#7 of 10: Top Reasons for Poor Premium IOL Outcomes and How to Remedy Them: Managing IOL Calculations

A 10 year study conducted by OMIC revealed that IOL power calculations represent the single largest malpractice risk to a practicing ophthalmologist.¹ In the most recent series on managing the ocular surface, the PHACO study² was revisited in that poor data acquisition in terms of keratometry, corneal topography, and/or biometry often leads to IOL power calculation errors of up to 1-2 diopters.

Assuming the patient has a pristine and/or stable ocular surface, the emphasis should then be on the actual biometry device utilized and IOL calculation software chosen.  To make matters even more complicated, if the patient has had previous refractive surgery and IOL, calculation decisions become more challenging.

With rising patient expectations, especially in the premium IOL world, IOL calculations need to be exact or the added costs of piggybacking IOLs, laser vision correction, and/or limbal relaxing incisions as enhancement options will become more apparent.

There are general principles in IOL calculation that must be avoided: if axial length is measured too short, the patient will have a myopic surprise and, if measured too long, a hyperopic surprise.  In a normal 24 mm or so eye, for every 1 mm error in axial length (AL) measurement, there is typically a 2.5-3.0 diopter error in IOL power.  In shorter eyes, less than 22 mm, there can be up to a 7.5 diopter error in IOL power for every mm error in AL measured.³  Keratometry error of 1 diopter in curvature readings can mean an additional error of 1 diopter in IOL power.

To carry out successful cataract surgery with the proper IOL power, four variables are critical to obtaining the premium outcome: axial length, keratometry, effective lens position (ELP), and desired postoperative refraction.  The ELP is the effective position of the IOL (the principal plane of the thin lens) relative to the anterior corneal vertex, which is different from the old term, anterior chamber depth (ACD), which ignores central corneal thickness.

Other variables such as lens thickness (LT) and horizontal white to white corneal diameter measurement (HWTW) are additional variables in newer generation IOL formulas, such as Holladay II, to better predict ELP and master the intended targeted refraction postop.  Devices such as the newest IOLMaster 500 (Carl Zeiss) finally implement Holladay II software, especially helpful for short and long eyes.

The challenge with longer eyes are myopic staphyloma, and devices that utilize optical coherence tomography (OCT), such as the IOLMaster 500 or LenStar, will more accurately measure axial length than ultrasound in these difficult situations.  Overall, accurate measurements of keratometry, axial length, ACD, HWTW, LT are critical to obtaining the correct IOL power.

In post refractive surgery situations, the challenges and expectations from patients are even greater.  Conventional keratometry and topography measurements of the cornea in these patients are inaccurate: in myopic LASIK/PRK, the anterior corneal curvature is flatter than the posterior curvature, often resulting in an IOL power underestimation, or hyperopic surprise postoperatively. 

In hyperopic LASIK/PRK, the reverse is true due to the anterior corneal curvature being steeper than its posterior curvature.  In RK patients, both the anterior and posterior corneal curvatures are flatter due to the peripheral weakening in the cornea, and typically a hyperopic surprise occurs in these patients as well.

There are several methods available for IOL power determination in post refractive surgery patients.  The clinical history method (CHM) basically subtracts the patient’s surgically induced refractive change from the pre-refractive surgery keratometry reading to determine current corneal power utilizing the general formula [K = Pre-RS K + (Pre-RS SE – Post-RS SE)].  The drawbacks with this formula are the need to obtain preop refractive surgery data if still available and the potential inaccuracy of the postop refraction due to a myopic shift from the cataract.

The contact lens method (CLM) utilizes a rigid gas permeable contact lens to perform an over-refraction using the general formula [K=BC + D + (ORcl – MRSEnocl)]. The major drawback of this method is it does not compensate for the change in the back-to-front surface ratio and it requires additional chair time.  Both of these methods are available at the ascrs.org website for inputting information as well as in the IOLMaster 500 software template for post refractive surgery.

The Masket regression method⁴ utilizes the formula K= LSE x (-0.326) + 0.101 where LSE is the excimer laser spherical equivalent treatment applied.  With this formula there is approximately 1 diopter adjustment for every 3 diopters of excimer laser treatment applied.  The formula I use consistently now without any additional chair time or need to obtain past data is the Haigis-L formula as part of the IOLMaster 500 program.  The Haigis-L formula avoids using corneal power readings to determine the effective ELP.

On the IOLMaster 500, the only criterion the surgeon must select is whether or not the patient had previous myopic or hyperopic laser vision correction.  The only drawback with the Haigis-L formula is it cannot be used for previous RK patients.  For RK patients, I still use the ascrs.org website and utilize my APP corneal power reading from my OPDIII (Marco Ophthalmics) and look at angle kappa and spherical aberration data from the same system to help me select the correct IOL power and premium IOL type for these patients.

One last evolving technology especially helpful in post-refractive surgery cataracts is intraoperative aberrometry (Wavetec ORA) to measure for correct IOL power on the OR table itself.  On average, it only takes an additional 2-3 minutes of operating time to ensure the precise IOL power.

In the end, managing IOL calculations is critical in obtaining the premium result in a premium patient. With patient expectations ever-growing, especially in the post refractive surgery subpopulation of cataract patients, IOL power and hitting the correct targeted refraction is mandatory.

Stay tuned for my next remedy on how to handle the unsatisfied patient who has undergone a premium IOL surgery.

¹10 year OMIC study. Survey of Ophthalmology 43:356-360, 1999.

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