RPG astigmatism lenses Flashcards
(18 cards)
Residual astigmatism
remains when a lens has been when a lens has been used to corrrect ametropia
residual astigmatism = Total astigmatism- corneal astigmatism
problems with spherical contact lenses on toric corneas
poor vision
poor centeration
cl rocking on flat meridian
unctable cl fitting
cornal distortion
spec blur
discomfort
poor blinking
epithelial damage
3 &9 oclock staining
back surface toric
corneal cyl >2.00 D
physically compatibal with cornea, good for corneal astigmatism
stable meridian orientation
optomise contact lens to cornea relationship
Front surface toric
spherical back surface and cyl front surface
circular design or turncated design
good for lenticular astigmatism
front surface circular design
centered optical zone
base down prism
easy manafacture
good comfort and physiological performance
Front surface truncated design
inferior zone of cl is turncated
rests against lower zone for stability
prism ballast
optical zone decentered superiorly
now uncommon
Bitoric
used when front and back surface resuklt in an unacceptable level of residual astigmatism
toric BS for physical surface
toric FS for full astigmatic correction
rotational stabiltiy
peripheral toric
spherical back optical zone
toric back peripheral zone
spherical front optical zone and peripheral curves
oval shaped optical zone
material selection
Dimensional stability. The CL must maintain its shape to ensure physical compatibility between the back surface & the cornea is maintained. With some materials the degree of back-surface toricity may vary over time &, as a result, the quality of vision may deteriorate with wear.
Oxygen transmissibility. As toric CLs are thicker, the Dk/t is less than an equivalent spherical CL of similar power. Selection of a CL with moderate to high oxygen permeability is necessary to ensure that the cornea’s physiological requirements are satisfied.
Optical stability. A stable CL material minimizes the risk of CL warpage. An irregular change in the shape of the CL would result in a ↓ of the quality of vision.
Manufacturing problems. Some GP CL materials are difficult for the manufacturer to cut & polish. It is prudent to ask the CL lab about the materials they use & to ask them to suggest which materials best combine the requirements of strength & durability with ease of manufacturing.
used targer TD for
normal lid margin locations
stronger lid forces
flatter or larger corneas
plus power
advantages
stable cl fitting
increased corneal - cl fittting relationship
better corneal physiological than toric SCLs
disadvanatages
relaticvally thick CLs
lower control over CL edge profile
possibe misalignment of the corneal and spectacle Rx cyl axes
BItoric fitting
calc refractive error along each meridian after allowing for vertex distance
aim for full allignment so in theory no tear lens effect
note down BVP along the steep and flat meridian for ordering
back surface toric fitting
- spherical lens selected to “align” along the flat K so select a BOZR the same as a flat K
- steeper radius is determined for k reading
- spherical power is determined by over refraction with minus cyls
- cyl is obtained by calcualtion
Optimum fitting characteristics
alignment of static central fluroscein pattern
inferior decentraion not beyong the limbus
some post blink cl movement is required
adequate pupil coverage
sable rotation position
assisment of CL fit
Stability of vision
VA / subjective response
Keratometry with CL in place:
Good fit - mires smooth & regular
Poor fit - mires distorted after blink
Orientation - markings on the CL
Horizontal line at 9 o’clock
Vertical line at 6 o’clock
Radial markings at 75 & 105
CL rotation: measurments
LARS - (left ADD, right SUBTRACT)
CAAS - (clockwise add , anticlockwise subtract)
Effect on tear film
if lens is steep - positive tear lens, more minus required
if lens is flat - negative tear lens, more plus required
