TEN Essential Aspects to Check in Optical Biometry to Avoid Refractive Surprise Post IOL Implantation
Optical biometry has transformed the way ophthalmologists calculate intraocular lens (IOL) power before cataract surgery. Despite advances, refractive surprises—unexpected vision outcomes after IOL implantation—still occur. These refractive surprise can lead to patient dissatisfaction and sometimes require additional procedures. To minimize this risk, it is crucial to carefully analyze the optical biometry chart and ensure all key parameters are accurate and reliable before you jump to your favorite IOL calculation formula.
This post highlights ten essential aspects to check in the optical biometry chart by way of which you can largely avoid refractive surprise. Understanding these factors helps surgeons make informed decisions, improving refractive outcomes and patient satisfaction.
1. Validating the measurements to avoid refractive surprise - A measurement is only as good as our ability to validate the data generated
Before you accept the patient's optical biometry measurements, validate that the measurements can be relied upon.
Always do a two eye biometry. This would help you to corelate the two eye measurements and rule out abnormality, that may need remeasurement.
For Tomey OA-2000 or IOL Master keep an eye on the SNR values. For axial length measurements on the ZEISS IOL Master 500, the Signal-to-Noise Ratio (SNR) indicates the quality and reliability of the interferometric peak used for axial length calculation.
Typical interpretation of IOL Master 500 SNR values:
SNR | Interpretation | Clinical meaning |
<2.0 | Poor | Unreliable; repeat scan or do an immersion biometry |
2-5 | Borderline | Use cautiously; repeat if possible, corelate with immersion biometry |
>5 | Good quality | Reliable |
>10 | Very good quality | High confidence |
OA- 2000 generate tomographic images of retina that can be useful in data validation.
A good reading with Tomey OS-2000 should have the following features:
Distinct peaks depicting B ellipsoid zone, C Retina and D sclera. Additionally, ILM membrane (A) may be visible
Additionally, the SNR value should be higher than 3.
2. Corneal Curvature (Keratometry) Values
Keratometry measures the corneal curvature, which affects the eye’s refractive power. Ensure the following:
The difference of astigmatism is less than 1 diopter between the two eyes.
The corneal spherical equivalent (SE) difference between both eyes is less than 1 diopter.
Too steep a cornea (>47D) or too flat a cornea (<41D) may need re measurement or validated with topography to rule out irregularity. In the image 4b, a way out of the whack value of 49.12 diopters in the right eye should raise suspicion and be followed up with corneal topography.
In image 4d, we see very steep corneal values as reflected in Lenstar reading (K2 49.41 @152 deg and K1 47.32@67 deg. The Sirrus topography image ( image 4c) confirms high superior inferior asymmetry, confirming an irregularity of cornea. The Keratoconus screening index confirms 'suspect keratoconus' with high values.
(for more understanding on the Sirrus map indices follow https://www.quickguide.org/post/sirius-topography-indices)
Too low an astigmatism (<1 diopter) with high standard deviation of K readings may indicate dry eyes.
In the Tomey OA-2000, check for KRI (corneal regularity/irregularity index) and KAI (corneal asymmetry index values).
KAI | 0 - 23.4 |
KRI | 0- 4.4 |
KAI and KRI values should be between the above values to indicate a healthy cornea quickguide.org | |
3. Axial Length (AXL values)
Every .1 mm of error in axial length measurement in an average eye, can lead to .3 diopter of difference in refractive error.
Therefore, the difference in axial length of .3 mm or more in an average to long eye, should be rechecked.
