U.S. patent number RE29,229 [Application Number 05/309,443] was granted by the patent office on 1977-05-24 for contact lens having an index of refraction approximating that of human tears.
This patent grant is currently assigned to E. I. Du Pont de Nemours and Company. Invention is credited to Louis J. Girard, Whitney G. Sampson, Joseph W. Soper.
United States Patent |
RE29,229 |
Girard , et al. |
May 24, 1977 |
Contact lens having an index of refraction approximating that of
human tears
Abstract
A wettable contact lens of the type applied to the human eye to
correct vision deficiencies, said lens formed of a transparent,
dimensionally stable, solid material characterized by an index of
refraction approximating that of the human tears.
Inventors: |
Girard; Louis J. (Houston,
TX), Sampson; Whitney G. (Houston, TX), Soper; Joseph
W. (Houston, TX) |
Assignee: |
E. I. Du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
26976821 |
Appl.
No.: |
05/309,443 |
Filed: |
November 24, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
684364 |
Nov 20, 1967 |
03542461 |
Nov 24, 1970 |
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Current U.S.
Class: |
351/159.01 |
Current CPC
Class: |
G02C
7/04 (20130101) |
Current International
Class: |
G02C
7/04 (20060101); G02C 007/04 () |
Field of
Search: |
;351/160 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Apt, Article in Transactions of the Am. Academy of Ophthalmology
and Otolaryngology, vol. 69, Mar.-Apr. 1965, p. 357. .
Girard et al., Article in American Journal of Ophthalmology, May
1966, pp. 1109-1111 & 1113-1123..
|
Primary Examiner: Rubin; David H.
Attorney, Agent or Firm: Arnold, White & Durkee
Claims
We claim:
1. A contact lens for providing optical correction to a human eye,
said lens having a generally concavo-convex cross section with the
concave surface adjacent to the eye and comprising a transparent,
dimensionally stable, nontoxic polymer characterized by an index of
refraction no greater than 1.40 and closely matching the index of
refraction of human tears, and selected from the group consisting
of polymers of hexafluoroacetone-tetrafluoroethylene-ethylene,
polymers of perfluoroalkylethyl methacrylates, and polymers of
perfluoro-2-methylene-4-methyl-1.3-dioxolane.
2. The contact lens of claim 1 in which the posterior surface of
said lens conforms with the topography of a human eye, and said
polymer is characterized by an index of refraction of no greater
than 1.37.
3. The contact lens of claim 2 in the form of a scleral lens.
4. The contact lens of claim 1 characterized by an index of
refraction no greater than 1.35.
5. The contact lens of claim 1 in which said transparent
dimensionally stable, nontoxic polymer comprises a polymer of
hexafluoroacetone-tetrafluoroethylene-ethylene.
6. The contact lens of claim 1 in which said transparent,
dimensionally stable, nontoxic polymer comprises a polymer of a
perfluoroalkylethyl methacrylate.
7. The contact lens of claim 6 in which the transparent,
dimensionally stable, nontoxic polymer comprises a polymerized
perfluoroalkylethyl methacrylate in which the alkyl group contains
from .Badd..[.3 to 14.]..Baddend. .Iadd.4 to 12 .Iaddend.carbon
atoms.
8. The contact lens of claim 7 in which the transparent,
dimensionally stable, nontoxic polymer comprises a polymerized
perfluoroalkylethyl methacrylate in which the alkyl group contains
.Badd..[.3.]..Baddend. .Iadd.8 .Iaddend.carbon atoms.
9. The contact lens of claim 1 in which said transparent,
dimensionally stable, nontoxic polymer comprises a polymer of
perfluoro-2-methylene-4-methyl-1.3-dioxolane.
10. The contact lens of claim 9 in which the transparent,
dimensionally stable, nontoxic polymer comprises a copolymer of
perfluoro (2-methylene-4-methyl-1.3-dioxolane) and
tetrafluoroethylene.
11. The contact lens of claim 10 in which said copolymer is
composed of from 50 to 95 percent by weight of perfluoro
(2-methylene-4-methyl-1.3-dioxolane) and from 5 to 50 percent by
weight of tetrafluoroethylene. .Iadd. 12. A solid, dimensionally
stable corneal contact lens for providing optical correction to a
human eye; said lens comprising a transparent, dimensionally stable
non-toxic polymer and being characterized by reduced aberrations
due to reflected light; said lens having both an anterior surface
and a posterior surface shaped to provide optical correction; said
lens having a refractive index no greater than 1.40 and having
sufficient permeability to oxygen and carbon dioxide to permit
circulation therethrough for nutrition of the cornea.
.Iaddend..Iadd. 13. The contact lens defined in claim 12 wherein
said lens has a Knoop hardness of at least 2. .Iaddend..Iadd. 14.
The contact lens defined in claim 12 wherein said lens has a Knoop
hardness of at least 4. .Iaddend..Iadd. 15. The contact lens
defined in claim 12 wherein said lens includes minute holes to
permit transport of oxygen to the cornea. .Iaddend. .Iadd. 16. The
contact lens defined in claim 12 wherein said polymer has
sufficient permeability to oxygen and carbon dioxide to permit
circulation therethrough for nutrition of the cornea.
.Iaddend..Iadd. 17. The contact lens defined in claim 12 wherein
the periphery of the lens is rounded to provide a comfortable fit.
.Iaddend. .Iadd. 18. The contact lens defined in claim 12, wherein
said lens has a refractive index closely matching the index of
refraction of human tears. .Iaddend.
Description
BACKGROUND OF THE INVENTION
Although known and used for some years, contact lenses have not
achieved full acceptance by most members of the public having
vision difficulties requiring correction. Contact lenses of the
scleral type cover a substantial area of the eye during use,
thereby sealing off circulation of tears and the atmosphere. This
causes partial asphyxiation which affects the metabolism and vision
of the eye. The more recent corneal lenses, because of their
relatively smaller size and lighter weight, usually cause less
irritation to the cornea, and have achieved greater acceptance.
However, the "break-in" period necessary to accustom the cornea of
the wearer to support a corneal lens, while varying from person to
person, depending upon the individual's sensitivity of the cornea,
usually extends over a considerable period of time. Some persons
have found contact lenses to be intolerable because of the eye
irritation resulting during and after any extended period of
continuous wear.
Effort has been made to design both scleral and corneal lenses to
fit more comfortably. For example, U.S. Pat. Nos. 2,240,157-Gagnon
et al and 2,330,837-Mullen suggest methods for producing a scleral
lens in which the scleral band fits the contour of the eye of the
wearer. However, the corneal portion of the lens clears the cornea
and limbus, so that the lens never contacts the cornea when worn.
Such lenses have generally required the use of an accessory fluid
in the space between the cornea and the ground-out corneal area of
the lens. Application of such lenses to the eye is difficult and
painful, and they have not achieved wide popularity.
It has been suggested that eye irritation could be reduced by
designing the contact lenses with various radii of curvature. U.S.
Pat. No. 2,544,246-Butterfield suggests that the corneal lens have
an inner spherical central area and an outer marginal portion
formed by a series of separate and distinct steps to introduce a
parabolic fit. U.S. Pat. No. 2,510,438-Tuohy, discloses a contact
lens having a radius of curvature on its concave side slightly
greater than the radius of curvature of the cornea, with an
increasing clearance at the marginal areas of the lens. Feinbloom
U.S. Pat. No. 3,227,507 discloses the production of a corneal
contact lens having an inner ellipsoidal surface. While lenses
disclosed in the foregoing patents offer an improvement over
scleral type lenses, they nevertheless tend to cause considerable
irritation to the eye, and on the average cannot be worn in excess
of ten hours.
Use of softer material in the production of contact lenses to
provide a more comfortable lens has also been suggested. U.S. Pat.
No. 3,228,741-Becker discloses the use of a filler, transparent,
hydrocarbon substituted polysiloxane rubber as a contact lens
material. Lenses of this material are said to possess a softness
similar to that of the upper lid of the human eye, a high
permeability to carbon dioxide, oxygen, and water vapor, and an
index of refraction ranging from 1.49 to 1.56, depending upon the
amount of filler material used. Because of their soft, elastic
characteristics, such materials do not lend themselves to
consistent production of high quality contact lenses, nor are they
dimensionally stable.
Wichterle et al. U.S. Pat. No. 3,220,960, discloses a hydrogel
material consisting of from 20 to 90 percent of an aqueous liquid,
and a cross-linked hydrophilic polymer. However, lenses made of
such materials are handicapped by optical problems. Notably, the
effective power of the lens changes as it is worn. Also, such
lenses may be torn easily while in the hydrated state, or broken in
the dehydrated state, thus adding a handling problem to their use.
Moreover, such porous hydrogels are receptive to bacterial invasion
and proliferation. Despite assertions to the contrary, it has been
found that such hydrogel materials do not afford extended periods
of comfortable wear in a high percentage of cases. An excellent
discussion of the disadvantages of such lenses is found in
Precision-Cosmet Digest, Vol. 5, No. 9, April 1965.
Due to the limitations of the foregoing materials, contact lenses
in commercial use at this time are almost universally manufactured
from polymethyl methacrylate, an optically clear, moldable,
synthetic polymer material characterized by an index of refraction
of 1.49. However, as a result of the problems noted above in
producing a lens which fits comfortably, lenses made from
polymethyl methacrylate can be worn only for relatively limited
periods of time.
It is therefore a primary object of the invention to provide an
improved contact lens.
Another object of the invention is to provide a contact lens which
affords a comfortable fit, and which may be worn for extended
periods of time without causing irritation to the eye.
Yet another object of the invention is to provide a contact lens
having an index of refraction approaching that of human tears.
A further object of the invention is to provide a contact lens
whose posterior surface conforms with the anterior surface of the
human cornea and/or sclera.
It is still another object of the invention to provide a contact
lens which significantly diminishes the aberrations due to
reflected light occurring with conventional contact lenses of the
prior art.
SUMMARY OF THE INVENTION
It has now been discovered that contact lenses which can be worn
comfortably over extended periods of time are attained if a
wettable lens is fabricated from a transparent, dimensionally
stable, solid material characterized by an index of refraction
approximating that of the human tear. This may be achieved in
accordance with one aspect of the present invention by providing a
contact lens of such a material in which the posterior surface of
the lens conforms with the varying topography of the anterior
segment of the human cornea and/or sclera.
The present invention takes advantage of the fact that differences
between the indices of refraction on either side of an interface of
a given curvature will result in a refractive or vergence power
change at this interface depending upon the curvature and the
indices of refraction. The refractive indices of the synthetic
polymer materials most widely used as contact lenses vary from a
low of 1.49 to a high of about 1.55. However, the total or combined
index of refraction of the eye as an optical instrument averages to
about 1.3375, or almost equal to that of the human tears
(1.336).
The significant difference between the index of refraction of the
eye and that of the material of which present lenses are composed
limits the number of modifications, or so-called fitting curves
which may be placed on the posterior aspect of the lens before the
optical quality of the lens is impaired. Thus, when contact lenses
made of a material having a higher index of refraction than the
cornea are modified excessively within the optical zone of the
lens, the resulting aberration due to the difference between the
indices of refraction will significantly alter the visual acuity.
This is particularly true in the case of flush-fitting therapeutic
lenses where the curvature markedly departs from sphericity.
It has now been discovered that by using a transparent,
dimensionally stable, solid material, characterized by an index of
refraction approximating the index of refraction of the human tears
and "workable" to the extent that it can be either molded or ground
into a lens, any number of fitting curves or modifications can be
made on the posterior aspect of the lens in order to give the best
lens-corneal relationship and thus provide the optically perfect
lens. The only optical requirement of lenses made in accordance
with the invention is that the front surface of the lens be
characterized by a curve of such power necessary to correct the
refractive error of the eye. In air, lenses of the invention are
aberrated when the anterior surface is spherical and the posterior
surface is aspherical or irregular, i.e., conforms with the varying
topography of the anterior segment of the human cornea and/or
sclera. However, once the irregularity of the posterior surface is
optically eliminated by placing it in contact with a medium of
similar index of refraction, as for example the tear film, the
aberrations optically disappear.
The present invention affords the production of contact lenses of
both the corneal and scleral type which may be worn for periods of
time considerably exceeding the average 10 hour period which is
characteristic of conventional lenses of the prior art in
accordance with the invention, "flush-fitting" lenses can be
provided whose posterior surfaces precisely match the contour of
the cornea of the wearer, thus affording a perfect, and hence
comfortable fit.
In addition, lenses may be constructed in accordance with the
invention in which the posterior aspects of the lenses are ground
or molded by standard methods to provide an optical curve thereon.
While such lenses do not afford the full advantages of comfort of
the "flush-fitting" lenses, they nevertheless exhibit a marked
improvement over the conventional lenses of the prior art. Notably,
and quite unexpectedly it has been found that such lenses
manufactured in accordance with the invention considerably
diminish, and in some cases eliminate, the aberrations due to
reflected light as manifested by halos, glare and streamers which
normally occur with conventional prior art contact lenses when the
wearer is exposed to bright lights shining into the eyes.
A critical feature of the invention is that the lenses thereof be
constructed from a transparent, dimensionally stable, solid
material characterized by an index of refraction approximating that
of human tears. The transparency of the material should be adequate
to assure good vision of the wearer of the lens, although the
material need not necessarily be crystal clear. Materials having
transparency characteristics similar to polymethyl methacrylate are
preferred.
The refractive index of the material suitable for use in the
practice of the invention should approximate that of human tears,
i.e., must be no greater than 1.40. It is preferred, however, that
the materials exhibit indices of refraction more nearly approaching
the value of 1.336, the index of refraction of human tears.
Generally speaking, when lenses are produced which are
characterized by posterior surfaces matching the topography of the
cornea of the wearer, indices of refraction of below 1.37 are
preferred. Of course, optimum results are achieved when the
material used exhibits an index of refraction below 1.34 and
approaches the value characteristic of human tears (1.336).
Materials useful in the practice of the invention must also be
characterized by good dimensional stability under the conditions
normally encountered in the manufacture and use of the contact
lenses made therefrom. In this connection it is important that the
materials retain their dimensional stability under conditions that
would be encountered when the lenses are removed from the eye and
stored. Thus, the materials contemplated herein differ
significantly from hydrogel materials which lose their dimensional
stability when taken out of an aqueous environment.
The materials used to provide lenses in accordance with the
invention should also be solid, i.e., have a Knoop hardness of at
least 2, and preferably should be characterized by a hardness of 4
or more in order to facilitate the manufacture of contact lenses by
the conventional methods known in the art. Of course, it is
essential that the materials exhibit nontoxic properties so as to
be compatible with the living tissue of the human eye.
Included among the materials which can be used in the practice of
the invention are the terpolymers of
hexafluoroacetone-tetrafluoroethylene-ethylene (HFA/TFE/E
terpolymers) obtained from a reaction medium containing ethylene
and tetrafluorethylene in 1:1 molar ratio and hexafluoroacetone in
large molar excess, with the hexafluoroacetone as a complex with
ROH in which the molar ratio of ROH with hexafluoroacetone may vary
from 0.8-2.5 R may be hydrogen or alkyl.
Such HFA/TFE/E terpolymers have been found to be characterized by
an index of refraction of the order of 1.39 and a Knoop hardness of
about 8.6. Contact lenses in accordance with the invention can be
molded or ground from such materials by methods well known to those
skilled in the art.
A further class of materials useful in the practice of the
invention include the polyperfluoroalkylethyl methacrylates of the
type disclosed in U.S. Pat. No. 3,282,905, Fasick et al. Such
methacrylates have been produced exhibiting indices of refraction
of the order of 1.368-1.374 and a Knoop hardness of above 2. Such
polymers are characterized by excellent clarity and are moldable at
130.degree.-140.degree. C.
By far the most preferred materials for use in the production of
lenses in accordance with the invention are the copolymers and
terpolymers of perfluoro-2-methylene-4-methyl-1.3-dioxolane,
referred to hereinafter as PMD. The preparation of PMD as well as
copolymers thereof is described in U.S. Pat. Nos. 3,307,330,
Niedzielski et al. and 3,308,107, Selman et al. The refractive
index of PMD homopolymer is in the range of 1.3308-1.3345, or
somewhat below the optimum value of 1.336, the index of refraction
of human tears. It has been found in this respect that the
copolymers of PMD and tetrafluorethylene provide materials which
are especially suitable in the practice of the present invention in
that they exhibit refractive indices in the desired range, are
characterized by good clarity, absence of color and haze, and
further exhibit good hardness properties. As noted in the patent to
Niedzielski et al. (3,307,330), such copolymers also exhibit
permeability to oxygen and carbon dioxide. Accordingly, when
contact lenses constructed of such materials are worn, oxygen and
carbon dioxide are circulated through the lens, as well as
underneath and around it, thus increasing the nutrition of the
cornea. It has been found that a copolymer containing 92 parts by
weight of tetrafluoroethylene with eight parts by weight of PMD was
characterized by an index of refraction of 1.349 and a Knoop
hardness of 4.9. The molding temperature of the copolymer was
350.degree. C. In this respect, copolymers of PMD and
tetrafluoroethylene characterized by optimum indices of refraction
and lower molding temperatures as well as higher values of Knoop
hardness may be prepared by copolymerizing from 10 to 20 weight
percent of tetrafluoroethylene with from 80 to 90 weight percent of
PMD.
It is essential that the lenses made in accordance with the
invention be wettable by human tears. In this respect wettability
may be determined by placing a drop of liquid on the substance
whose wettability is to be tested and observing and measuring the
receding and advancing contact angles, i.e., the angles the liquid
makes on the solid surface at the three phase boundary as measured
through the liquid in order for the material to be designated
"wettable" the receding angle must be 0.degree. and the advancing
angle must be less than 90.degree..
While many of the materials useful in the practice of the invention
may exhibit inherent wettability, as for example, the HFA/TFE/E
terpolymer mentioned above, it may be necessary to impart
wettability to the surface of contact lenses made from other
materials useful in the practice of the invention which do not
exhibit wettability. All of the materials mentioned above can be
treated to impart wettability, and thus render the surfaces of
contact lenses made therefrom hydrophilic. This may be achieved
with some polymers by treating the materials with a wetting agent
such as, for example, a 0.04 percent aqueous solution of
alkyldimethylbenzylammonium chloride. Such treatment simply
involves applying the wetting solution onto the lens or polymer
surface and subsequently washing the lens with water to remove the
applied solution. If a continuous film of water remains, the
polymer wets satisfactorily. If beading occurs, wettability is
questionable. Polymethyl methacrylate lenses of the type
conventionally used are wetted by this procedure, and remain
wettable after being placed in the eye. Other materials, notably
the TFE/PMD copolymers and polyperfluoroalkyl ethyl methacrylates
require a more vigorous treatment, such as exposure to corona
discharge or a sodium etchant, to attain wettability. Other means
for rendering contact lenses of the present invention wettable will
readily occur to those skilled in the art.
Contact lenses of the invention may be produced from the above
materials by the use of commercial contact lens grinding equipment
and techniques well known to those skilled in the art. The lenses
may be molded, machined and polished by known methods. In the event
it is desired to produce a flush-fitting lens, i.e., a lens whose
posterior surface precisely matches the varying topography of the
cornea of the eye, the molding technique described in the article
"Flush-Fitting Scleral Contact Lenses" appearing in the American
Journal of Opthalmology, Volume 61, No. 5, Part II, May, 1966, may
be used. Flush-fitting scleral lenses produced by that method are
direct copies of the anterior segment of the eye, including both
cornea and sclera, and follow in detail the surface contour of
both. Only a capillary layer of tears separates the lens thus
formed from the eye. Flush-fitting corneal lenses may be obtained
by using the foregoing method to provide a flush-fitting lens, and
thereafter cutting away the scleral band. The edges of the
remaining corneal portion may then be machined to provide
comfortable fit.
DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS
For further objects and advantages of the invention and for a more
detailed discussion of preferred embodiments thereof, reference is
to be had to the following description taken in conjunction with
the accompanying drawings in which:
FIG. 1 is a vertical cross section of a portion of a human eye to
which there has been applied a scleral contact lens of the type
disclosed in the prior art;
FIG. 2 is a vertical sectional view of a portion of the human eye
to which there has been applied a contact lens embodying one form
of the present invention;
FIG. 3 is a vertical section of a portion of the human eye to which
there has been applied a contact lens embodying another form of the
invention; and
FIG. 4 is a vertical section of a portion of the human eye to which
there has been applied a contact lens forming still another
embodiment of the invention.
Referring now to FIG. 1, the standard scleral contact lens 10 of
the kind disclosed in the prior art, when fitted to the human eye
12, parallels the curvature of the anterior sclera 14 and then
vaults the cornea 16, leaving between the anterior surface 18
(whose irregular contour has been exaggerated) of the cornea 16 and
the posterior surface 20 of the lens 10 a space 22 filled with
tears or a buffered solution. The posterior surface 20 of the
contact lens 10 is an optical surface which, with the anterior
curvature 24 of the lens, provides optical correction to the eye
12.
FIG. 2 illustrates a scleral contact lens 30 forming one embodiment
of the invention. Lens 30 is of generally concavo-convex cross
section and is constructed of a transparent, dimensionally stable
solid material characterized by an index of refraction
approximating that of human tears. By virtue of the fact that the
index of refraction of the material of which the lens 30 is
composed approaches that of the tear film of the eye 32, the lens
30 may be constructed such that it is flush-fitting, i.e., the
posterior segment 34 follows in exact detail the irregular surface
contour 36 of the anterior segment of the eye, including both the
cornea 38 and the sclera 40. Only a capillary layer of tears
separates the lens from the eye. Thus the lens shown in FIG. 2 does
not include an optical surface on its posterior base 34. Since the
posterior aspect of the lens 30 matches in detail the surface
contour of the cornea 38 and sclera 40 of the eye, it provides
virtually a "glove fit." Optical correction is achieved by grinding
a power surface on the interior surface 41 of the lens 30. Such a
lens is most satisfactory from the wearer's standpoint and can be
worn continuously for periods of time well exceeding 10 hours.
In FIG. 3, a "flush-fitting" corneal contact lens 60 of the
invention is shown fitted to a human eye. Lens 60 of generally
concavo-convex cross section is constructed of a material
characterized by an index of refraction approaching that of human
tears, i.e., not greater than 1.40 and preferably of a value more
closely approximating 1.336. The posterior surface 62 of the
corneal lens 60 conforms with the topography of the anterior
segment 64 of the eye 66. Such a corneal lens 60 may be constructed
by cutting away the scleral band of a flush-fitting scleral lens
such as shown in FIG. 2, and subsequently smoothing the edges 68 of
the corneal portion to provide a comfortable fit.
FIG. 4 illustrates a generally concavo-convex contact lens 80 of
the invention having both its posterior surface 84 as well as its
anterior surface 86 ground to provide optical correction. The lens
80 thus differs from the flush-type lenses shown in FIGS. 2 and 3.
While the lenses of the type shown in FIG. 4 do not provide as
comfortable a fit as those illustrated in FIGS. 2 and 3 and for the
reason that their posterior surfaces do not conform precisely with
the anterior surface contour of the cornea 88, the lenses
nevertheless are a marked improvement over the conventionally
available corneal contact lenses in that they significantly
diminish and often eliminate the aberrations due to reflected
light, as manifested by halos, glare and streamers which normally
occur with conventional contact lenses when the wearer is exposed
to bright lights shining into the eyes, as may occur during night
driving. The lens shown in FIG. 4 can be constructed from the
materials contemplated for use in accordance with the invention by
methods and procedures well known to those skilled in the art. If
desired, the lens 80 may be provided with minute channels or holes
90 to permit transport of tears and oxygen to the cornea. Such
channels may be drilled in the lens by conventional methods without
detracting from the optical properties of the lenses.
Generally speaking, lenses of the type illustrated in FIGS. 1-3
will be formed of polymer films having thicknesses varying from
about 3 to about 25 mm. Preferably, the lenses will have a
thickness of the order of 0.1-0.2 mm. minus lenses, and 0.15-0.50
mm. plus lenses.
The method of molding contact lenses of the type illustrated in
FIGS. 2 and 3 can be the same as has been used in the past for
standard scleral lenses. However, the subsequent manufacturing
technique differs considerably from past methods since the
posterior surface of a lens made in accordance with the invention
is not modified into an optical surface as in a standard scleral
lens, but remains a perfect replica of the cornea and sclera.
As mentioned above, a molding method which can be used to prepare a
flush-fitting scleral contact lens of the invention is described in
Volume 61, No. 5, Part II of the May, 1966, issue of the American
Journal of Ophthalmology. Generally speaking, this method involves
fitting a plastic casting shell device onto the eye, through the
stem of the funnel is injected the impression material consisting
of an opaque, rather viscous liquid prepared by mixing an alginate
powder with water. An example of such a material is ophthalmic
Moldite, a product manufactured by Obrig Laboratories, New York,
N.Y., and consisting of an alginate gelling agent which reacts when
mixed with water. Gelling of the material is retarded by an added
preparation which gives sufficient time for the material to be
placed on the eye and the molding shell centered.
After the alginate material is injected into the funnel and onto
the eye, sufficient time (about 3 minutes) is permitted to elapse
to permit the liquid to set to a gell impression which is then
removed from the eye. A mixture is prepared from water and powdered
plaster casting material and is then poured into the moist soft
alginate impression still attached to the funnel. The alginate mold
is vibrated while the mixture is poured into it, to prevent
formation of bubbles. After about ten minutes, the stone mold has
hardened enough so that it can be stripped from the alginate. The
resulting stone mold can then be used to make the lens into a
negative replica of the anterior corneal and scleral surfaces.
Flush-fitting scleral contact lenses of the invention may then be
produced by heating a sheet of material characterized by an index
of refraction approaching that of human tears to a temperature
sufficiently high to permit the sheet material to conform to the
surface of the mold upon the application of pressure. After
cooling, the resulting lens is taken from the die and the anterior
surface machined and polished to provide the desired optical
surface.
As above noted, a flush-fitting corneal lens, as illustrated in
FIG. 3, can be prepared by cutting away the scleral band of the
flush-fitting scleral lens, and treating the edges of the corneal
lens so that a comfortable fit may be attained.
As will be appreciated by those skilled in the art, it is also
possible to prepare contact lenses in accordance with the invention
through the use of conventionally available measuring devices such
as the Radioscope, the Keratometer and the Toposcope. While such
devices may be used to produce contact lenses whose posterior
portions closely approximate the anterior segment of the eye, such
lenses will not provide as satisfactory a fit as will be obtained
from the flush-fitting lenses described above.
In order that those skilled in the art may better understand how
the present invention may be practiced, the following examples are
given by way of illustration and not by way of limitation. The
lenses of examples 1-3 inclusive were prepared from the respective
polymer films which were heated to their softening temperature and
subsequently draped over the stone mold of a patient's eye. A
female mold roughly conforming to the shape of the stone mold was
then quickly pressed onto the polymer film and held tightly against
it until the polymer hardened. Except for the HFE/TFE/E terpolymer
(which was found to be inherently hydrophilic) the lenses
constructed from the other polymers were treated to impart
wettability.
EXAMPLE 1
A terpolymer was prepared by reacting hexafluoroacetone,
tetrafluoroethylene and ethylene in a mole ratio of 1.6:1.0:1.0,
the hexafluoroacetone being in the form of a 1:1 mol complex with
methanol. The resulting polymer was characterized by an index of
refraction of 1.392, a Knoop hardness of 8.6 and a molding
temperature of 230.degree. C. As previously mentioned, the polymer
exhibited inherent wettability. A good scleral lens was molded from
this polymer and appeared frosted, due to the irregular contour of
the posterior surface of the lens. This frosting was substantially
reduced when the lens was placed in water.
EXAMPLE 2
A scleral lens was molded from polyperfluoroalkylethyl methacrylate
of the formula ##STR1##
The polymer was obtained by polymerizing a mixture of monomers of
the above formula in which mixture the monomer weight fractions
were as follows: n=1, 1 percent; n=2, 50 percent; n=3, 35 percent;
n=4, 7 percent; n=5, 1 percent. The resulting polymer had an index
of refraction of 1.374 and a Knoop hardness of 2.03. Its molding
temperature was about 130.degree. C. A good scleral lens was
produced from this polymer which appeared partly frosted on its
posterior side and was opaque when viewed in air. However, the lens
became fairly transparent when placed in water.
EXAMPLE 3
By the method outlined above, a scleral lens was obtained from a
polyperfluoroalkylethyl methacrylate of the formula set forth above
in which n equaled 3. The resulting polymer was characterized by an
index of refraction of 1.368, a Knoop hardness of 2.08 and a
molding temperature of 140.degree. C. A good scleral lens was
obtained from the material. The posterior aspect of the lens
appeared frosted and the front surface was fairly smooth. The lens
was opaque when viewed in air but became essentially clear when
immersed in water.
EXAMPLE 4
A contact lens was prepared from a copolymer of tetrafluoroethylene
and perfluoro-2-methylene-4-methyl-1.3-dioxolane (PMD). The
refractive index of the copolymeric material was 1.3380 and the
Knoop hardness was 4.9.
By virtue of the present invention there are provided contact
lenses which are significantly improved over the lenses disclosed
in the prior art. Notably, the contact lenses of the invention, by
virtue of being constructed of materials characterized by indices
of refraction approximating that of human tears, can be contour
fitted to the eye to provide a perfect fit and thus enable the
lenses to be worn for periods of time considerably exceeding the
periods during which conventional lenses may be worn. Furthermore,
the lenses of the invention have been found to reduce and often
eliminate the aberrations due to reflected light which occur with
ordinary contact lenses.
It should be understood that while the present invention has been
described in considerable detail with respect to certain specific
embodiments thereof, it is not to be considered limited to those
embodiments, but may be used in other ways without departure from
the spirit of the invention or the scope of the appended
claims.
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