Mark Lobanoff, MD, is the co-founder and medical director of OVO Lasik + Lens in Minnesota. He is also the founder-inventor of Phorcides, topography-guided LASIK surgery planning software and Lochan cataract surgery planning software. He told us that his experience with the new Emmetropia Verifying Optical (EVO) toric calculator has been excellent.
EVO uses a thick lens formula
Most other toric calculator formulas are thin lens formulas that assume the IOL is infinitely thin. One advantage of the EVO toric calculator is that it uses a thick lens formula that considers lens thickness. This is important because, in reality, lenses have a thickness.1
“The EVO toric formula used in the new enVista toric calculator developed by helps me make accurate lens decisions. The residual refractive error my patients experience is lower than what I experienced with older formulas. This is one of the reasons other surgeons should consider using EVO formulas for their toric calculations.”- Dr. Mark Lobanoff
EVO toric calculator considers posterior corneal astigmatism
Many surgeons only use measurements of anterior corneal astigmatism. However, the posterior surface has an independent magnitude/meridian of astigmatism that should be considered.2 Failing to consider posterior corneal astigmatism (PCA) is associated with sub-optimal outcomes.2
The EVO toric calculator incorporates a theoretical model for PCA and takes different intraocular lens (IOL) geometries into account.1 The EVO formula predicts the spherical equivalent refractive error and recommends an IOL power based on biometry. With the EVO formula, a higher percentage of patients, about 80%, are in the 0 to 0.5 D range compared with traditional vergence formulas.1 In a retrospective analysis of 200 eyes with axial length of at least 25 mm, the EVO formula had the highest proportion of eyes ±0.5 D, which was statistically greater than Kane.3
Some toric calculators don't consider PCA or theoretically calculate it. According to Dr. Lobanoff, “To ignore that PCA exists is at your peril. I think some of the other toric formulas ignore it, at the peril of perhaps not hitting the refractive target. The EVO toric formula is basic and offers unique insights into IOL power that are missing from some of the older formulas.”
Red line denotes negative cylinder axis. Anterior corneal astigmatism on the right, posterior on the left. Note that in this case the posterior astigmatism is 90 degrees away from the anterior astigmatism. It will reduce the refractive effect of the anterior corneal astigmatism and reduce the necessary power of the toric IOL required.
Pearls for using the EVO toric calculator
Dr. Lobanoff recommends confirming the calculator inputs to optimize the outputs. If two diagnostic machines, such as a corneal topographer and a biometer, are available, he would review the keratometric readings from both. Two diagnostic machines may give slightly different readouts.2 If both diagnostics give the same reading, confidence is increased.
Additionally, it is helpful to look at corneal topography. A symmetrical topographic bow tie that is regular and oriented on a straight axis is ideal for a toric lens. In a case with an irregular corneal topography, a toric lens is not appropriate.
Two patients with similar amounts of astigmatism. The one on the right, with a normal symmetric straight "bow tie" pattern, is a good candidate for a toric IOL. The second on the left, with no clearly discernible bow tie, is not a great candidate for a toric IOL.
EVO formula outperforms legacy calculator in low astigmatism
Data from 109 eyes compared postoperative refractive astigmatism using the EVO formula to a formula that only considered anterior corneal astigmatism. 89% (97 eyes) were implanted with a toric IOL with an effective astigmatism power of 1.4 D or less. The proportion of eyes with an absolute refractive astigmatism of less than or equal to 1.0 D was statistically superior with the EVO formula. Additionally, the proportion of eyes in which the orientation of the predicted refractive astigmatism matched the actual refractive astigmatism was statistically significantly improved with the EVO formula compared to the legacy formula.4
“Patients want premium outcomes, especially if they're paying extra for these lenses, and that means it is beholden on the surgeon to understand how small additions like posterior astigmatism matter.”- Dr. Mark Lobanoff
enVista toric IOL for astigmatism
All enVista lenses are glistening-free with neutral asphericity to deliver consistent refractive power.5 enVista lenses also unfold quickly, in about 20 seconds (MX60E).6 In patients with astigmatism, the enVista toric IOL is useful because it is rotationally stable with 110 degrees of contact with the capsular bag.6 This reduces the amount of IOL rotation postoperatively and provides very reliable astigmatism correction and great patient outcomes.6
The enVista toric IOL offers some of the lowest toric power on the US market, thus it enables surgeons to use a toric IOL in patients who may be borderline candidates for other toric lenses.7 This allows a broader group of patients to experience the benefits of a toric lens. Dr. Lobanoff noted that “the EVO toric calculator essentially opens the market for toric IOLs up to a greater proportion of patients that can benefit from this technology. This includes lower astigmatic patients.” enVista is the only monofocal toric platform commercially available in the US that can treat <1 D of astigmatism at the corneal plane with the 1.25 D model.6
Mark Lobanoff, MD, completed an ophthalmology residency at Emory University and a fellowship in LASIK and refractive surgery at the University of Minnesota. He lectures on LASIK surgery around the world. In addition, he is involved in FDA research trials to improve the effectiveness of ophthalmic surgeries.
Financial disclosure: consultant to Bausch & Lomb