The human eye is like a fully automated digital camera. The cornea and the lens in the eye are the lenses in this camera. Light passes through these two “lenses” and is focused on the retina, where the photoreceptor cells in the retina transmit the light information to the brain through the optic nerve fibers, and we see the colorful world.
Figure 1 Our eyes work very similarly to a camera
Figure 1a: The principle of the eye
Figure 1b: The principle of a camera
The lens in the eye is a biconvex lens-like transparency, like the lens in a camera. When it becomes cloudy due to aging, trauma, radiation, heredity, immunity, metabolic abnormalities, etc., it blocks light from entering the eye and affects people’s vision, which is often referred to as cataract. This is often referred to as a cataract. In this case, people may have blurred vision with double vision, difficulty reading in low light, halos when looking at lights, or have to change their glasses frequently. The clouding of the lens due to aging is called “senile cataract”, which is a natural process of aging.
What is a cataract?
Figure 2a: A normal person’s lens is able to focus light clearly on the retina
Figure 2b: A cloudy lens (cataract) cannot focus light clearly in the retina
The initial stage of cataract has little effect on vision, and some patients can improve their vision by wearing glasses. When it reaches the middle and late stage of cataract, when the corrected vision with lenses is <0.3 and affects the patient's normal work and life, cataract surgery can be performed. This is called ultrasonic cataract extraction combined with IOL implantation, which removes the cloudy lens (cataract) and replaces it with an artificial lens. It is currently the most effective means of treating cataracts. With the continuous development of microsurgical techniques, cataract surgery has made leaps and bounds from simply replacing a cloudy lens with an IOL to meeting various needs through IOLs with different design features. Proper selection of IOLs is a very important part of the process.
Rigid and Soft IOLs
IOLs are divided into two types of IOLs: rigid and soft, depending on the material used to make them. Rigid IOLs have been used in clinical practice for a long time and the clinical results have been fully verified, and the price is relatively cheap. Soft IOLs, also known as foldable IOLs, are characterized by their foldability, small incision during implantation, generally 2 to 3 mm, no suturing, and fast recovery of vision after surgery. With the development and popularization of cataract ultrasound phacoemulsification surgery, foldable IOLs are now commonly used in clinical practice.
Figure 3: Ultrasonic cataract surgery combined with implantation of foldable IOLs
Figure 3a: Ultrasonic emulsification nucleus
Figure 3b: Implantation of foldable IOL
Figure 3c: IOL in the lens capsule
Spherical and Aspheric IOLs
The surface design of IOLs can be divided into spherical and aspheric IOLs. When it comes to spherical aberration, we must first understand what spherical aberration is. When buying a camera, the clerk will introduce us to the lens of a high-grade camera, which is composed of aspherical lens. Why use aspheric? It turns out that the lens will produce aberration around the lens. Simply put, is the light through the spherical lens will produce focus error. Elementary school physics tells us that light rays passing through a convex lens converge on a point called the focal point. In reality, the light from the center of the lens and the light from the periphery do not converge on a single point. Thus, the difference between the peripheral light convergence and the central focus is called aberration. The human eye is a precision optical instrument created by nature, and its own crystal is aspherical, so the center and the periphery of the crystal itself are no longer on a sphere, but have a difference, which overcomes the spherical aberration and allows the light to converge to a point.
At the beginning of IOL design, the optical surfaces were all designed to be spherical, and the lens was fully capable of meeting the need for restoring vision after cataract removal, but the positive spherical aberration it produces at night when the pupil is large leads to reduced visual quality. In recent years, the concept of wavefront aberration in the human eye has been introduced in the field of refractive ophthalmology. The complementary nature of this positive and negative spherical aberration reduces the total spherical aberration of the human eye, which results in good quality of vision. As people’s demand for quality of life continues to improve, just as the lens of a camera is constantly being updated, the design of IOLs also transitions from the original spherical surface to aspheric surface. Through the aspheric design of the IOL surface, it has zero or negative spherical aberration to balance the positive spherical aberration of the cornea and reduce the total spherical aberration after cataract surgery, thus improving the night vision quality of patients after cataract surgery.
Since aspheric IOLs are aspheric in the periphery of the optical portion, it means that only when the pupil is larger than 4mm light passing through the peripheral portion will show imaging differences from spherical IOLs. Therefore, only cataract patients with normal pupil size and response to light will have improved postoperative visual quality if they choose aspheric IOLs.
Figure 4 Visual image differences between spherical IOLs and aspheric IOLs
Fig. 4a: Visual acuity after spherical IOL implantation Fig. 4b: Visual acuity after aspheric IOL implantation
IOLs for Astigmatism Correction
Astigmatism occurs in the cornea in a percentage of normal people. The percentage of people with astigmatism greater than 1.5D ranges from 15% to 29%. For this group, if a regular IOL is implanted, the original corneal astigmatism will remain after surgery, affecting the quality of vision. This is because conventional IOLs do not have the ability to correct astigmatism. Therefore, for cataract patients with combined corneal astigmatism, doctors will recommend a toric IOL, or TORIC IOL, to correct the astigmatism. This type of IOL has an optical spherical surface with an additional column lens, which can be used to correct different degrees of astigmatism, ranging from +1D to +6D. Astigmatism is divided into retrogressive astigmatism and cisgender astigmatism. The doctor will tell the patient the astigmatism status at the time of optometry. When a patient has a corneal antegrade astigmatism >0.75 or a cis-regulated astigmatism >1.5D, the TORIC IOL can be selected to obtain the best postoperative vision.
Figure 5 Acrysof Toric IOL
Multifocal IOLs
The vast majority of IOLs in clinical use today are monofocal IOLs. It has only one focal point and can only see objects at a fixed distance. After surgery, nearsighted glasses or presbyopic lenses are required to meet the needs of both distance and near vision. As older people work and study for longer years, more and more cataract patients desire to have both good distance vision and near vision after surgery. Multifocal IOLs have emerged in recent years to meet this need. The light passing through this type of lens can be focused to form multiple focal points to achieve both near and far vision, which is closer to a physiological lens. However, this type of lens divides the light energy entering the eye into two, half for near vision and half for far vision; some patients may suffer from nighttime visual disturbances, glare, and decreased contrast sensitivity. Some patients with multifocal IOLs will have a learning and adaptation process after surgery.
Figure 6: Restor multifocal IOL
Individualized IOL Selection
The calculation of the IOL’s degree is complex. Before cataract surgery is performed, the corneal curvature, anterior chamber depth and axial length of the operated eye must be measured. Based on these data, the surgeon selects the appropriate IOL formula for the patient’s calculation. The calculated prescription also needs to be matched with different IOL constants to finalize the proposed IOL implantation prescription.
Basically, all of these IOLs are suitable for the average elderly cataract patient. The specific IOL to be used needs to be combined with your own situation. Folding IOLs are commonly used nowadays. Patients who have low requirements for visual quality and are not financially well-off can consider ordinary spherical monofocal IOLs. Patients who are financially well-off and seek quality vision can consider specially designed IOLs including aspheric, multifocal or astigmatism-correcting IOLs.
Some special patients need to choose specially treated IOLs. For example, patients with diabetic retinopathy, patients with uveitis, and patients after glaucoma surgery may choose IOLs made of hydrophobic acrylic or with heparin-treated surfaces to reduce post-operative inflammatory reactions. These IOLs are more biocompatible and have less inflammatory reactions. When diabetic patients develop fundus lesions that require treatment such as laser, this material can also tolerate laser well and ensure minimal laser energy loss. When one eye has been operated, it is better to use the same type of artificial lens for the second eye. Patients with post-retinal detachment complicating cataracts cannot choose to implant IOLs with special features such as multifocal because of the possible distortion of vision, and it is better to choose to implant ordinary monofocal crystals. For patients with iris loss who have lost the light regulating effect of the pupil, an IOL with iris needs to be implanted.
Patients with senile cataracts should inform their doctors in advance if they have some special needs. The doctor will analyze the specific situation and evaluate whether the special need is compatible with the implantation of a specially designed IOL, so that the design of the surgical plan and the selection of the lens can be better customized for each individual examination result.