| dc.description.abstract | ABSTRACT Glaucoma encompasses a group of conditions that result in damage to the optic nerve and can cause loss of vision and blindness. The nerve is damaged due to an increase in the eye’s internal (intraocular) pressure (IOP) above the nominal range of 15 – 20 mm Hg. There are many treatments available for this group of diseases depending on the complexity and stage of nerve degradation. In extreme cases where drugs or laser surgery do not create better conditions for the patient, ophthalmologists use glaucoma drainage devices to help alleviate the IOP. Many drainage implants have been developed over the years and are in use; but two popular implants are the Baerveldt Glaucoma Implant and the Ahmed Glaucoma Valve Implant. Baerveldt Implants are non-valved and provide low initial resistance to outflow of fluid, resulting in post-operative complications such as hypotony, where the IOP drops below 5 mm of Hg. Ahmed Glaucoma Valve Implants are valved implants which initially restrict the amount of fluid flowing out of the eye. The long term success rates of Baerveldt Implants surpass those of Ahmed Valve Implants because of post-surgical issues; but Baerveldt Implants’ initial effectiveness is poor without proper flow restriction. This drives the need to develop new ways to improve the initial effectiveness of Baerveldt Implants. A possible solution proposed by our research team is to place an insert in the Baerveldt Implant tube of inner diameter 305 microns. The insert must be designed to provide flow resistance for the early time frame [e.g., first 30 – 60 post-operative days] until sufficient scar tissue has formed on the implant. After that initial stage with the insert, the scar tissue will provide the necessary flow resistance to maintain the IOP above 5 mm Hg. The main objective of this project was to develop and validate an experimental apparatus to measure pressure drop across a Baerveldt Implant tube, with and without inserts. This setup will be used in the future to evaluate custom inserts and their effects on the pressure drop over 4 – 6 weeks. The design requirements were: simulate physiological conditions [flow rate between 1.25 and 2.5 μl/min], evaluate small inner diameter tubes [50 and 75 μm] and annuli, and demonstrate reliability and repeatability. The current study was focused on benchmarking the experimental setup for the IOP range of 15 – 20 mm Hg. Repeated experiments have been conducted using distilled water with configurations [diameter of tube, insert diameter, lengths of insert and tube, and flow rate] that produce pressure variations which include the 15 – 20 mm Hg range. Two similar setups were assembled and evaluated for repeatability between the two. Experimental measurements of pressure drop were validated using theoretical calculations. Theory predicted a range of expected values by considering manufacturing and performance tolerances of the apparatus components: tube diameter, insert diameter, and the flow-rate and pressure [controlled by pump]. Benchmarking trials for Poiseuille flow used tubes [without inserts] that have inner diameters of 50 and 75 microns. The experimental data were within the theoretical range of 48.2 – 103.2 mm Hg for 50 μm tubes and 9.2 – 16.8 mm Hg for 75 μm tubes for experiments run at 2.5 μl/min. The two setups differed by about 1 mm Hg for a 15 mm Hg pressure drop [about 6%] in a 75 micron tube. Further benchmarking trials for annular flow were conducted using a standard size wire [diameter 0.270 mm] inserted in a syringe needle [inner diameter of 0.340 mm]. The two pieces of apparatus produced results [with an average of 2.98 ± 0.32 mm Hg] which were within the theoretical pressure range [0.13 to 3.23 mm Hg] and had a difference of about 0.5 – 1 mm Hg in the measured pressures. Trials were also conducted with different types of sutures placed in the implant tubes to form annular flow. The following suture properties were varied: absorbable/non-absorbable, flow rates [1.5 and 2.5 μl/min], lengths [4 and 8 mm] and diameters [0.15, 0.2, and 0.3 mm]. The results from these trials indicated that the pressure profiles of absorbable sutures increase over time [from 3 to 5 mm Hg], probably because the sutures expand as they become hydrolyzed. However, the pressure profiles for non-absorbable sutures demonstrated a steady pressure [from 0.5 to 1.5 mm Hg]. Hence, the two setups can be used to verify the pressure responses of different types of inserts when they are placed within tubes of dimensions similar to those of Baerveldt Implant tubes. It is recommended that trials be conducted with different needle and wire combinations to generate a good data base to further benchmark the annular flow. In addition, preliminary experiments evaluated the dissolution of suture samples in a balanced salt solution and in distilled water. The balanced salt solution approximates the eye’s aqueous humor properties, and it was expected that the salt and acid would help to hydrolyze sutures much faster than distilled water. Suture samples in a balanced salt solution showed signs of deterioration [flaking] within 23 days, and distilled water samples showed only slight signs of deterioration after about 30 days. These preliminary studies indicate that future dissolution and flow experiments should be conducted using the balanced salt solution. Also, the absorbable sutures showed signs of bulk erosion/deterioration in a balanced salt solution after 14 days, which indicates that they may not be suitable as inserts in the implant tubes because flakes could block the tube entrance. Further long term studies should be performed in order to understand the effects of constant fluid movement over the surfaces of the absorbable sutures, by better means of rocking/shaking test suture samples to simulate flow conditions. | |
