Note: Descriptions are shown in the official language in which they were submitted.
$
- This invention relates to soft hydrogel contact lenses
and more specifically, is directed to soft contact lens design.
As is known in the art, contact lenses are frequently
made from polymethyl methacrylate. Such lenses are known as the
"hard lenses". Many people cannot adapt to the presence of a
hard lens in the eye and with others, the lens compromises the
physiolo~ical processes required for corneal metabolism. More
recently, new soft lens materials have been developed which avoid
some o the problems associated with the hard lens. One class of
such lens materials is described in U.S. Patents Nos. 2,976,576
and 3,220,960. These materials are hydrogels of a
sparingly cross-linked hydrophilic co~olymer
comprising a major amount of a monoester of an olefinic acid
from the group of acrylic and methacrylic acids ha~ring a single
olefinic double bond and a minor amount of polymerizable diester
of one of said acids, the diester having at least two olefinic
double bonds. A preferred hydrogel disclosed in the aforesaid
~atents is a slightly cross-linked material comprising a pre-
dominant quantity of 2-hydroxyethyl methacrylate. The hydrogel,
known as "hema", is used for contact lens fabrication because of
its ability to absorb water of hydration, typically from about
35 to 65% by weight of the hydrogel. The water renders the lens
flexible ar.d soft which properties enable it to mold to the
curvature of the eye. This is in contrast to the conventional
hard lens which maintains a rigid confiyuration that does not
always conform to the eye's curvature.
In V.S. Patent No. 4,056,496, hydrogels are disclosed
which are also suitable for soft lens fabrication. The hydrogels
are formed from a hydrophilic monomer from the group of dihydroxy-
alkyl acrylates and methacrylates, a substantially ~ater insol-
uble monomer from the group of alkyl acrylates and methacrylates
and preferably, a minor amount of an epoxidized alkyl acrylate
~9~s~
or methacrylate by a free radicaJ, bulk polymeri~ation process
in the substantial absence oE solvent.
It is known in the art that the conventional hard
contact lenses and many of the contemporary soft hydroyel lenses
may only be worn for a short duration of time, typically Eor
periods of time up to twelve hours. It is further known that
extended wear, for periods in excess of -twelve hours, particularly
during closed-eye periods (during sleeping hours) may cause long
term injury to the eye.
Perhaps -the most serious cause of injury arisin~ from
~xtended wear of the aforesaid lenses is oxygen deprivation due
to the lens covering a significant portion oE the corneal surface r~
thus acting as a barrier to contact of the cornea with an
oxygenated tear layer. This results in oxygen deprivation at
the cornea and interferes with the metabolic and physiological
requirements of the cornea.
It is known that the cornea requires a supply of
oxygen at its surface and relies upon oxygen diffusion from a
tear layer over its surface for almost all of its required supply.
During open-eye periods, the tear layer is oxygenated by atmos-
pheric oxycJen. During closed-eye periods, the tear layer is
oxygenated by the capillaries of the eyelid rather than oxygen
rom the atmosphere as when the eye is open. The partial pressure
o oxygen supplied from the capillaries is less than about 1/3
that supplied by the atmosphere. In the absence of a lens
acting as a barrier, -the oxygen supply to the cornea, both during ~ ~`
open-eye and closed-eye periods, is sufficient.
kh o w ~ r
It is-~w~r~ that a contact lens capable of continuous
wear should provide at least 3.5 I~l/cm -hr, preferably 6 1ll/cm2-
hr of oxygen to the corneal surface to avoid the physiological
complication arising from oxygen deprivation. ~lard contact
lenses r such as -those of methyl methacrylate, are not permeable
-- 2 --
g~
to oxygen, but through known lens design, permit some circulation
of ~ir to the corneal surEace. Contemporary hydrogel lenses,
though permeable to oxygen through the interstitial sp~ces of the
hydrogel material, are not sufficiently permeable to fully
oxygenate the cornea in -the cross-sections in which they are
fabricated. Hence, oxygen deprivation is also encountered with
these lenses.
The circulation of some oxygen using the aforesaid
lenses permits daily wear of the same with minimal non-reversible
damage to corneal physiology. I~owever, during closed-eye periods
when the oxygen supply is reduced to less than 1/3 the level of
opened-eye condition, known lens design does not permit sufficient
transfer of oxygen to the corneal surface to permit wear without
oxygen deprivation.
For purposes of definition herein, the term "daily
wear lens" and like terms are intended to mean a lens normally
worn during open-eye periods but not during closed-eye periods
(i.e., during periods of sleep.) The term "continuous wear
lens" and like terms are intended to mean a lens tha-t may be worn
as a daily wear lens if desired but which can also be worn for
extended periods of time (i.e., both during open-eye and closed-
eye~ periods), if desired.
It is an object of the invention -to provide a minus
prescription hydrogel contact lens tha-t can be worn on a
continuous or daily basis, if desired, withou-t removal from the
eye, both during opened-eye and closed-eye periods without
damage to the cornea.
Another object of this invention is to provide a
hydrogel minus prescrip-tion contact lens which can be removed
from the eye by the patient, handled and reinserted in the eye
without damage to the lens.
A further object of this invention is to provide a
-- 3
hydro~el minus prescription contact lens capable of continuous
wear which lens permits oxygen diffusion to the cornea in
sufficient quantlty to avoid the adverse effects of oxygen
deprivation; avoids physiological complications arising from
damaqe to the bulbar conjunctivia due to compression of the lim-
bal capi:Llaries; and avoids corneal-scleral wetting deficiencies.
~ n additional object of the invention is to provide
a hydrogel minus prescription contac-t lens which conforms to -the
shape of the eye.
The objec-ts of the invention are accomplished with a
combination of design features and hydrogel properties that .~
enable fabrication of the lens in substantially reduced cross- '~i
sectional thickness and mass (weight). In this respect, -the
maximum cross-sectional thickness of the lens does not exceed
0.15 mm for a daily wear lens and 0.10 mm for a continuous wear
lens. Other design features of the lens include a minimum dia-
meter of at least 12 mm and preferably, ranging between 13 and
17 mm; a reduced posterior peripheral curve width not to exceed
1.5 mm and preferably, total elimination of all posterior peri-
pheral curves so that the base curve is a monocurve that is
smooth, uninterrupted and preferably spherical; an anterior
lenticular curve preferably extending from the edge of the
optical zone to the periphery of the lens and havi.ng a radius
such that the edge thickness of the lens does not exceed 0.08 mm
and preferably, does not exceed 0.06 r~m; and physical properties
such that the lens is capable of handling and conforming -to the
curvature of the eye, at least in the periphery of the lens.
The hydrogel used for lens constructi.on ls one capable
of containing at least 35% water of hydration and must be
sufficiently rigid so as -to maintain i-ts shape in -the required
thin cross section while confo~ming to the eye. A preferred
class of suitable ma-terials is disclosed in United States Patent
~,9~
No. 4,056,496 supra.
With reference to the drawing, there is shown a cross
section of a hydrated minus soft lens fabricated in accordance
with the mos-t preferred embodimen-t of the inven-tion.
The inner surface of the lens, frequently referred to
as posterior surface, comprises base curve 1 having radius 2.
In accordance with this invention, -the base curve is preferably a
monocurve -- i.e., the posterior surface of the lens is smooth
and of a single radius. flowever, as is known in -the art, the
base curve may be provided with one or more peripheral curves
(not shown in the drawing), if desired, -though this is a lesser
preferred embodiment. ~lowever, if present, the peripheral curve
has a maximum band width of 1.5 mm and more preferably, a maximum
of 1.0 mm.
The front surface of the lens, frequently referred to
as the anterior surface, comprises power or prescription curve 3
having radius 4. The width of power curve 3 is known as the
optical zone of the lens (defined between points 5 and 6 of the
clrawing). This width is generally sufficient to cover most,
preferably all, of the cornea and hence, usually varies between
about 8 and 10 mm and more typically between 8 1/2 and 9 1/2 mm.
The anterior surface of the lens also is provided with
lenticular curve 7 having radius 8. The lenticular curve extends
from its junction with the optical curve to the outer periphery
or ed~e of the lens.
The overall diameter or chord of the lens is defined
as the distance between points 9 and 10. This diameter is at
least 12 mm, preferably varies between 13 and 17 mm and most
preferable is about 13.5 mm.
For daily wear, the lens has a maximum cross-sectional
thickncss at any point on its circumference not exceeding 0.15 mm
preferably not exceeding 0.10 mm and most preferably varying
5~
..~
between about 0.05 and 0.10 mm. For con-tinuous wear,the maximum
cross-sectional thickness of the lens at any point on its cir-
cumference does not exceed 0.10 mm, preferably does not exceed
0.08 mm and most preferably varies between about 0.03 and 0.08 mm.
The lenses of the subject invention are thinner than those of the
prior art, and the reduced thickness represents departure from
the prior art. The reduced cross-section permi-ts increased oxygen
diffusion through the lens thus avoiding corneal swelling tha-t
would otherwise result as a consequence of oxygen deprivation on
the corneal surface for an extended period of time. In this
respect, it should be noted that oxygen deprivation at the corneal
surface is a serious problem while depriving the conjunctivia of ~r
oxygen is not a problem since it receives oxygen from the vascular
system. Ilence, though a lesser preferred embodiment of the
invention, the cross-sec-tional thickness of the peripheral portion
of the lens, beyond the optical zone of the lens, can be thicker ~$
than the remainder of the lens without seriously compromising
corneal metabolism though it is preferred to maintain the overall
cross-sec-tional thickness of the lens as small as possible as this
reduces the mass of the lens that rests on the surface of the eye.
The edge of the lens is also of reduced cross-section
and preferably edge thickness as defined between points 11 and 12
of the drawing varies between about 0.03 and 0.08 mm and more
preferably, is about 0.06 mm. A reduced edge thickness within
the limits set forth is desirable as the edge will not interfere
with the eyelid with normal blinking and hence drying of the
scleral tissue is minimized.
The radius 2 of base curve 3 is within prior art
limits and is to some extent dependent upon the shape of the eye
to which the lens is fitted. The radius 4 of the power curve 3
is also within the prior art and is dependent upon the correction
provided by the lens. Finally, -the radius 8 of the lenticular
~i9~i~
~ curve 7 is that necessary to provide the desired edge thickness
of the lens. The lenticular curve extends from its junction
with the power curve to the outermost edge of the lens and to
reduce edge thickness, the curve must be steeper than the power
curve. Ilence, the radius 8 of the lenticular curve 7 is shorter
than radius 4 of power curve 3. Preferably, radius 8 is at
least 0.2 mm shorter than radius 4 and more preferably, a-t least `
0.5 mm. It should be noted that where the lenticular curve is
steeper than the power curve and particularly, in the absence
of posterior peripheral curvatures, the mass of the lens is
substantially reduced.
The hydroyel used to fabricate the lens is one capable
of retaining its struc-tural integrity in the thin cross-sections
required for the lens, is sufficiently rigid to retain a sub-
stantially constant optical surface and is sufficiently flexible
to permit the lens to conform to the surface contour of the eye.
The ability to conform to the surface contour of the eye is most
important as it is responsible for the lens remaining firmly
affixed to the eye without substantial movemen-t and change in
optical surface as is frequently encountered with blinking~
Preferably, the hydrogel has a water of hydration of a-t least
35~. and preferably a water of hydration varying between 35 and
50% and more preferably between 40 and 4S%.
Preferred hydrogels are a terpolymer of a hydrophilic
dihydroxy acrylate, a water-insoluble acrylate and an epoxidized
acrylate.
The hydrophilic dihydroxyalkyl acrylate comonomer
conforms to the general formula:
R O Oll
ll l
2 C C - o - (C~12)n - CH - Cf-12~
ba~
where ~ is hydrogen or methyl and n is a whole in-teger having a
value of from 0 to 4, preferably from 1 -to 4. A preferred
59L~
dihydroxyacrylate is 2,3-dihydroxypropyl methacrylate.
The second comonomer is a substantially w~ter insol-
uble alkyl acrylate or methacrylate corresponding to the general
formula:
R O
Cil2 = C - C ~ OR
where R is hydrogen or methyl and R' is alkyl having from 1 to
6 carbon atoms. Alkyl acrylates conforming to this formula are
readily available. Examples of suitable acrylates include
methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl
methacrylate, propyl methacrylate, butyl acrylate and butyl _ ;~
methacrylate. Methyl methacrylate is most preferred.
The third comonomer is the epoxidized alkyl acrylate
conforming to the formula:
R O O
,
2 C C - O - (CH2)n - CH - C~2
where R and n are as above defined. A preferred epoxidized
acrylate is 2,3-epoxypropyl methacrylate.
The molar ratio of dihydroxyalkyl acrylate to alkyl
acrylate varies within broad limits. Preferably, the dihydroxy-
alkyl acrylate at least equals or exceeds the alkyl acrylate
ancl a preferred ratio varies between about 1:1 and 10:1, more
preferably between 1.2:1.0 and 2:1. The most preferred molar
ratio is about 1.5:1Ø
The amount of epoxidi2ed acrylate used may vary
within broad parameters, generally from 0 to 30% by weight of
the dihydroxy acrylate, more preferably, from 0.1 ~o 15~ by
weight and most pre~erably, from 1.0 to 7.5% dependent upon the
monomers used, their ratio and -the like. A more definite amount
is the amount sufficient to provide a polymer capable of absorb- r
ing water of hydration in an amount of from 35 to 50%, and more
preferably, from 40 to 46%.
-- 8
5~
'I'he polym~rs are formed hy bulk polymerization using
suitable catalysts. The monomers are mixed in the absence oE
solvent and maintained under reduced pressure at an elevated
temperature for a period of time sufficient to solidify -the
reaction mixture. Typically, the temperature of reac-tion varies
between 20 and 60C. The catalyst concentration ma~ vary
within broa~ limits dependent upon the particular catalyst used,
but generally varies between about 0.001 and 0.2 weigh-t percent
of the hydroxyalkylacrylate, and preferably between 0.01 and 0.0
weight percent. A preferred catalyst is isopropyl percarbonate
in an amount of about 0.02 weight percent.
Other suitable lens materials would be obvious to ~Ir
those skilled in the art given the property requirements set
`forth herein. Thus, for example, the polymers formed by the
polymerization of 2-hydroxyethyl methacrylate is described in
U.S. Patent No. 3,220,960, supra, could be made suitable by an
increase in the concentration of cross-linking agent so that a
more highly cross-linked structure and hence, a more rigid
hydrogel, would be obtained.
v-
