The evolution of tube shunts presents a captivating journey through time, revealing the foundational theories that underpin current models and showcasing remarkable strides in biomaterials and bioengineering. These advancements have paved the way for the creation of contemporary tube shunts with superior flow control, diminished inflammatory reactions, and reduced fibrosis. This transformative progress has revolutionized the landscape of glaucoma management, positioning tube shunts at the forefront of therapeutic strategies. Crucial determinants in selecting the appropriate implant include ocular anatomy and the desired intraocular pressure.
Key points
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Significant advances in biomaterials and bioengineering have enabled the development of modern tube shunts with superior flow control and reduced inflammatory reaction and fibrosis.
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Shunts can be categorized according to design, dimensions, materials, and the presence or not of both an external reservoir and a flow-controlling valve.
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The most commonly used glaucoma drainage device materials are silicone and polypropylene, which may affect tissue reaction, degree of bleb encapsulation, aqueous absorption resistance, and thus, intraocular pressure (IOP) levels.
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Current research focuses on IOP control and reduced failure and complication rates.
Video content accompanies this article at http://www.advancesinophthalmology.com .
Introduction
Recently, surgical treatments for glaucoma have advanced considerably. Options now range from classical trabeculectomy to advanced minimally invasive glaucoma surgeries. Among these options, tube shunt implantation remains central. Aqueous shunts (glaucoma drainage devices [GDDs], drains, implants, tubes) have a long history in surgical attempts to control intraocular pressure (IOP). Advances in tube shunt design have been remarkable since the first reported attempt to mechanically drain aqueous from the anterior chamber in 1907 [ ]. Many devices have been introduced into clinical practice, concentrating on device design to try to improve flow control, resistance mechanisms, and plate structure, with the use of advanced biocompatible materials.
Initially, GDDs were reserved for secondary procedures when other glaucoma procedures had been unsuccessful or for patients with secondary glaucomas such as uveitic and neovascular glaucoma. With greater safety, they have become more popular and are now sometimes used as a primary procedure for medically uncontrolled glaucoma. This was supported by data from several studies, which compared GDDs with other devices and with “gold-standard” trabeculectomy, particularly the Tube Versus Trabeculectomy [ ] and the Primary Tube Versus Trabeculectomy [ ] studies. These 2 large randomized controlled trials compared the safety and efficacy of the Baerveldt glaucoma implant with traditional trabeculectomy in pseudophakic eyes with or without previous glaucoma surgery and changed the surgical treatment paradigm for glaucoma treatment.
Despite its importance in glaucoma management, detailed reviews of tube shunt history are limited [ ]. Understanding the evolution of fundamental concepts guiding tube shunt design might facilitate future modifications, thereby improving care for glaucoma patients. We reviewed the history of tube shunt design and development, currently available tube shunts in clinical practice, and suggest future directions.
Historical perspective
The development of an optimal GDD started at the beginning of the twentieth century and has accelerated. All shunts aimed to lower IOP by draining aqueous humor from the anterior chamber and then into the lymphatic system. The first report of mechanical drainage was in 1907 [ ]. Inserting a horsehair through a corneal paracentesis to the anterior chamber showed poor outcomes. Similarly, attempts to drain aqueous with organic materials were unsuccessful. These included calf arteries, conjunctiva strips, lacrimal canalicular fragments, and egg membrane pieces [ ]. The various shunts that followed can be categorized differently, including design, dimensions, materials, and the presence or not of both an external reservoir and a flow-controlling “valve.”
Aqueous destinations
Aqueous can be drained from the anterior or posterior chamber to the subconjunctival and suprachoroidal spaces. Attempts have been made to drain into the intraorbital space, the vortex veins [ ], or the nasolacrimal duct [ ].
Subconjunctival space
Early attempts to use the subconjunctival space to drain aqueous used artificial simple threads or wires via a translimbal approach. Results were poor, with early failure, mainly from little flow control and tissue inflammatory reaction. Among the materials used were fine glass rods [ ], silk [ ], gold [ ], and platinum wires [ ].
Suprachoroidal space
Drainage to the suprachoroidal space via cyclodialysis clefts seemed to offer advantages: no blebs are created, thus avoiding bleb-related complications; such filtration resembles physiologic drainage. However, cleft patency and flow control could not be achieved reliably, hindering success [ ]. Implants placed in the suprachoroidal space provoke tissue reaction and implant failure from tissue fibrosis. In 1955, Bietti used a polyethylene thin plate to improve cyclodialysis patency to the suprachoroidal space and first described polyethylene tubes to drain to the suprachoroidal space [ ]. Over time, other materials included absorbable gelatin film (Gelfilm) [ , ], platinum threads [ ], magnesium wires [ ], and tantalum [ ]. More recent devices used for suprachoroidal drainage include the CyPass Micro-Stent (ALCON, Fribourg, Switzerland), iStent Supra (GLAUKOS, San Clemente, CA, USA), STARflo glaucoma implant (iSTAR Medical SA, Wavre, Belgium), Aquashunt (OPKO Health Inc, Miami, FL, USA), and the Gold Micro Shunt Plus (SOLX Ltd, Boston, MA, USA) [ , ].
Vortex vein drainage
Lee and Wong used a collagen tube to drain through a translimbal incision into a vortex vein [ ]. As success was only moderate, this was later abandoned.
Device design and dimensions
To try to create and maintain a reliable aqueous drainage and absorption outflow system to optimize IOP control in the long term, 2 major modifications have been assessed.
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Connecting the draining tube to a reservoir plate to increase the surface area for absorption
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Adding flow-restricting elements, such as valves
GDDs often are categorized according to the presence or absence of both. Specific device materials, sizes, and resistance mechanisms are summarized in Table 1 .
| Device | Design Year | Tube Material | Tube Diameter Outer/Inner | Plate Material | Surface Area (mm 2 ) | Resistance Mechanism |
|---|---|---|---|---|---|---|
| Molteno | 1969 Molteno single-plate | Silicone | 0.63 mm/0.33 mm | Polypropylene | 135 | Valveless |
| 1981 Molteno double-plate | 270 | |||||
| 1990 Molteno Pressure Ridge | 135 and 270 | |||||
| 2000 Molteno3 G-Series | 175 and 230 | |||||
| 2012 Molteno3 S-Series | SS model: 185 SL model: 245 P1 model: 80 | |||||
| Schocket Tube | 1982 | Silicone | 0.64 mm/0.30 mm | Silicone band | No. 20 or 30 silicone band | Valveless |
| Baerveldt | 1990 | Silicone | 0.64 mm/0.30 mm | Silicone | 250 and 350 | Valveless |
| Aurolab Aqueous Drainage Implant | 2013 | Silicone | 0.64 mm/0.30 mm | Silicone | 300 | Valveless |
| PAUL Glaucoma Implant | 2018 | Silicone | 0.467 mm/0.127 mm | Silicone | 342.1 | Valveless |
| Ahmed ClearPath | 2019 | Silicone | 0.63 mm/0.30 mm | Silicone | 250 and 350 | Valveless |
| Susanna | 2016 | Silicone | 0.53 mm/0.23 mm | Silicone | 200 | Valveless |
| Krupin Eye Valve | 1976 | Silicone | 0.58 mm/0.38 mm | No reservoir | No reservoir | Slit valve |
| 1990, Krupin Eye Disk | Silicone | Silicone | 180 mm 2 | Slit valve | ||
| Ahmed | 1993 | Silicone | 0.635 mm/0.305 mm | Silicone; polypropylene; porous polyethylene | S2, FP7, PS2, PC7 models: 184 S3, FP8, PS3, PC8 models: 96 B1 and FXI models: 364 M4 model: 160 | Venturi valve |
| OptiMed Glaucoma Pressure Regulator | 1995 | Silicone | 0.56 mm/0.30 mm | Polymethylmethacrylate | 140 | Microtubules |
| eyeWatch and eyePlate | 2019 | Silicone | 0.63 mm/0.30 mm | Silicone | 200 and 300 | Deformable silicone tube |
Nonvalved tube-plate glaucoma drainage devices
The Molteno implant: the first nonvalved tube-plate glaucoma drainage device
In 1969, Dr Anthony Molteno (1938–2023), from South Africa, described his GDD [ ]. He realized that subconjunctival fibrosis was the cause of failure with simple stents, with both risk of poor control of flow resulting in early hypotony, followed by a foreign body, and aqueous-induced inflammatory response, which could lead to failure to control IOP. The Molteno implant (Molteno Ophthalmic Ltd, Dunedin, New Zealand) consists of a tube to drain fluid from the anterior chamber onto a plate that acts as a reservoir to facilitate absorption of aqueous ( Fig. 1 A). Molteno soon realized that a more posteriorly positioned reservoir was advantageous over drainage to a more anterior location: it bypassed the surgical site scar tissue, provided a large surface area for drainage, and reduced the risk of late infection [ ]. In a rabbit model, Molteno reported a patent fistula provided by the tube with the formation of a fibrous bleb over the plate, resulting in low IOP [ ]. Molteno suggested that when the bleb wall “matured,” the implant could be removed, expecting continuous drainage while avoiding possible complications from a retained ocular device. The tube and plate design concept provided the basis for future GDD designs.
Molteno later improved on his initial device, which was composed of acrylic, and with the advent of more biocompatible materials and designs, he no longer advocated removing the implant. The Molteno implant surgery involved a 2 step procedure of plate implantation between the rectus muscles, waiting for capsule formation around the plate, followed by the nonvalved tube placement draining the anterior chamber to the plate [ , ]. The 2 step procedure reduced the risk of early postoperative hypotony-related complications such as choroidal effusion, choroidal hemorrhage, a flat anterior chamber, and corneal endothelial cell loss. In 1980, the technique progressed to a single step with a 5-0 Vicryl suture to ligate the tube for temporary flow restriction until a fibrous capsule formed around the plate [ ]. Additional surgical improvements included a longer silastic tube connected to a polypropylene plate. The plate was moved further posteriorly to 7 mm from the limbus and secured to the underlying sclera with either an 8-0 nylon or Vicryl suture under Tenon’s capsule to reduce the risk of Dellen formation or late device extrusion [ , ].
A 2 plate model in 1981 doubled the absorption area from the plate reservoir, but a 4 plate model to double this again did not prove clinically superior [ , , ]. In 1990, Molteno converted the lead plate into 2 chambers with a V-shaped ridge to reduce early aqueous drainage and absorption, to try to reduce the risk of postoperative hypotony ( Fig. 1 B) [ , ]. Aqueous fills the small 15 μL antechamber and then, when IOP increases, percolates between the ridge area and the overlying Tenon’s capsule to the larger central chamber with its greater absorption surface for aqueous drainage, thus providing some flow control [ ].
The current Molteno3 device is made of a polypropylene plate with primary and secondary drainage areas, facilitating staged bleb formation [ , , ]. A smaller plate has been designed for standard cases, with a larger plate for cases requiring a large drainage area, such as larger eyes or younger adults with an increased inflammatory response. A pediatric size also is available. The silicone tube can be inserted via a translimbal or pars plana approach. It should be tied with a Vicryl thread to reduce postoperative hypotony until a capsule forms, reducing absorption rates from the reservoir to the lymphatic system, by which time the Vicryl has dissolved.
The progress of nonvalved glaucoma drainage devices
The Schocket tube: a nonvalved non-plate glaucoma drainage device
In 1982, Stanley Schocket, a vitreoretinal surgeon from Baltimore, Maryland, presented a device with a silastic tube draining aqueous to an equatorially placed encircling silicone band secured to the globe under the recti muscles ( Fig. 2 A, B) [ , ]. The band is inverted with its groove facing the sclera. Drainage to the orbital space was considered preferable to the subconjunctival space with less exposure to pathogens and less prone to trauma. Modifications to the Schocket procedure included placing a shorter 90° band under 2 recti muscles or using a preexisting band following scleral buckling procedures [ , ].
Baerveldt glaucoma implant: a nonvalved tube-plate glaucoma drainage device
In 1990, Dr George Baerveldt from South Africa, introduced the Baerveldt glaucoma implant (BGI, Abbot Medical Optics, Inc, Santa Ana, CA), a nonvalved barium-impregnated silicone tube draining to an oval silicone plate positioned over the equatorial subconjunctival space ( Fig. 3 ) [ , ]. The plate extends beneath 2 adjacent rectus muscles and is secured to the underlying sclera with either an 8-0 nylon or Vicryl suture 8 mm from the limbus and is available in adult and pediatric sizes [ ]. Compared with the double-plate Molteno device, the BGI occupies only one quadrant, is made of silicone, which possibly is less inflammatory than polypropylene, and is more flexible and easier to insert. Plate fenestrations were added to allow fibrous in-growth, limiting bleb height [ ]. Similar to the Molteno, the BGI can be implanted in 2 stages or a single stage if the tube is ligated or stented.
The Aurolab aqueous drainage implant: a nonvalved tube-plate glaucoma drainage device
In 2013, the Aurolab aqueous drainage implant (AADI) was introduced in India [ ]. Developed by Aurolab, the manufacturing division of the Aravind Eye Care System in Madurai, India, this device is a cost-effective, nonvalved GDD ( Fig. 4 ). It features a tube and plate design, conceptually and structurally akin to the BGI [ ]. While its primary use is in India, the AADI has also been made available in countries such as Egypt and in the Middle East [ ].
The AADIs tube and plate are made of medical-grade silicone infused with barium. However, the concentration of barium in the AADI differs from the BGI, rendering the implant more flexible. The plate is positioned 8 mm from the limbus [ ]. A 23 gauge needle is used to create a track for tube insertion into the anterior chamber. Once the tube is secured to the sclera, it can be shielded with corneal, scleral, or pericardial tissue to prevent exposure. Techniques such as tube stenting, ligature application, and venting incisions are similar to other nonvalved GDDs. These methods are instrumental in managing early postoperative hypotony and regulating IOP.
The AADI frequently is employed either as a secondary intervention following a failed trabeculectomy or as a primary treatment in cases of refractory glaucoma where the likelihood of trabeculectomy failure is high. Numerous studies have scrutinized the efficacy and complication rates of AADI in both pediatric and adult patients, comparing its performance with more commonly used devices like the Ahmed glaucoma valve (AGV) and the BGI. The success rates observed for the AADI in adult glaucoma cases spanned from 54% to 92% at 1 year, 43% to 89% at 2 years, and 50% at 4 years, while in the pediatric demographic, these rates ranged between 41% and 77%. Although hypotony-related complications are a primary concern, most of these complications did not significantly elevate the risk of surgical failure and were resolved during the follow-up period [ ].
The Susanna glaucoma drainage device: a nonvalved tube-plate glaucoma drainage device
In 2016, Dr Remo Susanna from Brazil introduced the Susanna Silicone GDD (SGDD). The tube and plate design of the SGDD is similar in principle to the Ahmed ClearPath and BGI, yet with some notable differences ( Fig. 5 ). For easier scleral fixation, the SGDD footplate is secured 4 to 5 mm from the limbus, with the plate positioned 8 to 9 mm from the limbus, enabling anterior fixation with a posteriorly located plate. The tube is thinner, requiring a 26.5 gauge needle for scleral tunneling to the anterior chamber in contrast to the Molteno or Baerveldt, which requires a 23 gauge needle. The thinner tube was designed for a safer intrascleral path and reduced chances for tube exposure [ , ]. Similar to other modern GDDs, the SGDD plate has fenestrations that allow fibrous scar tissue to form, thereby reducing bleb height [ ].
