IOL Power Calculation

Abdelmonem M. Hamed, M.D .Professor of ophthalmologyBenha College of Medicine

Fellow of Baylor College of Medicine ,Texas Medical Center, Houston, USA

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

– What is the ACD?

Types of formulas calculating IOL power

• Regression formulas like:– SRK: P=A-0.9K-2.5L– 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– Effective lens position

(distance from cornea to lens)

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

• Use of an older IOL formula• Incorrect measurement of the eye's axial length • Incorrect keratometry readings • Mistakes in entering the data into the IOL

calculation program • Incorrect labeling of the IOL by the

manufacturer • Mistakes in providing the IOL at the time of

surgery

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

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. – what is the expected myopia with

development?

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)– 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• You can get the correct mean K reading from:

– Historical methods (Douglas k.)– Holladay Double K

• Post lasik K = Preop K + (prelasik Rx – postlasik Rx)EX: preop K = 45 D, prelasik Rx. = -5.0 D, postlaik Rx. = -1Then the pt. Rx. correction = -4.0DPostlasik K = 45 – 4 = 41 D

What you have to do?– Total corneal refractive

power (Hamed et al., ophthalmology j., thin lens optics), taking in consideration during calculating the corneal power, the corneal thickness, and power of the posterior corneal surface.

– Net power map from pentacam machine

Ophthalmology J. 2002

What you have to do?

− Shammas No-History formula.–Using regression analysis, this method

estimates the post-LASIKcorneal power by adjusting the measured post-LASIK K (Kpost): 1.14 x post lasik K- 6.8 = Post-LASIK corneal power.

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

Which formula is accurate

LENS STAR LS 900• All in one

• biometer, • keratometer, • pachymeter, • pupillometer and • IOL calculator

• Using a diode as the laser source, enables measurement of the axial length of the patient’s eye, precisely on the patient’s visual axis

QUIZ

• 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 you should choose?