Clear Corneal Incision in Cataract Surgery

Abstract

Since the introduction of sutureless clear corneal cataract incisions, the procedure has gained increasing popularity worldwide because it offers several advantages over the traditional sutured scleral tunnels and limbal incisions. Some of these benefits include lack of conjunctival trauma, less discomfort and bleeding, absence of suture-induced astigmatism, and faster visual rehabilitation. However, an increasing incidence of postoperative endophthalmitis after clear corneal cataract surgery has been reported. Different authors have shown a significant increase up to 15-fold in the incidence of endophthalmitis following clear corneal incision compared to scleral tunnels. The aim of this report is to review the advantages and disadvantages of clear corneal incisions in cataract surgery, emphasizing on wound construction recommendations based on published literature.

INTRODUCTION

Several surgical approaches have been suggested to allow for a faster and easier phacoemulsification technique. The introduction of clear corneal incisions to enter the anterior chamber and remove the cataract using phacoemulsification revolutionized cataract surgery.¹ This is perhaps the most popular and widely accepted approach to perform this surgery. The use of clear corneal wounds has transformed cataract surgery by dramatically reducing surgical time, offering faster postoperative recovery, and lowering the induced astigmatism in comparison to scleral tunnel incisions.

ADVANTAGES OF CLEAR CORNEAL INCISIONS IN CATARACT SURGERY

In a survey of the members of the American Society of Cataract and Refractive Surgery (ASCRS) conducted in July 2003, 72% of respondents reported using clear corneal incisions for phacoemulsification.² This rate steadily increased from 1.5% in 1992 to 12.4% in 1995 and to 47% in 2000.³ Sutureless closure was the preferred technique for 92% of the responding physicians.² Reasons for this trend are based on several advantages compared to scleral tunnel incisions, such as:

Shorter procedure time and complications

Scleral incision location for phacoemulsification was initially introduced by Gerard and Hoffman.⁴ Traditionally the incision is placed superiorly; however, with surgeons paying increasing attention to preexisting astigmatism adjusting the location to meet the steep meridian is common. The incision size is usually 3-7 mm in chord length.⁵ Large scleral tunnel incisions can be sutured, whereas small incisions may be sutureless, but in all cases, a periotomy shall be performed.

Various problems may occur during scleral tunnel construction. An initial scleral tunnel that is too deep will create scleral disinsertion with exposure to the ciliary body. This can lead to problems with hemostasis, poor wound stability, early or posterior entry to the anterior chamber, and iris prolapse. In contrast, an incision that is too shallow may result in tear of the tunnel roof and problems with water tightness of the wound. The length of scleral tunnel is also important. A dissection that is too far into the cornea creates an anterior entry into the anterior chamber resulting in decreased maneuverability and corneal striae that interfere with visibility during subsequent steps. An incision that is too short would create problems with wound closure and iris prolapse. Filtration, hyphema, Descemet's membrane detachment, and induced astigmatism are other complications. All these steps and complications can be avoided by performing clear corneal incisions. The corneal entry avoids conjunctival trauma, cauterization, and tunnel creation with a reduction in surgical time, as fewer steps are required. It would also allow to utilize only topical anesthesia as a primary method.

Lower induced astigmatism compared to scleral tunnel

The effect of clear corneal incisions on induced astigmatism has been of great interest to cataract surgeons. Bar-Sela and Spiere⁶ compared astigmatism in congenital cataract surgery using clear corneal versus scleral tunnel incisions. Sixty-seven eyes of children 2 months-12 years of age undergoing cataract surgery between 1996 and 2001 were retrospectively studied. Mean astigmatism was measured at 1 and 3 weeks and 3 months postoperatively. In this study, scleral tunnel incisions were shown to have a higher induced astigmatism than clear corneal incisions at all time points. The amount of astigmatism reduced spontaneously with time in both types of incisions.

Bilinska et al.,⁷ evaluated the astigmatic effect of scleral tunnel incisions and clear corneal incisions in adults. Three groups of 30 patients each were evaluated. The first group underwent a sutured 6 mm scleral tunnel incision at 12 O’clock (group I). The second group received a sutureless 3.2 mm scleral incision at 12 O’clock (group II). The third group underwent a sutureless 3.2 mm superotemporal incision in clear cornea (group III). Postoperative astigmatism was examined by keratometry after 1 day, 1 week, and 1 and 3 months post-surgery. The surgical induced astigmatism was 0.71 diopter (D) in group III, 1.08 D in group I, and 0.95 D in group II. The difference between group III vs groups I and II was statistically significant.

Giansanti et al.,⁸ assessed the astigmatic effect of 2.75 mm clear corneal incisions in patients with preoperative astigmatism. Authors prospectively studied 146 eyes of 146 patients who received cataract surgery through three different surgeons. Two of the three surgeons used temporal approach and one used the superior approach. Corneal astigmatism was measured using computerized videokeratography at 1, 4, and 12 weeks. This study showed that clear corneal incisions at all incisions sites caused a small change in the preoperative corneal cylinder.

Minimal induced astigmatism when performing temporal clear corneal incision

Simsek et al.,⁹ studied the effect of superior and temporal clear corneal incisions on astigmatism after sutureless, small-incision phacoemulsification. In this prospective study, 40 eyes of 20 patients were evaluated. Preoperative and postoperative astigmatism (1 day, 1 week, and 1 and 3 months postoperatively) were measured for each eye. The induced astigmatism was 1.44 D (±0.31) in patients who received superior corneal incisions, and 0.62 D (±0.28) in patients who had a temporal incision. Upper eyelid pressure on the superior corneal incision was thought to be the mechanism for the induced against-the-rule astigmatism. Overall, clear corneal incisions were found to minimally affect astigmatism, especially when placed temporally.

Tailoring incision location to reduce preexisting astigmatism

Some studies have proposed performing clear corneal wounds to reduce preoperative astigmatism. In a study performed by Rao et al.,¹⁰ temporal clear corneal incisions were enlarged to determine the effect on preexisting against-the-rule astigmatism. In this prospective study of 21 eyes, enlargement of the wound to 4.5 or 5 mm was shown to reduce preexisting against-the-rule astigmatism. Another study performed by Xie et al.,¹¹ demonstrated that a clear corneal incision placed on the steepest meridian significantly reduced postoperative astigmatism. Manipulating incision location is unique to clear corneal incisions as scleral tunnels are classically placed superiorly.

Faster visual recovery

Cataract surgery was once considered a very complicated procedure where patients have to be admitted prior to surgery and follow long-term instructions in regard to postoperative head position and activity in order to reach a satisfactory visual outcome. Since the introduction of clear corneal incisions in phacoemulsification all those have changed. Cataract surgery became a day-case procedure where patients can resume their regular activities soon after.

Kershne¹² analyzed the data for 690 consecutive cataract procedures performed between March 1993 and March 1995. Each patient underwent cataract removal with topical anesthesia, clear corneal incision fashioned as an arcuate keratotomy to correct preexisting astigmatism, intercapsular phacoemulsification, and microinjection of a single-piece intraocular lens (IOL) into the capsular bag to correct spherical error. All patients were able to resume normal unrestricted activities within 24 h of surgery. In another study performed by von Jagow et al.,¹³ visual recovery was assessed in 45 consecutive patients with senile cataract and no concomitant eye disease. Those patients underwent clear corneal phacoemulsification with insertion of a foldable IOL in the capsular bag under topical anesthesia. The average preoperative uncorrected visual acuity (UCVA) was 0.23 ± 0.39 and best-corrected visual acuity (BCVA) was 0.38 ± 0.23 SD. Four hours postoperatively UCVA and BCVA improved significantly to 0.48 ± 0.24 and 0.68 ± 0.18, respectively. One day after surgery, the average UCVA was 0.65 ± 0.15 and the BCVA was 0.89 ± 0.07. Seventy-five percent (n = 34) of the patients improved to an UCVA of 0.5 or better. The visual acuity improved in all patients 1 day after surgery, with 60% (n = 27) of the patients achieving a BCVA of 0.9 or better. The postoperative corneal astigmatism had a significant role on visual recovery (P = 0.01).

In a prospective study by Watson and Sunderraj,¹⁴ a comparison of small-incision phacoemulsification through a 5 mm scleral incision vs standard extracapsular cataract surgery was made. Fifty eyes had extracapsular cataract extraction (ECCE) and 47 eyes had phacoemulsification. UCVA of 6/9 or better was achieved in 25% of eyes on the 1^st day following phacoemulsification, 36% at 1 week, and 57% at 12 weeks. Although these results were significantly better than those following ECCE, phacoemulsification performed through scleral tunnel showed slower visual recovery than that reported in clear corneal incisions.

It seems that wound structure and location are the major factors in shortening visual recovery time rather than the type of surgery itself (i.e., phacoemulsification or manual cataract surgery). Zheng et al.,¹⁵ compared astigmatism and visual recovery after large incision extracapsular cataract surgery and scleral tunnel incisions for phacoemulsification. Maximum visual acuity was reached after a mean of approximately 6 weeks after ECCE, 2 weeks after 6 mm superior scleral tunnel incisions, and between 1 day and 1 week after 3 mm superior or temporal scleral tunnel incisions. Gogate et al.,¹⁶ reported no statistical significance between phacoemulsification and the small-incision cataract surgery techniques in terms of safety and visual rehabilitation. This concludes that the smaller the incision the faster the visual rehabilitation. It goes without saying that phacoemulsification through clear corneal incision can be performed through much smaller incisions compared to scleral tunnels. In addition, the time from intraocular lens placement to the end of surgery may be shorter in phacoemulsification (vs. scleral tunnel incisions or manual cataract surgery); accordingly, the light irradiation from the microscope is focused on the macula for less time.¹⁷

Disadvantages of clear corneal incision in cataract surgery

Although the above discussion makes performing clear corneal incisions very appealing, they are not devoid of problems. Reported disadvantages included induction of irregular astigmatism,¹⁸ poor wound healing,¹⁸ increased loss of endothelial cells,¹⁹ and risk of wound dehiscence following trivial trauma.²⁰ The later applies for scleral tunnel incisions as well.²¹ Therefore, scleral tunnel cataract wound still has a role in cases with low endothelial cell counts and poor countries were small incision cataract surgery is primary utilized due to lack of the expensive phacoemulsification machines.

Sutureless clear corneal incision in cataract surgery: What is the best wound and blade design?

Does self-sealed clear corneal wound exist?

A well-sealed clear corneal incision is essential to decrease the risk of postoperative hypotony and wound leakage, thus eliminating the ingress of microorganisms from the lids and lashes. Incision construction and architecture are the keys to achieve a water-tight wound. The ultimate goal is the creation of a perfect, self-sealed corneal tunnel. Temporal corneal tunnels, 3 mm in width or less, that are square or nearly square shaped seem to be the most stable and refractively neutral. Ernest et al.,²² showed that square-shaped cataract incisions are more stable than rectangular-shaped wounds. In their study of cadaver eyes, square-shaped incisions (3.2 mm × 3.2 mm) were stable up to an external pressure of 525 pounds per square inch (psi) compared to rectangular incisions (3.2 mm × 2.0 mm), which leaked at 13 psi. Unfortunately, larger incision lengths often preclude a square construction due to encroachment on the visual axis. Masket and Belani²³ reported that square-shaped clear corneal wounds that are meticulously checked for sealing were stable postoperatively as demonstrated by the absence of hypotony and wound leakage.

Several studies demonstrated the presence of wound leakage or gaping in the early postoperative period. Behrens et al.,²⁴ have evaluated wound dynamics in the immediate postoperative period after phacoemulsification surgery using a small incision clear cornea approach. Eight patients underwent standard postoperative evaluation 24 h after uneventful phacoemulsification surgery performed through a temporal or nasal clear corneal incision. Examination of the wound with noncontact optical coherence tomography was also done. One patient showed partial spontaneous gaping in different areas of the incision, undetected at slit-lamp evaluation. Another patient showed localized gaping of the internal aspect of the corneal wound. Four other incisions showed some degree of localized Descemet's membrane detachment in the vicinity of the wound, also undetected by slit-lamp evaluation.

In another laboratory model, Sarayba et al.,²⁵ evaluated the self-sealing properties of standard clear corneal cataract incisions during two events: Application of mechanical external pressure, or controlled fluctuation of intraocular pressure. Eight fresh human donor globes were utilized. India ink was placed on the wound surface to assess inflow of fluids. Four of seven eyes demonstrated intraocular presence of ink, three of them after external manipulation, and another after varying the intraocular pressure (IOP). A similar study on four cadaveric human eyes was conducted by Mehran et al.,²⁶ where India ink was applied to the corneal surface while the intraocular pressure was varied, so as to simulate the intraocular pressure fluctuations secondary to blinking or eye squeezing. The optical densities from aqueous samples of globes were measured both before and after India ink application using a spectrophotometer. Three globes with sutureless clear corneal wounds revealed a significant increase in spectrophotometric readings, in contrast to the sutured wound, which did not show an increase in absorbance level relative to the baseline.

Herretes et al.,²⁷ evaluated inflow of extraocular fluid after phacoemulsification with use of sutureless corneal incisions in eight living human eyes. External pressure simulating patient manipulation was applied before and after wound hydrosealing with an irrigation cannula and inflow of blood-tinged tear fluid into the anterior chamber through the wound was monitored by using digital video. Inflow of extraocular fluid was observed in all eyes when the cannula was released, even after wound hydrosealing. Two patients showed spontaneous fluid inflow.

Sutureless clear corneal cataract wounds seem to be prone to allow ingress of fluid into the anterior chamber even in well-constructed wounds. However, studies have shown that the rates of postoperative wound leak and anterior chamber reaction in phacoemulsification surgery were the same for sutured and sutureless corneal incisions.²⁸

Phacoemulsification surgical technique is a factor in selecting wound size and structure as surgeon's preference could vary among coaxial small incision cataract surgery (SCIS) and bimanual microincision cataract surgery (MICS).

BLADES AND MATERIALS

Over the past several decades, a variety of materials and techniques have been implemented in the manufacturing of microsurgical blades (diamond, sapphire, black diamond, ceramic composites, stainless steel). The following are important properties that should be considered in these blades.

Edge sharpness

The sharper the blade the more control a surgeon has, there is reduced tissue trauma and the incisions are more reproducible. Two techniques are employed to produce these blades. Hard crystalline blades made from different types of diamond and aluminum oxide are traditionally manufactured using a lapping process that can be thermally and chemically assisted.²⁹,³⁰,³¹ The manufacturing process involves initially cleaving the material along rough outlines of the desired shape. Once the rough shape is produced, grinding and lapping are used to manufacture the cutting edge. The harder the material the sharper the blade and therefore diamond, being the hardest material known to man, produces the sharpest edge. Diamond blades can also be produced using lasers and acid to etch the edges which is a more economical process. The major drawback is that these laser and acid etched blades cannot be rehoned and would need to be replaced if damaged. Stainless steel blades are manufactured through several different manufacturing processes. Traditionally, stainless steel blades were made through a multistep grinding and honing process that utilized methods such as ultrasonic slurrying, mechanical abrasion, and lapping. This process can produce fairly high quality blades but suffers from large inconsistencies in blade quality and edge radius. More recently the manufacture of stainless steel blades has improved through the use of coining and electrochemical polishing. Coining is a cold forming process typically used to produce currency that involves stamping steel into a near-net shape. The process is also easy to adapt to mass production for efficiency. After coining, the blades are cleaned up and sharpened using an electrochemical polishing technique.³² Despite these techniques, there remains some variation in quality and consistency from blade to blade and from manufacture to another. Ceramics and ceramic composites are currently developing but difficult to be designed as microsurgical blades.

Nonetheless, sharpness of a blade may not be the most critical factor to achieve self-sealing capabilities in a clear cornea wound. To date, there are no reports showing the advantages of a sharper blade promoting watertightness on clear corneal incisions compared to less sharp blades. Further studies are required to demonstrate the advantages of having a clean vs dull cut in phacoemulsification. With the advent of femtosecond lasers for cataract surgery, another type of incision cut quality will be introduced, which also will require comparative studies to demonstrate benefits/drawbacks compared to conventional blades.

Penetration

In the design process, the hardness of a material can affect mass, which in turn can affect penetration. Diamond, being the hardest material, lends itself to producing thin blades while retaining rigidity. Diamond blades are between 150 and 200 microns thick. Because of the relative softness of the coined metal blades, they reach their minimum thickness at about 150 microns. Despite equality in minimum thickness, diamond has an advantage of its sharper edge. This requires smaller blade and therefore less blade entry in the anterior chamber and thereby less chance of damaging adjacent ocular structures and greater safety. Metal blades have to compensate by producing a more acute angle, which requires a longer blade.³³

Cost

The diamond keratome costs are higher and require an average of one repair a year that costs roughly half of its purchase price. A premium single use metal keratome costs 100 times less than a diamond keratome and can be used up to three times. If the blade is used more than three times, performance usually is reduced.³³ However, blade design or material chosen to be used in surgery should be determined by what works best for the surgeon and patient.

Endophthalmitis following clear corneal cataract surgery: Is clear corneal incision associated with higher rate of endophthalmitis compared to sutured scleral incision?

Endophthalmitis is an uncommon but serious intraocular infection that occurs most commonly as a complication of intraocular surgery and often causes severe visual impairment or even the loss of an eye.³⁴

In a systematic review of studies published from 1979 to 1991, a period that predates the use of self-sealing clear corneal incisions, Powe et al.,³⁵ reported a 0.13% incidence of acute postoperative endophthalmitis following cataract extraction. However, recent reports suggest that the post cataract endophthalmitis rate maybe substantially higher, suggesting a greater risk of endophthalmitis coincident with the adoption of clear corneal incisions by cataract surgeons.³⁶,³⁷,³⁸,³⁹,⁴⁰,⁴¹,⁴²

Colleaux and Hamilton³⁶ reported a 0.129 and 0.05% incidence of endophthalmitis following cataract extraction with sutureless clear corneal and scleral tunnel incisions, respectively. Similarly, three retrospective, comparative, case-controlled studies found a significantly higher endophthalmitis rate associated with clear corneal incisions compared with scleral tunnel incisions.³⁷,³⁸,⁴² Nagaki et al.,⁴⁰ reported a statistically increased risk with clear corneal incisions (0.29%) compared with sclerocorneal incisions (0.05%).

In a systematic review by Mehran et al.,⁴³ 215 studies that addressed endophthalmitis were analyzed. A total of 3,140,650 cataract extractions were pooled resulting in an overall rate of 0.128% of postcataract endophthalmitis. The incidence of acute endophthalmitis changed over time, with a significant increase since 2000 compared with previous decades (relative risk, 2.44 (95% confidence interval, 2.27-2.61)). The rate of endophthalmitis was 0.265% in the 2000-2003 period, 0.087% in the 1990s, 0.158% in the 1980s, and 0.327% during the 1970s. Furthermore, an upward trend in rates after 1992 was noted, compared with 1991 and prior.

Thomas et al.,⁴⁴ reported the incidence of postoperative endophthalmitis in clear corneal cataract surgeries performed with and without suture closure in 815 consecutive eyes that underwent cataract surgery by a single surgeon over a 5-year period. Five cases developed culture-positive postoperative endophthalmitis in the unsutured group and none in the sutured group (P = 0.022).

Theories to account for more frequent postcataract endophthalmitis with sutureless clear corneal incisions are centered on the stability of the surgical wound because its integrity is believed to be a critical factor. In addition, the lack of conjunctiva covering the clear corneal incisions and a possible increased technical difficulty in constructing a stable, self-sealing incision in the cornea compared with the sclera may contribute to an elevated risk of endophthalmitis with clear corneal incisions. Sutures are also under debate in regard to provide extra protection to bacterial-sized particles inflow during sudden changes in IOP. May et al.,⁴⁵ reported superiority of a well-constructed unsutured two-step clear corneal incision over sutured clear corneal incisions.

Some other factors inherent to the cataract surgery may also play a role in the increase of endophthalmitis rates in the past few years. These might include changes in outpatient versus inpatient surgery, a move from hospital to ambulatory setting, changes in anesthesia delivery (injected to topical), changes in IOL design/materials, and changes in antibiotic prophylaxis/sensitivity of organisms.

Different modalities for sealing clear corneal incisions

Sutures are the standard for sealing corneal incisions and wound repair because of the efficiency and strength of the closure. However, they may not be the ideal method for wound closure, especially in the cornea. Therefore, many corneal surgeons recognize that sutures can be a source of potential problems. Because of the different levels of tension that the sutures produce, stress is placed on certain areas of the cornea and may lead to significant topographic distortions and high levels of astigmatism.⁴⁶ Loose sutures may harbor bacteria and cause local inflammation and tissue necrosis as a prelude to infection and possibly endophthalmitis.⁴⁷ When exposed, sutures bring forth significant patient discomfort and foreign-body sensation. Suture removal is also required when nonbiodegradable materials are used, with additional stress on the patient.

Several methods have been suggested to ensure proper sealing of the incision at the end of surgery. Surgeons often use stromal hydration to enhance wound sealing. Vasavada et al.,⁴⁸ suggested sealing the main incision by stromal hydration of the incision's side walls to help oppose the roof and floor by gently irrigating BSS Plus into the stroma. Additional BSS Plus was injected at the internal ends of the lateral walls until it sealed internal entry. Vasavada and Dholakia⁴⁹ have also reported persistence of stromal hydration for 24 h.

Tissue adhesives have evolved and are continuously being improved to surpass the problems that are encountered with sutures. As early as in 1968, adhesives for ophthalmic use had already been reported.⁵⁰ Fibrin-based adhesives and cyanoacrylate glues, although both far from being ideal, have been extensively used in ophthalmic surgery.

A novel self-polymerizing ophthalmic tissue adhesive was evaluated in corneal incisions, with the advantage of eliminating secondary sources of activation and possibly providing a more biocompatible effect. Reyes et al.,⁵¹ compared a modified chondroitin sulfate aldehyde adhesive with standard sutures for sealing corneal incisions in enucleated rabbit eyes. Glued eyes (n = 8) showed no leakage after sustaining a maximum IOP of 104.7 mmHg. The mean leak pressures in the single-suture and three-suture subgroups were 26.4 ± 6.0 and 44.3 ± 8.2 mmHg, respectively.

Temperature-controlled photothermal welding, in which collagen denaturation is used to cause tissue adhesion, has also been used experimentally on aortic,⁵² gallbladder,⁵³,⁵⁴ and corneal tissue.⁵⁵,⁵⁶ Photosensitizers with laser irradiation have likewise shown success in corneal repair.⁵⁷ A laser-activated biological tissue solder was found as effective as sutures when used as a patch and superior to sutures for clear corneal incisions in this animal ex vivo model.⁵⁸ All these are modalities that have potential to become clinically used in the near future. Wound construction, however, continues to be one of the most important factors in a successful watertight clear corneal incision.¹

CONCLUSION

Despite disadvantages clear corneal incisions might bring, they still seem to be very appealing because of the important benefits of faster visual recovery, shorter surgical time, and less manipulation. We think that clear corneal incisions in phacoemulsification cataract surgery represent a natural evolution in this procedure. We believe that the focus should be placed on eliminating the disadvantages of wound leakage with better architecture to make them really watertight in all conditions. Femtosecond lasers might play a major role in the next few years, allowing the creation of new clear corneal architecture to achieve perfect postoperative wound closure.