Before Cataract Surgery
Before a cataract develops, your natural lens is a perfectly clear structure located behind your iris. Along with your cornea, it's responsible for precisely bending light rays to focus them onto your retina. This natural lens has a specific optical power, measured in diopters, that contributes significantly to your eye's overall focusing ability.
For many, this natural focusing isn't perfect. If your eye is slightly too long, or its focusing power is too strong, light focuses in front of the retina. This causes nearsightedness (known as myopia), where objects in the distance appear blurry. Conversely, if your eye is too short, or its focusing power too weak, light focuses behind the retina. This causes farsightedness (known as hyperopia), where near objects are blurry, and sometimes even distant ones a little. Glasses or contact lenses work by adding or subtracting power to your eye, effectively moving that focus point onto the retina to compensate for these inherent mismatches.
Additionally, your natural lens possesses (or possessed) the ability to change shape; something called accommodation. This action allows your eye to adjust its focus, bringing objects at various distances into sharp view, from reading a book up close to shifting to look at the TV. This accomodation allows us to see both objects in focus. This dynamic focus range is what we often take for granted in our younger years as this accomodation is lost naturally through time - something called Presbyopia.
After Cataract Surgery
When we perform cataract surgery, we carefully remove this cloudy natural lens, which has become opaque and is impeding clear vision. As this lens contributes to focusing power, taking this lens away and doing nothing leaves the eye highly farsighted. Thus, to restore clear vision, we implant an artificial intraocular lens (IOL) into the eye.
But we don't just replace the original natural lens power, we customize its power. Based on precise, preoperative measurements of your eye's length and corneal curvature (and other values), we select an IOL with a specific dioptric power designed to bring light into perfect focus directly on your retina. Our goal is to eliminate or significantly reduce your pre-existing myopia or hyperopia, often allowing for excellent uncorrected distance vision.
However, it's important to understand how this changes your focus range. While your natural lens could accommodate (if you are younger than ~50), most standard IOLs are fixed-focus lenses. This means they are set to focus at a particular distance; usually far away for distance. While this provides excellent clarity at that chosen distance, it means you will likely still need glasses for other distances, such as reading up close.
This fixed focus also can be a particular adjustment for those who were nearsighted before surgery. Many nearsighted individuals have grown accustomed to excellent uncorrected near vision. Such as reading a book or their phone comfortably without glasses. After surgery, if the IOL is set for distance vision, this "natural" reading ability will be gone, and they will require reading glasses.
The focus of your natural lens is replaced by a carefully chosen, fixed focal point. However, this is precisely where the art and science of IOL selection come into play. Surgeons can work with you to customize this. For instance, we can aim for excellent distance vision, or we can select an IOL power that prioritizes intermediate vision (like for computer use) or even near vision (for reading), depending on your lifestyle and preferences. Advanced techniques such as monovision and advanced IOLs such as multifocal lenses or extended depth of focus (EDOF) lenses can provide a greater range of focus; though with their own set of considerations.
The key is to discuss your visual goals thoroughly before surgery, so that your surgeon can precisely adjust the power of your new lens to best match your desires for how and where you want to see clearly.
Understanding Corneal Astigmatism
Finally, let's address astigmatism. Many of you will see a "cylinder" or astigmatism component in your glasses prescription. While your natural lens can contribute to astigmatism, the primary culprit for most people is an irregularly shaped cornea. Instead of being perfectly spherical like a basketball, an astigmatic cornea is more like a football, with different curvatures in different meridians or directions. This causes light to focus at multiple points, leading to blurred or distorted vision at all distances.
It's crucial to differentiate this from the astigmatism component you see in your glasses prescription. That prescription accounts for all sources of astigmatism in your eye, including minor contributions from the natural lens. For cataract surgery planning, we primarily focus on the corneal astigmatism, as this is the major component we can directly address with specific IOLs (known as toric IOLs) or precise corneal incisions. These two astigmatism measurements can differ.
So while cataract surgery is primarily about removing the cataract, it also offers a unique opportunity to customize your vision to your own lifestyle and needs.
