Figure 7
Schematic illustration of optic nerve head (ONH) canal in absence (A) and presence (B) of outer-wall shift. The intervening choroid is omitted for clarity. (Top) Three-dimensional view. A black overlying plane indicates the Bruch’s membrane opening (BMO), and the underneath cuboid with central hole indicates the scleral part of the ONH canal with the anterior scleral opening (ASCO) on the surface. (Bottom) Cross-sectional view. The red dots indicate the BMO margin, the blue dots the ASCO margin, the green dots the anterior lamina cribrosa (LC) insertion site, and the red lines the central retinal vascular trunk (CRVT). The peripapillary border tissue of the choroid (PBT-C) is the black line between the red and blue dots, and the peripapillary border tissue of the scleral flange (PBT-S) is the black line between the blue and green dots. The ASCO/BMO offset is the misalignment between the red and blue dots, and the LC/BMO offset, between the red and green dots. As the LC is located in a lower part of the ONH canal than the ASCO is, the LC/BMO offset is generally larger than the ASCO/BMO offset if the PBT-C and the PBT-S share a common directionality (B, Middle). If the PBT-S rotates further than the PBT-C, however, the LC/BMO offset can increase independently of the ASCO/BMO offset (B, Bottom; green arrow). We speculated that this could happen, due to the fact that the upper layer (the posterior polar retinal structure) is selectively affected by the restriction against expansion, whereas the lower layer (sclera and LC) is not.
