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WAVEFRONT-GUIDED LASIK OVERVIEW

Anand A Shroff
Hon. Ophthalmic Surgeon, Bombay Hospital.

Although the human eye is an optical marvel, from an enginnering point of view the eye is a rather poor optical instrument. Besides common known image errors such as myopia, hypermetropia and astigmatism, that the optical quality of the human eye suffers from ocular errors (aberrations) has been recognized as early as the middle of the 19th century.1 Shape factors of the refractive surfaces such as the cornea and the lens and their decentrations degrade the retinal image quality cause aberrations that cannot be corrected by spectacles or contact lenses. Since the introduction of the flying spot excimer laser, monochromatic ocular aberrations can be individually treated during LASIK (laser assisted stromal in-situ keratomileusis) procedures along with myopic, hypermetropic and astigmatic treatments.

Principle of Wavefront Imaging

The leading edge of incident light upon the cornea is considered to be a smooth curvature - referred to as a ‘wavefront’. As this smooth wave front courses through the various refractive surfaces and their imperfections, ocular aberrations are induced. It results in an irregular wavefront curvature incident on the retinal surface, which degrades the retinal image quality. An aberration-free eye would result in a smooth wavefront image on the retinal surface, reaching an optical quality that matches or exceeds the mosaic of the photoreceptors of the retina. This results in what is referred to as ‘supernormal’ vision that is significantly better than 6/6 or results in a gain of visual acuity in eyes incapable of 6/6 vision. Wavefront-guided customized LASIK, also referred to as ‘customized ablation’ performs corrections based on individual wavefront measurements. Which potentially increases the chances of patient visual acuity improvement by 1 or 2 Snellen lines over best corrected pre-operative visual acuity. [2]

Current refractive procedures correct lower order aberrations such as spherical and cylindrical refractive errors. However, higher order aberrations affect the quality of vision and may not significantly affect the Snellen visual acuity. It is the subtle deviations from the ideal optical system, which can be corrected by wavefront procedures.

Lower Order Aberrations

These constitute the basic refractive errors such as myopia, hypermetropia and astigmatism, which can be effectively treated by spectacles, contact lenses or conventional LASIK procedures.

Higher order aberrations - These constitute about 15-20% of total ocular aberrations.

Spherical aberrations - Rays entering through the periphery of the pupil may not converge on the geometric image point, therefore contributing to the spread of light in the image beyond that caused by diffraction. Wavefront aberrations measurements of the eye are performed after pharmacological dilation of the pupil to reduce spherical aberrations induced by the pupil and expose aberrations induced by peripheral parts of the cornea and lens.

Chromatic aberrations - Chromatic aberrations of the eye prevent the simultaneous focusing of all visible wavelengths. This may result in poor contrast. However, some defocused eyes compensate by blurring wavelengths that are phase-shifted. Which in effect means that the retinal image quality of such eyes may be better left untreated.

Coma - Light that is not incident on the visual axis tends not to focus on the focal plane. Depending on the point of convergence they may be classified as positive or negative.

Other higher order aberrations - Distortions and Petzval field curvature.

Diffraction - According to the wave theory of light, limitation of the aperture causes a spread of light even in a fully focused system. Whenever the pupil is < 2 mm, the actual image spread is equal to the diffraction image, and other factors can usually be ignored. Therefore aberrations become significant when the pupil is dilated as in poor light conditions.

Wavefront sensing

Aberrations or optical errors of the eye can be measured and analyzed using an instrument called the aberrometer. Various methods of aberrometry have been suggested which are listed below.

The Tscherning type [2]

A collimated laser beam source (frequency-doubled Nd : YAG laser) projects a grid of 128 thin parallel and equidistant rays onto the retinal surface. The pattern of distortion of the regular grid determines the optical errors of the eye. In an aberration-free eye the grid pattern projected on the retina would remain unchanged. This retinal spot pattern is imaged onto the sensor of a low-light CCD camera and the deviation of each sport measured by means of a computer. This method is used with the Wavelight excimer laser.

The Hartmann-Shack type [3]

A single laser beam is projected as a spot on the retina. The rays reflected off the retina are captured by the aberrometer’s objective lens, which is an array of tiny lenses (rather like the compound eye of an insect). The displacement of each spot of reflected light from its corresponding lenslet axis is measured. Mathematical integration of this information yields the shape of the aberrated wavefront. This aberrometer is used in conjunction with the Bausch and Lamb Technolas excimer laser.

Customized treatments

Standard LASIK treatments for all patients based on their refractive error alone treats the lower order optical errors alone - i.e. myopia, hypermetropia and astigmatism. In certain cases it could lead to an increase in the magnitude of higher order aberrations, such as those discussed above. Unpredictable results may result, more so in treatments with lasers that are not technologically advanced. Customized treatments address many factors that contribute to poor treatment, such as anterior and posterior corneal surfaces and thickness, pupil size, anterior chamber depth and anterior and posterior lens shape.

Laser / Wavefront interface [4]

The aberrometer captures five consecutive wave front measurements through a dilated pupil (to minimize spherical aberrations) on the day of surgery. These maps are then compared statistically and the three in closest agreement are used to generate a composite profile, which in turn creates the wavefront guided laser ablation profile and spot pattern.

The wavefront map is broken down into a calculation of the position of each excimer laser pulse to achieve the desired corneal profile. This requires perfect laser energy delivery (fluence) with each pulse, a small laser spot size for accurate placement as well as a very sensitive active eye tracker for a well-centred ablation profile despite eye movement during the treatment.

Aberrometer information is downloaded onto a floppy disc and interfaced with the laser software. The eye tacker is then engaged to exactly position the pupil in X, Y and Z-axes alignment. The wavefront determined pulsing sequence is made to correspond with the exact position of the aberra tions as seen at the level of the cornea. The WaveLight excimer laser tracks not only the pupillary position but also alters the contrast so that light and dark irides can have individually tailored eye-tracking (at 200 Hz) for dead-centre treatments.

The basic steps of LASIK treatment remain unaltered viz. A microkeratome is used to create a corneal flap so that the treatment can be performed in the inert stromal layer of the cornea, resulting in no discomfort or pain during the procedure. Laser ablation is performed on the stromal bed. The flap is repositioned perfectly in the same manner.

Advantages of wavefront-guided LASIK over conventional LASIK procedures

Recent studies indicate that upto 16% of wavefront-guided LASIK patients achieved a post-procedure uncorrected visual acuity of 6/3.4 None of the patients lost a line on their pre-operative best corrected vision and 70% of patients gained one to two lines of visual acuity on their pre-operative best corrected vision. These results remained stable when monitored over a 12 month period.

Refractive surgical procedures are known to cause an increase in higher-order wavefront errors. The aberrometer-linked treatments increased these errors by an average factor of 1.4 (some patients had almost non-aberrated retinal images which would have been compounded by conventional LASIK) as compared to conventional treatments where these errors were increased upto 40 times.

Wavefront treatment results were found to deliver stable post-operative results. This is probably more a factor of new generation excimer lasers, which treat with small spot sizes. They deliver homogeneous laser energy (fluence) with each pulse, accurately placed well-centred ablation profiles with almost non-existent acoustic shock to the corneal tissue.

The incidence of post-operative symptoms (due to large positive spherical aberrations and increased coma-like aberrations from small decentrations in treatment with conventional laser surgery - PRK and LASIK), such as glare, haloes, decreased contrast sensitivity and poor night vision are greatly minimized with wavefront treatments.

Wavefront-guided treatment has applications in complicated post-refractive surgery (RK, PRK, LASIK) patients (with significant symptoms, decentred treatments), as well as in patients professionally requiring acute vision.

Limitations of customized treatment

Aberrations induced by the optical system of the eye when it is non-accommodated would be different from those induced during accommodation. Therefore an eye corrected for distance vision may not have the same ‘super vision’ for near. Studies on patients who have undergone conventional LASIK indicate that this is not significant.

Aberrations in the optical system change with age. As the anterior chamber depth changes, the crystalline lens undergoes changes in volume and refractive index, and the vitreous undergoes liquefaction, the nature of aberrations may change, compromising vision to a small extent.

Surgical procedures such as cataract extraction would change the optical behaviour of the eye because the crystalline lens is replaced. The ‘super vision’ that the patient had before the cataract may not be possible to achieve after cataract surgery.

Patients may get accustomed to super normal visual acuity. Various factors could decrease the visual acuity (minor changes in the optical system of the eye, changes in corneal shape from the use of cosmetic lenses etc.). The customized LASIK patient may be happy in the event of a drop in visual acuity to 6/6 (conventional normal vision).

A relative limitation may be the prohibitive cost of equipment for the surgeon and the procedural cost for the patient. As with other technological instruments, it is likely that costs may lower as they have the world over with conventional refractive surgery.

CONCLUSION

The optical image quality of the human eye suffers from ocular errors (aberrations) which affect its visual potential. An aberration-free eye would result in a smooth wavefront image on the retinal surface, reaching an optical quality that matches or exceeds the mosaic of the photoreceptors of the retina. Conventional refractive procedures such as RK, PRK and conventional LASIK may increase aberrations causing symptoms such as glare, holes, decreased contrast sensitivity and poor night vision. These are particularly minimized by newer wavefront-guided lasers, which employ a small laser spot size for accurate placement and a sensitive active eye tracker for a well-centred ablation profile. Aberrations or optical errors of the eye are measured and analyzed using an instrument called the aberrometer. Wavefront-guided customized LASIK, also referred to as ‘customized ablation’, performs corrections based on individual wavefront measurements. Initial reports are very encouraging in terms of increased visual acuity and contrast sensitivity, predictability and stability of results, and minimal or no symptoms. Ophthalmic wavefront sensing coupled with new technology lasers have made wavefront-guided LASIK very reproducible with quick visual rehabilitation.

REFERENCES

1. Helmholtz H. Handbuch der physiologischen optik. Leipzig : Leopold Voss. 1867; 137-47.

2. The Dresden Wavefront Analyzer. Mrochen M et al. Department of Ophthalmology and Institute of Anatomy, TU Dresden, Germany.

3. Thibos LN, Hong X. Clinical applications of the Shack-Hartmann aberrometer. Optom Vis Sci 1999; 76 : 817-25.

4. Mrochen M, et al. Wavefront-guided LASIK : A brief report on feasibility and results. J Refract Surg. In press.


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