Contact Lenses

Overview of Contact Lenses

Contact lenses are like artificial covers for your eyes – they sit on the front part of your eye and act like your natural outer eye layer, the cornea. They can help correct vision problems and irregularities of the cornea, which is like the window of your eye. There are many factors that define a contact lens, such as its overall size, the size of the lens area that corrects your vision, the lens curvature, edge, power, thickness, and color. There’s a variety of contact lenses you can choose from, with options ranging from soft lenses to hard ones, and ones that let gas pass through them.

Contact lenses are used for many different reasons. They can be used to correct vision, treat eye conditions, prevent eye issues, help diagnose eye problems, support during eye surgery, change the appearance of your eyes, and accommodate specific job requirements. The behaviour of contact lenses is influenced by certain properties. Factors like how easily they stay wet, how much water they contain, how much oxygen they let through, how well they transmit light, how well they resist changes in temperature, their sizes, and how stable they are under pressure, all define how well the lens works.

Contact lenses come in two materials: focons and filcons. The perfect contact lens should be ‘eye-friendly’ or biocompatible, let gases pass, have excellent vision correction properties, be comfortable, easy to shape, sterile, stable, and have a good surface texture. Each type of contact lens has its pluses and minuses. It’s also important to note that using contact lenses can sometimes cause complications affecting different parts of the eye.

Anatomy and Physiology of Contact Lenses

The health of the front surface of your eye, also known as the cornea, primarily depends on the tear film that covers it. This tear film nourishes the cornea, keeps it lubricated, and protects it. Keep in mind, the cornea’s job is to stay clear and help the eye focus light. However, when you wear contact lenses, they can disrupt the helpful relationship between the tear film and the cornea. This could lower the functionality of your cornea, and also affect how comfortable and effective the contact lenses are.

Contact lenses can’t be glued or physically forced to the cornea. Instead, they stay in place because of the tear film’s sticky properties. When you place a lens on your cornea, it rubs against the moist conjunctival surface. Then, tear fluids spread across the lens, covering it including its edges, and help the lens stay in place. The water molecules in the tear film stick together and create a film that keeps the lens close to the surface of the cornea. This process also makes a sort of surface membrane between the tear film and the air surrounding the eye, which acts as a storage area for the lens.

The tear film and the lens work together to create something called a tear lens. This tear lens aids in the functioning of the cornea. The transparency of the tear and the cornea help in bending light, an important process for vision. To be exact, the tear’s refractive index, or its ability to bend light, is about 1.33 while the cornea’s is approximately 1.37. The minor difference in these figures means contact lenses can correct for irregularities in the cornea shape up to around 5 diopters, leaving only a small left over error of about 0.5 diopters.

Contact lenses have several key effects on the cornea. They help prevent tear evaporation and protect against having too much salt in your tears. They are also a barrier for delivery of oxygen and assist in removing waste by trapping it. They can cause trauma to the cells on the cornea’s surface. Rigid (“hard”) contact lenses cover between 50 to 80% of the cornea and the area they cover needs nutritional support from the tear film.

Lastly, blinking is crucial for maintaining the tear film’s relationship with the contact lens. Each blink works like a tiny pump that refreshes the film between the lens and your tears. This refreshing motion is influenced by how much tears you produce, how often you blink, and how much of the tear film is replaced with each blink.

Why do People Need Contact Lenses

Contact lenses can help with various eyesight issues, including strong nearsightedness (high myopia), having one eye that is missing a lens (unilateral aphakia), a scar on the clear front surface of the eye (corneal scar), differences in vision between the eyes (anisometropia), a cone-shaped cornea (keratoconus) and unevenly shaped eyes (irregular astigmatism). People also sometimes use lenses to improve their appearance if they have uneven eyesight (refractive error).

They can also be used to help treat a range of eye health issues, such as problems with the surface of the eye, including swollen, blister-filled corneas after lens implant surgery (pseudophakic bullous keratopathy), infections on the cornea that won’t heal (non-resolving corneal ulcers), recurrent surface damage to the cornea (recurrent corneal erosion syndrome), dry eyes, and any problems with the surface layer (epithelial defect). Contact lenses can also be useful after surgery to the cornea (post keratoplasty) or to fix a puncture (perforation), after the removal of the top layer of the cornea (post superficial keratectomy) or to fix small leaks after repairing a tear in the cornea (post-corneal tear repair).

Eye conditions that affect the colored part of the eye (iris), like absence of the iris (aniridia), a hole in this part of the eye (coloboma), and lack of color in the skin, hair, and eyes like in albinism, can also be helped with contact lenses. They can help deliver medication in certain cases of increased pressure in the eye (glaucoma).

For children with “lazy eye” (amblyopia), opaque contact lenses can help provide better vision by blocking the use of the weaker eye. In some cases, contact lenses can also help following surgery to the retina, the thin layer of tissue lining the back of the eye, especially to fix problems in the epithelial layer.

Diagnostic uses for contact lenses can include looking at the angle where the iris meets the cornea (gonioscopy), testing the electrical responses of various cells within the eyes (electroretinography), examining the part of the eye in people who have unevenly shaped eyes (fundus examination in patients with astigmatism), taking pictures of the back of the eye (fundus photography), and checking various parts of the interior of the eye (Goldman 3 mirror examination).

Contact lenses can also be used to prevent problems like in-turned eyelashes scratching the eye (trichiasis), dryness and inflammation of the cornea caused by exposure to air (exposure keratitis), and adhesion of the eyelid to the eyeball (symblepharon).

Furthermore, they can be useful for people who need clear and unobstructed vision for their job, such as sportsmen, police, and pilots. Contact lenses are used cosmetic reasons as well like in a case of a corneal scar or a shrunken, non-functioning eye (phthisis bulbi).

Finally, during certain surgical procedures, such as removal of the vitreous humor (pars plana vitrectomy), the use of a laser to seal off leaking blood vessels (photocoagulation), or opening up the front part of the eye to lower pressure in the eye (goniotomy), contact lenses can be used.

When a Person Should Avoid Contact Lenses

There are certain conditions that might completely prevent someone from getting a specific eye procedure:

If the patient has cerebral palsy or mental challenges, chronic eye infections (dacryocystitis), or dry eyes, the procedure may not be suitable for them. Other conditions such as eyelid-related problems (Hordeolum Internum, Hordeolum externum, Blepharitis, Trichiasis), eye infections (Conjunctivitis, Corneal ulcer), and inflammation of the eye (Episcleritis, Scleritis, Uveitis) can also prevent them from getting the procedure. Similarly, Fifth and Seventh nerve palsy, eye allergies (Allergic Conjunctivitis), and abnormal growth of cells on the cornea (Corneal dystrophies and degenerations) are other absolute reasons a patient cannot have this procedure. Finally, certain types of eye inflammation (Superficial punctate keratitis, Subepithelial keratitis) are also significant roadblocks.

On the other hand, there are other conditions that might not completely prevent the procedure, but they might make it riskier or less likely to succeed. These include being pregnant, having an eye condition with large bumps under the eyelid (Giant papillary conjunctivitis), or having a misalignment of the eyes (Strabismus).

Equipment used for Contact Lenses

Contact lenses come in a variety of types. Here’s some information about the different kinds you might come across:

Based on Where They Sit on the Eye:

• Scleral: These cover the whole front part of the eye including the cornea, sclera (white part), and conjunctiva (thin layer covering the front part of the eye).
• Corneal: These only cover the cornea (the clear front part of the eye).
• Semi-Scleral: These cover the cornea, overlap the limbus (border between cornea and sclera), and partially cover the conjunctiva.

Based on Water Content:

• Low – 0 to 40% water
• Medium – 40 to 55% water
• High – more than 55% water

Based on When You Can Wear Them:

• Daily wear: You put these in each morning and take them out each night.
• Extended wear: You can sleep in these for a certain number of nights as directed by your doctor.
• Disposable: These are thrown away after a single use.

Based on What They’re Made Of:

• Soft/Hydrogel contact lenses: These are made of hydroxyethyl methacrylate, a type of plastic that can absorb and hold onto water.
• Rigid gas permeable contact lenses: These are made of silicon and cellulose acetate butyrate. They allow oxygen to pass through to your eye.
• Rigid non-gas permeable contact lenses: These are made of polymethylmethacrylate and do not allow oxygen to pass through.

The Designs of Contact Lenses vary as well, here are two examples:

• Single cut lenses: These have two surfaces, the front is curved and the back has both a main and a secondary curve.
• Lenticular cut lenses: These also have two surfaces, but the front part has a thinner peripheral area and a flat central optical area while the back part has a main and a secondary curve.

There are also some key terms to understand when talking about contact lenses:

• Wettability: This is a measure of how well liquid binds (or “wets”) the surface of a lens. Good wettability means the lens can be worn comfortably.
• Water content: This refers to how much of the lens is made up of water. Higher water content tends to make the lens more comfortable and breathable but also thicker and less mechanically robust.
• Transmission of Light: A lens’s ability to transmit light is an essential optical property to ensure clear vision.
• Refractive Index: This is a measure of how much the lens bends light.
• Heat Resistance: This is a lens’s ability to withstand high temperatures without breaking down or deforming.
• Dimensional Stability: This refers to how well the lens maintains its original shape under different conditions.
• Flexural Stability: This pertains to the lens’s ability to flex without breaking.
• Oxygen Permeability: This term is about how well lenses allow oxygen to pass through. Oxygen is crucial for your eye’s health.
• Oxygen Transmission: Related to the above point, this term refers specifically to the rate oxygen passes through a lens of a particular thickness.

Who is needed to perform Contact Lenses?

An optometrist or a contact lens expert, as well as other eye health workers, are expected to know the basics of contact lenses. They should understand the structure and function of the eye when it’s wearing a contact lens, how to prescribe and fit them, and the importance of cleaning your contact lenses. They should also be aware of the different materials used to make contact lenses, their cost, when they are needed, and when they should not be used. If you do decide to use contact lenses, they should also know about any problems that could occur.

When we talk about contact lenses, we use certain terms to describe their characteristics. These include:

• Diameter of the lens: its overall size and measurement of the area that enables vision.
• Curve of the lens: This describes different parts of the lens curve, including the base (the part that contacts your eyeball) and the peripheral area (the parts at the edges).
• Blend: the lens can be light, medium or heavy blend.
• Edge of the lens: the outer boundary of the lens.
• Power of the lens: how much the lens corrects your vision.
• Central thickness of the lens: the thickness of the lens in its central part.
• Tint or colour: contact lenses can have different hues or shades.

When discussing contact lenses, British and American professionals might use different terms. For instance, what British eye health professionals refer to as the ‘Back optic zone’, Americans simply call the ‘Optic zone’. Understanding these terms can help you understand your contact lens prescription more completely.

Preparing for Contact Lenses

The materials for contact lenses have come a long way since the early days when they were made of glass. These were heavy, fragile, and difficult to make. Things changed in 1938, when lenses were first made from a clear plastic called methyl methacrylate, and then again in 1961 with the creation of soft, water-loving (or “hydrophilic”) lenses. This revolutionized how contact lenses were prescribed and used.

Contact lenses fall into two categories: “filcons,” which are soft and water-loving, and “focons,” which are hard and water-repelling (or “hydrophobic”). Filcon lenses are made from silicone rubber and are grouped into nine different categories (from 1a to 5). Focon lenses are made from PMMA and RGP lenses, and are grouped into six categories (from 1a to 5).

The contact lens material that’s best for you should offer several key features:

• Biocompatibility: This means the lens won’t hurt your eye or contain anything that could potentially harm it.
• Stability: The lens should have a consistent thickness and curvature to provide clear vision.
• Molding: The lens should be able to be reshaped and duplicated for easy manufacturing.
• Sterility: The lens material should be able to be sterilized or should resist contamination.
• Tolerance: How well the lens tolerates being worn depends on its design and how well it lets oxygen through.
• Surface chemistry: The surface of the lens should attract water so tears can spread across and stay on the lens.
• Gas permeability: Gas should be able to pass through the lens to ensure it doesn’t interfere with the oxygen supply to the cornea, enabling you to wear the lens for longer and increasing comfort.
• Optical property: The lens should be clear, and the way light passes through it should be similar to a tear film.

There are many different materials available for contact lenses these days. Silicone hydrogel lenses, for example, have a high oxygen transmission capacity due to their silicone content. Hydrogel lenses include hydroxyethyl methacrylate (HEMA) lenses, which are resistant to chemical or enzymatic breakdown, and HEMA-VP lenses, which have higher hydration than standard HEMA lenses.

Rigid gas permeable lenses are less flexible than the above-mentioned lenses, but are more oxygen-permeable than PMMA lenses. The material choices for these lenses have also evolved over the years, from cellulose acetate butyrate, which were the first to be used, to the current options of silicon acrylate and fluoropolymers.

All these materials have their own benefits and drawbacks, and the best choice for you will depend on your own unique eye health and lifestyle. With all these improved lens material choices, contact lenses are now more comfortable, breathable, and safe to wear than ever before.

How is Contact Lenses performed

Contact lenses are usually manufactured through a few different processes, like lathe cutting, molding, and spin casting.

Lathe Cutting: This is a technique where a machine, known as a lathe, is used to shape the front and back surface of the lens. Hard lenses are shaped using wax and soft lenses are shaped using aluminum oxide lubricated with a type of wax called distilled paraffin. After the initial shaping, the edges and curves of the lenses are further modified for optimal comfort and fit. One of the advantages of this method is that soft lenses can be engraved for identification, and they can be polished to have a smooth surface.

Molding: In molding, the lens is heated until it becomes flexible. Then, it is placed into a mold which gives it a desired shape.

Spin Casting: This method involves spinning the lens in a mold that constantly rotates. The lens shape changes as it spins, with the speed and shape of the mold determining its final curvature. The components of the lens that haven’t hardened yet are placed in water for 24 hours to finish the shaping process.

There are several steps to manufacturing contact lenses. First, the process starts with a clear piece of material, which could be a type of plastic known as PMMA or a combination of other types of plastic, cut into the rough dimensions of a lens. The next stage involves creating a posterior (back) curve in the lens blank. Then, they create an anterior (front) curve and polish the surface of the lens until it’s optically clear. Finally, the lens is finished by optimizing the curves on the front and back, as well as shaping the diameter and edges for fit and comfort.

Possible Complications of Contact Lenses

There are several eye conditions that may occur in different parts of your eye. Let’s simplify them by parts of the eye:

In Conjunctiva (the clear tissue covering the white part of your eye and the inside of your eyelids):
– Allergic conjunctivitis: This is inflammation of the eye caused by allergies.
– Giant papillary conjunctivitis: A condition where small, round bumps called papillae grow on your inner eyelid because of a foreign object like a contact lens.
– Superior limbic keratoconjunctivitis (SLK): A rare condition where your superior limbic, part of the eye, becomes inflamed.

In the Cornea (the clear, rounded-front surface of the eye that focuses light into your eye):
– Epithelial edema : This is swelling in the cornea which can blur vision.
– Epithelial defect: A condition where there is a wound or damage to the surface of the cornea.
– Epithelial microcysts: These are tiny, fluid-filled bumps on the surface of the cornea.
– Abrasions: Scratches on the topmost layer of the cornea.
– Superficial punctate keratitis: A condition causing tiny corneal erosions to appear.
– 3 and 9 o clock staining: This is a pattern of staining on your cornea that can indicate dry eye or other disorders.
– Sterile corneal infiltrates: These are tiny spots in your cornea caused by inflammation and not by an infection.
– Corneal neovascularization: This is an abnormal growth of blood vessels in the cornea.
– Warpage: Changes in the shape of the cornea due to mechanical stress like rubbing eyes or wearing unfitting glasses.
– Microbial keratitis: An eye infection in the cornea. This can be caused by bacteria or parasites (like acanthamoeba) especially in contact lens wearers.

Now, let’s talk about some issues specifically related to wearing contact lenses:
– Physical damage: Your lenses can get damaged by scratches or tears which can cause discomfort or vision problems.
– Lens discoloration: The lens changes color which could potentially obscure your vision.
– Lost lens: The lens gets lost inside your eye and might need medical help to remove.
– Lens deposits: Tiny pieces of debris or deposits build up on your contact lenses. This can cause discomfort or vision issues.
– Tight lens syndrome: This is discomfort from a contact lens that fits too tightly on your eye.
– Acute hypoxia: Not enough oxygen reaches your cornea when you wear your contacts for too long.
– Chronic hypoxia: This is when your cornea lacks oxygen over a long period, usually because of prolonged contact lens wear. It can lead to more serious eye problems.

What Else Should I Know About Contact Lenses?

Contact lenses have significantly changed the eye care field. Many people now choose contact lenses instead of glasses for their looks. Young people often prefer contact lenses because they are required at work, they look better, and they don’t depend on glasses.

In older people, main reasons for using contact lenses are to treat problems caused by injuries, hole in the cornea (the clear front surface of the eye), eye pain due to a condition after cataract surgery, and scars on only one cornea. However, the costs, the need for patients to follow care instructions, and possible complications can limit contact lens use. Some places also avoid giving contact lenses to children because of the risk of inflammation of the cornea caused by the lenses, and challenges with following the treatment plan.[53]