Lecture 4 - anatomy and physiology of a CL

Question 1

what is the base curve radius (BCR)?

Answer 1

radius of curvature of the central posterior optical section in mm

Question 2

what is the back vertex power (BVP)?

Answer 2

used to express power of CL -Determined from a fixed position with the concave surface of the lens against the lensometer lens stop

Question 3

what is the contact lens power?

Answer 3

dioptric power of CL - usually expressed as back vertex power

Question 4

what is edge clearance?

Answer 4

actual distance from lens edge to cornea (not inherent property of lens) - less than calculated edge lift due to corneal asphericity

Question 5

what is effective power?

Answer 5

power of CL at the corneal plane

Question 6

what is the Flat-K?

Answer 6

flatter (longer) of the 2 principal meridians (GP lenses are generally selected with BCR slightly steeper or flatter than Flat-K)

Question 7

what is fluoro-silicone/acrylate (FSA)?

Answer 7

used in modern GP CL - consists of fluorine, silicone and methyl methacrylate

Question 8

what is front vertex power (FVP)?

Answer 8

power determined from fixed position with convex surface against lens stop

Question 9

what is the lacrimal lens (LL)?

Answer 9

tear lens between GP and corneal surface

Question 10

what is the optical zone diameter (OZD, FOZD, BOZD)?

Answer 10

provides the visual optics for both GP and soft lenses

Question 11

what is the overall diameter (OAD, TD)?

Answer 11

linear edge-to-edge measurement of the lens in mm

Question 12

what is oxygen permeability (Dk)?

Answer 12

potential of a CL material to transmit oxygen, considers both solubility and diffusion (unit = barrer)

Question 13

what is oxygen transmissibility (Dk/t)?

Answer 13

describes oxygen transmission for a specific CL

Question 14

what is prism (in CL)?

Answer 14

produced by varying thickness from superior to inferior region of CL while maintaining the same front and back surface curvatures (used for cylinder lenses to center themselves)

Question 15

what is radial edge lift?

Answer 15

measured from the lends edge perpendicular to an extension of the BCR (inherent property)

Question 16

what is relative spectacle magnification (RSM)?

Answer 16

compares corrected ametropic retinal image to a standard emmetropic eye

Question 17

what is residual astigmatism?

Answer 17

spherical GP = refractive cylinder - corneal cylinder

spherical soft = refractive cylinder

Question 18

what is a rigid GP CL?

Answer 18

has rigid material (methyl methacrylate, silicone, fluorine), small in diameter (smaller than cornea) and higher optical quality than soft lenses

Question 19

what is a silicone hydrogel (SiHy) lens?

Answer 19

current generation of soft lenses - uses silicone in matrix for higher oxygen permeability

Question 20

what is a soft toric lens?

Answer 20

designed for refractive astigmatism - conforms to cornea (must be stabilized)

Question 21

what is a soft CL?

Answer 21

flexible (hydrogel or SiHy material), large in diameter (larger than cornea) and some water content

Question 22

what is spectacle magnification?

Answer 22

ratio of retinal image size of a corrected ametropic eye to retinal image size of the same eye uncorrected

Question 23

what is the OAD for a GP lens? soft lens?

Answer 23

GP OAD = 9mm - 10mm

soft OAD = 14mm

Question 24

what is the Bennett scale for Dk values?

Answer 24

low = 25-50
high = 51-99
hyper = equal to or more than 100

Question 25

how is water content and oxygen related in hydrogel lenses? what about in SiHy lenses?

Answer 25

``` hydrogel = directly related (higher water = higher Dk) SiHy = inversely related ```

Question 26

what is the HEMA material used in?

Answer 26

soft lenses - only used with other materials to increase water content

Question 27

what is EGDMA used in?

Answer 27

soft lenses - makes material stiffer/less stretchable

Question 28

what are NVP and MAA used in?

Answer 28

soft lenses - both increase water content

Question 29

what is MMA used in?

Answer 29

soft lenses - lowers water content and increases hardness/strength

Question 30

what is GMA used in?

Answer 30

soft lenses - increases wetability and deposit resistance (smaller pore sizes)

Question 31

what is PVA used in?

Answer 31

soft lenses - increases water content and resistance to deposits, Dk, hardness and strength

Question 32

what gives SiHy a high Dk value?

Answer 32

the silicone (Si-O) component

Question 33

what are SA and FSA used in?

Answer 33

modern GP lenses

Question 34

what are CAB and PMMA used in?

Answer 34

other GP lens materials

Question 35

what are 2 types of multi-curve GP lenses used in the US?

Answer 35

tri-curve or tetra-curve - within the zones the radii are spherical

Question 36

what happens to the radii as the multi-curve zones go from center to periphery (BCR - SCR - ICR - PCR)?

Answer 36

goes steepest to flattest

Question 37

if you had a 7.80 BCR lens that was tri-curve, what would be the radius for SCR and PCR?

Answer 37

SCR = 8.80 (1mm flatter) PCR = 10.80 (2mm flatter) each are 0.3mm wide

Question 38

if you had a OAD = 9.00mm of a tri-curve lens, what is the OZD if the SCR and PCR are each 0.30mm?

Answer 38

OZD = 9.00mm - 2 x (0.30 + 0.30) = 7.80mm

Question 39

what is the blend radius for a transition zone between BCR and SCR (BCR = 7.60mm and SCR = 8.60mm)

Answer 39

radius is average of two curves forming junction = 8.10mm

Question 40

if a lens is more than +/-5D, what happens to the positioning and central thickness?

Answer 40

if CT is too high (plus power) the lens is too heavy for lid and it drops in high minus - edge thickness may be too high and lid will push lens down

Question 41

what happens to the center of gravity with a thick central thickness (CT)?

Answer 41

it moves forward and lens moves down

Question 42

what is the range for CT (central thickness)?

Answer 42

0.13 - 0.19mm

Question 43

what happens to the fit of a multi-curve GP lens with a larger OAD?

Answer 43

the larger lens = fits tighter (moves less) | smaller = fits looser

Question 44

what is the single most important factor in GP lens comfort?

Answer 44

the edge design (anterior edge)

Question 45

what are the most comfortable GP edge designs?

Answer 45

round anterior/round posterior | round anterior/square posterior

Question 46

what is axial edge lift?

Answer 46

distance from edge of CL to hypothetical circle of BCR (parallel to axis) - manufacturer specific

Question 47

what is radial edge lift?

Answer 47

distance from edge of CL perpendicular to BCR - manufacturer specific

Question 48

what is axial edge clearance?

Answer 48

relationship between lens edge and corneal surface (parallel)

Question 49

what is radial edge clearance?

Answer 49

relationship between lens edge and corneal surface (perpendicular)

Question 50

what is a minus carrier?

Answer 50

used to reduce center thickness and increase edge thickness in high plus lenses (more than 3D)

Question 51

what is a plus carrier or CN bevel?

Answer 51

used to reduce edge thickness of high minus lenses (CN = -4 to -6 and carrier = more than -6)

Question 52

what is a fenestration in a lens?

Answer 52

ventilation hole drilled in large GP lenses - provides additional oxygen

Question 53

what is the general fitting rule for soft lenses?

Answer 53

BCR = Flat-K + 1mm (Never fit on-K)

Question 54

if you have a Flat-K of 44.50D, what would the BCR be for a soft lens?

Answer 54

44.50D (7.60mm) + 1mm = 8.60mm

Question 55

does the BCR have any influence on a soft lens fit?

Answer 55

no

Question 56

what is the general fitting rule for GP lenses?

Answer 56

BCR = Flat-K

Question 57

how much movement should be seen in soft lenses? GP lenses?

Answer 57

``` soft = no movement GP = about 1-2mm with each blink ```

Question 58

if you had a Flat-K of 44.50D and you were fitting a GP lens, what BCR would you use?

Answer 58

44.50D (7.60mm) = 7.60mm BCR