6.4 A-scan biometry & IOL calculation basics

Why biometry matters

Biometry is the measurement of the eye's anatomy needed to choose the correct intraocular lens (IOL) power before cataract surgery. The goal is to implant a lens that delivers the surgeon's target refraction, usually emmetropia or a small planned myopia. Three primary measurements feed every modern IOL formula: axial length (AL), the front-to-back length of the eye; keratometry (K), the corneal power; and anterior chamber depth (ACD). Accurate values are what separate a happy postoperative patient from a costly refractive surprise, so the technician's care at this step directly shapes the surgical result.

A-scan ultrasound biometry

A-scan (amplitude-scan) ultrasound sends a 10 MHz sound pulse along the visual axis and measures the time echoes take to return from each ocular interface: cornea, anterior and posterior lens surfaces, and retina. The machine converts travel time into distance using assumed sound velocities for the cornea, aqueous, lens, and vitreous, so a clean series of tall, steeply rising spikes is essential for a trustworthy axial length.

Contact versus immersion

There are two ultrasound techniques:

• Contact (applanation): the probe touches the cornea directly. It is fast and simple, but the probe can indent the cornea, compressing the globe and falsely shortening the axial length. Even a fraction of a millimeter of indentation shifts the calculated IOL power. Approach gently along the visual axis, barely touch, take several readings, and discard the short outliers.
• Immersion: a saline-filled scleral shell couples the probe to the eye without touching the cornea. Because there is no corneal compression, immersion is more accurate and reproducible and is preferred when precision matters, especially in very short or very long eyes.

Whatever the method, accept the reading only when values are consistent (typically within about 0.1 mm of each other), the retinal spike is tall and steep, and the standard deviation is low.

Reading the waveform

A valid axial-length trace shows a clean sequence of spikes: a corneal spike, then the anterior and posterior lens spikes, then a tall retinal spike, followed by lower scleral and orbital echoes. The retinal spike must rise steeply from the baseline; a stepped or gradually sloping spike often means the beam is off-axis or reflecting from the optic nerve rather than the fovea, which lengthens the reading. Ask the patient to fixate on the probe's internal light and average several concordant scans. The machine assumes fixed sound velocities for each ocular segment, so a silicone-oil-filled or pseudophakic eye needs the correct velocity mode selected, otherwise the axial length will be systematically wrong.

Optical biometry

Optical biometry (IOLMaster, Lenstar) measures axial length with light rather than sound, using partial coherence interferometry (PCI) or optical low-coherence reflectometry. Its advantages are decisive:

• Non-contact: nothing touches the cornea, so there is no indentation error and negligible infection risk.
• Higher resolution and reproducibility than ultrasound, measuring to the fovea and matching the true visual axis.
• Captures AL, K, ACD, white-to-white, and lens thickness in a single acquisition.

Its main limitation is dense media: a mature or brunescent cataract, dense vitreous hemorrhage, or a corneal scar can block the light. When optical biometry fails, A-scan ultrasound is the fallback. For most eyes, optical biometry is now the standard of care.

A-scan versus optical biometry

From measurements to IOL power

Once AL, K, and ACD are known, an IOL formula predicts the implant power. Modern choices include SRK/T, Barrett Universal II, Holladay 1 and 2, Hoffer Q, and Haigis. Barrett Universal II is widely favored for accuracy across a broad range of eye lengths. The older regression SRK formula is worth knowing conceptually:

P = A - 2.5L - 0.9K

Here P is the IOL power, A is the lens constant unique to each IOL model, L is axial length in millimeters, and K is average keratometry in diopters. The A-constant encodes the effective lens position for that specific implant, which is why entering the correct lens model is as important as the measurements.

The other inputs

Keratometry supplies the corneal power in two meridians; a difference between them is corneal astigmatism, which guides toric IOL planning. Take K readings before any drops that dry the cornea, and repeat if the mires look distorted. Anterior chamber depth and, in newer formulas, lens thickness and white-to-white help predict the effective lens position (ELP), the depth at which the implant will finally sit. Because the surgeon cannot know ELP until after surgery, formulas estimate it from these measurements, and this estimation is the second-largest error source after axial length.

Why axial length dominates

Axial-length error is the single largest source of IOL power error. A 1 mm error in axial length produces roughly 2.5 to 3 D of postoperative refractive error, far more than a comparable keratometry error. This is precisely why contact-probe indentation is dangerous and why immersion or optical biometry is preferred. Short (hyperopic) eyes amplify every error, so they demand the most meticulous measurement of all.