Lecture 4 - anatomy and physiology of a CL Flashcards
what is the base curve radius (BCR)?
radius of curvature of the central posterior optical section in mm
what is the back vertex power (BVP)?
used to express power of CL -Determined from a fixed position with the concave surface of the lens against the lensometer lens stop
what is the contact lens power?
dioptric power of CL - usually expressed as back vertex power
what is edge clearance?
actual distance from lens edge to cornea (not inherent property of lens) - less than calculated edge lift due to corneal asphericity
what is effective power?
power of CL at the corneal plane
what is the Flat-K?
flatter (longer) of the 2 principal meridians (GP lenses are generally selected with BCR slightly steeper or flatter than Flat-K)
what is fluoro-silicone/acrylate (FSA)?
used in modern GP CL - consists of fluorine, silicone and methyl methacrylate
what is front vertex power (FVP)?
power determined from fixed position with convex surface against lens stop
what is the lacrimal lens (LL)?
tear lens between GP and corneal surface
what is the optical zone diameter (OZD, FOZD, BOZD)?
provides the visual optics for both GP and soft lenses
what is the overall diameter (OAD, TD)?
linear edge-to-edge measurement of the lens in mm
what is oxygen permeability (Dk)?
potential of a CL material to transmit oxygen, considers both solubility and diffusion (unit = barrer)
what is oxygen transmissibility (Dk/t)?
describes oxygen transmission for a specific CL
what is prism (in CL)?
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)
what is radial edge lift?
measured from the lends edge perpendicular to an extension of the BCR (inherent property)
what is relative spectacle magnification (RSM)?
compares corrected ametropic retinal image to a standard emmetropic eye
what is residual astigmatism?
spherical GP = refractive cylinder - corneal cylinder
spherical soft = refractive cylinder
what is a rigid GP CL?
has rigid material (methyl methacrylate, silicone, fluorine), small in diameter (smaller than cornea) and higher optical quality than soft lenses
what is a silicone hydrogel (SiHy) lens?
current generation of soft lenses - uses silicone in matrix for higher oxygen permeability
what is a soft toric lens?
designed for refractive astigmatism - conforms to cornea (must be stabilized)
what is a soft CL?
flexible (hydrogel or SiHy material), large in diameter (larger than cornea) and some water content
what is spectacle magnification?
ratio of retinal image size of a corrected ametropic eye to retinal image size of the same eye uncorrected
what is the OAD for a GP lens? soft lens?
GP OAD = 9mm - 10mm
soft OAD = 14mm
what is the Bennett scale for Dk values?
low = 25-50 high = 51-99 hyper = equal to or more than 100
how is water content and oxygen related in hydrogel lenses? what about in SiHy lenses?
hydrogel = directly related (higher water = higher Dk) SiHy = inversely related
what is the HEMA material used in?
soft lenses - only used with other materials to increase water content
what is EGDMA used in?
soft lenses - makes material stiffer/less stretchable
what are NVP and MAA used in?
soft lenses - both increase water content
what is MMA used in?
soft lenses - lowers water content and increases hardness/strength
what is GMA used in?
soft lenses - increases wetability and deposit resistance (smaller pore sizes)
what is PVA used in?
soft lenses - increases water content and resistance to deposits, Dk, hardness and strength
what gives SiHy a high Dk value?
the silicone (Si-O) component
what are SA and FSA used in?
modern GP lenses
what are CAB and PMMA used in?
other GP lens materials
what are 2 types of multi-curve GP lenses used in the US?
tri-curve or tetra-curve - within the zones the radii are spherical
what happens to the radii as the multi-curve zones go from center to periphery (BCR - SCR - ICR - PCR)?
goes steepest to flattest
if you had a 7.80 BCR lens that was tri-curve, what would be the radius for SCR and PCR?
SCR = 8.80 (1mm flatter)
PCR = 10.80 (2mm flatter)
each are 0.3mm wide
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?
OZD = 9.00mm - 2 x (0.30 + 0.30) = 7.80mm
what is the blend radius for a transition zone between BCR and SCR (BCR = 7.60mm and SCR = 8.60mm)
radius is average of two curves forming junction = 8.10mm
if a lens is more than +/-5D, what happens to the positioning and central thickness?
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
what happens to the center of gravity with a thick central thickness (CT)?
it moves forward and lens moves down
what is the range for CT (central thickness)?
0.13 - 0.19mm
what happens to the fit of a multi-curve GP lens with a larger OAD?
the larger lens = fits tighter (moves less)
smaller = fits looser
what is the single most important factor in GP lens comfort?
the edge design (anterior edge)
what are the most comfortable GP edge designs?
round anterior/round posterior
round anterior/square posterior
what is axial edge lift?
distance from edge of CL to hypothetical circle of BCR (parallel to axis) - manufacturer specific
what is radial edge lift?
distance from edge of CL perpendicular to BCR - manufacturer specific
what is axial edge clearance?
relationship between lens edge and corneal surface (parallel)
what is radial edge clearance?
relationship between lens edge and corneal surface (perpendicular)
what is a minus carrier?
used to reduce center thickness and increase edge thickness in high plus lenses (more than 3D)
what is a plus carrier or CN bevel?
used to reduce edge thickness of high minus lenses (CN = -4 to -6 and carrier = more than -6)
what is a fenestration in a lens?
ventilation hole drilled in large GP lenses - provides additional oxygen
what is the general fitting rule for soft lenses?
BCR = Flat-K + 1mm (Never fit on-K)
if you have a Flat-K of 44.50D, what would the BCR be for a soft lens?
44.50D (7.60mm) + 1mm = 8.60mm
does the BCR have any influence on a soft lens fit?
no
what is the general fitting rule for GP lenses?
BCR = Flat-K
how much movement should be seen in soft lenses? GP lenses?
soft = no movement GP = about 1-2mm with each blink
if you had a Flat-K of 44.50D and you were fitting a GP lens, what BCR would you use?
44.50D (7.60mm) = 7.60mm BCR
