ICL Sizing & EVO ICL — FAQ
Straight answers to the questions surgeons and patients ask about ICL sizing, vault prediction, and the EVO Visian Implantable Collamer Lens.
What sizes does the EVO ICL come in?
The EVO Visian ICL (STAAR Surgical) comes in four overall lengths: 12.1 mm (small), 12.6 mm (medium), 13.2 mm (large), and 13.7 mm (XL). Choosing between these four sizes for a given eye is the central technical challenge of ICL surgery, because the same eye can vault very differently across adjacent sizes — the choice ICL Fit is built to get right for each eye from a single Pentacam scan.
How is ICL size determined?
Surgeons select ICL size from ocular biometry — historically white-to-white (WTW) horizontal corneal diameter plus anterior chamber depth (ACD), fed into a manufacturer nomogram. WTW is only a rough proxy for the sulcus-to-sulcus (STS) distance where the haptics actually rest. Modern approaches add anterior-segment imaging (Pentacam Scheimpflug tomography, AS-OCT, or UBM) to capture angle-to-angle distance, ACD, and crystalline lens rise, which predict post-operative vault more accurately than WTW alone. ICL Fit is a Pentacam-based AI nomogram built on exactly these anterior-segment features.
What is ICL vault and why does it matter?
ICL vault is the gap between the back of the ICL and the front of the natural crystalline lens, measured in micrometers on anterior-segment OCT. Vault is really a readout of fit: it runs low or high when the lens size doesn't match the eye's internal anatomy. That is why ICL Fit is built around predicting the best-fitting size for each individual eye — the size most likely to sit well — rather than trying to hit one universal vault number.
What causes a high vault or a low vault?
A high vault usually results from an oversized ICL relative to the sulcus (or a deep anterior chamber), and a low vault from an undersized ICL relative to the sulcus (or a shallow anterior chamber). Because WTW does not measure the sulcus directly, two eyes with identical WTW can require different sizes — which is why sizing errors happen and why imaging-based AI tools like ICL Fit aim to reduce them.
What is the difference between white-to-white and sulcus-to-sulcus?
White-to-white (WTW) is the external horizontal corneal diameter, measured from limbus to limbus. Sulcus-to-sulcus (STS) is the internal diameter of the ciliary sulcus behind the iris, where the ICL haptics seat. STS is what actually governs fit and vault, but it is harder to measure and correlates only moderately with WTW — so WTW-only nomograms carry inherent sizing error that direct sulcus or angle imaging reduces. This is why ICL Fit sizes from anterior-segment imaging rather than WTW alone.
How accurate is AI or Pentacam-based ICL sizing compared with traditional nomograms?
AI and image-based sizing methods are trained on preoperative imaging paired with measured post-operative vault, so they learn the geometry that manufacturer WTW nomograms approximate. In peer-reviewed and presented work — including the VAULT-OCT deep-learning model — image-based machine learning aims to reduce vault-related ICL exchanges and improve refractive predictability versus vendor formulas. ICL Fit is an AI nomogram of this type, taking Pentacam scan data and outputting an optimal size with a predicted vault for each candidate lens.
Which formulas and nomograms are used for ICL sizing?
Common ICL sizing tools include the STAAR/manufacturer WTW nomogram, the KS-formula and NK-formula, the LASSO formulas, the Reinstein sizing formula (based on very-high-frequency ultrasound), the Kane ICL formula, and device-specific regressions such as the ANTERION-based formula. Newer machine-learning nomograms (for example VAULT-OCT and ICL Fit) predict vault directly from anterior-segment imaging rather than relying on a fixed WTW regression.
How is ICL vault measured after surgery?
Post-operative vault is measured on anterior-segment OCT, where the vertical distance between the posterior ICL surface and the anterior crystalline lens is read in micrometers, typically at the first post-op visits. Slit-lamp estimation in corneal-thickness units is a rough bedside alternative, but OCT is the standard for a precise, repeatable vault number — the same outcome ICL Fit predicts for each candidate size before surgery.
How is EVO ICL different from LASIK?
The EVO ICL is a phakic intraocular lens placed between the iris and the natural crystalline lens; it adds a lens without removing corneal tissue and is removable. LASIK reshapes the cornea with an excimer laser and is permanent. ICL is often preferred for higher prescriptions, thinner corneas, or dry-eye-prone eyes, while LASIK is common for moderate prescriptions with very fast visual recovery. Neither is universally 'safer' — candidacy depends on the individual eye.
Learn more
See the ICL Education overview, the research behind ICL Fit, or the plain-language guide for patients.