the procedure. Cautery can be used to stop episcleral bleeding. The superficial flap should measure at least 5 5 mm and can be in any shape, but most surgeons choose a square or semicircle. The flap should be approximately one-third scleral thickness (200 to 250 m in depth) and should extend anteriorly into the clear cornea 0.5 to 1.00 mm. A second scleral flap measuring approximately two-thirds scleral thickness (500 to 550 m in depth) is created beneath the superficial scleral flap. It should extend into the cornea 0.5 mm to 1 mm anterior to Schwalbe’s line. The sclera in black patients is approximately 80 m thicker than in the Caucasian population and this necessitates a thicker deep scleral flap to get into the correct tissue plane.26 This plane is usually evident when the underlying choroid is visible beneath the remaining thin translucent layer of sclera. As the deeper scleral flap dissection proceeds anteriorly, Schlemm’s canal becomes visible as a dark line approximately 1 mm posterior to the limbus. Stegman27 points out that in a highly hyperopic eye, Schlemm’s canal may be located at or near the limbus, as opposed to a highly myopic eye, where it may be located as far posteriorly as 2.5 mm posterior to the limbus. In congenital glaucoma, Schlemm’s canal may be as far as 4 mm posterior to the limbus. Further dissection of this deep flap unroofs Schlemm’s canal and then reveals Descemet’s membrane.

Once the deep scleral flap is advanced into the clear cornea 0.5 to 1.0 mm, it is excised. This deeper scleral flap is usually made triangular in shape whether the superficial flap is square or semicircular. At this time the surgeon may replace the superficial flap without sutures or place the Staar collagen sponge (Aquaflow) in the space created by the deep sclerectomy. The superficial flap is then loosely sutured into position with two 10-0 nylon interrupted sutures.19–21 The sponge is oriented radial to the limbus and sutured into position with one 10-0 nylon suture. Deep sclerectomy with placement of the Staar collagen sponge is depicted in Figure 27–1A–E.19 Another variation in the deep sclerectomy operation is the placement of a reticulated hyaluronic acid implant into the space created by the sclerokeratectomy (Fig. 27–2).28 When the implant is used, the superficial flap is sutured tightly to prevent bleb formation. This approach to deep sclerectomy promotes drainage of aqueous through the two surgically created ostia into Schlemm’s canal, just as in the viscocanalostomy operation. It remains to be seen whether lower IOPs are achieved by injecting sodium hyaluronate into the ostia of Schlemm’s canal or if the hyaluronic implant, which slowly releases sodium hyaluronate for more than 4 months in humans,28 can achieve the same or better result.

VISCOCANALOSTOMY PROCEDURE

The viscocanalostomy procedure has been developed by Stegman, and has been in its current form for the past 5 years.22 The initial part of this procedure is similar to the deep sclerectomy with some exceptions.

First, the superficial scleral flap should optimally be placed between aqueous collector channels. The collector channels are frequently identified by focal pigmentation on the scleral surface.

Second, the aqueous veins, which drain the collector channels, should not be cauterized, because the success of the operation is contingent upon the patency of the veins and the collector channels.22 To control bleeding, Stegman uses a topical drug, POR-8, which is not available in the United States.22 A similar drug, vasopressin, can be applied topically with good results.26

Third, the deep scleral flap dissection should be temporarily halted after Schlemm’s canal has been unroofed. At this time high molecular weight sodium hyaluronate, Healon GV or Healon5, is gently injected into the two ostia of Schlemm’s canal for a distance of 4 to 6 mm on each side.22,26 A special cannula with an outer diameter of 150 m (Greishaber no. 149.40) facilitates this procedure.22,26 A paracentesis incision should be made, and the anterior and posterior chambers decompressed, prior to the injection of viscoelastic or dissection of the Descemet’s window.22,26 By decompressing the anterior and posterior chambers, the paper-thin Descemet’s window is less likely to be perforated during the anterior dissection of the deep scleral flap.22,26 Decompression prevents iris prolapse into the window if an inadvertent perforation of the window should occur. Finally, if the canal perforates into the anterior chamber because of too forceful an injection of sodium hyaluronate, iris prolapse into the canal is avoided.22,26

The deep scleral flap is then excised, followed by tight suturing of the superficial flap with interrupted 11-0 Mersilene sutures22,26 (Fig. 27–3). Even though both the deep sclerectomy and the viscocanalostomy procedures are nonperforating in nature, the former procedure when performed with the Starr collagen sponge relies on subconjunctival aqueous filtration and bleb formation to be successful in lowering IOP, and the latter procedure does not.

TRABECULECTOMY PROCEDURE

The trabeculectomy operation described by Cairns14 in 1968 has been modified over the years with the addition of antimetabolite usage as well as by the use of externalized releasable sutures.

Initially a fornix-based, limbus-based, or mixed conjunctival flap is created. The conjunctival dissec-

tion is performed carefully to prevent a buttonhole. Low-intensity cautery is applied. A one-half scleral thickness flap (square or triangular) measuring approximately 5 5 mm is created with its base at the limbus. The flap is then dissected toward and into the limbus. Perforating vessels, which are commonly encountered, are lightly cauterized. Mitomycin-C, if indicated, is applied. A paracentesis is created. The paracentesis provides access to the anterior chamber (AC) for introduction of viscoelastic or BSS. It is easily fashioned prior to depressurizing the AC during the trabeculectomy. The AC is filled with viscoelastic and is entered centrally at the base of the scleral flap. With a punch or blade, a deep block of tissue is then excised. This block must extend from the level of the scleral spur to just anterior to Schwalbe’s line. It will be composed of the canal of Schlemm and trabecular meshwork. An iridectomy is then achieved. If the iridectomy is too basal, bleeding may be encountered. If necessary, cautery is applied with a sharptip instrument to control hemorrhage. The corneoscleral flap is sutured back into position allowing controlled egress of anterior chamber fluid. This can be achieved with 10-0 nylon interrupted sutures, or with removable sutures as described below. Finally the conjunctiva is sutured. An 8-0 Vicryl suture on a vascular needle (Ethicon TG 160), employing a horizontal running mattress suture, is ideal to create a watertight closure. The AC is then pressurized with BSS. The amount of fluid egress is evaluated. If outflow is excessive, the flap is tightened. If inadequate, the flap is loosened. The conjunctivae are also evaluated for leaks, which are immediately repaired with 10-0 nylon or 8-0 Vicryl purse-string sutures.

promote epithelialization of the knot. This is important for patient comfort postoperatively. This suture often produces some astigmatism until it is either released or the nylon relaxes. However, in this author’s experience, it consistently achieves a tighter closure, and is easier to release, than the Cohen and Osher suture. However, the Wilson suture cannot be easily used at the apex of a triangular trabeculectomy flap. The Cohen and Osher releasable suture can. Therefore, it may be helpful to use both suturing techniques in closing trabeculectomy flaps.

Postoperatively these sutures should be released only if the IOP is higher than desired, and then only after some conjunctival and episcleral healing has occurred. Otherwise, hypotony may result. These releasable suture techniques are a valuable adjunct to trabeculectomy surgery and help to decrease the incidence of postoperative hypotony.

COMBINED PHACOEMULSIFICATION

AND TRABECULECTOMY

If phacoemulsification with intraocular lens (IOL) implantation is combined with either of the NPTSs or with a trabeculectomy, the cataract incision can be either (1) a separate clear corneal location (a necessity when combined with NPTS because by definition there is no anterior chamber entry), at a site away from the glaucoma surgery, or (2) under the scleral flap of a trabeculectomy (Fig. 27–6).

INTRAOPERATIVE COMPLICATIONS

CONJUNCTIVAL BUTTONHOLE

RELEASABLE SUTURE TECHNIQUES

Various releasable suture techniques have been reported by Cohen and Osher,29 Wilson,30 Shin, 31 Johnstone and co-workers,32 Hsu and Yarng,33 and Maberley and coworkers.34 These suture techniques may be used with either limbusor fornix-based conjunctival flaps. The Cohen and Osher technique consists of multiple four throw slipknots of 10-0 nylon, which are used to close the trabeculectomy flap (Fig. 27–4). This suturing technique consists of taking three bites proceeding from the cornea posterior through the scleral flap and emerging from the sclera. The free end of each slipknot is buried within the corneal stroma.

The Wilson releasable technique also involves taking three suture bites with a square knot (3-1-1-1). This is tied on the epithelial surface of the cornea (Fig. 27–5). It is important to cut off the excess suture flush with the knot with a 15-degree blade. This will

This complication is most likely to occur if a conjunctival flap is being prepared in tissue that has undergone previous surgery. Subsequent to any surgical procedure, adhesions develop between the conjunctiva and the episclera/sclera, making the dissection demanding. The surgeon should always use blunt scissors and nontoothed forceps when dissecting. When conjunctiva is adherent to underlying tissue, it may be necessary to perform a superficial scleral dissection, removing a thin layer of sclera along with the conjunctiva, to prevent a buttonhole from occurring. The best way to prevent a buttonhole is to choose a surgical site with loosely adherent conjunctiva. If a hole in the conjunctiva does occur, it should be repaired immediately with 8-0 or 9-0 Vicryl interrupted sutures on a blood vessel needle. Purse string sutures work well for this purpose. The needle will minimize the size of the microperforations caused by the needle passage. Incorporating a small amount of Tenon’s fascia into each bite makes the repair more

secure. If a large buttonhole occurs, it is reasonable to repair the hole and move to a different site on the sclera, especially if the surgeon is planning on performing a glaucoma procedure that produces a bleb, or if the use of an antimetabolite is planned. If the buttonhole occurs in conjunctiva that is gossamer in nature, sutures sometimes do not hold. If this occurs, abandonment of this surgical site for a new one is advisable, unless the planned procedure is one that does not produce a bleb.

SCLERAL FLAP PROBLEMS

Trabeculectomy

There are a variety of flap complications that can occur in various glaucoma operations. In trabeculectomy surgery the thickness of the scleral flap helps to determine the amount of aqueous egress beneath the flap. A thin flap usually allows more aqueous egress than a thick flap, assuming that other variables (sclerectomy size, number and tension of sutures) are the same. A thin flap is acceptable provided there is adequate tissue for flap closure. This author attempts to create a one-half scleral thickness flap, in trabeculectomy surgery, by creating the outline of the flap with a guarded trifacet diamond blade set at 375 m. A diamond blade or a metal disk blade is used to dissect the flap. The blade angle is maintained almost parallel to the sclera. If the flap begins to become thin during its creation, redirection

of the blade to a deeper plane is easily achievable. This will produce a thicker flap.

If a flap begins to become thick during its posterior dissection, there is a danger of entering the

FIGURE 27–2 Reticulated hyaluronic acid implant placed into the space created by the deep sclerectomy space.

Ostia of Schlemm's canal

Deep scleral flap (excised)

Descemet's

Endothelium

Schlemm's canal (medial wall)

Scleral spur

F

Corneal stroma

Scleral flap

Conjunctiva

Scleral lake

(filled with Healon GV)

Residual scleral bed

Pars plana