Researchers in Italy followed 31 patients who underwent either unilateral or bilateral PRK in 1991 to 1993. A total of 49 nearsighted eyes were included in the study, and check-ups were performed every two years over a period of 14 years.

Patients were divided into two groups — a “low myopia” group of subjects with less than -6.00 diopters (D) of myopia (range: -1.50 to -5.75 D) prior to surgery, and a “high myopia” group of subjects with at least -6.00 D of pre-operative myopia (range: -6.00 to -13.00 D).

PRK study: Results

Results at the final post-operative visit 14 years after surgery included the following:

Low myopia group:

• Mean residual refractive error was -0.17 D of myopia.
• Mean uncorrected visual acuity (UCVA) was between 20/20 and 20/25.
• Mean best spectacle-corrected visual acuity (BSCVA) was 20/20.
• 84 percent of subjects said they were satisfied with the results.

High myopia group:

• Mean residual refractive error was -0.67 D of myopia.
• Mean UCVA was nearly 20/25.
• Mean BSCVA was nearly 20/20.
• 75 percent of subjects said they were satisfied with the results.

In both groups, some degree of corneal haze increased three to six months after surgery, then declined over the subsequent six months.

Complications included minor corneal haze (two eyes), transient unequal pupil size (nine eyes), and iron-containing deposits in the corneal epithelium (four eyes).

No occurrences of ectasia or irregular astigmatism were noted, and no eyes had abnormal endothelial cell count or changes in endothelial cell shape (changes in the inner cell layer of the cornea that could lead to corneal clouding and vision loss). Three patients (9.7 percent) reported night vision problems.

Results achieved six months after surgery were maintained for up to 14 years with no evidence of regression of myopia in eyes in either group.

PRK study: Conclusions

The researchers concluded that PRK is a safe and effective procedure for the correction of myopia.

They noted, however, that results are less predictable for correction of high amounts of myopia.

Editor’s note: The PRK procedures in this study were performed with laser technology and techniques used in 1991 to 1993. It is possible that patients undergoing PRK with today’s modern laser technology may experience fewer night vision problems.

SOURCE:  A 14-year follow-up of photorefractive keratectomy. Journal of Refractive Surgery. June 2009.

Typically, fewer higher-order aberrations means better visual outcomes after LASIK surgery. The study suggests wavefront-guided LASIK offers the best results for most patients, with a lower incidence of re-treatments.

What are higher-order aberrations?

Higher-order aberrations (HOAs) are subtle optical defects in the focusing system of the eye that can cause blur, glare, halos and other visual disturbances. Higher order aberrations differ from common refractive errors — nearsightedness, farsightedness and astigmatism — which are easier to identify and can be corrected with eyeglasses and contact lenses.

Higher-order aberrations that are significant enough to affect vision can be present in eyes that have little or no refractive error. Vision problems caused by HOAs typically are most noticeable in low-light conditions, such as night driving.

Spherical aberration is one particular type of higher-order aberration. Uncorrected spherical aberration can cause glare and halos around street lights and headlights at night.

What is wavefront-guided LASIK?

Wavefront-guided LASIK uses pre-operative aberrometry measurements to determine the level of pre-existing higher-order aberrations and to develop a computerized laser treatment (ablation) to reduce these aberrations.

What is wavefront-optimized LASIK?

Wavefront-optimized LASIK, on the other hand, is an enhanced version of conventional LASIK. It does not address pre-operative HOAs, but the laser ablation is adjusted, based on the patient’s eyeglasses prescription, to minimize any increase in spherical aberration induced by the treatment.

So whereas the goal of wavefront-guided LASIK is to identify and treat multiple HOAs (including spherical aberration) and refractive error during the corneal reshaping of the eye, the goal of wavefront-optimized LASIK is only to limit any increase in spherical aberration during the treatment of refractive error by the laser.

Study design

To evaluate the effect of wavefront-guided and wavefront-optimized LASIK on higher-order aberrations, Jack T. Holladay, MD, clinical professor of ophthalmology at Baylor College of Medicine (Houston, Texas) and colleagues reviewed the results of 109 wavefront-guided procedures and 102 wavefront-optimized procedures performed in 2006 and early 2007.

Results and conclusions

Findings of the study revealed:

• Overall, the wavefront-guided LASIK treatments induced significantly less higher-order aberrations (including spherical aberration) than the wavefront-optimized procedures.
• Among eyes treated with wavefront-guided LASIK, approximately 12 percent had less HOAs after surgery than they had prior to surgery, 76 percent had essentially the same amount before and after surgery, and 12 percent had more HOAs after LASIK.
• Among eyes treated with wavefront-optimized treatment, approximately eight percent had less HOAs after surgery, 51 percent had essentially the same amount before and after surgery, and 41 percent had more HOAs after LASIK.
• There was more variation in outcomes among eyes receiving wavefront-optimized procedure.

The researchers concluded that wavefront-guided LASIK reduces or induces fewer higher-order aberrations than wavefront-optimized LASIK and therefore is the better procedure for optimizing post-surgical vision quality.

SOURCE:  Fewer higher-order aberrations yields better visual performance. EyeWorld. May 2009. (Supplement: Premium Clinical Options for Cataract and Refractive Surgery, sponsored by Abbott Medical Optics.)