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appearance were achieved in 51% of eyes (593).

Aphakia is another factor that adversely affects all types of filtering procedures, including trabeculectomies (594, 595). Patients with advanced COAG also have a worse outcome than the general glaucoma population, with approximately one third requiring a second operation within 3 years (596). In all these types of high-risk cases, however, the use of adjunctive antimetabolite therapy appears to generally improve the surgical outcome.

KEY POINTS

Glaucoma filtering procedures lower the IOP by creating a limbal fistula through which aqueous humor drains into a subconjunctival space and subsequently filters through the conjunctiva to the tear film or is absorbed by surrounding tissues.

The standard filtering techniques use common principles regarding preparation of the conjunctival flap and the iridectomy. They differ primarily according to the method of creating the fistula, with the earlier procedures using a fullthickness fistula and the technique most commonly used today incorporating a guarded fistula beneath a partialthickness scleral flap (trabeculectomy).

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Considerable attention has been given to the pharmacologic modulation of wound healing to minimize bleb failure.

Complications may be encountered during filtering operations (e.g., tearing the conjunctival flap, hemorrhage, and choroidal effusion) and in the early postoperative period (e.g., hypotony, pressure elevation, uveitis, and hemorrhage) or late postoperative course (e.g., bleb failure, bleb leak, endophthalmitis, and cataracts).

REFERENCES

1.Picht G, Grehn E Classification of filtering blebs in trabeculectomy: biomicroscopy and functionality. Curr Opin Ophthalmol. 1998;9(2):2-8.

2.Cantor LB, Mantravadi A, WuDunn D, et al. Morphologic classification of filtering blebs after glaucoma filtration surgery: the Indiana Bleb Appearance Grading Scale. J Glaucoma. 2003;12(3):266-

3.Addicks EM, Quigley HA, Green WR, et al. Histologic characteristics of filtering blebs in glaucomatous eyes. Arch Ophthalmol. 1983; 101(5): 795-798.

4.Hutchinson AK, Grossniklaus HE, Brown RH, et al. Clinicopathologic features of excised mitomycin filtering blebs. Arch Ophthalmol. 1994;112(1):74-79.

5.Kim JW. Conjunctival impression cytology of the filtering bleb. Korean J Ophthalmol. 1997;11 (1):25-31.

6.Benedikt O. The effect of filtering operations [in German]. Klin Monatsbl Augenheilkd. 1977;170 (1):10-19.

7.Powers TP, Stewart WC, Stroman GA. Ultrastructural features of filtration blebs with different clinical appearances. Ophthalmic Surg Lasers. 1996;27(9):790-794.

8.Kronfeld FC. The chemical demonstration of transconjunctival passage of aqueous after antiglaucomatous operations. Am J Ophthalmol. 1952;35(5:2):38-45.

9.Galin MA, Baras I, McLean JM. How does a filtering bleb work? Trans Am Acad Ophthalmol Otolaryngol. 1965;69(6):1082-1091.

10.Teng CC, Chi HH, Katzin HM. Histology and mechanism of filtering operations. Am J Ophthalmol. 1959;47(1, pt 1):16-33.

11.Jinza K, Saika S, Kin K, et al. Relationship between formation of a filtering bleb and an intrascleral aqueous drainage route after trabeculectomy: evaluation using ultrasound biomicroscopy. Ophthalmic Res. 2000; 32(5):240-243.

12.Avitabile T, Russo V, Uva MG, et al. Ultrasound-biomicroscopic evaluation of filtering blebs after laser suture lysis trabeculectomy. Ophthalmologica. 1998;212(suppl 1):17-21.

13.Hill RA. Tenon's traction sutures: an aid for trabeculectomy and aqueous drainage device implantation. J Glaucoma. 2002;11(6):529-530.

14.Vesti E, Raitta C. Trabeculectomy at the inferior limbus. Acta Ophthalmol. 1992;70(2):220-224.

15.Caronia RM, Liebmann JM, Friedman R, et al. Trabeculectomy at the inferior limbus. Arch Ophthalmol. 1996;114(4):387-391.

16.Luntz MH. Trabeculectomy using a fornix-based conjunctival flap and tightly sutured scleral flap. Ophthalmology. 1980;87(10):985-989.

17.Brincker P, Kessing SV. Limbus-based versus fornix-based conjunctival flap in glaucoma filtering surgery. Acta Ophthalmol. 1992;70(5):641-644.

18.Faggioni R. Trabeculectomy with conjunctival flap in the fornix: 12 months' follow-up [in German]. Klin Monatsbl Augenheilkd. 1983; 182(5):385-386.

19.Kozobolis VP, Siganos CS, Christodoulakis EV, et al. Two-site phacotrabeculectomy with intraoperative mitomycin-C: fornixversus limbusbased conjunctival opening in fellow eyes. J Cataract Refract Surg. 2002; 28(10):1758-1762.

20.Shingleton BJ, Chaudhry IM, O'Donoghue MW, et al. Phacotrabeculectomy: limbus-based versus fornix-based conjunctival flaps in fellow eyes. Ophthalmology. 1999;106(6):1152-1155.

21.Tezel G, Kolker AE, Kass MA, et al. Comparative results of combined procedures for glaucoma and cataract: II. Limbus-based versus fornix-based conjunctival flaps. Ophthalmic Surg Lasers. 1997;28 (7):551-557.

22.Berestka JS, Brown SV. Limbusversus fornix-based conjunctival flaps in combined phacoemulsification and mitomycin C trabeculectomy surgery. Ophthalmology. 1997;104(2):187-196.

23.Lemon LC, Shin DH, Kim C, et al. Limbus-based vs fornix-based conjunctival flap in combined glaucoma and cataract surgery with adjunctive mitomycin C. Am J Ophthalmol. 1998;125(3):340-345.

24.el Sayyad F, el-Rashood A, Helal M, et al. Fornix-based versus limbal-based conjunctival flaps in initial trabeculectomy with postoperative 5-fluorouracil: four-year follow-up findings. J Glaucoma. 1999;8(2):124-128.

25.Auw-Haedrich C, Funk J, Boemer TG. Long-term results after filtering surgery with limbal-based and fornix-based conjunctival flaps. Ophthalmic Surg Lasers. 1998;29(7):575-580.

26.Shuster JN, Krupin T, Kolker AE, et al. Limbus- v fornix-based conjunctival flap in trabeculectomy. A long-term randomized study. Arch Ophthalmol. 1984;102(3):361-362.

27.Traverso CE, Tomey KF, Antonios S. Limbalvs fornix-based conjunctival trabeculectomy flaps. Am J Ophthalmol. 1987;104(1):28-32.

28.Grehn F, Mauthe S, Pfeiffer N. Limbus-based versus Fornix-based conjunctival flap in filtering surgery. A randomized prospective study. Int Ophthalmol. 1989;13(1-2):139-143.

29.Reichert R, Stewart W, Shields MB. Limbus-based versus fornix-based conjunctival flaps in trabeculectomy. Ophthalmic Surg. 1987;18(9): 672-676.

30.Agbeja AM, Dutton GN. Conjunctival incisions for trabeculectomy and their relationship to the type

of bleb formation—a preliminary study. Eye. 1987;1( 6):738-743.

31.Miller KN, Blasini M, Shields MB, et al. A comparison of total and partial tenonectomy with trabeculectomy. Am J Ophthalmol. 1991;111(3):323-326.

32.Kapetansky FM. Trabeculectomy, or trabeculectomy plus tenectomy: a comparative study. Glaucoma. 1980;2:451-453.

33.Zigiotti GL, Savini G, De Caro R, et al. The features of Tenon's capsule at the limbus. Ital J Anat Embryol. 1997;102(1):5-11.

34.Lerner SF. Small incision trabeculectomy avoiding Tenon's capsule. A new procedure for glaucoma surgery. Ophthalmology. 1997;104(8): 1237-1241.

35.Das JC, Sharma P, Chaudhuri Z, et al. Small incision trabeculectomy: experiences with this new procedure for glaucoma surgery in Indian eyes. Acta Ophthalmol Scand. 2001;79(4):394-398.

36.Ophir A. Mini-trabeculectomy without radial incisions. Am J Ophthalmol. 1999;127(2):212-213.

37.Hung SO. Role of sodium hyaluronate (Healonid) in triangular flap trabeculectomy. Br J Ophthalmol. 1985;69(1):46-50.

38.Teekhasaenee C, Ritch R. The use of PhEA 34c in trabeculectomy. Ophthalmology. 1986;93(4):487-

39.Vesti E, Raitta C. A review of the outcome of trabeculectomy in openangle glaucoma. Ophthalmic Surg Lasers. 1997;28(2):128-132.

40.Wand M. Viscoelastic agent and the prevention of post-filtration flat anterior chamber. Ophthalmic Surg. 1988;19(7):523-524.

41.Wand M. Intraoperative intracameral viscoelastic agent in the prevention of postfiltration flat anterior chamber. J Glaucoma. 1994;3(2):101-105.

42.Raitta C, Vesti E. The effect of sodium hyaluronate on the outcome of trabeculectomy. Ophthalmic Surg. 1991;22(3):145-149.

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Shields > SECTION III - Management of Glaucoma >

39 - Glaucoma Drainage-Device Surgery

Authors: Allingham, R. Rand

Title: Shields Textbook of Glaucoma, 6th Edition Copyright ©2011 Lippincott Williams & Wilkins

> Table of Contents > SECTION III - Management of Glaucoma > 39 - Glaucoma Drainage-Device Surgery

39

Glaucoma Drainage-Device Surgery

In an attempt to maintain patency of a drainage fistula in glaucoma filtering operations, a wide variety of foreign materials have been implanted in the eye, extending from the anterior chamber to a subconjunctival space. These were once referred to as setons, because the implants consisted of solid structures, such as threads, wires, or hairs, that were placed in a wound to form a drainage, permitting aqueous to run alongside the surface of the inserted material. These procedures were uniformly unsuccessful in maintaining a patent fistula. Most devices use tubes that drain aqueous out of the eye to external reservoirs and have been clinically beneficial. This chapter reviews the most commonly used drainage implant devices, the surgical techniques of implantation, the complications and their management, and the comparative merits and indications for this group of glaucoma surgical procedures.

PHYSIOLOGY OF DRAINAGE IMPLANTS

Most current drainage implant devices (Fig. 39.1) have the same basic design, which typically consists of a silicone tube that extends from the anterior chamber (or, in some cases, the vitreous cavity) to a plate, disc, or encircling element beneath conjunctiva and Tenon capsule. The edge of the external plate has a ridge, through which the distal end of the tube inserts onto the upper surface of the plate. The ridge decreases

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the risk for obstruction of the posterior opening of the tube with the surrounding tissue and fibrous capsule. The plates of the glaucoma drainage devices have large surface areas and promote the formation of the filtering bleb posteriorly, near the equator.

Figure 39.1 Examples of glaucoma drainage devices. A: Ahmed FP-7 (silicone). B: Ahmed S2 (polypropylene; image shown at left), S3 pediatric (polypropylene; middle), and B1 (polypropylene—

double plate; right). C: Single-plate Molteno device. D: Baerveldt 250 mm2 (left) and 350 mm2 (right). The mechanism by which drainage implant devices control the intraocular pressure (IOP) relates to a fibrous capsule that forms a filtering bleb around the external portion of the draining device and, to some degree, the surface area of the implant plate. The morphology of this filtering bleb differs from that of the blebs seen after trabeculectomy.

After insertion of the drainage device, a thin collagenous capsule, surrounded by a granulomatous reaction, is present at 1 month. The granulomatous reaction resolves after 4 months, capsule thickness remains relatively stable, and the collagen stroma becomes less compact. The fibrous capsule matures over time and becomes thinner after 6 months in rabbit eyes (1). Although the bleb histology in the rabbit model is similar to that of humans and other primates, the eventual development of a fibroblastic inner lining in the rabbit model differs from that in humans, in whom the inner lining remains only as a meshwork of collagenlike bundles at some areas of the inner bleb wall (1, 2). Even though the filtering bleb around the implant is lined with a thick layer of connective tissue, microcystic spaces within that layer, seen on light and electron microscopy, may serve as the channels for aqueous drainage (2). Studies of monkey eyes with single-plate Molteno implants indicate that the capsule functions by a passive mechanism, shunting the flow of aqueous humor to the surrounding orbital tissues (3). All surfaces of the fibrous capsule contribute to filtration, which is consistent with echographic studies in human eyes that reveal bleb formation on both sides of the plate in successful cases (4). Histopathologic study of human eyes enucleated 2 to 6 years after Molteno implant surgery revealed patent tubes with no appreciable anterior chamber reaction and minimal inflammatory reaction in the outer layers of the bleb wall (5).

Measurement of the flow resistance using modified Baerveldt plates in rabbits showed a direct

relationship between the surface area of the implants and the filtering capacity of their surrounding capsule (6). At the same time, reduction of the bleb diameter decreases surface tension on the bleb, capsular fibrosis, and thickness, which increases the effectiveness of the filtering surface (7, 8).

Drainage devices with open tubes are likely to be complicated by early postoperative hypotony and therefore require temporary closure with a ligature or stent. The vast majority of glaucoma drainage devices develop an elevated IOP in the weeks to months after implantation as a result of capsule formation around the implant plate. This is frequently termed the hypertensive phase (9, 10).

The filtering bleb may fail after surgery due to the increased thickness of fibrous capsule around the drainage implant. Movement of the drainage plate against the scleral surface may be the mechanism of glaucoma implant failure resulting from the stimulation of the low-level wound healing response, increased collagen scar formation, and increased fibrous capsule thickness (11).

IMPLANT DESIGNS

Performance of similar drainage devices can vary significantly, depending on the standards in manufacturing. This causes a wide range of clinical outcomes and indicates a strong need for enhanced quality-control procedures in the device-manufacturing process (12). The glaucoma drainage devices also differ according to the size, shape, and materials from which the external component and tube are constructed. External portions of glaucoma drainage devices are made from materials that prevent fibroblast adherence. Different materials may influence the amount of inflammation in surrounding tissues. Polypropylene, used in some Ahmed and Molteno implants, may produce more inflammation than the silicone used in Baerveldt, Krupin, and Ahmed devices. Flexible plates caused less inflammation in the subconjunctival space of rabbit eyes than in the rigid ones (13, 14).

Alternative materials, such as hydroxylapatite (15) and expanded polytetrafluoroethylene (16, 17), that increase vascularization of the fibrous capsule around the plate, may offer a theoretical advantage by enhancing the efficacy, decreasing the capsule size, and increasing the functional lifetime of the implant (15).

One of the most fundamental design differences, however, is whether the device has an open, unobstructed drainage tube or one that contains a pressure-regulating valve. Baerveldt, Molteno, and Schocket implants are examples of open-tube implants. Ahmed and Krupin implants are designed to have a flow-restricting valve mechanism.

Open-Tube Drainage Devices Baerveldt Implant

The unique feature of this series of popular nonvalved drainage implants is the large surface area of the plates, which are designed in such a way that they can be easily implanted through a one-quadrant conjunctival incision. A silicone tube is attached to a soft barium-impregnated silicone plate with a

surface area of 250 mm2 (20 mm × 13 mm) or 350 mm 2 (32 mm × 14 mm) (18). In an 18-month prospective study, the 350-mm2 implant had a similar rate of success but lower risk for complications

than did the 500-mm2 model (19), which is no longer available.

The plate is typically positioned under the rectus muscle insertions, typically in the superotemporal quadrant (Fig. 39.2). The Baerveldt plate has fenestrations that allow growth of fibrous tissue through the plate, serving to reduce the height of the bleb, which reduces the risk for diplopia and helps secure the implant (19). A fibrous capsule forms after the first 3 to 6 postoperative weeks into which fluid can drain and from which fluid can be absorbed by the surrounding tissues.

After a previously failed trabeculectomy or cataract surgery, Baerveldt drainage devices were found to be more likely to control IOP, compared with trabeculectomy, but at the cost of greater ocular motility disturbances after 1 year of follow-up in a randomized, controlled trial (20, 21). In retrospective P.526

case-control or consecutive case series, a 350-mm2 Baerveldt implant had lowered IOP by a similar amount as a double-plate Molteno implant or Ahmed implant (discussed later) in patients with

uncontrolled, complicated glaucoma (22, 23 and 24). In a retrospective study, the 350-mm2 implant

maintained IOP below 21 mm Hg in 87% of the eyes, compared with 70% with the 500-mm2 implant

after 3 years (25). The success rate declined to 79% in the 350-mm2 group and to 66% in the 500-mm2 group after 5 years. The rate of complications was similar between the two groups, but complications

occurred slightly more often in the 500-mm2 group (25). Retrospective studies are limited by selection bias and may not detect small or mild differences.

Figure 39.2 Baerveldt implant is positioned under the superior and temporal rectus muscles. Molteno Implant

This is the prototype drainage implant device and has had the longest and most extensive clinical experience since Molteno introduced it in 1969 (26). The original design consists of a single plate of thin

acrylic with a diameter of 13 mm and an area of 135 mm2. A silicone tube with an external diameter of 0.62 mm and an internal diameter of 0.30 mm connects to the upper surface of the plate. The plate has a thickened rim, which is perforated to allow suturing to the sclera.

Subsequent modifications addressed various problems encountered with the original design. Success rates with singleplate Molteno implantation for glaucomas with poor surgical prognoses (aphakic or pseudophakic eyes, prior failed filters, neovascular glaucoma, and patient age younger than 3 years) ranged from 25% to 46% in one study but rose to 40% to 71% with implantation of a second plate (27). A double-plate Molteno implant combines two plates, one of which is attached to the silicone tube in the

anterior chamber, whereas a second tube connects the two plates, giving an increased surface area of 270

mm2 (28). In a randomized trial comparing single-plate and double-plate implants, the latter provided better IOP control but was associated with a greater risk for complications, most of which were related to hypotony (29). Another modification, which addresses the problem of hypotony, is the dualchamber, single-plate implant, in which a V-shaped “pressure ridge” on the upper surface of the plate encases a n

area of 10.5 mm2 around the opening of the silicone tube (30). In concept, the pressure ridge and overlying Tenon capsule regulate the flow of aqueous into the main bleb cavity during the early postoperative period, thereby minimizing excessive filtration and hypotony. The validity of this concept was supported in one study of 40 consecutive patients (31), but the ridge effect was found to be unpredictable in a more recent study (32). A thirdgeneration implant, named Molteno 3, has a bowlshaped structure on the implant plate immediately at the opening tube. It is designed to function as a biologic valve by limiting the available area of filtration during times of low aqueous production. To date, no data on the effectiveness of the Molteno 3, which is available with plate sizes of 175 and 230

mm2, have been published.

Figure 39.3 Schocket glaucoma drainage device. Schocket Tube Shunt

Schocket and associates (33, 34) developed a technique in which a silicone, or silastic, tube is extended

from the anterior chamber to a 360-degree encircling silicone band, as used in retinal detachment repair (Fig. 39.3), which functioned in developing the reservoir for aqueous drainage. Modifications have included insertion of the tube into a band extending for only 90 degrees beneath two rectus muscles or into the preexisting encircling band in eyes with glaucoma after scleral buckling surgery (35, 36). A long Krupin-Denver valve implant (discussed later under” Krupin Implants”) has also been used in combination with a 180-degree scleral band (37).

Two randomized trials compared Schocket tube shunts with double-plate Molteno implants. Although the Schocket shunt typically provides a larger surface area of the reservoir than the Molteno implants do, the latter provided lower final IOP in both studies (38, 39).

Flow-Restricted Drainage Devices

Little resistance is offered to aqueous outflow until the plate becomes encapsulated. The incorporation of a valve mechanism in implants seems to decrease early postoperative hypotony by

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providing resistance to the flow and therefore regulating the pressure within a desired range.

Figure 39.4 Ahmed glaucoma drainage device. Ahmed Glaucoma Valve

The Ahmed glaucoma valve implant is one of the most commonly used flow-restricted implants in difficult glaucomas. In this valved drainage implant design, a silicone tube is connected to a silicone sheet valve, which is held in a polypropylene body (40) (Fig. 39.4). The body of the S2 and FP-7 models

has a surface area of 184 mm2 (16 mm × 13 mm) and is 1.9 mm thick; the reservoir plate of the S2 model is made from polymethylmethacrylate, whereas that of the FP-7 model is made from silicone. A small retrospective comparison suggested that the FP-7 model may lower IOP at 1 year more than the S2 model does (41). The valve mechanism consists of two thin silicone elastomer membranes, 8 mm long

and 7 mm wide, which allows one-way regulation of the flow with a goal of keeping the IOP between 8 and 10 mm Hg in the early postoperative period. A second plate can be connected to the reservoir plate

and implanted in a second quadrant to increase the surface area by 180 mm2. These plates are made from both silicone and polypropylene, models FX1 and B1, respectively, to connect to corresponding

valved plate FP-7 or S2. Furthermore, a smaller valved implant of 96 mm2 is available and made from silicone (FP-8) or polypropylene (S3).

The inlet cross section of the chamber is wider than the outlet, which offers a theoretical small pressure differential between the anterior chamber and subconjunctival space, which is claimed to enable the valve to remain open even when only a small difference in pressure exists. However, no definitive proof of this has been given, and application of the Bernoulli equation (flow rate of a fluid is inversely proportional to pressure of the fluid) to the parameters that exist within the physiologic IOP range shows that the Bernoulli effect is almost nonexistent in either the Ahmed glaucoma valve chamber or the Krupin eye valve (discussed later). Calculations indicate that there is no significant pressure drop across the “valves” and that the critical site for pressur e drop is at the capsule surrounding the glaucoma implants (42).

One study evaluated obstruction to aqueous flow with the Ahmed implant. The obstructions were separated into tube-related and capsuleor valve-related obstructions. The Ahmed implant, as with other implants, has a hypertensive phase, which is a transient phase of low capsule permeability seen at 4 to 8 weeks postoperatively. The authors also introduced the concept of the “no-touch zone” on the Ahmed glaucoma valve, which is the area of the implant covering the chamber with the silicone leaflets. If the implant is grasped with forceps along the center line, it may separate the valve cover from the implant. The external pressure on the valve chamber can cause a defect in closure of the valve with consequent early postoperative hypotony and fibrovascular membrane ingrowth between the leaflets (43). This may lead to a failure of the valve due to adhesion of the valve membranes (44).

In a retrospective review, the double-plate Molteno implant with mitomycin C was more likely than the Ahmed drainage device with mitomycin C to result in an IOP lower than 15 mm Hg (45). Success rates at 1 year were 80% for the Molteno implant, 39% for the Krupin eye valve with disc, and 35% for the Ahmed drainage device. However, the Ahmed device was less likely to cause complications requiring another surgery (45).

Krupin Implants

In 1976, Krupin and associates introduced the concept of a oneway valve that opens at a predetermined IOP level to avoid the early postoperative complications of excessive drainage and hypotony (46). The original Krupin-Denver valve was composed of an internal Supramid tube cemented to an external silastic tube (46). The valve effect was created by making slits in the closed external end of the silastic tube, designed to open at an IOP between 9 and 11 mm Hg. The tube was short, extending only a few millimeters subconjunctivally, and had no external plate. Although preliminary experience was encouraging (47, 48), fibrosis eventually closed the subconjunctival portion of the valved tube (49), which led to failure in most cases.

In a subsequent technique, a long Krupin-Denver drainage tube, with the same one-way valve design, was attached to a 180-degree Schocket-type scleral explant, as previously described (37). This led to development of the Krupin eye valve with disc, which is the design in current use. A silastic tube is attached to an oval silastic disc, conformed to the curvature of the globe, 13 mm × 18 mm, with side walls that are 1.75 mm high (50). The valve at the distal end of the tube is the same design as in the earlier Krupin implants and is manometrically calibrated to open at pressures between 10 and 12 mm Hg. In the newer design of the Krupin implant, the valve lies inside the rim of the plate at its insertion and, as such, is exposed directly to the subconjunctival tissues (51). A review of 113 patients with the Krupin eye valve with disc implants identified 8 patients with primary valve malfunction requiring surgical revision, which involved manipulation, replacement of the valve, and amputation of the valve. Transient postoperative hypotony was noted in three patients, and chronic hypotony with loss of light perception in one patient. One explanted valve was examined and found to have partially fused leaflets, possibly related to the sterilization process and prolonged storage before implantation (51).

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Other Drainage Devices

The Ex-PRESS glaucoma drainage device is a more recently introduced implant, but it differs significantly from the aforementioned devices. The other implants have the basic design of a silicone tube that connects an intraocular space, most commonly the anterior chamber, with a subconjunctivally located reservoir plate, whereas this reservoir plate allows for the development of a delimited potential space and the formation of a fibrous capsule to create the resistance to outflow. The Ex-PRESS device does not have a reservoir plate, is implanted under a traditional trabeculectomy flap, and is subject to all of the considerations of a trabeculectomy (see Chapter 38).

At the time of publication, two other drainage devices are under investigation for approval in the United States. The Solx Gold Shunt (Solx Inc., Waltham, MA) is made from 24-karat gold and works to connect the anterior chamber and suprachoroidal space. Although the device is implanted by using an ab externo approach, no subconjunctival drainage occurs (i.e., no bleb). Effectiveness data have yet to be published. The iStent trabecular microbypass stent (Glaukos Corporation, Laguna Hills, CA) is a stainless-steel stent with a lumen that is implanted from an ab interno approach. The device traverses the trabecular meshwork and drains aqueous from the anterior chamber into the Schlemm canal, enhancing aqueous drainage (52). According to early studies, the iStent appears to lower IOP in chronic open-angle glaucoma as a stand-alone procedure and when used in conjunction with cataract extraction (53, 54).

IN VITRO COMPARISON OF DEVICES

The Krupin, Baerveldt, Ahmed, and OptiMed implants were compared at physiologic flow rates in vitro and in vivo in rabbits (55). With all devices, opening pressures were higher in vivo than in vitro because of tissue-induced resistance around the explant. Pressures with all devices dropped to 0 mm Hg after conjunctival wound disruption. In air, the Krupin and Ahmed implants had opening pressures of 7.2 and 9.2 mm Hg and closing pressures of 3.9 and 5.2 mm Hg, respectively. The OptiMed implant had the highest resistance values, with IOPs of 19.6 mm Hg, compared with 7.5 mm Hg with the Ahmed implant in vivo. The resistance was similar for the Baerveldt, Krupin, and Molteno dual-chamber devices implanted in vivo. Both Ahmed and Krupin valves functioned as flow-restricting devices, rather than true valves at the flow rates studied, but did not close after initial perfusion with fluid. Neither the Ahmed nor Krupin device had demonstrable opening or closing pressures in balanced salt solution. In another comparative study, the Joseph implants provided slightly lower IOPs and had significantly fewer failures than did the Schocket devices, although the Molteno implants provided the lowest pressures at 12 months among eyes with successful IOP control (56).

Resistance and pressure responses of the OptiMed, Krupin, and Ahmed drainage devices were compared by using a 30-gauge cannula as a simple resistor to determine whether the devices function as true valves. Resistance remained relatively constant for the Krupin and OptiMed implants, whereas the Ahmed offered a variable resistance over a range of flow rates and pressures between 12 and 15 mm Hg. The Ahmed device functioned as a valve that closely regulated pressure within a desired range by decreasing or increasing resistance as a function of flow (57).

SURGICAL TECHNIQUES Basic Principles

Although certain variations of surgical techniques are required for implantation of the different implant designs, the basic surgical principles apply in general to all glaucoma drainage devices.

Adequate surgical exposure is dependent on proper placement of a traction suture. A 6-0 polyglactin (Vicryl) or silk traction suture on a spatulated needle is placed through superficial cornea near the superior limbus and attached to the drape beneath the eye.

A fornix-based conjunctival-Tenon capsule flap is created, usually in the superotemporal quadrant, to expose the scleral bed (Fig. 39.5A). The flap is slightly elevated to allow for blunt dissection between Tenon and episclera with blunt Westcott

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