Contact lenses-history, types, materials

* Attention! contains terms and specific concepts.

Contact lenses are an increasingly popular medical device. Its use raises many questions and doubts, but before answering the most common among them, let's understand how it all begins.

We will also ask Dr. Galatea Tsvetkova for advice, recommendations and guidelines on tearing contact lenses.

History of contact lenses

It is a curious fact that some of the first records of contact lenses date back to 1508. Then Leonardo da Vinci, pushed the idea of vision correction by placing a hemisphere made of glass filled with water, on the The Cornea. It is well known that da Vinci had an increased interest in the anatomy of the human body and in particular the eyes, as they are closely related to art and painting.

It is also believed that he himself had a problem with one eye that helped him see images in a completely different way.

Christopher Tyler, an ophthalmologist at City University London believes that Leonardo da Vinci suffered from strabismus or in other words squint. It is a binocular vision disorder in which there is deviation of one or both eyes from the visual axis.

1508 г.Но нека се върнем на контактните лещи, където през 1887 г. Август Мюлер, немски майстор-стъклар, произвежда първото средство, подобно на контактна леща, което покрива роговицата.

1887 г.Неговият колега Адолф Фик, въз основа на откритията на Мюлер, създава първата контактна леща от стъкло. Но поради спецификата на материала, който бил тежък и дебел, лещите били носени върху цялото око или върху склерата. Това ги правело твърди и следователно неудобни.

1888 г.През 1934 г. е разработен първият пластмасов материал, полиметилметакрилат (PMMA).

1934 г.В края на 1940 г. е патентована първата контактна леща от PMMA.

The introduction of hard plastic was a big step in the development of contact lenses.These were the first hard contact lenses.

Over time, the diameter and thickness decreased until we reached the design we know today.

1940 г.През 1961 г. чехословашките фармацевти разработват първата контактна леща от мека пластмаса, изработена от хидроксиетил метакрилат (HEMA с 38% водно съдържание). След което, били представени, първите налични в търговската мрежа, меки контактни лещи от Bausch & Lomb.

Types of contact lenses

According to the principle of correction

• Afocal

They have no refractive power. The correction is carried out by the "tear lens", which occurs between the cornea of the eye and the contact lens.

• With a certain refraction

This lens rests on the cornea of the eye, and its front surface is processed so that the lens has a precisely defined refraction.

According to the intended purpose

Corrective

Their use is the most widespread. They are used every day and serve to correct vision.

• Spherical

They correct myopia and hypermetropia

• Toric (astigmatic).

They are corrected with them astigmatic eyes.

• Bifocals

They are used to correct presbyopia. With them, you can see both at a close distance and at a distance.

• Multifocal

Multifocal corrective lenses are used to correct presbyopia (age-related farsightedness). They give the possibility to see at all distances, by means of circular zones. In the center is the dioptric value for near vision, in the middle for PC, and the outermost part is positioned for the far zone.

Therapeutic contact lenses

They serve as a dressing for the cornea, for example after surgery, moisturizing and protecting it.

Cosmetic contact lenses

They are used, for example, in the case of a difference in the iris of the two eyes.

According to their construction

Lenses with a supporting scleral part

They have a larger diameter. Their peripheral part is shaped so that it rests on the sclera.

Cornea

They have a small diameter and are placed on the cornea. They should not cross the limbus of the eye (the area where the opaque sclera reaches its back and passes into the transparent cornea. In the limbus ends the conjunctiva, where many blood and lymphatic vessels are located. In the limbus conjunctiva there are accumulated a significant amount of lymphoid cells that participate in corneal immune processes.).

According to the wearing mode

• Normal, extended and continuous.
• One-day, two-week, one-month and three-month.

According to the material from which they are made

Contact lens materials are transparent polymers that can be obtained from a given monomer by polymerization. Also, they could be obtained from more number of monomers.

The monomers included in the composition of a given polymer influence its chemical, physical and mechanical properties.

Materials for hard contact lenses

Modern materials are gas permeable (RGP) and as we have already said, they were created in 1934. Nowadays, hard contact lenses are used to correct keratoconus. An eye disease in which the cornea has an irregular cone-like convex shape.

Soft contact lens materials

Hydrogel

Their main material is polymerized HEMA. They should not have a high water content, as this leads to deterioration of the physical properties of the material. This material is used for lenses with a three-month shelf life.

Silicone hydrogel

It is a material derived from silicone and hydrogel. They are biphasic. The silicone component provides high permeability, and the hydrogel facilitates fluid transfer and movement of the lens.

Modern materials are divided into 4 groups, called generations by manufacturers. They depend on water content and ion transportability.

• Low water content < 50%, non-ionic;
• High water content >50% non-ionic;
• Low water content < 50%, ionic;
• High water content >50% ionic;

Water gradient lenses

This is a new generation material that is used for daily contact lenses. At their core, they have a silicone hydrogel with low water content and high oxygen transportability.

Properties of contact lens materials

• Refractive index

A basic indicator of the visible area.

• Transmittance rate

This refers to the transparency of the material.

• Modular elasticity

It is determined by measuring the resistance of the material when force is applied. The unit of measurement is megapascals (mPa). The modulus ranges from (0.2 to 1.6)*106 Pa. For harder materials, the modulus has higher values.

• Hydration level

Refers to soft contact lenses and represents the amount of water expressed as a percentage of the lens material.

• Wettability

It is estimated by the contact angle of a drop of water on the given surface. For a hydrophobic material, the angle is greater than or equal to 90°, and for a hydrophilic material, it is , than 90°.

• Gas permeability (oxygen permeability)

It shows the amount of oxygen passing through the contact lens under certain conditions (DK*10-11). It is recommended to be a three-digit number and depends on the water content.

• Oxygen transportability

Same as oxygen permeability, but depends on lens thickness. The standard requires that the value be measured for a lens with a dioptric value of -3.00 dpt.((DK(oxygen permeability)/t(thickness))*10-9)).

• Tensile strength

It is characterized by the maximum force at which the lens breaks. Applies to soft contact lenses (SCL). For hard contact lenses (RCL) is the maximum force at which deformation occurs.

• Biological tolerance

This is the ability to moisten the eye.

• Minimum deposit retention

Contraindications to wearing contact lenses

• Заболяване на нервната система.
• Ендокринни заболявания.
• Инфекциозни заболявания.
• При бременност не се назначават първи контактни лещи, тъй като по време на бременност диоптричните стойности могат да дадат отклонение.
• При употреба на някои контрацептиви.
• Заболяване на клепачите.
• Проблеми със слъзния филм.
• Заболяване на роговицата и конюнктивата.
• Глаукома.

Advantages and disadvantages of contact lenses

Advantages

• Wide field of view. Wearing soft contact lenses (SCLs), you don't see a restriction of vision as you do with glasses. This is one of the main reasons that makes them suitable during sports.
• The size of the image obtained on the retina is close to normal.
• Contact lenses become part of the optical system of the eye.
• They are not affected by climatic features.
• They provide a good aesthetic appearance. Something extremely suitable for people who can't get used to wearing glasses and how they look.
• They hide defects on the iris and pupil.

Disadvantages

• Affect the sensitivity of the eye and in particular the The Cornea.
• The refractive power of contact lenses is different from that of spectacles at diopters above 3.50-4.00 Therefore, often at minus dioptric values above -4.00 dpt for spectacles, the parameters of contact lenses are reduced, and at +4.00 dpt - increased. This is done at the discretion of the eye care professional.
• Additional solution and storage container costs are required.
• Thorough removal and laying down is necessary.

In conclusion, we can add that a good awareness of contact lenses can ensure their smooth use and selection.