2. CHARACTERISTICS OF LOOSE FITTING
Excessive movement on blinking (more then 1mm).
Poor centration.
Poor comfort because of lid sensation.
Visual acuity variable.
Retinoscopy reflex clear at center & distorted at
periphery.
3. CONTI……….
Refraction variable because of lens movement.
Also keratometer mires distorted at periphery.
Buckling of lens edge.
Base curve(flat).
Diameter(small).
Sagittal depth(small).
4. WETTING ANGLE
The wetting angle is the angle formed at the
surface of the contact lens material when a drop of
liquid is placed on the surface.
5. CONTI………
When the wetting angle is greater , the material is
more hydrophobic. For example, polymethyl
methacrylate (PMMA) is a relatively hydrophobic
material with a wetting angle of approximately 60
degrees.
6. DK VALUE
Oxygen permeability in a lens material
D (diffusion coefficient)
Oxygen molecules dissolve in a material.
K (soluble coefficient)
Number of molecules in a material
