IOL Power IOL Power CalculationCalculationAbdelmonem M. Hamed,

M.D.Assistant professor of

OphthalmologyBenha College of Medicine

Types of formulas calculating IOL power

• Theoretical

• Regression

Types of formulas calculating IOL power

• Theoretical:– It is based on the optics of the eye( i.e.

corneal power, RI of cornea, distance of cornea to IOL, axial length, etc…)

– 1st and 2nd generation formulas is less accurate than regression formulas in normal length of the eye (22 – 25.5mm)

– 3rd and 4th generation formulas is more accurate than regression and 1st and 2nd generation theoretical ones

Types of formulas calculating IOL power

• Regression:– It depend on the regression analysis of acual

results of many uncomplicated IOL implantation

– How ever it is less accurate than theoretical

formulas in short and long eyes (‹22 –

›25.5mm)

Types of formulas calculating IOL power

• 1st and 2nd generation theoretical formulas like:– Colenbrander– Fyodorove– Van der Heijde– Binkhorst– Hoffer– Shammas fudged

Types of formulas calculating IOL power

• 3rd and 4th generation theoretical formulas like:– Holladay II– SRK/T– Hoffer Q

Types of formulas calculating IOL power

• Regression formulas like:– SRK: P=A-2.5L-0.9K– SRK/T T= theoretical

– What is the A-constant?

Types of formulas calculating IOL power

• All the 3rd and 4th generation formulas consider:– ACD depth– Whit to whit length– Actual position of IOL

(distance from cornea)

Potential errors leading to the placement of a wrong IOL include:

• Use of an older, outdated IOL formula for the patient • Incorrect measurement of the eye's axial length

(biometry) • Incorrect keratometry readings • Mistakes in entering the data into the IOL calculation

program • Incorrect labeling or packaging of the IOL by the

manufacturer • Mistakes in providing the IOL at the time of surgery, such

as a mix-up with an IOL for another patient

Consider repeat measurements if

• Axial length ‹22.0 or ›25.0 mm

• Average corneal power ‹ 40 D or › 47 D

• Between 2 eyes, there is a difference in:

– Corneal power › 1 D

– AL › 0.3MM

– Emmetropic IOL power › 1 D

• Immersion technique gives better results or IOL master

Suggestions for Improving Patient Safety

• It found that – 54% of the error was attributed to axial length

measurement errors, – 8% to corneal power measurement errors, – 38% to anterior chamber depth (ACD)

estimation errors

Suggestions for Improving Patient Safety

• Note: The approximate IOL power, if emmetropia is desired, can be quickly calculated by using an initial power of 18 D and adding the patient's preoperative refractive state multiplied by 1.6

• This can be used as a quick check for the calculated IOL power

• If preop Rx = -8.0 D (-8.0 X 1.6 = -13.0)• 18 – 13= 5D

What you have to do?

• if you have an eye filled with silicon oil

• if the patient had lasik

• If you have a child Pt. what is the ideal postop. Rx for them. what is the expected myopia with development?

• If you have to insert multi-focal accommodative, or Piggyback IOL.

What you have to do?

• if you have an eye filled with silicon oil– The RI of SO = 1.42 (higher than vitreous

1.34) …the SO eliminates 10D effect of biconvex IOL

– Velocity of SO = 1000– Do not use silicone IOL if there is SO inside

the eye.

What you have to do?

• if the patient had lasik– Historical methods– Total corneal refractive power– Net power from pentacam machine– Holladay report “Equivalent K-Readings” – Others etc…

What you have to do?

• If you have a child Pt. what is the ideal postop. Rx for them. what is the expected myopia with development?– Ideal Rx: plano to -1.0 D– Expected induced myopia with development -4D

(myopic shift from age 2 – 20 ys)– Role of thumb: it is much easier to correct myopia at

age 20 than amblopia– Some surgeons choose to aim for postoperative

hypermetropia in young children and infants to allow for myopic shift as the eye grows

What you have to do?

• If you have to insert multi-focal:– Calculate IOL power for distance =23 D– Calculate IOL for near 27 D– IOL add = 4D– choose nearest available add (for examble

3.5D)

What do you know about IOL registry

• ACD

• A-constant

• Surgeon factor

QUEEZE

• Pt has got cataract, AL 28mm, average corneal power 43 D– Emmetropic IOL power with SRK II = +7D– Emmetropic IOL power with SRK/T = +4D

• Which power i should choose?

Thank you for your attention