Key Components of IOL Biometry

Axial Length (AL) Measurement:

• The distance from the corneal surface to the retinal pigment epithelium.
• Accurate measurement is critical; even small errors can lead to significant refractive outcomes.
• Measured using:
  • Optical coherence biometry (e.g., IOLMaster, Lenstar).
  • Ultrasound biometry (A-scan) when optical devices cannot obtain readings.

Keratometry (K):

• Measures the curvature of the anterior corneal surface.
• Essential for determining the corneal refractive power.
• Devices used include:
  • Manual keratometers.
  • Automated keratometers.
  • Topography systems for detailed corneal mapping.

Anterior Chamber Depth (ACD):

• The distance between the corneal endothelium and the anterior lens capsule.
• Influences the effective lens position (ELP), which impacts IOL power calculations.

Lens Thickness (LT):

• Measured with advanced optical biometers.
• Helps refine calculations, especially for highly myopic or hyperopic eyes.

White-to-White (WTW) Distance:

• Horizontal corneal diameter.
• Used in IOL sizing, especially for toric or multifocal IOLs.

Refraction:

• Preoperative refractive error assessment is vital for IOL calculations.

IOL Calculation Formulas

IOL power calculations rely on biometric data and mathematical formulas. Common formulas include:

• Older Formulas:
  • SRK II.
  • SRK/T (used for a wide range of axial lengths).
• Modern Formulas (more accurate for extreme biometric values):
  • Barrett Universal II.
  • Holladay II.
  • Hoffer Q (preferred for shorter eyes).
  • Haigis.
  • Hill-RBF (machine learning-based).

Advanced Biometry Techniques

Optical Low-Coherence Reflectometry (OLCR):

• Non-contact, precise measurements (e.g., IOLMaster).
• Measures AL, K, ACD, and more.

Swept-Source Optical Coherence Tomography (SS-OCT):

• Offers enhanced imaging and biometry of all ocular structures.
• Examples: IOLMaster 700, Argos.

Ultrasound Biometry:

• A-scan ultrasound is used when optical methods fail (e.g., dense cataracts).

Special Considerations in IOL Biometry

Post-Refractive Surgery Eyes:

• Altered corneal curvature makes calculations challenging.
• Specific formulas (e.g., Shammas, Barrett True-K) or adjustment methods are needed.

Toric IOLs:

• Require detailed astigmatism analysis.
• Tools like corneal topography or tomography aid in precise axis measurement.

Pediatric Biometry:

• Requires special consideration for growth-related changes in eye dimensions.

Sources of Error and Tips for Precision

Errors in Measurement:

• Ensure consistent alignment during keratometry and AL measurements.
• Use modern devices to minimize operator dependency.

Corneal Irregularities:

• Advanced corneal mapping helps identify and account for irregular astigmatism.

Lens Calculation Adjustments:

• Adjust IOL power based on surgeon-specific outcomes and preferences.