Different types of Cataract surgeries and IOLs
This article will provide some basic information on the different types of cataract surgeries and IOLs available.
More than 50% of the Indian population above 60 years of age is found to have reduced visual clarity due to cataract (clouding of the lens), making cataract surgery one of the most common surgeries performed on the eye. Over the years, cataract surgeries have advanced immensely having a focus on improved safety and efficacy. With increasing expectation on good clarity of vision after surgery, various artificial lens (Intraocular lens) has come up in the market providing great benefits. This wide range of availability makes it difficult for the general people to decide on the type of surgery and design of intraocular lens (IOC) to be chosen. By the end of this article, you should get some clarity.
A little about our natural lens:
A transparent lens is present inside the eyeto clearly visualize the objects that we see. This lens has two zones, the outer and the inner zone. The lens, like a camera, focuses on images at different distances by changing its shape. This is achieved through coordinated eye muscles movements (accommodation). Naturally, with ageing, the outer zone develops new layers and the old existing layers start accumulating in the central inner zone. With age, the lens turns cloudy and eventually becomes completely opaque. This affects the visibility and makes it uncomfortable to see bright lights directly. Cataract development may vary from person to person but is often dependent on their lifestyle and other metabolic changes of the human system. As the cataract becomes severe, the natural lens has to be removed and replaced with an artificial lens for better clarity. The lens also needs to be clear so that the retina specialist can see the back portion of the eye better, especially when a problem in the retina is suspected.
Figure 1: Central white haze due to cataract development
Figure 2. Light scattering as seen in cataract
Cataract surgery has advanced extensively through the years and is one of the simple and effective surgeries nowadays. The surgery will be performed after injecting a local numbing agent (anesthesia) into the eye. There are many procedures of performing cataract surgery, depending on the status of the eye and the visual needs of the patient.
Although the procedure differs depending on the type of surgery and the type of cataract, here’s the general procedure of the surgery:
• Creating an opening (incision) in the white portion of the eye(sclera) near the transparent layer(cornea) to gain access to the lens inside the eye
• Insertion of a surgical knife or probe to cut or breakdown the cataract formed inside
• Removal of cataract
• Placement of an artificial lens inside the eye with the help of a forceps
Figure 3. General cataract surgery procedure
Different types of surgical procedures include
1. Intra-capsular cataract extraction (ICCE): It is one of the oldest procedures where the entire lens is removed and a new artificial lens called the intraocular lens (IOL) is placed in the front part (anterior chamber) of the eye. A large opening is made to pull out the entire lens and hence the wound takes a longer time to heal. This procedure is outdated but is preferred in certain cases.
2. Extra-capsular cataract extraction (ECCE): This procedure is useful in severe forms of cataract. The opaque material might be very hard for it to be broken down into small parts and hence the opaque region will be cut and removed as a single piece. The back cover of the lens is left behind to provide support for the IOL to be placed. To accomplish this, the opening would be comparatively large (10 12mm) and stitches are to be put for it to recover. These stitches could cause minor changes in the shape of the transparent layer (cornea) leading to the need for wearing spectacles for both distance and near after surgery.
Figure 4. Extra-capsular cataract extraction
3. Phacoemulsification: It is the most common type of cataract surgery done. A small opening of about 3 5.5mm is created and ultrasound beams are used to soften the hard lens and break it into small pieces. A suction tool attached to the probe will remove those small pieces. The IOL is then placed in the left out back cover (capsule) of the natural lens. Since the opening is much smaller there will not be a need for additional stitches to be put after the procedure and hence healing time is faster.
Figure 5. Phacoemulsification procedure
4. Small incision cataract surgery (SICS): It is a manual procedure similar to the ECCE surgery and the success rate of this technique is dependent on the surgeon’s skill. This is preferred in cases where advanced equipment required for phacoemulsification is not available. It is much safer and has a quicker healing time compared to ECCE.
5. Micro incision cataract surgery (MCS): MICS is an advanced type of cataract surgery where the opening is very small (less than 1.8mm) aiming to minimize the need for glasses after surgery. Due to reduced incision size, it has a quick healing time with minimal complications.
Note: The location and size of the incision will also affect the residual refractive power after surgery. The smaller the incision size lesser will be the residual astigmatic power after surgery.
Intraocular lenses (IOL)
Intraocular lens (IOL) is an artificial lens placed inside the eye after the removal of the natural crystalline lens. IOLs are chosen based on the ocular health and visual needs of the patient. There are certain tests that are to eliminate the dependency on spectacles and for better visual performance post-surgery, certain tests are done to gather measurements of the eye. These tests are comprehensively called digital biometry reading (DBR) and include the following:
Keratometry: This technique is used to measure the curvature of the transparent layer of the eye (cornea) and its respective power. It can be done either manually or using automated instruments. Keratometry values can also be obtained using advanced instruments like Pentacam, which would provide a shape map of the cornea.
Figure 6. Manual Keratometer: Instrument used to measure the corneal shape
Figure 7. Topography: An automated instrument to measure corneal shape factors
A-scan: This technique uses an ultrasound wave to measure the inner length of the eye. The speed at which an ultrasound wave returns after hitting the back portion of the eye is converted to distance in mm that gives us the axial length measurement of the eye
Figure 8. Axial length measure of a normal eye
The eye’s refractive power is majorly based on these two measurements and specific formulas are to be used to calculate the appropriate power of IOL to be placed. The material of the IOL used also influences the power of the IOL. Even a minimal error in the calculation might cause a major difference in the clarity of vision after surgery. IOL power will be decided after considering all the parameters influencing the power as per the formula.
Similar to glasses, IOLs also have different properties based on the material and design of the lens. The IOL has two parts, the central lens region called the optic and the peripheral holding region called the haptic. IOL can be classified based on different designs of optics and haptics.
Figure 9. IOL with central optics and peripheral haptics
Based on the optic design, types of IOLs include:
Single vision IOL: It is an IOL with single power throughout the optic that corrects only for one particular distance (Mostly distance vision alone). For other regions, the patient might require spectacle correction.
Figure 10. Single vision IOL
Other advanced designs developed to provide clarity at varied distances are termed fall under ‘Premium
IOLs’
Toric IOLs. These IOLs are prescribed for those with a significant amount of astigmatism (different powers in different meridians of the cornea). To place toric IOLS, Accurate corneal shape measurements before surgery are essential for perfect vision after surgery.
Figure 11. Toric IOL with marking in the optics
Multifocal IOLs. These IOLs will have multiple rings with a gradual difference in power in each ring. This will allow focusing of objects at a wider range of distance with options of bifocal (distance and near) and trifocal (distance, near and intermediate). This IOL has one disadvantage that it reduces the contrast of the image and may not be ideal for those with pre-existing retinal problems.
Figure 12. Multifocal IOL with rings
Extended depth of focus (EDOF) IOL: These lenses come with a unique design that allows the bending of light helping in sharp focus. This allows a wider range of vision at both distance and an intermediate region. These IOLs provide better night vision as it reduces glare and is preferred for patients with increased visual tasks at night.
Accommodative IOLs. With a special design of haptics (hinge type) the IOLs will have the ability to move forward and backwards on the contraction of the eye’s muscles. It will act as the natural lens and power will change according to the object that is focused.
A combination of IOLs can also be opted for in specific cases. In case if a person is in need of better near vision without glasses, he can be placed with an IOL for distance correction in one eye and near correction in the other eye (monovision). For this to be implemented, a trial with similar contact lenses prior to surgery is a requirement to monitor the ability of the person to adapt and use both eyes according to the visual need.
The haptics play a major role in holding the IOL in position and hence based on the needs, different designs have been developed. In conditions where simple hooks cannot hold the IOL, specialized design is required for better placement. Based on the haptic design, IOLs can be classified as
• Single piecewith both optic and haptic made of one continuous material
• Multi-piece including different haptic loops attached (C-loop, J-loop and modified J-loop)
• Plate and plate loop
Figure 13. Different haptic designs
The advanced design of the haptic helps in positioning the IOLs at different locations. There are cases where the IOL must be placed in the front portion of the eye due to a lack of support in the back. In such cases, a unique design (angled and claw-shaped) of the haptic will assist in holding the lens in place.
Figure 1 5. IOL with claws placed in the front portion of the eye (Iris-fixation)
IOL Materials
There are different material of optics including poly-methyl-methacrylate (PMMA), acrylic, silicone, and hydrogel. PMMA is not of wide use nowadays as it is a rigid material. Water-repelling type of IOL materials (hydrophobic) including PMMA and hydrophobic acrylic are would be prone to dust deposition. The waterloving type (hydrophilic) of the acrylic lens and the hydrogels unfold gradually when placed inside the eye and will have lesser deposition of dust and microbes. The silicone material is also dust prone. The hydrogels are a roll-able type of IOL due to their ultrathin (100microns) lens.
Conclusion
In view of providing utmost comfort and clarity after cataract surgery, various types of IOL have been developed. With a wide range of options in the type of cataract surgeries and IOLs, appropriate choice based on your visual needs, eye condition and the doctor’s suggestion are essential for better results.
Summary
Cataract is a condition where old lens materials get deposited at the central portion of the lens with ageing. This cloudy formation tends to affect the visibility and has to be removed for better clarity in vision. The natural hazy lens is removed and an artificial lens is placed through a small opening (incision). Phacoemulsification is a widely in case use where the lens particles are broken down and removed. Size of incision can affect the shape of the cornea and hence advanced techniques including small incision or micro- incision surgeries are performed for a better and safe outcome.
For deciding the power of IOL, keratometry and axial length of the eye are calculated for it to be utilized in the IOL formula. The type of IOL plays a role in deciding the power and hence appropriate choice of IOL is essential. Based on the visual needs, the type of IOL (monofocal, multifocal, toric or extended depth of focus) is decided.
