**2. Classification**

Based on the materials, intraocular lenses can be classified as:

1.Rigid (PMMA)


## **2.1 PMMA**

One of the first materials to be used for the purpose of intraocular lenses, polymethyl methacrylate (PMMA) is a rigid, non-foldable, hydrophobic material (**Figure 2**). Hydrophobic nature of PMMA lenses makes them more likely to adhere to corneal endothelial cells during insertion, thus causing potential endothelial loss. The refractive index is 1.49 and the usual optic diameter is 5–7 mm. They are usually single piece and have low memory haptics.

Due to their property of rigidity, a large incision is required for its implantation. An incision size of about 5.5–6 mm or a large corneoscleral tunnel is required for its

#### **Figure 2.**

*(a) MMA (methyl methacrylate) forms the basis for acrylic IOLs. (b) Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic; it was initially developed as a lightweight and shatter-resistant alternative to glass.*

**Figure 3.** *(a) Polydimethylsiloxane and (b) polydimethyldiphenylsiloxane.*

implantation. Large sized incisions are associated with delayed healing and astigmatic refractive errors. Hence PMMA is seldom used today except in developing countries due to economic reasons.

One piece variant of PMMA lens means that optics and haptics are made from a single mold of the same material. It is said to be three piece when the optics and the haptics are made from different materials and are attached together (**Figure 3**).

Penetration by aqueous humor has been noted to cause small vacuoles within the lens optic. This "glistening" phenomenon is rarely seen with PMMA.

After the advent of phacoemulsification in 1967, by Charles Kelman, the size of the incision did decrease significantly. However, the incision still had to be extended for implantation of the rigid IOL. The obligation of downsized incision was still amateur. This made way for the flexible and foldable breed of IOLs.

#### **2.2 Silicone**

Since 1950s, silicone has been used in a variety of medical device applications including contact and intraocular lenses. The malleable nature of silicone makes it chemically stable as well as imparts diverse mechanical properties. Also, due to its excellent biocompatibility and versatile properties, desired optical clarity and specific viscosity can be attained.

The first foldable silicone IOL was implanted in human eyes in the 1978 by Kai-yi Zhou. Silicone is hydrophobic, that is, it makes a contact angle of 990 with the water droplet on its material surface and therefore must be handled dry before implantation. This property allows a smaller incision than the IOL size. The refractive index of silicone lens is between 1.41 and 1.46 and the optic diameter is 5.5– 6.5 mm. Because of the low refractive index, the optics are rather thick especially for high refractive powers. Such lenses may require an incision of size up to 3.2 mm. Although there are injectors available for safe and dry handling of silicone lenses, premature and abrupt opening of the lenses remains a dispute for most surgeons.

After implantation, the anterior capsule rim opacifies quickly, while the posterior capsule may remain clear for many years. Despite the low posterior capsular opacification (PCO) rate and the good resistance to Nd:YAG laser shots, silicone is less used today because it is not suitable for micro incision cataract surgery (MICS).

Adhesiveness is a property by which the IOL fuses with anterior and posterior capsule and hence reduces the risk of decentration. An important point about silicone lenses is that it has poor adhesive property and it is kept in place by the virtue of its haptics and capsule coalescence. The character of "glistening" is seen in silicone lenses as well.

**7**

**Figure 4.**

*methacrylate, and (c) PEA – poly(ethyl acrylate).*

*Basic Science of Intraocular Lens Materials DOI: http://dx.doi.org/10.5772/intechopen.92573*

capsule [3].

**2.3 Acrylic**

or hydrophobic.

its side chain molecule.

IOLs than with the hydrophobic acrylic IOL [4].

properties of the material.

Silicone lenses are available in two variants depending on the type of haptics. The two kinds of haptics include modified C loop and plate haptics. Of these, the plate haptics have a higher tendency to decenter in eyes with defective anterior

Silicone is a synthetic polymer made up of periodically repeated silicon-oxygengroups (siloxane). This structure is the backbone for a polymer, which is identical for all silicone IOLs. Bound to the silicon atom are side chains, which influence the

The rigid PMMA lens is acrylic in nature. However the side chain molecules attached to the main polymer confer certain properties to the IOL. So, substituting the side chains in PMMA to hydroxyethyl or polyethyl groups alters the rigidity of the material. The newly formed polymers are now flexible and clear and this is the material that makes newer generation IOLs foldable. Furthermore, depending on the side-chain chemistry, the flexible acrylic material can be made to be hydrophilic

Most hydrophilic IOLs utilize the same material as contact lenses: hydroxyethylmethacrylate (HEMA) (**Figure 4**). Poly HEMA containing IOLs are also called hydrogels. With a water content of approximately 38%, they are flexible. Because of the high water content, they have a low refractive index. These lenses are highly foldable and can be injected through incisions approximately 1.8 mm in length or smaller, allowing for microincision cataract surgery (MICS). Because of hydrophilic nature of hydrogels, they are flexible and inert. Hydrophobic lenses have a low water content (<1%) and they carry a lesser risk of posterior capsule opacification. Higher uveal biocompatibility was achieved with the modern hydrophilic acrylic

A salient property of these acrylic materials is glass transition temperature or Tg. It is essentially the temperature at which the material changes its rigidity and becomes more flexible. Tg is different for different acrylic materials depending on

Foldable acrylic lenses tend to be more robust than their silicone equivalents. They undergo less post-implantation decentration or rotation. If posterior segment surgery is likely to be necessary at a later date, they are a better choice, as silicone oil – which would ruin silicone-based IOLs – can be used. However, this comes at the

The three piece hydrophobic acrylic foldable intraocular lens consists of a truncated hydrophobic optic and polymethylmethacrylate (PMMA) haptics. The single piece IOL is a new version of the hydrophobic acrylic foldable IOL, with both

*Flexible acrylic lenses can be made from (a) HEMA – (hydroxyethyl) methacrylate, (b) PEMA – (polyethyl)* 

cost of a slightly larger incision size being necessary for implantation.

#### *Basic Science of Intraocular Lens Materials DOI: http://dx.doi.org/10.5772/intechopen.92573*

Silicone lenses are available in two variants depending on the type of haptics. The two kinds of haptics include modified C loop and plate haptics. Of these, the plate haptics have a higher tendency to decenter in eyes with defective anterior capsule [3].

Silicone is a synthetic polymer made up of periodically repeated silicon-oxygengroups (siloxane). This structure is the backbone for a polymer, which is identical for all silicone IOLs. Bound to the silicon atom are side chains, which influence the properties of the material.

#### **2.3 Acrylic**

*Intraocular Lens*

**Figure 3.**

countries due to economic reasons.

*(a) Polydimethylsiloxane and (b) polydimethyldiphenylsiloxane.*

specific viscosity can be attained.

**2.2 Silicone**

implantation. Large sized incisions are associated with delayed healing and astigmatic refractive errors. Hence PMMA is seldom used today except in developing

One piece variant of PMMA lens means that optics and haptics are made from a single mold of the same material. It is said to be three piece when the optics and the haptics are made from different materials and are attached together (**Figure 3**).

Penetration by aqueous humor has been noted to cause small vacuoles within the

After the advent of phacoemulsification in 1967, by Charles Kelman, the size of the incision did decrease significantly. However, the incision still had to be extended for implantation of the rigid IOL. The obligation of downsized incision was still

Since 1950s, silicone has been used in a variety of medical device applications including contact and intraocular lenses. The malleable nature of silicone makes it chemically stable as well as imparts diverse mechanical properties. Also, due to its excellent biocompatibility and versatile properties, desired optical clarity and

The first foldable silicone IOL was implanted in human eyes in the 1978 by Kai-yi Zhou. Silicone is hydrophobic, that is, it makes a contact angle of 990

the water droplet on its material surface and therefore must be handled dry before implantation. This property allows a smaller incision than the IOL size. The refractive index of silicone lens is between 1.41 and 1.46 and the optic diameter is 5.5– 6.5 mm. Because of the low refractive index, the optics are rather thick especially for high refractive powers. Such lenses may require an incision of size up to 3.2 mm. Although there are injectors available for safe and dry handling of silicone lenses, premature and abrupt opening of the lenses remains a dispute for most surgeons. After implantation, the anterior capsule rim opacifies quickly, while the posterior capsule may remain clear for many years. Despite the low posterior capsular opacification (PCO) rate and the good resistance to Nd:YAG laser shots, silicone is less used today because it is not suitable for micro incision cataract

Adhesiveness is a property by which the IOL fuses with anterior and posterior capsule and hence reduces the risk of decentration. An important point about silicone lenses is that it has poor adhesive property and it is kept in place by the virtue of its haptics and capsule coalescence. The character of "glistening" is seen in

with

lens optic. This "glistening" phenomenon is rarely seen with PMMA.

amateur. This made way for the flexible and foldable breed of IOLs.

**6**

surgery (MICS).

silicone lenses as well.

The rigid PMMA lens is acrylic in nature. However the side chain molecules attached to the main polymer confer certain properties to the IOL. So, substituting the side chains in PMMA to hydroxyethyl or polyethyl groups alters the rigidity of the material. The newly formed polymers are now flexible and clear and this is the material that makes newer generation IOLs foldable. Furthermore, depending on the side-chain chemistry, the flexible acrylic material can be made to be hydrophilic or hydrophobic.

Most hydrophilic IOLs utilize the same material as contact lenses: hydroxyethylmethacrylate (HEMA) (**Figure 4**). Poly HEMA containing IOLs are also called hydrogels. With a water content of approximately 38%, they are flexible. Because of the high water content, they have a low refractive index. These lenses are highly foldable and can be injected through incisions approximately 1.8 mm in length or smaller, allowing for microincision cataract surgery (MICS). Because of hydrophilic nature of hydrogels, they are flexible and inert. Hydrophobic lenses have a low water content (<1%) and they carry a lesser risk of posterior capsule opacification. Higher uveal biocompatibility was achieved with the modern hydrophilic acrylic IOLs than with the hydrophobic acrylic IOL [4].

A salient property of these acrylic materials is glass transition temperature or Tg. It is essentially the temperature at which the material changes its rigidity and becomes more flexible. Tg is different for different acrylic materials depending on its side chain molecule.

Foldable acrylic lenses tend to be more robust than their silicone equivalents. They undergo less post-implantation decentration or rotation. If posterior segment surgery is likely to be necessary at a later date, they are a better choice, as silicone oil – which would ruin silicone-based IOLs – can be used. However, this comes at the cost of a slightly larger incision size being necessary for implantation.

The three piece hydrophobic acrylic foldable intraocular lens consists of a truncated hydrophobic optic and polymethylmethacrylate (PMMA) haptics. The single piece IOL is a new version of the hydrophobic acrylic foldable IOL, with both

#### **Figure 4.**

*Flexible acrylic lenses can be made from (a) HEMA – (hydroxyethyl) methacrylate, (b) PEMA – (polyethyl) methacrylate, and (c) PEA – poly(ethyl acrylate).*

the optic and haptics consisting of a foldable acrylic material. The table below gives a comparison based on their different properties [5, 6]:




### **2.4 Collamer**

Another subset of hydrophilic foldable acrylics is the Collamer lens. This Collamer material is a patented copolymer of hydrophilic acrylic and porcine collagen (<0.1%) hydroxyethyl methacrylate copolymer with a UV absorbing chromophore. In theory, the porcine collagen improves the biocompatibility of the lens when implanted in human eyes. It is a foldable phakic IOL consisting of a plate haptic with a central convex/concave optical zone and a forward vault to reduce the contact with the lens.
