Biometry is the process of measuring the power of the cornea (keratometry) and the length of the eye, and using this data to determine the ideal intraocular lens power by A scan.

Introduction to Biometry

Cataract Surgery: Visual Outcomes

Prerequisites for IOL Calculation

Sources of Errors

Types of Biometry

Ultrasound Biometry Principles

Procedure & Probe Orientation

IOL Power Formulas

Special Cases (Silicone Oil)

MCQs (1–20)

MCQs on Biometry

Biometry is a method of applying mathematics to biology. In ophthalmology, it deals with IOL power calculation.

It is the process of measuring the power of the cornea (keratometry) and the length of the eye (axial length) to determine the ideal intraocular lens power. A-scan ultrasound biometry (Amplitude scan) is a routine diagnostic test used to provide data on the length of the eye.

What is Biometry?

Modern cataract surgery aims not just at removing the cataractous lens, but at giving the patient excellent refractive outcomes through IOL implantation.

Refractive outcomes depend on calculating:
1. Corneal power
2. Lens power
3. Media
4. Axial length

Essential measurements include:
1. Axial Length
2. Central Corneal Power
3. Anterior Chamber Depth (ACD)

Other measurements:
- Lens thickness
- Effective lens positioning (ELP)
- White to white (WTW)
- Corneal thickness (pachymetry)

Percentage responsibility for inaccuracy:
- Prediction of ELP: 35.5%
- Postoperative refraction: 27%
- Axial length measurement: 17%
- Keratometry measurement: 10%

Impact of errors:
- 1mm error in AL leads to +/- 2.5D of refractive error.
- 0.1mm error in AL leads to ~0.25D post-op error.

Sources of Errors in IOL Power Calculation

Longer eyes are more "forgiving" of errors, while small eyes are least forgiving.

- 1mm error in 30mm eye ≈ 1.75D post-op error
- 1mm error in 20mm eye ≈ 3.75D post-op error

Normal Range: 22.0mm – 24.5mm (Average: 23.5mm)

Normal Axial Length

Biometry methods are divided into:
1. Ultrasound Biometry (Contact vs. Immersion/Non-contact)
2. Optical Biometry (PCI i.e. IOLMaster 500, or OLCR i.e. Lenstar 900, OA 2000)

Works on the PIEZOELECTRIC EFFECT (from Greek "piezein" meaning squeeze). Mechanical deformation of a piezoelectric material generates electricity.

Note: Ultrasound biometry does NOT measure corneal power.

The probe sends ultrasonic waves that travel at specific speeds in various ocular media. Returning echoes are transformed into "spikes" or echoes.

Frequency used is about 10 MHz (Human audible range is only up to 20,000 Hz). Distance is calculated using: Speed × Time = Distance.

Working Mechanism

The probe vibrates before pausing to allow the tips to receive all returning echoes. This timing allows for precise axial length, ACD, and lens thickness measurements.

Working Mechanism (cont.)

Axial length measurements are based on sound velocities through different ocular structures.

Other Principles

Velocities used for calculation:
- Cornea/Lens: 1641 m/s
- Aqueous/Vitreous: 1532 m/s

Average sound velocities:
- Phakic eye: ~1550 m/s
- Aphakic eye: ~1532 m/s
- Pseudophakic eye: 1532 m/s + correction factors

Velocities in Different Parts of Eye

Proper technique is vital for accuracy. Immersion method is preferred over contact to avoid corneal compression.

Procedure for Ultrasound Biometry

Alignment is critical. The probe must be perpendicular and centered on the visual axis to obtain reliable and reproducible spikes.

Probe Orientation to Visual Axis

The quality of the echoes depends on the shape and smoothness of the ocular interfaces (cornea, lens, retina).

Shape and Smoothness of Interface

A high-quality retinal spike is essential for determining the total axial length precisely.

Good Retinal Spike

Different generations of formulas:
1st Gen: SRK I, Binkhorst I (K, AL)
2nd Gen: SRK II, Binkhorst II (K, AL)
3rd Gen: Holladay I, SRK/T, Hoffer Q (K, AL)
4th Gen+: Holladay 2, Olsen, Haigis (Include ACD, LT, WTW, etc.)

Formulas to Calculate IOL Power

Silicone oil has a much lower refractive index/sound velocity than vitreous. Using standard velocity can lead to errors up to 8mm.

Expect 2-5D induced myopia after oil removal.

Biometry in Silicone Oil Filled Eyes

Sources of errors:
- Globe irregularities: Posterior staphyloma
- Intraocular material: Silicone oil
- Poor ocular fixation: Nystagmus
- Human error: Lack of calibration, incorrect settings

Introduction to Biometry

Course Outline