5. Advanced Topography Selecting Premium IOLs and Complex Cases - - PowerPoint PPT Presentation

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• 5. Advanced Topography

Selecting Premium IOLs and Complex Cases

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Today we will learn

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Why K readings are important to IOL calculations

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How to take accurate measurements

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How to compare readings from different devices

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How to investigate irregular K readings

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Understanding different ways to measure corneal curvature

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Understanding Astigmatism front and back

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Using reports to screen out unsuitable patients

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Topography & IOL

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IOL = A constant - 2.5 AL - 0.9K

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Axial Length specific location for measure

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A constant - Manufacturer or Optimised

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K readings ??????

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0.1mm in K reading = 0.5D change

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Today we will look at K readings and curvature

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Curvature

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The cornea is not a uniform shape

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It varies depending on where we measure the K reading

• Auto Keratometer 3-3.3 mm
• IOL Master 2.4mm
• Pentacam 3mm

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Measuring Accurately

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Most readings are reflections of the tear film

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Make sure tear film is consistent across the surface

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Use Artificial Tears

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Haigis says don’t use ANY drops on cornea prior to measurement

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Use an instrument that measures cornea directly?

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Pentacam

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Pentacam measures cornea not tear film, so should be unaffected by artificial tears

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But … the cornea will change shape as it is lubricated

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0.1mm change in curvature equates to 0.5D change in power

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Solution

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The drier the eye the more it will be affected by artificial tears

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Take measurements before and after drops

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Discuss with patient. If they are likely to keep with dry eye regime after surgery then use the lubricated reading

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If not use the pre drop reading

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Comparing K Readings

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Check quality

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Review Images

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Were artificial tears used?

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Comparing K Readings

Set Zone to Diameter you wish to compare. Eg 2.4 for IOL master Choose Apex or Pupil. Eg for IOL Master choose pupil Choose Zone or Ring. Eg for IOL Master choose ring

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Compare K Readings

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Compare eg Pentacam with IOL Master

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If the readings are close use the IOL master readings

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If the readings are different you may need to investigate further

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Investigating K Readings

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Using Pentacam because

• Most popular
• Images from and back surfaces of cornea
• I have most experience with it

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Check Zone and Ring are Similar

X X XX/2=K2 X X XX/2=K1

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If not - BAD

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Tips

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Asymmetry is suspicious

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Check other reports like Belin Ambrosio

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Check for Artefacts

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Egg Shaped pupil suggests movement

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Power Distribution

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Corneal Curvature

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Sim Ks

• Anterior taken at 3mm (2.4 IOL Master)

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True Net Power

• Add Anterior and Posterior power together
• Thin Lens Formula

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Total Corneal Refractive Power

• Ray trace through all surfaces
• Thick lens formula

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True Net Power

Cornea n=1.376 Aqueous n=1.336 Air n=1 PowerD = n-1 x1000 rmm

+

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Total Corneal Refractive Power

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Astigmatism

ATR 0-30 ATR 150-180 WTR 60-120 OBL 30-160 O B L 1 2

• 1

5

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Astigmatism

Anterior surface-based keratometry compared with Scheimpflug tomography- based total corneal astigmatism.

Tonn B1, Klaproth OK1, Kohnen T1.

https://www.ncbi.nlm.nih.gov/pubmed/25468887

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Axis

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Cataract Pre-Op

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Also

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Chord Distance

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Difference between Pupil Centre and Apex (where the patient is looking)

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Jack Holladay - over 0.5 mm not suitable for MF

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For more info www.gatinel.com

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Example

๏ Lets review the topography

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4 Maps Refractive

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Power Distribution

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BAD

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Holladay Report - Chord

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Refractive - Chord

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Cataract Report

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Today we have learnt

✓

Why K readings are important to IOL calculations

✓

How to take accurate measurements

✓

How to compare readings from different devices

✓

How to investigate irregular K readings

✓

Understanding different ways to measure corneal curvature

✓

Understanding Astigmatism front and back

✓

Using reports to screen out unsuitable patients

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Thank you!

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We will email you some questions.

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Please send your questions and comments to emma@medsalesacademy.com

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We look forward to seeing you on the next course - A Constants and Formula