Contact Lenses: Types and Functions

What are Contact Lenses?

Contact lenses are medical devices that are applied to the ocular surface for therapeutic or aesthetic reasons. Once positioned, they adhere to the tear film that covers the front of the eye; when the eyelid blinks, it passes over the contact lens, inducing a slight movement of the same that allows the tears to bring the necessary oxygen and lubrication to the cornea. below.

Depending on lifestyle, patient motivation and eye health, contact lenses are an effective and versatile alternative to glasses for correcting refractive errors, such as hyperopia, astigmatism and myopia.

Often, these devices are chosen for aesthetic reasons, but they can also have very practical advantages in certain sports or professional situations, in which the glasses could be damaged or prevent the adequate use of protective devices. Contact lenses can also be used for the treatment of certain eye diseases. For example, scleral lenses can correct the anisometropia (inequality in the power of refraction between the two eyes) and the irregular curvature of the corneal surface in the case of keratoconus.

What to know before buying them

Contact lenses are medical devices that require special care and attention. Most people are able to wear contact lenses, while others cannot tolerate them due to their susceptibility to eye infections or a particular hypersensitivity.

If you decide to use contact lenses, the first step is to see an eye doctor for a thorough examination. All contact lenses require a valid medical prescription, drawn up according to an individual protocol. It is important to follow a regular follow-up program, as recommended by the doctor (approximately after one week, one month and six months, and then once a year). Respecting the recommendations regarding cleaning, disinfection and storage methods allows you to avoid the risk of potentially dangerous complications for vision. Contact lenses should not normally be worn for more than a certain number of hours daily (this depends on the characteristics of the lens and what is specified by the doctor) and should be removed before night rest.

Contact Lenses vs. Glasses

For many people, contact lenses offer flexibility and convenience. Compared to glasses, they provide better peripheral vision: they move with the eye and allow you to see a natural field of view. They also have no structures that obstruct vision and greatly reduce distortions. Contact lenses do not fit. they mist up and do not collect moisture, such as rain, snow or sweat; this makes them ideal for sports and other outdoor activities.

Types of Contact Lenses

Different contact lenses are available to suit a variety of needs and preferences as well as a specific treatment regimen. These devices vary in material (hard or soft), size (corneal, scleral, etc.), mode of wear (daily or long-term) and relative duration of application [conventional (annual replacement) and "disposable" (daily replacement or monthly)].
Based on the materials they are made of, it is possible to distinguish two general categories of contact lenses: soft and hard.

Soft contact lenses

Soft contact lenses are made from flexible hydroxyethyl methacrylate (HEMA) polymers, silicone and other similar materials. These polymeric components are hydrophilic (composed of a maximum of 80% water) and ensure good oxygen permeability and wettability of the lens, thus offering better overall comfort.

Soft contact lenses are the easiest to handle and wear, but they cannot give the best refractive result. These adapt to the shape of the eye and tend to stay well in place, so they are a good choice if you participate in a sport or lead an active life. Thanks to their structure, they are comfortable at a short distance from the implant and can be worn for longer periods than rigid contact lenses. Some disadvantages include a shorter life span and the inability to correct some vision defects. New soft lens materials (silicone-hydrogel) have improved oxygen permeability; however, some factors can affect the comfort of these devices: they are slightly more rigid and the surface of the lens can be hydrophobic, therefore less "wettable".
Soft contact lenses are available in different versions, such as:

Soft disposable contact lenses. Soft contact lenses are generally the least expensive option. They require frequent replacement and can be worn for the prescribed period (for example, 7 to 30 days), before being thrown away. Disposable soft contact lenses are not they are only convenient, but they eliminate many of the worries associated with prolonged use, such as eye infections.When removing, you need to make sure you clean them properly before applying them again.
Soft contact lenses for prolonged use. The soft lenses for extended wear are designed to be worn for long periods of time (approximately seven days) without having to resort to removal and cleaning. These lenses are made of silicone hydrogel, a material that allows six times more oxygen to pass through than disposable lenses. Soft contact lenses for prolonged use can be worn during sleep, but must be removed at least once a week. to subject them to a thorough cleaning and disinfection. They are less frequently recommended, as despite being approved for prolonged use, the risk of corneal abrasion and infection increases with nighttime use of any contact lens.
Soft "disposable" contact lenses. Disposable lenses are probably the most expensive, but also the most comfortable and easy to maintain: they are used when they are replaced and then thrown away. These are removed and replaced with a new pair of lenses on a daily, weekly or monthly basis. Use of disposable devices may be considered if you only wear contact lenses occasionally.

Rigid contact lenses

Rigid contact lenses are made from glassy polymers and other materials, such as silicone or fluoropolymers. These devices can be more difficult to apply and remove, but they can correct most vision problems; they are also durable and resistant. In particular, rigid contact lenses can be the best choice in case of astigmatism or very high myopia and hyperopia, as they allow superior vision compared to a soft contact lens. Furthermore, they are indicated when a person suffers from allergies or tends to form protein deposits.

Hard contact lenses are initially less comfortable than soft contact lenses and require a period of adaptation before the wearer can get used to wearing them (up to a week). In addition, rigid contact lenses are more likely to slip out of the center of the eye, which could lead to discomfort and blurred vision. A particular type of rigid (spherical) contact lens is able to replace the natural shape of the cornea with a new refractive surface, which means that a rigid spherical lens can correct astigmatism or corneal irregularities, such as keratoconus.

Rigid contact lenses are made with both non-permeable material and with permeable materials. The lenses based on polymethyl methacrylate (PMMA) are commonly known. Their disadvantage consists precisely in the poor passage of oxygen through the cornea, which causes a series of adverse clinical events. For this reason, PMMAs are now rarely used and are largely replaced with rigid gas-permeable contact lenses or soft contact lenses. However, this type of device still plays a role in the management of severe cases of keratoconus, irregular astigmatism and some ocular surface diseases.

Rigid gas permeable contact lenses (RPG)

Rigid gas permeable (RPG) contact lenses are similar to rigid ones, offer sharp vision and correct most vision problems by maintaining their shape; they are also resistant and have a longer lifespan than soft contact lenses. If your prescription doesn't change, you can use the same pair of lenses for up to two to three years. Like hard lenses, RPGs are not recommended for intense physical activities with a high risk of trauma. Rigid gas permeable contact lenses are often more breathable than soft ones, which reduces the risk of eye infections. In addition, they are easier to manage and offer excellent visual acuity, making them more suitable for astigmatic patients.

Other types of contact lenses

Depending on your vision needs, particular types of contact lenses may be considered, such as:

Hybrid contact lenses. Hybrid contact lenses have a hard (gas permeable) central disk surrounded by a soft outer ring. These may be an option if you have an irregular corneal curvature (keratoconus) or if you have difficulty wearing conventional rigid lenses.
Toric contact lenses. A toric contact lens has a different focusing power on its surface. Usually, these devices cost more than other contact lenses, but they are particularly suitable for correcting astigmatism. A toric lens must have the correct orientation to improve visual acuity and to correct the refractive defect, therefore it must possess characteristics additional graphics. Some of these, in fact, have marks or incisions to facilitate the application and prevent rotation of the lens far from ideal alignment.
Bifocal or multifocal contact lenses. Bifocal contact lenses contain different zones for near and far vision to correct presbyopia (similar to bifocal corrective glasses). These devices, soft or hard, are comparable to eyeglasses with bifocal or progressive lenses, as they have multiple focal points. Multifocal contact lenses are able to correct myopia, hyperopia and astigmatism, in combination with presbyopia. Visual quality is often not as good as with single vision contact lenses, however for some people they are a "practical alternative to correcting presbyopia.
Colored contact lenses. Some contact lenses are tinted for cosmetic or therapeutic purposes, to improve color perception or to help compensate for color blindness. Tinted lenses are available by prescription and should only be worn following the directions of an ophthalmologist. These devices change the color, the appearance of the eye, or both.Websites often advertise colored contact lenses as if they were cosmetic products or fashion accessories, with whimsical packaging and names; However, it should be remembered that the use of contact lenses without an eye exam and a medical prescription can lead to serious eye problems, which in extreme cases could even cause permanent loss of vision.

Important things to remember

The ophthalmologist can discuss with the patient the best handling and storage methods, depending on the type of contact lens used. Devices that are not properly cleaned are a potential source of infection. Worn or incorrectly fitted contact lenses In addition, a lens can deform over time and the cornea can change shape, so the degree and fit of the cornea must be re-evaluated on a regular basis. Follow-up visits will be scheduled according to visual needs and specific eye condition.
A patient may not be a good candidate for contact lens wear in case of:

Frequent eye infections;
Severe allergies;
Treatment-resistant dry eye;
Very dusty working environment;
Inability to manage and care for contact lenses.

Any eye drops can interact with contact lenses, so it is best to avoid instillation, except for drops recommended by your eye doctor. Homemade saline solutions should not be used for cleaning contact lenses, as they are associated with severe corneal infections due to possible microbial contamination.