Overview
This lecture reviews types of biometry used in preoperative cataract measurements, focusing on methods, key measurements, and their impact on intraocular lens (IOL) calculations.
Biometry Basics
- Biometry applies mathematics to biology; in ophthalmology, it refers to precise eye measurements.
- Common biometry systems: A-scan and B-scan ultrasound, optical coherence tomography (OCT), and laser interferometry.
- Preoperative biometry is critical for accurate IOL power selection in cataract surgery.
Methods of Biometry
- A-scan ultrasound uses a 10 MHz sound wave for one-dimensional measurements, usually in contact or immersion technique.
- B-scan ultrasound generates 2D images using a fan of sound waves; used for more comprehensive imaging.
- Optical interferometry uses laser light (e.g., IOL Master, Lenstar) for high-precision, non-contact measurements.
Key Biometric Measurements
- Axial length: Most important measurement; errors of 0.33 mm can cause a 1 diopter refractive error.
- Keratometry: Measures the corneal curvature; 1 diopter error in keratometry equals a 1 diopter refractive error.
- Anterior chamber depth (ACD): Third most important; 1 mm error results in 1.5 diopter refractive error.
- Lens thickness: Useful in advanced formulas for IOL power prediction.
- Corneal diameter (white-to-white): Related to ACD and newer calculation formulas.
Comparison of Biometry Methods
- A-scan ultrasound
- Portable, less expensive, requires skill; measures to internal limiting membrane (ILM).
- Contact method can compress cornea and cause errors.
- Only measures up to ILM, not the photoreceptor or RPE layer.
- Optical interferometry
- Accurate to 0.01 mm, no corneal contact, measures to the foveal RPE.
- Not effective for dense cataracts or uncooperative patients.
- Expensive and less portable.
Devices
- IOL Master: First optical biometer; improved with time, but still struggles with dense cataracts.
- Lenstar: Measures more zones and integrates advanced IOL formulas for multifocal/toric lenses.
Key Terms & Definitions
- Biometry — Application of mathematics to biological measurements.
- Axial length — Distance from corneal apex to retinal pigment epithelium.
- Keratometry — Measurement of corneal curvature.
- Anterior chamber depth (ACD) — Distance from corneal endothelium to the lens.
- Optical interferometry — Measurement technique using split laser beams for precise length measurements.
- A-scan ultrasound — Technique measuring distances in the eye using sound waves.
Action Items / Next Steps
- Review examples of A-scan and optical biometry devices.
- Practice interpreting scan results and comparing with expected values.
- Familiarize yourself with advanced IOL formulas and key measurement techniques.