1.Basic concepts
1.1, wavefront aberration
Light is traveling electromagnetic waves, a point source of light emitted by the light, will propagate in all directions as a wavefront, the propagation of light in the process of phase consistent points connected to the composition of the surface is called the wavefront, also known as wavefront. In the ideal imaging, a point light source through the optical system into the image, should be an ideal image point as the center of the sphere, but in the actual imaging, due to the imaging characteristics of the optical system or due to defects in the optical system, resulting in the actual wavefront and the ideal wavefront is not the same wavefront, the difference between the actual wavefront and the ideal wavefront, that is, the wavefront aberration.
Aberrations exist in all optical systems and can be divided into chromatic aberrations and monochromatic aberrations from the perspective of geometric optics. Monochromatic aberration can be divided into five types: spherical aberration, coma, field curvature, image dispersion and aberration, which are formed due to the structure of the refractive medium itself. From a physical optics perspective, aberration can be defined as wavefront aberration (or wavefront surface aberration). Wavefront aberration is an important indicator of the imaging quality of optical systems.
1.2, the aberration of the human eye
The normal human eye is an extremely complex optical system, which also has aberrations. The sources of wavefront aberration in the human eye are mainly.
(1) The surface of the cornea and lens is not ideal;
(2) The cornea is not on the same axis as the lens and vitreous humor;
(3) Unevenness of the inner material of the cornea and lens and vitreous humor, resulting in local deviations in refractive index.
For the human eye, it is most meaningful to have a clear image of the retina, so it is important to accurately measure the aberration of the human eye. The aberration is divided into 2 parts: low-order aberration and high-order aberration, relative to the clear image of the retina. Low-order aberration refers to the blurring or distortion of the retinal image caused by refractive problems such as out-of-focus (including myopia and hyperopia) and astigmatism; high-order aberration refers to other optical defects of the refractive system such as spherical aberration, comet aberration, irregular astigmatism, etc.
The total higher-order aberration and spherical aberration of the whole eye increase significantly with age, while the corneal origin does not change significantly and the lens origin increases significantly with age and gradually develops from negative to positive values.
1.3 Evaluation and measurement of wavefront aberration
Zernike polynomial is a serial function orthogonal to the unit circle, which can decompose the wavefront aberration into multiple order images, and quantify the aberration of the ocular optical system by Zernike polynomial. The Zernike polynomial is divided into 7 orders of 35 terms, namely, 1st order tilt (including 2 terms, denoted by C1, C2, and the same below), 2nd order defocus, astigmatism (C3-C5), 3rd order (C6-C9), 4th order (c10-c14), 5th order (c15-c20), 6th order (c2l -The aberrations after the second order are higher order aberrations, which are the hotspots of research in recent years, among which the more important ones are coma aberration of the third order and spherical aberration of the fourth order, in addition to secondary astigmatism, clover aberration, etc. In addition, there are secondary astigmatism, clover aberration, etc. Except for the spherical aberration in the fourth order, which can be corrected by the elimination of spherical aberration lens, the rest of the higher order aberration cannot be corrected by optical glasses. In general, the lower order aberration has a greater impact on the quality of optical imaging than the higher order aberration. the impact of Zernike aberrations on optical quality in descending order is: spherical aberration, defocus, astigmatism, coma aberration, clover, clover, five-leaf clover, etc.
Wavefront aberrometry is a new type of measurement instrument that works by allowing a parallel beam of light to be directed into the eye, focused on the retina to produce a light source point, and then reflected back from the retina. The actual direction of the outwardly reflected light can be measured using a matrix of lenses that form a wavefront dot matrix. Because the reflected light undergoes all the aberrations of the ocular optic system, the direction must deviate in optical range from the ideal parallel reflected light direction. By comparing the deviations between them, the aberration distribution of the whole ophthalmic system can be calculated and displayed on the screen in the form of a three-dimensional image, which is a more intuitive wavefront aberration map.
2.Wavefront aberration and keratoconus
2.1 Wavefront aberration after keratoconus surgery
Optical defects such as myopia, hyperopia and regular astigmatism that reduce the visual function of the human eye can be corrected by frames, contact lenses or keratoconus surgery. Higher order aberrations such as spherical aberration and comet aberration, which also reduce the visual function of the eye, can only be corrected with corneal refractive surgery. While corneal refractive surgery improves vision, other visual problems such as glare, halos, and poor night vision also occur. According to the literature, there is a significant increase in higher-order aberrations after RK, PRK, and LASIK, especially in the third-order emmetropia and fourth-order spherical aberration, and there is a significant correlation between postoperative aberration and pupil size.
The reasons for the increase in wavefront aberration after clinical refractive surgery were found to be: (1) unsatisfactory change in corneal curvature; (2) off-center cutting; and (3) corneal irregularity, corneal haze clouding and wound healing reaction. This shows that the application of wavefront aberration detection technology after refractive surgery is of great importance, i.e., it can make an accurate evaluation of the surgical results, help improve the surgical method, and also recognize and deal with patients who have good vision but have other visual quality complaints.
2.2. Wavefront aberration-guided individualized refractive surgery
Due to the relatively homogeneous nature of traditional laser refractive surgery procedures, postoperative increases in ocular aberration can occur in varying degrees and forms for different individuals. Myopic patients often have complaints and grievances regarding visual quality after surgery. As a result, many scholars have proposed “individualized cutting”. Individualized cutting refers to the targeted laser cutting for individual patient differences, including two meanings: one is the targeted cutting for patient-specific wavefront aberration, to achieve not only the correction of myopia, hyperopia and astigmatism in the traditional sense of refractive error, but also the correction of comet aberration and other high aberrations, so that patients can obtain extraordinary vision after surgery. The second is the targeted correction of wavefront aberration secondary to laser surgery to improve postoperative wavefront aberration and visual acuity.
Wavefront aberration-guided individualized cutting is based on the information provided by the wavefront aberrometer, where the measured aberration is expressed in the form of irregularities in the corneal surface, and then the excimer laser is used to precisely shape the corneal surface with fine structures, aiming to make every point projected to the cornea accurately focused at the macula, thus maximizing the development of the potential visual acuity of the human eye. Therefore, the design of the cutting solution is critical.
Numerous clinical studies have shown that wavefront-guided individualized cutting not only reduces the original aberration but also reduces the incidence of surgically-derived aberrations. The reduced depth of cut compared to conventional surgery prevents corneal swelling and a large amount of surgical-derived aberrations, improves the postoperative nocturnal suboptimal results, and provides a wider postoperative visual field, resulting in better naked eye vision and visual quality. In addition, the wavefront aberration technique can be used to detect post-refractive complications such as central island and off-center cuts, and then guide the laser to further precisely shape the corneal surface to correct the patient’s aberration and improve the patient’s visual acuity and quality of vision.
However, wavefront aberration-guided individualized cutting is not suitable for all people, and the technique is more effective in correcting low to moderate refractive errors, but less effective for high and ultra-high refractive errors. Individualized cutting is not yet able to eliminate the effects of diffraction and chromatic aberration; eliminating higher order aberrations increases the visual quality of central vision, but reduces the visual quality of peripheral vision, and requires a balanced choice.
3.Wavefront aberration and cataract, IOL
The lens plays an important role in compensating for corneal aberration, and changes in the lens can have a certain effect on the aberration of the human eye. Thus the aberration technique can be used to evaluate the effect of cataract and its surgery on visual quality as well as to study the design and selection of IOLs and the improvement of cataract surgery methods to ensure the best visual quality for post-cataract surgery patients.
3.1. Wavefront aberration in cataract eyes
Differences in the location and severity of lens clouding cause different changes in wavefront aberration. Comet aberration is the main component in cortical cataract, while different spherical aberration is the main component in nuclear cataract. All nuclear cataracts have negative spherical aberrations, and all cortical cataracts have positive spherical aberrations. Many cataract patients still have good vision in the early stage, but have symptoms of blurred vision, photophobia, monocular diplopia or even triple vision, which may be the result of the increase of spherical aberration as well as secondary astigmatism.
3.2. Wavefront aberration in cataract IOL eyes
Studies have shown that the aberrations of IOL eyes are greater than those of natural lens eyes. There are 3 main reasons for this.
(1) The optical properties, size and structure of the two crystals are very different. The density, refractive index and refractive power of the natural lens are different, its thickness changes with adjustment, and the refractive index of the peripheral part of the lens is smaller than that of the central part, which can offset the spherical aberration. An artificial lens does not have these characteristics. The spherical aberration of the IOL also varies depending on its material and design, and increases when the pupil increases, which is the reason for the blurred vision and glare at night in the IOL after cataract surgery.
(2) Changes in the relative relationship between the IOL and the cornea; the aberration of the human eye is mainly composed of corneal aberration and internal aberration (mainly the lens). The cornea has positive spherical aberration with less variation, and the transparent lens is negative spherical aberration, which can compensate for the positive spherical aberration of the cornea. The IOLs currently used in clinical practice are biconvex or plano-convex structures, which do not balance the aberration of the cornea and increase the spherical aberration, causing a decrease in visual quality.
(3) Surgery has an impact on the cornea.
3.3. Wavefront aberration and IOL design
It is generally believed that the ideal IOL should be designed to not only improve the patient’s visual acuity, but also to compensate for corneal aberration and to minimize overall aberration as much as possible. Implanting an aspheric IOL with negative spherical aberration can reduce the spherical aberration of the human eye, increase the contrast sensitivity of amblyopic patients, and give them better visual quality. With the application of wavefront aberration technology in cataract field and IOL design in recent years, aspheric IOLs are gradually becoming mature. The main design concepts of aspheric IOLs now used in clinical practice are divided into 3 categories.
(1) Zero spherical aberration IOL: implantation in the eye does not change the original spherical aberration in the eye.
(2) Aspheric lens with -0.27µm spherical aberration: implanted in the eye to offset the positive spherical aberration of the cornea, so that the whole eye spherical aberration is zero.
(3) Aspheric IOL with a spherical aberration of -0.20µm: After implantation, it partially cancels the positive spherical aberration of the cornea and preserves +0.10µm spherical aberration in the whole eye, which is consistent with the spherical aberration characteristics of the young population. Preoperative measurement of corneal spherical aberration is necessary when selecting an IOL.
4. Outlook
The application of wavefront aberration technology in ophthalmology is a milestone, which can accurately and objectively describe the imaging characteristics of the human eye and make it possible for people to have perfect vision, and its application in ophthalmology clinics is becoming more and more important. However, in refractive surgery, the problems of how to design the elimination of aberrations in individualized cutting and which aberrations to eliminate are still unsolved, and how to optimize wavefront-guided refractive surgery is still a very important issue. The application of wavefront aberration technology in the field of cataract surgery cannot be truly individualized yet. Therefore, there is still a long way to go for the ideal application of wavefront aberration in ophthalmic clinical practice.