Download Physical properties of experimental vitreous membranes. I

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glucose deprivation on function of isolated
mammalian retina, J. Neurophysiol. 26: 617,
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time of retinal cells when deprived of their
blood supply by increased intraocular pressure, Am. J. Ophthalmol. 35: 133, 1952.
Reinecke, R. D., Kuvvabara, T., Cogan, D. C ,
et al.: Retinal vascular patterns. Part V. Experimental ischemia of the cat eye, Arch.
Ophthalmol. 07: 470, 1962.
Popp, C : Die retinafunktion nach intraocularer ischemie. v. Craefes Arch. Ophthalmol. 156: 395, 1955.
Lamb, A.: Ocular changes occurring during
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Drance, S. M.: Personal communication,
April, 1974.
Physical properties of experimental vitreous
membranes. I. Tensile strength. TETSUTO
NUMATA, IAN J. CONSTABLE, AND DANIEL
E. WHITNEY.
The tensile strength of vitreous membranes,
induced by silk sutures imbedded in the vitreous
body and removed one to two weeks later, was
investigated in rabbits. Three types of membranes
were distinguished by their appearance under
an operating microscope: dense, opaque, cylindrical membranes, (Type 1), broken with 2 to 16
Cm. of force and elongated from 127 to 200 per
cent before breaking; thin, cylindrical membranes,
(Type 2), broken with 200 mg. to 2 Cm. of force
and elongated from 51 to 152 per cent before
breaking; and thin film-like membranes imbedded
in formed vitreous gel, (Type 3), broken with
200 mg. to 5.5 Cm. of force and elongated from
53 to 1S6 per cent before breaking. Vitreous
membranes formed two to eight weeks after surgery resisted forces greater than those required
to detach rabbit retina. The results of the experiment are rclcvatit in the design of vitrcctomy
instruments.
A satisfactory surgical instrument for vitreous
membrane removal by the closed transcleral route
Ophthalmology
February 1975
must possess properties such as small size (maximum 2.0 mm. diameter tube), a separate infusion
chamber to maintain pressure, and a suction line
to remove cut membranes without pulling on the
easily detachable retina. These requirements have
been quite well recognized by others.'••s However, knowledge of the mechanical properties of
the materials being cut would be very useful for
the systematic design of specific details such as
shape, material, and finish of the cutting edge,
and speed and stroke of the cutting mechanism.
The present experiment was carried out to investigate the tensile strength of vitreous membranes
produced in rabbit eyes.
Materials and. methods.
Production of vitreous membranes. Ten rabbits
were used for this experiment weighing between
1.7 to 4.2 kilograms. Animals were anesthetized
with intravenous pentobarbital sodium (25 mg.
per kilogram) and pupils were dilated prior to
surgery with phenylephrine 10 per cent and
scopolamine 0.3 per cent. The eyes were proptosed
and held by forceps applied to the superior and
inferior recti. Three to six sutures (4-0 silk) were
passed through conjunctiva and sclera across the
vitreous cavity at the level of the equator by
means of a straight sewing needle. After one or
two weeks, sutures were removed and, thereafter,
mcmbiane formation was observed by indirect
ophthalmoscopy and by slit lamp biomicroscopy.
Membrane preparation for testing. Two rabbits
were killed every week by an overdose of anesthetic and the eyes were enucleated and put in
normal saline at 25" C. Each eye was sutured into
a molded plastic hemisphere, then opened by an
incision around the limbus. The anterior segment
was lifted with forceps and the lens cut away from
the vitreous face. The membranes were then dissected out from the eye under an operating microscope and transferred onto a flat, black plexiglass plate. Then, membranes were cut into test
specimen sizes. The individual form clamps were
made by cutting soft polyurethane form into
1.6 by 1.6 by 0.4 mm. blocks and glueing them
onto strips of aluminum foil (Fig. 1). Membrane
test specimens were transferred from the black
plexiglass plate onto form clamps submerged in
normal saline at 25U C , and the ends of each
membrane were then pressed between two such
forms (Fig. 1). Form clamps prevented membranes from slipping during pulling and from
being pinched directly by the plexiglass clamps.
Pinching might create a local weakness in membranes where they would break with very small
forces. Each membrane specimen secured at both
ends by form clamps was carefully transferred
into a plexiglass testing bath filled with normal
saline at 25° C , and secured tightly into two
plexiglass clamps as shown in Fig. 1. The specimen was then pulled until broken by a trans-
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Volume 14
Number 2
Reports
149
Polyurethone foam
Upper
plexiglass
clamp
Membrane
Aluminum-form
clamps
Lower
plexiglass
clamps
Fig. 1. Apparatus for testing tensile strength, The transducer (above) moves up at constant
speed, stretching the vitreous membranes which are held between two plexiglass clamps (lower
left). The form clamps (lower right) prevent uneven force from being applied to the vitreous
membrane.
ducer mounted on a microscope stand, which
was driven up and down by a motor at V2 rev.
per hour (0.3 mm. per minute extension) and
Vi rev. per minute (9 mm. per minute extension).
The transducer was calibrated using milligram
weights and its outputs were recorded on a Sanborn 350 recording machine.
Results.
Ohaervatumx mudv on the membranes formed.
The number of membranes obtained from each
eye varied between nine and twenty-two. Membranes formed were classifiable into three categories on the basis of appearance under the
operating microscope (Fig. 2). Dense, opaque,
white membranes approximately 1 mm. in diameter
or less were found bridging two nearby suture
perforation sites (Type 1). Incomplete and thin
films of vitreous gel surrounded these membranes.
This type of membrane tended to be thicker and
wider near the perforation sites tapering down
in width toward its middle. Fine, cylindrical
membranes, approximately 0.05 mm. in diameter,
wider near the perforation sites, tapering down
(Type 2). Diameters, however, varied and were
never constant throughout the length of the
membranes. Therefore, small sections of each
membrane specimen were selected for testing so
that they were as regular as possible. Considerable amounts of formed gel surrounded this type
Fig. 2. Schematic diagram of three vitreous membrane types observed under an operating microscope. Type 1 bridged adjacent perforation sites,
Type 2 extended along the suture tracks, and
Type 3 was embedded in formed vitreous gel.
of membrane, but most of it was broken down
by the preparation for testing. These membranes
were often transparent and hard to observe under
the operating microscope. Thin, film-like mem-
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150 Reports
Investigative Ophthalmology
February 1975
Fig, 3. Serial photographs of a Type 1 membrane being pulled and broken.
branes imbedded in formed vitreous gel were
found at random locations in the vitreous cavity
(Type 3). This material was difficult to handle
and trim into the small size necessary for testing.
These membranes were often quite transparent,
and could only be seen by means of oblique
illumination.
Variable amounts of vitreous gel surrounded
each type of membrane and could not be easily
separated from them, but was continuous only
in Type 2 and Type 3 membranes. Although,
Type 1 and Type 2 membranes were found radiating out from suture tracks, Type 3 membranes
bore no direct relationship to them, being present
at random sites within formed vitreous gel. All
membranes tended to increase in density up to
approximately four weeks, then became thinner
and more transparent. None of the membranes
had caused actual retinal detachment by pulling
on the retina at local attachment points within
the period of observation.
Tensile strength and membrane extension.
Membranes first elongated, then finally broke at
a random site (Fig. 3). The tensile strength and
the extent of elongation of membranes before
breaking are depicted in Fig. 4 as a function of
age. Two different extension speeds, 0.3 mm.
per minute and 9.0 mm. per minute, did not reveal any strength differences. Type 1 membranes
showed a breaking strength of 2 to 16 Cm. and
a breaking elongation of 127 per cent to 200 per
cent. Type 2 membranes showed a breaking
strength of 200 mg. to 2 Cm. and breaking
elongation of 51 per cent to 152 per cent. Type
3 membranes showed a breaking strength of
200 mg. to 5.5 Cm. and a breaking elongation
of 53 per cent to 186 per cent. All membranes
tested were stronger than the force required to
detach the retina.!l< in No strength differences
were observed between two-week-old and eightweek-old membranes. A clear association between
membrane age, strength, and elongation could
not be made.
Discussion, The method of producing experimental vitreous membranes by means of sutures
was found to be reliable. This method has several
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Reports 151
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Number 2
10,000
1000
100
I
10
200
150
t
I
Uj
50
SI
S2
S3
S4
S5
S6
Fig. 4. Tension data (above). Si to S« represent
six rabbits tested. Forces measured are plotted on
a long scale in milligrams. S, and S.> were twoweek-old membranes pulled at 0.3 mm. per minute speed. S:, and Si were four-week-old membranes pulled at 9.0 mm. per minute speed. Sr,
represents two-week-old membranes pulled at
9.0 mm. per minute speed. S,: represents eightweek-old membranes pulled at 9.0 mm. per minute
speed. Elongation data (below). Elongation data
of each membrane specimen are plotted in the
same order as the tension data.
advantages over the commonly used method of
injecting blood. The vitreous body remains clear,
allowing detailed clinical examination during formation of membranes and during experimental
surgery. A reproducible variety of membranes
are forme:I and, most importantly, formed vitreous
gel is not radically destroyed as occurs after
blood injection in the rabbit. This last point is
particularly relevant when assessing the efficiency
of vitrcctomv instruments.
Type I membranes were usually much stronger
than either Type 2 or Type 3: Type I membranes represent ingrowth of scar tissue at perforation sites. Their greater strength was probably
due principally to larger amounts of these materials rather than to actual structural differences.
The fine strands bridging suture tracks (Type
2) were of similar strength to lacelike membranes
embedded in formed gel (Type 3). It was difficult to be certain to what degree their tensile
strength was dependent on the newly formed
membranous tissue and to what degree it was
dependent on the formed gel. Attempts to mea-
sure the tensile strength of normal formed
vitreous gel were unsatisfactory for several reasons. Normal vitreous gel could not easily be
cut into small regular sizes suitable for testing,
repeated handling tended to break up the gel,
and the clear gel was invisible when immersed
in saline. However, handling indicated that the
formed gel alone was not strong enough to contribute much of the strength of Type 2 and Type
3 membranes.
It was inevitable that a wide range of tensile
strength and extension would be obtained since
individual membranes varied in diameter along
the segments being tested, and since only gross
trimming of surrounding formed vitreous gel
and thicker Type I membranes was done. No
attempt was made to measure cross-sectional
areas and express results as force per unit area,
because it was felt that the range of breaking
strength was more relevant for instrument designs than these idealized values.
Implications of the results for design of vitrcctomy instruments. The present experiments
showed that even the finest vitreous membranes
visible only with the aid of the operating microscope were stronger than the forces necessary to
detach the rabbit retina in vitro and in vivo and
membianes can invariably be broken if stretched
a maximum of 200 per cent. These facts suggest
cutting mechanism designs in which membranes
are first clamped before cutting. With such a
design, the dependence on perfectly fitting shearing surfaces would become less critical. Systematic
determination of the clamping surface and clamping force necessary would require data on adhesive force and frictional force characteristics
of vitreous membranes. Experiments to define
these properties for rabbit vitreous membranes
will be reported later.
The authors thank Charles L. Schepens, M.D.,
for his support of this project, and Mr. Oleg
I'omerantzell and Dr. F. Delori for their advice
and assistance.
From the Department of Retina Research, Eye
Research Institute of Retina Foundation, Boston
and the Department of Mechanical Engineering,
Massachusetts Institute of Technology, Cambridge,
Mass. Supported by the |ohn A. Hartford Foundation, Massachusetts Lion's Eye Research Inc., and
by Grant EY 01030-01 from the National Eye
Institute of Health. Submitted for publication
Aug. 27, 1974. Reprint requests: Editorial Services Unit, Eye Research Institute of Retina
Foundation, 20 Staniford St., Boston, Mass. 02114.
Key words: tensile strength, vitreous membranes,
vitreous gel, vitrectomy.
REFERENCES
1. Couvillion, C. C, Freeman, H. M., and
Schepens, C. L.: V. Modification of the
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Machemer, R., Parel, J. M., and Buertner,
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Brighbill, F. S , Kaufman, H. E., and Levenson, J. E.: A vitreous suction cutter for
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Duvas, N. C : The cataract roto-extractor,
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Tolentino, F. I., Bunko, A., Sehepens, C. L.,
et al.: Vitreous surgery. XII. A new surgical
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deGuillebon, H., and Zauberman, H.: Experimental retinal detachment. Biophysical
aspect of retinal peeling and stretching, Arch.
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Qunntitation of pilocarpine flux enhancement across isolated rabbit cornea by
hydrogel polymer lenses. DAVID L.
KHOHN AND JULIANNA M. BHEITFELLEM.
The comparative effect on pilocarpine flux across
rabbit cornea induced by two hydrogel polymer
lenses containing ci/ual doses was quantitated in
a transport chamber. This closed system featured
continuous flow of a tear analog but excluded
variables of the internal eye influencing concentration. Flux induced by both lenses increased
linearly with time. At 240 minutes total flux was
a whole order greater than that induced bij the
same pilocarpine dose in free fluid. Analysis of
pilocarpine in tear analog effluent showed the
flux to be independent of the available dose retained in the hydrogel polymer lens, suggesting
that eorneul transport of pilocarpine to the aqueous
may involve mediation by a carrier system.
Clinical studies1"' and animal experiments1' r>
have demonstrated enhanced transcorneal pilocarpine flux induced by presoaked hydrogel polymer
lenses. The mechanism of this enhancement has
February 1975
been uncertain. In addition, exact measurements
of the effect of these polymers relative to that
of ordinary drop administration or constant flow
delivery have been unavailable, since aqueous concentration in the living eye after topical administration of any type is variably influenced by
tear elution and multiple intraocular events. The
latter include chamber inflow and outflow, preferential uptake of the drug by tissue in the
internal eye," isomerization and hydrolysis, "• s
anil, in the case of radioisotope determinations,
interchange of isotope with solvent."
This report describes <|iiantitation of the effect
on pilocarpine flux across isolated rabbit cornea
of two well-known hydrogel polymer lenses. A
transport chamber system was used which incorporates the ellect of tear How but excludes
the complex variables of pilocarpine concentration
in the living internal eye. It is a closed system
in that all administered pilocarpine can be accounted for. The intention was to compare relative flux efficiency of the two polymer lenses at
various points in time. It was anticipated that
some inference as to mechanism of transport
enhancement relative to equal dose "drop" administration might be available from this comparison.
The transport chamber and llux data lor single
drop administration ol pilocarpine were described
in a previous publication.1" All parameters reported, including flush and control runs in advance of the pilocarpine administration, were
held constant for experiments now described.
However, single fluid pilocarpine instillations were
replaced by delivery by presoaked hydrogel polymer lens buttons. Each of these was trephined
when fully hydrated with pilocarpine solution.
The experiment was designed so that the total
available pilocarpine close in each lens button was
equivalent to the "secondary" dose in the upper
chamber in the previously described single-fluid
dose experiments."'
Lens buttons were presoaked in 30 to 50 ml.
16.0 per cent pilocarpine—HCI in saline for 48
hours with a change of solution at 24 hours.
Each button was blotted in a standard manner,
then eluted for a minimum of 1 week in 100
ml. of saline at 4° C. The buttons were then
matched on the basis of pilocarpine capacity independent of weight or thickness. Matching buttons were accumulated until a "pool" ol prematched buttons was available. Each lens button
from this "pool" was used randomly for determination at various time intervals. Long and short
intervals were rim sequentially. From time to
time, a group of lenses were taken Ironi the
"pool" and re-tested by full soaking and elution
for constancy of piloearpine-HCI space. Buttons
were discarded if any evidence of fraying or
mechanical damage was present.
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