An intraocular lens (IOL) is an artificial lens that replaces the extracted natural lens during cataract surgery to provide refractive correction. An IOL consists of a central optic and haptics that fixate the IOL within the capsular bag.
A one-piece IOL (single-piece IOL / one-piece IOL) is a lens in which the optic and haptics are made of the same material (acrylic, silicone, or PMMA) and molded as a single unit. In contrast, a three-piece IOL (3-piece IOL) has the optic and haptics made of different materials and manufactured separately.
In modern cataract surgery, foldable IOLs have become mainstream. Foldable IOLs can be made from silicone, hydrophilic acrylic, or hydrophobic acrylic, all of which cause minimal foreign body reaction 1). Using an injector, they can be inserted through a small incision.
When a one-piece IOL is functioning normally, patients typically experience improved vision and good visual outcomes after cataract surgery. However, if the following problems occur, characteristic subjective symptoms appear.
Complications due to ciliary sulcus fixation
If a one-piece IOL is mistakenly placed in the ciliary sulcus, the following subjective symptoms may occur.
IOL decentration or subluxation
If the IOL becomes displaced from the capsule, diplopia, increased optical aberrations, and decreased visual acuity occur.
Normal One-Piece IOL Findings
In-the-bag fixation: The IOL is correctly positioned within the capsular bag.
IOL centration: Under mydriasis, the center of the optic coincides with the center of the pupil.
Rotational stability: Confirm no axis deviation in toric IOL
Posterior capsule clarity: Check for posterior capsule opacification
Findings of one-piece IOL complications
Findings due to ciliary sulcus fixation: Pigment dispersion, iris defects on transillumination, elevated intraocular pressure, recurrent hyphema
UGH syndrome: Triad of uveitis + glaucoma + hyphema
IOL decentration/tilt: Check for haptic extrusion out of the capsule under mydriasis
Posterior capsule opacification: Opacification of the posterior capsule
The main difference is the integrity of the material and the limitation of fixation location. A one-piece IOL has the optic and haptics made of the same material as a single piece and is designed exclusively for in-the-bag fixation. A three-piece IOL has haptics made of a different material (usually PMMA or PVDF) manufactured separately and can be used for both in-the-bag and ciliary sulcus fixation. When the posterior capsule is ruptured, out-of-the-bag fixation of a one-piece IOL is contraindicated; a three-piece IOL should be used or surgery should be postponed.
The major risk factors for complications related to one-piece IOLs are listed below.
| Risk Factor | Associated Complication |
|---|---|
| Incorrect ciliary sulcus fixation | UGH syndrome, pigment dispersion, elevated intraocular pressure, intraocular hemorrhage |
| Used during posterior capsule rupture | IOL displacement, dislocation, secondary glaucoma |
| Pseudoexfoliation syndrome | Zonular weakness → late in-the-bag IOL dislocation |
| Prior vitrectomy | Zonular weakness → IOL dislocation |
| High myopia | Increased risk of toric IOL axis misalignment |
| Anterior capsule contraction | IOL tilt and decentration |
The haptics of one-piece IOLs are thick and have a square-edge design; when placed in the ciliary sulcus, they contact the back surface of the iris. This contact causes mechanical irritation to the iris → pigment dispersion → iris transillumination defects, leading to elevated intraocular pressure, recurrent inflammation, and intraocular hemorrhage (UGH syndrome)1).
Assessment of Intraocular Conditions
Slit-Lamp Examination
Under dilation, check the position of the IOL (optic and haptics). Evaluate whether the haptics are outside the capsule or if the IOL is tilted.
Ultrasound Biomicroscopy (UBM) and Anterior Segment OCT
Useful when IOL-iris contact is suspected. Helps diagnose UGH syndrome1).
When a one-piece acrylic IOL is fixated outside the capsule, complications such as IOL decentration, pigment dispersion, elevated intraocular pressure, intraocular hemorrhage, and cystoid macular edema (CME) may occur. If complications arise, surgery is required in over 90% of cases, and IOL exchange is performed in 83% of those. Final visual acuity is often good (20/20), but the surgical invasiveness is significant. As a preventive measure, it is recommended to have a three-piece IOL available when posterior capsule rupture occurs.
The appropriate insertion and fixation technique for a one-piece IOL is described below.
Insertion Using an Injector
Foldable one-piece IOLs are preloaded in an injector and inserted into the anterior chamber through a small incision (2–3 mm). Preloaded injectors reduce the risk of contamination during IOL insertion and prevent problems such as IOL damage, haptic breakage, and IOL inversion 1).
In-the-Bag Deployment and Fixation
The IOL is deployed within the capsular bag, and the haptics are securely fixed in the bag. Optimal positioning is achieved when the anterior capsulotomy (CCC) evenly covers the IOL optic (capsulorrhexis overlap).
Toric one-piece IOLs have higher rotational stability than early-generation silicone plate IOLs 1). However, if IOL decentration or axis misalignment occurs, reoperation for axis alignment is necessary. Axis misalignment is most likely to occur in the early postoperative period (within a few hours) 1).
The main causes of IOL axis misalignment include the following 1):
Foldable IOL Materials
In modern cataract surgery, foldable IOLs are mainstream, and the following three types of materials are used1).
Aspheric Design
In spherical IOLs, the focal points of paraxial and peripheral rays differ (spherical aberration). Aspheric IOLs correct this and improve contrast sensitivity.
Tinted IOL (Blue-Light Filtering)
Uncolored UV-blocking IOLs transmit a large amount of visible light, especially short-wavelength light. Tinted IOLs have transmission characteristics close to those of the human crystalline lens, reducing postoperative discomfort in color and brightness and potentially suppressing retinal phototoxicity from short-wavelength light.
Uveitis-Glaucoma-Hyphema (UGH) syndrome is a complication particularly associated with modern one-piece acrylic IOLs1). The thick square-edged haptics of a one-piece IOL placed in the ciliary sulcus repeatedly irritate the posterior iris and ciliary body, causing pigment dispersion, iris transillumination defects, elevated intraocular pressure, recurrent inflammation, and intraocular bleeding1). Pseudophacodonesis is also a risk factor for UGH syndrome1).
One-piece acrylic IOLs have been confirmed to have higher rotational stability than early-generation silicone plate-type IOLs, but rare cases of decentration can occur depending on the design 1). Some extended depth of focus (EDoF) IOLs (e.g., Mini Well Ready) have soft closed-loop haptics and may dislocate from the capsular bag. Since decentration is difficult to detect under non-mydriatic conditions, confirmation using the retroillumination method under mydriasis is important.
The IOL exchange rate within one year after multifocal IOL implantation is reported to be higher than that for monofocal IOLs. Thorough preoperative evaluation, patient education, and IOL selection based on a full understanding of the differences in characteristics depending on IOL type and optical design are required 1).
