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Transcript
The scleral spur and scleral roll
Robert A. Moses and Walter J. Grodzki, Jr.
The scleral spur and scleral roll together form a fibrous hoop ivhich may be detached from
adjacent structures as a unit. Tensile tests of the spur-roll resemble reported data on sclera.
It is proposed that the circular structure of the spur-roll prevents action of the meridional
portion of the ciliary muscle on the trabecular meshwork from collapsing the canal of
Schlemm.
Key words: scleral spur, scleral roll, canal of Schlemm, trabecular meshwork, meridional
ciliary muscle.
A
scleral spur contained elastic fibers; Iwamoto4 tended to agree. Fine and Yanoff5
considered the spur to be collagen, as did
Hogan, Alvarado, and Weddell.6
Since the scleral roll lies at the junction
of the scleral curvature with the corneal
curvature, it may have structural importance in bracing this portion of the eye,7
and since the scleral spur lies between the
meridional portion of the ciliary muscle
and the trabecular mesh on which the muscle must act if it is to influence facility of
outflow, it would seem desirable to gain
more information about these structures.
The spur and roll together may be separated from adjacent structures and studied
as a unit. The present study is of the tensile
properties of the spur-roll ring.
limbus-parallel ring of fibers forms
the inner surface of the sclera at the junction of the scleral and corneal curvatures
and projects inward to interdigitate with
the tendon fibers of the meridional ciliary
muscle. The sessile group of limbus-parallel fibers of the sclera is called the scleral
roll and the inward-projecting group of
limbus-parallel fibers is the scleral spur.1
The scleral roll thus forms the posterior
wall of the canal of Schlemm and the roll
and spur together form the posterior wall
of the internal scleral sulcus. The spur extends inward from the inner sclera toward
the axis of the eye for about 0.09 mm.2
Salzmann3 was of the opinion that the
From the Department of Ophthalmology and the
Oscar Johnson Institute, Washington University
School of Medicine, St. Louis, Mo.
This work was supported in part by National Eye
Institute Grant EY 00256 from The National
Institutes of Health, Bethesda, Md.
Submitted for publication Aug. 9, 1976.
Reprint requests: Robert A. Moses, M.D., Department of Ophthalmology, Washington University School of Medicine, St. Louis, Mo.
63110.
Method
The eye is bisected at the equator and the
anterior segment is everted over a rounded cork.
The zonules are cut and the lens is removed. The
choroid and ciliary body are reflected forward
and pressure is applied to the attachment of the
scleral roll to the main body of the sclera with a
Tooke knife, a cornea-splitting instrument (Fig.
1, A). The spur and roll are seen after separation
as a narrow, glistening, white stripe on the outer
925
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926
Moses and Grodzki
Invest. Ophthalmol. Visual Set.
October 1977
Fig. 1. A, With the choroid and ciliary body reflected forward, pressure is made at the base
of the roll. B, Spur-roll attached to ciliary body, black arrow; partially detached, white arrow. C, The scleral spur-roll: uveal tissue has been cut away, tags of trabecular mesh remain
attached.
surface of the ciliary body (Fig. 1, B). The ciliary
body and shreds of trabecular mesh are then
teased and cut away, leaving the scleral spur and
roll as a loop (Fig. 1, C).
Alternately, when the choroid and cihary body
are reflected forward, gentle pressure with the
half-round Tooke knife is applied to the external
(scleral) surface of the ciliary body at its at-
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tachment to the scleral spur, separating the ciliary
body from the spur. The spur and roll are thus
left attached to the sclera, from which they may
be detached as above.
Series 1. The isolated spur-roll loop is placed
over the hooks of the apparatus shown in Fig, 2,
in which the lower hook is fixed to the stainless
steel frame and the upper hook is suspended from
Volume 16
Number 10
Scleral spur and scleral roll 927
Fig. 3. Lubricated scleral spur-roll loop over
semicircles of force-length instrument.
Fig. 2. Apparatus for force-elongation study of
scleral spur-roll consists of fixed hook (below),
and moveable hook supported by force transducer.
Scleral spur-roll over hooks. (Micrometer not
shown.)
a force transducer (Sanborn FTA-10-1). The
transducer is then raised in small steps by means
of its micrometer support and the force on the
spur-roll loop is recorded. The spur-roll loop is in
saline solution at room temperature (23° to 24°
C.) throughout the measurement.
Series 2. The hooks of the apparatus are replaced by polished stainless steel semicircles 11.1
mm. in diameter. The spur-roll loop and the
semicircles are lubricated with oil-in-water emulsion8 (Petrogalar; Wyeth Laboratories) diluted
with an equal volume of water. The spur-roll
loop is placed over the semicircles and immersed
in saline at room temperature (Fig. 3). The upper
semicircle is raised in small steps and imposed
force recorded as above.
In both series, correction of the distance between hooks or semicircles is made for the elongation of the force transducer spring.
Results
Spur-roll loops were dissected from 16
eyes obtained at autopsy. The loops were
used 2 to 4 days after death.
Series 1 (12 eyes). Force-length curves
such as shown in Fig. 4 were obtained.
The curves all showed the loop to be easily
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4.0
u 3.2
a:
o
170
17.2
W.i
176
178
18.0
1B.2
18.1
18.6
18.8
Fig. 4. Typical force-length relation of scleral
spur-roll loop. Line has been fit to curve for force
> 0.3 gm. Increasing force, closed circles; decreasing force, open circles.
stretched at low tensions, with increasing
resistance to stretch as tension was increased. Most loops withstood at least 8
gm. of tension before breaking.
Series 2 (4 eyes). The results were essentially similar to those of series 1. The
data from this series were not analyzed
because the wide spread of extension per
gram made us suspect that friction of the
loop against the steel forms introduced
additional error. However, it simply may
be that eyes 356 and 369 had weak spur-
928 Moses and Grodzki
Invest. Ophthalmol. Visual Sci.
October 1977
Table I. Least-squares fit of scleral roll-spur force length curve to y = mx + b*
for force >0.3 gm.
Eye
Age at
death
(yr.)
Series 1 (Hooks):
209
75
228
70
238
74
255
265 R$
266L
278
279R
280L
295 R
296L
310
80
62
62
72
72
72
54
54
58
Mean
S.D.
S.E.
No.
68.5
8.6
2.9
9
Series 2 (semicircles):
356
84
. 369
40
382
49
393
63
Decreasing force
Increasing force
m
(mm./gm.)
0.213
(a)0.190f
(b) 0.176
(a) 0.190
(b) 0.213
0.169
0.230
0.208
0.247
0.231
0.262
0.204
0.177
0.160
0.207
0.0315
0.0091
12
0.569
0.563
0.108
(a) 0.146
(b) 0.120
b
(mm.)
radius =
(b/ir mm.)
m
(mm./gm.)
b
(mm.)
17.07
18.16
18.15
19.13
19.13
18.51
17.69
17.69
18.32
17.60
17.66
17.93
18.07
17.98
5.43
5.78
5.78
6.09
6.09
5.89
5.63
5.63
5.83
5.60
5.62
5.71
5.75
5.72
0.217
0.196
0.187
0.266
0.226
0.176
0.205
0.233
0.223
0.180
0.210
0.190
0.217
0.184
17.15
18.25
18.20
19.02
19.21
18.57
17.86
17.72
18.50
17.86
17.96
18.10
18.09
17.98
5.46
5.81
5.79
6.05
6.11
5.91
5.69
5.64
5.89
5.69
5.72
5.76
5.76
5.72
17.98
0.52
0.151
12
5.72
0.167
0.048
12
0.208
0.0256
0.0074
12
18.09
0.471
0.136
12
5.76
0.149
0.043
12
20.36
17.62
18.04
17.56
17.62
6.48
5.61
5.74
5.59
5.61
0.396
0.405
0.121
0.124
0.117
20.56
17.81
18.23
17.89
17.84
6.54
5.67
5.80
5.69
5.68
radius =
(b/ir mm.)
°y = length of loop (mm.), m = elongation per unit force (mm./Gm.), x = force (gm.), b = length of loop (mm.
calculated) at 0.0 gm. of force.
fa = first run, b = repeat run.
JR, L = right and left eyes of same cadaver.
roll loops whereas eyes 382 and 393 had
strong loops.
In both series, viscoelastic behavior was
demonstrated.
In both series, the low-tension part of
the stretch curve was considered to be
gross form rearrangement such as kink
straightening. When this portion of the
curve (force <0.3 gm.) was ignored, the
remainder was moderately well fit (mean
r2 = 0.94) by a linear equation of the
form
y = mx + b
in which y = hook-to-hook distance (mm.),
x = force (Gm.), m = increase in distance per gram of force (mm. Gm.-1), and
b = spur-roll loop length extrapolated to
0.0 Gm. of tension (mm.). The results are
given in Table I.
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Discussion
Kupfer7 in 1962 pointed to the importance of the spur-ring as a group of fibers
well placed at the junction of scleral and
corneal curvatures to help maintain the
shape of the eye. However, the ring is
placed on the inner surface of the sclera
where, if the spur-roll is to resist the tendency of intraocular pressure to round out
the limbal region, the wall of the eye must
hang from the spur-roll ring. There is
little in the relatively loose attachment of
the roll to the sclera to suggest that the
limbus hangs from the outside of the roll.
It would seem more likely for a groove
in the globe to be braced from the outside by a constricting band.
Rather, the spur-roll placement on the
inner side of the sclera suggests that it is
concerned with changing the direction of
Volume 16
Number 10
Scleral spur and scleral roll
929
Fig. 5. Tension along sclera would swing unsupported trabecular meshwork outward. Outward component of ciliary muscle tension indicated by resultant arrow.
action of the meridional ciliary muscle and
of holding the canal of Schlemm open.
The meridional portion of the ciliary
muscle rests against the anterior sclera;
the direction of action of the muscle is
parallel to the inner surface of the sclera.
The trabecular meshwork lies in the curve
of the cornea just posterior to the intersection of the scleral and comeal curves
in an obtuse angle at Schwalbe's line. In
the absence of a scleral spur, contraction
of the meridional ciliary muscle would
tend to swing the meshwork outward, its
attachment to the cornea acting as a hinge.
This action would narrow the canal of
Schlemm (Fig. 5). All of the tendon fibers
which join the trabecular mesh pass axial
to some circular band or hoop of collagenous tissue against which they bear and
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which prevents their outward migration.
This collagenous ring thus changes the
direction of tension of the meridional muscle from along the inner sclera to along
the inner cornea as schematically shown
in Fig. 6.
That ciliary muscle contraction enhances
the facility of outflow is suggested by the
lens depression studies of Van Buskirk and
Grant,9 the action of topical parasympathomimetic drugs, and by the accommodation
studies of Armaly and Burian.1() If the muscle is to act on the meshwork, either the
tendon fibers must slide over the spur rings
or the spur must also move, as suggested
by Rohen and Unger.11 The present study
has shown that the scleral spur-roll is a
ring of considerable strength. Septa across
the canal lumen12 and septa dividing the
Invest. Ophthalmol. Visual Set.
October 1977
930 Moses and Grodzki
fact that even in severe, long-standing glaucoma the posterior portion of Schlemm's
canal is patent.
Although Young's modulus of elasticity
cannot be accurately stated in the case of
viscoelastic materials, approximate figures
for comparision are given in Table II. 13 ' 14
It is seen that the spur-roll has an elastic
constant of the same order of magnitude as
sclera but considerably less than that of
tendon.
REFERENCES
Fig. 6. Schematic representation of scleral spur
hoop and meridional ciliary muscle. The tendons
bend inside the hoop to join the trabecular meshwork.
Table II. Modulus of elasticity of fibrous
structures
Substance A nimal
Assumption
Young's
Modulus of
Elasticity Ref.
(dyne/cm.2) No.
2.9 x 10'
Rabbit Thickness
0.28 mm.
Tendon
Rat
800 x 10'
13.7 x 10'
Spur-roll Man
Circular crosssection diameter 0.2 mm
Sclera
13
14
canal into parallel channels5 may help to
keep the canal open, while septa across
collector channel ostia12 may tend to prevent prolapse of the inner canal wall into
the collector channels after canal collapse,
but it is the presence of the strong spurring that is undoubtedly responsible for the
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1. Virchow, H.: Mikroskopische Anatomie der
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2. Nesterov, A. P., and Batmanov, Y. E.: Study
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Its Development and Senescence (Brown, E.
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4. Iwamoto, T.: Light and electron microscopy
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5. Fine, B. S., and Yanoff, M.: Ocular Histology,
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6. Hogan, M. J., Alvarado, J. A., and Weddell,
J. E.: Histology of the Human Eye, a Text
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7. Kupfer, C : Relationship of ciliary body
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1962.
8. Moses, R. A., and Grodzki, W. J.: Theory
and calibration of the Schi0tz tonometer.
III. Friction between tonometer footplate
and cornea, INVEST. OPHTHALMOL.
10:589,
1971.
9. Van Buskirk, E. M., and Grant, W. M.:
Lens depression and aqueous outflow in
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10. Armaly, M. F., and Burian, H. M.: Changes
in the tonogram during accommodation,
Arch. Ophthalmol. 60:60, 1958.
11. Rohen, J., and Unger, H-H: Zur Morphologie
und Pathologie der Kammerbucht des Auges,
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Akademie der Wissenschaften und der Literatur, Abhandlungen der MathematischNaturwissenschaftlichen
Klasse, Jahrgang
1959, Nr. 3.
12. Bill, A., and Svedbergh, B.: Scanning electron microscopic studies of the trabecular
meshwork and the canal of Schlemm—an attempt to localize the main resistance to out-
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Scleral spur and scleral roll
931
flow of aqueous humor in man, Acta Ophthmol. 50:295, 1972.
13. Gloster, J., Perkins, E. S., and Pommier,
M.-L.: Extensibility of strips of sclera and
cornea, Br. J. Ophthalmol. 41:103, 1957.
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Eyring, H.: The mechanical properties of rat
tail tendon, J. Gen. Physiol. 43:265, 1959.