Incision Construction - Cataract Surgery, 3rd Edition

Cataract Surgery, 3rd Edition

PART IV – Phacoemulsification

Chapter 13 – Incision Construction

Howard Fine,MD,
Richard S. Hoffman,MD,
Mark Packer, MD, FACS

Contents

	•	Evolution of Small Incisions
	•	Surgical Techniques for Scleral Tunnel Incisions
	•	Development of Clear Corneal Incisions
	•	Indications for Clear Corneal Incisions
	•	Classification of Clear Corneal Incisions
	•	Preoperative Evaluation
	•	Techniques
	•	Intraoperative and Postoperative Complications
	•	Postoperative Clinical Course and Outcomes
	•	Profiles of Clear Corneal Incisions
	•	Controversies Surrounding Clear Corneal Incisions
	•	Endophthalmitis: Is there an Increased Risk?
	•	Conclusion

CHAPTER HIGHLIGHTS

	≫	Principles of self-sealing incisions
	≫	Development of clear corneal incisions
	≫	Techniques and profiles of clear corneal incisions
	≫	Controversies in self-sealing and sutured incision techniques

During the decade between the 1960s and the early 1970s, most cataract surgery in the United States and Europe was performed by the intracapsular cataract extraction technique using a limbal incision under a conjunctival flap. With few exceptions, there was little interest in reducing, minimizing, or altering surgically induced astigmatism.^[1,][2] The last 25 years have produced a rapid advancement in cataract surgery wound architecture. As the technology for removing cataracts has advanced, there has been a gradual trend towards smaller incisions, moving from the superior scleral to the temporal clear corneal location, in an attempt to reduce intraoperative complications and postoperative astigmatism.

Evolution of small incisions

With the advent of phacoemulsification, Kelman^[3] predicted that incisions 3mm wide would be astigmatism neutral because of their reduced size. However, within a very short time of the introduction of phacoemulsification, intraocular lens (IOL) implants became more commonplace. This situation necessitated the enlargement of the phacoemulsification incision to 6.5–7mm for lens implantation.

Kratz is generally credited as the first surgeon to move from the limbus posteriorly to the sclera in order to increase appositional surfaces thus enhancing wound healing and reducing surgically induced astigmatism (Figure 13-1).^[4,][5] Girard and Hoffman^[6] were first to call the posterior incision a scleral tunnel incision and were, along with Kratz, the first to make a point of actually entering the anterior chamber through the cornea creating a corneal shelf. This corneal shelf was designed to prevent iris prolapse. Maloney, who was a fellow of Kratz, advocated a corneal shelf to his incisions, which he described as strong and waterproof.^[7]



Figure 13-1 The scleral tunnel incision and running suture closure.

With the availability of small-incision lenses that could be introduced through incisions of 4–mm or less, the stage was set for the development of techniques that resulted in the achievement of both relative astigmatism-neutral and self-sealing incisions. In 1989, Shepherd^[8] introduced the single horizontal suture, which was actually a vertical mattress suture, for the closure of 4mm scleral tunnel incisions in phacoemulsification and foldable lens implantation (Figure 13-2). The achievement of astigmatism neutrality was impressive. Others rapidly recognized that the compressive force of the single horizontal suture was tangential to the limbus and, therefore, exerted no force on the cornea, which would alter its curvature. As a result, variations of the Shepherd single stitch were soon developed for closure of incisions 5–7mm wide, including the Fine infinity suture (Figure 13-3),^[9] Masket's horizontal anchor suture (Figure 13-4),^[10] and Fishkind's horizontal overlap suture (Figure 13-5).^[11]



Figure 13-2 The single horizontal suture.



Figure 13-3 The infinity suture.



Figure 13-4 The horizontal anchor suture.



Figure 13-5 The horizontal overlap suture.

In 1989, McFarland^[12] utilized the corneal shelf incision architecture and recognized that these incisions sized for foldable IOLs allowed for the phacoemulsification and implantation of lenses without the need for suturing. This involved lengthening the scleral tunnel and, in his early attempts, creating partial-thickness grooves in the floor of the scleral tunnel parallel to the long axis of the tunnel so that the incision could be reversibly stretched to admit a foldable lens.

Ernest^[13] observed McFarland's surgery and recognized that McFarland's long scleral tunnel incision terminated in a decidedly corneal entrance and that the posterior lip of the incision, the so-called corneal lip, acted as a one-way valve imparting to this incision its self-sealing characteristics (Figure 13.6). Koch^[14] described what he called the incisional funnel (Figure 13.7), indicating that there were certain characteristics of seal-sealing incisions with respect to length and configuration that imparted not only self-sealability, but also astigmatism neutrality to these incisions.



Figure 13-6 The self-sealing “corneal lip” scleral tunnel incision.



Figure 13-7 Incisional funnel with two possible incisions illustrated: both astigmatism neutral.

Self-sealing scleral tunnel incisions have varied with respect to width and the configuration of the groove (which represents the external or scleral incision as opposed to the internal or corneal portion of the incision). The groove has varied from circumlimbal to straight (Figure 13.8), frown (Figure 13.9) or chevron-shaped.^[15–18]



Figure 13-8 Scleral tunnel incision with straight groove.



Figure 13-9 Frown incision.

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Surgical techniques for scleral tunnel incisions

In the following passage, we describe in detail the construction of a self-sealing scleral tunnel incision using a straight external scleral groove and a tunnel width of 4mm, recognizing that the same tunnel can be made 7mm wide with enlargement of the internal opening from 3 to 7mm following completion of phacoemulsification and cortical cleanup, and just before lens implantation.

A conjunctival flap is made precisely by marking the width of the scleral tunnel at the limbus (Figure 13.10), and making vertical releasing incisions in the conjunctiva and Tenon's at exactly that width. These vertical releasing incisions extend back approximately 5mm. The sub-tenon's space is bluntly dissected with a scissors (Figure 13.11) before a peritomy (Figure 13.12). After the peritomy, the conjunctiva-Tenon's flap is folded at its base upside down on top of the posterior conjunctiva. The peritomy leaves approximately a 0.5–1mm lip of conjunctiva attached to the limbus. This acts as a buttress postoperatively to prevent anterior migration of the conjunctiva so that the flap never overhangs the limbus.



Figure 13-10 Caliper marks on conjunctiva to indicate position of vertical releasing incisions.



Figure 13-11 Blunt dissection of sub-Tenon's space with closed scissors.



Figure 13-12 Peritomizing the conjunctival flap.

Mild cautery is performed near the limbus. Posteriorly, however, heavier cautery is used. The large vessels emanating from the rectus muscle and perforating the sclera between the muscle and the beginning of the tunnel are cauterized directly and adequately. (If these perforating vessels are cauterized before they enter the sclera, the tunnel should be dry during the entire procedure and there should be no bleeding either intraoperatively or postoperatively resulting in hyphema.)

Following cautery, a Fine millimeter marker (Rhein Medical No. 8-12106) is stamped in methylene blue and then pressed to the scleral bed, creating a 5 × 8mm grid of dots 1mm apart starting 1mm posterior to the anterior edge of the corneal vascular arcade (Figure 13-13). This allows selection of an incision length, location and shape with great precision and reproducibility.^[17] The globe is fixated with a twist grip (Weck No. 7640) posteriorly in the area of bared sclera and the sclera is cut perpendicularly to make a groove by incising the appropriate dots (Figure 13-14). The groove is sufficiently deep that the surgeon can look down the groove and pick the depth within the sclera at which he or she will dissect the scleral tunnel. A slight anterior edge is elevated with the No. 64 Beaver blade that is used to make the groove, and from that point on an Alcon bevel-up crescent knife (Figure 13-15) (Alcon 8065-940002) is used to dissect the scleral tunnel. (It is important to keep the leading edge of the knife down, whether cutting anteriorly or to either side, as one moves the knife. This is a sharp knife that makes a very clean dissection in the scleral plane.) The dissection is carried forward to the Descemet's membrane at the anterior edge of the vascular arcade (Figure 13-16).



Figure 13-13 Millimeter grid on bare scleral bed.



Figure 13-14 Initiation of the groove or external incision.



Figure 13-15 Initiation of the tunnel with a crescent knife.



Figure 13-16 Dissection of the scleral tunnel into clear cornea.

At this point, a side port is made with a trifacet freehand diamond knife (No. KOI KM218R). Viscoelastic is exchanged for aqueous humor through the side port by injecting the viscoelastic into the distal angle. As the expanding wave of viscoelastic moves towards the paracentesis, aqueous humor is expressed. This results in a very stiff and stable anterior chamber. A 3.5mm keratome blade (Beaver No 5530) is lubricated with viscoelastic and brought into the tunnel. The blade is advanced so that its point is just at the anterior edge of the vascular arcade. The point is tipped slightly posteriorly, resulting in a dimple on the anterior surface of the cornea, whose center is directly on the anterior edge of the arcade. The dimple is frequently outlined by a semicircular light reflex (Figure 13-17) with the tip of the keratome at the center. The keratome is then advanced horizontally, parallel to the iris, which results in a linear horizontal cut through Descemet's membrane into the anterior chamber, 0.5mm anterior to the edge of the vascular arcade (Figure 13-18).



Figure 13-17 Dimpling of the cornea by depressing the point of the keratome.



Figure 13-18 Straight-line incision in Descemet's membrane 0.5 mm anterior to the vascular arcade.

The surgeon must continuously guide the tip of the keratome as it is brought into the anterior chamber. If it is pointed too posteriorly, the cut in Descemet's membrane will start to curve posteriorly at the ends in a “frown” configuration. On the other hand, if the tip of the keratome is elevated too much, the cut in Descemet's membrane will start to curve forward in a “smile” configuration rather than proceeding straight across and parallel to the groove. If the keratome is tilted to one side or the other, an “S-shaped” configuration may result. In all instances, observation of the cut as it proceeds in Descemet's membrane by advancing the keratome can allow for correction of the orientation of the keratome. A straight cut in Descemet's membrane is necessary for the correct architecture of the incision.

The incision is complete when the parallel shoulders of the keratome enter the anterior chamber. The incision can be characterized by the presence of a short posterior lip of clear cornea that acts as a one-way valve.^[19] Following the completion of the case, this valve is held closed by intraocular pressure which also acts to collapse the scleral tunnel.

If one goes more anteriorly into clear cornea before incising Descemet's membrane, the visualization during phacoemulsification is markedly impaired because of the striae that occur as the phaco tip is tilted down for endolenticular phacoemulsification.

It is important to avoid putting traction on the roof of the scleral tunnel with a forceps. A bridle suture is used during incision construction and the twist grip is placed posterior to the dot grid to stabilize the globe during construction of the scleral tunnel. The forceps is used to elevate the tunnel roof in placing the keratome inside the tunnel, but countertraction is placed on the posterior lip of the groove rather than the anterior lip during the cutting of Descemet's membrane with the keratome.

Phacoemulsification and later evacuation of viscoelastic take place with the bridle suture unattached to minimize stretching of the tunnel roof. After cortical cleanup and expansion of the bag with viscoelastic, the incision in Descemet's membrane is widened with a 4mm blunt-tip keratome (Figure 13-19) (Beaver No. 374732) for folded silicone lenses. For 6mm lenses, the initial keratome incision is enlarged with a super-sharp knife (15° Alcon ophthalmic knife No. 8065-921502) taking care to incise Descemet's membrane as a continuation of the straight-line cut made by the 3.5mm keratome.



Figure 13-19 Enlargement of the 3.5 mm incision to 4.0 mm with a blunt tipped keratome.

Following IOL implantation and evacuation of residual viscoelastic, the anterior chamber is fully repressurized with BSS through the side port. The lips of the wound are tested by applying pressure with a Weck cell sponge against the posterior lip of the wound (Figure 13-20) in an effort to make the incision leak. If it does leak, which happens less than 5% of the time, a single horizontal suture is placed (in the case of incisions 5mm or larger, an infinity suture is placed). If no leakage is observed, the conjunctival flap is unfolded back over the incision and smoothed in place. Its corners are returned to the corners of the bed from which they were derived, up against the remaining lip of conjunctiva attached to the limbus (Figure 13-21). The flap is frequently adherent within 1 h, as observed in one-eyed patients who are not patched at the conclusion of surgery. A Maloney keratometer is used to estimate astigmatism at the conclusion of the surgery (Figure 13-22).^[15]



Figure 13-20 Testing the scleral incision for being watertight.



Figure 13-21 Conjunctival flap repositioned in its bed.



Figure 13-22 Estimation of corneal curvature using a Maloney intraoperative qualitative keratometer.

Development of clear corneal incisions

There have been many surgeons who have favored corneal incisions for cataract surgery prior to their recent popularization. In 1968, Kelman^[3] stated that the best approach for performing cataract surgery was with phacoemulsification through a clear corneal incision utilizing a triangular-tear capsulotomy and a grooving and cracking technique in the posterior chamber. Harms and Mackenson^[20] in Germany published an intracapsular technique using a corneal incision in 1967 in an atlas called Ocular Surgery Under the Microscope. Troutman was an early advocate of controlling surgically induced astigmatism at the time of cataract surgery by means of the corneal-incision approach.^[21] Arnott^[22] in England used clear corneal incisions and a diamond keratome for phacoemulsification, although he had to enlarge the incision for introducing an IOL. Galand^[23] in Belgium utilized clear corneal incisions for extracapsular cataract extraction in his envelope technique and Stegmann of South Africa has a long history of having used the cornea as the site for incisions for extracapsular cataract extraction (Stegmann R. Personal communication, December 3, 1992). In April of 1992, Fine presented his self-sealing temporal clear corneal incision at the annual meeting of the American Society of Cataract and Refractive Surgery.^[24] In May of 1992, at the Island Ophthalmology Seminar, Kellan demonstrated on video a technique that he referred to as the scleral-less incision. It was essentially a corneal limbal stab incision through conjunctiva and the limbus, entering the anterior chamber through clear cornea, leaving a corneal shelf or lip (Figure 13-23A and B). Finally, perhaps the leading proponent of clear corneal incisions for modern era phacoemulsification was Kimiya Shimizu of Japan.^[25]



Figure 13-23 A, Kellan's corneal limbal stab incision through conjunctiva and the limbus. B, Corneal limbal incision following removal of the steel keratome.

Fine's personal experience with corneal incisions began in 1979 when the temporal clear cornea was used as the site for secondary anterior chamber IOL implantation. The temporalapproach was preferred because of the unpredictable nature of the disturbed anatomy present at the superior limbus in eyes that had previous intracapsular cataract extraction. As soon as foldable lenses were available, in 1986, he used sutured clear corneal incisions for phacoemulsification and foldable IOL implantation in patients who had pre-existing filtering blebs. After these procedures, a marked reduction in surgically induced astigmatism was noted despite the fact that these incisions were corneal rather than scleral. In 1992, Fine began routinely utilizing clear corneal cataract incisions for phacoemulsification and foldable IOL implantation with incision closure using a tangential suture modeled after John Shepherd's technique.^[8] Within a very short period, the suture was abandoned in favor of self-sealing corneal incisions.^[26]

Indications for clear corneal incisions

Initially, the utilization of clear corneal incisions were limited to those patients with pre-existing filtering blebs, patients taking anti-coagulants or with blood dyscrasias, or patients with cicatrizing disease such as ocular cicatricial pemphigoid or Stevens–Johnson syndrome. Subsequently, because of the natural fit of clear corneal cataract incisions with topical anesthesia, the indications for clear corneal cataract surgery expanded. With the ability to avoid any injections into the orbit and utilization of intravenous medications, those patients who had cardiovascular, pulmonary, and other systemic diseases that might have contraindicated cataract surgery became surgical candidates. Subsequently, through the safety and increasing utilization of these incisions by some pioneers in the United States, including Williamson, Shepherd, Martin, and Grabow,^[27] these incisions became increasingly popular and utilized on an international basis.

Studies by Rosen^[28] using topographical analyses of these incisions demonstrated that clear corneal incisions sized 3mm in width or less were topographically astigmatism-neutral. This led to an increasing interest in these incisions because of an increasing utilization of techniques including T-cuts, arcuate cuts, and limbal relaxing incisions for managing pre-existing astigmatism at the time of cataract surgery. Without astigmatism neutrality in the cataract incision, the predictability of adjunctive astigmatism-reducing procedures would be decreased, making it more difficult to achieve the desired result. In the initial studies and ultimate utilization of multifocal IOLs, the need for astigmatism neutrality was again a factor for stimulating interest in clear corneal incisions. Finally, the availability of phakic IOLs and the need for control of astigmatism at the time of implantation of these lenses has driven many surgeons to consider clear corneal incisions as the route for phakic IOL implantation.

Other advantages of the temporal clear corneal incision include:

	•	better preservation of pre-existing filtering blebs^[29]
	•	preservation of options for future filtering surgery
	•	increased stability in the refractive results because of the neutralization of the forces from lid blink and gravity
	•	the ease of approach to the incision site
	•	the lack of need for bridle sutures and resultant iatrogenic ptosis
	•	the location of the lateral canthal angle under the incision which facilitates drainage.

Classification of clear corneal incisions

Early on there was criticism surrounding the use of self-sealing clear corneal incisions because of the fear of a possible increase in the incidence of endophthalmitis secondary to poor wound healing and sealability. This potential controversy stimulated many studies into the strength and safety of clear corneal incisions compared to limbal and scleral tunnel incisions. Unfortunately, because of a lack of standardization in the definition of what constitutes a limbal versus clear corneal incision, considerable confusion has been generated in this area making it difficult for surgeons to communicate and compare the relative claims of their individual techniques. Based on Hogan's Histology of the Human Eye: “The conjunctival vessels are seen with the slit lamp as fine arcades that extend into clear cornea for about 0.5mm beyond the limbal edge”,^[30] and topographical studies of incisions done by Menapace^[31] in Vienna, Fine has categorized these incisions using the parameters of location and architecture.^[32] An incision is termed clear corneal when the external edge is anterior to the conjunctival insertion, limbal corneal when the external edge is through conjunctiva and limbus, and scleral corneal when it is posterior to the limbus (Figure 13-24). In addition to the anatomic designation of the external incision, these incisions are also classified by their architecture as being single plane when there is no groove at the external edge of the incision, shallow groove when the initial groove is less than 400 μ, and deeply grooved when it is deeper than 400 μ (Figures 13-25 and 13-26). To reduce the confusion and facilitate communication regarding these incisions, we believe they should be classified as clear corneal, limbal corneal, or scleral corneal incisions and as single planed, shallow grooved, or deep grooved.



Figure 13-24 Classification of corneal tunnel incisions by external incision location.



Figure 13-25 Classification of corneal tunnel incisions by wound architecture.



Figure 13-26 Cross-sectional view of single plane (A), shallow groove (B), and deep groove (hinged) clear corneal incisions (C).

Preoperative evaluation

Certain studies that may be of value as part of a preoperative work-up include endothelial cell counts in patients with endothelial dystrophies, and perhaps computerized corneal topography when refractive surgical procedures are going to be combined with cataract surgery in the management of pre-existing astigmatism. This is especially true when refractive and keratometric measurements do not coincide. There has been a recent trend for surgeons to use fourth-generation fluoroquinalone drops four times per day for 3 days prior to the day of surgery.

Techniques

Single plane incisions, as first described by Fine,^[33] utilized a 3mm diamond knife.

A Fine-Thornton 13mm fixation ring (Mastel Instruments, Rapid City, SD) (Figure 13-27) stabilizes the globe and allows manipulation without creating conjunctival tears, subconjunctival hemorrhages, or corneal abrasions (Figure 13-28). Aqueous humor is replaced by viscoelastic material through the side-port incision (Figure 13-29). After pressurization of the eye with viscoelastic, a 300 micron groove may be placed at the anterior edge of the vascular arcade (Figure 13-30), however, this is optional. If the groove has been placed, an incision is made by depressing the posterior edge of the groove with the diamond blade, flattening the blade against the surface of the eye. The knife is moved in the plane of the cornea until the shoulders, which are 2mm posterior to the point of the knife, touch the external edge of the incision and then a dimple down technique is used to initiate the cut through Descemet's membrane. After the tip enters the anterior chamber, the initial plane of the knife is re-established to cut through Descemet's in a straight-line configuration (Figure 13-31). Following phacoemulsification, lens implantation, and removal of residual viscoelastic, stromal hydration of the clear corneal incision can be performed in order to help seal the incision.^[26] This is performed by placing the tip of a 26- or 27-gauge cannula in the side walls of the incision and gently irrigating balanced salt solution into the stroma (Figure 13-32). This is performed at both edges of the incision in order to help appose the roof and floor of the incision. Once apposition takes place, the hydrostatic forces of the endothelial pump will help seal the incision. In those rare instances of questionable wound integrity, a single radial 10-0 nylon suture is placed to ensure a tight seal.



Figure 13-27 The Fine Thornton ring, shown in partial profile. Temporal limbus is seen inferiorly.



Figure 13-28 Purchase of the globe by the Fine-Thornton ring.



Figure 13-29 Paracentesis being made.



Figure 13-30 Grooving of the peripheral cornea.



Figure 13-31 Construction of the corneal tunnel.



Figure 13-32 Stromal hydration of the incision.

Williamson^[34] was the first to utilize a shallow 300–400 micron grooved clear corneal incision. The rationale for the Williamson incision was that it led to a thicker external edge to the roof of the tunnel and less likelihood of tearing. Langerman^[35] later described the single hinge incision in which requirements for the initial groove were 90% of the depth of the cornea anterior to the edge of the conjunctiva. Initially he utilized a depth of 600 μ and subsequently made the tunnel itself superficially in that groove, believing that this led to enhanced resistance of the incision to external deformation. Minimal differences in surgically induced astigmatism have been demonstrated between beveled and hinged clear corneal incisions.^[36]

Adjunctive techniques were utilized to combine refractive surgery incisions with clear corneal cataract incisions. Until recently, Fine used the temporal location for the cataract incisions and added one or two T-cuts made by the Feaster Knife (Rhein Medical No. 05-8200) with a 7mm ocular zone for the management of pre-existing astigmatism. Others, including Lindstrom and Rosen, rotated the location of the incision to the steep axis in order to achieve some increased flattening at the steepest axis to address pre-existing astigmatism. Kershner^[37] utilized the corneal incision in the temporal half of the eye by starting with a nearly full-thickness T-cut through, which he then made his corneal tunnel incision. For large amounts of astigmatism he used a paired T-cut in the opposite side of the same meridian. Finally, the popularization of limbal relaxing incisions by Gills^[38] and Nichamin^[39] added an additional means of reducing pre-existing astigmatism by using the groove for the limbal relaxing incision as the site of entry for the clear corneal cataract incision. This has been found to be a simple and practical approach for reducing pre-existing astigmatism at the time of cataract surgery.^[40] At this time, Fine places all of his incisions at the temporal periphery and addresses pre-existing astigmatism with limbal relaxing incisions at the steep axis and/or toric IOLs.

New technology blades have been developed which have helped perfect incision architecture. The Fine Triamond Knife (Mastel No. 0851913191) was developed in conjunction with Mastel Precision Instruments (Rapid City, SD) so that the incision could be made with an extremely sharp, thin and narrow knife without a necessity for dimpling down, which resulted in some tendency for there to be tearing of tissue or scrolling of Descemet's membrane. Subsequently, in conjunction with Rhein Medical (Tampa, FL), the 3-D blade (No. 05-5083) was developed, which had differential slope angles to the bevels on the anterior versus the posterior surface (Figures 13-33A, B, C) resulting in an ability to just touch the eye at the site of the external incision location and advance the blade in the plane of the cornea. The differential slopes on the anterior versus posterior aspects of the blade allowed the forces of tissue resistance to create an incision that was characterized by a linear external incision, a 2mm tunnel, and a linear internal incision without the need to dimple down or distort tissues to create the proper incision architecture.^[41] The trapezoidal 3-D blade (Rhein No. 05-5086) also allows enlargement of the incision to 3.5mm for IOL insertion without altering incision architecture (Figure 13-34). Histologic studies of clear corneal incisions performed with steel keratomes and diamond keratomes have shown more disruption of corneal stromal tissue with steel keratomes and more likelihood of severe stromal damage after insertion of foldable IOLs, suggesting that diamond keratomes may have a beneficial effect on incision healing.^[42,][43]



Figure 13-33 Schematic representation of top view (A) and bottom view (B) of the 3 mm Rhein 3-D diamond keratome. The front profile of the keratome (C) demonstrates the differential slopes on the anterior versus posterior aspects of the blade which allow the forces of tissue resistance to create the proper incision architecture.



Figure 13-34 The Rhein 3-D Trapezoidal Blade with 2.5–3.5 mm blade dimensions.

Many companies, in addition to Rhein Medical, are designing diamond knives for clear corneal incisions. Mastel Precision Surgical Instruments have designed a sleek trapezoidal blade that they have named the Superstealth (Figure 13-35). The Stealth blade is an ultra-thin diamond with asymmetric facets that result in a self-directing bevel similar to the Rhein 3-D blade. ASICO (American Surgical Instruments Company, Westmont, Illinois) has designed two new diamond knives for clear corneal incisions: the Pathfinder (Figure 13-36) and the Clearpath (Figure 13-37). Both blades contain a shelf on the surface of the blade that creates an inner corneal valve of consistent length by forcing the leading edge of the blade into the anterior chamber when the shelf reaches the external incision. Stromal hydration of the wound is claimed to be unnecessary with the Clearpath blade due to the facet design.



Figure 13-35 Mastel Trapezoidal Diamond Stealth Blade.



Figure 13-36 ASICO Pathfinder Blade.



Figure 13-37 ASICO Clearpath Blade.

A recent study by Mamalis^[44] has revealed small predictable enlargements of clear corneal incisions after insertion of foldable IOLs with both forceps and injectors. The degree of wound enlargement increased with higher IOL powers when lenses were inserted with forceps but did not increase with increasing IOL powers when injectors were used. In general, injectors were associated with a smaller percentage increase in wound stretching than forceps, making them a preferable choice for foldable lens insertion through clear corneal incisions.

Intraoperative and postoperative complications

Although clear corneal and scleral incision cataract surgery share many of the same intraoperative and postoperative complications, clear corneal incisions, by nature of their architecture and location, have some unique complications associated with them. If one accidentally incises the conjunctiva at the time of the clear corneal incision, ballooning of the conjunctiva can develop, which may compromise visualization of anterior structures. When this develops, the use of a suction catheter is usually required by the assistant to aid in visualization. Early entry into the cornea might result in an incision of insufficient length to be self sealing, and thus a single suture may be required in order to ensure a secure wound at the conclusion of the procedure. A late entry may result in a corneal tunnel incision sufficiently long that the phacoemulsification tip would create striae in the cornea and compromise visualization of the anterior chamber. In addition, incisions that are too short or improperly constructed can result in an increased tendency for iris prolapse.

Manipulation of the phacoemulsification handpiece intraoperatively may result in tearing of the roof of the tunnel, especially at the edges, potentially compromising the ability for the incision to self seal. Tearing of the internal lip can also occur, resulting in compromised self-sealability or, in rare instances, small detachments or scrolling of Descemet's membrane in the anterior edge of the incision. Of greater concern has been the potential for incisional burns.^[45,][46] When incisional burns develop in clear corneal incisions, there may be a loss of self-sealability, corneal edema, and severe induced astigmatism.^[47] In addition, manipulation of the incision can result in an epithelial abrasion which can compromise self-sealability because of the lack of a fluid barrier by an intact epithelium. Without an intact epithelial layer, the corneal endothelium does not have the ability to help appose the roof and floor of the incision through hydrostatic forces.

Postoperatively, hypotony might result in some compromised ability for these incisions to seal. Wound leaks and iris prolapse have been very infrequent postoperative complications^[48] and are usually present in incisions greater than 3.5mm in width. In a large survey performed for the American Society of Cataract and Refractive Surgery by Masket and Tennen,^[49] there was a slightly increased incidence of endophthalmitis in clear corneal cataract surgery compared to scleral tunnel surgery. However, the survey failed to note the incision sizes in those cases where endophthalmitis in clear corneal incisions had occurred, and thus it is possible that any increase in the incidence of endophthalmitis is associated with unsutured clear corneal incisions greater than 4mm in width.

Postoperative clinical course and outcomes

The usual postoperative regime involves examination on the first postoperative day and a second examination at 10 to 14 days at which time spectacle correction is prescribed. Use of drops postoperatively includes instillation two to three times a day of a fluoroquinalone, prednisolone acetate and a topical non-steroidal anti-inflammatory drug (NSAID). The antibiotic and steroid are discontinued at 10 to 14 days and the NSAID is continued for an additional 10 days.^[50]

Numerous studies have been performed documenting the safety and low magnitudes of astigmatism induced by these incisions depending on their size. Masket and Tennen^[51] have documented by vector analysis 0.50 diopter (D) of induced cylinder and less than 0.25 D of cylinder change in the surgical meridian using 3.0 × 2.5mm self-sealing temporal clear corneal incisions. They were also able to demonstrate the refractive stability of these incisions 2 weeks following surgery. Kohnen, Dick, and Jacobi^[52] compared the surgically induced astigmatism of 3.5, 4, and 5mm grooved temporal clear corneal incisions and found a mean induced astigmatism of 0.37 D, 0.56 D, and 0.70 D respectively after 6 months. A similar study by Pfleger et al.^[53] revealed even smaller amounts of induced astigmatism from 3.2, 4, and 5.2mm temporal clear corneal incisions with the 3.2mm incision demonstrating astigmatic neutrality with only 0.09 D of induced cylinder.

In addition to comparing the effects of different-sized temporal clear corneal incisions on induced astigmatism, numerous studies have evaluated the relative astigmatic effects of incision location in regard to clear corneal incisions versus corneoscleral incisions, and of the temporal versus superior meridian. Nielsen^[54] evaluated surgically induced astigmatism from 3.5mm and 5.2mm temporal and superior clear corneal incisions, and compared them with 3.5mm and 5.2mm corneoscleral incisions at the superior location. The 3.5mm clear corneal incisions induced roughly 0.5 D of with-the-rule or against-the-rule drift, depending on temporal or superior location. Larger amounts of astigmatism were induced with the larger clear corneal incisions. He found that the refractive effect of clear corneal incisions was stable between postoperative day 1 and postoperative week 6, making their astigmatic keratotomy effect more useful and predictable if one wished to consider preoperative cylinder when selecting incision type or location.

Cillino et al.^[55] compared the astigmatic effects of unsutured 5.2mm temporal clear corneal incisions with 5.2mm superior corneoscleral incisions and found comparable amounts of induced astigmatism. Rainer et al.,^[56] however, has found a small but significant amount of surgically induced astigmatism continuing up to 5 years postoperatively with 5mm superior scleral incisions. Although the use of unsutured 5.2mm clear corneal incisions is considered unsafe because of a possible increase in rates of wound complications and endophthalmitis, Holweger and Marefat^[57] have demonstrated that absorbable sutured 5-clear corneal incisions were topographically comparable to 3.5mm sutureless clear corneal incisions, 6–8 months postoperatively, making this incision and closure technique a viable option for surgeons.

When temporal clear corneal incisions of 3.2mm or less have been compared with superiorly placed scleral tunnel incisions of the same size, similarly low numbers of induced astigmatism have been documented for the two incision locations.^[58,][59] In contrast, similarly sized incisions when compared in regard to temporal versus superior clear corneal location have demonstrated more meridional flattening in the superior axis than the temporal axis.^[60–62] This has also been demonstrated in the oblique superolateral clear corneal incision compared with a temporal incision, confirming the bias for the temporal location for clear cornea incisions when astigmatic neutrality is desired.^[63]

Although small clear corneal incisions appear to have similar astigmatic effects as superior corneoscleral incisions, recent concern has surrounded the possibility of increased endothelial cell loss with these incisions. Grabow^[64] reported an increased incidence of endothelial cell loss for superior clear corneal incisions, which increased linearly with increasing ultrasound times. Amon et al.^[65] discovered a significant increase in endothelial cell loss in 3.5mm temporal clear corneal incisions when compared to 3.5mm superior scleral tunnel incisions. However, a recent study by Dick et al.^[66] found that the total endothelial cell loss at 1 year with clear corneal incisions compared favorably with endothelial cell loss rates of other cataract extraction techniques. As ultrasound times decrease in the future with advancing technologies and techniques, such as lens chopping and the use of power modulations,^[67] endothelial cell loss rates should become insignificant.

Dick et al.^[68] have also recently demonstrated that cataract extraction through a clear corneal incision results in less inflammation in the immediate postoperative period when compared to surgery through a sclerocorneal incision. This may ultimately have a beneficial effect in reducing posterior capsule opacification, cystoid macular edema, and keratopathy.

Profiles of clear corneal incisions

Clear corneal incisions involving an incision in the plane of the cornea with a length equal to 2mm are still being constructed in the same manner today. In 1992, the incisions were as wide as 4mm, but have more recently been reduced to a maximum width of 3.5mm, if not sutured. Figure 13-26 shows an artist's view of what the profile of clear corneal incisions were thought to look like. Part A shows the single plane incision and its apparent inherent lack of stability as one surface can easily slide over another. Charles Williamson, MD, from Baton Rouge, innovated an alteration of that incision which involves a shallow, perpendicular groove prior to incising the cornea into the anterior chamber (Part B). David Langerman, MD, deepened the perpendicular groove with the belief that it led to greater stability (Part C). These grooved incisions have been abandoned by the authors in favor of a paracentesis-style incision due to the difficulties associated with a persistent foreign body sensation in the grooved incisions and the pooling of mucus and debris in the gaping groove. More importantly, the grooved incisions represent a disruption in the fluid barrier that intact epithelium create, which allows for a vacuum seal as a result of endothelial pumping.

Initial incision construction technique began with a blade applanated to the surface of the eyeball with the point at the edge of the clear cornea, which advanced for 2mm into the plane of the cornea before incising Descemet's membrane (Figure 13-38). These early incisions were made with knives with straight sides; however, these knives were subsequently replaced by trapezoidal-shaped knives in order to allow the enlargement of the incision without violating the architecture by cutting sideways. From the onset of the use of clear corneal incisions, stromal hydration of the incisions, which thickens the cornea, forcing the roof of the incision onto the floor of the incision and facilitating endothelial pumping to the upper reaches of the cornea, was strongly advocated. Testing the seal of the incision with a Seidel test using fluorescein (Figure 13-39) was also strongly advocated. These practices have not changed since 1992, except that we now infrequently depress the posterior lip of the incision.



Figure 13-38 Clear corneal incision construction with the blade completely inserted.



Figure 13-39 Testing the seal of the incision with a Seidel test using fluorescein and tactile pressure.

To obtain a better understanding of the architecture of clear corneal incisions, the authors conducted a study of the profiles of clear corneal incisions using the Zeiss Visante Optical Coherence Tomography (OCT) Anterior Segment Imaging System (Figure 13-40). This technology has allowed the first view of the clear corneal incision in the living eye in the early postoperative period. All previous views were in autopsy eyes sectioned through the incision, which introduces artifacts. Figure 13-41 shows an example of the corneal periphery in a control eye which includes the anterior chamber angle. The regularity of the corneal epithelium blending in the conjunctiva and the clear corneal stroma blending into sclera can be clearly seen.



Figure 13-40 The Zeiss Visante Optical Coherence Tomography Anterior Segment Imaging System.



Figure 13-41 OCT image of a control eye showing the corneal periphery including the anterior chamber angle.

A variety of knives were used to create the clear corneal incisions during cataract surgery. All clear corneal incisions were made by one surgeon (IHF). OCT images of each operative eye were taken on the first postoperative day, within 24 h of cataract surgery and are representative of multiple images from multiple patients.

As seen in Figure 13-42, which was taken on the first day postoperatively, the clear corneal incision is actually curvilinear, not a straight line, as seen in the artist's depiction of clear corneal incisions (Figure 13-26). It is an arcuate incision which is considerably longer than the chord length originally estimated for the length of the incision. It is very important to note that the architecture of the incision allows for a fit not unlike tongue and groove paneling, which adds a measure of stability to these incisions and makes sliding of one surface over the other considerably less likely. Figure 13-43 shows an incision that was made with a 300 micron groove at the external edge of the incision prior to incision construction. The incision itself still has a similar curved or arcuate configuration, but the gaping of the external groove, which is noted on the first day postoperatively, is accompanied by a similar offset of the internal lips of the incision, which appears to be somewhat less stable than a paracentesis-style incision.



Figure 13-42 OCT image of a clear corneal incision made with the Rhein 3D Trapezoidal 2.5–3.5 mm Blade. Image of the blade is inset.



Figure 13-43 OCT image of a clear corneal incision with a 300 micron groove at the external edge of the incision. Image of the Rhein 3D blade is inset.

These images also demonstrate the persistence of stromal swelling from stromal hydration on the first postoperative day, which many critics of clear corneal incisions believed disappeared within 1 or 2 h.

Figure 13-44 shows a clear corneal incision made with the Rhein Medical (Tampa, FL), the Rhein 3D Trapezoidal blade, 2–2.5mm (#05-5088), for incision construction using single-piece acrylic lenses with a Royale injector (ASICO, LLC, Westmont, IL, #AE-9045). Once again, the very advantageous architecture of the incision is observed. It is interesting to note that the arc length is considerably longer than the chord length and is probably a hyper-square incision in that it is only 2mm wide. As Figures 13-45–48 demonstrate, all clear corneal incisions made with a variety of blades demonstrated a similar, arcuate architecture.



Figure 13-44 OCT image of a clear corneal incision made with the Rhein 3D Trapezoidal 2–2.5 mm Blade. Image of the blade is inset.



Figure 13-45 OCT image of a clear corneal incision made with the Accutome Simplicity Blade. Image of the blade is inset.



Figure 13-46 OCT image of a clear corneal incision made with the Accutome Black Blade. Image of the blade is inset.



Figure 13-47 OCT image of a clear corneal incision made with the ASICO Clear Cornea Fixed Angle 2.8–2.8 mm Blade. Image of the blade is inset.



Figure 13-48 OCT image of a clear corneal incision made with the Mastel Superstealth Blade. Image of the blade is inset.

The BD Kojo Slit (BD Medical-Ophthalmic Systems, Franklin Lakes, NJ, #372032) is a blade that is curved in the direction of the width of the incision. This creates an arcuate incision paralleling the curvature of the peripheral cornea with a chord length whose width is considerably smaller than the incision itself, which may add a greater degree of stability. The first few times that this blade is used, its unusual configuration makes it somewhat more difficult to create an incision in the plane of the cornea and the incision can end up considerably shorter than anticipated (see Figures 13-49 and 13-50). However, as one learns how to use this blade, the desired architecture is much easier to achieve (Figure 13-51).



Figure 13-49 OCT image of a clear corneal incision made with the BD Kojo Slit Blade during the learning curve. Image of the blade is inset.



Figure 13-50 OCT image of a clear corneal incision made with the BD Kojo Slit Blade during the learning curve. Image of the blade is inset.



Figure 13-51 OCT image of a clear corneal incision made with the BD Kojo Slit Blade after using the blade for 1 month. Image of the blade is inset.

One of the surprising findings was that proper incision construction resulted in a longer incision than the chord length that was measured and in greater stability (like tongue in groove paneling) of the incision. Another surprising finding was that stromal swelling does, indeed, last for at least 24 h. These findings demonstrate those characteristics that have contributed to an added measure of safety in clear corneal incisions that can result in the absence of endophthalmitis.

Controversies surrounding clear corneal incisions

One of the most controversial criticisms of clear corneal incisions has been their relative strength compared to limbal or scleral incisions. Ernest et al.^[69,][70] demonstrated that rectangular clear corneal incisions in cadaver eye models were less resistant to external deformation utilizing pinpoint pressure than were square limbal or scleral tunnel incisions. Subsequently, Mackool and Russell^[71] demonstrated that once the incision width was ≤3.5mm and the length ≥2mm, there was an equal resistance to external deformation in clear cornea incisions as compared to scleral tunnel incisions. In Ernest's work as well, as incision sizes became increasingly small, the force required to cause failure of these incisions became very similar for limbal and clear corneal incisions, and thus this could be used to further document the safety of incisions sized 3mm or less.

A major criticism of these cadaver studies is that there is a lack of functioning endothelium contributing to wound sealing. Others have also indicated that cadaver eye incision strength cannot be compared to incisions in vivo.^[28] Ernest and Neuhann^[72] have compared in vivo posterior limbal incisions with clear corneal incisions and found that deep-grooved incisions performed better than shallow-grooved or single-plane incisions, in addition to finding that posterior limbal incisions performed better than clear corneal incisions when challenged by pinpoint pressure.

Many surgeons have called into question the validity of pinpoint pressure as a clinically relevant test for cataract wound strength because the probability that anyone would challenge their own incision by pressing on it with something as fine as the instruments utilized to apply pinpoint pressure in these studies is highly unlikely. Regardless of whether more posteriorly placed incisions demonstrate increased strength compared to clear corneal incisions, the real question is whether that added strength is clinically significant or relevant. Fine^[73] and others have demonstrated the stability of clear corneal incisions when a knuckle or a finger tip, the most likely way patients would challenge these incisions, was used. In addition, it is a well-known fact that a 1mm “hypersquare" paracentesis will leak the day after surgery if pinpoint pressure is applied to its posterior lip; however, the likelihood of any paracentesis incision leaking spontaneously or with blunt pressure the day following surgery is extremely low.

Another point of controversy is in regard to the studies in cat eyes performed by Ernest et al.^[74] These studies revealed a fibrovascular response in incisions placed in the limbus with extensive wound healing in 7 days compared to a lack of fibrovascular healing in clear corneal incisions. This study has been used to propose an increased safety for limbal incisions as compared to clear corneal incisions. Unfortunately, the real issue for these various incisions is not healing but sealing. We believe that as long as an incision is sealed at the conclusion of surgery, and it remains sealed thereafter, the time before complete healing of the incision is accomplished is almost irrelevant, especially since there is still a 7-day period in which limbal incisions are not truly “healed." An analogy can be drawn to the sealing which takes place during laser-assisted in-situ keratomileusis (LASIK) in which there is no fibrovascular healing of the clear corneal interface, which has little effect on the strength, effectiveness or safety of the wound and, in fact, is an advantage by limiting scarring and an inflammatory healing response.

One of the clear disadvantages of limbal corneal incisions is the greater likelihood of ballooning of conjunctiva, which can make visualization of anterior chamber structures during the surgical procedure more difficult. In addition, studies by Park et al.^[29] demonstrated that violation of the conjunctiva threatens the integrity not only of pre-existing filtering blebs but of the conjunctiva which would participate in filtering surgery at some future date. Finally, the presence of subconjunctival hemorrhage, although not important with respect to the ultimate function of the eye, may be of importance from a cosmetic perspective to the patient as well as to the survival of filtering blebs.

Contraindications for clear corneal incisions include the presence of radial keratotomy incisions that extend to the limbus that might be challenged by clear corneal incisions,^[75]marginal degenerations associated with thinning of the peripheral cornea and, perhaps, advanced corneal endothelial dystrophy.

Endophthalmitis: is there an increased risk?

Endophthalmitis prophylaxis involves a large number of factors including:

	•	a proper preoperative antibiotic regime
	•	preparation of the surgical field, including Betadine and draping
	•	incision construction
	•	surgical technique, including atraumatic surgery
	•	power modulations to avoid heating the incision
	•	avoiding grasp of the roof of the incision with a toothed forceps, which would abrade the epithelium and disrupt the fluid barrier for endothelial pumping
	•	incision closure
	•	testing for leakage
	•	postoperative antibiotics.

The authors have practiced for longer than 10 years, on over 9000 cases without a single case of infectious endophthalmitis.

The role of unsutured clear corneal incisions for cataract surgery and the apparent increased incidence of postoperative endophthalmitis in many reports are under rather intense scrutiny.^[76–86] The role of changing antibiotic sensitivity has been an issue (Figure 13-52). In Sweden, there has been a decreased incidence of endophthalmitis associated with an increased use of clear corneal incisions.^[87,][88] The recent ESCRS study of endophthalmitis showed an 80% reduction with the use of intracameral cefuroxime.^[89] There are other reports of no increased incidence of endophthalmitis with the use of clear corneal incisions.^{[76,][90–92]}



Figure 13-52 Graph illustrating the role of changing antibiotic sensitivity to infectious agents in the incidence of postoperative endophthalmitis.

Attention to all of the details for endophthalmitis prophylaxis is of primary importance. Incision construction leading to proper architecture is of primary importance among all of the variables that are part of endophthalmitis prophylaxis, and certainly not all clear corneal incisions are the same. An incision in the plane of the cornea with a chord length of at least 2mm appears to give uniquely advantageous architecture for adequate self-sealability.

Conclusion

Clear corneal cataract incisions are becoming a more popular option for cataract extraction and IOL implantation throughout the world. Through the use of clear corneal incisions and topical and intracameral anesthesia, we have achieved surgery that is the least invasive of any time in the history of cataract surgery, with visual rehabilitation that is almost immediate. Just 25 years ago, inpatient intracapsular cataract surgery, often performed under general anesthesia, followed by aphakic spectacles, was the standard of care. It is striking to realize how far we have come in such a short time.

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Contents

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Cataract Surgery, 3rd Edition

PART IV – Phacoemulsification Chapter 13 – Incision Construction

PART IV – Phacoemulsification

Chapter 13 – Incision Construction