Catch The Wave With Custom Refractive Surgery
BY BRIAN D. MARSHALL, O.D., F.A.A.O., Fairfield, Conn.
"Doctor, why is it that I see 20/20 in your office but I don't see as well at night?" After finding nothing clinically to explain the patient's complaint, you scratch your head and honestly say that you don't know. I'm sure we have all had a few patients like that and dread when they come in and complain.
Finally we have some answers. The new wavefront technology measures optical distortions that we couldn't measure before. Up till now, optometrists have only been able to correct lower-order aberrations in the visual system. We've used sphere, cylinder and prism in various combinations to provide the clearest vision for our patients. This article will explain how wavefront will revolutionize refractive care.
Fig. 1: Wavefront scans provide far more information than
Wavefront makes it possible
Wavefront technology has revolutionized refractive care by not only allowing us to measure and diagnose higher-order aberrations, but to correct them as well. These include spherical aberration, coma, trefoil and others -- otherwise known as irregular astigmatism. Wavefront aberrometer measurements provide a detailed map of every point within the pupil. By comparing the topography to wavefront information, it's even possible to analyze aberrations of the crystalline lens and vitreous.
Now the limiting factor for visual acuity is the spacing of photoreceptors and the neural connections of the fovea. The wavefront analyzer also analyzes the lower-order aberrations, making it a super
autorefractor. However, the technology will not perform refractions, nor tell the optometrist what sphere and cylinder are needed to best correct the eye.
Made for each other
Wavefront aberrations are directly related to pupil size, which is why we take measurements in a darkened room. The visual system is most sensitive to light rays entering the center portion of the pupil as described by the Stiles-Crawford effect. Therefore, patients who have smaller pupils, typically less than 6.0 mm, tend to have fewer aberrations in their optical system.
Dilating those patients will induce more aberrations in the system in the form of starburst, halos, etc. Patients who complain of glare, halos and poor night driving often have increased higher-order aberrations. These same patients also tend to demonstrate irregular corneal astigmatism, higher amounts of astigmatism and larger refractive errors in general, making them excellent candidates for
Wavefront is also well suited to former refractive surgery patients -- the procedure has been shown to induce more aberrations. (Smaller ablation sizes and wound healing responses contribute to their genesis.) Finally, demanding patients (engineers, pilots, gas permeable lens wearers, etc.) who have precise visual needs are good candidates for wavefront ablations.
Maps with more
Wavefront scans look similar to corneal topography maps, but they provide much more information. Figure 1 shows a scan created by the VISX
Wavescan. It's important to compare the wavefront refraction found in the upper left corner to the manifest refraction to validate the measurements.
The upper color images represent both lower- and higher- order aberrations while the lower images show only the higher order. The system measures the speed-of-light photons traveling through the entire visual system, thus, the red areas indicate phase advancement and the blue areas indicate phase retardation. A lack of symmetry in the colors indicates more influence of higher-order aberrations. The variations in elevations of the surface are generally within 1 µm for an eye with few aberrations.
Figure 1 also shows the percentage of higher-order aberrations in the top box along with pupil size and the wavefront refraction. This represents the percentage of aberrations caused by higher-order aberrations only, but this can be deceiving. If an eye has a significant refractive error, such as -8.00D, then the percentage of aberrations attributed to the higher-order component will usually be lower in the normal eye than an eye with a refractive error of -1.00D.
It's important to note the RMS Error shown on the scan next to the color image demonstrating only the higher-order aberrations: This is the root mean square, which represents the total optical quality of the visual system. It compares the visual system's wavefront with that of the ideal
wavefront. Values above 0.20 RMS represent a significant number of higher-order aberrations in the visual system.
Fig. 2: Results from VISX's Wavescan
Refractive surgery benefits
The biggest application of wavefront technology has been in refractive surgery. By communicating the wavefront data with a laser, a surgeon can place the precise correcting shape that will compensate for the eye's natural aberrations on the cornea. Before he can do this, however, consistently reproducing stable wavefront maps is the most crucial step. It's important to have patients remove their soft contact lenses at least one week before the procedure. Soft toric and extended wear patients should go two weeks without their lenses, and GP wearers even longer.
Careful pachymetry measurements are necessary because more tissue will be removed than with the traditional procedure. The amount will depend on the extent of aberrations present. Cycloplegic refractions are still necessary to accurately define the natural refractive status.
How is it different?
For the surgeon, the custom laser-assisted in situ keratomileusis
(LASIK) procedure is nearly the same. The only difference is that the laser is doing the extra work of correcting the eye's aberrations. Typically this results in slightly longer ablation times. Early Food and Drug Administration studies demonstrated that custom LASIK ablations show better results then those of conventional photorefractive keratectomy
(PRK) and LASIK in terms of increased accuracy and reduced aberrations.
Conversely, custom PRK results were better than custom LASIK results; creating a flap even with a microscopically smooth blade still creates small amounts of aberrations. The data for hyperopes is not as large but indications thus far are encouraging.
Wavefront gets results
Today two excimer laser systems are approved by the FDA for the treatment of myopia and astigmatism: Alcon's LADARWave with the LADARVision4000 Laser System and VISX's Wavescan with the VISX Star S4 Laser System. Neither platform yet has FDA approval to treat
hyperopia, high degrees of myopia or astigmatism.
VISX's multi-center study consisted of 351 eyes of varying amounts of myopia and astigmatism. It analyzed the effectiveness of wavefront ablations based on several criteria including uncorrected visual acuity, refractive stability, predictability, higher order analysis, and a subjective questionnaire on night vision.
Figure 2 shows that after six months, 74% had 20/16 or better and 94% had 20/20 or better uncorrected visual acuities. In fact, 69% of the eyes had the same or better uncorrected visual acuity
(UCVA) as compared to their best corrected visual acuity (BCVA) before surgery.
Figure 2 also shows that 85% were satisfied with their night vision six months after surgery as opposed to 65% before surgery. Finally, VISX's data showed that 90% of the eyes were within 0.50D of the intended correction.
Alcon's LADARWave results have been equally impressive. The company's multi-center study included 139 eyes treated for spherical myopia. At six months, 95.7% of these were within 1.00D of the intended correction. The patient satisfaction responses indicated that 76.5% of patients rated their vision since surgery "better" or "significantly better." And improved contrast sensitivity, reduced aberrations and superior optical quality were noted in the Alcon FDA labeling.
Both studies clearly indicate that LASIK is both safe and very precise with wavefront technology. Alcon and VISX are currently in studies to support extension of the parameter ranges.
Remedy LASIK failures
The most exciting application of
wavefront-guided LASIK is for the retreatment of refractive surgery patients who've had less-than-perfect results. These are the most unhappy patients. They originally expected great vision and now suffer with ghosting, halos, night vision disturbances and poor visual acuities.
For this group, the key is to allow the cornea to fully stabilize. This is monitored with visual acuity, corneal topography and wavefront mapping. In the meantime, GP contact lenses are a great tool to demonstrate the potential of vision improvement and relieve patient anxiety; stabilization can take three months or longer.
Have realistic expectations
Wavefront technology isn't able to solve all refractive problems. Unfortunately, the sensors used in the wavefront scans currently can't measure severe aberrations in conditions such as
keratoconus, previous radial keratotomy or corneal transplants. Therapeutic
wavefront-guided LASIK is effective for enlarging optical zones, but it's not that helpful for striae or central islands.
For previous PRK patients, it's best to enhance with LASIK if the corneal thickness is sufficient. For previous LASIK patients, re-lift the original flap: Tissue fibers can peel off and become lodged in the space between the new and old flap, inducing irregular astigmatism. Night glare secondary to smaller ablation zones can be dramatically reduced by using larger zones with custom
LASIK. After the enhancement, the patient should see an improvement in their UCVA over their previous BCVA within 24 hours.
However, healing-induced aberrations can still happen, especially in higher-correction enhancements. Wavefront mapping monitors these aberrations. As the smoothing process promotes epithelial thinning over the bumps and thickening over the divots, these aberrations usually diminish with time.
As of now, only pre-existing aberrations are treated with enhancements. More important than treating pre-existing aberrations, however, is to avoid causing them in the first place. Predictive algorithms using healing adjusted ablations will enable us to do this in the near future.
The future is even brighter
Within the next several months, the parameters of
wavefront-guided refractive surgery will expand, allowing those who have hyperopia as well as larger degrees of myopia to be safely corrected. Today the biggest downside to the procedure is the healing response.
If the cornea were a piece of plastic, the results would be perfect every time; however the cornea is a living, biomechanical structure. While technology isn't yet able to accurately predict healing, researchers are looking at algorithms that can be programmed into the laser's software for predicting the healing response.
Greater integration of wavefront technology with corneal topography will someday permit optometrists to measure and predict shape factor information outside of the pupil which influences the shape within the pupil. Researchers are also investigating the use of wavefront technology in customized contact lenses and even
It won't be long before those aberrometers you may remember learning about in school or seeing in some dreary, cluttered research department find their way into your office.
Dr. Marshall is in private practice at Shoreline Eye Associates in East Haven, Conn. You can contact him by phone at (203) 468-8800 or by fax at (203) 468-0183.
Optometric Management, Issue: October 2003