contact lens terminology some basic definitions

contact lens terminology some basic definitions

1. SHAHLA THESNIM K Contact lens terminology

2. Basic definitions BLANK SEMIFINISHED BLANK SEMIFINISHED LENS FINISHED LENS

3. 1. Blank or the button from which the rigid or a soft lens is fabricated by lathe

4. 2. Semi finished blank A blank where base curve is pre-generated. 3. Semi finished lens A lens that has the back and the front curve cut. The peripheral curves and the edge is not yet fabricated. 4. Finished lens A lens ready to be worn on the eye.

5. KEY DESIGN FEATURES OF CONTACT LENS 1. BASE CURVE 2.POWER 3.LESN DIAMETER 4.PERIPHERAL CURVES 5.OPTIC ZONE 6.CENTRAL THICKNESS 7.EDGE DESIGN

6. BASE CURVE(BC) Base curve or BC is also called as BOZR —Back optic zone radius, BCOR —Back central optic radius or CPC —Central posterior curve. This is the back curve of the contact lens, which contours the front surface of the eye. To achieve a proper fit the base curve of the contact lens should be aligned with the curvature of the cornea.

7. Base curve is expressed in mm ( millimeters ) or diopters.Example—8.1 mm, 8.3 mm. 8.6 mm, etc. Also 8.1 mm is a steeper base curve than 8.3 mm, and 8.3 mm is steeper than 8.6 mm. In diopters 44.0 D, is said to be steeper than 41.0 D radius of curvature.

8. Base curves can be spherical or aspheric. An aspheric base curve as the name suggests flattens from center to periphery and matches the corneal asphericity . The apshericity is specified by e-value . Following are the e-values and their shape correlation:

9. e-value shape Zero spherical Between 0-1 elliptical 1.0 parabolic > 1.0 hyperbolic Most aspheric contact lenses have e-values from 0.3 to 1.1

10. Larger e-values are fitted in multifocal designs. Since the base curve in case of aspheric lenses varies from Center to periphery, the base curve of the aspheric lens is denoted as PAR-posterior apical radius which is a single point curve of the geometric center of the lens. This has little significance as value because the fitting will depend upon the ultimate design and shape of the back curve.

11. Overall Diameter (OD) The length of the lens across its widest diameter is called the overall diameter or the lens diameter. It is specified in millimeters . A soft lens is usually 12-15 mm and a rigid lens is of 8 to 10 mm diameter. Larger the corneal diameter greater is the overall diameter of the lens required.

12. Peripheral Curve/Curves A lens with a single base curve is called a Monocurve Lens. A lens with two curves at the back is called a Bicurve lens. A lens with three curves is called as a Tricurve lens. A lens with more than three curves is called a Multicurve lens.

13. So a Monocurve lens will have one base curve and no peripheral curve. A Bicurve lens will have 1 base curve and 1 peripheral curve . A tricurve lens will have 1 base curve and 2 peripheral curves and so on.

14. The peripheral curve or PC is the curve surrounding the curve on the posterior surface of the lens. If there is more than one peripheral curve then the inner curves are called secondary or intermediate curves.

15. PC is also alternatively called as PPCR – posterior peripheral curve radius. The peripheral curves are flatter than the base curve, and the peripheral most curves will be the flattest one in a normal contact lens design, which is done to match the corneal shape.

16. Bevel /Blend The junction between the base and the peripheral curves are blended properly to give a smooth transition and junction. Blending is the smoothening of the junction of the base curve and the peripheral curves. A well-blended junction is important for the comfort of the lens. Blending is soft, medium or heavy.

17. Peripheral Curve Width The peripheral and the intermediate curve have a fixed width or size. It is usually 0.3 to 0.5 mm wide It depends upon the optic zone diameter and the overall diameter.

18. Optic Zone (OZ) The central optic portion, which carries the base curve of the lens, is called the optic zone. It is the central circular portion of the lens where the power of the lens is located. The optic zone should cover the pupil properly both in scotopic and mezopic condition of light to avoid glare and flare problems.

19. Optic Zone Diameter It is the diameter of the optic zone specified in millimeters . It is also the overall diameter minus the peripheral curve width . The average size of the optic zone is 7 to 8.5 mm in case of rigid lenses and 7 to 12.0 mm in case of soft lenses.

20. Power The power of the lens is ground on the front surface of the lens. A plus lens will be thicker in the center and the minus lens will be thicker in the periphery .

21. Central Thickness It is the center thickness of the contact lens or the distance between the anterior and the posterior surface of the geometric center of the lens usually specified in millimeters .

22. Center thickness has its impact on fitting. The thickness of the lens also affects the oxygen transmissibility. Each lens material has its critical thickness, reducing beyond that leads to flexure problems .

23. Sag values The lens with a greater sag value will behave steeper than the lens with the lesser Sag. Sag values are dependent on the diameter and the curvature. If the diameter of the lens is kept constant increasing the base curve radius decreases the sag or flattens the lens. suppose there are 2 lenses of same 14 mm diameter, and if the radius of curvature is increased from 8.3 to 8.6 the lens with 8.6 mm radius will have lesser Sag or will behave flatter. Sagittal Depth: Sag (Fig. 2.6) Sagittal depth or Sag is the perpendicular distance between the geometric center of the back of the lens surface and the diameter of the lens.

24. Similarly if the base curve is kept constant and the diameters are changed, the lens with smaller diameter will have lesser Sag or will behave flatter. Suppose there are 2 lenses with 8.6 BC but one lens has a diameter of 13.0 mm and the second lens has a diameter of 14 mm the 13 mm lens will have lesser Sag.

25. Increasing the sagittal height tightens the lens, which can be done by either decreasing the base curve or by increasing the diameter .

26. Single Cut vs Lenticular Lens Design Single cut design is the lens design, which has a single continuous curve in the front. The back surface is either monocurve ( with single curve), bicurve (with two curves—base curve and peripheral curve), tricurve (with three curves—base curve,intermediate curve and peripheral curve ) or multicurve ( basecurve and more than 2 peripheral curves).

27. A lenticular design is a lens designed with the optically powered portion of the front surface confined to the middle of the lens surrounded by the peripheral portion . The portion of the lens- which does not carry the optic zone, is called the carrier. It is just like a spectacle lenticular designed lens.

28. Edge Lift The terms axial edge lift (AEL) and the radial edge lift(REL) are used to describe the distance between the lens and the cornea for each of the lens zones.

29. The distance between an extension of the BCOR (back central optic radius) and the absolute edge of the lens; when measured parallel to the optical axis, axial edge lift (AEL); when measured along the radius, radial edge lift (REL).

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