3 - power, center + edge thickness, edge lift

What are the two types of edge left?

1. axial edge lift

2. relative edge lift

not evaluated clinically

What is edge lift/edge clearance?

distance between peripheral curve and peripheral cornea

What is the purpose of edge lift?

1. encourage tear exchange behind lens

2. assist in centration

3. aids in CL removal

4. prevents indentation of peripheral cornea

What are some clinical ways to describe edge lift?

1. adequate

2. excessive

3. minimal

What is the ideal peripheral fluorescein width/edge lift?

0.5-1.0mm width

What edge lift widths are unacceptable?

too small - <0.3mm

too large - >1.0mm

What are two ways to increase edge lift? And by how much?

1. flatten (increase) SCR/PCR by at least 1.0mm

2. increase SCW/PCW by at least 0.2mm

Why do we try to avoid adjusting the SCW/PCW to change edge lift?

because changing the SCW or PCW will alter the size of lens

What are two ways to decrease edge lift? And by how much?

1. steepen (decrease) SCR/PCR by at least 1.0mm

2. decrease SCW/PCW by at least 0.2mm

what is the purpose of the front curve?

provide the appropriate power

Why is the thickness of the lens important?

1. influences Dk/t

2. influences flexibility

3. affects lens mass

How does OAD (overall diameter) affect the thickness of the lens?

increased OAD → increased lens thickness

How does power affect the thickness of the lens?

increase power → increased lens thickness

what power GP lens is the thinnest?

~-2.00D

Where is the lens the thickest for a myopic shape?

edges

Where is the lens the thickest for a plus shape?

center

At what lens powers does the lens take on a minus shape?

≥-4.00D

At what lens powers does the lens take on a plano shape?

-2.00 to -3.00D

At what lens power does the lens take on a plus shape?

anything equal to or more positive than -1.00D

What is the typical center thickness for a ≥-3.00D lens (minus lens)?

0.13-0.18mm

What is the typical center thickness for a Plano shaped lens?

0.20-0.21mm

What is the typical center thickness for a ≥+3.00D lens?

0.30-0.60mm

What is the typical edge thickness for an unpolished lens?

0.10-0.12mm

What is the typical edge thickness for a polished lens?

~0.08mm

What is a lenticular curve?

peripheral curve on the front of the lens

what happens to the power curve after you add a lenticular?

power curve is no longer across the whole front surface

what is the anterior optic zone?

diameter of the power curve that is inside of the lenticular curve

For plus-shaped lenses, the lenticular is _________ than the power curve?

flatter

What is the purpose of lenticular on plus shaped lenses?

1. thinner lenses → decreases lens mass and improves Dk/t

2. better fit → interacts better with lid, moves center of gravity back

What are the two types of plus-shaped lenticulars?

1. regular carrier

2. minus carrier

What is a regular carrier lenticular?

edge thickness = juctional thickness

image - top lens

What is a minus carrier lenticular?

edge thickness > junctional thickness

image - bottom lens

How does the relationship between edge thickness and junctional thickness affect the lens?

the greater the edge thickness is compared to the junctional thickness → the more the lens will interact with the lid

For minus lenses, the lenticular is _____________ than the power curve?

steeper

What is the purpose of a minus-shaped lenticular (lenticular on a minus lens)?

1. thinner lenses → decreases lens mass, improves Dk/t

2. better fit → decreases lid awareness, improves comfort

How does the optic zone change the wider the lenticular curves get?

the size of the optic zone decreases