12. Contact Lenses Flashcards
(54 cards)
why are CLs better in some cases for patients
- reduce or elminiate the aberrations assocaited with glasses used in the correction of high refractive errors
- reduce aniseikonia associated with anisometropia and high degrees of atigmatism
examples of CLs used in ophthal
- bandage contact lenses - precroneal membranes
- used in electroretinography
what determines the power of the optical (central) zone of the contact lens
- anterior curvature
- posterior curvature
- thickness
- refractuve index
when is a CL described as spherical
same radius of surgace curvature in each meridian
when can cyclindrical refractive erros be corrected by contact lenses
front surfac,e, back surface or both are toric
how is torsion of a toric CL prevention
either
incorporating an up to 2.00 D base down prism to weight the lower pole or lens
OR
removing the lower 0.5-1mm of the lens (truncation) to allow it sit on the edge of the lower eyelide
what is the base curve of a CL
posterior surface of the optical zone is defined by the posterior central curvature
how is a correct fit ensured for CL
base curve should conform closly to the aspheric surface of the cornea
how is an aspheric shape created
encircling the optical zone with one or two concentrix zones of increasing radius of curvate to produce a bircurve or tricurve C;
how are the junctions between teh zones made smooth
blneding
or computer controlled production
corneal contact lenses have a smaller diameter than the cornea on which they are supported - T/F
true
sclearal contact lenses
rarely used
have a peripheral rim which is supported by the sclera
how is the corneal surface oxygenated
by the tear film
CL are made of gas-imperable materials
thf incorporate fenestrations slot or grooves to facilitate the circulation of tears behind the lens
why do high power concave (minus) lenses ride high
tendency of the upper lid to grip the thick upper edge of a high power lens
countered by a periheral bevel
why do high power convex (plus) lenses drop lower
due to weight
counted by a minus peripheral carrier postion which tends to be lifted by the upper lid
refractive index of the precorneal tear film
1.333
refractive index of the cornea
1.3375
refractive index of precorneal tear film = cornea
YES
tear film helps to reduce what
corneal surface irregulalrity
deinfe the tear lens
tear film between the posteruor surface of the CL and anterior surface of the eye
if the tear lens has a uniform thickness it has
plano power
a steeper base curve of the CL (i.e more vaulted CL) does what
increases the axial ehigh of the tear lens making it more strongly positive
what does the tear lens help
allows a speherical CL to neutralise corneal astigmatism
