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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 Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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- Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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 Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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. Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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 Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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 Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 1. Virchow, H.: Mikroskopische Anatomie der ausseren Augenhaut und des Lidapparates. In Graefe, A. K., and Saemisch, T., editors: Handbuch der gesamtem Augenheilkunde I, Leipzig, 1910, Engelmann, pp. 280 ff. 2. Nesterov, A. P., and Batmanov, Y. E.: Study on morphology and function of the drainage area of the eye of man, Acta Ophthalmol. 50:337, 1972. 3. Salzmann, M.: The Anatomy and Histology of the Human Eyeball in the Normal State: Its Development and Senescence (Brown, E. V. L., Tr.), Chicago, 1912, University of Chicago Press, pp. 20, 47. 4. Iwamoto, T.: Light and electron microscopy of the presumed elastic components of the trabeculae and scleral spur of the human eye, INVEST. OPHTHALMOL. 3:144, 1964. 5. Fine, B. S., and Yanoff, M.: Ocular Histology, a Text and Atlas. New York, 1972, Harper & Row, Publishers, p. 215, and p. 218 footnote. 6. Hogan, M. J., Alvarado, J. A., and Weddell, J. E.: Histology of the Human Eye, a Text and Atlas, Philadelphia, 1971, W. B. Saunders Company, p. 169. 7. Kupfer, C : Relationship of ciliary body meridional muscle and corneoscleral trabecular meshwork, Arch. Ophthalmol. 68:818, 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 enucleated primate eyes, Am. J. Ophthalmol. 76:632, 1973. 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, Volume 16 Number 10 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- Downloaded From: http://iovs.arvojournals.org/ on 05/13/2017 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. 14. Rigby, B. J., Hiroi, N., Spikes, J. D., and Eyring, H.: The mechanical properties of rat tail tendon, J. Gen. Physiol. 43:265, 1959.