Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to improvements in contact
lenses and more particularly to the improvement of contact
lenses made Erom so~t hydrophilic plastics.
The development of soft hydrophilic plastics contact
lenses has made it possible for the wearer to wear the
lenses for lengthy periods of time without any irritation
of the eye. However, a major difficulty has been exper-
ienced with such lenses as, owing to the flexibility of
the plastic lens, the lens tends to conform to the corneal
shape and thereby transmits any corneal astigmatism through
the lens to its outer surface. Thus,for example, a
circular spherical hydrophilic lens of the correct fitting
parameters, when placed on an eye with a corneal astigmat-
ism, will tend to conform to this astigmatic shape of the
cornea and thus transmits this astigmatism to the anterior
surface of the lens resulting in ineffective visual
correction.
Various attempts have been made to modify the lens
design by providing a lens with a prism ballast or by
truncation and by combining therewith an equivalent
astigmatic surface which may be on the posterior or
anterior surface of the lens.
The prism ballast and/or the truncation, is then
located at a required axis so as to locate the axis of the
~ astigmatic surface in such a manner that the resultant
cylinderof the cornea is neutralised.
Although optimum visual correction can be achieved in
this manner, difficulties arise in keeping the lens located
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in the proper position. Not only may the action of the lids
in blinking rotate the lens off the desired axis, but also
` the lens may fail to remain located in the required position
when the head is tilted due to the gravity effect on the base
of the locating ballast prism. On the other hand, it is not
possible to lock the circular conventional hydrophilic lens
in any predetermined axis, whilst maintaining an ade~uate tear
flow beneath the lens at all ti~les.
It is an object of this invention to overcome these
problems and to assure the position of the lens on the cornea
irrespective of movements of the eye lids and movements of the
head. To achieve this object, the invention takes advantage
of the variation in scleral shape caused by the ocular muscles
attached to the external surface of the eye ball.
Four ocular muscles are located in different directions
on the eye ball and are generally defined as superior and
inferior muscles attached to the eye ball in vertical opposition
to each other and nasal and temporal muscles sometimes also
- referred to as lateral and medial muscles and attaahed
horizontally opposed to each other. These muscles, which are
inserted in the sclera of the eye ball, form between them
depressions which according to the invention are used to locate
the contact lens on the cornea. The object of the invention is
therefore achieved by providing a soft hydrophilic plastic corneal
- contact lens comprising a substantially circular op-tical
correction zone which is shaped for the correction of astigmatism,
said lens being soft enough to tend to conform to the shape of
the cornea when placed on an eye with corneal astigmatism, at
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least one discrete lug or flange extending from the periphery of
said correction zone, said lug having a radial width measured
from the periphery o said correction zone which is less than the
radius of said correction zone, said lug being adapted to engage
the sclera of the eye adjacent to the cornea at one of the
depressions on the eyeball formed between the four rectus muscles
attached to the sclera of the eye to thereby prevent rotation of
the lens to maintain the re~uired astigmatism correction. The
lens itself can then be p.rovided with the required astigmatic
anterior and/or posterior surface to correct any residual
astigmatism due to corneal and/or optical surfaces posterior to
the cornea to provide optimum correction of the ametropia.
By adapting the lug or flange to the depression on the
cornea to which the lens is fitted, the position of the lens can
be secured without inhibiting the tear flow beneath the lens.
Although the location of the lens can be achieved wi.th
one single lug or flange, it may be advantageous to provide the
lens with two opposing lugs or four lugs so as to balance the lens
and thus avoid any strain on the lens due to movement of the head r
and at the same time to engage more than one depression formed
between the ocular muscles. It is, however, possible also to
provide the lens with three lugs or flanges which may facilitate
the proper location of the lens on the cornea.
The particular shape of the lugs or flanges can be
determined in each case by the use of clinical trial lenses struc-
tured with various radii of curvature for the scleral lugs or
flanges.
Several embodiments of the invention will be described
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in detail hereinafker in connection with the drawings in which:
Figure 1 shows schematically an eyeball with muscles
attached thereto and the positioning of a lens according to
the invention in relation to these muscles.
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Fig. 2 shows under A, s, C and D various forms of
a corneal lens according to the invention,
Fig. 3 shows a cross~section of the lens shown in
ig. 2D along line III-III, and
In Fig. 1 the eyeball or its sclera is shown
schematically supported by its four ocular muscles namely
the superior muscle 2, the inferior muscle 3 attached to
the eyeball 1 in vertical opposition and the lateral or
nasal muscle 4 and the medial or temporal muscle 5 attached
to the eyeball 1 in horizontal opposition embedded in the
sclera O
~` A corneal lens 6 is positioned on the cornea 7 of the
eye and is located by lugs 8 and 9 of the lens which fit
into depressions between the muscles 2 and 5 and 3 and 4
respectively. Although generally two lugs will be suffic-
ient to locate the corneal lens on the eyeball, any other
number of lugs, for example four lugs, can be employed as
indicated by the additional lugs 10 and 11 shown in dotted
lines in Fig. 1.
In E'ig. 2 various preferred forms of the corneal
lens accord1ng to the invention are shown. While Fig. 2A
shows a lens with a central optical correction zone 22 and
a single flange 12. Fig. 2B shows a double flanged lens
having oppositely arranged flange's 13 and 14.
25 ~ In some cases a triple flanged lens wi-th flanges 15,
16 and 17 may be required as shown in Fig. 2C. To achieve
a comple-tely balanced lens arrangement the four- lug
arrangement as shown in Fig. 2D can be used whereby the
lugs 18, 19, 20 and 21 attached to the optical correction
zone 22 fi-t into the depressions in the sclera as shown
in Fig. 1 by the lugs 8, 9/ 10 and 11.
A cross-section through the lens arrangement of
Fig. 2D is shown in Fig. 3. The central optical correction
zone 22 covers the corneal zone 23, while the lugs 18, 20
and 21 (lug 19 not being shown in this cross-section) are
adapted to contact the depressions in the sclera as
explained a~ove. ~
The dimensions of the central optical correction zone
22 and the ~lange or flanges 12 to 21 are selected according
to -the ocular shape of the sclera, corneal curvature/s
and corneal diameter.
The power of the central optic zone 22 is determined
by accepted professional procedures for power and axis
of cylinder.
The parameters of the lens once determ.ined either
by calculation or trial lens fitting may be generated by
conventional lathing and polishing or moulding of the lens
plastic to the required shape and dimensions.
The lens design permits the selection of a range of
basic lens parameters which covers the majority of eyes
requiring this type of amptropic correction. An example
oE a fitting set would be :
25` ABBREVIATIONS
.:
BASE CURVE O~TIC RADIUS BCOR
RADIUS SCLERAL FLANGE RSF
DIAMETER OF OPTIC ZONE DO
FLANGE WIDTHSTANDARD 3 MM.
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COR _ _ RSF
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7.8 13.0 12,0
7.8 13.5 12.0
8.1 13.5 12.0
8.1 14.0 12.5
8.1 14.5 13.0
8.4 13.5 12.0
8.4 14.0 12.5
8.4 14.5 12.5
8.7 14.0 12.5
: 8.7 14.5 13 ~
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`The size of the fitting set is limited only by the
number of lenses and variatlons in paramaters desire.
The foregoing detailed description of some forms
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of the invention is not to be considered as limiting the
invention to the particular examples. It must be understood
that variatlons can be made without departing from the
scope of the invention.
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