Laser refractive surgery has matured so that we are now able to go beyond simple correction of refractive errors. The very concept of refractive error is perhaps oversimplistic as a basis for vision correction. When sphere and cylinder are corrected with corneal reshaping, accurate correction of these lower-order aberrations (LOAs) can provide improvement in spectacle independence. However, a better strategy is to consider the total aberrations of a visual system and to address these as part of the vision-correction procedure. Wavefront analysis and its incorporation into the planned treatment allow greater control and better visual outcomes. The use of corneal topographic information in combination with whole-eye wavefront analysis promises to make refractive correction even more accurate and reliable, in normal eyes as well as those with irregular corneas after previous refractive surgery.
A system of wavefront treatment comprises three components: measurement of the wavefront, planning, and laser delivery
Measurement of the wavefront. Wavefront measurements are a representation of the aberrations in a visual system. As each ocular system is different, these measurements quantify how the individual eye handles light rays. For instance, an unaberrated system would have a flat wavefront. The wavefront is the sum of individual aberrations and is measured in its deviation from a flat wavefront.
Planning. Wavefront data are combined with other metrics of an eye as well as patient age, refractive error, topographic data, and visual needs.
Laser delivery. Corneal reshaping to correct refractive error requires predictable laser tissue removal and controlled placement of the reshaping. Registration and alignment of the laser ablation is essential if higher-order aberrations (HOAs) are to be corrected.
Ironically, the greatest contribution to laser vision correction through the application of wavefront data has been the improvements achieved in ancillary technologies such as ablation profiles, laser tracking, and registration. With these elements incorporated into the treatment, results are significantly improved even without the use of wavefront-guided correction.
Alignment and registration of the ablation are crucial if HOAs are to be effectively corrected. Zernike polynomials (ie, the mathematical expression of a wavefront) are referenced to the pupil center that was present when the wavefront was captured. This is the point that must be used for centering the ablation. Most laser systems are able to compensate for the pupil centroid shift that occurs with changes in pupil size. Centration becomes more complex when LOAs (sphere and cylinder) are treated separately from HOAs.
Registration tolerances of 200 μm (x and y) and 5° rotation are suggested as a requisite for predictable correction of HOAs. To put this into context, consider that the average pupil centroid shift (vectoral) was measured at 370 ±130 μm in one study.1 In another, angle kappa was shown to have a mean value of 553 ±127 μm.2
WAVEFRONT AND TOPOGRAPHY
We have learned to control and correct spherical aberration and coma, the elements of wavefront error that have the most effect on vision, with wavefront-guided or wavefront-compensated treatments. There are two distinct approaches to wavefront treatment, one of which is to measure the wavefront error and fully correct it as part of the refractive correction (wavefront-guided). The other is to compensate for the wavefront errors that will be induced as part of the laser ablation, thus minimizing their increase as part of the treatment (wavefront-optimized).
In addition to the use of wavefront measurements of total aberrations of the eye, diagnostic software is now incorporating corneal wavefront data as well. Topographic wavefront-based treatments are growing in application. This strategy has particular merit in retreatments, as it is on the cornea that wavefront degradation will have been created during laser ablation. Topography-based treatments may be best approached using a target asphericity rather than attempting to reduce corneal wavefront to zero.
An interesting application of our knowledge of wavefront errors and their impact on vision is to selectively alter different HOAs to enhance vision. Carl Zeiss Meditec’s (Jena, Germany) system of presby-LASIK is an example of a targeted increase in spherical aberration to enhance depth of focus (Figure 1).
The development and application of wavefront technology has driven improvements in laser refractive surgery outcomes. Laser reshaping is more accurate with better alignment and registration of the ablation, and our ability to correct and control HOAs gives our patients better vision.
Patrick Versace, MD, practices at the Vision Eye Institute, Sydney, Australia. Dr. Versace states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +61 2 93863666; e-mail: p.versace@ unsw.edu.au.
- Park SH,Kim M,Joo CK.Measurement of pupil centroid shift and cyclotorsional displacement using iris registration. Ophthalmologica.2009;223(3):166-171.
- Hashemi H,Khabazkhoob M,Yazdani K,Mehravaran S,Kafarzadhpur E,Fotouhi A,et al.Distribution of angle kappa measurements with Orbscan II in a population-based survey.J Refract Surg.2010;26(12):966-971.
- Rocha KM,Vabre L,Chateau N,Krueger RR.Expanding depth of focus by modifying higher-order aberrations induced by an adaptive optics visual simulator.J Cataract Refract Surg.2009;35(11):1885-1892.