Contact lens theory 1 course 2

Contact lens theory 1 – Course 2
19/30/2020 Tatiana Mighiu
TOPIC
1. Instrumentation
1.1 Keratometer
1.2 Radiuscope
2. MOR as it relates to contact lens fitting
Readings:
N.Efron – Contact Lens Practice
Clinical instruments page 55-56

1.1 Keratometer ( Ophthalmometer )
The keratometer is a diagnostic instrument used to measure the central
curvature of the anterior surface of the cornea.
There are 2 different designs of keratometers on the market. Some are
based on Bausch & Lomb principles (Fig. 1) and the others are based on
the Javal-Schiotz principles (Fig. 2).
Fig. 1 Fig 2

Keratometric mires alignment for B&L Keratometer
Horizontal alignment
Misalignment
Alignment

Keratometric mires alignment for the Javal-Schiotz keratometer
(www.ciom.it)
Aligned Misaligned

Keratometer Functions
a. Corneal apex measurement (aprox 3 mm in diameter)
- used to determine the contact lens base curve selection
- used to monitor any corneal changes
- detect contact lens surface irregularities or poor lens wetting
b. Amount of corneal astigmatism is determined
(as difference between the horizontal and vertical readings)
c. Nature and axis of astigmatism is determined
WTR – flatter meridian is in the horizontal meridian
ATR – flatter meridian is in the vertical meridian
Oblique – axes between 70°- 90° and 110°-160°
Irregular – axes are not 90° apart and can’t take accurate readings

d. Determine the nature of ametropia
- AXIAL - high myopia with low keratometric readings
- REFRACTIVE – high myopia with high keratometric readings
e. Assess corneal edema(mires will appear watery)
f. Assess dry eye (mires will appear broken, not clear)
g. Assessment of tear layer by evaluating the mires after blink

h. Assessment of the fit of a soft contact lens by evaluating the
mires after the blink
- clear mires – steep fit
- blurry mires – flat fit
i. Assessment of the flexure of a rigid contact lens
(rigid lens bends on the eye with blinking)
j. Measurement of a rigid lens curvature (BCOR, FCOR) with the
use of accessory devices ( E.g. : conta-check)
k. Diagnosis of pathology
(E.g. keratokonus – very steep curves, corneal ulcer, pterygium,
scarring – distorted mires)

B&L Keratometer parts and principle
Parts:
Lock
Focusing knob
Height elevation
knob
Canthus
marker
Meridian
axis drum
and scale
Horizontal
drum and
scale
Vertical
drum and
scale
Occluder
Chin rest
Head rest
Eyepiece

The B&L Keratometer is calibrated in diopters.
Range of the B&L Keratometer 36.00D to 52.00D.
B&L Keratometer assumed corneal refractive index of
1.3375.

9/30/2020 Tatiana Mighiu 10

Steps in taking K readings:
Adjust
the
eyepiece
Position
patient
- with the
chin in the
chin rest
and with
the head
firmly
against the
head rest
Position
yourself
Align the
instrument
in front of
the right eye
Occlude left
eye (if not,
the right eye
is not
looking
directly at
the
instrument
Instruct
patient to:
- keep eyes
wide open
and blink
normally
-do not move
or speak
-look at the
reflection of
own eye in the
instrument
Adjust
focusing
knob

To accurately read central corneal curvature MUST:
1. Superimpose the doubling mire.
2. Rotate keratometer body until horizontal + signs
of lower and center mires are on the same
plane and read axis.
3. Adjust until + signs coincide and then read
power on the horizontal drum.
4. Adjust until “-” signs coincide and read power.
Re align instrument in front of the left eye and
repeat procedure.

Patient side Practitioner side
(www.dfv.com)

Horizontal mires in spherical vs astigmatic corneas
Horizontal mires Vertical mires
─ ─
+ ++ +
─ ─
- Vertical alignment of “+” signs
- Tips of “+” signs are on the same horizontal
line
Since the tips of the “+” signs are aligned the
axis can be read from the axis wheel
To read the horizontal meridian the horizontal
meridian drum has to be rotated as to
superimpose the 2 horizontal “+” signs
─
─
+ + +
+ +
─
─
- Vertical misalignment of “+” signs
- Tips of “+” signs are NOT on the same
horizontal line
Since the tips of the “+” signs are not aligned
the axis wheel has to be rotated until the tips
of the “+” signs align and then the axis can be
read from the axis wheel
To read the horizontal meridian the “+” signs
have to be now superimposed.

Superimposed horizontal and vertical mires
Spherical cornea Astigmatic cornea
Horizontal mire Doubling mire
─ ─
+ + +
─ ─
- Horizontal meridian drum has been rotated
and the “+” signs between the mires are
superimposed.
─
Vertical mire
+ + +
─
Doubling mire + +
─
- Vertical meridian drum has been rotated
and the of “- ” between the mires are
superimposed

Recordings and interpretation of findings
Normal corneal curvature readings are between
41.00 - 46.oo D
Average corneal curvature is between 43.00D - 44.00D
Associated
with myopia
Ex: 41.00D
STEEP CORNEAS
Associated
with
hyperopia
Ex: 46:00D
FLAT CORNEAS

Recordings and interpretation of findings
Recordings
a. Generally the horizontal meridian is recorded first at its axis
followed by the vertical meridian at its axis.
If astigmatism is regular horizontal meridian value is
recorded first and is followed by the vertical meridian value
at its axis.
b. Values can be recorded in diopters or millimeters.
Most common is to have them recorded in diopters as
42.75@180/43.75@090 or 42.75/43.75@090 or written in
millimeters as 7.89/7.71@090
For accuracy 3 readings MUST be taken and the mean
value recorded. The 3 readings should be within 0.25D in
each principal meridian.

Recordings
Writing the corneal astigmatism in sphero-cyl form
E.g. OD 40.50D @ 180 / 43.50@090
From the K readings, the difference between the horizontal meridian
and the vertical meridian represents the amount of corneal
astigmatism.
Corneal astigmatism is 3.00D
a. Minus cylinder (-)cyl – the axis is the axis of the lower dioptric
power
-3.00 x 180
b. Plus cylinder (+) – the axis is the axis of the highest dioptric power
+3.00 x 90

Interpretation of findings
E.g.1 42.00@180/43.75@090
- the flatter meridian is at 180°, therefore the nature of astigmatism
is WTR
- the amount of corneal astigmatism is 1.75D
- astigmatism is regular because the axes are 90° apart
E.g.2 44.62 @165/43.25 @075
- the flatter meridian is at 75°, therefore the nature of astigmatism is
ATR
- the amount of corneal astigmatism is 1.37D
- astigmatism is irregular because the axes are not 90° apart
E.g. 3 45.00/46.37@60
- the flatter meridian is at 150, therefore the nature of astigmatism is
Oblique
- the amount of astigmatism is 1.37D
- astigmatism is regular because the axes are 90° apart

Sources of inaccurate readings
Instrument
• From instrument own construction
- uses para-axial theory
- assumes cornea is sphero-cylindrical
- assumes focal length = image distance
• Inadequate instrument calibration
- cross-reticle is of center – measurement taken will not be
of corneal apex
- the drum readings do not match steel ball readings –
instrument can be recalibrated or error be taken into
readings recordings

Operator
- uncorrected refractive error – will cause fluctuating
accomodation (check own vision and eyeglasses if done
properly)
- not focusing the eyepiece properly – can induce an error up
to 1.00D (make sure the focusing steps are done properly)
- incorrect measurement technique
- the tips of “+” signs are not aligned – axis off
- “+” signs not superimposed horizontal reading off
- “-” signs not superimposed – vertical readings off
- inaccurate readings (do not read measurement properly
from the drums or axis wheel)

Patient
- has fixation problem (ensure that the eye not measured is occluded)
- has corneal abnormalities
- irregular cornea (mires will not superimpose) – can’t take accurate
readings
- inadequate or excessive tear film (broken or watery mires) - ask
patient to blink several times before taking the measurement
- ointment on cornea
- eyelid or eyelashes interference (ask patient to open the eyes wider)
As a general rule the keratometer readings are taken at the beginning of
the pre-fitting protocol before bio-microscopy and ocular tests (tear film
evaluation and ocular staining)

4.Keratometer attachments
a. Lens Co Meter – is a steel ball of known radius
The steel balls have the following radii with the
corresponding K values:
Radii K
7.14 mm 47.25 D
7.54 mm 44.75 D
7.94 mm 42.50 D
8.38 mm 40.25 D
8.73 mm 38.62 D
9.13 mm 37.00 D
USES:
- to calibrate the instrument
- to measure the optic radius of a gas permeable lens
only

Lensco - Meter

Question 1
What will be the reading error of a keratometer,
if readings of 44.67@180/44.87@090 where
recorded when a 44.75D steel ball was
measured?

.
b. Con Ta Check
- is a mirror that
holds a lens
with a known
radius
• Used to:
- calibrate instrument
- check the front and
back curve of a rigid lens

c. Inns Disc
– keratometer attachment used to extend the range of the instrument
to +30.00 D….. +61.00D
How it works:
A spherical lens is placed over the aperture on the front of the keratometer
(that side facing the patient), the normal range of 36.00 D to 52.00 D can be
extended.
A +1.25 sphere extends the range of the keratometer 9.00 D to 61.00 D
A -1.00 sphere extends the range an additional 6.00 D flatter to 30.00D

d. Topogometer
- attachment used to measure larger areas of the cornea
- has a vertical and horizontal scale attached to the front of
the keratometer
- has been used in the analysis of keratoconic cornea
How it works:
- the patient looks on the target and the instrument is
focused on the central keratometry reading, the target
can be moved laterally along the track until the focused
reading begins to change
- an optic cap zone can be calculated and a nomogram is
used to assess the lens design

Topogometer
(www.wotol.com)
319/30/2020
Tatiana Mighiu

Contact Lens Theory 1 Course 10
2.Radiuscope
(Optic Spherometer or Binocular Digital Radius Gauge)
Advantages - more accurate than the keratometer
- simple to use
Disadvantage - costly
32
Optical system used in conjunction with a
microscope to measure the curvature of a
surface in radius of millimeters
Uses Drysdale method
9/30/2020

Radial target as seen looking through the microscope
Objective lens
Real image
Aerial image Concave lens mount

Radiuscope uses
a. Measures the base curve of rigid lenses
(using the concave lens mount)
b. Measures the convex radius of a spherical rigid lenses
(using the concave lens mount)
c. Measure of a secondary (peripheral) curve
d. Lens inspection ( edge thickness, scratches)
e. Determine lens warpage

Verification of a rigid contact lens

AO Radiuscope
Radiuscope

2. MOR as it relates to contact lens fitting
Types
1. Rx is spherical
MOR is spherical
2. Rx is sphero-cylindrical
MOR is spherical
MOR is sphero-cylindrical Axes 90 apart
MOR is spher0-cylindrical Axes NOT 90° apart

1. RX is a spherical prescription
MOR is a spherical prescription
The MOR is added algebraically added to the Rx
E.g.1 Rx +2.25 D E.g.2 Rx -4.00 D
MOR +0.50 D MOR +0.75 D
______________ _____________
New Rx + 2.75 D New Rx -3.25 D

2. Rx is a sphero - cylindrical prescription
MOR is a spherical prescription
The MOR is added algebraically to the spherical
portion of the Rx
E.g.1 Rx -3.25 – 0.50 x 90
MOR +0.75
___________________
New Rx -2.50 -0.50 x 90

3. Rx is a sphero – cylindrical lens
MOR is a sphero-cylindrical lens
Axes 90° apart
The RX and MOR are transposed so that both axis are
the same and then the prescriptions are added
algebraically
Step 1. Transpose MOR so that axis is the same
with Rx
Step 2. Add Rx prescription to the transposed
MOR

E.g.1 Rx -2.00 -1.25 x 180
MOR +0.25 -0.50 x 90
Step 1 Transpose MOR axis at 180 as original
prescription
-0.25 + 0.50 x 180
Step 2 Add Rx with transposed MOR
Rx - 2.00 - 1.25 x 180
New MOR - 0.25 + 0.50 x 180
_______________
New Rx -2.25 – 0.75 x 180

4. Rx is sphero-cylindrical lens
MOR is a sphero-cylindrical lens
Axes NOT 90° apart
Step 1 -Transpose Rx and MOR into a “+” cyl form
Step 2 - Prescription with the axis closest to “0” is A
Step 3 - Determine δ as the difference of the 2 axis
NEW CYLINDER
Step 4 - Calculate the resultant cylinder with the formula:
__________________________
Rcyl = √ A2
cyl + B2
cyl + 2Acyl Bcyl cos 2δ

NEW SPHERE
Step 5 – Calculate the added sphere with formula:
Acyl + Bcyl – Rcyl
s = ---------------------
2
Step 6 – Calculate the new sphere with formula:
New sphere = A sph +B sph + s

NEW AXIS
Step 7 – Determine added axis φ from the following
formula
Bcyl sin 2δ
sin 2φ = ──────
Rcyl
Step 8 – Determine New axis with formula:
New axis = Acyl axis + φ
Step 9 – Write the new Rx in (-) cyl form

Homework 2 MOR Problems
1. A patient has the following spectacle prescription refracted at 10 mm
Rx OD - 2.75 + 1.00 x 045
OS - 3.25 + 1.00 x 065
He is fitted with contact lenses and the best visual acuity was
achieved when the following MOR was performed
MOR OD -0.50 + 0.75 x 135
OS +0.50 – 0.75 x 155
Calculate the new power required to fit this patient with contact
lenses. Give your answer in (-) cyl form and round to 1/8 of a
diopter.

Rx OD - 5.75 - 2.00 x 180
OS - 6.25 - 1.75 x 168
MOR OD -0.75 + 0.75 x 090
OS +0.25 + 0.75 x 078
diopter.

Rx OD + 3.75 + 1.00 x 045
OS + 6.25 + 1.00 x 065
MOR OD -0.50 + 0.75 x 80
OS +0.50 – 0.75 x 120
diopter.

4. A patient has been fitted with a contact lens with the following
power +15.75 -3.00 x 180. His visual acuity was not satisfactory with
this contact lens and he was over refracted and found that his
VA will be substantially improved if a contact lens that will carry an
additional power of plano +1.75 x 140 will be ordered.
What would be the power of the contact lens that has to be
ordered?

Contact lens theory 1 course 2

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