Glaucoma Implants: Results

Introduction

Modern glaucoma implants have come to solve the difficult task of maintaining a surgical fistula between the anterior chamber (AC) and the subconjunctival space for prolonged periods of time. Initial attempts using different materials had poor and unpredictable results, failing partly because they aimed to drain aqueous near the limbus, as in usual filtering surgeries at the time. The result was to obtain the same old complications plus new ones derived from the implants.

Historical Background

Molteno saw the need to provide a means to maintain the draining space despite scarring, adding an 8 mm acrylic plate for that purpose, and for his first cases he placed his implant near the limbus. The idea was for a temporary bleb to form, then removal of the implant would be done a couple of months later. Results were not very good due to frequent irritation, dellen formation, and tube and plate extrusions. Around 1973, he introduced the concept of using a posteriorly placed larger implant, away from the limbus, connected through a long tube to the anterior chamber, starting a new era in implant surgeries.

Acceptance of the new implant took a very long time, especially because the initial postoperative period was plagued with hypotony-related complications. The first strategy to avoid these complications was to divide the surgery into two stages: placing the plate under the sub-Tenon's space in one operation, and about 4–8 weeks later placing the tube inside the anterior chamber. This strategy was very useful, but the requirement for a second surgical procedure did not make it very popular.

An alternative method to avoid hypotony was developed by Krupin, where the closed end of a supramid tube, attached to an intracameral Silastic tube, had a couple of slit openings that permitted flow with intraocular pressures (IOP) between 11 and 16 mmHg, and closed when IOP fell after a bolus of aqueous had left the AC. This group later attached the system to a grooved scleral explant (effectively adding a restrictive mechanism to a Schocket procedure) to increase and maintain the filtration area, and a few years later (around 1990) finally attached the system to a silicone filtering disk. Tendency for failures related to slit valve occlusion, which has been reported with all its designs, has made it fall in popularity.

The next step that finally sparked further research by many other groups around the world and new valve designs, was the introduction of the absorbable Vicryl suture, tying off the tube, allowing one surgery to behave like the two-stage procedure. The tube is inserted into the AC at the time of implantation, but it does not drain aqueous at the beginning, avoiding early hypotony. Filtration starts when the Vicryl suture is absorbed by the surrounding tissues, usually 3–6 weeks after primary implantation. Adding slit cuts anterior to the ligature also permits partial control of IOP during the first weeks by letting some aqueous escape anterior to the plate.

The use of internal partial tamponade with chromic 4/0 gut was introduced as an alternative technique that also reduced the risk of early hypotony and permitted a single-stage implant procedure. The suture was threaded through the tube, and then left partially visible under the conjunctiva for easy removal at a later time. Concerns about the risk of infection have reduced trust in leaving the suture exposed and motivated the use of alternative suture material that permitted easy removal despite conjunctival coverage.

Another option was the introduction of the pressure ridge to the Molteno implant. The ridge, coupled to episcleral tissues, serves as a pressure-sensitive device that permits early flow, reducing the risk of early hypotony, albeit not in a very consistent manner, probably due to differences in surgical technique for stretching Tenon's capsule over the implant.

In the 1980s, a couple of devices with mechanisms to regulate IOP were tried and failed to give adequate results, namely the Mendez glaucoma seton and White's glaucoma pump.

The introduction of the Ahmed glaucoma valve at the beginning of the next decade finally delivered a more reliable one-way valve within a glaucoma seton, providing a means to achieve IOP control in a one-stage procedure without using extraneous material in or around the tube. The mechanism allowed for closing of flow if the IOP went below 8 mmHg, working as a real valve, and thus reducing the risk of early hypotony.

The other problem encountered with all implants has been bleb fibrosis and loss of long-term IOP control. The first strategy to overcome this problem was also proposed by Molteno and involved a combination of oral steroids, oral nonsteroidal anti-inflammatory agents, and colchicine. His next strategy was to increase the size of the drainage area, introducing his double-plate implant in 1981. The new implant, with a total surface area of 274 mm ² , proved to be more effective to maintain IOP control, at the expense of increased surgical time and risk of complications, but not as many as when he used 3- or 4-plate implantations.

George Baerveldt introduced his implant in 1992. Based on his previous experience with other implants he incorporated a large surface area (up to 500 mm ² ), and also changed the biomaterial of the end plate to barium-impregnated silicone. The need to isolate the rectus muscles, place the plate under them, and the very large blebs with the first model, probably account for the early publications showing frequent strabismus as a complication. This complication was minimized when fenestrations to the end plate were introduced, limiting the height of the bleb, thus reducing muscular imbalance.

The Joseph and the Optimed Glaucoma Pressure Regulator (see also Chapter 114 ) were also introduced during the 1980s and 1990s, but although initial results were encouraging, clinical results were less than optimal, and their in vitro characteristics were also not better than what was already available.

Newer modifications to existing implants have been introduced. The Hoffman Elbow for pars plana tube insertion and New World Medical's Pars Plana Clip facilitate placing the tube in the posterior chamber, maintaining a radial and perpendicular position with respect to the sclera. The addition of a second plate to Ahmed's implant increased the surface area to 364 mm ² , with the advantage of having the possibility of deciding the position of the second plate on either side of the primary body during surgery.

Recent Advances

Probably the most significant modification to the Ahmed valve has been the change of the plate's biomaterial to silicone (see also Chapter 113 ). The new model is also available in two sizes and a dual-plate system, and evaluation of its results compared to the older polypropylene valves has shown an effect that is at least 1 mmHg lower ( Table 117-1 ).

A third-generation Molteno implant is also available. It is still a nonvalved device, maintains the pressure ridge (now with an ellipse shape), is more flexible, thinner, with a reduced height of the outer ridge and injection molded polypropylene as a smoother biomaterial. Results of a supra-Tenon's technique show similar IOP to the classic Molteno in this location, but with a slightly better survival rate. A conventional sub-Tenon's implant technique for the Molteno3, using tube ligature and scleral tunnel, did not show such promising results.