L15 chapter 13 keratometry and keratoscopy 2 2007 2008

Keratometry: Main Points so Far ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

OBJECTIVE h  ½ h C MIRE F CORNEA ½ h PRISM (P  ) IMAGE PLANE Fig 13.17, Page 13.18 Moving prism toward image plane decreases image displacement ( x ) Previously doubled images are no longer doubled (now overlap) What new corneal radius would this prism position “suit”? What happens if we move the prism?  x h  < h  P

Q1: Based on the previous figure, how could the keratometer prism be used to yield a measure of anterior cornea radius? ,[object Object],[object Object],[object Object]

Fig 13.22, Page 13.27 ILLUMINATED MIRE HORIZONTAL & VERTICAL PRISMS EYEPIECE OBSERVER APERTURE PLATE CORNEAL MIRE IMAGE P H P V OBJECTIVE LENS Schematic View of the B & L Optical System OBJ GRATICULE PLANE Two prisms means two deviated images

Topcon Keratometer What the Clinician Sees V 90 / H 180

B & L: Oriented to Measure r 90 and r 18 0 Question: If most corneas are aspheric, what is one drawback with a keratometer? Answer: only measuring radius at one location (annulus) on cornea; and it is NOT central radius OBJ h' 90 Corneal vertex h' 180

B & L: Oriented to Measure r 60 and r 15 0 h' 60 Corneal vertex h' 150

Q2: What does this appearance indicate? ,[object Object],[object Object],[object Object],[object Object],[object Object]

B & L: Oriented to Measure r 90 and r 18 0 Question: What does the above appearance indicate? Answer: anterior corneal astigmatism. What type? Against-the-rule OBJ h' 90 Corneal vertex h' 180

Estimation of Total Corneal Power ,[object Object],[object Object],[object Object],[object Object],Page 13.23 OBJ

[object Object],Basis of Corneal Power Estimate ,[object Object]

Basis of Corneal Power Estimate - Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D Page 13.23 n cornea 1.376

Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only n cornea 1.376 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D

Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D

Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Keep true anterior corneal radius - this is what keratometry measures Want single surface cornea to give same +43.05 D as the Exact Eye cornea Using n  = 1.3315, the +7.7 mm Exact Eye anterior corneal radius yields correct total corneal power +43.05 D Why is new n  < 1.336? Only option is to change n aqueous

Estimation of Total Corneal Power ,[object Object],[object Object],[object Object],[object Object],Page 13.24

Calibration Refractive Indices - Real Keratometers ,[object Object],[object Object],[object Object],[object Object]

Calibration Refractive Index - B & L Keratometer ,[object Object],[object Object],[object Object],[object Object],OBJ Page 13.24-25

Intraocular Implant Design ,[object Object],OBJ: when applying formula, the basis (n cal ) of the K value must be consistent with the ‘A’ value (design constant) IOL power for emmetropia Constant based on IOL type Axial length in mm Average total corneal power based on keratometry

Corneal Power Estimate - Routine Applications ,[object Object],[object Object],[object Object],Page 13.25

L15 chapter 13 keratometry and keratoscopy 2 2007 2008

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