Biometry

Biometry is 3% of the COA exam, small on paper and large in consequence. Its measurements feed the formula used to choose the power of the intraocular lens (IOL) implanted at cataract surgery, and an implant of the wrong power can leave the patient needing glasses they did not plan on, or another procedure to correct it. The formula cannot tell a good measurement from a bad one, so errors have to be caught at the instrument.

Why it matters — axial length plus K readings drive the IOL power calculation; a 1.0 mm axial-length error shifts the postoperative refraction roughly 2.5–3 D.

Two ways to measure axial length — optical biometry uses light and never touches the eye; ultrasound A-scan uses sound and needs contact or an immersion shell. Optical is today's first choice whenever the eye's media let light through.

The assistant's role — capture clean, repeatable measurements on both eyes, cross-check them, and flag anything unusual. Choosing the lens is the surgeon's job.

Newer formulas also use extra measurements, such as anterior chamber depth and lens thickness, which optical biometers capture in the same session.

How errors carry through — about 2.5–3 D per millimeter of axial length means even 0.1 mm is worth roughly a quarter diopter, which is why axial length is recorded to two decimal places. An error of 1 D in the K readings produces roughly 1 D of refractive error. Short eyes punish error the most.

How it works — a light-based instrument measures the eye while the patient looks at its fixation target, so the measurement runs along the visual axis to the fovea. It is fast, non-contact, needs no anesthetic, and is highly repeatable.

Setup — chin and forehead in place, steady fixation, and a blink just before capture, since the K readings depend on a smooth tear film. Contact lens wearers leave their lenses out for as long as the physician directs before measuring, because lenses temporarily reshape the cornea.

Sequence — optical biometry and keratometry come before drops, dye, applanation tonometry, contact ultrasound, or anything else that touches or disturbs the cornea.

When it fails — light must reach the retina and come back. A mature or very dense cataract, a corneal scar, blood in the vitreous, or poor fixation can each block the measurement. The answer is ultrasound, not a guess and not the fellow eye's number.

The principle — the probe sends sound along the eye's axis and times the returning echoes; the machine turns time into distance using an assumed speed of sound. Each spike on the display is an interface: cornea, front and back of the lens, retina, then sclera and orbital fat.

Velocity settings — sound travels faster through the crystalline lens than through aqueous and vitreous, so the machine must be told the eye's lens status: phakic, aphakic, or pseudophakic (with the implant material). An eye filled with silicone oil needs its own setting. The wrong setting produces a clean-looking, wrong number.

Contact technique — the probe rests on the anesthetized cornea. Even slight pressure indents it, which shortens the measured eye. The formula then calls for too much lens power, and the patient ends up nearsighted after surgery — the classic myopic surprise.

Immersion technique — the patient reclines, and a scleral shell filled with saline keeps the probe from touching the cornea, so nothing compresses it. It is the more accurate ultrasound method and the preferred one whenever ultrasound is needed.

Reading the tracing — a good scan shows tall, sharp, steeply rising spikes, especially the retinal spike, meaning the beam is perpendicular and on axis. A short retinal spike that rises gradually signals a beam that is off axis or striking the optic nerve; discard that reading. Repeat measurements should cluster tightly; scattered readings are re-measured, not averaged.

Fixation still matters — the formula wants the cornea-to-fovea distance, so coach the patient's fixation, especially in long myopic eyes, where an off-axis reading can land in a bulge of the back wall (a posterior staphyloma).

Infection control — probes and shells are cleaned and disinfected between patients per protocol.

Measure both eyes, even when only one is having surgery, and check the results against each other and against the patient:

Record the method — note whether each axial length was optical or ultrasound, and any difficulty, so the surgeon knows what the numbers rest on.

Be ready to state the 2.5–3 D per millimeter rule, compare optical and ultrasound biometry, explain why contact compression causes a myopic surprise, describe the immersion setup, choose the velocity setting by lens status, recognize a good A-scan tracing, give the normal axial length range, and list an IOL formula's inputs.