CONTACT LENSES

1. CONTACT LENS PARAMETERS

2. Some Basic Definitions
 Blank or the button from which the rigid or a soft
lens is fabricated by lathe(Fig 2.1)
 Semi finished blank—A blank where base curve is
pre-generated
 Semi finished lens—A lens that has the back and
the front curve cut. The peripheral curves and the
edge is not yet fabricated.
 Finished lens—A lens ready to be worn on the eye.

4. The Overall Contact Lens has Following
Key Design Features
 1. Base curve
 2. Power
 3. Lens 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.
So a longer base curve produces flatter base curves. In diopters 44.0 D, is said to be
steeper than 41.0 D radius of curvature
 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

8.  The apshericity is specified by e-value. Following are the
e-values and their shape correlation:
 e-value shape
Zero spherical
Between 0-1 elliptical
1.0 parabolic
> 1.0 hyperbolic

9.  Most aspheric contact lenses have e-values from 0.3 to 1.1.
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

10. Overall Diameter (OD) (Fig. 2.3)
 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.
 Diameter depends upon the corneal diameter and the palpebral aperture.
 Larger the corneal diameterThe 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. Diameter depends upon
the corneal diameter and the palpebral aperture.
 Larger the corneal diameter greater is the overall diameter of the lens required. 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.  The peripheral curve or PC is the curve surrounding the base 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.
 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.

14. 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.

15. 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.

16. Optic Zone (OZ) (Fig. 2.5)
 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.
 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.

18. Optic Zone Diameter (Fig. 2.3)
 It is the diameter of the optic zone specified in millimeters. It is
also the overall diameter minus the peripheral curve width.

19. 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.

20. 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.
 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.

21. 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 ( base curve and more than
2 peripheral curves)
 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.

22.  In this case the front optic zone diameter is reduced and a front
peripheral curve or curves are added.
 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.
 Lenticular lens reduces the thickness of the lens and thus provides
greater comfort and improves the transmissibility.
 Lenticular design is commonly made in high-powered lenses.

23. Sagittal Depth: Sag
 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.
 Sag values have their importance in fitting soft contact lenses. 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.

24.  Let us 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.
 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 (Fig. 2.7).
 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.
 Increasing the sagittal height tightens the lens, which can be done by either
decreasing the base curve or by increasing the diameter.

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

27.  AEL is the distance between the apex of the lens edge and the
continuation of the base curve, measured parallel to the lens axis.
 REL is the distance between the apex of the lens edge and the
continuation of the base curve.
 Edge clearance, is the distance between the peripheral curves
highest point (the lens apex edge) and the peripheral cornea

28.  Edge lift and edge clearance are measured in hundredths of a
millimeter.
 For example a normal peripheral system would have a radial edge
lift of 0.08 to 0.10 mm.
 This would be equal to an axial edge lift of approximately 0.11 to
0.14 mm (depends upon the diameter).
 The rule is that the axial edge lift or clearance will always be more
than radial edge lift or clearance.