3. CONTACT LENSES
Optical devices placed directly in front
and adjacent to cornea
To rectify refractive errors
To provide protection
To improve cosmesis
5. In 1508, Leonardo da Vinci
sketched the first forms of
new refracted surface on the
cornea.
In 1887, Adolf Fick was
apparently the first to
successfully fit contact lenses,
which were made from brown
glass.
6. Theses lenses because the developer had
little knowledge of the metabolic need and
physiology of the cornea.
Did not conform to the shape of the cornea.
Causes corneal abrasions.
Wearing time short and frequently painful
8. Materials used in RGP&RNGP lense-
polymethyl methacrylate(PMMA)
Silicon acrylate
Fluropolymers
Cellulose,silicone,styrene used earliar.
Materials used in soft CL:
Hydrogel is the main ingrediant-
Hydroxyethyl methacrylate(HEMA)
HEMA-venyl pyrrolidone(HEMA-VP)
Glycidyl methacrylate
MMA-PVD
Silicon hydrogel lenses(new generation)
9. An ideal Contact lens material is the one,
which would meet the following conditions:
It provides sufficient oxygen to the cornea to
meet its requirements.
Is optically transparent.
Has stable dimensions.
10. Contd.
Has good wettability when on the eye.
It requires minimum patient care &
maintenance
It resists spoliation
Is easily merchantable or easy to
manufacture.
11. • Wetability-It is the ability of the
tears to form a complete film
over the lens surface.
• Flexibility- The more the lens is
rigid the less it will bend when
placed over the cornea. A highly
flexible material will contour to
the cornea.
12. Optical quality- The lens should be optically
homogenous & transparent with minimum
transmission loss.
Biocompatibility- The lens should not induce
any inflammatory or immunological responses
and should be inert.
13. Manufacturing ease- The process of
lens manufacturing should be easy
and cost effective.
Stable parameter- Lens material
should be dimensionally stable and
be easily polished.
19. E) FDA Contact Lens Classification:
Group 1: Low water content (<50%) non-
ionic polymers.
Group 2: High water content (>50%) non-
ionic polymers.
20. Group 3: Low water content
(<50%) ionic polymers.
Group 4: High water content
(>50%) ionic polymers.
31. Teminologies to know
Base curve(BC):
The primary curveture of central posterior surface of
CL,adjacent to the cornea.
• It is measured by its radius of curveture(mm)or is
sometimes converted to diopters(D) by taking of
reciprocal of its radius.
Peripheral curve(PC):
Secondary curve just outside of the BC at the edge
of a CL.
• They are typically flatter than the BC to
approximate normal peripheral flat cornea.
33. cont..
Tear lens:
The optical lens formed by tear film layer between the
posterior surface of a CL and anterior surface of
cornea.
• In soft CL:It conform to corneal curveture & form
plano power tear lens.
• In Rigid CL:It varies with base curve of lens.
BC equal to K reading→Plano power tear lens
BC steeper than K→Plus power tear lens
BC flatter than K→Minus power tear lens
34.
35. cont..
Vertex distance:
Distance between back surface of the lens and front
surface of cornea.
• Formula of vertex distance correction:-
Dl
Dc= 1+d*Dl
Where,
Dc=Power of CL
Dl=Original lens power
d=Vertex distance in metre
37. A lens has the following basic parameters:
Posterior surface curveture (Base curve)
Anterior surface curvature(Power curve)
Power
Diameter.
Type of the lens.
38.
39. Fitting procedure of rigid CL
History:
• Exclusion of medical contraindication.
• Reason of CL wearing
• General & emotional status of patiet.
Ocular examination:
• Conjunctival,corneal,limbal injection
• Tearfilm break up time(TBUT)
• Blink charecterstic is noted
• Measure-Corneal,pupil diameter&papebral
width for CL diameter.
40. cont..
Refraction:
Retinoscopy,subjective refraction,back vertex
distance.
Keratometry:
For measuring corneal curveture in 2 principle
meridian.
Trial lens fitting:
At least with 2 different diameters lenses with
increments of 0.25D in base curve value.
Diameter:
9mm in a patient with avg. corneal
diameter&palpebral aperture.
41. cont..
Base curve(BC):
Usual practice is to fit the trial lens based on flatter
'K' reading.
• In astigmatism:BC steeper than flatter K.
• If astigmaism:
0.5-1D→BC 0.25D steeper than flatter K.
1-2D→ BC 0.5D steeper than flatter K.
>2D→1/3rd toricity should be added to K for BC.
eg.if K1=44 k2=47, then BC=44+[47-44]/3=45D
42. cont..
• Power of trial lens=spectecle power in minus
cylinder form corrected for “0”vertex distance.
eg.if specs power: -9.25/+0.50x90°
Minus power form: -8.75/-.50x180°
vertex distance:15mm
After vertex correction,
CL power: -7.75/-0.50x180°
43. cont..
• After adaptation period evaluate trial lens for-
position of the lens-
High ride
Low ride
Horizontal decentering
BC determination-
Flat
Steep
Ideal
• Finalization of diameter
• Finalization of power
44.
45. Post fitting management
• Ordering rigid lens:
After finalization of ideal fitting trial lens,order the rigid
contact lens including these variables-
BC radius
Optic zone diameter(OZD)
1st back peripheral zone(BPZ)
1st & 2nd peripheral curve radius(PCr)
Overall diameter of the lens(OD)
Power of the lens(D)
• So,A Prescription looks like-
BC:OZD/1stPCr:1st BPZ/2nd PCr:OD,D
46. Cont..
• Examining ordered lens
• Evaluatiion of ordered lens fit
• Educate the patient about-
CL care
Wearing schedule
How to insert & remove,recentration
• Post fit follow up
47. Fitting of soft CL
Measure corneal curvature in mm
Add 1 to the mean K reading. (but thin lens
need lesser addition to K reading)
Manufactures nowadays usually do not supply
curves in 0.1 mm increments. There are 2 to 3
BASE CURVE MEASUREMENT
48. base curves available. Select the lens
closest to the required BC.
Where only 2 base curve are available,
select the steeper BC for steep cornea and
Flatter BC for flat cornea.
49. 2) Power/Back Vertex Power
When spectacle power is less than +4 or
-4 then no change of the power.
There are 3 basic steps to calculate the
expected power of the soft lens.
Transpose into minus cylinder form.
50. Spherical equivalent calculation-
spherical power+cylinder power/2
Vertex distance calculation- If the
spectacle prescription is more than
4.0 diopter. Power must be adjusted
for vertex distance.
51. Contd.
Spectacle power Remarks Contact Lens
Power
-2.50 DS Same as spectacle -2.50
-2.50/-0.50 Half the cylinder
& add to sph
-2.75
-8.0 DS More than 4, so refer
to BVP
-7.25
-8.0/-1.0 DS @ 90 Sph equivalent= -8.5
Next compensate for
vertex distance.
-7.75
52. 3) DIAMETER
Typical soft lens diameters range from
13.0 to 14.5 mm (in 0.50 steps)
The basic rule is to cover the cornea
adequately, so that there is no exposure of
the limbus on blinks leading to discomfort
and epithelium staining.
53. The diameter is selected on the basis of
the HVID measurements. Add 2 mm to the
HVID and the lens, diameter should be at
least that much.
*HVID=horizontal visible iris diameter
55. Uses of BCL:
The five main aims of therapeutic CL are
summarised as:
1. Relief of ocular pain;
2. Promotion of corneal healing;
3. Mechanical protection and support;
4. Maintenance of corneal epithelial hydration;
5. Drug delivery
56. Diff Between RGP & Soft CL
RGP Soft CL
Oxygen Delivery Good Poor
Visual Acuity Better than soft CL Poor than RGP
Initial Comfort Less More
Long-Term Comfort More Less
Durability More Less
Deposit Resistance More Less
