It is an artificial device placed on cornea for optical or therapeutic purpose. It’s made of soft, flexible plastics that allow oxygen to pass through the cornea.
ย ย โข Soft contact lenses are made up of ย HEMA (hydroxyethylmethacrylate). HEMA is hydrophilic. This hydrophilic nature is because of cross linked polymers with carbon to carbon backbone, to which are attached hydrophilic groups. It’s refractive index 1.43.
โขPHEMA (poly- hydroxyethylmethacrylate) is still the basic lens material in use aa the soft lens material.
โขPhysical properties of this lens material-
I) Water content – โช 38 to 80%
โช <40% low water content lenses.
โช >55% high water content lenses.
โช 40 to 55% mid water content lenses.
II) Elasticity – The soft contact lens material has to be elastic so that it can survive the repeated stress of insertion and removal. It should recover it’s shape very rapidly after stress.
III) Ionicity – Two groups based on iconicity- (a) Ionic, (b) Non- ionic
โข Several conpounds are added to hydrogens to improve it’s properties like wettability, water content, oxygen permeability on ionicity.
The design of soft contact lens depends upon the following parameters of a soft contact lens.
a) Diameter – Average size 9.2 to 9.4mm.
b) Thickness.
c) Curvature – Front and back with thin flexible lenses design almost irrelevant.
Back surface design –
I. Monocurve – Usually are not made in monocurve designs.
ย ย II. Bicurve – It is common, an central optic zone design and one peripheral curve(0.8 to 1mm flatter) .ย ย ย ย ย ย ย ย ย ย ย ย ย ย III. Multicurve – lenses which have manyย curves, flexible lenses don’tย need a multicurve design.
IV. Aspheric – Shapes cornea better, fewer base curves need to be made with aspheric back curve design, it is difficult to manufacture.
Front surface design –
Front surface design contains the front optic zone depends upon the power. The front surface design varies in bifocals/ multifocals.
Edge of soft contact lens – The edge has a little effect than rigid lenses as it is mostly under the lid. Yet, thick is less comfortable and a very thin lens edge cause mechanical cutting.
Manufacturing technique and design – There are three main techniques of soft contact lens manufacturing, different techniques create different designs like~
A soft contact lens made with lathe is used to make simple designs.
The spin technique creates simple back surface designs.
๐. ๐๐ฅ๐๐๐ง๐๐๐๐ก – A spherical soft contact lens bends light evenly in every direction(horizontally, vertically etc) . They are typically used to correct myopia and hyperopia.
๐๐. ๐ผ๐จ๐ฅ๐๐๐ง๐๐ โ Aspheric soft contact lens have different interaction between the lens and light entering the eye and make a clearer image, have varying curvatures across the surface, changing from the edge to the center of the lens.
๐๐๐. ๐๐ค๐ง๐๐ –
โขCorrect for astigmatism,
โขMore expensive than spherical,
โขMarks or etching for fitting,
โขDifferent focusing power horizontally/ vertically.
Successful fitting involves complete set of parameters. The parameters are –
๏ธDiameter – It is the 3rd basic parameter of the soft contact lens. Typical soft contact lens diameters range from 13 to 14.5 mm. Smaller corneas may need smaller diameters.
๏ธBase curve – It is chosen on the basis of the keratometry. Base curves range from 8.1 to 9.1mm.
๏ธPower – Lens should have selected as close as possible to the spectacle power. Before fitting we must have to know that the power is cylindrical or spherical.
