In the evolution of cataract surgery, the incision has become increasingly smaller, sturdier, and more anatomically correct. As recently as 1972 the cataract incision was typically created at the limbus, limbus parallel, short in length, and beveled. It was closed with sutures of varying number and material. These incisions healed with an unpredictable wound gape. They created erratic amounts of astigmatism, and, on occasion, were difficult to close in a watertight method.

In 1977, Richard Kratz introduced the concept of a scleral tunnel incision. This incision began approximately 2 mm posterior to the limbus. A tunnel was created through sclera to the cornea, where the plane of the incision was adjusted and intraocular entry occurred near the iris root. The incision width was required to be 5.5 to 7 mm depending on the style of polymethylmethacrylate (PMMA) intraocular lens (IOL) selected. This incision was closed with an interrupted or running suture of different materials. The benefit of the long tunnel was that it was easier to create a watertight closure. In addition, there was improved wound healing through the increased surface area of the scleral tunnel, due to the posterior location of the external incision, it was more astigmatically neutral.1,2

All scleral incisions shared the potential problems of difficulty manipulating the phaco tip within the incision, hemorrhage within the wound creating hyphema, and wound leaks with subsequent filtering blebs.

“ONE-STITCH” CLOSURE AND

FOLDABLE INTRAOCULAR LENS

In October 1989, the first foldable IOL gained Food 28 and Drug Administration (FDA) approval. The

AMO SI-18 lens could be folded and inserted through an incision of 4 mm in width. The smaller incision required only a single suture for closure, was easier to make watertight, and, due to its small width, was more astigmatically neutral. The increased wound stability resulted in faster visual rehabilitation for the patient.

Suturing techniques similarly evolved. John Shepherd3 introduced the first horizontal mattress suture. This technique attached the roof of the incision to the floor with a single 10-0 nylon limbus parallel mattress suture. This resulted in even greater wound stability, with resultant enhanced visual restoration of stable vision.

SUTURELESS CATARACT SURGERY

THE SCLEROCORNEAL INCISION

In January 1990, theorizing that the suture closure of the incision caused hyphema, Michael McFarland4 introduced the concept of sutureless incision construction. He created a funnel-shaped scleral tunnel with an internal width of 4 to 4.5 mm and an external width of 3.0 mm. He created vertical cuts in the floor of the incision near the external aspect to allow passage of the folded silicone IOL.

Paul Ernest5 shortened the length of the scleral aspect of the tunnel and modified the internal aspect of the incision to lengthen it, thus creating an internal corneal lip. This modification caused the prevention of iris prolapse and assured a secure closure. These incisions were watertight and did not require a suture.

Cadaver studies confirmed the tensile strength of the properly constructed sutureless incision with an

internal corneal lip. Gonioscopic studies confirmed anterior chamber entry anterior to the trabecular meshwork, therefore preserving this structure.

THE CLEAR CORNEAL INCISION

In the spring of 1991, Howard Fine6 introduced sutureless clear corneal incisions. His original wound design was for a temporally placed external incision, at the anterior aspect of the limbus, with a width of 4 mm (due to implant insertion requirements at that time). The internal incision also had a width of 4 mm. The length of the tunnel was 1.75 mm. This wound construction was difficult to make entirely watertight. Sporadic reports of endophthalmitis gave credence to the issue of inadequate wound seal allowing backward flow of tear into the anterior chamber with commensurate bacterial contamination.

Cadaver studies of this wound architecture by Ernest7 revealed that square corneal wound construction, regardless of incision size, was as stable as sclerocorneal tunnel incisions with the corneal lip. The stable incision was thought to be more watertight and would therefore prevent endophthalmitis.

New IOL technology and insertion techniques utilizing injectors have allowed IOL insertion through increasingly smaller incisions.8 Fine introduced the

concept of stromal hydration.9 Therefore, at present, small clear corneal tunnels that, although nonsquare, approach square dimensions are now created and are immediately watertight through the use of stromal hydration. These incisions are created at the temporal limbus, have a width of 2.8 to 3.2 mm, and a length of 1.75 to 2.00 mm (Fig. 4–1).

A modification of clear corneal incisions termed “near clear” locates the corneal external incision at, or just posterior to, the sclerocorneal junction at the limbus. The posterior location allows for a longer, more stable tunnel, without causing the internal aspect of the incision to invade the visual axis7 (Fig. 4–2).

A modernized form of the Sinskey scleral tunnel is the sclerocorneal frown incision. This incision is variably curved away from the limbus. At the external incision the closest point is 2 mm posterior to the limbus and has a chord length, dependent on the optic diameter of the IOL to be inserted, of 3.6 to 4.6 mm. The external incision will stretch and straighten to allow implantation of a 5.5- to 6.5-mm optic of a PMMA IOL. The internal incision is limbus parallel 1 mm anterior to the limbus. This generates a corneal valve to prevent iris prolapse. The internal incision will measure 5.2 to 6.2 mm and will stretch minimally during IOL implantation (Fig. 4–3).

From the above discussion it can be seen that, in general, cataract incisions fall into two categories:

sclerocorneal incisions, including tangential and frown incisions; and clear corneal incisions, including near-clear incisions.

ANATOMY

The surgical limbus is approximately 2 mm wide. The anterior limbal border ends at the most anterior reflection of conjunctivae on the cornea. This can be delimited microscopically as fine conjunctival blood vessels terminating in clear cornea. This overlies the termination of Bowman’s membrane. Just posterior, extending for 1 mm, is a blue area that terminates at the end of Descemet’s membrane (Schwalbe’s line). This part of the limbus overlies the cornea. It blends posteriorally with a 1-mm white area that overlies the scleral spur and iris root. The posterior aspect of this area marks the posterior limit of the limbus. The limbal zones are wider and therefore more pronounced in the vertical meridian than in the horizontal meridian8 (Fig. 4–4).

There are three anatomic components of the surgical incision. The external incision is the posterior aspect of the incision. The internal incision is the anterior aspect of the incision through Descemet’s membrane. In between is the tunnel part of the incision.

The external incision of the clear cornea incision is created just anterior to the fine limbal arcades of conjunctival blood vessels seen on slit-lamp examination. That of the near-clear corneal incision (limbalcorneal) is located through to the conjunctival reflection on the cornea, at the most anterior aspect of the blue zone.

Scleral spur (thin white line)

Schwalbe's line

Trabecular meshwock

Schlemm's canal

Conjunctiva

Tenon's fascia

Scleral spur

FIGURE 4–4 Limbus anatomic relationships.

TABLE 4–1 CLASSIFICATION OF CORNEAL

TUNNEL INCISIONS BY EXTERNAL INCISION

LOCATION9

Clear corneal incision—entry anterior to conjunctival insertion

Near-clear corneal incision (limbal corneal)—entry through conjunctiva and limbus

The external incision of the clear or near-clear corneal incision can be created with or without a hinge (Table 4–1). A direct entry through the cornea has the benefit of ease of construction and speed. This may be of benefit during topical anesthesia. The lip of this incision, however, is thin and fragile. It should not be manipulated directly with instruments, as tearing is likely. Alternatively, a hinge of varying depth may be created. If the hinge is deep (0.6 mm), as described by Langerman,10 the floor of the incision is reportedly more flexible so that it will seal more efficiently, creating a more watertight closure. The hinge also allows for the construction of a thicker roof, making it less fragile and less likely to be damaged by holding instruments. However, there have been reports that a deep hinge leads to wound instability and override, which causes the patient to perceive foreign body sensation for a variable amount of time.

The external incision of the sclerocorneal frown incision is located in the white zone posterior to the limbus (Table 4–2 and Figs. 4–1 to 4–3).

The tunnel portion of the wound through the sclera or corneal stroma must be in the same vertical plane as the eternal incision. If one side is deeper into the tissues than the other, a watertight closure is more difficult.

The internal incision in all cases consists of a sharp entry through Descemet’s membrane and endothelium. If Descemet’s is not sharply incised, it may be pushed ahead of the blade, leading to a Descemet’s detachment.

The ideal incision would be one that would induce no astigmatism, allow easy access to the anterior chamber for all aspects of the procedure yet allow the egress of fluid necessary for successful phaco, and allow for a sutureless watertight closure at physiologic anterior chamber pressures.11

TABLE 4–2 SCLEROCORNEAL INCISION—ENTRY

POSTERIOR TO THE LIMBUS9

Classification of corneal tunnel incisions by wound architecture

Single plane—no groove Shallow groove— 400 m Deep groove—>400 m