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Contact Lenses 1 > Lecture 4 - anatomy and physiology of a CL > Flashcards

Lecture 4 - anatomy and physiology of a CL Flashcards

1
Q

what is the base curve radius (BCR)?

A

radius of curvature of the central posterior optical section in mm

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2
Q

what is the back vertex power (BVP)?

A

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

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3
Q

what is the contact lens power?

A

dioptric power of CL - usually expressed as back vertex power

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4
Q

what is edge clearance?

A

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

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5
Q

what is effective power?

A

power of CL at the corneal plane

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6
Q

what is the Flat-K?

A

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

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7
Q

what is fluoro-silicone/acrylate (FSA)?

A

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

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8
Q

what is front vertex power (FVP)?

A

power determined from fixed position with convex surface against lens stop

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9
Q

what is the lacrimal lens (LL)?

A

tear lens between GP and corneal surface

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10
Q

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

A

provides the visual optics for both GP and soft lenses

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11
Q

what is the overall diameter (OAD, TD)?

A

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

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12
Q

what is oxygen permeability (Dk)?

A

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

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13
Q

what is oxygen transmissibility (Dk/t)?

A

describes oxygen transmission for a specific CL

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14
Q

what is prism (in CL)?

A

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)

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15
Q

what is radial edge lift?

A

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

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16
Q

what is relative spectacle magnification (RSM)?

A

compares corrected ametropic retinal image to a standard emmetropic eye

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17
Q

what is residual astigmatism?

A

spherical GP = refractive cylinder - corneal cylinder

spherical soft = refractive cylinder

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18
Q

what is a rigid GP CL?

A

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

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19
Q

what is a silicone hydrogel (SiHy) lens?

A

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

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20
Q

what is a soft toric lens?

A

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

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21
Q

what is a soft CL?

A

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

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22
Q

what is spectacle magnification?

A

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

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23
Q

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

A

GP OAD = 9mm - 10mm

soft OAD = 14mm

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24
Q

what is the Bennett scale for Dk values?

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

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

A 
hydrogel = directly related (higher water = higher Dk)
SiHy = inversely related
26
Q

what is the HEMA material used in?

A

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

27
Q

what is EGDMA used in?

A

soft lenses - makes material stiffer/less stretchable

28
Q

what are NVP and MAA used in?

A

soft lenses - both increase water content

29
Q

what is MMA used in?

A

soft lenses - lowers water content and increases hardness/strength

30
Q

what is GMA used in?

A

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

31
Q

what is PVA used in?

A

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

32
Q

what gives SiHy a high Dk value?

A

the silicone (Si-O) component

33
Q

what are SA and FSA used in?

A

modern GP lenses

34
Q

what are CAB and PMMA used in?

A

other GP lens materials

35
Q

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

A

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

36
Q

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

A

goes steepest to flattest

37
Q

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

A

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

38
Q

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?

A

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

39
Q

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

A

radius is average of two curves forming junction = 8.10mm

40
Q

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

A

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

41
Q

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

A

it moves forward and lens moves down

42
Q

what is the range for CT (central thickness)?

A

0.13 - 0.19mm

43
Q

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

A

the larger lens = fits tighter (moves less)

smaller = fits looser

44
Q

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

A

the edge design (anterior edge)

45
Q

what are the most comfortable GP edge designs?

A

round anterior/round posterior

round anterior/square posterior

46
Q

what is axial edge lift?

A

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

47
Q

what is radial edge lift?

A

distance from edge of CL perpendicular to BCR - manufacturer specific

48
Q

what is axial edge clearance?

A

relationship between lens edge and corneal surface (parallel)

49
Q

what is radial edge clearance?

A

relationship between lens edge and corneal surface (perpendicular)

50
Q

what is a minus carrier?

A

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

51
Q

what is a plus carrier or CN bevel?

A

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

52
Q

what is a fenestration in a lens?

A

ventilation hole drilled in large GP lenses - provides additional oxygen

53
Q

what is the general fitting rule for soft lenses?

A

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

54
Q

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

A

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

55
Q

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

A

no

56
Q

what is the general fitting rule for GP lenses?

A

BCR = Flat-K

57
Q

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

A 
soft = no movement
GP = about 1-2mm with each blink
58
Q

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

A

44.50D (7.60mm) = 7.60mm BCR

Contact Lenses 1 (6 decks)

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