Biomedical Engineering Reference
In-Depth Information
Appendix D: Javal's Rule
The cornea and crystalline lens are the primary sources of ocular astigmatism.
Corneal toricity can be measured directly with a keratometer or corneal topog-
rapher. When the keratometry readings are known, the total amount of ocular
astigmatism, as measured in the spectacle plane, can be estimated using Javal's rule:
Est. ocular astig
=
(corneal astigmatism)(1.25)
+
(
−
0.50
×
090)
where Est. ocular astig is the estimated ocular astigmatism in the spectacle plane
and corneal astigmatism
is the
correction
for corneal toricity in minus cylinder form
(as measured in the corneal plane).
What is the rationale for this rule? Corneal astigmatism is multiplied by 1.25 to
correct for lens effectivity (Chapter 7). In addition, Javal's rule assumes that the
average crystalline lens has a small degree of against-the-rule astigmatism that is
correctable with a spectacle lens power of pl
−
0.50
×
090.
Let's look at an example.
A patient's K's are as follows:
44.00/090
43.00/180
Based on Javal's rule, what is the estimated ocular astigmatism in the spectacle
plane?
Figure D-1A shows the corneal powers on a lens cross. The required cylindrical
correction is also given (in minus cylinder form). While the cornea has 1.00 D of
with-the rule-astigmatism, this is partially mitigated by the presumed against-the-
rule lenticular astigmatism, resulting in a predicted with-the-rule
ocular
astigma-
tism of
−
0.75 D.
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