Ahmed Glaucoma Valve and Setons

Setone; literally means “thick and hard hair”, but 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 formed the basic idea of ​​future seton surgery by placing “horsehair” in a paracentesis opening to reduce intraocular pressure (IOP) in a patient with absolute glaucoma. Zorab did a similar procedure with a silk thread in 1912 and called it “aqueoplasty”. In the following years, gold, tantalum, platinum, cartilage and silicon 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 with 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 the tube implants used today.

IMPLANT PHYSIOLOGY
The main purpose of implant physiology; It is the transport of aqueous humor to the episcleral plate surface in the postequatorial region with the help of a tube placed in the anterior chamber. Since a capsule has not yet formed around the episcleral plaque in the first 4-6 weeks, there is no resistance to fluid passage under the tenon. After this period, a fibrovascular capsule develops around the episcleral plaque. There is no tight connection between the episcleral plate and the capsule, and there is a filtration area where aqueous humor circulates. The aqueous humor passes through the space between epithelial cells in the bleb capsule by passive diffusion and reaches the orbital capillaries and lymphatic vessels. It has been shown that 0.2 μm diameter latex molecules pass through the capsule wall. The pressure in the capsule is equal to the anterior chamber pressure. The most important factors affecting the success of the implant are the bleb surface area and the permeability of the capsule wall. So 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 maximally tolerated medical treatment and filtration surgery with antifibrotic agents. Although it is applied as the first choice for neovascular glaucoma seton surgery, it is generally indicated in cases of open-angle, closed-angle and congenital glaucoma that cannot be controlled despite multiple filtration surgeries. 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 filter surgery can be successful.

IMPLANT TYPES
Implant designs are divided into two parts according to whether there is a system that restricts the fluid flow through the anterior chamber tube.
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 16 mm long silicone tube (0.64 mm outer diameter / 0.30 mm inner diameter) and an attached round polypropylene plate with 13 mm diameter and 1.65 mm thickness. The end of the tube opens into the upper part of the episcleral plate. The plate area is 135 mm2. There are different variations of the Molteno implant. Two plates of the same size are combined with a silicone tube to create a larger filtration area in the double plate type. Thus, the surface area increases to 270 mm2. This type can be considered in neovascular glaucoma cases 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 caused by the implant not having a valve system. Without changing the dimensions of the implant, a triangular chamber was made on the upper surface of the implant, and it was thought that the first accumulation of the liquid in the small chamber of 10.5 mm2 and then the overcoming the resistance of the upper tenon capsule and passing into the large area would create a certain resistance in the liquid flow. The disadvantage is that it is the oldest implant produced and it does not contain an effective valve system, although long-term results are obtained in various types of glaucoma.
Today, the use of more flexible biomaterials instead of rigid in the plate materials of seton implants has come to the fore. Molteno3 type, produced as the third generation, has a rectangular plate design made of silicon with a thickness of 0.7 mm and a surface area of ​​175 mm2 or 230 mm2. It is the thinnest implant available, with an increased surface area compared to the old model, a certain inclination for easy implantation, and produced from a flexible material such as silicone. This type of implant also has a “pressure ridge” system, but 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 silicon tube with an inner diameter of 0.30 mm and an outer diameter of 0.64 mm, and a kidney-shaped silicone plate of 0.84 mm height, coated with barium. It is produced in 2 different lengths with a surface area of ​​250 mm2 (BG-103-250) and 350 mm2 (BG-101-350). Holes were added on the plate to prevent bleb swelling. Fibrous tissue progresses through these holes and reduces bleb swelling. Due to its large surface area, thin flexible structure, it has advantages such as easy implantation and also radiological visualization. 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 Ahmed Glaucoma Valve, which is another frequently used implant today, more successful results are obtained due to its larger surface area, due to the fact that it does not contain a valve. It is the disadvantage of hypotonia that can be encountered in the early period.
Schocket Implant: Also known as Anterior Chamber Tube Shunt to an Encircling Band (ACTSEB), this implant consists of a 30 mm long (0.30 mm inner diameter 0.30 mm outer diameter 0.64 mm) silastic tube (Storz N-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 surfacing tape to the groove in it with 10/0 nylon sutures. The prepared strip is placed on the equator as 360°. Although it is the cheapest implant and provides a wide filtration area, its use is limited because 4 quadrant dissection is required. However, the success rate of the implant, which is obtained as a result of combining the existing band with a tube, has been reported as 86% in 1 year, especially in cases that had previously undergone scour due to detachment.
II. Valved Implants
They contain various systems (valve, membrane, resistant matrix, etc.) that will ensure the liquid passage in the tube within certain pressure values.
Krupin Valve: The first example of this implant consisted of a translimbal tube placed under the scleral flap 2-3 mm behind the limbus and forming a collapsible valve system with horizontal and vertical slits at the tube end. 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 1.75 mm and its surface area is 180 mm2. Valve working pressure is between 9-11 mmHg.
Joseph Valve: Similar to the Shocket implant, it consists of a 9 mm wide, 85 mm long and 1 mm thick silicone strip and an attached silicone silicone tube (0.38 mm inner diameter, 0.58 mm outer diameter). An elongated slit on the upper surface of the silicone tube functions as a valve. The opening pressure of the tube is 4 mmHg. Although it was made in two different types (360°/surface area 765 mm2 and 180°/surface area 383 mm2), 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 0.64 mm and an inner diameter of 0.32 mm, suturable side wings and two unidirectional valves and an outer tube (1.4 mm outer diameter, 0.6 mm inner diameter) connecting them. Its surface area is 280 mm2. Valve mechanism works between 5-15 mmHg. It has no use today.
Optimed Glaucoma Pressure Regulator: It is a modification of the first applied translimbal implants. Its surface area is 18 mm2. It consists of a tube of polymethacrylate matrix and a silicon body attached to it. It has three models that vary according to the number of capillary passageways it contains. 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 (inner diameter 0.32 mm, outer diameter 0.64 mm) attached to it. Its height is 1.9 mm, and its surface area is 184 mm2 (Model S2). On the upper surface of the plate body, 2 thin silicone elastomer membranes are mounted in front of the tube inlet, stretched. The aqueous humor in the tube passes between these membranes and the liquid flow encounters a certain resistance with the effect of the “venturi diaphragm” formed by these silicone leaves placed in tension. The force between 8-12 mmHg created by the stretched silicone sheets creates a valve effect and the liquid flows towards the reservoir inside the valve. According to the “Bernoulli hydrodynamic principle”, the velocity of the fluid passing from a large pipe to a smaller outlet area increases. Accordingly, the reservoir pool was built according to a gradually narrowing design. There are pediatric type (Mode S3) with a surface area of ​​96 mm2 and a double plate type (Mode B1) with a surface area of ​​364 mm2. Single (Model FP7), pediatric (Model FP8) and double plate (Model FX1) types of these models, which are made of silicone materials, are now replacing the old polypropylene rigid materials.
Special extenders21, 22 and special attachments for pars plana application have also been produced to extend the tube length.
Ahmed Glaucoma Valve is the most applied tube implant today with its early and long-term results. Although the surface area is a disadvantage for the success of this implant, the use of silicone material in the new models, a good valve system and double plate application are the most important features. are important advantages.

SURGICAL TECHNIQUE
Biomicroscopic examination evaluates anterior segment formations such as conjunctiva, anterior chamber angle and lens. For implant placement, the region where the conjunctiva is most mobile is selected. Attention is paid to peripheral anterior synechia (PAS), absence of neovascularization and corneal transparency where the tube enters the anterior chamber. Conjunctival opening is related to implant dimensions. In one-piece implants, the superior 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 can cause “acquired superior oblique syndrome”.
A non-absorbable suture (5/0 polyester or nylon) is fixed to the sclera through the holes on the episcleral plate 10-12 mm behind the limbus. For implants without a valve system, “temporary tubal ligation” is necessary to prevent hypotonia in the early postoperative period.
The main ones of these techniques are;
Two-Stage Initiative: The episcleral plate is sutured to the sclera, but the tube is not placed in the anterior chamber. A trabeculectomy is performed from another quadrant. As the second step, the tube is placed in the anterior chamber within 4-6 weeks.
Vicryl Binding Technique: The tube is closed with a 6/0 vicryl suture just in front of the episcleral plate. As this suture dissolves spontaneously in 4-6 weeks, encapsulation develops on the plate during this time.
Latina Suture: 5/0 or 6/0 chrome catgut is passed through the tube. It is taken out from the part close to the episcleral plate and embedded in the lower fornix. If necessary, the tube lumen is opened by pulling the suture. Alternatively, additional sutures are placed around the tube.
Tubal Ligation in the Anterior Chamber: The tip of the tube that will enter the anterior chamber is closed with 10/0 nylon sutures. Then, at a desired time, suture opening is performed with Nd:YAG laser.
Pneumatic Stent: Tube ligation can be performed by using the tube end implanted in the pars plana after vitrectomy, by delivering expanding gases such as perfluoropropane to the vitreous.
After one of these procedures or if a valved implant is applied, the tip of the tube is placed directly in the anterior chamber in such a way that it moves 2-3 mm from the iris surface and does not exceed the pupillary area. With the presence of vitreous in the anterior chamber, vitrectomy should be performed and cleaning should be performed. For tube entry, a 22 G (0.72 mm) or 23 G (0.65 mm) needle is inserted 1-2 mm behind the limbus into the anterior chamber, parallel to the iris plane. If pars plana vitrectomy has been performed before, the tube can be inserted 3.5 mm behind the limbus into the pars plana. Another modification is the placement of the tube in the scleral sulcus if there is intense PAS formation in pseudophakic cases. The tube surface can be closed by preparing a 4×4 mm scleral flap to enter the anterior chamber or by using tissue occlusive materials such as dehydrated human allograft dura mater, sclera, and pericardium over full-thickness sclera. Tenon and conjunctiva are closed separately with 8/0 or 9/0 vicryl sutures.

RESULTS
Pediatric Glaucomas
Pediatric glaucomas, especially aphakic pediatric glaucomas, constitute an important group of tube implantation indications. In these cases, it is important to first determine the tube size to be inserted. While it is appropriate to implant pediatric designs with an axial length of less than 21 mm, it is more appropriate to insert adult tubes in eyes with an axial length above this. Performing a detailed anterior vitrectomy in the presence of vitreous in the anterior chamber in eyes with aphakic glaucoma will also increase the chance of surgical success. While the success of Ahmed glaucoma valve in pediatric glaucomas is 85% in the first year, this rate drops to 42% at the end of the fourth year. The most common problem in the pediatric age group is tube erosion. and endophthalmitis. Therefore, these cases should be followed closely; If necessary, it is appropriate to repeat the examinations under general anesthesia. Again, the success of tube implants in the pediatric age group decreases as the age of the patient decreases.
Glaucomas Secondary to Penetrating Keratoplasty
In glaucoma cases after penetrating keratoplasia, preservation of graft transparency is as important as IOP control. In these cases, the tube to be placed in the anterior chamber may mechanically lead to corneal endothelial cell loss. Therefore, pars plana tube implantation may be a suitable alternative during or after keratoplasty. Ritterband et al. In a study they carried out, they achieved 59% graft transparency and 83% IOP control in patients who underwent penetrating keratoplasty and pars plana tube placement in a 2-year follow-up. Placing the tube in the ciliary sulcus in pseudophakic cases is also beneficial in preventing chronic endothelial damage and providing corneal transparency.
Uveitic Glaucomas
Uveitic glaucomas constitute a very successful group within the tube implant group. In these cases, the initiation of topical and, if necessary, systemic steroid treatment before the operation and the intensive use of medical anti-inflammatory treatment after the operation make significant contributions to the surgical success. Papadaki et al. They reported the 4-year success of the Ahmed glaucoma valve as 50% in uveitic glaucomas 36
Neovascular Glaucoma
Neovascular glaucomas are the group at the top of the tube implantation indication. Because the success of classical filter surgery with antimetabolite is limited in these cases. In addition to serious complications such as inflammation, vision loss and hypotonia in the laser cyclophotocoagulation procedure to be performed again, the necessity of repeating the procedure over time creates its disadvantages. As a result, we found it to be 63% to 25% in Ahmed glaucoma valve and 37% and 30% in single plate Molteno tube implantation cases 40 . When all cases were evaluated, surgical success was lower in patients with preoperative visual acuity below 0.1 and diagnosed with diabetic retinopathy and IOP above 35 mm Hg. Despite all this, tube implants play an important role in the treatment of neovascular glaucoma. Complete preoperative retinal ablation and combined use of anti-vascular endothelial growth factors will also increase surgical success.
Traumatic Glaucomas
In a study we conducted, it was determined that 17.6% of glaucoma surgery was required after blunt or penetrating trauma, and tube implantation surgery was performed at a rate of 5%. Although it varies according to the type and severity of trauma, surgical success in cases with Molteno implant after 10 years of follow-up was found in a study conducted. It was found to be 76%.

COMPLICATIONS
Excessive Filtration and Hypotonia

In valveless implants, if fluid flows from the anterior chamber without encapsulation around the episcleral plate, shallow anterior chamber, hypotonia and choroidal detachment may develop due to excessive filtration. In addition, sudden hypotonia after very high IOP values ​​can lead to widespread retinal hemorrhages called decompression retinopathy, especially in young cases. To prevent this, tubal ligation should be performed for the first 4 weeks in implants without a valve system.
Low Filtration and IOP Increase
If tube ligation is performed, an increase in IOP is expected in the early period. In addition, a temporary “hypertensive phase” may be encountered in the 6-8 weeks period, since the encapsulation bleb has not yet vascularized. It may be necessary to use glaucoma drugs during this period and will pass when the capsule development is completed. Also, in this case, proximal or distal tube obstruction should be investigated. Occlusion of the proximal end of the tube may occur due to iris, fibrin, blood, or vitreous. This situation can be easily understood in biomicroscopic examination and tube irrigation can be performed or the obstruction can be opened with a laser. If the obstruction is in the distal area close to the episcleral plaque area, bleb needling or opening the encapsulation area can be performed.
Wound Separation and Tube Erosion
Since tube implantation is performed in eyes with severe conjunctival scarring and contraction, separation between the wound lips is more common. These openings can cause epithelial ingrowth and fistula formation. Therefore, care should be taken to cover the tenon and conjunctiva separately in primary suturing, and if there is an early wound opening, it should be repaired. In a study by Lankaranian et al., while no conjunctival erosion was detected in 8-year follow-up with double layer pericardium closure, they found conjunctival erosion with a rate of 16% with single layer pericardium coating. In cases where single layer pericardium covering was applied, the mean time of erosion was 9 months. Again, in scleromalacia and glaucoma cases where the sclera was thinned, double layer pericardium application in the form of a sandwich can be applied under and over the tube. In cases of tube erosion, in cases where the conjunctiva is severely damaged, closure of the open parts with an amniotic membrane can also be applied as an alternative.
Blep Encapsulation
Long-term bleb encapsulation is one of the most important causes of implant failure. In a study, bleb encapsulation was reported as 23% after Ahmed glaucoma valve. In order to prevent this, antimetabolite administration during the implant has been recommended, but in controlled studies, it has been observed that antimetabolite administration in tube implants does not increase surgical success. Since the most important factor in bleb encapsulation is the tenon tissue, it has been thought that placing the plate on top of the tenon tissue after the tenon tissue has been separated from the conjunctiva will create a healthier capsule and it has been suggested that it will reduce the risk of bleb encapsulation.
Corneal Decompensation and Graft Rejection
Corneal decompensation is difficult to occur unless there is direct tube-endothelium contact. However, chronic endothelial loss continues after implantation. In the study of Kim et al., this rate was found to be 10.5% in one year. Where the tube is inserted, endothelial loss is maximum, while this rate is lower in the central cornea.
Strabismus and Diplopia
This complication occurs especially with large implants that come into contact with the extraocular muscles. While this rate was 77% in the first and wide Baerveldt implant, it decreased to 6% with the opening of the fenestrations. The causes of strabismus and diplopia are mass effect due to very large bleb, muscle stretching, adipose tissue herniation, Faden effect due to scarring under the rectus muscles, and “acquired superior oblique syndrome”.
Cataract
Occurs due to tube-lens contact. The tube should be placed in the anterior chamber so that it does not exceed the iris surface. If cataract has developed, cataract surgery can be performed safely in the later period.
Tube Migration
It occurs as a result of wound contraction around the plate during wound healing and poor suturing of the plate. If the episcleral plate is contracting posteriorly, the tip of the tube may disappear from the anterior chamber or the tube may move, causing endothelial loss. For this reason, it is necessary to suture the episcleral plate to the sclera with non-absorbable suture material and at the same time to fix the tube with suture over the sclera.
Endophthalmitis
Endophthalmitis is a rare complication of tube implants. It usually occurs in the late period due to conjunctival erosion around the tube and plate. Sometimes it may be necessary to remove the tube to control the infection. It should be differentiated from sterile endophthalmitis. This condition may respond to steroid therapy.
Optical Nerve Trauma
Today, the size differences of newly produced implants have brought up the risk of trauma to the optic nerve at the back. The critical distance is 2 mm to avoid compression of the optic nerve. Implants that can specifically exceed this limit are Ahmed Glaucoma valve S2, FP7 and Baerveldt 350 mm2 implants. The placement of these implants in the upper nasal quadrant, especially in eyes with axial lengths less than 20 mm, poses a serious risk of optic nerve trauma. Implant surface area; It is an important parameter in terms of the bleb morphology it will form; but in the studies carried out, implants with very large surface area; It has been observed that (for example, 500 mm2 Baerveldt implant) does not change the surgical success much. If implants that cannot be placed in the upper temporal quadrant are placed in the lower quadrant, there is no significant difference in surgical success.

EK İŞLER VE YURT DIŞI İLE İLGİLİ İLANLAR İÇİN MUTLAKA UĞRAYIN

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