Optical and ultrasound biometry

Principles of Optical vs Ultrasound Biometry

Accurate determination of ocular dimensions—particularly axial length (AL), anterior chamber depth (ACD), and corneal curvature (K)—is the paramount determinant of refractive success following cataract surgery. A biometrical error of just $0.1\text{ mm}$ in axial length produces approximately $0.25\text{ to }0.30\text{ D}$ of refractive error at the spectacle plane in an average eye.

Optical Biometry Technologies

Optical biometry utilizes non-invasive, high-resolution interferometric principles to measure distance along the visual axis:

1. Partial Coherence Interferometry (PCI, e.g., Zeiss IOLMaster 500): Employs a dual-beam infrared semiconductor laser diode ($\lambda = 780\text{ nm}$) with a coherence length of approximately $160\,\mu\text{m}$. It measures the optical path length from the anterior corneal vertex to the retinal pigment epithelium (RPE). An internal regression algorithm calibrated against immersion ultrasound converts optical path length into geometric axial length.
2. Optical Low-Coherence Reflectometry (OLCR, e.g., Haag-Streit Lenstar LS900): Uses an $820\text{ nm}$ superluminescent diode to generate a complete axial profile in a single measurement sweep, capturing central corneal thickness (CCT), ACD, lens thickness (LT), and axial length.
3. Swept-source optical coherence tomography biometry: Long-wavelength swept-source systems improve acquisition through some dense cataracts, but failure can still occur. Features such as fixation checks, macular scans and measured total corneal power depend on the instrument and software. Do not attribute every feature of one branded device to all swept-source biometers; repeat inconsistent results and use an alternative method when necessary.

Ultrasound Biometry: Acoustic Physics & Media Velocities

Ultrasound biometry relies on piezoelectric transducers emitting high-frequency acoustic pulses ($10-15\text{ MHz}$). Sound propagates through ocular media at velocities determined by physical tissue density and bulk compressibility according to the acoustic impedance formula:

where $Z$ is acoustic impedance, $\rho$ is tissue density, and $v$ is sound velocity. When acoustic waves strike an interface between two tissues of differing acoustic impedance (e.g., aqueous to anterior lens capsule, or vitreous to neurosensory retina), an echo is reflected back to the probe.

Optical versus Acoustic Measurement Landmarks

• Optical Biometry: Measures from the anterior corneal vertex to the retinal pigment epithelium (RPE) because infrared light traverses the transparent sensory retina and reflects off the pigmented melanin layer.
• Ultrasound Biometry: Measures from the anterior corneal vertex to the internal limiting membrane (ILM) of the retina, because the acoustic impedance change occurs between the liquid vitreous and the neurosensory retinal surface.
• Device calibration: Optical and ultrasound devices detect different retinal interfaces, but reported lengths incorporate device-specific conversion/calibration. Do not add a universal retinal-thickness offset manually. Use the appropriate device settings and optimised lens constants.

Contact Applanation versus Immersion A-Scan

• Contact Applanation A-Scan: The ultrasound probe is placed directly against the anesthetized cornea. Manual pressure mechanically indents the cornea, causing variable compression of the anterior chamber depth by $0.14\text{ to }0.33\text{ mm}$. This artifactual shortening of the measured axial length causes the biometry formula to calculate an inappropriately higher IOL power, resulting in a disastrous postoperative myopic surprise.
• Immersion A-Scan (Prager Shell): A small cylindrical plastic scleral shell filled with balanced salt solution is positioned over the eye. The probe is submerged in the fluid without touching the cornea. Corneal compression is completely eliminated, producing crisp, separate echoes for the corneal front and back surfaces, accurate ACD, and reproducible axial length.

Biometrical Pitfalls: Staphyloma & Silicone Oil

1. Posterior staphyloma: Ultrasound may measure an off-axis deepest point rather than the fovea. Check fixation and scan alignment, repeat inconsistent readings and use appropriate optical or image-guided ultrasound methods. No instrument guarantees the correct endpoint in every eye.
2. The Silicone Oil-Filled Eye:
   • Sound travels dramatically slower through silicone oil ($980-1049\text{ m/s}$) than through normal vitreous ($1532\text{ m/s}$). If an ultrasound biometer operates on the standard $1550\text{ m/s}$ phakic calibration, the prolonged transit time through silicone oil is falsely interpreted as a massive, artifactual elongation of the eye (frequently adding 3 to 6 mm to the measured AL).
   • If this artifactual measurement is used, an underpowered IOL is selected, producing a severe hyperopic surprise ($+5.0\text{ to }+10.0\text{ D}$). Clinicians must switch ultrasound machines to dedicated "silicone oil mode" or apply mathematical velocity correction: $\text{AL}_{\text{true}} = \text{ACD} + \text{LT} + \left( \text{VCD}_{\text{measured}} \times \frac{v_{\text{oil}}}{1532} \right)$
   • Optical effect: Silicone oil changes the refractive-index difference at the posterior IOL surface. The shift depends on IOL shape, power, position and oil contact; use an oil-specific calculation rather than a fixed 2–3.5 D addition or a compulsory lens design.