Wound construction is critical in modern cataract surgery because this initial step of the procedure serves as the foundation on which the rest of the surgical steps are built. A poorly constructed wound will make subsequent steps more difficult and increase the risk of complications. A properly constructed wound not only facilitates the surgery but more importantly ensures a self-sealing, watertight wound.
Numerous surgical knives can be used to fashion the incision. These blades are available in different materials, sizes, shapes, and bevels. Typically either a keratome alone or in conjunction with a preset blade for the groove is used to create the incision. Disposable keratomes of the correct size can be prepackaged with the phaco packs to ensure that the incision width matches the size of the phaco needle. Alternatively, diamond knives create excellent reproducible incisions and are available in a variety of shapes and sizes. These are obviously more expensive initially but are reusable and have a long lifespan if well cared for.
There has been debate about whether or not clear corneal incisions increase the risk of endophthalmitis. Incisions in vascular tissue (sclera and anterior limbus) produce a rapid fibroblastic response that strengthens and stabilizes the wound in the early posteroperative period. Although this reaction does not occur in the cornea, I do not believe it is simply the location of the wound (corneal vs. scleral vs. limbal) but rather the water tightness of the incision that is responsible. There are many ways of creating a corneal incision and not all incisions are equal.
The various components of the cataract incision include location, size, shape, and architecture:
Location refers to the position relative to the limbus and the clock hour position at which the incision is centered. Incisions produce tissue gape, causing corneal flattening in the meridian of the incision and steepening in the perpendicular meridian. The degree of astigmatism depends on the distance of the incision from the center of the cornea, the size of the incision, and its shape. Astigmatism is inversely proportional to the incision’s distance from the limbus and directly proportional to the cube of the incision’s length. Incision are now typically created in a temporal location rather than superiorly and in clear cornea or at the anterior limbus (“near clear”) as opposed to traditional scleral tunnel incisions. However, some surgeons prefer to place the incision on the steep meridian and will therefore operate at various locations.
Size refers to the width and length of the wound. The width is determined by the equipment: gauge of phaco needle and IOL design. The advantage of a smaller incision is reduced astigmatic effect, and those shorter than 3 mm are astigmatically neutral. The most stable wound configuration is square, with the length of the tunnel as long as the width of the incision.
Shape refers to the external appearance. The larger scleral tunnel incisions were made in a variety of shapes including curved, straight, frown, keyhole, or chevron. Current small and micro-incision cataract surgery incisions are straight.
Architecture refers to the number of incision planes. Biplanar wounds have a larger surface area than uniplanar ones, and may be more stable. The initial partial-thickness groove is made perpendicular to the cornea followed by a beveled entry into the anterior chamber. Triplanar incisions (perpendicular-beveled-perpendicular) consist of an internal corneal lip created by redirecting the keratome tip (“dimple down” maneuver followed by reorienting the keratome parallel to the iris). This functions as a one-way valve and produces a self-sealing, water tight wound when made correctly. If, however, the keratome is angled downward, upward, or tilted sideways while incising Descemet's membrane, then instead of being straight, the shape of the internal incision will be an arrowhead, V, or S, respectively, and the internal valve function may be compromised.
Regardless of the type and method of wound construction, it is imperative to evaluate the incision carefully at the conclusion of surgery. The wound must be tested for watertightness, which is usually assessed by applying pressure with the broad edge of a microsurgical sponge just posterior to the incision or by depressing the central corneal dome with the smooth handle of the same sponge. This should be done with the eye at a physiologic intraocular pressure and prior to stromal hydration of the wound. If the wound leaks then suture closure is recommended. If there is only a drop of fluid that can be expressed with pressure behind the incision, then stromal hydration at the internal corners of the wound will almost always seal the wound and suturing may not be necessary.
The importance of mastering the cataract incision cannot be overemphasized. Once the aforementioned components are understood, a variety of wound designs can be created and tailored to the individual case. A well-constructed wound is the first step in successful surgery for both the surgeon and the patient.