Wide palpebral fissure
>10.5mm
Narrow palpebral fissure
<9.0mm
Large HVID
>11.5mm
Small HVID
<11.5mm
Steep Ks
>45.00D
Flat Ks
<41.00D
Interpapebral fit
- >2.00 corneal cyl
- wide palpebral fissure
- large HVID
- steep Ks
- tight upper lid
- smaller OAD
- BC fit steeper than K
Lid attachment fit
- narrow palpebral fissure
- small HVID
- flat Ks
- larger OAD
- BC slightly steeper than K
Small OAD
<9.00mm
Large OAD
>9.4mm
BC for lid attached fit
Slightly flatter than K
BC for interpalpebral fit
slightly steeper than K
Lens lag w/ lid attached fit
1-3mm
Avg OAD
9.0-9.2mm
Normal palpebral aperture
9.0-10.5mm
Lid tension
- lid attached fit: if loose �> use larger OAD
- interpalpebral fit: if tight �> use smaller OAD
Hyperopia lens design
Has a thicker center & larger OAD
Myopia lens design
Has a thinner center & a smaller OAD
Corneal curvature design
- flatter K = larger OAD
- steeper K = smaller OAD
OZD
65-80% of lens OAD
Must be 1-2mm larger than pupil size in dim illumination
= BCR in mm
OZD w/ .4/.3 widths
#NAME?
OZD w/ .5/.3 widths
#NAME?
For every 0.1mm radius of curvature change
~ 0.5D change in power
For every 0.05mm radius of curvature change
~0.25D change in power
Dx BC for plano to 0.50DC
0.25 flatter to on-K
Dx BC for 0.75 to 1.50DC
On K to 0.50D steeper
Dx BC for 1.75 to 2.50DC
0.25 to 0.50D steeper
Dx BC for >2.75DC
Consider toric BC
Mean K + 0.1mm =
BCR
If OZD smaller than normal
Consider steeper BCR
If OZD greater than normal
Consider flatter BCR
Changing the OAD and keeping the FP the same:
- flatten BC by 0.25D (0.05mm) for every OAD inc of 0.4mm
- steepen BC by 0.25D (0.05mm) for every OAD dec of 0.4mm
- SAMFAP the new power
Each peripheral curve gets progressively ____ as we move toward the periphery
Flatter
role of peripheral curve
midperipheral alignment
Edge clearance inc if
- PCR flattened
- PCW widened
- move upper lid interaction
- superior decentration
Edge clearance dec
- PCR steepened
- PCW narrows
- little upper lid interaction
- inferior decentration
Lenticulation - what surface is altered
Front surface altered to enhance edge shape and lens mass
Lenticulation in plus lenses
inferior decentration so
- create a minus carrier to make a thicker edge
- easier lid attachment
lenticulation in what lenses creates minus carrier
plus lenses and low minus <4
why would u want to lenticulate high minus?
>=-4D has thick edges causing discomfort, too much lid attachment, and heavy lens may decenter
Lenticulation in minus lenses
- creates plus carrier
- decreases lid attachment
- could have a plano center
If GP CT too thick
Inferior decentration, decrease O2
If GP CT too thin
Flexible, instability, easily warped
TAP
Thin as possible
Regarding GPs: the higher the Dk
The lower the modulus
Inc CT by ___mm for Dk > 50 for each diopter of corneal cyl
0.02mm
Meaningful BC change to dec movement and improve centration
At least 0.1mm (0.50D) steeper
Meaningful BC change to inc movement, but may induce decentration
At least 0.1mm (0.50D) flatter
Meaningful OZD change to inc sag
0.3/0.4mm increase
For every OZW dec of 0.4mm,
Steepen BC by 0.05mm (-0.25D)
For every OZW inc of 0.4mm
Flatten BC by 0.05mm (+0.25D)
Lateral decentration may be due to
- ATR astigmatism
- decentered apex
- lid influence
How to alleviate lateral decentration
- steeper BC
- larger OAD/OZD
- soft lenses
How to alleviate superior decentration
- inc CT
- change to thinner lens
- consider bitoric if high corneal cyl
- change to higher mass material
- steepend BC by 0.1mm
- dec OAD (to move away from upper lid)
How to alleviate inferior decentration
- dec CT
- change material to lower mass
- flatten BC by 0.1mm (OAD/OZW constant)
- consider bitoric if >2.50 corneal cyl
If lens fits too steep:
- flatten BC by at least 0.50D
Or
- dec OAD by at least 0.4mm
If lens fits too flat:
- steepen BC by at least 0.50D
Or
- inc OAD by at least 0.4mm