Hydrophobic acrylic IOL

[caption id='attachment_601' align='alignright' width='400' caption='EnVista lens in cartridge - Courtesy of Richard Packard MD'][/caption]

Cataract surgeons report favourable early experiences with a new, single-piece IOL constructed of a novel hydrophobic acrylic material (enVista MX60, Bausch+Lomb).

In a free paper session during the XXIX ESCRS Congress, Roberto Bellucci MD, Department of Ophthalmology, Hospital and University of Verona, Italy, and Richard Packard MD, Prince Charles Eye Unit, Windsor, UK described the features of the new IOL, reviewed its handling characteristics, and reported postoperative outcomes.

The IOL has a 6.0mm optic with an aspheric (aberration-free) design and 360-degree square edge. It has modified C-haptics, an overall length of 12.5mm, lies flat on the bench, and has a five-degree posterior vault when compressed.

Its hydrophobic acrylic material is a highly cross-linked acrylic copolymer designed specifically for IOL use. It has an optimised refractive index (1.54), a high Abbe number (40.5), and low silicone oil affinity. The IOL is hydrated to an equilibrium water content of four per cent and packaged in physiological sterile saline so that its water content is maintained.

[caption id='attachment_602' align='alignright' width='400' caption='EnVista lens emerging into eye with elbow of haptic forward'][/caption]

'The refractive index of this novel acrylic material allows for a thin lens and is similar to that of the acrylic of the AcrySof IOLs, but it has a higher Abbe number. Hydration of the material to equilibrium prior to implantation should eliminate the driving force for water diffusion into the IOL in the eye, theoretically eliminating the potential for haze, glistenings and other material defects,' said Dr Bellucci.

'Surface hardness is also excellent, making it resistant to damage from surgical instruments, and the surface properties of the IOL are more stable after implantation compared to other hydrophobic IOLs that are packaged dry,' said Dr Bellucci.

Design features

Dr Packard also commented on the unique material and design features. 'The Abbe value of this material is important because the higher the number, the less the chromatic aberration and therefore the better the retinal image quality. Its four per cent equilibrium water content, which is much higher than the 0.35 per cent to 0.5 per cent water content of other hydrophobic acrylic IOLs, should allow the material to remain stable in the eye without water exchange, which is believed to cause glistenings,' he said.

The hydrophobic acrylic is not as sticky as some lens materials, but it does have a good contact angle and with its 360-degree square posterior edge, it should have good PCO characteristics, he added.

Dr Bellucci reported that the BCVA outcome in his series of 10 patients was 'very satisfying'. At six months, logMAR BCDVA was -0.02 ±0.06. Slit-lamp examinations performed throughout the follow-up period confirmed continued centration and absence of any internal vacuoles or surface scratches.

Findings from wavefront aberrometry showed a pattern consistent with that of neutral aspheric IOLs. Optical quality was investigated by determining the point-spread function (Strehl ratio) measured using both Hartmann-Shack (Topcon) and double-pass (OQAS) aberrometers, and the results were satisfactory.

'These first optical and anatomical results are encouraging, and because the IOL could have absorbed water in its packaging prior to surgery, even this short-term study is important as initial confirmation of its potential to remain glistening-free. Longer observation is needed,' said Dr Bellucci.

Dr Packard's study included 20 patients, 10 of his own and 10 operated on by David Spalton MD, St Thomas' Hospital. The surgeries were done under topical anaesthesia through a 2.75mm clear corneal incision. Ease of insertion using a proprietary injection device was rated with an overall score of four (best possible = five).

'The injector system worked well through a 2.75mm incision, and the IOL unfolded slowly, which made it easy to position. Now, a new injector system is available allowing implantation through a 2.2mm incision,' Dr Packard said.

No adverse events

He also reported that consistent with the surface hardness of the hydrophobic acrylic material, he observed no markings from instrument handling in slit-lamp examination, and there were no adverse events during a mean follow-up of 17.3 days.

Mean Snellen UDVA improved from 21.9/6 to 10.4/6 and mean CDVA improved from 14.3/6 to 8.2/6. However, Dr Packard observed the results were skewed because visual prognosis was guarded in two patients with ocular co-morbidities. Mean MRSE was +0.4 ±0.9 D compared with a formula predicted target of -0.27 ± 0.2 D.

Power calculations were performed using the SRK/T formula with the manufacturer's original recommended A-constant of 118.7 for optical biometry. However, based on the refractive outcomes, the manufacturer has revised the A-constant to 119.1, Dr Packard said.

Dr Packard also presented a video to demonstrate IOL loading and implantation through a 2.2mm incision.

After the IOL is placed in the cartridge with the haptics tucked under the edge, the implant is rotated to align its long axis, ie, along the optic-haptic junction, with the long axis of the cartridge. This causes the leading haptic to turn into an elbow on the top of the lens and assures that when the lens is released, the leading haptic is properly positioned and the IOL can be pushed directly into the capsular bag.

'Proper loading of the IOL in the cartridge is important for ease of positioning. In addition, to enable pushing the lens into the bag, surgeons should fill the whole capsular bag with viscoelastic before filling the rest of the anterior chamber,' Dr Packard said.