Choosing an Intraocular Lens (IOL) Accurately for Cataract Surgery or Refractive Lens Exchange (RLE)

The foundation of accurate IOL selection lies in comprehensive ocular measurements:

• Axial Length (AL)
  Represents the distance from the corneal surface to the retinal pigment epithelium. It determines the total eye length, a primary variable in every IOL formula. An error of just 0.1 mm can cause around a 0.27 dioptre refractive shift. This is measured by optical biometers such as the Zeiss IOLMaster 700 or Lenstar LS900.
• Keratometry (K readings)
  Measures the anterior corneal curvature in dioptres, typically across two meridians (flat K and steep K). It defines corneal power and helps calculate IOL power; differences between meridians define corneal astigmatism. Measured with devices like a keratometer, corneal topographer, OCT, or Scheimpflug system.
• Corneal Topography or Tomography
  Provides a three-dimensional map of the anterior and posterior corneal surfaces. This is crucial for detecting irregularities or asymmetry that impact toric or multifocal lens planning. Measured using Pentacam AXL, Anterion, or Revo FC devices.
• Total Corneal Power (TCRP)
  Combines refractive power from both corneal surfaces, offering more accuracy than anterior K readings alone. This measurement is essential for toric or premium IOLs, and is obtained via Scheimpflug (Pentacam) or OCT (Anterion).
• Anterior Chamber Depth (ACD)
  Measures the distance from the corneal endothelium to the anterior lens capsule. This affects effective lens position (ELP) and formula accuracy, measured by biometer or OCT.
• Lens Thickness (LT)
  Indicates the axial thickness of the crystalline lens and improves ELP predictions, particularly with formulas such as Barrett Universal II, Olsen, and Kane. Measured by IOLMaster 700, Lenstar, or Anterion.
• White-to-White (WTW)
  Measures horizontal corneal diameter, aiding in ELP prediction, enhancing IOL centration, and guiding sizing for phakic IOLs. Measured by IOLMaster 700, Pentacam, or Anterion.
• Central Corneal Thickness (CCT)
  Indicates the central corneal thickness in microns, influencing keratometric precision and refractive conversion accuracy. Measured via pachymetry or OCT.
• Posterior Corneal Curvature
  Represents power contribution from the posterior corneal surface, critical for precise toric axis alignment and overall astigmatism correction. Measured by Scheimpflug or OCT.
• Pupil Diameter (Photopic and Mesopic)
  Measures functional pupil size under bright and dim lighting, determining suitability for multifocal or extended depth of focus (EDOF) lenses. Larger pupils may increase the risk of halos or glare. Measured by corneal topographer or pupillometer.
• Angle Kappa / Chord Mu
  Assesses offset between visual and pupillary axes, important in multifocal and trifocal lens alignment to avoid optical decentration. Measured using a topographer or aberrometer.
• Axial Anterior Segment Length (ASL)
  Measures the distance from cornea to the posterior lens capsule, combining ACD and LT for improved formula precision. Measured by OCT biometers like IOLMaster 700 or Anterion.
• Ocular and Corneal Aberrations
  Assess higher-order aberrations such as coma and spherical aberration, guiding the choice between aspheric and spherical IOLs. Measured using devices such as iTrace or OPD-Scan III.
• Refraction (Pre-operative Subjective or Cycloplegic)
  Defines baseline refractive status and guides target refraction for distance, near vision, or monovision outcomes. Performed by subjective clinical testing.
• Dominant Eye
  Identifies the preferred fixation eye and determines monovision planning, with the dominant eye typically targeted for distance vision. Confirmed via clinical dominance tests.