Force and Geometry Determine Structure and Function of Glaucoma Filtration Capsules

Abstract

Background/Aims: Glaucoma shunts vent aqueous humor to orbital tissue across a fibrous capsule. The capsule consists of collagen and fibroblasts, and its formation was assumed to be a cicatricial process. Capsule fibrosis remains the leading cause of device failure. We tested the wound healing hypothesis using morphometry and polarization microscopy and found the capsule consists not of scar tissue but rather highly organized extracellular matrix. Methods: Polarization microscopy reveals collagen orientation within the extracellular matrix. Rotation of the specimen in the plane of polarized light reveals collagen bundle orientation with respect to capsule surface geometry. Capsule thickness was measured and correlated with device dimension. Results: Capsule collagen is highly organized and not scar tissue. Relative orientation is predictable from simple engineering principles and shows tissue deposits reinforcing collagen parallel to its principle stress axes along the capsule surface. Capsule thickness and collagen orientation correlate with device dimension and surface geometry. Conclusion: The corollary is true. Geometry can be used to manage the amount, distribution and orientation of collagen and thus control hydraulic conductivity and surface area of the filtration capsule.

Magnetic Resonance Imaging Study on Blebs Morphology of Ahmed Valves

“… 15 16 This thickened wall provides increased resistance to being crossed by the aqueous humor may thus result in an IOP increase. 17 18 As the aqueous humor (AH) content of inflammatory cytokines is related to the IOP level, then at least initially, the subconjunctival space of high-IOP patients would be bathed by AH with higher concentrations of inflammatory cytokines. 6 This might be an explanation as to why valved implants are associated with greater thickness of the capsular wall 19 and more probability of bleb dysmorphia and creation of giant reservoirs, 20 21 as nonvalved surgery is usually done resorting to techniques that prevent, at least in an early stage, a free drainage of aqueous humor.…”

Surface Topographies of Glaucoma Drainage Devices and Their Influence on Human Tenon Fibroblast Adhesion

“…It has been shown that fibroblasts react to mechanical stress by increasing the expression of actin stress fibers as well as ECM components. 41 Wilcox and Kadri 14,15 presented evidence that this mechanism is active in tenon fibroblasts around the GDD, showing that collagen fibers and cells align along the direction of tension caused by sharp edges or bulky base plates. The RhoA/Rock signaling pathway appears to play a prominent role in this response to mechanical stimuli.…”

“…Among these, the choice of biomaterial is believed to influence the initial inflammatory response to the implant, 12 whereas shape, size, and rigidity of the base plate are thought to influence fibroblast growth and extracellular matrix (ECM) deposition through mechanically induced activation of the cells. [13][14][15] One aspect of implant design that has been largely ignored is the surface texture of the base plate. Since the surface constitutes the major site of interaction with the surrounding tissue, topographic features as well as surface chemistry may well influence wound healing and the fibrotic response.…”

Resonant magnetoelastic microstructures for wireless actuation of liquid flow on 3D surfaces and use in glaucoma drainage implants

“…The tube drains AH into the space created by the plate body, located in a subconjunctival space over the sclera (Figure 1a). Over time, a fibrous capsule of tissue, or "bleb," is formed around the AGDD, serving as a reservoir for the extra AH that drains from the eye [23][24][25][26][27][28][29] . In a well-functioning implant with a controlled IOP, the bleb has a thickness of less than 500 μm 24 .…”