring and not a zone with Keratometers still stands true with this machine.
The Holladay Equivalent K Reading or the EKR - Having described the challenge above let us look into the alternative at hand. The EKR comes with the Pentacam which is a Scheimflug based topographer ( tomography ). In the EKR mode, the true indices of the cornea and the aqueous is used. That is, in the EKR mode, as light passes through the air and enters the anterior cornea passing the tear film, and thereby leaves refracted passing through the posterior cornea to the aqueous, the true index of refraction of each medium is taken into consideration. It uses Snell's Law of Refraction to account for the spherical aberration that adds to the positive power of cornea at each of the points when you move from the center to the periphery. This is not accounted by Keratometers or simulated Ks from Saggital or axial maps.
That was the first step with EKR. As the true net power of the cornea is derived by applying the corneal indices for both anterior and posterior cornea, it now back calculates this net power of cornea to what would have been the anterior corneal values with a refractive index of 1.3375 that is used by Keratometers. Remember, this is the index of refraction used by IOL calculation formulas and hence the Holladay EKR can be used with any non post refractive IOL calculation formula. That is, you can now use this corneal reading derived from EKR with the Holladay I or II, or SRK T, or Hoffer Q IOL calculation formula. What should be your formula of choice among the IOL calculation formulas, is beyond the scope of this write up.
True Net Power of the Cornea ( TNP ) - This is measured by Scheimflug based devices and uses the original corneal refractive indices together with the measurement of both anteiror and posterior corneal curvature. Since the true corneal refractive indices are used, you cannot use this measurement with IOL calculation formulas like Holladay II or SRK T or Hoffer Q, as they are all based on the refractive indices of 1.3375. Add to this, sine it derives the net power of the cornea, if you key this value to these formulas, it would lead to double correction as all IOL formulas assume the anterior to posterior ratio as 82.5%.
In a different context, let me state that TNP does not account for the spherical aberration of cornea and therefore cannot be totally be relied for peripheral corneal power.
Total Corneal Refractive Power Map ( TCRP ) - If you are interested in calculating corneal readings for simple IOL calculation, you need not look beyond EKR. The TCRP is however an accurate measurement of the True Refractive Power of cornea as it accounts for not only the correct corneal indices of refraction, but also account for spherical aberration, that is the change in corneal power as you move from center to periphery of the cornea despite the cornea having a regular curvature.
The TCRP Map uses ray tracing to calculate power . The True Net Power described earlier assumes that all rays passing through the cornea and emerging in aqueous are running parallel. In TCRP parallel light beams are transmitted through the cornea and focal length resulting from their refraction at the anterior and posterior cornea is calculated using Snell' Law of refraction. This refraction is based on true indices of air, cornea and the aqueous. In this way the ray tracing simulation takes into account principle planes of refraction of both sides of cornea. From this focal length one can then calculate the power at each point in cornea in diopters.
Thus TCRP takes into account - True Indices of Cornea and aqueous , refractive effect resulting from Spherical Aberration and anterior to posterior corneal curvature ratio. Needless to say, the TCRP map should show higher power towards the periphery due to spherical aberration.
However, unlike the EKR, you cannot use this into IOL calculation formula, again like TRP it uses true corneal indices of refraction and not the indices of refraction used by IOL calculatio of formulas.
Use the EKR from Holladay to account for anterior to posterior corneal changes for post Myopic Lasik, Keratoconus, etc patients and then use the values to your daily IOL calculation formula