CAPSULORHEXIS RECONSIDERED

In the six years since the dawn of femtosecond laser-assisted cataract surgery (FLACS), precise capsulotomies have been touted as one of its biggest advantages. Perfectly sized, shaped and centred anterior capsule openings should help position and stabilise intraocular lenses (IOLs), leading to more predictable effective lens position and better visual outcomes, proponents say.

“A large capsulotomy or capsulorhexis allows the IOL to bulge forward very slightly and a small capsulotomy keeps the lens more posterior, about a half-dioptre difference. Removing that variation is helpful,” Julian Stevens MRCP, FRCS, FRCOphth, DO, consultant surgeon at Moorfields Eye Hospital, London, told the XXXII Congress of the ESCRS in London, UK.

This reasoning appears to resonate. Early data from the ongoing ESCRS Femtosecond Laser-Assisted Cataract Surgery Study show it is the most frequently used FLACS capability. Nearly 98 per cent of more than 2,000 FLACS procedures reported through August 6th, 2014 involved laser-cut capsulotomy, with laser nuclear fragmentation second at 90 per cent. By comparison, only one in three FLACS procedures involved laser-cut corneal incisions and one in five astigmatism correcting incisions.

Yet data on the subject are mixed. Many studies document the clear superiority of FS capsulotomies over manual rhexes in terms of circularity, intended size and centration. A few even show slightly better mean visual outcomes for FS laser capsulotomies. For example, a 2012 prospective study involving 372 patients by Harvey Uy MD, Manila, Philippines and Warren E Hill MD, Mesa, Arizona, US, showed a mean advantage of about 0.2 dioptres for FS-treated eyes.

Other studies cast doubt on the impact of capsulotomy perfection on lens position. A 2013 retrospective study of manual capsulorhexes involving 635 patients by Oliver Findl MD of Hanush Eye Hospital, Vienna, and Moorfields, comparing cases with “optimal” rhexes of 4.5mm to 5.5mm and 360-degree optic overlap with eccentric and smaller rhexes found no significant difference in lens tilt or anterior chamber depth change, and a mean difference in centration of just 0.08mm in the eccentric group, three months after surgery.

Dr Findl said its not surprising rhexis morphology has little effect on lens position because most modern IOLs fixate in the capsular bag equator, centring the optic in the bag.

“But it becomes a different issue if you fixate the optic in the capsulotomy. You would think that such a lens is not going to change position,” he noted.

Inspired in part by the precision offered by laser-cut capsulotomies, at least two new lenses designed to clip into anterior capsule openings are now available in Europe, with large-scale clinical trials pending. Proponents say they hold the potential to solve many IOL refractive issues, from enhancing predictability of effective lens position to reducing higher order aberrations to eliminating negative dysphotopsias along with lens tilt and decentration.

Only large prospective trials will determine whether these and other capsulotomy-fixated lenses will fulfil the tantalising refractive promise of femtosecond laser-assisted surgery. But even if they succeed, they still may not completely vindicate FLACS. Technologies that enhance the precision of manual capsulorhexis as well as anterior capsulotomy lasers that bolt on to surgical microscopes may well deliver the benefits – without the steep femtosecond laser price.

On the visual axis

One problem with IOLs that centre in the capsular bag is the bag centre is usually not in line with the visual axis, and therefore the lens is not centred on the visual axis, where it provides the best optical performance, said Jack T Holladay MD, Bellaire, Texas, US. While every patient is different, on average the bag centre is about 0.3mm temporal to the pupil centre while the visual axis is about 0.3mm nasal and slightly inferior to the pupil centre.

The resulting displacement of the lens centre off the visual axis by about 0.6mm is inconsequential with monofocal lenses, inducing a slight horizontal coma, Dr Holladay said. But it is critical for multifocal diffractive optics, where even a small misalignment results in patient complaints of glare, “waxy” vision and decreased corrected visual acuity.

The solution is moving the lens centre as close to the visual axis as possible, Dr Holladay said. In experiments conducted with Paolo Vinciguerra MD and Eric Donnenfeld MD, glare and blur disappeared and vision improved from 20/30 to 20/20 or better in several patients after their lenses were pushed nasally on to the visual axis using a 30-guage needle or moving the pupil using an argon laser.

Indeed, experienced refractive surgeons often nudge the superior haptic of multifocal lenses slightly nasally during initial implantation to achieve this effect, Dr Holladay noted. The manoeuvre generally works but it is more art than science and is not always stable. The lens may shift after surgery as the haptics settle and push the optic back toward the centre of the bag.

Lenses that fixate on a capsulotomy centred over the visual axis will likely be more reliable and precise, Dr Holladay said. “It should stabilise the lens so it cannot decentre with respect to the visual axis and that is good.”

Creating such a capsulotomy with a femtosecond laser system should also be simple, requiring no more than using a coaxial light source to the capsulotomy on the first Purkinje image, which marks the visual axis on the corneal surface.

The Oculentis Femtis lens appears to be truly capsulotomy fixated, Dr Holladay said. An aspheric lens designed to be aberration neutral, it features a groove around the optic edge into which the capsulotomy edge fits much as a bicycle tyre fits on a rim. Small anterior haptics and larger posterior haptics keep the lens in place, which has an overall length of 10.5mm. It is entering large-scale clinical trials this year, said Ludger Hanneken MD, Cologne, Germany, who implanted Femtis in several patients in early clinical trials.

Eliminating negative dysphotopsia

A second lens designed with femtosecond laser-cut capsulotomies in mind is the Morcher 90S. It also features a groove around the optic into which the capsulotomy edge is inserted. However, its purpose is primarily preventing negative dysphotopsias, said Samuel Masket MD, Los Angeles, US, who holds a patent on the design.

The literature suggests that up to 15 per cent of IOL patients experience negative dysphotopsia immediately after surgery, with the number declining to about two per cent after one year, Dr Masket said. He developed the grooved lens concept after conducting extensive examinations of the anatomic relationships of pseudophakic eyes of patients experiencing pronounced peripheral shadows. He concluded that the 360-degree overlap of the capsulotomy on to the IOL optic is the common pathway leading to negative dysphotopsia regardless of lens design or temporal incision placement.

This conclusion has been borne out by clinical experience that shows negative dysphotopsia can persist even after an in-the-bag lens exchange, but is invariably eliminated by placing the optic in front of the capsulotomy, Dr Masket said. This can be done either by reverse capturing the optic in front of the bag with the haptics in the bag, or implanting a lens in the ciliary sulcus. However, both techniques risk iris chafing and virtually guarantee rapid bag shrinkage with fibrotic PCO.

“My thought was to develop a lens implant that remains in the bag and has the stability to prevent fibrotic PCO, but with a portion of the optic overlaying the anterior capsule to prevent negative dysphotopsia,” Dr Masket said. The Morcher lens is based on the concept and includes these features, including a square posterior edge to impede lens epithelial cell migration, he added.

As of the end of 2014, about two dozen of the lenses had been implanted in humans, Dr Masket said. While a prospective trial involving 70 to 100 patients will be needed to statistically demonstrate the effect, so far no patients have suffered negative dysphotopsias or iris chafe.

Dr Masket allows that the design also enables centring the optic on the visual axis and that this might reduce higher order aberrations as well as lens tilt and decentration. It might also improve the predictability of effective lens position by preventing phimosis and the axial displacement that may result, though these effects also have yet to be demonstrated.

“My primary interest is eliminating negative dysphotopsia. We are pleased there are other advantages to the design and we hope surgeons will take advantage of them.”

Is FS laser really needed?

In Dr Masket’s mind the only question remaining is whether a femtosecond laser is really required. Indeed, Dr Findl has successfully implanted six of the Morcher 90S lenses using a manual capsulorhexis guided by a Zeiss Callisto system that superimposes an outline of the proposed opening through the surgical microscope, and holds it on target with an eye tracker.

Dr Findl believes that while following the projected guide requires a little extra effort, most surgeons should be able to do it. “Maybe a trainee or someone who does less surgery may have more difficulty, but for an experienced surgeon it is not a big deal,” he said.

Other observers are less confident that most surgeons can execute a manual capsulotomy with sufficient precision. “Dr Findl is an exceptionally skilled surgeon,” Dr Masket noted.

“To have the capsulorhexis perfectly round and centred on the visual axis – you can’t do that by hand,” Dr Holladay said.

Other technologies might also help attain the precision required without resorting to a femtosecond laser. A doughnut-shaped silicone ring with a sizing scale incised on the surface that adheres to the capsule surface designed by Malik Kahook MD, Denver, US, is helping residents and skilled surgeons create more precisely sized and shaped capsulotomies. A YAG laser for anterior capsulotomies that bolts on to the surgical microscope is also in development.

The utility of capsulotomy-fixated lenses may also be limited. Dr Findl noted that bothersome negative dysphotopsia is rare, presenting in just a handful of the 6,500 cases his centre operates annually. Therefore, a dysphotopsia-preventing lens might be reserved for those requiring a lens exchange, or in the fellow eye of a patient with severe negative dysphotopsia.

Dr Holladay believes capsulotomy fixation may be really useful only for premium lenses that are sensitive to precise lens placement. In descending order of sensitivity, these include multifocal diffractive, extended depth of focus and highly aspheric lenses. Toric lenses may also benefit from capsulotomy fixation.

Dr Holladay also sees potential for reduced surgical variation eventually leading to better visual outcomes. “We have all the ingredients to put the lens where we want it and have a 360-degree overlap even though it is decentred in the bag a little bit. It will improve the performance of multifocal and premium lenses. What we don’t know is if it will actually reduce the prediction error of where the lens ends up (axially). We don’t know if the repeatability of the femtosecond laser will actually do that,” he said.

Julian Stevens:
julianstevens@compuserve.com

Jack Holladay: holladay@docholladay.com

Samuel Masket: avcmasket@aol.com

Oliver Findl: oliver@findl.at

Ludger Hanneken: hanneken@sehkraft.de

Warren Hill: hill@doctor-hill.com