In recent years multifocal IOLs have been gradually applied to the clinic, improving the defects of monofocal IOLs that are difficult to read at close range. Despite significant advances in IOL design, there is still a gap between IOLs and natural lenses with accommodative power. Understanding the design principles and improving surgical technique is fundamental to the use of multifocal IOLs. Do multifocal IOLs affect contrast sensitivity? Is it possible to implant a multifocal IOL in a pediatric cataract? What happens if I have residual diopters after implantation of a multifocal IOL? These questions are further elaborated with respect to the use of multifocal IOLs. Cataract patients are often implanted with an intraocular lens (IOL) after cataract surgery to remove the lens. Although this traditional treatment is able to fully restore the patient’s distance vision, a monofocal IOL does not provide adequate near vision, and the patient is often required to wear low prescription distance lenses to see close up. Reading is an integral part of everyone’s daily life, and the loss of this ability means a diminished quality of life. Solutions to this problem include monovision, adjustable IOLs, and multifocal IOLs. monovision involves implanting an IOL in the dominant eye for distance vision and an IOL in the non-dominant eye for near and intermediate distances; however, not everyone is comfortable with monovision due to the loss of perceived depth. Current adjustable IOLs suffer from insufficient and uncertain amplitude of adjustment and a high incidence of posterior deficits. The multifocal IOL, which has been used in recent years, is a more complex and advanced IOL that is implanted in the eye to provide two main focal points for near and distance vision, which improves the ability to see near more than monofocal IOLs. The image from the second focus is out-of-focus and very blurry, and it has been clinically proven that patients primarily perceive the focal image. However, a very small number of patients may perceive these unwanted images such as glare, flashes, and halos [1]. Mid-distance vision is primarily provided by the two main lens degree out-of-focus characteristics, and there are IOLs (e.g., ReZOOM) that have been added to provide mid-distance vision by design. Early multifocal IOLs compromised surgical outcomes due to astigmatism caused by the ultrasonic emulsification technique and the off-set position of the IOL, and, in addition, there were many reports of patients with reduced contrast sensitivity and the presence of halos and glare.The emergence of the Array SA40N (AMO) led to significant improvements in visual quality after cataract surgery, with many patients obtaining a more satisfactory treatment outcome. The second generation of multifocal IOLs, including the Tecnis Multifocal (AMO) ReSTOR (Alcon), has allowed more patients to move away from eyeglasses while significantly reducing postoperative glare and halos. Nevertheless, the design of multifocal IOLs is still not perfect, and there are still some problems in the process of using them, which need to attract our sufficient attention. I. Understanding the design principle is the basis for using multi-focal IOLs The design principle of multi-focal IOLs is illustrated with the example of ReSTOR, which utilizes the concept of diffraction-refraction design and has two main foci, one far and one near. The near focus corresponds to an increase of 3.2 D in the plane of the eyeglasses. The base lens optic provides distance vision using the principle of refraction, while the 12 ring steps molded in one shot on the front surface of the optic provide the increase in diffraction. These diffraction rings are located in the center of the optic in an area of 3.6 mm in diameter, and the portion of the optic outside of the 3.6 ~ 6.0 mm is the refractive area, which is primarily for distance vision. The height of the diffraction steps decreases gradually from 1.3 µm in the center to 0.2 µm in the periphery, and there are 12 zones, with the central zone having a diameter of 0.75 mm, and the outermost zone having a diameter of 3.6 mm. The height of the central steps produces a delay of about half a wavelength of light in the atrial water, so that, with a small pupil, 41% of the light energy is divided between the two lens diopters, i.e., far and near focuses, respectively. i.e., the far and near foci. The position of the dividing line of the rings determines the magnitude of the increase in diopters, and ReSTOR increases to 4.0 D in the lens plane (equivalent to 3.2 D in the plane of the framed spectacles). The amount of light energy distributed between the two focal points depends on the value of the height of the step. As the pupil becomes larger, more steps of progressively lower height are exposed.12 The height of the steps decreases progressively, with the outermost step height being only 0.2 µm, with more and more light being divided between the far focus and less and less light being divided between the near focus. The lowering of the height of the steps leads to a gradual change in the energy balance of the light. 3.6 mm away is the refractive zone of the monofocal point, there is no diffraction, all the light is used to look away, that is, as the pupil increases, the distribution of the light energy is gradually favored by the focal point of the far distance. In low light conditions at night, patients sometimes experience an out-of-focus image from the second degree of the multifocal IOL, creating halos. ReSTOR is designed with a limited diffraction zone, which limits the energy of the out-of-focus light at large pupil sizes, and the progressive step ensures that the redistribution of the light between the two images is gradual, so that ReSTOR is designed to reduce glare and halos. As the height of ReSTOR’s steps decreases, the energy of the two focal points changes gradually with the size of the pupil, with more and more light being diverted to the far focus (>41%) and less and less light being diverted to the near focus (<41%) as the pupil dilates. The human pupil is mostly small in natural or artificial conditions with the right amount of light. When reading close up, often with bright lights, our pupils are also smaller. When working in close proximity, our adjustment reflexes also make our pupils smaller. In these cases, ReSTOR provides enough light energy to see close or far (41%). When we are in pupil dilation, dark light conditions, and we are generally not engaged in complex reading or near work, but more in distance vision oriented activities, such as driving at night, ReSTOR is relatively able to provide more light energy for distance vision, and this balanced regulation of energy aids in distance seeing. The Tecnis multifocal all-optical surface diffraction has different characteristics compared to the ReSTOR. The IOL step height is the same for all-optical surface diffraction, and the light energy at both foci does not change with pupil size, with 41% of the light energy going to distance and 41% going to near. Therefore, theoretically, under the condition of dark light pupil dilatation, the Tecnis multifocal has a little more chance of halo than ReSTOR, but the Tecnis multifocal retains a better near vision.The new model of ReSTOR, SN6AD3, and the Tecnis multifocal both add an aspheric design of the optical surface, which can increase the nighttime contrast sensitivity, and further improve the quality of vision. Second, strictly control the selection of indications Multifocal IOL is designed for ideal orthokeratology, which is formed by special optical imaging principle to form two main focal points, and only by selecting the right patients, can its function be fully utilized. In the case of corneal astigmatism greater than 1.0 D, previous refractive surgery, congenital ocular anomalies (e.g. microcornea, pupil anomalies, lens subluxation, etc.), acquired iris and pupil anomalies, corneal opacities, recurrent inflammation of the uveal membrane, and suffering from endophthalmos diseases (retinal detachment, proliferative diabetic retinopathy, AMD, etc.), the preexisting condition may be aggravated and may interfere with the observation or treatment, or bring new uncomfortable symptoms to the patient, so it is recommended not to use multifocal IOL. serious suspensory ligament rupture, posterior capsule rupture, or even vitreous detachment during the operation, implantation should also be abandoned. C. Surgical Technique Intraoperatively, serious vitreous detachment, pupil injury or operation of artificially enlarging the pupil, damage to the suspensory ligament, tear or rupture of the anterior capsule margin, rupture of the posterior capsule membrane, and hemorrhage in the anterior chamber should be avoided. The anterior capsule tear needs to be positively rounded and centered, with an optical surface of less than 6.0 mm (usually ≤ 5.5 mm). Thorough removal of the cortex all the way to the periphery of the capsular bag is required, and posterior capsule membrane polishing is necessary; even mild posterior capsule membrane clouding can have a significant effect on near vision, leading to a decrease in visual quality. Diffractive, multifocal IOLs, produced using a precise manufacturing process, are controlled to a surface accuracy of 0.2 µm. Do not damage the diffractive surfaces of the optical section when mounting the fold or implanting the IOL with a pusher. Small incisions should be used for implantation to minimize surgical source astigmatism. After implantation of the IOL, the residual viscoelastic agent in the capsular bag should be thoroughly removed, and suctioning of the viscoelastic agent should be done gently to minimize damage to the diffractive optical surfaces. The IOL position should be adjusted, and the edge of the optical plane should be covered by a circular tearing capsule to ensure that the IOL is located in the center of the lens capsular band in a long-term and stable manner. The turbidity of the posterior capsule membrane was found to be timely YAG laser incision. Fourth, does multifocal IOL affect contrast sensitivity? Ferrer-Blasco et al. showed that there is no statistically significant difference between the distance and near contrast sensitivity of multifocal ReSTOR in bright light or dim light conditions compared with monofocal IOL. It has also been reported that the multifocal IOL contrast sensitivity was not statistically different from the monofocal IOL, although it was slightly reduced. However, Vingolo et al. reported a decrease in ReSTOR contrast sensitivity compared to monofocal IOL. After cataract extraction, the patient's contrast sensitivity normally increases significantly. Implanted multifocal IOLs may experience a temporary decrease in contrast sensitivity and glare due to the light being dispersed to different foci, even though the depth of focus is increased. Over time, the patient's cerebral cortex gradually accepts the multifocal IOL after a period of adaptation, and the contrast sensitivity of the operated eye generally recovers in 3 to 6 months.The height of the ReSTOR peripheral diffraction step decreases from the center to the periphery from 1.3 to 0.2 µm layer by layer, and this design further reduces the effect on contrast sensitivity.Pieh et al. concluded that the patient's contrast sensitivity decreased with the multifocal IOL. sensitivity reduction is related to the dispersion of light intensity by multifocal IOLs. Dick et al., on the other hand, believed that IOLs with clear optical axes received 2 to 3 times the beam of a normal lens, and therefore concluded that the dispersion of light intensity was not sufficient to cause a change in the contrast sensitivity of multifocal IOLs. It has also been suggested that retinal ganglion cells would be tolerant to changes in image contrast and brightness.Haaskjold et al. concluded that visual acuity and contrast sensitivity tend to decline in the elderly with age, and that the decline in macular function has a greater effect on contrast sensitivity than the IOL optical surface. Therefore, it is reasonable to assume that multifocal IOL contrast sensitivity changes will not have a significant impact on patients' visual quality. V. Can multifocal IOLs be implanted in children with cataract? Congenital cataract surgery is mainly aimed at treating amblyopia and reconstructing the fusion function. However, cataract surgery in children has a series of difficult problems different from those in adults, such as determining the degree of IOLs, postoperative myopic drift, postoperative inflammation, posterior cataracts, and secondary glaucoma. Undoubtedly, multifocal IOLs provide better pseudoadjustment, especially in patients with monocular cataracts, which is significantly better than monofocal IOLs.Refractive errors after implantation of monofocal IOLs can be corrected with lenses, while multifocal IOL residual diopters are corrected with lenses, and can it be adapted to children who still have difficulty in expressing themselves? Does it affect the treatment of amblyopia? Currently there is not much clinical experience.Jacobi et al[15] concluded that, after implantation of multifocal IOL after cataract surgery in children, with appropriate undercorrection of 10%~15%, postoperative children retained a certain degree of farsightedness, and they could look at the distance with the near focus of the multifocal IOL without glasses, and then gradually transitioned to the distance with the far focus with the development of the eyeballs. The correction of distance vision by multifocal IOL is not much different from that of monofocal IOL, but the visual quality of near vision is not as good as that of monofocal IOL. Multifocal IOL requires higher centering, and cataract surgery in children needs to tear open the posterior capsule and anterior vitrectomy, so it is difficult to implant the IOL in the capsule and center it, and it is easy to cause the IOL to be shifted by the postoperative inflammatory reaction, and the eccentricity will affect the retina of the multifocal IOL. retinal imaging quality of multifocal IOLs. Children's eyes have thinner corneas and sclera, and surgical-derived astigmatism tends to be higher than in adults, with a somewhat greater likelihood of halos and glare. Therefore, caution needs to be exercised regarding the implantation of multifocal IOLs in children. What if I still have residual diopters after multifocal IOL implantation? The purpose of implanting multifocal IOL is to avoid wearing glasses after cataract surgery, but there are still a small number of patients with some errors after surgery. If the residual IOL is large, it is recommended to use Holladay II formula for hyperopia and SRK/T formula for myopia to re-calculate IOL and then replace the multifocal IOL with another surgery. visual acuity can be improved by more than one line, the patient may be dissatisfied, and currently available correction methods include PRK, LASIK, and femtosecond laser-guided LASIK, which can reduce the surgical risk associated with IOL replacement.Alfonso et al. reported that 6 months after implantation of the ReSTOR in 53 eyes (31 patients), the residual equivalent spherical lens dioptricity was 0.5 D or more, with a mean equivalent spherical lens (0.20 ± 0.49) D (-2.00 ~ +1.00 D), and 6 months after reoperation with femtosecond laser LASIK, all patients' naked-eye distance visual acuity was improved to 0.83 ± 0.20 compared with the preoperative level without any surgical complications. Therefore, it is concluded that the refractive residual error after multifocal IOL can be accurately corrected with femtosecond laser LASIK surgery. Femtosecond laser LASIK surgery can predict the thickness of the corneal flap, reduce corneal epithelial damage, and improve the naked eye distance vision, which is safer and more accurate than PRK or LASIK surgery. In summary, multifocal IOLs provide patients with higher quality distance and near vision, average mid-distance vision, and are especially recommended for bilateral implantation with a higher postoperative off-set rate. The current multifocal IOL is not yet suitable for all patients, it cannot ensure 100% lens removal, a very small number of patients will have visual disturbances at night, and pediatric cataract patients are not yet suitable for implantation. Accurate biometric measurements and perfect surgical techniques are important guarantees to give full play to the function of multifocal.