Overview
This lecture covers ultrasound biometry in ophthalmology, focusing on its principles, measurement techniques, sources of error, and practical considerations for accurate intraocular lens (IOL) power calculation.
Basics of Biometry in Ophthalmology
- Biometry applies mathematics to biology, primarily for intraocular lens (IOL) power calculation.
- Accurate IOL calculation requires measurement of axial length, corneal power, anterior chamber depth, lens thickness, and effective lens position (ELP).
- Errors in these measurements lead to significant post-operative refractive errors.
Sources and Impact of Measurement Errors
- ELP prediction is the largest source of IOL power calculation error.
- A 1 mm error in axial length gives a 2.5 D refractive error in average eyes (23.5 mm), higher in shorter and lower in longer eyes.
- A 1 D error in corneal power results in 1 D postoperative refractive error.
Types of Biometers and Principles
- Two main types: ultrasound biometers (sound waves) and optical biometers (light-based).
- Ultrasound biometers: contact (probe touches cornea) and immersion (probe in saline shell, avoids corneal compression).
- Optical biometers often integrate corneal power measurement, unlike ultrasound devices.
Ultrasound Biometry: Physics and Techniques
- Operates on the piezoelectric effect: crystals convert electrical energy to ultrasonic waves and vice versa.
- Sound travels at varying speeds through different eye tissues; average velocity in phakic eyes is ~1550 m/s.
- Amplitude (A) scan displays echo spikes from tissue interfaces, used to measure distances.
Practical Technique: Contact and Immersion Methods
- Contact (applanation) method: probe gently touches the cornea; compression must be avoided.
- Immersion method uses a saline-filled shell (Prager/Hansen), eliminating corneal compression and improving accuracy.
- Five high-amplitude spikes (contact) or six (immersion) represent key ocular interfaces.
Ensuring Accurate Measurements
- Probe alignment with the visual axis is vital for high-quality spikes and accurate readings.
- Normal macular anatomy is necessary for reliable echo spikes.
- Sound absorption (e.g., dense cataracts) and incorrect gain settings can affect spike quality and measurement accuracy.
- Electronic calipers (gates) on the biometer mark measurement points for automated calculations.
Special Considerations and Common Errors
- Biometer has modes for phakic, aphakic, and pseudophakic eyes; specific correction factors may be needed.
- Silicone oil presence alters velocity and can cause false long axial length readings.
- Major errors: corneal compression (contact method), probe misalignment, variable readings in high myopes or posterior staphyloma, absent scleral spikes when scanning through the optic nerve.
Advantages of Immersion Over Contact Biometry
- Avoids corneal compression, improving accuracy.
- Separate probe and corneal spikes support better interface analysis.
- Less operator-dependent and generally yields slightly longer—but more accurate—axial length measurements.
Key Terms & Definitions
- Biometry — Application of mathematics/statistics to biological measurements.
- Axial Length — Distance from corneal vertex to retina, critical for IOL calculation.
- Effective Lens Position (ELP) — Predicted postoperative IOL position.
- A-scan (Amplitude scan) — One-dimensional ultrasound measurement of ocular distances.
- Piezoelectric Effect — Property of certain crystals to convert electrical energy to mechanical vibrations and vice versa.
- Gain — Amplification of returning ultrasound signals to improve spike visibility.
Action Items / Next Steps
- Review the basic principles and differences between contact and immersion ultrasound biometry.
- Remember key error impacts (axial length and corneal power) for exam settings.
- Practice identifying high-quality A-scan spikes and recognize common artifacts/errors.
- Refer to additional resources on corneal topography and B-scan techniques for further study.