Multifocal or monofocal

Multi vs monofocal

Both options can be effective for patients following previous corneal refractive surgery. Howard Larkin reports

Should cataract patients with previous corneal refractive ablations receive multifocal (MFIOL) or monofocal intraocular lenses? The answer depends upon the type and degree of correction previously applied, surgeons’ corneal measurement capabilities and informed patient preferences, according to debaters at the JCRS Symposium at the 38th Congress of the ESCRS Virtual.

Arguing for monofocal lenses was Ruth Lapid-Gortzak MD, PhD, cornea, cataract and refractive surgeon at the Amsterdam University Medical Centers, of the University of Amsterdam, and medical director of Retina Total Eye Care, Driebergen, the Netherlands. Dr. Lapid presented the opinion that monofocal lenses are preferable in most cases due to the difficulty of achieving acceptable visual outcomes with MFIOLs, though she does not hold this position herself.

Arguing in favour of judicious use of MFIOLs was José F Alfonso MD, PhD, head of cornea and lens surgery at the Fernández-Vega Ophthalmological Institute, Oviedo, and lecturer at the University of Oviedo, Spain. His extensive research suggests that the key to meeting patient demands for spectacle independence is analysing how refractive surgery has changed the cornea, and finding a lens that can work with the aberrations present.

ABERRATED EYES Dr Lapid pointed out that LASIK and PRK change the cornea in ways that make it difficult to measure its power. The ratio between the anterior and posterior surfaces changes, and there is a forward shift of the posterior cornea that makes it difficult to correctly estimate effective lens position.

“So, it doesn’t work with regular formulae.” Dr Lapid said.

Other complicating factors include decentred ablations and induced higherorder aberrations, particularly spherical aberrations (SA). Small treatment zones, which are common for older ablations, are troublesome because they induce moreacute changes in cornea shape, Dr Lapid said. “All these factors go together.”

Multifocal IOLs have a host of sideeffects that make them unsuitable for patients with high visual demands, Dr Lapid said. For example, even in extended We are really affecting the quality of vision with multifocal IOLs. The bottom line is I should be saying monofocal lenses are the only option, but there is no one-size-fits-all

Ruth Lapid-Gortzak MD, PhD

depth of focus lenses the gain in depth of focus was offset by a decline in both peak VA and contrast sensitivity (Bellucci et al. JCRS 2019;45:919-926). Modulation transfer function is also lower in multifocal IOLs than in monofocal IOLs and is highly dependent on pupil size (Tandogan et al. JRS 2017;33:808-812).

“We are really affecting the quality of vision with multifocal IOLs. The bottom line is I should be saying monofocal lenses are the only option, but there is no one-size-fits-all, and judicious use of preoperative screening make multifocal lenses an option.” Good biometry and shared consent are essential to reach patients goals, she advised.

OFFSETTING SA Dr Alfonso pointed out that controlling SA is essential for a good cataract surgery outcome after laser refractive surgery. Myopic photoablation induces positive SA commensurate with the dioptres of error corrected. For example, a -3.0D correction induces about +0.1 microns SA, while a -9.0D correction induces about +0.3 microns. Hyperopic ablation is the reverse with a +2.0D correction inducing about -0.1 microns SA and a +4.0D about -0.2 microns.

While a small amount of SA results in greater defocus tolerance and depth of field in ablated eyes – indeed, a young healthy, unablated eye has about +0.1 microns – it is essential to keep it within ±0.2 microns, which results in a loss of visual acuity of about one line and a gain in depth of focus of about 0.5D.

So, Dr Alfonso chooses a lens that will balance the existing SA of the postrefractive surgery cornea with a lens that will offset it to bring total SA within ±0.2 microns, which maintains good distance and acceptable intermediate vision. For example, a patient who received a myopic LASIK correcting -1.0 to -4.0D can do well with a trifocal diffractive lens with -0.1 microns, such as the FineVision.

“We obtain standard distance vision and preserve the trifocal performance of the lens.” A higher myopic correction of -7.0D to -10.0D might induce a +0.3 microns cornea, requiring a lens with greater negative SA, such as the Eyehance with -0.27 SA, to achieve best results.

Dr Alfonso’s research suggests that diffractive trifocal lenses do not perform as well after higher LASIK myopic corrections resulting in mean corneal power equal or less than 38D, limiting distance-corrected vision. With myopic corrections greater the -7.0D, two lines of vision can be lost.

“This problem does not occur with nondiffractive extended depth of focus lenses such as the Vivity, which would be indicated in a patient who received a myopic LASIK correction between -5.0 and -6.0D.”

Conversely, in patients after hyperopic photoablation, negative corneal SA is induced. Implanting a neutral monofocal lens does not increase the corneal aberration. With a mild myopia refractive target of -0.5D, this results in standard distance vision and functional intermediate vision. With diffractive lenses, Dr Alfonso’s research shows a loss of one line of corrected distance vision in corneas with high hyperopic corrections.

Dr Lapid congratulated Dr Alfonso on the depth of his research. She pointed out that assessing higher-order aberrations involves extra steps. “It is expensive and time-consuming, so the patient has to understand what you are doing and why.” Her research with instruments available in every cataract clinic has shown that with simpler screening methods one can evaluate which post-LASIK eye will do well with multifocal IOLs (Vrijman et al JCRS 2017; 43:909-914).