Evaluation of the ultrastructural and in vitro flow properties of the PreserFlo® microShunt

Abstract

To measure the in vitro flow properties of the PRESERFLO implant for comparison with the theoretical resistance to flow. Methods:The PRESERFLO was designed to control the flow of aqueous humor according to the Hagen-Poiseuille (HP) equation. Scanning electron microscopy (SEM) was performed to analyze the ultrastructure, and flow measurements were carried out using a gravity-flow setup.Results: SEM images of the PRESERFLO showed luminal diameters of 67.73 × 65.95 μm and 63.66 × 70.54 μm. The total diameter was 337.2 μm, and the wall was 154 μm wide. The theoretical calculation of the resistance to flow (R) for an aqueous humor (AH) viscosity of 0.7185 centipoises (cP) was 1.3 mm Hg/(μL/min). Hence, assuming a constant AH flow of 2 μL/min, the pressure differential across the device ( P) was estimated to be 2.6 mm Hg. The gravity-flow experiment allowed us to measure the experimental resistance to flow, which was R E = 1.301 mm Hg/(μL/min), in agreement with the theoretical resistance to flow R given by the HP equation. Conclusions:The experimental and theoretical flow testing showed that the pressure drop across this device would not be large enough to avoid hypotony unless the resistance to outflow of the sub-Tenon space was sufficient to control the intraocular pressure in the early postoperative period.Translational Relevance: The fluid properties of glaucoma subconjunctival drainage devices determine their specific bleb-forming capacity and ability to avoid hypotony and therefore their safety and efficacy profile.

Efficacy and Safety of the Preserflo Microshunt With Mitomycin C for the Treatment of Open Angle Glaucoma

“…In this sense, it is important to remark that the authors reported 3 out of 32 cases of surgical revision in the 0.2 mg/mL MMC group (9.3%), while in our series the percentage was higher (15.6%), due to early failure (3%) or late fibrosis (12.5%) of the filtering blebs. It is very likely that the rate of bleb fibrosis was actually comparable among their 0.2 mg/mL MMC subgroup of patients and ours, given that Preserflo provides a standardized flow rate of aqueous humor (theoretical and experimental resistance to flow=1.3 mm Hg/μL/min, pressure drop 2.6 mm Hg23), but the way in which it was managed was probably different. In fact, 1 year after surgery they reported 0.9 mean number of medications versus 0.2 in our series, suggesting a probable trend toward the use of medication instead of surgical revision for the treatment of bleb failure in their study.…”

“…Compared with its main competitor, XEN45 (XEN-GGM; Allergan Plc, Parsippany, NJ, EEUU), most studies have reported hypotony rates comparable to PSM (Table 7), despite resistance to flow is 3.4-fold higher through XEN45. 23 Interestingly, in the study conducted by Kim et al, 29 63.2%-83.3% of the patients experienced hypotony, probably due to the high concentration of injected MMC used in the study (70% of the patients received an injection of 0.4 mg/mL without wash-out during an "ab interno"-closed conjunctiva technique). Regarding other filtering surgeries, Tables 8 and 9 show the hypotony rates reported by some studies about trabeculectomy, Express (Alcon, Fort Worth, TX, EEUU) and glaucoma drainage devices.…”

“…It is interesting to point out that the rate of hypotony reported in clinical studies ranges widely between 0% and 69%,8,9,13–16,20 even though most of them report transient hypotony, with nearly no cases of persistent hypotony. Compared with its main competitor, XEN45 (XEN-GGM; Allergan Plc, Parsippany, NJ, EEUU), most studies have reported hypotony rates comparable to PSM (Table 7), despite resistance to flow is 3.4-fold higher through XEN45 23. Interestingly, in the study conducted by Kim et al,29 63.2%–83.3% of the patients experienced hypotony, probably due to the high concentration of injected MMC used in the study (70% of the patients received an injection of 0.4 mg/mL without wash-out during an “ab interno”—closed conjunctiva technique).…”

“…5 The review conducted by Bell et al 22 concluded that with PSM, the trend is to achieve higher rates of success with higher concentrations of MMC, increasing the rate of device-related AEs and reoperations as well. The clinical study with the longest follow-up has been carried out by Batlle et al, 9 reporting the outcomes of 23 patients with primary open angle glaucoma (POAG) 5 years after surgery using 0.4 mg/mL MMC for 3 minutes. The authors concluded that the reduction of the mean IOP and the number of hypotensive medications on the long term after DOI: 10.1097/IJG.0000000000002052 PSM implantation was sustained over time, with a low rate of postoperative complications and no long-term sight-threatening AEs.…”