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Hitting the post-refractive surgery IOL power bull’s-eye
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A 10-year study conducted by OMIC revealed that IOL power calculations represent the single largest malpractice risk to a practicing ophthalmologist. In the premium sector, missing your target refraction can not only increase your liability, but also end up deducting from your overall profitability in terms of doing additional refractive enhancements, such as limbal relaxing incisions, piggybacking IOLs, and/or laser vision correction. In the end, word-of-mouth referrals will drop off drastically.
Like any anterior segment surgical procedure, addressing the ocular surface preoperatively and/or postoperatively is critical to optimizing the postoperative visual outcome. The PHACO study ultimately revealed that poor diagnostic data acquisition (keratometry, corneal topography and biometry) due to unrecognized ocular surface disease preoperatively can lead to greater IOL power calculation errors, up to 1 D to 2 D. 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.
Add in that the patient has had previous refractive surgery and IOL calculation decisions become more challenging. Patient expectations in this modern refractive cataract arena are underrated and, unfortunately, the competitive environment in which we practice has created an overwhelming hurdle to nail the IOL bulls-eye.
Variables for premium outcomes
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 and desired postoperative refraction. The effective lens position is the effective position of the IOL (the principal plane of the thin lens) relative to the anterior corneal vertex; this is different from the old term “anterior chamber depth,” which ignores central corneal thickness. There are many reports and publications that support an improved effective lens position with femtosecond laser cataract removal, one of the primary advantages of this technology.
Lens thickness and horizontal white to white corneal diameter measurement are additional variables in newer-generation IOL formulas, such as Holladay II, to better predict effective lens position and master the intended postoperative targeted refraction. Devices such as the newest IOLMaster 500 (Carl Zeiss Meditec) finally implement Holladay II software, which is especially helpful for short and long eyes, as well as post-RK eyes.
The challenge with longer eyes are myopic staphyloma, and devices that utilize optical coherence tomography, such as the IOLMaster 500 or LenStar (Haag-Streit), will more accurately measure axial length than ultrasound in these difficult situations. Overall, accurate measurements of keratometry, axial length, anterior chamber depth, horizontal white to white corneal diameter measurement and lens thickness are critical to obtaining the correct IOL power.
A recent publication by Douglas Koch, MD, and colleagues revealed a simple adjustment formula for eyes longer than 25 mm in axial length to avoid a hyperopic surprise. My go-to Koch adjustment formula overall is 0.8829 multiplied by the measured axial length + 2.825; I then take this adjusted axial length and plug it into my preferred IOL power formula and aim for slight myopia of −0.5 D to −1 D, avoiding the dreaded hyperopic surprise in eyes with high axial length of greater than 25 mm. In my OR setting, I have the additional advantage of utilizing intraoperative wavefront aberrometry with ORA (WaveTec) to obtain a real-time IOL power reading in the aphakic mode to further fine-tune this Koch adjustment if necessary before injecting my premium advanced IOL.
Post-refractive patients
In post-refractive surgery situations, conventional keratometry and topography measurements of the cornea are typically inaccurate. In myopic LASIK/PRK, the anterior corneal curvature is flatter than the posterior curvature, often resulting in an IOL power underestimation or a 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 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 preoperative 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 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 that 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 on the American Society of Cataract and Refractive Surgery website for inputting information, as well as in the IOLMaster 500 software template for post-refractive surgery. The ASCRS calculator utilizes a variety of formulas, such as the Holladay II, Shammas and double-K methods, from which to select your desired IOL power.
The Masket regression method utilizes the formula K = LSE × (−0.326) + 0.101, where “LSE” is the excimer laser spherical equivalent treatment applied. With this formula, there is approximately 1 D adjustment for every 3 D of excimer laser treatment applied.
The formula I now use consistently 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 lens position. On the IOLMaster 500, the only criterion the surgeon must select is whether the patient had previous myopic or hyperopic laser vision correction. The only drawback with the Haigis-L formula is that it cannot be used for previous RK patients.
For RK patients, I still use the ASCRS website and the average pupil power and/or effective central corneal power reading from my OPD III (Marco Ophthalmics), as well as 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. As mentioned earlier, I always implement intraoperative wavefront aberrometry with the ORA device in post-refractive surgery patients and use the Haigis L or ASCRS calculator as my baseline to select my initial IOL power prior to entering the OR.
In the end, setting expectations with post-refractive surgery patients undergoing cataract surgery is critical. Make sure the patients clearly understand the real chance for needing an IOL exchange, piggyback IOL or further corneal enhancement procedure. As the surgeon, make sure a corneal topography is performed preoperatively to be sure a corneal enhancement procedure is even possible. Our current formulae and technological advances have made hitting the post-refractive surgery IOL bulls-eye easier, but work still remains.
My next column will focus on the “Jackson rules” for setting patient expectations, especially in the premium patient.