New adaptive optics system reduces higher order aberrations and previews custom ablation outcomes

Three colour SLO image of patient with diabetic retinopathy who has undergone laser treatment. Image shows the choroid (yellow arrow) and scars left by laser (blue arrow).

A NEW diagnostic device which uses a wavefront-driven micromirror array can significantly reduce higher order aberrations with associated improvement in visual function, while providing patients with a preview of what they might expect from customised ablation.

"This is a small, inexpensive and easy to use system which rapidly corrects higher order aberrations and thereby allows refractive surgeons to demonstrate to patients their best possible visual performance.

"In addition, it can be used to test the results of wavefront-guided ablations in a subjective as well as objective way," reported Ulrich von Pape PhD at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery.

Dr von Pape and colleagues used the adaptive optics system to analyse changes in root mean square (RMS) data in 60 eyes. Total baseline higher order RMS in that cohort, based on measurement of aberrations up to the sixth order, ranged from 0.12 microns to 0.81microns and averaged 0.3 microns.

After correction with the wavefront-driven micromirror array, total RMS was reduced to a mean of 0.13microns. RMS values were less than 0.35 microns in 100% of eyes and were decreased to less than 0.15 microns in more than half of the cohort.

Baseline RMS values for third and fourth order aberrations averaged 0.15 microns and 0.11 microns, respectively, and they were also each reduced by about 50%. The eyes had low amounts of fifth and sixth order aberrations (baseline RMS 0.04 microns), and those aberrations were not affected by the micromirror array system.

Changes in BSCVA were evaluated in 14 eyes, all of which had a BSCVA of 20/20 or better. Of the eyes in that sub-group, the best wavefront-corrected visual acuity represented a one line improvement from baseline in six eyes. Most patients whose visual acuity improved had a 5.0mm or larger pupil and a total RMS greater than 0.3 microns, Dr von Pape noted.

"Since the proband for this study were persons with healthy eyes, the visual acuity results are still very satisfying. However, based on our initial calculations of modulation transfer function (MTF) in a very limited number of eyes, we expect that even individuals with an RMS less than 0.3 microns will probably have improvement in contrast sensitivity even if they do not show a measurable change in visual acuity," he said.

The researchers performed MTF and point spread function (PSF) calculations in two typical eyes. The PSF showed an improvement in retinal image quality. Plotting of the MTF curves for the uncorrected and corrected wavefronts with the neural threshold function showed the wavefront correction would allow detection of higher spatial frequencies.

However, even a larger improvement could be expected in contrast sensitivity at lower frequencies. This would make it possible for an individual to recognise a pattern with one quarter of the original contrast, Dr von Pape observed.

The researchers first evaluated the performance of this system for correcting wavefront aberrations in artificial eyes manifesting different higher order aberrations. Aberrations with a total RMS of up to 1.0 microns could be reduced below 15% of their original value, Dr von Pape reported.

The subsequent clinical studies confirmed the system’s ability to reduce higher order aberrations in human eyes. Those results suggest that wavefront correction would have potentially greater universal benefit for improving contrast sensitivity compared to visual acuity, Dr von Pape explained.

"However, in using this system as a pre-surgical tool, ophthalmologists and patients must recognise that the wavefront correction made with the micromirror array is not necessarily identical to the correction made with wavefront-guided LASIK since the precision of the surgical correction is limited by the relatively large size of the laser beam and the final outcome may also be influenced by flap positioning and corneal healing," he said.

The wavefront-driven micromirror array system integrates a deformable mirror with a commercially available Hartmann-Shack type wavefront sensor (VISX WaveScan) and a continuous spherocylindrical pre-compensation unit.

The micromirror array was designed and manufactured by the Fraunhofer Institute IPMS, Dresden, Germany. It consists of 48,000 micromirrors, each measuring 40x40 microns, and has overall dimensions of 8.0mm x 9.6mm.
Each mirror is mounted on four flexible arms and can be moved up and down independently using electrostatic forces to correct higher order aberrations detected by the aberrometer. Each mirror has a maximum travel range of 400 microns and a refresh rate of 250Hz.

The system works in conjunction with the Visx WaveScan wavefront sensor. The wavefront data provides surface information to the active mirror. The patient is viewing a scene, reflected on the specific adaptive mirror. The mirrors deform, literally at the push of a button, to provide a corrected wavefront with reduced aberrations. This allows direct assessment of the visual capabilities of the corrected eye of a patient, while giving a preview to a patient at the same time.

The WaveScan, including the adaptive optics for correcting wavefront errors, is being developed by 20/10 Perfect Vision in Heidelberg, Germany. The US laser company Visx recently acquired the technology from 20/10 Perfect Vision for $5.9 million.

REFERENCES • Ayyakkannu Manivannan, Jan Van der Hoek, Pedro Vieira, Allison Farrow, John Olson, Peter F Sharp and John V Forrester. Clinical Investigation of a True Colour Scanning Laser Ophthalmoscope. Arch Ophthalmol 2001 119: 819-824. • Ross Ashman, Ayyakkannu Manivannan and Peter F Sharp. Omitted References on Colour Scanning Laser Ophthalmoscopy. Arch Ophthalmol 2002 120: 1601-a. Dirk-Uwe Bartsch, William R Freeman, Ann M Lopez, Ayyakkannu Manivannan and Peter F Sharp. A False Use of "True Colour". Arch Ophthalmol 2002 120: 675-676. • P Vieira, A Manivannan, PF Sharp and JV Forrester 2002. True colour imaging of the fundus using a scanning laser ophthalmoscope. Physiol. Meas. 23 1-10. PF Sharp, A Manivannan, P Vieira, JH Hipwell. Laser imaging of the retina. British Journal of Ophthalmology 1999; 83(11):1241-5.

Ulrich von Pape PhD
20/10 Perfect Vision, Heidelberg, Germany
Email: vonpape@2010pv.com

Top