Critical Measurements and Parameters for Accurate Laser Eye Surgery Planning

• Manifest Refraction
  This is the patient’s subjective refractive error including sphere, cylinder, and axis. It defines the baseline prescription to treat and guides the excimer laser ablation plan. Measurements are obtained by clinical subjective refraction.
• Cycloplegic Refraction
  Taken with accommodation relaxed to detect latent hyperopia or accommodative spasm, thus ensuring accurate refractive targets. This is performed under cycloplegia using standard refraction methods.
• Keratometry (K readings)
  Assesses anterior corneal curvature, including flat and steep meridians. This defines corneal power and helps calculate residual stromal thickness and ablation centration, measured using a topographer or optical biometer.
• Corneal Topography / Tomography
  Creates a 3D map of the anterior and posterior corneal surfaces. It detects keratoconus, ectasia risk, asymmetry, or irregular astigmatism, measured via Pentacam, Anterion, Revo FC, or Galilei devices.
• Corneal Thickness (CCT)
  Refers to central corneal thickness in microns. It determines ablation depth limits; the residual stromal bed must remain at or above 250–300 microns for structural safety. Measured with pachymetry, OCT, or Scheimpflug imaging.
• Epithelial Thickness Profile
  Maps epithelial layer distribution across the cornea, identifying early keratoconus through epithelial masking and predicting post-operative healing response. Measured by OCT (Anterion or Revo FC).
• Residual Stromal Bed (RSB)
  Indicates stromal tissue remaining after flap creation and ablation. It is crucial for biomechanical safety; less than 250 microns suggests high ectasia risk. Calculated as CCT minus flap and ablation depth.
• Corneal Diameter (HVID / WTW)
  Measures the horizontal visible iris diameter, important for optical zone placement and ablation centration, measured by biometer or corneal topographer.
• Pupil Size (Photopic / Mesopic)
  Evaluates pupil diameter under bright and dim lighting. The pupil must fit within the treated optical zone to minimise postoperative glare or halos, measured with pupillometer or topographer.
• Corneal Asphericity (Q-value)
  Describes corneal shape tendencies (prolate or oblate), impacting ablation geometry and postoperative optics quality, measured by topographer or aberrometer.
• Higher-Order Aberrations (HOAs)
  Measures complex optical distortions beyond simple sphere and cylinder. These guide the choice between wavefront-guided or topography-guided laser treatments, measured using aberrometers such as iDesign, iTrace, or OPD-Scan III.
• Angle Kappa / Chord Mu
  Defines distance between visual axis and pupil centre, preventing decentration during laser ablation, measured with topographer or aberrometer systems.
• Corneal Biomechanics
  Indicates corneal stiffness and elastic response, detecting subclinical keratoconus and predicting ectasia risks, assessed with Corvis ST or Ocular Response Analyser.
• Tear Film Stability (TBUT, Schirmer)
  Assesses tear film quality, breakup time, and aqueous production to ensure accurate topography and support postoperative recovery, measured via tear film analysis or Schirmer testing.
• Wavefront Aberrometry (Total Ocular)
  Captures total ocular optical aberrations, guiding customised wavefront-based treatments and optical zone shaping, measured with iDesign, Zywave, or iTrace devices.
• Axial Length / Anterior Chamber Depth (optional)
  Measures overall eye length and anterior chamber depth, useful in high myopia or to verify posterior segment safety, measured by optical biometer.
• Retinal / Macular OCT
  Provides cross-sectional imaging of retina and macula, detecting subtle retinal disease or macular pathology, especially important in high myopes, performed with devices such as Revo FC or Spectralis.

Meticulous safety calculations confirm that the cornea can sustain treatment without risking structural integrity:

• Residual Stromal Bed (RSB)
  Calculated as CCT minus flap thickness and ablation depth. Safety threshold is minimum 250–300 microns, ideally above 300 microns.
• Percent Tissue Altered (PTA)
  Calculated as (flap + ablation depth) divided by CCT, then multiplied by 100. Should remain below or equal to 40% to minimise ectasia risk.
• Optical Zone Diameter
  Defines treated central ablation zone, which should exceed mesopic pupil size by at least 0.5 mm to limit glare.
• Transition Zone
  The blended zone beyond the optical zone that smooths curvature changes, improving optical smoothness and reducing halos.
• Ablation Depth
  Depth of laser tissue removal in microns, typically 12–14 microns per dioptre of myopia, with a safe residual stromal bed.
• Epithelial Healing Profile
  Monitors epithelial thickness changes after surface ablation to predict regression risk or haze development, tracked via OCT.