Biometry 101: Capturing the accurate power

Reviewed by Dr Julie Schallhorn.

Knowledge of the optics of the eye is essential for the understanding of biometry, according to Dr Julie Schallhorn, an associate professor of ophthalmology at the University of California San Francisco, California, United States.

Dr Schallhorn shed some light on the topic for new ophthalmologists. “Most of the refraction of light that is entering the eye takes place at the air–tear film interface, and there also is slight divergence at the back corneal surface–aqueous interface, after which refraction occurs through the lens. At [this] point the light then converges onto the fovea,” she explained.

Given this movement of light, clearly what happens at the various surfaces in the eye is important for determining the power of the lens that will be implanted in the eye. As Dr Schallhorn pointed out, the corneal curvature, anterior chamber depth, effective lens position and axial length must all be considered. “Accurate measurements of the eye are critical to predicting the lens power,” she emphasised.

Understanding lens power

The first-generation formula used to determine lens power was the Sanders-Retzlaff-Kraff, generally referred to as the SRK. In this formula, the IOL power is equal to an A constant that compensates for all unmeasured factors minus 2.5 times the axial length minus 0.9 times the keratometry.

“Axial length measurements are incredibly important to the obtaining of the correct IOL power,” Dr Schallhorn said. She explained that if there is an error in the measurement of the axial length, there will be a big error in the power of the IOL, resulting in an amplification of the error by 2.5 times. Likewise, an error in the keratometry results in an error in the IOL power, as does an error in the A constant.

With the modern light-based biometers, utilising Michelson interferometers, the ophthalmologist should look for an interference pattern between the light entering the eye and a reference arm. The result is a spectrum of interference that is insensitive to patient movement; therefore, despite the normal biological noise that is present, the result is a very accurate measurement of what is actually going on inside the eye.

Because of this capability, a light-based biometry system is far superior to ultrasound biometry for predictability and reliability. Dr Schallhorn added that biometry can also evaluate the corneal curvature by assessing a number of points on the ocular surface as well as the difference in the keratometry reading between the two principal axes.

Relevance in the real world

Dr Schallhorn outlined the steps that must be taken to ensure successful measurements. She emphasised that, all things considered, it is important to be fastidious about biometry measurement.

She referred to what she considers to be a valuable tool: Warren Hill’s IOLMaster checklist. The checklist lays out all the factors involved in obtaining accurate measurements.

In addition, the ocular surface must be considered because of the potential effect on the biometry measurements resulting from, for example, epithelial basement dystrophy and Salzmann nodular degeneration and the effects before and after treatment. Topography is important to paint a clear picture of the corneal shape, which ultimately affects the vision, such as in patients with keratoconus.

Lastly, Dr Schallhorn advised double-checking the biometry printout for the correctness of every factor for every patient. “It is important to know how your biometer works, check the cornea carefully, obtain a topography map, be fastidious in your measurements and double-check your work,” she concluded.

Julie Schallhorn, MD, MS

E: Julie.schallhorn@ucsf.edu

This article is adapted from Dr Schallhorn’s presentation at the Real World Ophthalmology Autumn 2021 meeting. Dr Schallhorn is a consultant to Carl Zeiss Meditec.