Hydrophobic Acrylic
Features: High adhesion to the capsule, currently the standard material.
PCO risk: Lowest with sharp-edge design2).
Note: Glistening (dot-like reflections) may occur but usually does not affect visual function2).
Monofocal Intraocular Lens
1. What is a Monofocal Intraocular Lens?
Section titled “1. What is a Monofocal Intraocular Lens?”A monofocal intraocular lens (IOL) is an intraocular lens with a single focal distance in the optical part, which is an artificial lens inserted into the capsular bag after removal of the clouded natural lens during cataract surgery.
An eye with an IOL is called pseudophakia. Since there is no image magnification, it can be used in one eye without problems, and provides the most natural and physiological visual function compared to glasses or contact lenses. This is in contrast to the approximately 30% image magnification caused by aphakic glasses.
Cataract surgery has become a nearly perfected procedure using phacoemulsification and a small incision (about 2-2.5 mm) with a foldable IOL. As a result, the meaning of cataract surgery has shifted from “an eye-opening surgery to treat cataracts” to “a vision-restoring surgery to achieve higher quality postoperative visual function.”
Monofocal IOLs can be used within the scope of health insurance. Multifocal IOLs are selected medical treatment and require patient out-of-pocket costs. In the functional classification of IOLs, they fall under the Monofocal category among the four categories (Monofocal / Toric / SVL / Accommodating) based on ESCRS guidelines and ISO 11979-7:2024, and are positioned as PARTIAL-RoF narrow1).
Worldwide, approximately 20 million cataract surgeries are performed annually2), and in Japan, the number exceeds 1.5 million per year.
Q
Will I no longer need reading glasses after receiving a monofocal intraocular lens?
A
Since a monofocal IOL focuses only at a fixed distance, if it is set for distance vision, presbyopic correction glasses will be needed for near vision. Monovision (setting one eye for mild myopia) may reduce dependence on glasses. Switching to a multifocal IOL can provide near vision, but it carries risks of glare and halos and involves out-of-pocket costs.
2. Classification and Types
Section titled “2. Classification and Types”Monofocal IOLs are classified along three axes: shape, material, and optical design.
2-1. Shape Classification
Section titled “2-1. Shape Classification”Previously, three-piece designs with different materials for the optic and haptics were common, but currently, one-piece designs that allow smaller incisions using an injector are increasingly used.
One-piece IOL: The optic and haptics are integrated from the same material. Suitable for small-incision insertion via injector, and is currently the mainstream.
Three-piece IOL: The optic and haptics are made of different materials. Selected for cases requiring extracapsular fixation (ciliary sulcus fixation). However, fixing a one-piece acrylic IOL in the ciliary sulcus is contraindicated due to risks of decentration, iris chafing, pigment dispersion, increased intraocular pressure, and recurrent hyphema2).
2-2. Material Classification
Section titled “2-2. Material Classification”Hydrophilic acrylic
Features: High flexibility.
PCO risk: Higher than hydrophobic acrylic and silicone2).
Precautions: Risk of calcium deposits (calcification). It is recommended to avoid in cases of corneal transplantation or vitreous surgery where gas or air is used in the anterior chamber, as calcification risk increases2).
Silicone
Features: Low PCO rate.
PCO risk: Low.
Precautions: Avoid in cases where silicone oil is used after vitreous surgery. Compared to acrylic materials, the risk of endophthalmitis is 3.13 times higher (95% CI 1.47–6.67)1).
PMMA
Features: Rigid material. Cannot be folded.
PCO risk: Limited reference data.
Precautions: Requires a large incision, so it is rarely used now except in special situations.
Q
What is the best material for intraocular lenses?
A
Currently, hydrophobic acrylic is the standard material. It has the lowest incidence of posterior capsule opacification and can be folded for insertion through a small incision. Hydrophilic acrylic carries a risk of calcification, and silicone requires caution due to interaction with silicone oil during vitreous surgery.
2-3. Optical design classification
Section titled “2-3. Optical design classification”Spherical IOL: A conventional standard design with positive spherical aberration 2).
Aspherical IOL: A design that counteracts the positive spherical aberration of the cornea. It improves contrast sensitivity depending on pupil diameter, but is susceptible to decentration and tilt, and its functional benefits are debated 2).
The cornea has positive spherical aberration, while the young crystalline lens has negative spherical aberration, canceling out the overall spherical aberration of the eye. With aging, the spherical aberration of the lens becomes positive, increasing the overall positive spherical aberration of the eye. Aspherical IOLs are designed to vary the curvature of each refractive surface to focus peripheral and paraxial rays at the same point.
Tinted IOL (blue-light filtering): Has a spectral transmittance similar to the human crystalline lens, reducing transmission of short-wavelength light (which may cause retinal phototoxicity). A 2018 Cochrane review did not clearly demonstrate a protective effect on the macula, but also found no adverse effects on color vision 2).
Toric IOL: Adds cylindrical power to correct corneal astigmatism. 15–29% of cataract patients have corneal astigmatism of 1.5 D or more 2). The model, power, fixed axis, and incision location are calculated using the manufacturer’s online calculator, and at the end of surgery, the weak axis of the IOL is aligned with the steep meridian of the cornea.
Enhanced monofocal IOL: A randomized controlled trial of 218 cases (435 eyes) showed significantly improved intermediate and near corrected visual acuity compared to standard aspherical monofocal IOLs (P < 0.001), while distance corrected visual acuity and photic phenomena were equivalent 1). According to the ESCRS functional classification, it corresponds to PARTIAL-RoF enhance 1).
Differences from multifocal IOLs: Monofocal IOLs theoretically have no loss of light energy. In diffractive multifocal IOLs, light is split into multiple foci, leading to risks of glare, halos, and reduced contrast sensitivity 2).
Q
Which is better: aspherical or spherical lenses?
A
Aspherical IOLs correct corneal spherical aberration and improve contrast sensitivity, but their functional benefits are debated. They are also more susceptible to decentration and tilt. Currently, many IOLs adopt an aspherical design.
3. Epidemiology
Section titled “3. Epidemiology”Cataract is the leading cause of preventable blindness worldwide, with approximately 37 million people blind (about 0.6% of the world population), about half due to cataract 2). In Europe, 7 million cataract surgeries are performed annually; in the US, 3.7 million; and worldwide, about 20 million per year 2).
In Japan, cataract surgery is one of the most commonly performed surgeries, exceeding 1.5 million cases per year. Monofocal IOLs remain the most frequently used lenses. Multifocal IOLs are elective treatments and incur patient costs.
The prevalence of cataracts (including early opacities) reaches approximately 45% in the 50s, 75% in the 60s, 85% in the 70s, and 100% in those aged 80 and older.
4. Power Calculation and IOL Selection
Section titled “4. Power Calculation and IOL Selection”4-1. Preoperative Examination
Section titled “4-1. Preoperative Examination”Essential preoperative examinations for IOL power calculation include measurement of axial length, corneal curvature radius (K value), and anterior chamber depth.
Axial length measurement: Optical biometry (e.g., IOLMaster) is the standard. It shows better refractive outcomes than immersion A-scan 2). When optical measurement is not possible (e.g., dense nuclear cataract, corneal opacity), ultrasound A-scan is used.
Corneal curvature radius measurement: Keratometer or corneal topographer. If tear film BUT is less than 10 seconds, keratometry accuracy decreases, and preoperative dry eye treatment may be necessary.
Anterior chamber depth measurement: Used to predict the position of the IOL after insertion.
4-2. Generations and Accuracy of IOL Power Calculation Formulas
Section titled “4-2. Generations and Accuracy of IOL Power Calculation Formulas”The standard deviation (SD) of prediction error in a large-scale comparison of 18,501 cases is shown below 1).
| Formula | Prediction Error SD | Rank |
|---|---|---|
| Barrett Universal II | 0.404 | 1 |
| Olsen | 0.424 | 2 |
| Haigis | 0.437 | 3 |
| Holladay 2 | 0.450 | 4 |
| Holladay 1 | 0.453 | 5 |
| SRK/T | 0.463 | 6 |
| Hoffer Q | 0.473 | 7 |
The generational evolution of calculation formulas is as follows. The first generation (Fyodorov / Binkhorst / Colenbrander formulas) had large errors due to individual differences in anterior chamber depth. The second-generation SRK formula (1980) and its improved version SRK II correct the constant according to axial length. The third-generation SRK-T and Holladay 1 formulas are still widely used clinically. The fourth-generation Holladay 2 formula uses seven factors: axial length, K value, age, corneal diameter, lens thickness, preoperative anterior chamber depth, and preoperative refractive error.
In a comparison of 949 eyes (Barrett Universal II vs Hill-RBF vs intraoperative aberrometry), Barrett Universal II had a MAE of 0.29 D (within ±0.5 D: 84%), Hill-RBF had a MAE of 0.31 D (within ±0.5 D: 83%), and intraoperative aberrometry had a MAE of 0.31 D (within ±0.5 D: 82%), with no significant difference among the three (P > 0.05)1).
Q
Which is the most accurate among the latest IOL power calculation formulas?
A
In large-scale comparisons, Barrett Universal II has the highest accuracy with a prediction error SD of 0.404. However, for special eyes (e.g., steep cornea, post-refractive surgery eyes), the optimal formula differs, so it is important to choose according to the case.
4-3. Power Calculation in Special Eyes
Section titled “4-3. Power Calculation in Special Eyes”Post-refractive surgery eyes (after LASIK/PRK): Standard formulas overestimate corneal power and cause hyperopic shift. Barrett True-K is the best, showing accuracy of within ±0.5 D in 67.4% and within ±1.0 D in 93%1).
Steep cornea (K > 46.00 D): Hill-RBF is the best (within ±0.5 D: 83.0%)1).
Flat cornea (K < 42.00 D): Barrett Universal II is the best (within ±0.5 D: 96.7%)1).
Long axial length (axial length > 25 mm): Wang-Koch adjustment is recommended, but not needed for Barrett Universal II or Hill-RBF2).
4-4. Target Refraction Selection Strategy
Section titled “4-4. Target Refraction Selection Strategy”The choice of target refraction is made according to the patient’s occupation and lifestyle.
Emmetropia target (0 D)
Target: Set postoperative refraction to 0 D.
Advantages: Good uncorrected distance vision.
Disadvantages: Requires presbyopic correction glasses for near vision.
Mild Myopia Target (−0.5 to −1.0 D)
Target: Set postoperative refraction to mild myopia.
Advantages: Enables near work without glasses.
Disadvantages: May require glasses for distance tasks such as driving.
Mini-Monovision
Target: Set dominant eye to 0 D and non-dominant eye to −0.25 to −0.75 D1).
Advantages: Reduces spectacle dependence while maintaining stereopsis.
Disadvantages: Not accepted by all patients.
Full Monovision
Target: Set non-dominant eye to −1.75 D or more. Acceptance rate is approximately 90%2).
Advantages: Good uncorrected near vision.
Disadvantages: Not suitable for patients with latent strabismus, macular disease, or optic nerve disease2).
For ciliary sulcus fixation, select a power reduced by 0.5 to 1.0 D compared to in-the-bag fixation 2).
5. Surgical Techniques and Postoperative Management
Section titled “5. Surgical Techniques and Postoperative Management”5-1. Standard Procedure
Section titled “5-1. Standard Procedure”Phacoemulsification with in-the-bag IOL implantation is the current standard procedure. A foldable IOL is inserted via an injector through a small incision of approximately 2 to 2.5 mm. Anesthesia includes topical, retrobulbar, or sub-Tenon’s anesthesia.
In-the-bag fixation of a posterior chamber IOL is recommended for most cases 2). Anterior capsulotomy (continuous curvilinear capsulorhexis: CCC) covering the entire circumference of the IOL optic suppresses the development of posterior capsule opacification 2). Insertion of a foldable IOL using an injector (including preloaded types) reduces the risk of microbial contamination during surgery 2). Preloaded injectors reduce the risk of IOL loading errors (scratches, haptic deformation, or inversion) 2).
5-2. Axis Alignment of Toric IOL
Section titled “5-2. Axis Alignment of Toric IOL”Use the manufacturer’s online calculator to determine the power, model, alignment axis, and incision location. At the end of surgery, align the IOL’s flat meridian with the steep corneal meridian. Using a calculator that accounts for posterior corneal astigmatism (PCA) significantly reduces residual astigmatism 1). For every 3 degrees of toric IOL misalignment, the corrective effect decreases by approximately 10%. In a study of 8,229 cases, misalignment of ≥5° occurred in 0.89% of cases 1).
5-3. Postoperative Refractive Outcomes
Section titled “5-3. Postoperative Refractive Outcomes”| Database | Number of cases | CDVA 20/40 or better | CDVA 20/20 or better |
|---|---|---|---|
| European registry | 368,256 cases | 94.3% | 61.3% |
| IRIS Registry | 33,437 eyes | 81.7% (1 month postop) | — |
| Eyes without ocular complications | — | 95% or more | — |
(From AAO PPP 2021) 2)
Postoperative CDVA worsened in only 1.7% of all cases 2). The 1-year incidence of serious complications (endophthalmitis, expulsive hemorrhage, retinal detachment, etc.) was 0.5% (endophthalmitis 0.16%, expulsive hemorrhage 0.06%, retinal detachment 0.26%) 2).
Q
What is the success rate of the surgery?
A
In a European registry of approximately 370,000 cases, 94.3% achieved corrected visual acuity of 20/40 or better, and only 1.7% had worsened vision after surgery. The 1-year incidence of serious complications (endophthalmitis, retinal detachment, etc.) was 0.5%, making it a generally safe procedure.
5-4. Indications for Extracapsular and Anterior Chamber Fixation
Section titled “5-4. Indications for Extracapsular and Anterior Chamber Fixation”When capsular support is insufficient (e.g., zonular dialysis, posterior capsule rupture), extracapsular fixation should be considered. For ciliary sulcus fixation, a 3-piece IOL is appropriate (sulcus fixation of a 1-piece acrylic IOL is contraindicated) 2). The incidence of IOL decentration or tilt after intrascleral fixation (e.g., Yamane technique) is reported to be 0.1–1.7% 1).
6. Postoperative Complications and Long-term Management
Section titled “6. Postoperative Complications and Long-term Management”6-1. Posterior Capsule Opacification (PCO)
Section titled “6-1. Posterior Capsule Opacification (PCO)”This is the most common postoperative complication, caused by residual lens epithelial cells proliferating and migrating onto the posterior capsule. It can be treated with Nd:YAG laser posterior capsulotomy.
The rate of Nd:YAG laser treatment varies widely, from less than 5% to 54% depending on the report2). Among IOL materials, hydrophobic acrylic (sharp-edge) has the lowest PCO rate. A 2013 meta-analysis of 9 RCTs found that hydrophobic sharp-edge IOLs had a lower PCO rate than hydrophilic sharp-edge IOLs2). PCO rates tend to be lower in elderly patients2). A 12-year follow-up RCT suggested that the protective effect of sharp-edge hydrophobic IOLs may only delay the onset of PCO2). Anterior capsule polishing increases PCO and may hasten the need for YAG laser2).
6-2. Glistenings
Section titled “6-2. Glistenings”These are punctate reflections seen in the optic portion of hydrophobic acrylic IOLs. Those occurring in the deep layers are called glistenings, while those on the surface are called subsurface nano glistenings (SSNG). They usually do not affect visual function, but rarely IOL removal and exchange may be required2).
6-3. Calcium Deposition (IOL Calcification)
Section titled “6-3. Calcium Deposition (IOL Calcification)”This is a complication characteristic of hydrophilic acrylic IOLs. It causes severe opacification and requires IOL exchange. Use of gas or air in the anterior chamber during corneal transplantation or vitrectomy can induce calcification2).
6-4. IOL Decentration and Dislocation
Section titled “6-4. IOL Decentration and Dislocation”The reported incidence is 0.1–1.7%1). Risk factors include history of vitrectomy, aging, high myopia, inflammation, retinitis pigmentosa, diabetes, mature cataract, history of acute angle-closure attack, and connective tissue diseases1).
6-5. Dysphotopsia
Section titled “6-5. Dysphotopsia”Positive dysphotopsia (glare, halos): Incidence is lower with monofocal IOLs than with multifocal IOLs2).
Negative dysphotopsia (crescent-shaped dark shadow): Has been reported with sharp-edge IOLs.
6-6. Endophthalmitis
Section titled “6-6. Endophthalmitis”The incidence is 0.16%2). Silicone IOLs have a 3.13 times higher risk (95% CI 1.47–6.67) compared to acrylic IOLs1).
7. Latest Research and Future Perspectives
Section titled “7. Latest Research and Future Perspectives”Enhanced monofocal IOL: Compared to standard aspheric monofocal IOLs, intermediate and near corrected visual acuity is significantly improved (P < 0.001). A randomized controlled trial of 218 cases (435 eyes) showed that distance corrected visual acuity and the incidence of photic phenomena are equivalent to standard monofocal IOLs1). It is available under insurance coverage and is expected to become more widespread.
Light-adjustable IOL (Power Adjustable IOL): A technology that allows postoperative adjustment of spherical and cylindrical power by moving unpolymerized photosensitive silicone macromers with ultraviolet light2). It is attracting attention as a means to minimize postoperative refractive error.
Refractive Index Shaping: A technology that locally changes the chemical properties of acrylic IOLs using femtosecond lasers to modify the refractive index and adjust the power2). It is expected to be applied for postoperative refractive adjustment.
AI-driven IOL power calculation: In addition to Barrett Universal II and Kane formulas, new generation machine learning-based calculation formulas are being developed and clinically evaluated1). The goal is to improve accuracy in cases with special axial lengths or corneal curvatures.
New generation IOL materials: Clinical introduction of hydrophobic surface IOLs with modified water content is progressing1). They aim to reduce glistening formation while maintaining the advantages of hydrophobicity (low PCO rate).
8. References
Section titled “8. References”- European Society of Cataract and Refractive Surgeons (ESCRS). ESCRS Guideline for Cataract Surgery. https://www.escrs.org/escrs-recommendations-for-cataract-surgery
- Miller KM, Oetting TA, Tweeten JP, et al. Cataract in the Adult Eye Preferred Practice Pattern®. Ophthalmology. 2022;129(1):P52-P94.