Comparison of Intraocular Lens (IOL) Materials

Key points at a glance

Key points of this disease

• Intraocular lens (IOL) materials include hydrophobic acrylic, hydrophilic acrylic, silicone, PMMA, and collamer.
• The most widely used IOL today is hydrophobic acrylic, which has a low incidence of posterior capsule opacification (PCO).
• Each material differs in refractive index, water content, biocompatibility, and complication profile, requiring selection based on the case.
• The main drawback of hydrophobic acrylic IOLs is glistening (microvacuoles), but its impact on visual function is rare.
• Hydrophilic acrylic IOLs are highly flexible and suitable for micro-incision surgery, but have a higher PCO rate and risk of calcification.
• Square-edge design, regardless of material, inhibits lens epithelial cell proliferation and helps reduce PCO.
• When selecting an IOL, consider the possibility of future vitreous surgery and the presence of uveitis.

1. Comparison of Intraocular Lens (IOL) Materials

An intraocular lens (IOL) is an artificial lens inserted after removing the cloudy lens during cataract surgery. In 1949, Harold Ridley first implanted a polymethyl methacrylate (PMMA) IOL in a human. Since then, IOL materials and designs have evolved significantly.

The main IOL materials currently available are as follows:

• Hydrophobic acrylic: Introduced in 1993. Currently the most widely used material.
• Hydrophilic acrylic: High water content and excellent flexibility. Suitable for micro-incision cataract surgery.
• Silicone: Has a long history as a foldable IOL.
• PMMA: The first IOL material. Rigid and non-foldable.
• Collamer: A collagen-containing copolymer. Mainly used for phakic intraocular lenses (ICL).
• PEG-PEA/HEMA/styrene copolymer: A new material combining hydrophilic and hydrophobic properties.

Foldable IOLs (silicone, acrylic) have largely replaced rigid PMMA IOLs because they can be inserted through a small incision ¹⁾. Surgeons must understand the advantages and disadvantages of each material when making a choice ¹⁾.

2. Characteristics and Classification of IOL Materials

Physical and Optical Properties

IOL materials are classified by properties such as Abbe number, refractive index, water content (hydrophilicity), and glass transition temperature.

Property	Definition	Clinical Significance
Refractive Index	Measure of light bending	Higher allows thinner lens
Abbe Number	Measure of chromatic dispersion	Higher means less chromatic aberration
Water Content	Water retention capacity	Higher reduces glistening

The refractive index of the natural crystalline lens is 1.4, and its Abbe number is 47. A higher refractive index allows for a thinner IOL design for the same power, which is advantageous for insertion through a small incision. However, as the refractive index increases, the Abbe number decreases, leading to increased chromatic aberration.

Biocompatibility

Biocompatibility is broadly divided into capsular biocompatibility and uveal biocompatibility.

• Capsular biocompatibility: Refers to the interaction between the IOL and residual lens epithelial cells (LECs). It is involved in the development of anterior capsule opacification (ACO) and posterior capsule opacification (PCO).
• Uveal biocompatibility: Refers to the ability to avoid immune reactions of the iris, ciliary body, and anterior choroid.

Hydrophobicity and Contact Angle

The hydrophilicity or hydrophobicity of the IOL surface is measured by the contact angle. A larger contact angle indicates higher hydrophobicity. Hydrophobic materials tend to adhere better to the posterior capsule, reducing the space for LEC migration and thereby suppressing PCO.

3. Characteristics of Each IOL Material

Hydrophobic Acrylic

This is currently the most widely used IOL material in the world. It is composed of a cross-linked copolymer of acrylic acid ester and other comonomers.

• Refractive index: High, ranging from 1.47 to 1.56. Allows for thin design.
• Water content: Low, 0.1–0.5%.
• Glass transition temperature: 16–55°C.
• PCO: Low incidence when combined with a square edge ¹⁾. Adheres to the posterior capsule via fibronectin binding, inhibiting LEC migration.
• Glistenings: Liquid-filled microvacuoles that appear inside the IOL. They are more likely to occur in low-water-content materials, but rarely affect visual function or require explantation ¹⁾.

In the AAO Cataract PPP (2021), square-edged hydrophobic acrylic IOLs are considered one of the materials with the lowest rates of PCO and Nd:YAG posterior capsulotomy ¹⁾.

Blue light-filtering IOLs

Tinted IOLs (blue light-filtering IOLs) were expected to have a protective effect on the macula, but a 2018 Cochrane Systematic Review did not show a clear protective effect ¹⁾. On the other hand, no adverse effects on color vision have been identified.

Hydrophilic acrylic

This material is made by introducing hydroxyl groups into the PMMA backbone, and flexibility is imparted by adding HEMA (hydroxyethyl methacrylate).

• Refractive index: Low, 1.40–1.43. The lens becomes thicker.
• Water content: High, 18–38%.
• Glare: Low incidence.
• PCO: Higher incidence than silicone or hydrophobic acrylic ¹⁾. This is thought to be partly due to difficulty maintaining a sharp posterior edge due to swelling.
• Calcification: Calcium-phosphate deposition, which was a problem in older generations, has been improved in newer generations. However, if air or gas enters the eye during corneal transplantation (DSEK, DMEK) or vitrectomy, there is a risk of calcification, so it is advisable to avoid these IOLs in eyes where such surgeries are anticipated ¹⁾.

Because it is very flexible, it can be inserted through an incision of about 1.8 mm, which is advantageous for micro-incision cataract surgery (MICS).

In a prospective study of 86 eyes with pseudoexfoliation syndrome, hydrophilic acrylic IOLs had the lowest LEC proliferation and excellent capsular biocompatibility, but had more debris deposition on the surface and the highest PCO rate, and uveal biocompatibility was inferior.

Silicone

Silicone is a synthetic polymer with a repeating silicon-oxygen backbone.

• Refractive index: 1.43. Lower than acrylic, so the lens is thicker for the same power.
• Water content: 0.38%.
• Contact angle: 97–120°. Strongly hydrophobic.
• Glass transition temperature: –120 to –90°C.
• PCO: LEC adheres poorly; with the sharpest square edge, some reports indicate a lower PCO rate than hydrophobic acrylic after long-term use (≥6 years).
• Disadvantages: Bacteria, cells, and silicone oil tend to adhere. May fog due to condensation during vitrectomy, so use cautiously in diabetic eyes ¹⁾. Should be avoided in eyes with silicone oil ¹⁾.

Q When should silicone IOLs be avoided?

A

Avoid use in cases where silicone oil or expansile gas may enter the posterior segment ¹⁾. The same applies to eyes at high risk for future vitrectomy, such as severe proliferative diabetic retinopathy.

PMMA

The first material used for IOLs, with excellent tissue tolerance and long-term stability.

• Refractive index: 1.49. High optical clarity.
• Water content: 0.4–0.8%.
• Contact angle: 65–71°.
• Glass transition temperature: 105–113°C.
• Disadvantages: Rigid and non-foldable. Requires a 5.5–6 mm incision for insertion, causing postoperative astigmatism and delayed healing.
• Complications: Long-term use may lead to snowflake degeneration (indication for IOL explantation). Due to hydrophobicity, it may adhere to corneal endothelial cells during implantation, potentially causing endothelial damage.

Currently, it is used in limited cases such as scleral-sutured IOLs when capsular bag fixation is not possible.

Collamer

It is a copolymer of HEMA (hydroxyethyl methacrylate) and porcine-derived collagen, mainly used as a posterior chamber phakic intraocular lens (ICL).

• Refractive index: 1.44.
• Water content: 40%.
• Features: Available as the EVO phakic posterior chamber lens family for spherical and astigmatic correction. FDA approved in the US in 2022. The central port design eliminates the need for peripheral iridotomy (PI) that was previously required.
• Indications: Ages 21–45, myopia with spherical equivalent -3.0 to -20.0 D, anterior chamber depth ≥3.0 mm, stable refraction with change ≤0.5 D within 1 year.
• Vault: Distance between the back of the phakic posterior chamber lens and the front of the crystalline lens. The optimal range is 50–150% of central corneal thickness (250–900 μm). Too low increases risk of anterior subcapsular cataract; too high increases risk of angle-closure glaucoma.

PEG-PEA/HEMA/Styrene Copolymer

This is a new-generation IOL material that combines hydrophilic and hydrophobic properties. It is used in the enVista MX60 IOL.

• Composition: PEG-PEA 40%, HEMA 30%, styrene 26%, EG-DMA 4%.
• Refractive index: 1.54.
• Water content: 4–5%.
• Hardness: 1.8 MPa.
• Features: PEG-PEA provides hydrophobicity, while HEMA provides hygroscopicity. It does not develop glistenings, and has been reported to have low rates of PCO and Nd:YAG capsulotomy.

Hydrophobic Acrylic

Most widely used: Current standard IOL material.

PCO rate: Low with square edge.

Glistenings: Main drawback, but rarely affects visual function.

Hydrophilic Acrylic

Excellent flexibility: Can be inserted through an approximately 1.8 mm incision.

PCO rate: Higher than other materials.

Calcification risk: Caution after air or gas injection.

Silicone

Long-term PCO rate: Some reports indicate it is lower than hydrophobic acrylic.

Caution: Avoid in eyes with silicone oil or gas use.

Condensation: May fog during vitreous surgery.

4. Intraocular Lens Material and Posterior Capsule Opacification

Posterior capsule opacification (PCO) is the most common long-term complication after cataract surgery, with reported incidence rates of 5–54% ¹⁾. It is treated with Nd:YAG laser capsulotomy, but IOL material and edge design significantly influence the incidence.

PCO Characteristics by Material

• Hydrophobic acrylic: When combined with a square edge, has the lowest PCO rate¹⁾.
• Hydrophilic acrylic: Has a higher PCO rate than silicone or hydrophobic acrylic¹⁾.
• Silicone: LEC adheres less easily, and long-term PCO rate is low.
• PMMA: Tends to have a high PCO rate, along with hydrogel IOLs.

Importance of Edge Design

A 2013 meta-analysis (9 RCTs) and several longitudinal studies showed that square-edged hydrophobic IOLs have lower rates of PCO and Nd:YAG posterior capsulotomy than square-edged hydrophilic IOLs¹⁾. Square-edged acrylic, PMMA, and silicone IOLs are reported to be equivalent in terms of the need for Nd:YAG posterior capsulotomy (evidence level I+, recommendation strength Strong)¹⁾.

However, one randomized trial suggested that the protective effect of square-edged hydrophobic lenses may only “delay” PCO development compared to round-edged silicone and PMMA IOLs after 12 years¹⁾.

Q Which IOL material is least likely to cause PCO?

A

Square-edged hydrophobic acrylic IOLs currently have the lowest PCO rate¹⁾. Edge design is as important as material, and a square edge contributes to PCO suppression regardless of material.

5. Selection and Precautions for IOL Materials

IOL selection should be based on the characteristics of each material and tailored to the individual patient’s situation.

General Principles of Material Selection

• Standard cataract surgery: Hydrophobic acrylic IOL is the first choice¹⁾.
• Micro-incision cataract surgery (MICS): Hydrophilic acrylic IOL may be suitable.
• Future risk of vitreous surgery: Silicone IOLs should be avoided¹⁾. Choose hydrophobic acrylic.
• Eyes scheduled for corneal transplantation: Hydrophilic acrylic IOLs carry a risk of calcification and should be avoided ¹⁾.
• Uveitic eyes: Acrylic IOLs or heparin surface-modified (HSM) PMMA IOLs are associated with better visual outcomes ¹⁾. They have been reported to perform better than non-HSM PMMA or silicone IOLs.

Clinical situation	Recommended material	Material to avoid
Standard surgery	Hydrophobic acrylic	—
Risk of vitreous surgery	Hydrophobic acrylic	Silicone
Scheduled corneal transplantation	Hydrophobic acrylic	Hydrophilic acrylic
Uveitis	Acrylic, HSM PMMA	Non-HSM PMMA, silicone

Precautions in IOL Material Selection

• Hydrophilic acrylic IOLs carry a risk of calcification after surgeries where air or gas is injected into the eye (e.g., vitrectomy, DSEK, DMEK) ¹⁾. In eyes at risk of corneal endothelial damage, other materials should be selected.
• Silicone IOLs may become coated and opacified by silicone oil if present in the posterior segment ¹⁾.
• Although rare, IOL explantation due to glistening in hydrophobic acrylic IOLs has been reported ¹⁾.
• One-piece acrylic IOLs have been associated with uveitis-glaucoma-hyphema (UGH) syndrome ¹⁾.

Q Which IOL material is suitable for patients with uveitis?

A

Acrylic IOLs (especially hydrophobic acrylic) or heparin surface-modified PMMA IOLs are associated with good outcomes ¹⁾. Preoperative control of uveitis and diagnosis of Fuchs heterochromic iridocyclitis are also favorable prognostic factors.

6. Physical Properties and Optical Design of IOL Materials

Refractive Index and Chromatic Aberration

The refractive index of an IOL depends on the chemical composition of the material. Addition of halogens, aromatic groups, or sulfur increases the refractive index. Refractive index and IOL thickness are inversely correlated; higher refractive index materials allow for thinner designs.

Chromatic aberration in pseudophakic eyes is determined by the Abbe number of the IOL material. Abbe numbers among IOL materials range from 37 to 55. Chromatic aberration also affects contrast sensitivity and emmetropization.

Glass Transition Temperature

The glass transition temperature is the temperature at which a polymer changes from a hard glassy state to a flexible rubbery state. IOLs are designed to have a glass transition temperature below physiological body temperature (37°C) and room temperature. If it exceeds body temperature, the lens may not unfold properly inside the eye.

Aspheric IOLs

Spherical IOLs have positive spherical aberration, which adds to the cornea’s positive spherical aberration, increasing overall ocular aberration. The young crystalline lens has negative spherical aberration that cancels this out, but with aging, the lens’s spherical aberration shifts toward positive.

An aspheric IOL is a lens designed so that the curvature of each refractive surface varies, allowing peripheral and paraxial light rays to converge at the same point. Currently, most IOLs adopt an aspheric design. Although aspheric IOLs improve contrast sensitivity by reducing spherical aberration, they can increase coma aberration if decentered or tilted. Therefore, in cases where IOL fixation is unstable, a spherical IOL may be more suitable.

Tinted IOLs

Conventional non-tinted UV-absorbing IOLs transmit a large amount of short-wavelength light. Tinted IOLs have a spectral transmittance closer to that of the human crystalline lens and are expected to have a protective effect against retinal phototoxicity. Previously, only PMMA IOLs were available, but foldable versions are now being developed.

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7. Latest Research and Future Perspectives (Investigational Reports)

For patients: Please read carefully

The following content is currently under investigation or in clinical trials and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Development of Hydrophilic-Hydrophobic Hybrid Materials

New materials that optimally balance hydrophilic and hydrophobic properties are being developed, such as the PEG-PEA/HEMA/styrene copolymer (enVista MX60). The goal is to overcome material-specific drawbacks such as glistening in conventional hydrophobic acrylic and PCO/calcification in hydrophilic acrylic.

IOL Surface Modification Technologies

Heparin surface-modified (HSM) PMMA IOLs have shown good results in eyes with uveitis ¹⁾, and improving biocompatibility through surface modification is considered an important direction for future IOL development. Research is ongoing on coatings and nanotexturing of IOL surfaces to inhibit lens epithelial cell (LEC) adhesion and biofilm formation.

Overcoming IOL Calcification

Calcification of hydrophilic acrylic IOLs is particularly problematic after corneal endothelial transplantation or vitrectomy. New-generation hydrophilic acrylic IOLs are reported to have a reduced risk of calcification, but complete resolution has not been achieved. Research is being conducted on improving material composition and surface treatments to enhance resistance to calcification.

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8. References

1. American Academy of Ophthalmology. Cataract in the Adult Eye Preferred Practice Pattern. Ophthalmology. 2022;129:P1-P126.