III. WOUND LOCATION
While wounds can be placed at any position theoretically, choice of location may be influenced by astigmatism considerations, preexisting ocular disease, and ergonomic comfort of the surgeon.
The closer the incision is placed to the optical center of the cornea, the greater the degree of astigmatism induced.Given the elliptical shape of the normal cornea, temporal incisions are generally further from the center and induce a smaller magnitude of astigmatism than superior incisions of equal size. This is true for both clear corneal and scleral tunnel incisions.Wound location can also be selected so as to correct preexisting astigmatism.
B. Ocular Disease
Ocular disease may play an important role in determining not only whether the wound is placed in the cornea or sclera, but also the clock hour or quadrant chosen. For example, a temporal pterygium may lead to a choice of placing a superior or superotemporal wound, while a superior filtering bleb or superotemporal tube shunt may preclude placement of superior wounds.
Ergonomic comfort of the surgeon is also important in planning the placement of the wound. For example, during phacoemulsification of the right eye, a right-handed surgeon has full temporal cornea access, but incisions in the superior cornea or sclera require the surgeon to sit at the head of the patient. In this position, a prominent brow or a globe positioned deep in the orbit may make surgical positioning and access difficult for the surgeon. Likewise, a high nose bridge can interfere with maneuverability of the second instrument. With the left eye, the surgeon has relatively improved access to the superotemporal globe, but a straight temporal incision requires the surgeon to be seated at the 4:30 position with spatial crowding from the patient’s upper torso.
D. Endophthalmitis Risk
While several prospective and retrospective studies have reported an increased risk of postoperative endophthalmitis with temporal wound incisions, others have found no statistically significant association between wound location and risk of endophthalmitis.
IV. WOUND ARCHITECTURE
A. Incision Width
As techniques and technology have advanced in cataract surgery, surgeons have created smaller incisions. In order to remove the lens, the incision must be of sufficient size to accommodate the sleeve diameter of the phaco probe. The development of smaller probes has led to initial incisions less than 2 mm and as small as 0.9 mm, such as those used in bimanual phacoemulsification.
The final wound dimension is determined by the size of the optic of the IOL to be inserted, or the size of the injector port when inserting foldable IOLs. Direct IOL insertion or insertion via an injector can stretch and permanently enlarge the incision, potentially resulting in loss of the designed self-sealing wound properties. As lenses inserted through wounds of less than 2 mm become more widely accepted and studied, the final wound size necessary to perform cataract surgery with IOL implantation will continue to decrease.
B. Incision Shape
The impact of incision shape on postoperative astigmatism is controver- sial. Some studies have revealed a significant change in postoperative astigmatism,while others found no such difference. Although the trend toward smaller, more astigmatism-neutral incisions has made wound-shape less relevant in routine phacoemulsification, incision shape is potentially important when shifts in postoperative astigmatism are desirable or when large incisions are unavoidable. Examples include the simultaneous correction of a preexisting astigmatism,the conversion of an operation to an extracapsular cataract extraction, or the decision to insert a nonfoldable IOL.
The 4 shapes described for scleral tunnel incisions are arcuate, straight, frown, and chevron. The arcuate incision follows a curved, circumlimbal trajectory that approximates the curvature of the adjacent limbus. The straight incision traces a simple linear trajectory in the direction tangential to the adjacent limbus. The frown incision follows a curved trajectory and can be thought of as an inverse-arcuate incision. Lastly, the chevron is a modification of the arcuate incision that approximates the curvature of the adjacent limbus with 2 straight lines in a V formation.
When an incision is made in the sclera, normal tension forces separate the 2 wound edges, causing gape. The curved incision allows the greatest degree of translational and rotational tissue movement,and therefore induces the greatest magnitude of postoperative astigmatism.The frown incision results in the least amount of tissue movement and postoperative astigmatism. In addition to the astigmatic benefit, the frown or chevron incision effectively shortens the length of the scleral tunnel, allowing less restriction to movement of surgical instrumentation without altering the chord length of the external incision.
C. Tunnel Length
Premature posterior or delayed anterior entry into the anterior chamber results in a tunnel that is too short or long, respectively, both of which can generate complications. A short tunnel can lead to poor control of anterior chamber depth and iris prolapse. Decreased tissue apposition and failure of the wound to self-seal may necessitate the use of sutures with potential post- operative astigmatic sequelae.Maximum architectural stability is obtained in wounds 3.5 mm wide or less and of at least 2 mm in length.
A long tunnel can decrease surgical instrument mobility in the anteri- or chamber due to oarlocking and impaired pivoting ability. The excessive manipulation of surgical instruments can tear the internal or external wound edges, induce scrolling or detachment of Descemet’s membrane, and create corneal striae, which in turn decreases intraoperative visibility and may damage corneal endothelium.The endothelial cells can be further damaged by phaco-induced trauma as the probe tip enters the anterior chamber in closer proximity to the corneal endothelium11 (see Figure 6-2).
V. WOUND CLOSURE
Both scleral tunnel and clear corneal incisions, when created properly, possess a self-sealing, valve-like tunnel design. If wound closure is felt to be inadequate, various techniques can be employed.
Sutures may be used to provide additional support in scleral tunnel and clear corneal incisions. However, suture placement has been associated with increased amounts of surgically induced astigmatism, and well-constructed clear corneal incisions have been reported to have superior closure when compared to sutured corneal incisions.Suture placement has not been found to decrease the rate of postoperative endophthalmitis, and one study evaluating the incidence of postoperative hypotony found no difference between wounds closed with stromal hydration and those closed with sutures.
B. Stromal Hydration
Corneal stromal hydration involves the forceful injection of fluid into the stroma adjacent to a clear corneal wound, causing stromal expansion and greater wound opposition. This technique has been shown to reduce the ingress of ocular surface fluid in the immediate postoperative period, but there is concern that wounds requiring hydration become inadequately sealed once the effect wears off.The duration of the effect of hydration is a matter of debate, with articles reporting durations of 15 minutes to 1 week.
C. Tissue Adhesives
A variety of synthetic and biologic tissue adhesives have been used for enhanced wound closure. Commonly used agents include cyanoacrylate- based and fibrin-based adhesives. The adjunctive use of cyanoacrylate-based adhesives has been shown to produce watertight scleral and clear corneal wounds whose resistance may be superior to sutured wounds.Both fibrin- based and cyanoacrylate-based adhesives are reported to have a tensile strength comparable to sutures used in cataract surgery.
The primary advantage of wound adhesives is enhanced wound closure whose effectiveness may exceed that of wound suturing. Disadvantages include an inconvenient preparation and application process, foreign body sensation, reactive conjunctival hyperemia, and, in the case of human fibrin glues, the theoretic risk of viral transmission.Novel agents, including chondroitin sulfate aldehyde-, acrylic copolymer-, dendrimer-, and dendritic macromer-based tissue adhesives, may offer improved biocompatibility and a more favorable side effect profile.
Other proposed techniques for enhancing wound closure include increasing the target case-completion intraocular pressure, using peribulbar anesthesia with lid-taping instead of topical anesthesia, using methylcellulose-based viscosurgical devices rather than chondroitin sulfate or sodium hyaluronate, and internal wound tamponade by an air bubble in the anterior chamber.
The type, location, and architecture of the wound constructed each influence the surgical procedure, patient recovery, and visual outcome of phaco- emulsification. While both scleral tunnel and clear corneal wounds may be used to successfully complete removal of the cataract, attention must be given to each individual patient and preexisting ocular disease, astigmatic profile, and facial anatomy. Wound architecture and surgical technique are also important in avoiding intraoperative complications, minimizing corneal endothelial damage, and proper wound closure. A thorough understanding of the advantages, disadvantages, and correct application of each element of wound construction is essential to the proper planning and ultimate visual outcome.
*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 59-63