HEALTH

Intraocular Lens (IOL) Materials

The first intraocular lens (IOL) was placed after cataract surgery in London in 1949 by Sir Harold Ridley. In the 1970s, IOL placement became a standard procedure after cataract surgery. The material from which the first IOLs were created was polymethyl methacrylate (PMMA). PMMA is a rigid material and the corneal incision had to be at least as large as the optics of the IOL, which became its major disadvantage in cataract surgery. The main goal of modern cataract surgery is the smallest possible incision, so IOLs need to be flexible and therefore foldable. This goal was achieved with improvements in IOL design and the materials that make them foldable.
The first foldable IOLs were made of hydrogel, but they were not enough and the development of the first silicone IOLs overcame this problem. Collapsible silicone IOLs were first implanted by Kai-yi Zhou in 1978. The advantages of the collapsible IOL are its compatibility with a small incision surgery, which is a self-closing procedure, and the possibility of placement with disposable applicators, which makes the surgery safer.

The IOL is designed and consists of the optic-central part and haptic-lateral structures that hold the lens inside the capsular bag. The first intraocular lens was placed after cataract surgery by Sir Harold Ridley at St Thomas’ Hospital in London in 1949, and the material from which the first IOLs were made was polymethyl methacrylate (PMMA). It is a rigid, non-foldable material that complicates the placement of the IOL. In the 1970s, new lighter posterior chamber IOLs were designed and feature propylene haptics for better stabilization and fixation of the ciliary sulcus. And after cataract surgery, IOL placement has become a standard procedure.

In the early 1980s Epstein started using lenses made of silicon to make them foldable. In this way, they can be inserted into the eye through incisions of 3 mm or less, compared to the 5-7 mm incisions required for the placement of non-foldable IOLs. The practice of IOL implantation was revolutionized in 1984 when Thomas Mazzocco began folding and implanting plate haptic silicone IOLs. Acrylic materials can be rigid (PMMA) and foldable made of hydrophobic acrylic materials (AcrySof – Alcon Laboratories, Sensar – Advanced Medical Optics -AMO) and hydrophilic acrylics (Centerflex, Akreos).
Each collapsible acrylic lens design has different refractive index, It is made of a different copolymer acrylic with glass transition temperature, water content, mechanical properties and other qualities. Hydrophobic acrylic lenses and silicone lenses have a very low water content (less than 1%). However, hydrophobic acrylic materials with a higher water content of around 4% are also available. Hydrophilic acrylic lenses are made from copolymers with higher water content ranging from 18% to 38%.
The first silicone material used in the IOL industry was polydimethylsiloxane with a refractive index of 1.41, while the newer silicone materials have higher refractive indices. Foldable acrylics have a refractive index of 1.47 or more, and for silicone lenses it is lower – 1.41 and higher. For this reason, acrylic lenses are thinner than silicones with the same refractive power.

Different Materials Used in Intraocular Lenses and Their Properties

Biocompatibility

Biocompatibility of a material , depends on the biological response to the foreign body material and depends on the design and material of the implant.

Materials used in ophthalmology should also be optically transparent for a long time, have a high resolving power or refractive index, and block ultraviolet rays.
The reaction of lens epithelial cells and capsule to IOL material and design is capsular biocompatibility. The response of the uvea to the IOL is uveal biocompatibility. During cataract surgery, the blood-aqueous barrier is broken and proteins and cells are released in the aqueous humor. The proteins are then adsorbed on the IOL surface, which influences subsequent cellular reactions on the IOL.

Flares

Flashes are a phenomenon that causes vacuole formation in the IOL optics due to penetration of aqueous humor into the IOL material. Flares are fluid-filled microvacuoles that form within the IOL optics when the lens is in an aqueous environment. They may be observed more frequently with any type of IOL with hydrophobic acrylic lenses. Factors that may affect the formation of shimmers include IOL material, manufacturing technique and packaging, as well as associated conditions of eye glaucoma, conditions that lead to disruption of the blood-aqueous barrier, and use of ocular drugs. expresses it as a cavitation.The flare develops over time and represents a dynamic process within the lens/eye system. The causes and long-term consequences are not entirely clear. The hydrophobic acrylic IOL has the highest degree of lens flare compared to the silicone and HSM-PMMA IOL at 11.3–13.4 years after surgery. Hardly any in HSM-PMMA IOL

Hydrophobicity and Hygroscopy

Hydrophobicity is a measure of the material’s tendency to separate itself from water. Each material has measurable hydrophobicity, which is graded using contact angle measurements, and this is a surface property. It ranges from just a few degrees for nearly perfectly hydrophilic surfaces, such as bare silica glass prepared with dangling hydroxyl groups, to almost 180° for superhydrophobic surfaces.
Since oxygen-hydrogen bonds in water are highly polar, hydrophobicity is highly dependent on the chemistry of the material. Partial electric charges in atoms tend to be attracted to opposite charges.Polymers are primarily composed of non-polar carbon-carbon and carbon-hydrogen bonds, so they are generally non-hydrophilic and are attracted to materials with partially charged bonds.
Hygroscopy describes a material’s tendency to absorb and retain water. A highly hygroscopic material absorbs water. In ophthalmology, hydrophobicity has been used to describe both the surface and interior of IOLs. The interaction of an IOL surface with water is a measure of hydrophobicity, and the ability of IOLs to absorb water is a hygroscopic one.

Polymethyl Methacrylate

The first IOL implanted in 1949 was made of PMMA . There have been reports of over 28 years that the original lenses implanted by Ridley remained perfectly clear and centered. There are also reports of some spontaneous dislocations of the vitreous. It is a solid, non-collapsible material that has less than 1% water content and is therefore hydrophobic. PMMA IOLs are usually one piece, large and therefore rarely used today. They have a refractive index of 1.49 and a normal optical diameter of 5-7 mm.The implantation of silicone IOLs was introduced in 1984. Silicon is a hydrophobic material with a refractive index of 1.41-1.46 and an optical diameter of 5.5-6.5 mm. Models are three-piece design with PMMA, polyvinyl difluoride (PVDF) and polyamide haptics. The problem with silicone is a sudden opening in the anterior chamber following implantation which can cause rupture of the posterior capsule.
Silicone IOLs are suspected to promote bacterial adhesion and therefore have a higher risk of postoperative infection. Silicone oil droplets adhere well to the silicone IOL in patients with retinal detachment or silicone oil tamponade used in diabetic retinopathy surgery. Therefore, silicone IOLs should not be implanted in myopic eyes with a high risk of retinal detachment.
Today, silicone IOLs are used less frequently because they are not suitable for microincision cataract surgery (MICS). There is also a slightly adjustable lens-two-component silicone IOL in use, in which power is adjusted after implantation with UV exposure. While aqueous silicone can penetrate the material, flares may occur with silicone optics.

Hydrophobic Collapsible Acrylic

Acrylic hydrophobic IOLs are the most widely used modern collapsible IOLs today.The purpose of the new design is to make the IOL collapsible. They can be manipulated during surgery and always return to their original shape in a short time. The first implanted IOL was in 1993. Hydrophobic Collapsible Acrylic can be in a three-piece and one-piece design, with an optical diameter of 5.5-7 mm and a total length of 12-13 mm, transparent or colored-yellow. The refractive index can be 1.44–1.55.
Single and multi-piece hydrophobic IOLs can be implanted with a small incision no larger than 2.2 mm and must be properly positioned because of their low self-centering ability. PCO is significantly lower than PMMA IOLs but generally slightly higher for hydrophobic acrylic lenses compared to silicone. they develop scintillations because they are easily penetrated, but are not always clinically significant unless they are intense or multifocal.IOLs made of this material are usually one-piece and designed for capsular bag implantation. The refractive index of the material is 1.43 and the water content ranges from 18% to 34%.
They are soft, compressible and provide excellent biocompatibility for their hydrophilic surfaces. They can be implanted through a small incision of less than 2 mm and are therefore ideal for MICS. The folding of poly-HEMA chains is dependent on the level of hydration and therefore the physical and optical properties of the polymer change as a function of water content. As the lenses hydrate, they absorb water and become soft and transparent. The main disadvantage is the higher optical opacification rate and lower resistance to capsular bag contraction compared to other materials.
Given the new knowledge and technological advances and achievements, we can expect new materials and designs of IOLs. We expect some changes in the shape of IOLs (discoid, plate-lamellar, ball-shaped) to improve biocompatibility and refractive quality, and thus some innovations in implantation possibilities.As a result, some new, different and innovative approaches in IOL design and materials and refractive ophthalmology are expected in the future.

References:
https://www.karger.com/ WebMaterial/ShowFileCache/883755
https://crstodayeurope.com/articles/2016-jan/does-iol-material-matter/
https://www.edinaeye.com/comprehensive-services/intraocular -lens-implants/types-of-intraocular-lenses-and-materials/