How to choose an IOL for cataract surgery
The lens in the eye is equivalent to the lens of a camera. The lens of an ortho-optic eye has a convex lens function of about 20D (1000 degrees of glasses), in addition to the adjustment function, which is equivalent to the zoom lens of a camera, so that we can see far and near clearly. After cataract surgery to remove the intraocular lens, the human eye lacks a convex lens of about 20D, and the focal point does not converge on the retina, so if an equivalent convex lens is not implanted, the patient cannot see anything after surgery. In the past, when IOLs were not popularly used, patients compensated by wearing glasses of about 1000 degrees after cataract surgery, but the weight of glasses with 1000 degrees was heavy, distortion of vision (spherical aberration), appearance of colored fringes (chromatic aberration), and reduction of visual field were serious.
IOL implantation is the most effective way for cataract patients to see again, and the invention of this technology brought a leap forward in the field of ophthalmology. The first IOL implantation in the world was done in 1949 by Harold Ridley, a famous British ophthalmologist, who replaced the original lens with an artificially manufactured lens (i.e., IOL implantation) to correct postoperative hyperopia, which opened the curtain of IOL development. The history of IOL implantation in China has also been more than 30 years. With the continuous progress of new material science and processing technology, IOLs have also been introduced. At present, there are many types of IOLs available, including ordinary hard IOLs, foldable IOLs, traditional spherical IOLs, aspheric IOLs, adjustable IOLs, multifocal IOLs, dye-added IOLs, surface-coated IOLs, astigmatic IOLs, micro-incision IOLs, etc., which can satisfy different patients. IOL, aspheric IOL, adjustable IOL, multifocal IOL, dye-added IOL, surface-coated IOL, toric IOL, micro-incision IOL, etc., can meet the needs of different patients’ conditions and economic conditions.
1. Hard and soft (foldable, push-injection) IOLs: The production material for hard IOLs is polymethyl methacrylate (PMMA, a special plastic material), and the production materials for soft IOLs, also known as foldable IOLs, are acrylate (special plastic material) and silicone. Hard IOLs have been used in clinical practice for a long time and their clinical results are well proven. They are inexpensive, but the surgical incision for implantation is large, usually around 6 mm. Soft IOLs are characterized by foldability, so the incision at implantation is small, usually at 2-3 mm, without sutures, and the recovery of vision after surgery is fast, but the price is relatively high. Soft (foldable) IOLs are currently used in ultrasound phacoemulsification procedures in major cities.
2. Spherical IOL and aspheric IOL: The radius of curvature of all points on the convex lens curvature surface of the traditional spherical IOL are equal, which will produce spherical aberration and chromatic aberration while imaging (Figure 1), especially in the large pupil state (at night, etc.); in addition, positive spherical aberration exists in the human cornea, while negative spherical aberration exists in the lens of young people, and the two cancel each other out for clearer vision; with ageing With age, the aging lens gradually increases its spherical aberration due to nuclear sclerosis and refractive index changes, losing the ability to compensate for positive corneal spherical aberration and increasing the spherical aberration of the human eye. Aspheric IOLs have aspheric design on the peripheral part of the anterior or posterior surface of the IOL (the peripheral part has unequal radius of curvature to the optical center, Figure 2), thus reducing spherical aberration and chromatic aberration and offsetting the positive spherical aberration of the cornea (Figure 3). IOL imaging with aspheric design reduces spherical aberration, chromatic aberration, and sharpens imaging, especially at night. It will be an essential design for IOLs, as will camera lenses. Spherical aberration and chromatic aberration are higher level aberrations, and only lower level aberrations need to be corrected after better correction, which means that patients with large postoperative astigmatism or large myopia or hyperopia, or preoperative estimation of large fundus problems, or cataract extracapsular extraction surgery do not need to implant aspheric IOLs. The implantation of aspheric IOLs requires a high level of ultrasound emulsification and IOL implantation skills, otherwise, the visual effect will be inferior to that of spherical IOLs due to postoperative deviation and tilting. Currently, imported single-focus spherical IOLs are about RMB 1500-2600 (the price varies depending on other design elements), and imported single-focus folding IOLs with aspheric design are about RMB 2500-3500. Figure 1 Figure 2 Figure 3
3. Monofocal with adjustable IOL and multifocal IOL: The vast majority of IOLs in clinical application at present are monofocal IOLs, which have only one focal point and the eye implanted with this IOL can only see objects at one distance clearly. Therefore, it is still necessary to wear nearsighted glasses or fancy glasses to meet the demand of seeing far or near after the surgery. Adjustable IOLs developed in recent years are based on two main principles: displacement adjustment and multifocal imaging, and the corresponding IOLs are also called adjustable IOLs and multifocal IOLs. These IOLs are useful for improving the vision throughout the post-cataract surgery period and are less dependent on glasses. (1) Adjustable Folding IOL: Designed according to the principle of human eye adjustment, it can provide better distance and near vision at the same time. Physiologically based accommodation refers to the ability of the eye to clearly image objects at any distance on the retina by the contraction of the ciliary muscle. In young, crystalline eyes, accommodation is accomplished by contraction of the ciliary muscle, relaxation of the suspensory ligament, increase in the thickness of the central lens, and change in the refractive power of the lens. Theoretically, the human eye requires at least 8.00D of accommodation under physiological conditions. The design of adjustable folding IOL adopts the design concepts of displacement adjustment, double optical surface adjustment and deformation adjustment, which produces the adjustment principle similar to that of human lens, i.e., the contraction of ciliary muscle leads to the relaxation of lens suspensory ligament, and the elastic retraction of lens capsule membrane leads to the adjustment of adjustable folding IOL with the subsequent deformation and thickening of refractive power. The design concept is closer to the natural state of the human eye, as it does not involve the distribution of optical energy at multiple intersections when imaging at different focal points; in addition, compared to multi-intersection IOLs, the adjustable folding IOL is less demanding on the eye when implanted. The biggest shortcoming of the adjustable folding IOL is its poor adjustment ability, which is about 0.5-1.5D, and it cannot form clear distance and near vision throughout the whole process. This lens is a good choice for patients who do not have high requirements for near vision. The implantation of adjustable folding IOLs also requires a high level of surgical skill in ultrasound emulsification. Currently, adjustable folding IOLs are imported and cost between 6000-8000. (2) Multi-intersection folding IOL: Improving the vision throughout the post-cataract surgery has become a concern for ophthalmology clinicians. Lens ultrasound emulsification combined with IOL implantation has evolved from a simple rejuvenation surgery to a refractive surgery, and the emergence of multifocal IOLs has provided us with a solution to this problem. Currently, there are two types of multifocal IOLs in clinical use, one is refractive and the other is diffractive. A) Refractive multifocal IOL: The Array type and ReZoom type IOLs from AMO are the representatives of refractive IOLs. Most of them are biconvex lenses, with the front surface consisting of 3 to 5 refractive zones of different refractive power, and the distant, intermediate and near focal points are arranged in concentric circles, with a difference of +3.50 D between the distant and near focal points (Figure 4). The advantage of this type of IOL is the simplicity of the concept using the traditional refractive technique. Each zone is responsible for imaging only the distal or proximal focal points, the imaging is dependent on the pupil size, and the image quality is influenced by the pupil size and the IOL deviation. The optical section is optimized to have less impact on the contrast sensitivity of the operated eye. The refractive multifocal IOL has better distal and mid-range visual acuity. Fig. 4 B) Diffractive multifocal IOL: Represented by the Restore from Alcon (USA) and the Tecnis multi-intersection IOL from AMO (USA), the optical section adopts a stepped progressive diffraction technique with a stepped design in multiple concentric circles, the height of which is between 1.3 and 0.2 μm (300 times the diameter of a hair), and the width of the step follows the same pattern The step width decreases with the same pattern, and the peripheral region is the refractive zone (Figure 5). The progressive diffraction structure blends with the peripheral refractive zone, allowing the distribution of light energy to gradually favor the distant focal point as the pupil increases, minimizing nighttime visual interference, while the progressive step design also significantly improves the quality of near imaging (Figure 6). Both IOLs incorporate an aspheric design, with the Restore being a yellow-dyed IOL. The diffractive multifocal IOL is superior for distance and near (50-20 cm) vision (Figure 7). This type of IOL is especially suitable for patients who work at close distances (reading books and using computers). The author observed that if the patient’s own eye condition is good, 90% of them do not need to wear fancy glasses to read computers and newspapers after surgery, and the effect of implantation in both eyes is even better. Patients with frequent night work, astigmatism >0.75D, significant refractive interstitial clouding, small pupils, and poor fundus are not suitable for implantation. The implantation of the above-mentioned multi-intersection IOL requires high surgical skills and precise IOL calculations, otherwise, the advantages of this IOL cannot be brought into play; in addition, the percentage of out-of-pocket expenses for the surgery is high. Currently, the cost of all surgeries for patients with medical insurance in Beijing is about RMB 11,000, of which about RMB 7,500 is out-of-pocket. Although multi-intersection IOLs take better care of medium and near vision than multi-intersection IOLs, patients who are picky about their vision still cannot get rid of their glasses. Figure 5 Figure 6 Figure 7
4. Dye-added IOLs: Some scholars believe that the high-energy part of visible light (violet and orchid light) may cause damage to the macula of the fundus. In addition, some patients who have clear IOLs implanted after cataract surgery will have more light entering their eyes, and although they can see clearly, they will have discomfort such as “shaking eyes”. For these reasons, many foreign IOL manufacturers have designed dye-added IOLs, like dyeing or color-changing glasses, to adjust the visible light entering the eye to meet the needs of different patients. Among them, yellow-dyed IOLs are represented by Hoya (Japan) and Alcon (U.S.), which add yellow-dyed materials to IOLs (Figure 8) to make patients see as they did when they were 50-60 years old after surgery. Medennium (USA) adds color-changing dyes to IOLs so that under strong high-energy light (outdoors), IOLs are yellow, and if high-energy light is reduced in the environment (indoors), IOLs are almost colorless (Figure 9 and 10). This way, the IOL filters out some of the high-energy light and protects part of the patient’s fundus while not affecting color vision and reducing visual sensitivity in dark environments. Dye added IOLs have been clinically observed to reduce the sensation of “wobbly eyes” in bright environments and may have a protective effect on the fundus. Some foreign scholars have reported that some color-sensitive patients have mild color deviation after implantation of these IOLs; in addition, for people with low visual sensitivity (e.g., elderly people), their visual acuity in dark environments may be reduced. Based on the above academic debate, it is inconclusive whether dye added IOLs are suitable for all patients. Figure 8 Figure 9 Figure 10
5. Surface drug-coated IOLs: This category is represented by the IOL surface coated with heparin, which acts as an anticoagulant that inhibits the formation of fibrin clots and has direct anti-inflammatory effects. IOLs treated with heparin surface can reduce the foreign body reaction after surgery, reduce the occurrence of posterior capsule clouding, etc., and have the advantages of quick recovery and short drug administration time.
6. Micro-incision push-in IOL: The IOL with the smallest incision at present can be implanted with 1.4 mm incision. 1.4 mm incision is required to remove the cataract before implantation of this IOL. The smallest incision for IOL implantation results in a smaller incision and less astigmatism of surgical origin. It has been reported abroad that the effect of 1.4mm versus 3.0mm incision on the quality of vision of patients is not significant. This procedure is not suitable for patients with a hard lens nucleus.
7. Toric IOL: represented by Toric toric IOL of Alcon, USA, the toric IOL adds astigmatism (column lens) to the spherical refractive power, and during surgery, the axis of toric IOL is positioned at the axis of the patient’s corneal astigmatism, thus correcting the corneal limbal astigmatism while removing the cataract and implanting the IOL.
The above mentioned IOL design elements can be repeated in one IOL, such as folded aspheric design diffractive multi-intersection IOL, folded heparin surface-coated aspheric design IOL, micro-incision aspheric design IOL, folded yellow-dyed aspheric design IOL, etc. In addition, there are many different designs (three-piece, one-piece, round-square edge design, how to climb type, IOL optical diameter, etc.) for the same type of IOL (such as aspheric folding type), and there are many varieties of IOLs after the combination of various designs. The more design elements of an IOL are superimposed, the higher the price. Currently, in Beijing, IOL patients are reimbursed 810 RMB for medical insurance and public medical expenses, and the difference is paid out of pocket. How to choose a suitable one among so many IOL varieties is difficult for the patient to do on his own and requires the guidance of a specialist. When choosing an IOL, first of all, the patient should clarify his or her lifestyle and eye habits, i.e., what basic functions the IOL needs, such as whether he or she often works at close range (requires near vision), whether he or she has a lot of nightlife (preferably aspheric design), etc.; secondly, inform the doctor whether you have other eye diseases and systemic diseases affecting your eyes; thirdly, your special lifestyle habits (e.g., not wanting to wear glasses when looking at close objects, photophobia etc.); fourth, the price of IOL you can accept. Based on the above requirements, the surgeon will take into account your age, eye condition, IOL refractive index calculation results, etc. to select a suitable IOL for you and seek your opinion.