MEASURE CORNEAL POWER
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• Central corneal power is the 2nd most important factor in the
calculation of formula
• With 1D error in corneal power cause 1D post operative
refractive error

CORNEAL POWER CAN BE MEASURED BY:
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a. Keratometry:
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keratometer is used to measure the curvature of
anterior surface of the cornea across a fixed chord length (2-3mm) which lies within the optical axis of the cornea.

PRINCIPLE OF KERATOMETER:
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i. Anterior surface of cornea acts as a convex mirror
ii. If curvature of cornea is increased image size will be decrease
iii. From the size of corneal image (1st purkinje image), radius of
curvature can be calculated by using formula,
r = 2i/o
HERE,
r = radius of curvature of cornea
i = image height
o = object height

iv. From the radius of curvature dioptric power of the cornea are
calculated using formula,

D = n1-n2/r
HERE,
D = Dioptric power of cornea
n1 = refractive index of 1st medium (cornea)
n2 = Refractive index of 2nd medium (air)
r = radius of curvature of cornea

b. Topography:
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Provide 3D image of cornea from which radius of curvature is detected along with other pathologies in cornea.

c. Pentacam:
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Use a single scheimpflug camera to measure the radius of curvature of anterior & posterior cornea as well as corneal thickness to measure corneal power.

c. Galilei:
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Similar fashion to pentacam

EEFECTIVE IOL POSSITION
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A. ANATOMICAL FACTORS:
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i. Include axial length, corneal steepness, limbal white to white
measurement, pre-operative AC and lens thickness
ii. Holladay showed in a study that the depth of AC had a positive
and partial relationship to limbal white to white measurement
B. IOL RELATED FACTORS:
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i. Include shape, length, flexibility, anterior angulation & material
of haptic IOL

C. SURGEON RELATED FACTORS:
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i. Individual surgeon technique can also influence effective Lens
position

D. BAG TO SULCUS SHIFT:
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i. In situation like posterior capsule rent or loss of anterior
capsule integrity, the IOL needs to place on ciliary sulcus
instead of normal “IN THE BAG” position.
ii. This require deduction from the calculated IOL power, 0.5-o.7
less by most surgeons.

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