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Ladas Super Formula aims to improve refractive accuracy of cataract surgery
Honing individual surgeon results may also help increase accuracy.
We have all seen it in our own patients: After a seemingly routine cataract surgery that is without complications, the patient ends up with a refractive miss. Even though we were aiming for a plano refraction, the patient is a diopter away from that. For a patient electing for cataract surgery alone, this is still a successful case because the patient was not electing for vision correction. But for those patients who are electing to pay out of pocket for treatment of astigmatism and a specific refractive outcome, being off by 1 D is unsatisfactory. We can fix this and achieve the plano result that we desire, but how did this happen in the first place?
Unusual causes
While lay people may think that the refractive surprise is the result of the “wrong” lens being put in the eye, this is actually a very unusual case. Certainly putting the lens for patient A into patient B is a serious medical mistake, but it is quite rare. Other errors such as not remembering to adjust the power when switching between IOLs with different A-constants are also infrequent. The error is in determining the IOL power and/or predicting the patient’s healing response, which determines the final resting position of the optic.
Biometric data
Image: Devgan U
We must have precise input data for accurate IOL calculations. Biometry principally consists of the keratometric power and the axial length, but also can include other data such as measured anterior chamber depth, white-to-white and more. Previously, ultrasound A-scan measurement of the axial length was more variable, with applanation producing a greater variability than immersion techniques. Now with optical coherence biometry we are able to more accurately measure the axial length in the majority of eyes. Keratometry has advanced as well, with newer devices able to measure closer to the center of the cornea at the visual axis. Any eyes, however, with irregular corneas or post-refractive surgery corneas can prove difficult to accurately measure. In addition, there is the refractive component of the posterior cornea, which is sometimes different from expected. In the vast majority of eyes, measuring the anterior corneal curvature can tell us the total power of the cornea because the ratio of the anterior to posterior corneal powers is in the typical range. In some eyes, such as those with prior LASIK, this ratio of anterior to posterior corneal power is changed, and it is helpful to directly measure the posterior cornea.
Effective lens position
Particularly in small eyes, the effective lens position (ELP) plays a very large role in determining the refractive outcome of the surgery. When we place the IOL in the capsular bag, the assumption is that the patient will heal as predicted without much anterior-posterior shift of the optic. In some patients, this is not the case. In a post-vitrectomy eye, the IOL may end up more posterior in the eye, and thus a slightly higher IOL power should be used to avoid a postop hyperopic surprise. In an eye with a very shallow anterior chamber, the optic may sit more anterior and require a slightly lower power for a plano outcome. Another important consideration is that many factors go into determining the architecture of the eye and that not all eyes with identical axial lengths and keratometry will have the same ELP. The shorter the eye, the more of a difference the ELP will make because the optic power is higher. The most challenging eye for determination of ELP is the eye with a short axial length and normal to deep anterior chamber. In these cases, the IOL power typically exceeds +30 D and can go even beyond +40 D.
Calculation variability
With multiple different IOL calculation formulas, it becomes important to choose the best one for the job, particularly if the eye is not a typical or average eye. For example, the SRK-T formula tends to be excellent, but when confronted with an eye with extreme keratometry measurements, it tends to be thrown off compared with another formula such as the Holladay 1. Remember that these formulas must perform two calculations: the determination of where in the eye the IOL optic will sit and the optimal power given the keratometry and axial length. When the printout from your biometry device shows a different ideal IOL power for each formula, how do you choose which is best?
A recent patient of mine had lens calculations that showed that the ideal IOL power was different for each formula on the printout. The Holladay 1 was +23.0, the Hoffer Q was +24.0, the SRK-T was +23.5, and the Haigis was +22.5 — all different, so which should we choose? While some would advocate averaging the formulas together, others would move toward more complicated multivariable formulas in the hope that more input variables would give a better result.
I opted to use the Ladas Super Formula, which is available to beta testers at www.IOLcalc.com, to help make the decision. Based on the Ladas Super Formula algorithm, which amalgamates multiple different formulas in order to maximize the strengths of individual methods while minimizing their weaknesses, a lens power of +23.0 was selected, and it turned out to be spot-on correct. How can we learn from this case to hone future IOL calculations?
Honing individual surgeon results
The Holladay IOL Consultant software has the ability to analyze postoperative data and hone the surgeon’s A-constant for each specific IOL. This allows the Holladay 2 formula to make a more accurate calculation for the patient population as a whole. If the results of this surgeon tend to produce patients with more postoperative myopia than expected, then the A-constant can be shifted lower in order to produce a calculation with a lower IOL power and thus less postoperative myopia.
Another approach proposed by John Ladas, MD, PhD, is to hone results of individual surgeons but also to crowdsource data from a large cohort of ophthalmologists. By plotting out thousands or even millions of eyes, we can learn where formulas agree and where they differ. We can also learn which types of eyes are most problematic, how each surgeon does compared to the group, or even what the normal distribution is across a population. We can keep increasing our accuracy, and that spells better vision for our patients.
You can become a beta tester and access the Ladas Super Formula at no cost. Go to www.IOLcalc.com and follow the prompts to sign up.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: devgan@gmail.com; website: www.DevganEye.com.
Disclosure: Devgan reports he and Ladas are principals in the www.IOLcalc.com website.