Seton; although it means “thick and hard hair” in its literal sense, refers to the synthetic and inert materials used to create an open drainage fistula in glaucoma surgery. In 1906, Rollet and Moreau placed a “horsehair” in a paracentesis opening to lower intraocular pressure (IOP) in a patient with absolute glaucoma, laying the foundation for future seton surgery. In 1912, Zorab performed a similar procedure using silk thread and named it “aqueoplasty.” In subsequent years, gold, tantalum, platinum, cartilage, and silicone were used for this purpose, but their long-term results were unsuccessful. In 1969, Molteno developed the first tube implant by placing an acrylic plate connected to a tube in the anterior chamber at the limbus level. Later, in 1976, he further refined the implant design with an equatorial region application, forming the basis for the tube implants used today.

Implant Physiology

The main purpose of implant physiology is the transportation of aqueous humor to the surface of the episcleral plate in the post-equatorial region through a tube placed in the anterior chamber. During the initial 4-6 weeks when no capsule has formed around the episcleral plate, there is no resistance to fluid passage under the Tenon’s layer. After this period, a fibrovascular capsule develops around the episcleral plate. There is a filtration area between the episcleral plate and the capsule where aqueous humor circulates without a tight connection. Aqueous humor diffuses passively through the gaps between the epithelial cells of the bleb capsule to reach orbital capillaries and lymphatic vessels. It has been shown that latex molecules with a diameter of 0.2 μm can pass through the capsule wall. The pressure inside the capsule is equal to the anterior chamber pressure. The most important factors that affect the success of the implant are the surface area of the bleb and the permeability of the capsule wall. A thin and wide capsule implies a lower intraocular pressure.

Indications

Indications for tube implants include resistant glaucoma cases where IOP control cannot be achieved despite the maximum tolerated medical treatment and filtration surgery with antifibrotic agents. While neovascular glaucoma is often used as the first option for seton surgery, it is generally indicated in cases of open-angle, closed-angle, and congenital glaucoma that cannot be controlled despite multiple filtration surgeries. Furthermore, it is also suitable for use in secondary glaucoma due to uveitis, pseudophakic glaucoma, iridocorneal endothelial syndromes, secondary glaucoma to penetrating keratoplasty, epithelial downgrowth, and secondary glaucoma to complicated retinal surgery. Due to the potential complications that can occur after tube surgery being more problematic than glaucoma filtration surgery, it should not be considered in situations where primary filtration surgery could be successful.

Types of Implants

Implant designs are divided into two parts depending on whether or not there is a system that restricts the fluid flow passing through the anterior chamber tube.
I. Valveless Implants

There is no system to hinder the fluid flow inside the tube from the anterior chamber to the episcleral region.

Molteno Implant: The first applied implant. It consists of a 16mm long silicone tube (outer diameter 0.64mm/inner diameter 0.30mm) and a round polypropylene plate with a diameter of 13mm and a thickness of 1.65mm connected to it. The end of the tube opens to the upper part of the episcleral plate. The plate area is 135 mm2. There are various variations of the Molteno implant. In the double-plate type, two plates of the same size are combined with a silicone tube to create a larger filtration area, thus increasing the surface area to 270 mm2. This type can be considered in cases of neovascular glaucoma that require a larger filtration area. In Pediatric Molteno implants, the plate diameter is 8mm. In recent years, the “Molteno Pressure Ridge” has been developed to overcome the problems associated with the implant not containing a valve system. Without changing the dimensions of the implant, a triangular chamber is made on the upper surface, allowing the fluid to first accumulate in a small reservoir of 10.5 mm2 and then overcome the resistance of the upper Tenon capsule to pass to the wide area, creating a certain resistance in the fluid flow. The disadvantage is that it does not contain an effective valve system, despite being the oldest produced implant and despite long-term results being obtained in various types of glaucoma.

In today’s world, the use of more flexible biomaterials in place of hard materials for the plate materials of seton implants has come to the forefront. In the Molteno3 type, which is produced as a third generation, there is a rectangular plate design made of silicone with a thickness of 0.7mm and a surface area of 175 mm2 or 230 mm2. It is the thinnest available implant, and it has been increased in surface area compared to the old model, given a certain incline for easy implantation, and produced from a flexible material like silicone. This type of implant also has a “pressure ridge” system, but it should still be considered a valveless implant.

Baerveldt Implant: It is an implant with a wide surface area that can be placed in a single quadrant. It consists of a silicone tube with an inner diameter of 0.30mm and an outer diameter of 0.64mm and a kidney-shaped silicone plate coated with barium with a height of 0.84mm. It is produced in 2 different sizes with a surface area of 250 mm2 (BG-103-250) and 350 mm2 (BG-101-350). Holes have been added to the plate to prevent bleb protrusion. Fibrous tissue advances through these holes, reducing bleb protrusion. It has advantages such as a wide surface area, easy implantation due to its thin, flexible structure, and radiological visibility. With the “Hoffmann elbow” attachment (BG-102-350), it can be implanted in the pars plana. Despite not having a valve structure, it has a wide range of use due to its wide surface area and easy implantation. Despite achieving more successful results compared to the widely used Ahmed Glaucoma Valve in comparative results due to its larger surface area, the disadvantage is hypotony that may be encountered in the early period due to the lack of a valve.

Schocket Implant: This implant, also known as Anterior Chamber Tube Shunt to an Encircling Band (ACTSEB), can be prepared by the surgeon by mounting a 30mm long (inner diameter 0.30mm outer diameter 0.64mm) silastic tube (Storz N-5941-1) onto a groove inside a silicone encircling band No:20 (surface area 300 mm2) or No:220 (surface area 450 mm2) with a 10/0 nylon suture. The prepared strip is placed 360° on the equator. Despite being the cheapest implant and providing a wide filtration area, its use is limited because it requires 4 quadrant dissection. However, a success rate of 86% has been reported in 1 year with the implant obtained by combining the existing band with a tube in cases where a scleral buckle has already been applied due to detachment.

II. Valved Implants

They contain various systems (valve, membrane, resistant matrix, etc.) that allow the fluid transition inside the tube within certain pressure values.

Krupin Valve: The first example of this implant included a translimbal tube, which was placed under a scleral flap 2-3 mm behind the limbus and formed a valve system that opens and closes with horizontal and vertical slits at the end of the tube. The latest model used today consists of a silicone (13×18 mm) episcleral oval disc and a tube with the same valve mechanism attached to it. Its thickness is 1.75mm and the surface area is 180 mm2. The working pressure of the valve is between 9-11 mmHg.

Joseph Valve: Similar to the Schocket implant, it consists of a 9mm wide, 85mm long and 1mm thick silicone band, and a silicone tube attached to it (inner diameter 0.38mm, outer diameter 0.58mm). The thin, long slit on the upper surface of the silicone tube functions as a valve. The opening pressure of the tube is 4 mmHg. It is made in two different types (360°/surface area 765 mm2 and 180°/ surface area 383 mm2), but it is not used today.

White Glaucoma Pump Shunt: It is a one-piece silicone implant. It consists of an inner tube with an outer diameter of 0.64mm and an inner diameter of 0.32mm, suturable side wings, and an outer tube (outer diameter 1.4mm, inner diameter 0.6mm) connecting two one-way valves. The surface area is 280 mm2. The valve mechanism operates between 5-15 mmHg. It is not used today.

Optimed Glaucoma Pressure Regulator: It is a modification of the first applied translimbal implants. The surface area is 18 mm2. It consists of a tube made of polymethacrylate matrix and a silicone body connected to it. It has three models that vary depending on the number of capillary passages it contains. As the length of the transition pathways increases, the fluid flow also decreases. It is not widely used today.

Ahmed Glaucoma Valve: It consists of a pear-shaped oval (13×16 mm) polypropylene plate and a silicone tube (inner diameter 0.32mm, outer diameter 0.64mm) connected to it. Its height is 1.9mm, and the surface area is 184 mm2 (Model S2). On the upper surface of the plate body, two thin silicone elastomer membranes are mounted tensely in front of the tube entry point. The aqueous humor in the tube passes between these membranes, and the fluid flow encounters a certain resistance with the “venturi diaphragm” effect created by these tensely placed silicone leaves. The power between 8-12 mmHg created by the taut silicone leaves creates a valve effect, and the fluid flows towards the reservoir in the valve. According to the “Bernoulli hydrodynamic principle”, the speed of the fluid passing from a large pipe to a smaller exit area increases. Accordingly, the reservoir pool is designed according to a gradually narrowing design. There are pediatric types (Model S3) with a surface area of 96 mm2 and double-plate types (Model B1) with a surface area of 364 mm2. The single (Model FP7), pediatric (Model FP8), and double-plate (Model FX1) types made of silicone materials of these models are replacing the old polypropylene hard materials today.

Special extenders21, 22 and special attachments for pars plana application have also been produced.

The Ahmed Glaucoma Valve is the most applied tube implant today, with early and advanced period results obtained. Although the surface area is a disadvantage for this implant’s success, the use of silicone material in new models, containing a good valve system, and the ability to apply a double plate when necessary are its most important advantages.

Surgical Technique

Biomicroscopic examination evaluates anterior segment formations such as the conjunctiva, anterior chamber angle, and lens. The most movable region of the conjunctiva is chosen for implant placement. Attention is paid to the absence of peripheral anterior synechiae (PAS), neovascularization, and corneal transparency at the location where the tube enters the anterior chamber. Conjunctival opening is related to implant dimensions. Single-piece implants are generally preferred in the upper temporal quadrant to maximize surface area and avoid contact with oblique muscles. Implantation in the upper nasal area should not be preferred because it can cause “acquired superior oblique syndrome”.

The implant is fixed to the sclera with a non-absorbable suture (5/0 polyester or nylon) from 10-12 mm behind the limbus to the holes on the episcleral plaque. For implants that do not contain a valve system, a “temporary tube ligation” needs to be performed to prevent hypotony in the early postoperative period.

The main techniques are;

Two-Stage Intervention: The episcleral plate is sutured to the sclera, but the tube is not placed in the anterior chamber. Trabeculectomy is performed from another quadrant. As a second stage, the tube is placed in the anterior chamber within 4-6 weeks.

Vicryl Ligation Technique: The mouth of the tube is closed with a 6/0 vicryl suture just in front of the episcleral plate. This suture dissolves on its own within 4-6 weeks, during which time encapsulation develops on the plate.

Latina Suture: 5/0 or 6/0 chromic catgut is passed through the tube. The part close to the episcleral plate is taken out and buried in the lower fornix. If necessary, the suture can be pulled to open the tube lumen. An additional suture can be placed around the tube as an alternative.

Tube Ligation in the Anterior Chamber: The part of the tube that will enter the anterior chamber is closed with a 10/0 nylon suture. Then, at a desired time, the suture is opened with an Nd:YAG laser.

Pneumatic Stent: Tube ligation can be performed using expanding gases such as perfluoropropane, which are given into the vitreous after vitrectomy, in the pars plana implanted tube.

After one of these procedures is performed or if a valved implant is being applied, the tip of the tube is placed in the anterior chamber so as to advance 2-3 mm from the iris surface and not to exceed the pupil area. If vitreous is present in the anterior chamber, a vitrectomy should definitely be performed for cleaning. For the tube entrance, an entry is made into the anterior chamber parallel to the iris plane with a 22 G (0.72 mm) or 23 G (0.65 mm) needle 1-2 mm behind the limbus. If pars plana vitrectomy has been performed before, the tube can be inserted into the pars plana 3.5 mm behind the limbus. Another modification is to place the tube in the scleral sulcus in pseudophakic cases with intense PAS formation. The entry into the anterior chamber can be covered with tissue covering materials such as dehydrated human allograft duramater, sclera, pericardium, by preparing a 4×4 mm scleral flap or through full-thickness sclera. The tenon and conjunctiva are closed separately with 8/0 or 9/0 vicryl suture.

Results

Pediatric Glaucoma
Pediatric glaucomas, especially aphakic pediatric glaucomas, constitute a significant group of tube implantation indications. In these cases, it is important to first determine the size of the tube to be implanted. While it is appropriate to implant pediatric designs in eyes with an axial length of less than 21 mm, it is more appropriate to implant tubes made for adults in eyes with an axial length above this. A detailed anterior vitrectomy should also be performed in aphakic glaucomatous eyes, especially in the presence of vitreous in the anterior chamber, which will increase the chance of surgical success. In pediatric glaucomas, while the success of the Ahmed glaucoma valve is 85% in the first year, this rate drops to 42% by the end of the fourth year. The most encountered problem in the pediatric age group is tube erosion and endophthalmitis. Therefore, these cases should be closely monitored; if necessary, their examinations should be repeated under general anesthesia. Also, the success of tube implants implanted in the pediatric age group decreases as the patient’s age decreases.

Glaucoma Secondary to Penetrating Keratoplasty

In glaucoma cases occurring after penetrating keratoplasty, maintaining graft transparency is as important as controlling IOP. In these cases, the tube to be placed in the anterior chamber can cause mechanical corneal endothelial cell loss. Therefore, pars plana tube implantation can be an appropriate alternative during or after keratoplasty. In a study conducted by Ritterband et al., in cases where penetrating keratoplasty and pars plana tube placement were performed, 59% graft transparency and 83% IOP control were provided in 2-year follow-up. Again, in pseudophakic cases, placing the tube in the ciliary sulcus also helps to prevent chronic endothelial damage and maintain corneal transparency.

Uveitic Glaucoma

Uveitic glaucomas constitute a quite successful group within the tube implant group. In these cases, starting topical and, if necessary, systemic steroid treatment before surgery and using intense anti-inflammatory treatment after surgery makes significant contributions to surgical success. Papadaki et al. reported the 4-year success of the Ahmed glaucoma valve in uveitic glaucomas as 50%.

Neovascular Glaucoma

Neovascular glaucomas are the leading group in tube implantation indications. Because in these cases, the success of classical filtration surgery done with antimetabolite is limited. Also, the necessity to repeat the laser cyclophotocoagulation procedure over time, in addition to serious complications such as inflammation, vision loss, hypotony, constitutes its disadvantages. In our study, we found the surgical successes of neovascular glaucoma cases as 63% and 25% in Ahmed glaucoma valve and 37% and 30% in single-plate Molteno tube implantation cases as a result of 1 and 5 years of follow-up. When all cases were evaluated, surgical success was lower in those with preoperative visual acuity below 0.1, diabetic retinopathy diagnosis, and IOP above 35 mm Hg. Despite all this, tube implants play an important role in the treatment of neovascular glaucoma. The completion of the preoperative retinal ablation procedure and the combined use of anti-vascular endothelial growth factors will also increase surgical success.

Traumatic Glaucoma

In a study we conducted, it was found that glaucoma surgery was required in 17.6% of cases after blunt or penetrating trauma, and tube implantation surgery was performed in 5% of cases. Depending on the type and severity of the trauma, in a study conducted, the surgical success was found to be 76% in cases where Molteno implantation was performed as a result of 10 years of follow-up.

Complications

Excessive Filtration and Hypotony
If fluid passage from the anterior chamber occurs before encapsulation forms around the valveless implants’ episcleral plate, shallow anterior chamber, hypotony, and choroidal detachment can develop due to excessive filtration. In addition, sudden hypotony after very high IOP values can lead to widespread retinal hemorrhages, called decompression retinopathy, especially in young cases. To prevent this, tube ligation should definitely be done in the first 4 weeks in implants without a valve system.

Low Filtration and Increased IOP

Increase in IOP in the early period is expected if tube ligation has been performed. Additionally, a temporary “hypertensive phase” can be encountered in the 6-8 week period due to the encapsulation bleb not being vascularized yet. This situation arises due to the vascular structures not developing yet in the capsule while the capsule is forming on the plate in the early period and the fluid not being able to be expelled outside the capsule. Glaucoma medications may need to be used during this period, and it will pass once capsule development is complete. Additionally, proximal or distal tube obstruction should be investigated in this situation. The proximal end of the tube may become obstructed due to iris, fibrin, blood, or vitreous, and this can easily be understood through biomicroscopic examination, and tube irrigation can be performed or the obstruction can be opened with a laser. If the obstruction is in the distal part near the episcleral plate area, bleb needling or opening of the encapsulation area can be performed.

Wound Separation and Tube Erosion

Since tube implantation is performed in eyes where there is serious conjunctival scar and contraction, separation between the wound lips is more common. These separations can lead to epithelial ingrowth and fistula formation. Therefore, care should be taken to close the tenon and conjunctiva separately during primary suturing, and if there is wound opening in the early period, it should be repaired. Lankaranian et al., in a study they conducted, found no conjunctival erosion in 8-year follow-ups with double-layer pericardium covering, whereas they found 16% conjunctival erosion with single-layer pericardium covering. The average erosion emergence time was 9 months in cases where single-layer pericardium coverage was applied. Again, double-layer pericardium can also be applied in a sandwich form under and over the tube in cases of scleromalacia and glaucoma, where the sclera has thinned. In cases of tube erosion, if the conjunctiva has also been seriously damaged, the open parts can be covered with amniotic membrane as an alternative.

Blep Encapsulation

Blep encapsulation, which occurs in the long term, is one of the most important causes of implant failure. In a study conducted, bleb encapsulation was reported as 23% after the Ahmed glaucoma valve. Although it has been suggested to apply antimetabolite during implantation to prevent this, in controlled studies, it has been observed that applying antimetabolite in tube implants does not increase surgical success. The most important factor in bleb encapsulation is the tenon tissue; therefore, in recent years, it has been thought that placing the plate part on it after the tenon tissue is thoroughly separated from the conjunctiva will create a healthier capsule and proposed that it will reduce the risk of bleb encapsulation.

Corneal Decompensation and Graft Rejection

It is difficult for corneal decompensation to occur unless there is direct tube-endothelium contact. However, endothelial loss continues chronically after implantation. In a study conducted by Kim et al., this rate was found to be 10.5% in a year. While the endothelial loss is maximum at the place where the tube is placed, this rate is lower in the central cornea.

Strabismus and Diplopia

This complication especially occurs in large implants that contact extraocular muscles. While this rate is 77% in the first and large Baerveldt implant, it has dropped to 6% with the opening of fenestrations. Causes of strabismus and diplopia include mass effect due to large bleb, muscle tension, herniation of adipose tissue, Faden effect due to scarring under rectus muscles, and “acquired superior oblique syndrome”.

Cataract

It occurs due to tube-lens contact. The tube should be placed in the anterior chamber so as not to exceed the iris surface. If a cataract has developed, cataract surgery can be performed safely in the later period.

Tube Migration

It occurs as a result of wound contraction formed around the plate during wound healing and the plate not being well sutured. If the episcleral plate contracts backwards, the end of the tube may disappear from the anterior chamber, or the tube may move and cause endothelial loss. Therefore, the episcleral plate should be sutured to the sclera with non-absorbable suture material, and at the same time, the tube part should also be fixed with a suture on the sclera.

Endophthalmitis

Endophthalmitis is a rare complication in tube implants. It usually occurs due to conjunctival erosion around the tube and plate in the late period. Sometimes, the tube may need to be removed to control the infection. It should be distinguished from sterile endophthalmitis. This condition may respond to steroid therapy.

Optic Nerve Trauma

Today, the size differences of newly produced implants have brought the risk of trauma towards the optic nerve to the agenda. The critical distance for there to be no pressure on the optic nerve is 2 mm. The implants that can especially exceed this limit are Ahmed Glaucoma valve S2, FP7, and Baerveldt 350 mm2 implants. Especially in eyes with axial lengths below 20 mm and when these implants are placed in the upper nasal quadrant, it brings a serious risk of optic nerve trauma. The surface area of the implant; while being an important parameter in terms of the morphology of the bleb it will form; it has been observed in the studies that the implants with a very large surface area; (such as Baerveldt implant of 500 mm2) do not change the surgical success very much. There is no significant difference in surgical success when implants that cannot be placed in the upper temporal quadrant are placed in the lower quadrant.