UNDERSTANDING ASTIGMATISM SOFT TORIC CONTACT LENS Flashcards
CAUSES OF ASTIGMATISM
Meridional differences in:
- curvature and/or flattening rates of cornea and crystalline lens
- refractive index of optical components
CAUSES OF ASTIGMATISM
Shape of posterior pole
Toric in shape
Tilted
Decentred
- K reading of the Vertical Meridian (or near) is the steepest (least)
- minus cyl axis horizontal
wtr
○ K reading of the Horizontal Meridian (or near) is the steepest (least)
○ minus cyl axis vertical
ATR
○ Two Principal Meridians lie between the axes of WTR or ATR
○ 20-70; 110-16
OBLIQUE
The greatest refractive power is between 60 and 120 meridians
WTR
The greatest refractive power is between 0 to 30 and 150 to 180 meridians
ATR
Rule of thumb - Corneal astigmatism
_______ mm difference in surface radii
approximately equals _________ diopters of
corneal astigmatisM
0.10 ; 0.50
CORRECTION OF ASTIGMATISM
- SPECTACLES
- CL: RGPs, Soft spherical, Soft toric, RGP toric
- Refractive surgery
Aims of CL correction
● Meet patients’ needs
● Good acuity
● High comfort levels
● Acceptable corneal response
Major features of soft toric lenses
All features of normal soft lenses, plus:
- correcting astigmatism
- rotationally stable
- predictable rotation
PRINCIPLES OF STABILIZATION
- Truncation
- Prism stabilisation
- dynamic stabilisation
DYNAMIC STABILIZATION
● Interaction between both lids = orientation and stability
● Thickness profile minimised
● Superior and Inferior Thin Zones
● Thickness difference varies with power
● Dual thin zone stabilisation
● Stabilisation thickness constant with power
Advantages of dynamic stabilization
- better comfort
- less visible
- thinner design
Disadvantage of dynamic stabilization
variable stability with different powers
