Biometry is a foundational investigation in cataract surgery planning. A small error here can lead to significant postoperative refractive surprises—frustrating both for the surgeon and the patient.
This post simplifies biometry for ophthalmology trainees and FRCOphth aspirants by first explaining the principles, then working through a real-world scenario of a -5.00 D myope, and ending with practical questions and learning points commonly asked in clinical exams and viva.
📖 What Is Biometry?
Biometry in cataract surgery refers to measuring the anatomical parameters of the eye to calculate the most accurate IOL power. The key measurements include:
- Axial Length (AL): Corneal epithelium to the RPE
- Keratometry (K): Corneal curvature
- Anterior Chamber Depth (ACD)
- Lens Thickness (LT) – in advanced formulas
- White-to-White (WTW) – used in some formulas
These values are entered into IOL power calculation formulas to estimate the ideal intraocular lens strength needed for emmetropia.
🛰️ How Does Non-Contact (Optical) Biometry Work?
Optical biometry is a non-contact technique that uses light instead of sound waves to measure ocular dimensions.
🔹 Most Common Devices:
- IOL Master 500 – Partial Coherence Interferometry (PCI)
- IOL Master 700 – Swept-Source Optical Coherence Tomography (SS-OCT)
- Lenstar LS 900 – Optical low-coherence reflectometry
🔍 Working Principle:
- A beam of infrared light is projected into the eye.
- It reflects off different ocular surfaces (cornea, lens, retina).
- The time delay and interference pattern of reflected light allows calculation of distances.
- This provides high-precision, high-reproducibility AL measurements without needing corneal contact.
✅ Advantages:
- Non-contact = no risk of infection or corneal compression
- Accurate & reproducible
- Measures multiple parameters simultaneously
- Integrates advanced IOL formulas (e.g. Barrett, Olsen)
❌ Limitations:
- Requires good fixation
- Can be blocked by dense cataracts or posterior subcapsular opacities
- Low Signal-to-Noise Ratio (SNR) means unreliable data
📁 Practice Case: -5.00 D Myope for Cataract Surgery
Let’s apply this knowledge through a clinical scenario often encountered during biometry interpretation:
👤 Patient Summary:
- Age: 60 years
- Refraction: -5.00 D in both eyes
- Cataract: NS + PSC
- Planned procedure: Phacoemulsification with IOL
📑 IOL Master Printout:
- Axial Length (AL): 24.8 mm
- SNR: Low
- Keratometry: Normal
- ACD: 3.2 mm
- IOL power calculated: +18.5 D (Barrett Universal II)
📋 A-Scan (Contact) Biometry:
- AL: 21.8 mm
- K and ACD not re-measured
- IOL power: +24.0 D (SRK-II formula used)
🧠 Discussion: Questions, Answers & Learning Points
❓ 1. When Should Biometry Be Repeated?
Repeat biometry if:
- SNR is low in IOL Master (optical reading is unreliable)
- AL mismatch between optical and ultrasound biometry > 0.2 mm
- Refraction doesn’t match AL (e.g. high myope with short AL)
- Dense cataracts prevent consistent optical measurement
✅ In this case: The myopic refraction suggests a long eye, but AL of 21.8 mm from A-scan indicates an error. Biometry must be repeated, ideally with immersion A-scan or reattempt optical biometry.
❓ 2. How Does IOL Master Measure AL?
The IOL Master uses:
- Partial coherence interferometry (PCI) in IOL Master 500
- Swept-source OCT in IOL Master 700
It measures AL from the tear film to the RPE using light interference patterns. High accuracy (~±0.01 mm), but sensitive to fixation and media clarity.
❓ 3. What Are the Advantages/Disadvantages vs A-Scan?
| Feature | Optical Biometry (IOL Master) | A-Scan (Ultrasound) |
|---|---|---|
| Contact | No | Yes (or immersion) |
| Accuracy | ±0.01 mm | ±0.10 mm |
| Media Clarity Required | Yes | No |
| Operator Dependency | Low | High |
| Preferred in | Clear cornea/cataract | Dense cataract |
Conclusion: Optical biometry is the gold standard when possible. Use A-scan in dense cataracts or when optical fails.
❓ 4. What Formula Was Used with AL 21.8?
If measured on an A-scan console, likely formula:
🔸 SRK-II or SRK/T
Problem: These formulas are less accurate for short eyes or if AL was under-measured.
❓ 5. What Formula Should Be Used?
| Axial Length | Formula |
|---|---|
| <22.0 mm | Hoffer Q, Holladay 2 |
| 22–24.5 mm | SRK/T, Holladay 1 |
| >24.5 mm | SRK/T, Barrett, Haigis |
Best choice now:
🟢 Barrett Universal II – accurate across all ranges.
🟢 For short AL (true or mismeasured): Hoffer Q > SRK/T.
According to the NICE guideline(2017) NG77: Also refer to Cataract Guidelines
- Hoffer Q: AL <22 mm
- SRK-T or Barrett Universal II: AL
22–26 mm - SRK-T or Haigis: AL >26 mm
- Special formulas for eyes that have had
previous laser refractive surgery, e.g.
Haigis-L
❓ 6. What Are the Commonly Used IOL Formulas?
| Formula | Best For | Notes |
|---|---|---|
| SRK-II | Obsolete | Simple regression |
| SRK/T | Long eyes | Widely used |
| Hoffer Q | Short eyes | Good for <22 mm AL |
| Holladay 1 | Average eyes | Basic theoretical |
| Holladay 2 | All eyes | Multiple variables |
| Haigis | Post-refractive & long eyes | Uses ACD |
| Barrett Universal II | All eyes | Most accurate overall |
| Olsen | All eyes | Requires C-constant |
Use ASCRS or APACRS IOL calculators to auto-suggest formulas based on AL.
✅ Take-Home Points
🔹 Biometry is not just a printout—always validate values logically
🔹 Cross-check AL with patient’s refraction
🔹 Low SNR = red flag. Always repeat or confirm
🔹 Know your formulas and when to apply each
🔹 Optical biometry is preferred, but immersion A-scan is still essential for dense cataracts