9.1 Biometry Overview

9.1 Biometry Overview

Biometry is the set of measurements an ophthalmic technician takes to calculate the intraocular lens (IOL) power implanted during cataract surgery. The three measurements that drive nearly every IOL formula are axial length (AL), the front-to-back length of the eye in millimeters; keratometry (K), the corneal curvature in diopters; and anterior chamber depth (ACD), the distance from the cornea to the lens. Get any of these wrong and the patient ends up needing glasses they were promised they would not need, the most common cause of a refractive surprise after cataract surgery.

Why precision matters

The single largest source of postoperative refractive error is axial length. As a working rule, a 1 mm error in axial length shifts the final spectacle refraction by roughly 2.5 to 3.0 diopters. Because the average eye is about 23.5 mm long, even a 0.3 mm error, the kind a sloppy applanation A-scan produces, can leave a patient a diopter off target. Keratometry is the second largest contributor; a 1 D error in K produces about 1 D of refractive error. ACD matters most in short and long eyes.

Two ways to measure axial length

Optical biometry uses partial coherence interferometry (PCI), an infrared 780 nm light beam, in devices such as the IOLMaster. It is non-contact, does not touch or compress the cornea, and is far more reproducible, the longitudinal resolution of light is roughly eight times finer than a 10 MHz ultrasound wave. Optical biometry has become the clinical standard for routine cataract cases.

Ultrasound A-scan biometry uses a 10 MHz sound wave. It is still required when optical biometry cannot acquire a reading, for example a dense (mature) cataract, a vitreous hemorrhage, or a corneal scar that blocks the light beam. A-scan comes in two flavors: applanation (contact), where the probe touches the cornea, and immersion, where a fluid-filled scleral shell separates the probe from the cornea.

How biometry appears on the COA exam

The COA blueprint folds these topics into a small biometry slice plus diagnostic ultrasound. Expect applied items: a stem describes a measurement workflow or an error and asks what caused it or what to do next. Typical traps include forgetting that applanation falsely shortens the eye, confusing axial length with corneal thickness (pachymetry), or selecting the wrong A-scan mode for a phakic versus pseudophakic versus silicone-oil-filled eye. Read every stem for the eye state, the device, and the immediate task before choosing.

Use the official IJCAHPO content outline and candidate handbook to confirm current domain weights and the Registered Ophthalmic Ultrasound Biographer (ROUB) scope before relying on secondary summaries: IJCAHPO COA Certification Page.

What the IOL formula actually uses

Every modern IOL power formula combines the same core inputs in different mathematical ways. The earliest regression formula was the SRK formula, written as P = A − 2.5L − 0.9K, where P is implant power, A is the lens-specific A-constant, L is axial length, and K is average keratometry. You will not be asked to compute it by hand, but the form makes the lesson concrete: axial length (L) carries a coefficient of 2.5, so every millimeter swings power by 2.5 D, and keratometry (K) carries 0.9, so each diopter of K moves power by about 0.9 D.

Theoretical and ray-tracing formulas, Hoffer Q, Holladay 1 and 2, the SRK/T, and the Barrett Universal II, refine the estimate of effective lens position (ELP), where the IOL will sit in the eye, by adding ACD, lens thickness, and white-to-white corneal diameter.

Matching the formula to the eye

A practical clinic rule is that formula accuracy depends on axial length:

• Short eyes (AL under ~22 mm): Hoffer Q or Barrett tend to perform best
• Average eyes (~22-26 mm): most modern formulas perform well
• Long eyes (AL over ~26 mm): SRK/T, Barrett, and axial-length-adjusted formulas reduce the hyperopic surprise that older formulas produce in myopic eyes

The technician's job is not to pick the formula, that is the surgeon's call, but to deliver inputs clean enough that the chosen formula can succeed. A flawless formula cannot rescue a compressed-cornea axial length or a keratometry taken on a dried-out tear film.

Keratometry in detail

Keratometry measures the curvature of the central cornea in two principal meridians, reporting each as a power in diopters or a radius of curvature in millimeters. A normal cornea is roughly 43 to 44 diopters, equivalent to about a 7.5 to 7.8 mm radius; remember that a flatter cornea has a longer radius and lower power, while a steeper cornea has a shorter radius and higher power. The difference between the two meridians is the corneal astigmatism, and the orientation of the steeper meridian is its axis.

Optical biometers measure keratometry automatically by projecting light rings onto the cornea and measuring their reflected size, the same principle as a manual keratometer or a corneal topographer. The technician confirms the rings are crisp and circular; a broken or distorted mire reflection signals dry eye, irregular cornea, or poor fixation and means the K value is unreliable.

Anterior chamber depth and effective lens position

Anterior chamber depth is measured from the corneal epithelium (or endothelium, depending on the device convention) to the front of the natural lens, normally 3.0 to 3.5 mm. It matters because modern formulas use it to predict the effective lens position, where the implanted IOL will finally rest. A shallower chamber tends to push the implant forward, increasing its effective power, so ignoring ACD in short or unusually shallow eyes is a known source of error. White-to-white corneal diameter (the horizontal visible iris diameter) and lens thickness feed the same ELP estimate in advanced formulas such as Holladay 2 and Barrett.