PURPOSE. The purpose of this study was to estimate human trabecular meshwork (hTM) stiffness, thought to be elevated in glaucoma, using a novel indirect approach, and to compare results with direct en face atomic force microscopy (AFM)...
morePURPOSE. The purpose of this study was to estimate human trabecular meshwork (hTM) stiffness, thought to be elevated in glaucoma, using a novel indirect approach, and to compare results with direct en face atomic force microscopy (AFM) measurements. METHODS. Postmortem human eyes were perfused to measure outflow facility and identify high-and low-flow regions (HF, LF) by tracer. Optical coherence tomography (OCT) images were obtained as Schlemm's canal luminal pressure was directly manipulated. TM stiffness was deduced by an inverse finite element modeling (FEM) approach. A series of AFM forcemaps was acquired along a line traversing the anterior angle on a radially cut flat-mount corneoscleral wedge with TM facing upward. RESULTS. The elastic modulus of normal hTM estimated by inverse FEM was 70 6 20 kPa (mean 6 SD), whereas glaucomatous hTM was slightly stiffer (98 6 19 kPa). This trend was consistent with TM stiffnesses measured by AFM: normal hTM stiffness ¼ 1.37 6 0.56 kPa, which was lower than glaucomatous hTM stiffness (2.75 6 1.19 kPa). None of these differences were statistically significant. TM in HF wedges was softer than that in LF wedges for both normal and glaucomatous eyes based on the inverse FEM approach but not by AFM. Outflow facility was significantly correlated with TM stiffness estimated by FEM in six human eyes (P ¼ 0.018). CONCLUSIONS. TM stiffness is higher, but only modestly so, in glaucomatous patients. Outflow facility in both normal and glaucomatous human eyes appears to associate with TM stiffness. This evidence motivates further studies to investigate factors underlying TM biomechanical property regulation. T he cause of increased outflow resistance leading to ocular hypertension in glaucoma remains unknown. However, several intriguing studies suggest that human trabecular meshwork (hTM) stiffness may differ in glaucomatous versus unaffected eyes. 1 These studies raise the possibility that TM stiffness might be involved in the pathogenesis of ocular hypertension in glaucoma. Last et al., in an important early study, used atomic force microscopy (AFM) in dissected TM samples from postmortem eyes to show that TM stiffness was markedly elevated in glaucomatous versus normal eyes. 2 Subsequently, Camras et al. examined the tensile stiffness of dissected human TM 3–5 via uniaxial testing. This study found that the glaucomatous TM was softer than normal TM, contrary to the results of the AFM testing. This discrepancy might be in part due to different testing modes used (tension versus compression). Johnson et al. (IOVS 2015;56:ARVO E-Abstract 3541) estimated hTM stiffness in vivo in normal eyes using OCT imaging and an indirect method based on changes in TM geometry. In all of the above studies, there were methodologic concerns, including dissection that may have damaged/altered the TM; different mechanical loading conditions that in some cases did not replicate the loads that occur in vivo; and/or oversimplified modeling assumptions. In view of the limitations of the above studies, it is important to explore alternate approaches for TM stiffness estimation to determine whether TM stiffness is truly altered in eyes with ocular hypertension. Optical coherence tomography (OCT) is a cross-sectional, three-dimensional imaging technique with high spatial resolution (<20 lm). 6 Recent studies have used OCT for characterization of tissue structure and movement inside the eye. 7–11 Specifically, spectral domain OCT (SD-OCT) was used to image radial limbal segments in ex vivo primate eyes. A cannula was inserted into Schlemm's canal (SC) to control pressure inside SC lumen. A pressure gradient was introduced across the TM that differed from the gradient expected due to standing, sitting, supine, or prone positions. Instead, pressure gradients were typical of those encountered during body inversion (e.g., gymnastics and yoga) and the spectrum of partial inversions
