Fitting
& Verification
Dyer Chart Method Examples
Example 1:
Spectacle Rx: -3.25-1.00
x 180
Vertex distance = 12 mm.
K: 43.50/44.50
Given the data above and referencing the Dyer nomogram, determine
the following rigid contact lens parameters: base curve, optical zone
size, peripheral curve size and surface power, over all diameter, and
power.
The first step is to notice if there is any residual astigmatism, or
if it is all contained in the cornea. In this case, since the amount
of astigmatism in the refraction equals that found in the cornea there
is no residual astigmatism present. This means that a spherical rigid
contact lens should completely correct this patient�s ametropia. After
transposing the Rx to minus cylinder form, we can now drop the cylinder
and axis. And since the power is less than � 4.00 diopters there is
no need to compensate for vertex distance. Therefore the combined power
of the tear lens and contact lens will need to equal -3.25 D.
To come up with the correct base curve, we first locate the flattest
corneal meridian in the left hand column of the Dyer Chart on page 12.
The flattest meridian in this case is 43.50. We then find the amount
of existing corneal astigmatism along the top of the chart which in
1.00 D in this case. So we can see that the indicated base curve is
equal to 44.00 D.
For other lens parameters such as diameter, peripheral curves and thickness,
we reference the charts on page 13. Finding 44.00 diopters in the left
hand column of the �Dyer Chart For Other Lens Parameters� we notice
that indicated diameter is 8.4 mm, with a 7.0 optical zone. If the lens
contains only one peripheral curve its curvature would be 40.50 and
contain a width of 0.2 mm. If the lens has two peripheral curves, the
second would contain a curve of 36.50 D while a possible third curve
would be the flattest at 30.00 D with a width of 0.3 mm.
The recommended base curve according to the Dyer chart is 44.00 D which
is in fact 0.50 D steeper than 43.50, the flatter of the two corneal
meridians. The approximate power of the tear lens, therefore will be
equal to +0.50 D. What do we need to add to the +0.50 D in the tear
to equal the desired combined power of -3.25? The answer is -3.75. Which
means the power of the contact lens in this case will need to be -3.25
D.
What about thickness? Lets reference the second chart on page 13. In
this case the lens diameter is 8.4 and the power is -3.75 D. We find
the power along the left hand column and the diameter across the top
and it yields a center thickness of 0.12 mm. We will use 8.4 mm as the
diameter since that is the closest diameter given on the chart.
Example 2:
Rx: +3.25 +1.75
x 010
Vertex distance = 10 mm
K: 44.75/46.50
Given the data
above and referencing the Dyer nomogram, determine the following rigid
contact lens parameters: base curve, optical zone size, peripheral curve
size and surface power, over all diameter, and power.
All of the astigmatism is in the cornea, no residual astigmatism. Therefore
a spherical rigid contact lens should correct this patient�s ammetropia.
Transpose to minus cylinder form, +5.00 -1.75 x 100
drop cylinder and axis, +5.00
compensate for vertex distance. +5.25 combined power of tear lens and
contact lens at the corneal plane.
Base curve according to Dyer Chart is 45.50 which is 0.75 D steeper
than K.
Power of
tear lens therefore is approximately +0.75 D.
Since the combined power of the contact lens and tear lens at the corneal
plane needs to equal +5.25 and we already have 0.75 D in the tear, the
power of the contact lens in this case need only be +4.50 D.
Other lens parameters according to the Dyer Chart:
Total diameter: 8.1 mm
Optical zone diameter: 6.7 mm
Peripheral curve: 42.00 D
Peripheral curve width: 0.20 mm
Thickness: 0.29 mm