Clinical Techniques
Biometry
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Get access- To predict postoperative refraction for an IOL implant
Note
The formulae only predict the post-operative spherical equivalent
- Partial coherence laser interferometry is superseding ultrasound biometry
- Greater accuracy
- Greater reproducibility
- Measures true foveal axial length
- Principles of Zeiss IOLMaster (partial coherence, non-contact biometry)
- Two coaxial laser beams that are partially coherent
- An interference pattern is produced
- This pattern is used to interpret measurements
Parameters in biometry
- Axial length
Significant disparity between the two eyes should be noted (ie. greater than 0.2mm difference)
- Aphakia, pseudophakia and silicone oil can all affect the readings generated
- Hoffer Q (or Haigis) tends to be used for shorter eyes
- Average keratometry: the mean of the powers of the highest and lowest curvatures
- If astigmatism is highly irregular, corneal topography must be studied preoperatively
Patients with previous keratorefractive surgery pose a challenge and biometry is much less accurate. Haigis-L formula is used for such patients
- A constant
- Correction factor applied to each lens type to achieve accuracy
Reflects position within the eye ie. AC lenses have a different (lower) A constant compared to PC lenses
Hot Topic
The more posterior a lens is placed, the stronger it needs to be. Hence if after posterior capsule rupture an AC lens is to be placed, it needs to be weaker than the planned intracapsular lens. A planned intracapsular lens in the sulus can produce myopic shift since it will be slightly too strong for this more anterior position
For the original SRK and the newer SRK-T formulae there is a 1:1 relationship between the A-constant and the IOL power. Therefore if two lenses have the same predicted outcome, but different IOL powers, this difference will be the same between the A-constants.
SRK-I formula: IOL power = A constant - 2.5 (axial length in mm) - 0.9 (keratometry reading)
The SRK-II formula introduced a modification scale for the A constant depending on the axial length.
Hot Topic
When to repeat biometry (RCOphth guidelines)
- AL <21.2 or > 26.6
- Mean K of <41 or >47
- Difference in inter-ocular mean K of >0.9
- Delta K or >2.5 (astigmatism)
- Difference in AL of >0.7
Inaccurate AL
- Too short
- Corneal compression in contact scans (eg. A scan)
- Beam misalignment
- Asteroid hyalosis
- Phakic settings in a pseudophakic eye
- Too long
- Silicone oil filled eye