Seton; Although its literal meaning is “thick and hard hair”, it refers to synthetic and inert materials used to provide an open drainage fistula in glaucoma surgery. For the first time in 1906, Rollet and Moreau created the basic idea of future seton surgery by placing "horse hair" into a paracentesis opening to reduce the intraocular pressure (IOP) in a patient with absolute glaucoma. Zorab performed a similar procedure with a silk thread in 1912 and called it "aqueoplasty". In later years, gold, tantalum, platinum, cartilage and silicone were used for this purpose, but their long-term results were unsuccessful. Molteno developed the first tube implant in 1969 by placing an acrylic plate connected to the tube placed in the anterior chamber at the limbus level. Later, with the new implant design he applied to the equatorial region in 1976, he formed the basis of tube implants used today.
IMPLANT PHYSIOLOGY
The main purpose of implant physiology; It is the transport of the aqueous humor to the episcleral plate surface in the post-equatorial region with the help of a tube placed in the anterior chamber. Since a capsule has not yet formed around the episcleral plate in the first 4-6 weeks, there is no resistance to fluid passage under Tenon. After this period, a fibrovascular capsule develops around the episcleral plate. There is no tight connection between the episcleral plate and the capsule, but there is a filtration area in between where the aqueous humor circulates. The aqueous humor passes through the space between the epithelial cells in the bleb capsule by passive diffusion and reaches the orbital capillaries and lymphatic vessels. Latex molecules with a diameter of 0.2 μm have been shown to pass through the capsule wall. The pressure inside the capsule is equal to the anterior chamber pressure. The most important factors affecting implant success are the bleb surface area and the permeability of the capsule wall. In other words, a thin and wide capsule means lower IOP.
INDICATIONS
Indication group for tube implants; These are refractory glaucoma cases in which IOP control cannot be achieved despite maximum tolerated medical treatment and filtration surgery with antifibrotic agents. Although neovascular glaucoma is performed as the first option for seton surgery, it usually requires more than one procedure. It is indicated in cases of open angle, closed angle and congenital glaucoma that cannot be controlled despite a filtration surgery. It is also suitable for use in glaucoma secondary to uveitis, pseudophakic glaucoma, iridocorneal endothelial syndromes, glaucoma secondary to penetrating keratoplasty, epithelial ingrowth, and glaucoma secondary to complicated retinal surgery. Since complications that may occur after tube surgery are more problematic than glaucoma filtration surgery, they should not be considered in cases where primary filtration surgery can be successful.
IMPLANT TYPES
Implant designs reduce the fluid flow passing through the anterior chamber tube. It is divided into two parts depending on whether there is a restrictive system or not.
I. Valveless Implants
There is no system to prevent the flow of fluid in the tube from the anterior chamber to the episcleral region.
Molteno Implant: It is the first implant applied. It consists of a silicone tube (outer diameter 0.64 mm/inner diameter 0.30 mm) with a length of 16 mm and a round polypropylene plate with a diameter of 13 mm and a thickness of 1.65 mm. The end of the tube opens into the upper part of the episcleral plate. Plate area is 135 mm2. There are different 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, the surface area increases to 270 mm2. This type can be considered in cases of neovascular glaucoma where a larger filtration area is needed. In the pediatric Molteno implant, the plaque diameter is 8 mm. In recent years, the "Molteno Pressure Peak" has been developed to eliminate the problems associated with the implant not containing a valve system. It was thought that by making a triangular-shaped chamber on the upper surface of the implant without changing its dimensions, the fluid would first accumulate in a small chamber of 10.5 mm2 and then overcome the resistance of the upper Tenon capsule and pass into a large area, which would create a certain resistance in the fluid flow. Although it is the oldest implant produced and has long-term results in various types of glaucoma, its disadvantage is that it does not contain an effective valve system. Today, the use of more flexible biomaterials instead of rigid ones in the plate materials of seton implants has come to the fore. The Molteno3 type, produced as the third generation, has a thickness of 0.7 mm and a thickness of 17 mm. It has a rectangular plate design made of silicone with a surface area of 5 mm2 or 230 mm2. It is the thinnest implant available and has an increased surface area compared to the old model, a certain slope for easy implantation, and is made of a flexible material such as silicone. This implant type also has a "pressure peak" system, but it should still be considered a valveless implant.
Baerveldt Implant: It is an implant with a large surface area that can be placed in a single quadrant. It consists of a silicone tube with an inner diameter of 0.30 mm and an outer diameter of 0.64 mm and a kidney-shaped silicone plate with a height of 0.84 mm coated with barium. 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 swelling. Fibrous tissue advances through these holes, reducing the bleb swelling. It has the advantages of being easily implanted due to its large surface area, thin and flexible structure, and also being visible radiologically. It can be implanted into the pars plana with the “Hoffmann elbow” attachment (BG-102-350). Although it does not have a valve structure, it has a wide area of use due to its large surface area and easy implantation. In comparison with the Ahmed Glaucoma Valvi, another frequently used implant today, more successful results are achieved due to its larger surface area, however, it does not contain a valve. The disadvantage of hypotony that may be encountered in the early period with 5941-1); It can be prepared by the surgeon by mounting the No:20 (surface area 300 mm2) or No:220 (surface area 450 mm2) silicone circumference band into the groove inside it with 10/0 nylon suture. The prepared strip is placed at the equator at 360°. Although it is the cheapest implant and provides a wide filtration area, its use is limited because 4 quadrant dissection is required. However, especially in cases where cerclage was previously performed due to detachment, the success rate of the implant obtained by combining the existing band with a tube has been reported as 86% in 1 year.
II. Valved Implants
The fluid flow in the tube is controlled by certain pressure values. They contain various systems (Valve, membrane, resistant matrix, etc.) that will provide internal flow.
Croupin Valve: The first example of this implant is placed under the scleral flap 2-3 mm behind the limbus and forms a collapsible valve system with horizontal and vertical slits at the tip of the tube. It contained a translimbal tube. The latest model used today consists of a silicone (13×18 mm) episcleral oval disc and a tube with the same valve mechanism. Its thickness is 1.75 mm and its surface area is 180 mm2. The operating pressure of the valve is between 9-11 mmHg.
Joseph Valvi: Similar to the Shocket implant, it consists of a 9 mm wide, 85 mm long and 1 mm thick silicone strip and a silicone tube (inner diameter 0.38 mm, outer diameter 0.58 mm) connected to it. . A long, thin slit on the upper surface of the silicone tube functions as a valve. The opening pressure of the tube is 4 mmHg. Although it is made in two different types (360°/surface area 765 mm2 and 180°/surface area 383 mm2), it is not in use today.
White Glaucoma Pump Shunt: It is a one-piece silicone implant. It consists of an inner tube with an outer diameter of 0.64 mm and an inner diameter of 0.32 mm, suturable side wings and two one-way valves, and an outer tube connecting them (outer diameter of 1.4 mm, inner diameter of 0.6 mm). The surface area is 280 mm2. The valve mechanism works between 5-15 mmHg. It is not used today.
Optimed Glaucoma Pressure Regulator: It is a modification of the first translimbal implants. The surface area is 18 mm2. It consists of a tube made of polymethacrylate matrix and a silicone body connected to it. There are three models that vary according to the number of capillary passages they contain. As the length of the passageways increases, the fluid flow decreases. It is not used much today.
Ahmed Glaucoma Valve: It consists of a pear-shaped oval (13×16 mm) polypropylene plate and a silicone tube attached to it (inner diameter 0.32 mm, outer diameter 0.64 mm). Its height is 1.9 mm, and its surface area is 184 mm2 (Model S2). On the upper surface of the plate body, two thin silicone elastomer membranes are tensioned and mounted in front of the tube entrance. The aqueous humor in the tube passes between these membranes and the liquid flow encounters a certain resistance due to the "venturi diaphragm" effect created by these silicone leaves placed in tension. Ger The force between 8-12 mmHg created by the thin silicone leaves creates a valve effect and the liquid flows towards the reservoir inside the valve. According to the "Bernoulli hydrodynamic principle", the speed of the liquid passing from a wide pipe to a smaller exit area increases. Accordingly, the reservoir pool was built according to a gradually narrowing design. There are the pediatric type (Mode S3) with a surface area of 96 mm2 and the double plate (Model B1) type with a surface area of 364 mm2. Single (Model FP7), pediatric (Model FP8) and double plate (Model FX1) types of these models made of silicone materials are now replacing the old polypropylene hard materials. special attachments have also been produced.
Ahmed Glaucoma Valve is the most applied tube implant today, with early and advanced results being obtained. Although the surface area is a disadvantage for the success of this implant, new models can use silicone material, contain a good valve system and The most important advantage is the ability to apply double plates when necessary.
SURGICAL TECHNIQUE
Biomicroscopic examination evaluates anterior segment formations such as conjunctiva, anterior chamber angle and lens. For implant placement, the area where the conjunctiva is most mobile is selected. Attention is paid to the absence of peripheral anterior synechiae (PAS), neovascularization and corneal transparency where the tube enters the anterior chamber. Conjunctival opening is related to implant dimensions. In single-piece implants, the upper temporal quadrant is generally preferred because it creates maximum surface area and does not come into contact with the oblique muscles. Implantation in the upper nasal region should not be preferred because it may cause "acquired superior oblique syndrome".
It is fixed to the sclera with a non-absorbable suture (5/0 polyester or nylon) through the holes on the episcleral plate, 10-12 mm behind the limbus. It does not contain a valve system. For implants, it is necessary to perform "temporary tubal ligation" to prevent hypotony in the early postoperative period.
The main of these techniques are;
Two-Stage Intervention: The episcleral plate is sutured to the sclera. However, the tube is not placed in the anterior chamber. Trabeculectomy is performed from another quadrant. As the second stage, the tube is removed within 4-6 weeks.
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