Analysis of Schwalbe’s Line (Limbal Smooth Zone) by Scanning Electron Microscopy and Optical Coherence Tomography in Human Eye Bank Eyes

Purpose: Implantation of intraocular devices may become critical as they decrease in size in the future. Therefore, it is desirable to evaluate the relationship between radial location and Schwalbe’s line (smooth zone) by examining its width with scanning electron microscopy (SEM) and to correlate this with observations by optical coherence tomography (OCT). Methods: Full corneoscleral rings were obtained from twenty-six formalin-fixed human phakic donor eyes. SEM of each eye yielded a complete montage of the smooth zone, from which the area was measured, and width was determined in each quadrant. In three different eyes, time domain anterior segment OCT (Visante, Carl Zeiss Meditec Inc., Dublin, CA, USA) and spectral domain OCT (Cirrus 4.0, Carl Zeiss Meditec Inc., Dublin, CA, USA) were used to further characterize Schwalbe’s line. Results: The overall smooth zone width was 79±22 μm, (n=15) ranging from 43 to 115 μm. The superior quadrant (103±8 μm, n=19), demonstrated significantly wider smooth zone than both the nasal (71±5 μm, n=19, P<0.001), and inferior (64±5 μm, n=18, P<0.0001) quadrants but not the temporal quadrant (81±7 μm, n=17, P>0.05). SEM findings of the smooth zone were correlated with visualization of Schwalbe’s line by Cirrus and Visante OCT imaging. Conclusion: The smooth zone appears widest superiorly and thinnest inferonasally, suggesting that as glaucoma surgical devices become smaller, their placement could be targeted clinically by using OCT with preference to the superior quadrant, to minimize damage to the corneal endothelium.