Phakic IOLs and refractive lens exchange

When refractive errors exceed the safe tissue limits of keratorefractive photoablation (such as myopia $> -8.00\text{ D}$ or hyperopia $> +4.00\text{ D}$), or when corneal thickness or topography is unfavourable, lens-based refractive surgery becomes the preferred modality. Lens-based refractive surgery is divided into two broad categories:

1. Phakic Intraocular Lenses (PIOLs): Implantation of an artificial lens into the anterior or posterior chamber while preserving the clear natural crystalline lens and crystalline accommodation.
2. Refractive Lens Exchange (RLE): Surgical removal of the clear crystalline lens via phacoemulsification followed by intraocular lens (IOL) implantation, analogous to cataract surgery.

1. Indications & Optical Advantages of Phakic IOLs

Phakic IOLs provide distinct optical and physiological benefits over excimer laser photoablation in high ametropias:

• Preservation of Accommodation: The patient's natural crystalline lens remains untouched, allowing young pre-presbyopic patients to maintain dynamic accommodation.
• Optical Superiority & Contrast Sensitivity: High myopic excimer ablation profiles induce significant positive spherical aberration ($Z_4^0$), degrade the natural prolate corneal asphericity, and reduce mesopic contrast sensitivity. Phakic IOL optics are placed closer to the eye's nodal point, producing larger retinal image magnification (reducing myopic minification) and superior visual quality without altering corneal topography.
• Reversibility: Unlike keratorefractive surgery, PIOL implantation is surgically reversible; the lens can be explanted or exchanged if refractive targets shift, cataracts develop, or complications occur.

Clinical Eligibility Criteria

• Age and range: Use the current model-specific local instructions for use, stable refraction and lens/angle status.
• Refractive Stability: Refractive shift $\le 0.50\text{ D}$ over the preceding 12 months.
• Correction range: Depends on model and authorisation; avoid generic ranges covering all phakic lenses.
• Contraindications to Cornea-Based Surgery: Central corneal thickness $< 480–500\,\mu\text{m}$, expected residual stromal bed $< 300\,\mu\text{m}$, expected $\text{PTA} \ge 40\%$, or suspicious topography (forme fruste keratoconus).

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2. Anatomical Classification & Lens Models

Phakic IOLs are categorised by their anatomical site of fixation within the eye:

1. Anterior Chamber Angle-Supported PIOLs (Historical / Obsolete)

Early models (e.g., NuVita MA20, ZSAL-4, AcrySof Cachet) featured flexible haptics positioned directly into the iridocorneal angle recess. Although surgically simple, they resulted in unacceptably high rates of long-term complications:

• Progressive Endothelial Cell Loss: Chronic micro-movement and peripheral contact with corneal endothelium during blinking or ocular rubbing led to late corneal decompensation.
• Pupil Ovalisation: Chronic mechanical compression and ischaemic atrophy of the iris root by angle footplates.
• Secondary Glaucoma: Progressive formation of peripheral anterior synechiae (PAS) and chronic low-grade trabeculitis.
• Current status: Many historical angle-supported designs were discontinued because of endothelial safety concerns; existing implants still require surveillance, and availability is jurisdiction-specific.

2. Anterior Chamber Iris-Fixated PIOLs (Artisan / Artiflex)

Designed by Jan Worst, these lenses feature two opposing claw haptics that enclave a fold of mid-peripheral, immobile iris stroma:

• Models: Artisan (rigid polymethylmethacrylate [PMMA], requiring a $5.2–6.2\text{ mm}$ incision) and Artiflex (foldable silicone optic with PMMA haptics, injectable through a $3.2\text{ mm}$ incision).
• Anatomical Safety: The optic is held elevated over the pupil, maintaining clearance from the delicate trabecular meshwork and angle structures.
• Mandatory Peripheral Iridectomy: Because the lens optic sits over the pupil, a surgical peripheral iridectomy (or preoperative Nd:YAG laser iridotomy) is mandatory to prevent pupillary block angle-closure glaucoma.
• Complications: Endothelial cell loss (requiring strict annual monitoring), pigment dispersion, nocturnal glare/halos, and traumatic disenclavation following blunt ocular injury.

3. Posterior Chamber Sulcus-Supported PIOLs: The Implantable Collamer Lens (ICL)

The Visian ICL (STAAR Surgical) is currently the predominant phakic IOL worldwide. It is positioned entirely within the posterior chamber, resting in the ciliary sulcus behind the iris and in front of the anterior crystalline lens capsule.

Material Science: Collamer

Collamer is a proprietary biocompatible polymer composed of poly-hydroxyethylmethacrylate (poly-HEMA), water ($34\%$), and a small quantity ($0.2\%$) of purified porcine collagen, integrated with a benzophenone UV-absorbing chromophore:

• The material carries a negative surface charge, which repels negatively charged circulating serum proteins and inflammatory cells.
• Biocompatibility reduces but does not eliminate inflammatory responses or other complications. Do not describe a material as immunologically invisible.

The EVO Visian ICL (Central KS-Aquaport Design)

Earlier ICL iterations (V4 model) required preoperative Nd:YAG laser iridotomies to prevent aqueous entrapment and pupillary block. The modern EVO Visian ICL (V4c and V5 models) incorporates a patented $360\,\mu\text{m}$ central hole (KS-Aquaport) along with two $360\,\mu\text{m}$ peripheral flow ports:

• Fluid Dynamics: Aqueous humour synthesised by the ciliary body flows unimpeded from the posterior chamber through the central port directly into the anterior chamber.
• Elimination of Iridotomy: Completely obviates the requirement for preoperative or intraoperative peripheral iridotomies, eliminating risks of laser-induced hyphema, iritis, or dysphotopsias.
• Preservation of Lens Physiology: Facilitates continuous aqueous nutrient circulation over the anterior crystalline lens capsule, significantly lowering the incidence of cataract formation.

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3. Preoperative Assessment & Safety Criteria

Confirm stable refraction, functional goals, clear crystalline lens, open angles and the absence of glaucoma or active inflammation. Obtain a dilated retinal examination and discuss myopia-related disease. Measure endothelial cell density, internal anterior-chamber depth and dimensions used for sizing. Internal ACD is measured from endothelium to anterior lens capsule, rather than epithelium.

Age range, ACD, endothelial thresholds and correction range depend on the particular model and current instructions for use in the intended jurisdiction. Do not copy a generic age/ECD table between devices. The FDA extended the US EVO/EVO+ age indication to 21–60 in 2026; this does not establish European labelling for every phakic IOL. See the FDA approval supplement for that specific change.

Sizing may use white-to-white and ACD according to the manufacturer; ultrasound biomicroscopy can add sulcus and ciliary-body information. Each method has measurement uncertainty. Repeat inconsistent dimensions and consider anatomy, including cysts and angle crowding. A thin cornea alone does not make a patient suitable for a phakic lens.

The central-port EVO design generally removes the requirement for preoperative iridotomy used with older non-port models. Port obstruction, retained viscoelastic and excessive crowding can still cause pressure problems. Follow the actual device's perioperative and monitoring instructions.

4. Post-ICL Vault Measurement & Clinical Management

The vault is defined as the perpendicular distance between the posterior surface of the ICL optic and the anterior crystalline lens capsule measured along the visual axis. It is evaluated quantitatively via Anterior Segment Optical Coherence Tomography (AS-OCT) or high-resolution Scheimpflug imaging.

Vault Categories & Clinical Consequences

• Vault interpretation: Approximately 250–750 micrometres is a commonly used clinical range, not a universal safety boundary. Interpret vault with model, angle, pressure, lens clarity and endothelial findings.

Longitudinal Endothelial Monitoring Protocol

• Normal physiological endothelial cell loss in adults is approximately $0.6\%$ per year.
• Following phakic IOL implantation, surgical trauma induces an acute drop of $1–5\%$ within the first 6 months, after which annual loss should plateau towards physiological levels.
• Endothelial surveillance: Monitor serial counts and corneal/angle status according to the device label. Progressive loss or other harm may require explantation; a universal annual percentage or cell threshold cannot replace model-specific and clinical assessment.

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5. Refractive Lens Exchange (RLE)

Refractive Lens Exchange (RLE)—also termed clear lensectomy—entails the removal of an unclouded crystalline lens via phacoemulsification and capsular bag implantation of an IOL (monofocal, toric, extended depth of focus [EDOF], or multifocal/trifocal).

Clinical Indications

1. Presbyopic hyperopia: Lens extraction can provide refractive correction and deepen the anterior chamber, reducing the lens-related component of angle closure. It does not eliminate every lifelong glaucoma mechanism.
2. Presbyopia: Selected patients may choose monofocal, toric or presbyopia-correcting IOLs after discussing optical compromises and surgical risk.

The Severe Retinal Hazard in Young Axial Myopes

Young high myopes need particular caution: extraction removes remaining accommodation and may increase retinal-detachment risk. Risk depends on axial length, age, peripheral retinal findings, vitreous status and surgical complications, so a single 10-year percentage or threefold multiplier is misleading. Discuss spectacles, contact lenses and suitability for corneal or phakic-lens procedures; a phakic IOL is not automatically suitable. A personalised informed decision is required.