The lens in the eye is equivalent to the lens of a camera. The lens of an orthoptic eye has a convex lens function of about 20D (1000 degrees of glasses), and in addition, it has an 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 focus does not converge on the retina. 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 the glasses with 1000 degrees was heavy, and the visual distortion (spherical aberration), colored fringes (chromatic aberration), and reduced 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 meet the needs of different patients. IOLs, etc., can meet the needs of different patients’ conditions and economic conditions, and the following is an introduction to the technically mature IOLs that have been used in clinical practice one by one.
1.Hard and soft (foldable, push-injection) IOLs.
The production material of hard IOL is polymethyl methacrylate (PMMA, a special plastic material), and the production material of soft IOL, also called foldable IOL, is acrylate (special plastic material) and silicone.
Hard IOLs have been used in clinical practice for a long time and their clinical results have been fully verified. 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 emulsion surgery in major cities.
2.Spherical IOL and aspheric IOL.
The radius of curvature of all points on the convex lens curvature surface of traditional spherical IOLs 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; as age grows, the aging lens gradually increases its spherical aberration due to nuclear sclerosis as well as refractive index change After cataract removal, the negative spherical aberration of the IOL is needed to compensate for the positive spherical aberration of the cornea.
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 can reduce spherical aberration, chromatic aberration and image sharper, especially at night. It will be a fundamental 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.
Aspheric IOLs are about 1000-1500 RMB more expensive than the same type of spherical design IOLs. Currently, imported single-focus spherical design folding IOLs in the market are about 1500-2600 RMB (prices vary due to different design elements), and imported single-focus folding IOLs with aspheric design are about 2500-3500 RMB.
3.Monofocal and adjustable IOLs, multifocal IOLs.
The vast majority of IOLs in clinical application at present are monofocal IOLs, which have only one focal point, and the eyes implanted with such IOLs can only see objects of 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 serve to improve 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 relying on the contraction ability 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 refraction 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 the poor adjustment ability, which is about 0.5-1.5D, and it cannot form a clear vision of both near and far 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 products and cost between 6000-8000.
(2) Multi-intersection folding IOL: Improving the whole vision after cataract surgery has become a concern for ophthalmology clinicians, and lens ultrasound emulsification combined with IOL implantation has developed into refractive surgery from a simple restorative 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 type and the other is diffractive type.
A) Refractive type multifocal IOL: The Array type and ReZoom type IOLs from AMO of America are the representatives, and the optical principle follows the law of refraction of light. It is mostly a biconvex lens, with the front surface consisting of 3 to 5 refractive areas of different refractive power, and the distant, intermediate and near focal points are arranged in concentric circles, and the difference in refractive power between the distant and near focal points is +3 and 50D (Figure 4). The advantage of this type of IOL is that it uses the traditional refractive technique with a simple concept, each zone is responsible for imaging only the distal or near focal points, imaging is dependent on pupil size, and image quality is affected by pupil size and IOL deviation. The optical section is optimized to have less impact on the contrast sensitivity of the operated eye. Refractive multifocal IOLs have better distal and intermediate visual acuity.
B) Diffractive multifocal IOL: Represented by Restore made by Alcon in the United States and Tecnis multi-intersection IOL made by AMO in the United States, the optical surface adopts step progressive diffraction technology, with a step-like 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 step width decreases with the same pattern The outer peripheral region is the refractive zone (Figure 5).
The progressive diffraction structure blends with the peripheral refractive zone, so that as the pupil increases, the distribution of light energy gradually favors the distant focus, minimizing visual interference at night, and 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 range (reading books, using computers). I 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 bilateral implantation is more.
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 expenses. 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.
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, and in addition, some patients have clear IOLs implanted after cataract surgery, and the light entering the eye increases, which may produce discomfort such as “shaking eyes” at the same time, although the vision is clear. 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 Corporation of Japan and Alcon Corporation of the United States, where yellow-dyed materials are added to the IOL (Figure 8) to make the patient’s postoperative vision as if he or she were 50-60 years old.
Medennium (USA) adds color-changing dyes to IOLs so that IOLs are yellow under strong high-energy light (outdoor) and nearly colorless in environments where high-energy light is reduced (indoor) (Figure 9), so that IOLs filter out some of the high-energy light in outdoor areas and protect 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 patients with color-sensitive vision have mild color deviation after implantation of these IOLs; in addition, for people with low visual sensitivity (e.g., elderly people), their vision in dark environments may be reduced. Based on the above academic debate, it is inconclusive whether dye-added IOLs are suitable for all patients.
5. Surface drug-coated IOLs.
This category is represented by the surface heparin coating of IOLs. Heparin, as an anticoagulant, can inhibit the formation of fibrin clots and has a direct anti-inflammatory effect. The IOL treated with heparin surface can reduce the foreign body reaction after surgery and reduce the occurrence of posterior capsule clouding, etc. It has the advantages of quick recovery and short medication time.
6.Smallest incision pushing IOL.
At present, the IOL with the smallest incision can be implanted with 1 or 4 mm incision, and this IOL needs to have 1 or 4 mm incision before implantation to remove the cataract with the new technology of cold ultrasound. The smallest incision push-in IOL has a small incision and less astigmatism of surgical origin. It has been reported abroad that the effect of 1,4mm versus 3,0mm incisions on the quality of vision of patients is not significant. This procedure is not suitable for patients with hard lens nuclei.
7. Astigmatic IOL.
Represented by Toric toric type IOL of Alcon, USA, toric (column lens) is added to the spherical refractive power, and the axis of toric IOL is positioned at the axis of patient’s corneal astigmatism during surgery, thus correcting corneal limbal astigmatism while removing cataract and implanting IOL.
Several design elements of the above mentioned IOLs can be repeated in one IOL, such as folded aspheric design diffractive multi-intersection IOL, folded heparin surface coated aspheric design IOL, tiny 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 (e.g. aspheric folding type), and after the combination of various designs, there are many varieties of IOLs, and Dr. Zhang Ship roughly estimates that there are nearly 100 types of IOLs tendered by the Beijing government. The more design elements of an IOL are superimposed, the higher the price. Currently in Tianjin, IOL medical insurance and public medical patients are reimbursed 1,900 yuan, and the difference is paid out of pocket.
How to choose a suitable one among so many varieties of IOLs 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 you often work at close range (requiring near vision), whether you have a lot of night life (preferably aspheric design), etc.; secondly, inform the doctor whether you have other eye diseases and systemic diseases that affect your eyes;
Third, your special habits (such as not wanting to wear glasses for near vision, photophobia, etc.); fourth, the IOL price you can accept. The surgeon will choose the IOL that is suitable for you based on your above requirements, combined with your age, eye condition, IOL refractive index calculation results, etc. and seek your opinion.