Soft Contact Lens

These Parameters are usefully for understand of soft Contact Lens

2. SCL Parameters.
Back surface
Front surface.
Base curve or BOZR (r
0).
Total diameter Ø
T.
Optic Zone diameter.
Sagittal depth.
Central thickness t
c.
Power (BVP).
Water content.

3. Front surface (ant.
surface)
Surface over which the
eyelid passed.
Curved to alter the refraction
of light.
Back surface (post.
Surface)
Nearest to cornea.
Shaped in relationship to
overall cornea.

4. Base curve (BOZR/BCOR/CPC)
Central back curve.
Larger, flatter the curve.
Can be spherical and
aspheric.
In mm of ROC or Diopters.

5. Asphericity is specified by e-value.
•Purpose
Aligned the CL to the ant. Corneal surface.
•Verification
Radiuscope
Measures within 0.01mm of radius.
e-value shape
0 Spheric
0-1 Elliptical
1.0 Parabolic
>1.0 Hyperbolic

6. Total diameter/Overall
diameter
Dimension from one edge
to the other edge.
Depends upon the corneal
diameter & the palpebral
aperture.
Larger corneal diameter -
Greater TD.
TD

7. Purpose
determines the centration of CL
In mm
Verification
lens magnifier with graticule or a
V gauge

8. Optic Zone
Carries lens power
Area of lens where base curve
located
Frontsurface
In Single Cut Lens
OZ = Entire Front surface of lens
In Lenticular Cut Lens
OZ = Front surface -Carrier width
Back surface Lens
OZ = TD –Width of IPC & PPC
OZ
IPC
PPC

9. Optic Zone Diameter-
Diameter of optic zone
OZD =TD –PCW
Purpose-
Carries optical power
Verification-
Lensometer
OZD
PCW

10. Power-
Focal length of CL in air
Dioptric power = CAC –CPC
Front Vertex Power–
Power at front surface
Back Vertex Power–
Power at back surface
Measured in diopters
Verification
Lensometer

11. Sagittal Depth-(Sag)
Distance from flat surface to
highest point on lens
OR
Perpendicular distance from central
posterior portion of CL to diameter
of lens
Depend on diameter & curvature

12. More Sag value more steeper
lens-
BC constant -Increasing
diameter, Increasing sag -
Steeper fit
Diameter constant-
Increasing BC, decreasing sag
-Flatter Fit

13. Center thickness-
Distance between Ant. & Post. surface
Measured at geometric center of lens
Classified as-
<0.06mm -Ultra thin
0.06-0.10mm -Thin
0.10-0.15mm -Standard
>0.15mm -Thick
0.035 -0.04mm -Hyper thin lens

14. Thick Lenses-
Better mask cyl
Easier to handle
Reduces Dk/t
Thin Lenses-
Excellent Dk/t
Do not mask astigmatism very well
Tendency to dehydrate & may
cause corneal desiccation
Not suitable for dry eyes

15. Purpose-
Determine optical power & fit of
lens
Measured in mm
Verification-
Thickness gauge

16. Water Content –
Percentage of CL constituted by water
Affects –
Physiological tolerance
Wearing regimen
Durability
Thickness
Increase in WC –
Increase in Dk/t
Dk doubled (20% increase in WC)
Increase in thickness
Decrease mechanical strength

17. TD
Selection
Measure HVID
Either add 2mm to HVID
Follow manufacturers
recommendation
Larger diameter for higher Rx
–improve fit and centration.
0.5mm added to trial lens
diameter -HWC
Parameter Selection
&
Effect of Parameter change

18. Measuring HVID
PD rule
Wessley keratometer
Graticule
Photographic or video image
Comparator scale

19. Changing TD
Increases –tighten the fit
Decreases –loosen the fit
Rule of Thumb
∆TD 0.5mm ≈∆BOZR 0.3mm
(Increases) (Increases)

20. BOZR
Selection
Measure Ks (mm)
Add either
0.3-0.9mm to flattest K
1.0mm to avg K
4D flatter then avg K
Nearest to Ø
Ttrial set

21. Effect of change
Decreasing –Steepen the fit
Increasing –Loosen the fit
Rule of Thumb
∆BOZR 0.3mm≈ ∆TD 0.5mm
(Increases) (Increases)

22. Centre thickness
Selection based on
BVP
Dk/t
Intended application (DW,EW,FW)
Wearing period
Durability
Handling

23. Effect of change
Excessive movement –Thick lens
Less movement –thin lenses
Better centration & more comfort –Thin
lenses
Increased peripheral rigidity –Tightens fit

24. Optic zone diameter
Selection
Defined by FOZD
Usually 8-11 mm
For high Rxs –as small as 7.5mm
Pupil size –in Photopic /scotopic
Effect of change
Smaller -More flexible periphery –
Less movement
Larger -Less flexible periphery –More
movement
FOZ

25. Water content
Higher Rx –HWC for Dk/t advantage
Lower Rx –LWC for easier handling & greater
durability
Effect of change
LWC –thin lens so more movement
Other factors affected
Dk/t
Durability
Thickness
Deposit resistance

26. Effect of method of manufacturing
Influences rigidity
Lens Fit
Lens rigidity depends on
Chemistry
Method of manufacturing
WC
Thickness
Other material properties

27. Lens rigidity least with
Thinner lenses
Spin-cast manufacture
HWC
Lens rigidity greatest with
Thick lenses
Lath cut lenses
LWC material
Material with MMA or PVA