July 2002
Volume 43, Issue 7
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Glaucoma  |   July 2002
Peripapillary Scleral Thickness in Perfusion-Fixed Normal Monkey Eyes
Author Affiliations
  • J. Crawford Downs
    From the Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and the
  • Richard A. Blidner
    From the Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and the
  • Anthony J. Bellezza
    From the Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and the
  • Hilary W. Thompson
    LSU Eye Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana.
  • Richard T. Hart
    From the Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and the
  • Claude F. Burgoyne
    From the Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and the
    LSU Eye Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana.
Investigative Ophthalmology & Visual Science July 2002, Vol.43, 2229-2235. doi:
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      J. Crawford Downs, Richard A. Blidner, Anthony J. Bellezza, Hilary W. Thompson, Richard T. Hart, Claude F. Burgoyne; Peripapillary Scleral Thickness in Perfusion-Fixed Normal Monkey Eyes. Invest. Ophthalmol. Vis. Sci. 2002;43(7):2229-2235.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

purpose. To characterize the thickness of the peripapillary sclera in perfusion-fixed normal monkey eyes so as to build accurate computational models of intraocular pressure (IOP)–related stress and strain within these tissues.

methods. Nine rhesus monkeys were perfusion fixed, each with one normal eye set to an IOP of 10 mm Hg by manometer. A 6-mm-diameter specimen containing the optic nerve head and peripapillary sclera was trephined from each scleral shell and cut into 4-μm serial sagittal sections across the scleral canal opening, either horizontally (four eyes) or vertically (five eyes). The thickness of the peripapillary sclera was measured on every 24th section at 100-μm intervals from the posterior scleral canal opening (PSCO) to the peripheral edge of the specimen. The data were pooled by quadrant (superior, inferior, nasal, and temporal), regions within each quadrant, and distance from the PSCO, overall and for individual eyes, and subjected to analysis of variance.

results. In terms of distance from the PSCO, the peripapillary sclera was thinnest nearest the PSCO (201 μm, nasal; 201 μm, temporal; 240 μm, inferior; 249 μm, superior), thickened progressively to a maximum in the midperiphery approximately 600 to 1000 μm from the PSCO (326 μm, nasal; 415 μm, superior; 420 μm, temporal; 422 μm, inferior), and thinned again peripherally in all quadrants. The peripapillary sclera was thinner in the nasal quadrant when compared with the other quadrants superiorly, inferiorly, and temporally (central region means of 291 μm, nasal; 369 μm, superior; 372 μm, inferior; and 369 μm, temporal; P < 0.0001).

conclusions. In the normal monkey eye, peripapillary scleral thickness varies significantly with distance from the posterior scleral canal opening and is thinner in the nasal quadrant than in the other quadrants. These differences are substantial and are likely to affect the magnitude of IOP-related stress and strain within these tissues for a given level of IOP.

As part of our ongoing studies of the optic nerve head (ONH) as a biomechanical structure, we recently reported the results of thickness measurements for the posterior scleral shell of perfusion-fixed normal and early glaucomatous monkey eyes. 1 These data are important for building finite element models of the load-bearing connective tissues of the perfusion-fixed monkey posterior pole. In the preparation of the scleral shells for that study, a 6-mm diameter trephination of the optic nerve and peripapillary sclera was performed to allow characterization of the thickness of the peripapillary sclera (0–1500 μm from the scleral canal). 
Accurate measurement of peripapillary scleral thickness is necessary for adequate modeling of the important transition between the peripapillary sclera, the scleral canal wall, and the peripheral laminar beams. Because scleral wall stress is inversely proportional to scleral thickness, variation in scleral thickness by location in the peripapillary region is likely to have a significant effect on the stresses in the scleral canal wall that are transferred to the lamina cribrosa. Accurate modeling of the conditions at the boundary zone between the lamina and the sclera is crucial to any models of the ONH. 
IOP-related stress within the immediate peripapillary sclera is also important, because the effects of such stress may diminish the volume flow of blood through the contained branches of the posterior ciliary arteries. 2 3 In addition, this stress is likely to be increased in myopia and other conditions in which the peripapillary sclera is thinned 4 5 and may underlie changes in the peripapillary scleral collagen that have been reported in human eyes with glaucoma. 6  
The purpose of the present study was to characterize the thickness of the peripapillary sclera within 1500 μm of the posterior scleral canal opening in this subset of nine normal monkey eyes that had been perfusion fixed at an IOP of 10 mm Hg. 
Materials and Methods
Animals
All animals were treated in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Nine male rhesus monkeys that had been killed in connection with another study (Table 1) were perfusion fixed and the eyes enucleated, dissected, and stored as described previously. 1 The normal eye of each monkey, which had been set to an IOP of 10 mm Hg by manometer for approximately 15 minutes before death and maintained at that level for 1 hour after perfusion fixation, was used in this study. 
Scleral Section Preparation and Sampling
The ONH and peripapillary sclera were removed from the posterior scleral shell with a 6-mm trephine, dehydrated, infiltrated, and embedded in historesin (Technovit 7100; Kulzer, Wehrheim, Germany). Serial sagittal sections 4 μm in thickness (model RM2165 Microtome; Leica, Bensheim, Germany) were cut, mounted on glass slides, stained for collagen with Van Gieson stain, and coverslipped. Of the nine normal-eye ONH specimens, four were sectioned horizontally (temporal–nasal) and five were sectioned vertically (superior–inferior; Fig. 1 ). 
The scleral canal opening of each ONH was defined to coincide with the opening in Bruch’s membrane. This definition was chosen because Bruch’s membrane is easily identified on histologic sections and is visible clinically in fundus images. Thus, the first and last histologic sections in which Bruch’s membrane was open established the boundaries of the scleral canal opening. Beginning with the first section at one end of the scleral canal opening, every 24th section was selected; therefore, the interval between the surfaces of any two consecutive selected sections was 96 μm (24 sections × 4 μm per section; Fig. 1 ). The number of sections selected for each ONH varied, depending on the size of the opening of Bruch’s membrane and whether the sectioning was vertical or horizontal (Table 1)
Image Acquisition and Scleral Thickness Measurements
Digital color images of the selected sections, along with a companion image of a slide-mounted micrometer scale to allow calibration of the exact pixel size, were acquired with a 3-CCD color camera (HV-C20; Hitachi, Tokyo, Japan) attached to a microscope (Optiphot-2; Nikon, Tokyo, Japan). All images were generated at a resolution of approximately 2 μm/pixel. 
All section images for a single specimen were marked by one of two masked operators (JCD, RAB) and processed using a custom image-analysis program, as follows (Fig. 2) : (1) The exact pixel size was calculated by using the companion image of a slide-mounted micrometer. (2) Bruch’s membrane opening was marked and then, for visual reference, marks denoting the anterior and posterior insertions of the lamina cribrosa and the laminar surfaces were imported from a separate study that used the same section images. (3) The posterior scleral canal opening (PSCO) and the anterior and posterior peripapillary scleral surfaces were marked from the scleral canal opening to the edge of the section. (4) All marks were classified as being located on the anterior or posterior scleral surface and in either the superior or inferior quadrant (vertical sections) or nasal or temporal quadrant (horizontal sections). (5) The opening of Bruch’s membrane and the PSCO were automatically measured. (6) On each side of the canal, scleral thickness at the PSCO was automatically measured by projecting a line perpendicularly from the anterior scleral surface to the posterior scleral surface at the PSCO. (6) Scleral thickness was then automatically measured in the same fashion at 100-μm intervals from the PSCO to the peripheral end of the section. (7) Data for each section image were exported to a spreadsheet file (Excel; Microsoft, Redmond, WA) for each ONH. 
This approach yielded scleral thickness measurements on a 96 × 100-μm grid on either the nasal and temporal (for horizontally sectioned specimens) or superior and inferior (for vertically sectioned specimens) sides, extending from the scleral canal boundary to the outer edge of the trephined specimen (Fig. 1) . To ensure that the same number of data points was used for each section, only the points from the PSCO to the 1500-μm point were included in the data analysis. In two monkeys, the ONH was off center in the trephination, resulting in no data beyond 1200 μm on one side of the scleral canal in one specimen and beyond 1100 μm in the other. 
Statistical Analysis by PSCO Distance, Quadrant, and Region
The data from each measurement point were identified by quadrant, region within quadrant, and distance from the PSCO (Fig. 1) . The quadrants were superior or inferior for vertically cut specimens and nasal or temporal for horizontally cut specimens. The quadrants were divided into three regions, one central (the central 50% of the histologic sections in each quadrant) and two peripheral (the remaining 50% of the sections, half on each side of the central region). Thus, for the horizontal sections, the regions were superior, central, and inferior, and for the vertical sections, the regions were temporal, central, and nasal. Regionalization within the quadrants was necessary to allow pooling of central region data from all quadrants with no overlap, while permitting acquisition of transitional data in the peripheral regions, where overlap between adjacent quadrants occurred in the area nearest the PSCO (Fig. 1)
Mean peripapillary scleral thickness at each PSCO distance was calculated for each of the three regions within each quadrant. Mean peripapillary scleral thickness for each region in each quadrant was calculated as the mean of all the regional peripapillary scleral thickness measurements across all PSCO distances. A factorial analysis of variance (ANOVA) was used to assess the effects of PSCO distance, quadrant, region by quadrant, and study eye on the dependent variable, peripapillary scleral thickness. The analysis assessed effects both overall (data from all sections from all eyes) and within the data from each study eye. 
Pooled Peripapillary Scleral Thickness Topology Map
The overall mean peripapillary scleral thickness by PSCO distance, quadrant, and region was plotted in three-dimensional space as follows. The maximum values for the vertical and horizontal dimensions of Bruch’s membrane opening for all nine study eyes were used to generate an average ellipse defining the scleral canal. The pooled mean peripapillary scleral thicknesses by PSCO distance for the three regions of each quadrant were mapped onto a coordinate system defined by this average ellipse. A custom program (Matlab; The Math Works, Inc, Natick, MA) was used to interpolate these irregularly spaced, discrete thickness data into a continuum thickness. 
Results
Reproducibility of Peripapillary Scleral Thickness Measurements
The mean scleral thicknesses of both quadrants of two monkey eyes were obtained by repeated measurement on three separate days at least 2 weeks apart by a single masked observer (Table 2) . By ANOVA, measurement day was significant overall and for monkey 7 (P < 0.05), but not for monkey 5. Overall, and within each monkey, all four quadrants were distinguishable from one another, even in the face of the variability introduced by measurement day (P < 0.0001). 
Mean Peripapillary Scleral Thickness by PSCO Distance, Quadrant, and Region
In individual eyes and in all eyes combined (Figs. 3 4) , the effect of distance from the PSCO was significant (P < 0.0001). The peripapillary sclera was thinnest at the PSCO, thickened to a maximum in the midperiphery of the specimens 600 to 1000 μm away from the PSCO, and thinned again somewhat toward the farthest data point analyzed (1500 μm from the PSCO). 
Similarly, the effect of quadrant was significant in most of the individual eyes (Table 3) and overall (Fig. 3 ; P < 0.0001). In general, the peripapillary sclera was thinner in the nasal quadrant and thicker in the temporal, superior, and inferior quadrants. For the individual monkeys (Table 3) , the central region of the nasal quadrant was significantly thinner than that of the temporal quadrant in all four horizontally sectioned specimens, but there was no consistent pattern in the central regions of the superior and inferior quadrants of the five vertically sectioned specimens: three eyes showed thicker inferior quadrants, one eye had a thicker superior quadrant, and one eye (monkey 3) showed no difference. Both the superior and inferior quadrant thicknesses for monkey 3 were substantially less than those of the other vertically sectioned eyes, suggesting that intersubject differences in peripapillary scleral thickness can be substantial. 
Within the pooled regional data (Fig. 4) , the thickness pattern seen in the other analyses was maintained. All three regions of the nasal quadrant were significantly thinner than any of the corresponding regions of any of the other quadrants, but were not significantly different from one another. The regions of the superior and inferior quadrants were thinnest on the nasal side and thickened toward the temporal side. The three regions of the thicker temporal quadrant were not significantly different from one another. 
Peripapillary Scleral Thickness Topology Map
The continuum thickness data shown in Figure 4 represents the thickness of the average normal monkey eye perfusion fixed at an IOP of 10 mm Hg. This figure illustrates the substantial differences in pooled peripapillary scleral thickness by quadrant and PSCO distance in the normal monkey eye. 
Discussion
The purpose of this study was to characterize the thickness of the peripapillary sclera within normal monkey eyes perfusion fixed at an IOP of 10 mm Hg. The principal findings of this report are as follows. First, in all quadrants, monkey peripapillary sclera was thinnest at the posterior scleral canal opening (201 μm, nasal; 201 μm, temporal; 240 μm, inferior; and 249 μm, superior) and progressively thickened to a maximum (326 μm, nasal; 415 μm, superior; 420 μm, temporal; and 422 μm, inferior) 600 to 1000 μm from the PSCO. Second, peripapillary sclera in the normal monkey eye was generally thicker in the superior, inferior, and temporal quadrants (central region means of 369, 372, and 369 μm, respectively) and thinner within the nasal quadrant (central region mean of 291 μm). 
Several researchers have reported thicknesses for the posterior sclera in human eyes that are substantially greater than those we report for these monkey eyes. 7 8 9 Olson et al. 7 reported scleral thickness in humans of 900 to 1000 μm near the optic nerve, and Fine and Yanoff 8 characterized human scleral thicknesses of 1000 μm in the foveal region. These researchers observed a progressive thickening of human sclera, proceeding posteriorly from the equator to the fovea. This is consistent with our findings in the monkey eye, and the relative (percentage) thickening is similar in both humans and monkeys. 1 However, our data suggest that although the monkey posterior sclera shell thickens as one proceeds posteriorly from the equator, it is thickest 600 to 1000 μm away from the posterior scleral canal opening and thins again immediately adjacent to the scleral canal. 
That the sclera is thinnest immediately adjacent to the canal is most likely the result of the enlargement of the nerve within the canal, presumably due to myelination. Not only does the scleral canal expand between Bruch’s membrane and the posterior aspect of the lamina cribrosa (note the relative lengths of line “H” and “I” in Fig. 2 ), but the neural tissues also expand rapidly immediately beyond the posterior aspect of the canal (Fig. 2) , where the arachnoid and pial sheaths insert into the sclera. This expansion of the scleral canal alone dictates that the peripapillary sclera will be thinner in these regions. 
That the sclera is thinner nasally is most likely caused by the angle of the expanded scleral canal and optic nerve posterior to the lamina, relative to the scleral wall. We are currently constructing the first digital three-dimensional reconstructions of the scleral canal in normal and early glaucoma monkey eyes perfusion fixed at varying levels of IOP. 10 Within these reconstructions, the scleral canal wall is not perpendicular to the scleral wall, but rather is obliquely angled toward the optic chiasm. Therefore, for an individual eye, the degree and extent of nasal scleral thinning most likely follows from the relative obliqueness of the scleral canal (i.e., the length and angle of the axons’ passage through the scleral wall). 
If the scleral shell were a perfect mechanical pressure vessel, we would expect the sclera to be thickest close to the scleral canal, so as to better withstand the stresses that are concentrated around any hole in a pressurized spherical vessel. 4 11 12 The thickened region of sclera away from the canal that we report may represent a reinforcing ring that shields the thinner tissues adjacent to the canal from higher levels of stress. In this scenario, it may be that in some eyes, deformation of the “thinned” peripapillary sclera central to this ring may accompany deformation of the lamina cribrosa within the scleral canal under conditions of elevated pressure. 13  
Histologic evidence for a ring of collagen and elastin fibrils around the scleral canal has been reported by several investigators. 14 15 16 17 Whether this ring includes the thicker band of sclera 600 to 1000 μm away from the canal found in our study remains to be determined. It is of interest to note that this thickened region of sclera is readily apparent in the superior, inferior, and temporal quadrants, but is less evident in the nasal quadrant (Figs. 3 4) . An additional contributing cause of the greater thickening in these three quadrants may be tensile forces generated by the inferior oblique muscle and/or the optic nerve sheath when the eye looks down and in. 
In general, scleral wall stress is inversely proportional to scleral thickness, although this relationship is imperfect owing to the viscoelastic and anisotropic material properties of scleral tissue and the nonspherical shape of the scleral shell. 11 12 Thus, within the superior, inferior, and temporal quadrants, IOP-related stress immediately adjacent to the canal should be as much as 42% greater than stress 600 to 1000 μm away from the canal at a given level of IOP. Separately, in the thinner nasal quadrant, IOP-related stress should be, on average, 21% higher than stress in the other quadrants. 
The three-dimensional geometry of the scleral canal (its size, shape, and oblique orientation relative to the scleral wall), as well as the thickness of the peripapillary sclera, probably affects the magnitude of mechanical stress within the connective tissues of the scleral canal wall and peripapillary sclera for a given level of IOP. 18 Whether these differences influence the clinical susceptibility of the axons within the scleral canal to a given level of IOP remains to be determined. 
For a given vascular perfusion pressure, elevated IOP-related stress within the peripapillary sclera should diminish flow within the contained portions of the short posterior ciliary arteries, which pass relatively directly through the sclera to the choroid. Blood flow may also be restricted in the branching vessels that pass for longer distances within the sclera to supply the prelaminar, laminar, and retrolaminar optic nerve. Hayreh et al. have demonstrated by fluorescein angiogram that choroidal flow in normal eyes decreases with elevated IOP 19 and can be variably delayed through the individual branches of the short posterior ciliary arteries. 20 Langham 2 has suggested that the influence of IOP-related stress within the sclera on the contained vessels may be a mechanism for IOP-dependent vascular damage to the axons in glaucoma. Occlusion of the posterior ciliary arteries is presumed to be the central pathophysiology of anterior ischemic optic neuropathy (AION), 3 and AION is more likely to occur in patients with elevated IOP. 3 However, AION commonly occurs in the small, “at risk” disc, which is classically associated with mild to moderate hyperopia and which is assumed to include thicker, rather than thinner, sclera. 
Whether the relative thinness of the nasal sclera has additional, quadrant-specific effects on the flow of blood within the nasal branches of the short posterior ciliary arteries remains to be determined. Most of the short posterior ciliary arteries pierce the sclera nasal and temporal to the ONH. 21 22 To our knowledge, there is no evidence that the nasal posterior ciliary arteries are more commonly involved in either AION or glaucoma. However, because the areas of the choroid and ONH that are fed by the posterior ciliary arteries vary greatly among individuals, 21 22 occlusion of the nasal short posterior ciliary arteries (or their intrascleral branches) caused by higher scleral stresses may not necessarily manifest as AION or glaucoma that begins or worsens in the nasal quadrant of the ONH. 
At present, in vivo quantitative measurement of peripapillary scleral thickness and the volume flow of blood within the short posterior ciliary arteries and their intrascleral laminar and retrolaminar branches is not possible. Thus, the relationship between peripapillary scleral thickness, peripapillary scleral IOP-related stress and strain, and the volume flow of blood within the short posterior ciliary arteries and their intrascleral branches remains to be determined. 
Finally, if physiologic or pathophysiologic thinning of the peripapillary sclera contributes to a given optic nerve head’s susceptibility to glaucomatous optic neuropathy, 12 it may underlie how axial myopia contributes to that risk. From an engineering standpoint, the myopic optic nerve head may be more susceptible to a given level of IOP on several mechanistic levels. First, the scleral canal may be unusually large, abnormally shaped, and/or tilted, leading to elevated levels of IOP-related stress for a given level of IOP. 18 23 Second, the peripapillary sclera may be unusually thin, leading to higher IOP-related scleral wall stress and deformation. Third, the extracellular matrix of myopic sclera may be abnormally weak, causing larger scleral deformations for a given level of IOP. Fourth, apart from the thinning of the sclera, the increased size of the axially myopic eye should increase IOP-related scleral stress for a given level of IOP. 11 12 18  
Two clinical relationships should be present to support the notion that abnormally thin peripapillary sclera contributes to the risk of glaucoma in a myopic eye. 12 Individuals with axial myopia should demonstrate an increased incidence of glaucomatous neuropathy occurring at “normal” levels of IOP and a peripapillary sclera that is thinner than that seen in well-matched normal eyes. 
The relationship between myopia, IOP, and glaucomatous optic neuropathy remains controversial. 12 24 25 26 27 28 29 However, at least two rigorous studies have found an increased risk of glaucomatous optic neuropathy that was not accompanied by an increased risk of elevated IOP. 27 28 Among the 3654 participants in the Blue Mountains Eye Study, Mitchell et al. 28 reported the presence of glaucomatous optic neuropathy in 4.2% of eyes with low myopia (≥ −1 D to < −3D, based on refractive error, not axial length), 4.4% of eyes with moderate to high myopia (≥ −3D), and 1.5% of eyes without myopia. Most important, although there was an increased risk of ocular hypertension in the low (but not the moderate to high) myopia group, the overall statistical analysis strongly suggests that in this population, the relationship between glaucoma and myopia was independent of IOP. Daubs and Crick, 27 in a case–control study of nearly 1000 eyes, found that moderate and high myopia, again defined by refraction, not axial length, substantially increased the risk of glaucomatous optic neuropathy, and that this effect was again independent of IOP. 
To our knowledge, although comparisons of the thickness of the intact ocular wall have demonstrated thinning in myopic eyes, 30 a definitive in vivo study of peripapillary scleral thickness with adequate resolution to detect differences of less than 100 μm has not been performed and, at present, must await the availability of instrumentation with a resolution of at least 20 μm. 
Our study is limited by the possibility of tissue shrinkage or swelling due to fixation. In a recent study of human sclera, thickness did not change significantly in response to fixation. 7 In fact, there is some evidence to suggest that acellular collagenous tissues swell with fixation. 31 32 However, Panda-Jonas et al. 33 reported 12.5% linear shrinkage in the optic disc after fixation. Thus, although there is literature to suggest that our ex vivo posterior scleral thickness measurements should accurately estimate in vivo monkey scleral thickness, the possibility remains that, because of tissue shrinkage, we are underestimating scleral thickness. However, assuming that any shrinkage would occur evenly over all specimens, our scleral thickness measurements should still legitimately model the relative variation in scleral thickness by location that would be present in nonfixed monkey eyes. 
The peripapillary scleral thickness data described here along with the previously reported data for the posterior scleral shell 1 will be used to construct the first finite element models of the monkey posterior scleral shell. These models will serve to establish the boundary conditions for future finite element models of the lamina cribrosa and scleral canal wall. 18 In addition, they will become the basis of future attempts to model IOP-related effects on blood flow within the peripapillary scleral branches of the posterior ciliary arteries that supply the anterior optic nerve, ONH, and peripapillary choroid. 
 
Table 1.
 
Animal Data: Normal Monkey Eyes at a Manometer-Controlled IOP of 10 mm Hg
Table 1.
 
Animal Data: Normal Monkey Eyes at a Manometer-Controlled IOP of 10 mm Hg
Monkey Weight (kg) Age, * (y) Eye Axial Length (mm) Section Orientation Number of Sections Measured
No. ID
1 1T 7.5 6 OS 21.4 Horizontal 26
2 1S 6.0 6 OS 20.9 Horizontal 19
3 1Z 7.5 6 OS 21.6 Vertical 16
4 2D 8.3 5 OS 21.5 Vertical 14
5 1W 5.7 6 OD 20.5 Horizontal 19
6 2A 5.8 11 OS 20.5 Horizontal 13
7 1U 6.8 7 OS 20.3 Vertical 15
8 1V 6.8 7 OS 20.8 Vertical 20
9 1R 6.7 10 OS 20.3 Vertical 15
Figure 1.
 
Diagram of a trephined ONH specimen showing the treatment condition (A), the distribution of specimens for horizontal and vertical sectioning and the approximate spacing between measured sections (every 24th serial 4-μm sagittal section) (B), the position of the peripapillary scleral thickness measurement points within each measured specimen (C), and the classification of the pooled measurement points by quadrant and region (D).
Figure 1.
 
Diagram of a trephined ONH specimen showing the treatment condition (A), the distribution of specimens for horizontal and vertical sectioning and the approximate spacing between measured sections (every 24th serial 4-μm sagittal section) (B), the position of the peripapillary scleral thickness measurement points within each measured specimen (C), and the classification of the pooled measurement points by quadrant and region (D).
Figure 2.
 
Representative digital image of a serial sagittal histologic section of an ONH specimen (A) showing the location of the marks placed by the observer: A, the termination points of Bruch’s membrane; B, the anterior scleral surface; C, the posterior scleral surface; D, the anterior insertions of the lamina cribrosa; E, the PSCO; F, the anterior laminar surface; and G, the posterior laminar surface. Within each marked image, custom, automated image analysis software was used (B) to estimate peripapillary scleral thickness: H, the width of Bruch’s membrane opening; I, width of the PSCO; J, the thickness of the sclera at the PSCO; and K, the thickness of the dense peripapillary sclera (separate from the adjacent loose episcleral tissues) measured every 100 μm from the PSCO. 0, 500, 1000, and 1500 indicate the distance (in micrometers) from the PSCO.
Figure 2.
 
Representative digital image of a serial sagittal histologic section of an ONH specimen (A) showing the location of the marks placed by the observer: A, the termination points of Bruch’s membrane; B, the anterior scleral surface; C, the posterior scleral surface; D, the anterior insertions of the lamina cribrosa; E, the PSCO; F, the anterior laminar surface; and G, the posterior laminar surface. Within each marked image, custom, automated image analysis software was used (B) to estimate peripapillary scleral thickness: H, the width of Bruch’s membrane opening; I, width of the PSCO; J, the thickness of the sclera at the PSCO; and K, the thickness of the dense peripapillary sclera (separate from the adjacent loose episcleral tissues) measured every 100 μm from the PSCO. 0, 500, 1000, and 1500 indicate the distance (in micrometers) from the PSCO.
Table 2.
 
Reproducibility Study: Peripapillary Scleral Thickness by Quadrant and Measurement Day in Both Quadrants of Two Monkey Eyes
Table 2.
 
Reproducibility Study: Peripapillary Scleral Thickness by Quadrant and Measurement Day in Both Quadrants of Two Monkey Eyes
Quadrant Monkey 7—Measurement Day Monkey 5—Measurement Day
1 2 3 1 2 3
Superior 249 ± 11 257 ± 11 259 ± 11
Inferior 273 ± 13 285 ± 13 296 ± 13
Nasal 244 ± 8 248 ± 8 240 ± 8
Temporal 319 ± 8 325 ± 8 333 ± 8
Figure 3.
 
Overall mean peripapillary scleral thickness in the central region of each quadrant plotted as a function of distance from the posterior scleral canal opening.
Figure 3.
 
Overall mean peripapillary scleral thickness in the central region of each quadrant plotted as a function of distance from the posterior scleral canal opening.
Figure 4.
 
Contour map of the mean peripapillary scleral thickness topology (micrometers) for all study eye data combined, showing the PSCO (ONH) and the mean positions of the central and peripheral regional measurement points within each quadrant.
Figure 4.
 
Contour map of the mean peripapillary scleral thickness topology (micrometers) for all study eye data combined, showing the PSCO (ONH) and the mean positions of the central and peripheral regional measurement points within each quadrant.
Table 3.
 
Peripapillary Scleral Thickness in the Central Region of Each Quadrant in Each Eye
Table 3.
 
Peripapillary Scleral Thickness in the Central Region of Each Quadrant in Each Eye
Monkey Quadrant
No. ID Superior Inferior Nasal Temporal
1 1T 319 ± 4* 418 ± 4
2 1S 280 ± 4* 311 ± 5, †
3 1Z 313 ± 3 309 ± 3
4 2D 456 ± 7 397 ± 7*
5 1W 244 ± 4* 319 ± 4
6 2A 308 ± 5* 376 ± 7, ‡
7 1U 365 ± 7* 390 ± 6
8 1V 358 ± 5* 382 ± 5
9 1R 364 ± 7* 386 ± 7
The authors thank Stephanie Hager, Budd Hirons, Juan Reynaud, and Lindell Skinner for technical assistance. 
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Figure 1.
 
Diagram of a trephined ONH specimen showing the treatment condition (A), the distribution of specimens for horizontal and vertical sectioning and the approximate spacing between measured sections (every 24th serial 4-μm sagittal section) (B), the position of the peripapillary scleral thickness measurement points within each measured specimen (C), and the classification of the pooled measurement points by quadrant and region (D).
Figure 1.
 
Diagram of a trephined ONH specimen showing the treatment condition (A), the distribution of specimens for horizontal and vertical sectioning and the approximate spacing between measured sections (every 24th serial 4-μm sagittal section) (B), the position of the peripapillary scleral thickness measurement points within each measured specimen (C), and the classification of the pooled measurement points by quadrant and region (D).
Figure 2.
 
Representative digital image of a serial sagittal histologic section of an ONH specimen (A) showing the location of the marks placed by the observer: A, the termination points of Bruch’s membrane; B, the anterior scleral surface; C, the posterior scleral surface; D, the anterior insertions of the lamina cribrosa; E, the PSCO; F, the anterior laminar surface; and G, the posterior laminar surface. Within each marked image, custom, automated image analysis software was used (B) to estimate peripapillary scleral thickness: H, the width of Bruch’s membrane opening; I, width of the PSCO; J, the thickness of the sclera at the PSCO; and K, the thickness of the dense peripapillary sclera (separate from the adjacent loose episcleral tissues) measured every 100 μm from the PSCO. 0, 500, 1000, and 1500 indicate the distance (in micrometers) from the PSCO.
Figure 2.
 
Representative digital image of a serial sagittal histologic section of an ONH specimen (A) showing the location of the marks placed by the observer: A, the termination points of Bruch’s membrane; B, the anterior scleral surface; C, the posterior scleral surface; D, the anterior insertions of the lamina cribrosa; E, the PSCO; F, the anterior laminar surface; and G, the posterior laminar surface. Within each marked image, custom, automated image analysis software was used (B) to estimate peripapillary scleral thickness: H, the width of Bruch’s membrane opening; I, width of the PSCO; J, the thickness of the sclera at the PSCO; and K, the thickness of the dense peripapillary sclera (separate from the adjacent loose episcleral tissues) measured every 100 μm from the PSCO. 0, 500, 1000, and 1500 indicate the distance (in micrometers) from the PSCO.
Figure 3.
 
Overall mean peripapillary scleral thickness in the central region of each quadrant plotted as a function of distance from the posterior scleral canal opening.
Figure 3.
 
Overall mean peripapillary scleral thickness in the central region of each quadrant plotted as a function of distance from the posterior scleral canal opening.
Figure 4.
 
Contour map of the mean peripapillary scleral thickness topology (micrometers) for all study eye data combined, showing the PSCO (ONH) and the mean positions of the central and peripheral regional measurement points within each quadrant.
Figure 4.
 
Contour map of the mean peripapillary scleral thickness topology (micrometers) for all study eye data combined, showing the PSCO (ONH) and the mean positions of the central and peripheral regional measurement points within each quadrant.
Table 1.
 
Animal Data: Normal Monkey Eyes at a Manometer-Controlled IOP of 10 mm Hg
Table 1.
 
Animal Data: Normal Monkey Eyes at a Manometer-Controlled IOP of 10 mm Hg
Monkey Weight (kg) Age, * (y) Eye Axial Length (mm) Section Orientation Number of Sections Measured
No. ID
1 1T 7.5 6 OS 21.4 Horizontal 26
2 1S 6.0 6 OS 20.9 Horizontal 19
3 1Z 7.5 6 OS 21.6 Vertical 16
4 2D 8.3 5 OS 21.5 Vertical 14
5 1W 5.7 6 OD 20.5 Horizontal 19
6 2A 5.8 11 OS 20.5 Horizontal 13
7 1U 6.8 7 OS 20.3 Vertical 15
8 1V 6.8 7 OS 20.8 Vertical 20
9 1R 6.7 10 OS 20.3 Vertical 15
Table 2.
 
Reproducibility Study: Peripapillary Scleral Thickness by Quadrant and Measurement Day in Both Quadrants of Two Monkey Eyes
Table 2.
 
Reproducibility Study: Peripapillary Scleral Thickness by Quadrant and Measurement Day in Both Quadrants of Two Monkey Eyes
Quadrant Monkey 7—Measurement Day Monkey 5—Measurement Day
1 2 3 1 2 3
Superior 249 ± 11 257 ± 11 259 ± 11
Inferior 273 ± 13 285 ± 13 296 ± 13
Nasal 244 ± 8 248 ± 8 240 ± 8
Temporal 319 ± 8 325 ± 8 333 ± 8
Table 3.
 
Peripapillary Scleral Thickness in the Central Region of Each Quadrant in Each Eye
Table 3.
 
Peripapillary Scleral Thickness in the Central Region of Each Quadrant in Each Eye
Monkey Quadrant
No. ID Superior Inferior Nasal Temporal
1 1T 319 ± 4* 418 ± 4
2 1S 280 ± 4* 311 ± 5, †
3 1Z 313 ± 3 309 ± 3
4 2D 456 ± 7 397 ± 7*
5 1W 244 ± 4* 319 ± 4
6 2A 308 ± 5* 376 ± 7, ‡
7 1U 365 ± 7* 390 ± 6
8 1V 358 ± 5* 382 ± 5
9 1R 364 ± 7* 386 ± 7
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