Elastic Modulus Determination of Normal and Glaucomatous Human Trabecular Meshwork

The recent publication, by Last et al. ¹ (with PR as a co-author) stimulated active discussion between us regarding modeling of the possible consequences of increasing stiffness of the trabecular meshwork (TM) on aqueous humor outflow. This area of investigation is a nascent one, and emerging from these discussions are considerations regarding the use of the model that investigators in the field may find of value. In the model presented in the article, a simple, elastic, porous membrane was used to determine the flow resistance associated with fluid flow through the TM. This model allowed an exploration of the hydrodynamic consequences of altering the stiffness of the TM, as observed in the glaucomatous TM. 

From our subsequent discussions, it became apparent that readers may benefit from a fuller explanation of the limitations of the model and an expanded discussion of issues related to aqueous outflow that the article did not address. We want to take this opportunity to highlight these elements to better inform discussions regarding the possible implications of the altered stiffness of the glaucomatous TM. 

The model used the size of the pores in the inner-wall endothelium of Schlemm's canal as a uniform opening throughout the entire TM. However, the length of these pores is much shorter than the spatial dimensions of the TM, which has a length of perhaps 50 to 100 μm (at least 100-fold longer than an inner-wall pore). The porosity (the volume of the pores divided by the total volume of the region of the TM) used in the model was approximately 0.03%, several orders of magnitude lower than the measured porosity of the juxtacanalicular connective tissue (greater than 30%). ² The use of measured values of juxtacanalicular connective tissue porosity in the model would, of course, yield predicted outflow resistances much lower than the measured values. This same discrepancy between calculated and measured outflow resistance has been reported in other models used to describe flow through the juxtacanalicular connective tissue. ^{2 –5} It is important to recognize that this simple model is far removed from the rich, three-dimensional architecture possessed by the TM in situ. The TM and the interface with Schlemm's canal are complex and have proteoglycans and matricellular proteins associated with them that almost certainly create openings for aqueous humor flow that differs from those found by morphologic evaluation. ^4,6  

The article presented a simple model that explores one component contributing to outflow resistance, but the dynamics of aqueous outflow are of course more complex than that represented by a simple membrane model. Another contributing factor not discussed by Last et al. ¹ is that the increased stiffness of the glaucomatous TM would lead to an increased stiffness of cells in this region, particularly the Schlemm's canal cells, and that this increased stiffness may be important in the process of pore formation. Indeed, cells grown on stiffer matrices increase stress fiber formation and increase their modulus. ^{7 –9} We note that the impact of meshwork stiffness on the process of being able to form pores is distinct from the modeling exercise presented wherein pores were assumed to be static in number. 

Another possible contributing factor is that the increased stiffness of the meshwork could alter gene and protein expression in TM cells and may alter phagocytosis by these cells. The cause of the increased TM stiffness in glaucoma is not yet known, but may become an important pharmacologic target. 

We appreciate the opportunity to provide this information above, which we hope will enrich the discussions regarding the role of the biomechanical attributes of the outflow pathway and its relevance to the onset and progression of glaucoma.