corneal incision
Clear Corneal Incisions in Cataract Surgery
This type of incision has revolutionized surgery, but associated risksmust be considered.
BY BHAIRAVI V. KHAROD, M.D., AND TERRY KIM, M.D.

Since its first introduction in 800 B.C., cataract extraction has evolved significantly. From the ancient art of couching to the modern use of phacoemulsification, many techniques have been employed to perfect cataract surgeries over the past few centuries.

A significant breakthrough in the surgical removal of cataracts occurred with the introduction of clear corneal incisions by I. Howard Fine, M.D., in 1992. The use of clear corneal wounds has transformed cataract surgery by reducing surgical time, offering faster postoperative recovery and lowering the rate of induced astigmatism in comparison to the once-dominant traditional scleral tunnel incision. Moreover, complications associated with scleral tunnel incisions, such as conjunctival manipulation and hyphemas, have been virtually eliminated with the utilization of clear corneal incisions.

There is some question as to whether clear corneal incisions are associated with higher rates of endophthalmitis, and there have been several reports, surveys and studies to investigate this association. This article will review the current data on the benefits and risks of the clear corneal incision in cataract surgery.

Clear Corneal Incisionsand Endophthalmitis

The advantages of clear corneal incisions have made them the preferred method of wound entry in cataract extraction. 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. This rate steadily increased from 1.5% in 1992 to 12.4% in 1995 and to 47% in 2000. Sutureless closure was the preferred technique for 92% of the responding physicians.1 The advantages of clear corneal incisions are widely recognized by ophthalmic surgeons; however, recent scrutiny has been placed on the potential increased risk of postoperative endophthalmitis with these wounds. The rate of postoperative endophthalmitis has been documented in the range of 0.189% to 0.38% in different studies.2 Many studies have evaluated the association and possible causes of endophthalmitis with clear corneal incisions.

A large prospective case series by Monica et al3 studied the safety of clear corneal incisions in cataract surgery. This study included 3,500 consecutive cataract surgeries performed with clear corneal incisions under topical anesthesia. All patients underwent a temporal 3-mm x 2-mm beveled clear corneal incision for phacoemulsification. No cases of endophthalmitis were noted in these patients.

A large study from the Bascom Palmer Eye Institute in Miami that reviewed cataract cases over a 7-year period showed no greater incidence of endophthalmitis with corneal incisions than with scleral incisions.4 Similarly, a recent peer-reviewed study published by Lundstrom et al5 reported that no conclusive evidence exists that demonstrates an association between clear corneal incisions and endophthalmitis.

Nevertheless, in a large, retrospective, comparative case-controlled study, Cooper et al6 evaluated the incidence of postoperative endophthalmitis in clear corneal incisions with or without sutures vs. scleral tunnel incisions. In this study, 38 patients with culture-positive acute post-cataract endophthalmitis were compared to randomly selected control patients who underwent uncomplicated cataract surgery. The incision type and use of suture during cataract surgery of endophthalmitis patients were compared with the controls. This study demonstrated that clear corneal incisions were associated with a threefold greater risk of endophthalmitis than scleral tunnel incisions. The placement of a suture did not affect the incidence of endophthalmitis.

Many other studies have evaluated the association of clear corneal incisions with endophthalmitis. In a recently published ASCRS White Paper, the authors summarize some of the leading articles published on post-cataract endophthalmitis.7 In this review article, the authors note that studies conducted by Taban and colleagues and West et al report a significant increase in the incidence of endophthalmitis since the introduction of clear corneal incisions. Furthermore, studies performed by Miller and colleagues, Nagaki et al and Colleaux and Hamilton show a marked increase in the rate of endophthalmitis when a clear corneal wound was employed for phacoemulsification compared to a scleral tunnel incision.

Most recently, a partially masked randomized placebo-controlled multinational clinical study was performed by the European Society of Cataract and Refractive Surgery board. This study reported that the rate of endophthalmitis in patients receiving standard prophylaxis with preoperative topical povidone iodine drops was 0.34% (23 cases in 6,862 patients). The rate was reduced significantly with the use of intracameral cefuroxime (5 cases in 6,836 patients). This study reports a much higher rate of endophthalmitis associated with clear corneal incisions than previous studies and raises a significant concern in the use of clear corneal wounds for cataract surgery.2

There are many studies that have evaluated the association of endophthalmitis with clear corneal incisions and, while many studies show an increased risk of endophthalmitis with clear corneal wounds, this association remains under debate.

Risk Factors for Endophthalmitisin Clear Corneal Incisions

If clear corneal incisions are indeed associated with a higher risk of endophthalmitis, what makes them a more susceptible route for microbial invasion? This question has been investigated in many studies that have evaluated potential risk factors for endophthalmitis in clear corneal wounds. For instance, Chawdhary et al8 studied cataract wound dynamics to determine the risk of intraocular contamination in early postoperative hypotony. In this prospective case series, the anterior chamber was decompressed in 30 patients with elevated postoperative IOP. Fluorescein dye was used to observe ocular fluid movements during and after the decompression procedure. This study showed that fluid ingress occurred immediately after the anterior chamber was decompressed and continued for a prolonged period. This suggested hypotony as one possible mechanism of bacterial introduction into the anterior chamber.

Taban et al9 conducted a similar study using India ink in cadaveric eyes. The authors varied the IOP via an infusion cannula to simulate physiologic fluctuations in IOP that may be caused secondary to blinking or eye squeezing. The investigators found that ink particles were both grossly and microscopically visible inside the clear corneal wounds. Herretes et al10 also demonstrated that sutureless corneal incisions allow ingress of extraocular fluid into the anterior chamber after phacoemulsification. The above studies demonstrated the fragility of a clear corneal incision in the early postoperative period and the possibility of contamination of the anterior chamber with extraocular fluid through these wounds.

Other risk factors that have been suggested as potential contributors for microbial entrance through clear corneal incisions include improper wound architecture and an improper placement of the clear corneal wound. An improperly constructed wound is defined by many leading surgeons as a wound that lacks an adequate length of corneal tunnel or a wound that is not square. Such a wound is thought to allow easier microbial access to the anterior chamber as it lacks the self-sealing properties of a properly constructed corneal incision. There has also been speculation by some surgeons that higher risk exists with temporal clear corneal incisions. Most surgeons who perform temporal clear corneal incisions make their wound in the inferior aspect of the temporal cornea, which could possibly allow direct contact with organisms and contaminants from the inferior conjunctival fornix.

Treatment of Endophthalmitis

Though no conclusive evidence exists to demonstrate a definite association between clear corneal incisions and higher risks of endophthalmitis, surgeons should pay close attention to making a properly constructed corneal wound with an adequate corneal tunnel. Elimination of the risk of endophthalmitis is essential in this age of bacterial resistance to existing therapies. Ciprofloxacin resistance had been one of the driving forces for the introduction of the fourth-generation fluoroquinolones in ophthalmology. However, recent studies have shown that resistance has increased significantly in the last 5 years, and the in vitro susceptibility to fourth-generation fluoroquinolones has declined.11

In a large retrospective study, Recchia et al analyzed the microbiologic spectrum and in vitro susceptibility of organisms isolated from the vitreous of patients with endophthalmitis following cataract surgery over a period of 11 years. In this study, records of 497 consecutive patients treated for clinically suspected post-cataract endophthalmitis from July 1989 through June 2000 were reviewed. The results of microbiologic culture and in vitro antibiotic susceptibility from 1989 to 1994 and 1995 to 2000 were compared. The authors report that there was a significant increase in the incidence of gram-positive bacteria (92% to 97% of bacterial isolates) in the vitreous samples of patients in the 1995 to 2000 group. Furthermore, all bacterialisolates in this group had increased resistance to ciprofloxacin. Coagulase-negative staphylococci showed increased resistance to both ciprofloxacin and cefazolin. However, resistance to bacitracin, trimethoprim-sulfamethoxazole and vancomycin remained unchanged. Vancomycin had an in vitro efficacy against more than 99% of gram-positive bacteria. Ceftazidime was shown to be effective against 100% of gram-negative bacteria tested.12

These studies show the changing spectrum of pathogens causing post-cataract endophthalmitis, while also indicating growing resistance to some antibiotics. The cataract surgeon should regard with caution the serious implications these have on the prevention and treatment of postoperative endophthalmitis.

Induced Astigmatism and Clear Corneal Incisions

The effect of clear corneal incisions on induced astigmatism has been of great interest to cataract surgeons. Bar-Sela et al13 compared astigmatism in congenital cataract surgery using clear corneal vs. scleral tunnel incisions. Sixty-seven eyes of children 2 months to 12 years of age undergoing cataract surgery between 1996 and 2001 were retrospectively studied. Mean astigmatism was measured at 1 week, 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.

A study performed by Bilinska et al14 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 sutured 6-mm scleral tunnel incisions at 12-o'clock. The second group received sutureless 3.2-mm scleral incisions at 12-o'clock. The third group underwent sutureless 3.2-mm superotemporal incision in clear corneas. Postoperative astigmatism was examined by keratometry after 1 day, 1 week, 1 month and 3 months post-surgery. The lowest mean postoperative corneal astigmatism was achieved in the group with the superotemporal clear corneal incision. This study demonstrated that clear corneal incisions induce less astigmatism than scleral tunnel incisions.

Simsek et al15 performed a similar study to evaluate 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 undergoing small incision surgery were evaluated. Preoperative and postoperative astigmatism (1 day, 1 week, 1 month and 3 months postoperatively) were measured for each eye. The induced astigmatism in this study was 1.44 D (+/- 0.31 D) in patients who received superior corneal incisions and0.62 D (+/- 0.28 D) in patients who had a temporalincision. 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.

Most recently, Giansanti et al16 assessed the astigmatic effect of 2.75-mm clear corneal incisions in patients with preoperative astigmatism. The 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.

Some studies have proposed performing clear corneal wounds to reduce preoperative astigmatism. In a study performed by Rao et al.,17 temporal clear corneal incisions were enlarged to determine the effect on pre-existing 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 pre-existing against-the-rule astigmatism. Another study performed by Xie et al18 demonstrated that a clear corneal incision placed on the steepest meridian significantly reduced postoperative astigmatism.

Minimizing Risks

Clear corneal incisions have revolutionized the field of cataract extraction in the past two decades. They have quickly become the preferred method of wound entry, replacing scleral tunnel incisions. Clear corneal incisions have gained favor among surgeons because they are easy, require less time, have minimal effect on astigmatism and are generally self-sealing. Their advantages are multifold, both for the surgeon performing the extraction and the patient undergoing the surgery. Many surgeons have raised awareness of the potential increased risks of endophthalmitis following clear corneal incisions. Though the association between clear corneal wounds and endophthalmitis remains under debate, the surgeon should try to eliminate any factors that may make these incisions more susceptible to microbial invasion. Appropriate preoperative chemoprophylaxis and draping should be performed. Furthermore, special attention should be paid to wound construction to allow self-sealing and offer a resistant path for microbial invasion. Factors such as postoperative hypotony and postoperative contamination should also be avoided.

Ultimately, the risks of clear corneal wounds should be assessed along with the benefits when one is making the decision for the incision type to be used during cataract extraction.

Bhairavi V. Kharod, M.D., is clinical associate, cornea and refractive surgery, at Duke University Eye Center in Durham, N.C. Terry Kim, M.D., is associate professor of
ophthalmology, cornea and refractive surgery, at Duke University Eye Center. Dr. Kim can be reached at (919) 681-3568 or via e-mail at terry.kim@duke.edu.

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