Corneal tensile strength in fully healed radial keratotomy wounds.

PURPOSE: To investigate the susceptibility to rupture of a human donor cornea with fully healed radial keratotomy (RK) incisions using biomechanical measurement methods and finite element analysis. METHODS: A human cornea 8 years after RK was cut into four strips and subjected to tensile testing until rupture occurred. The fellow cornea was pressurized on an artificial anterior chamber beyond the initiation of wound gape. The tensile strengths of nine strips from unincised corneas were measured as controls. To investigate the effect of epithelial plugs on the RK strip test results, a finite element model of a strip containing an epithelial plug was analyzed. RESULTS: Rupture occurred along the wound site with wide variability in the measured tensile strengths. There was no significant difference in tensile strength between the RK corneal strips (16.6 +/- 4.43 N/mm2) and the control strips (19.1 +/- 3.50 N/mm2). Four of the eight incisions of the fellow RK cornea gaped, but the cornea did not rupture up to a pressure of 2740 mm Hg. An epithelial plug of 10% of the corneal thickness was found in one of the incisions. The finite element results demonstrated higher stresses at the wound site that increased with the size of the epithelial plug. CONCLUSIONS: The presence of an epithelial plug in a fully healed radial keratotomy incision will create a stress concentration at the incision site that may predispose the cornea to rupture. The variability in the strength measurements indicates that the increase in rupture susceptibility due to RK may be hard to predict and may depend on factors such as the size of the plug and the strength of the wound collagen.

Keratocyte loss and repopulation of anterior corneal stroma after de-epithelialization.

OBJECTIVE: To quantitate corneal keratocyte loss and repopulation of anterior stroma after de-epithelialization. METHODS: Fourteen white New Zealand rabbits, each weighing 2.3 to 3.2 kg, were divided into seven groups. Each rabbit underwent a bilateral 6-mm mechanical deepithelialization procedure. The rabbits were killed two at a time after 1, 3, 6, 8, 10, 14, and 28 days. The corneas were labeled for mitosis using 5-bromo-2-deoxyuridine and stained for keratocyte quantification. Three untreated rabbits were added as controls. RESULTS: Corneas that were not operated on showed a higher density of keratocyte nuclei within the anterior corneal stroma than in the posterior stroma. Following de-epithelialization, there was a decrease in the ratio of anterior-posterior keratocytes, with maximal decrease on the third postoperative day. Keratocyte repopulation was completed by day 14. Mitotic activity was seen on days 1, 3, and 6 in the anterior half of the de-epithelialized stroma. CONCLUSIONS: Anterior stromal keratocytes are lost after epithelial removal, but repopulation is complete within 2 weeks. These findings demonstrate a loss of keratocytes and their recovery after corneal epithelial removal, as well as an interaction between corneal epithelium and stroma in rabbits.

Keratocyte loss after corneal deepithelialization in primates and rabbits.

PURPOSE: To evaluate the response of stromal keratocytes to central corneal deepithelialization. METHODS: Rabbits and monkeys underwent unilateral mechanical deepithelialization with a blunt instrument and were killed at intervals ranging from 15 minutes to 24 hours after surgery. Two rabbits underwent unilateral deepithelialization under a fluid bath containing corneal preservation medium. Two rabbits were treated unilaterally with corneal preservation medium topically applied every 15 minutes for 16 hours after epithelial removal. Four rabbits underwent linear keratotomy immediately after deepithelialization of the cornea or on normal unoperated corneas and were killed 1 day (two animals) and 14 days (two animals) after surgery. RESULTS: Deepithelialization resulted in severe ultrastructural changes in keratocytes within 30 minutes after surgery. After 24 hours, the number of keratocytes in the anterior stroma underneath the deepithelialized area had decreased significantly in rabbits (P = .0001) and in monkeys (P = .0007) compared with controls. The wound healing was altered and delayed when the epithelium was not present after keratotomy. The use of storage media during and after deepithelialization minimized the early keratocyte changes and appeared to stimulate reepithelialization. CONCLUSIONS: Removal of corneal epithelium causes loss of superficial stromal keratocytes in rabbits and monkeys. Keratocyte death may results from osmotic changes that alter the corneal wound healing response.

Arachidonic acid metabolites after excimer laser corneal surgery.

OBJECTIVE: We investigated early mediators of inflammation following excimer laser ablation in a rabbit cornea model. The ability of topical anti-inflammatory agents to influence these responses was also examined. METHODS: Adult New Zealand white rabbits were subjected to photorefractive keratectomy with a 193-nm argon fluoride excimer laser. Prostaglandin E2 and leukotriene B4 levels were measured using an enzyme immunoassay, and leukocyte infiltration was determined histologically. RESULTS: Prostaglandin E2 production was rapid and sustained, but we were unable to detect the presence of leukotriene B4. Relative to control, postoperative topical diclofenac sodium treatment caused a significant decrease in prostaglandin E2 levels and a significant increase in corneal leukocytes at 10 hours. Fluorometholone treatment did not significantly alter prostaglandin E2 levels but markedly depressed leukocyte ingress. CONCLUSIONS: Diclofenac reduces prostaglandin E2 levels but not leukocyte infiltration in the cornea following photorefractive keratectomy and thus may be useful clinically to reduce postsurgical pain.

Prevention of keratocyte loss after corneal deepithelialization in rabbits.

PURPOSE: To determine if keratocyte loss, after removal of the corneal epithelium, can be prevented by a collagen shield used alone or in combination with a topically applied corneal preservation medium. METHODS: Twenty-four New Zealand white rabbits were divided into six groups of four rabbits each. The central 6 mm of corneal epithelium was removed from one eye of each animal by means of a blunt spatula. Postoperatively, these eyes were treated every 4 hours for 24 hours with the following: group 1, balanced salt solution (BSS) drops (Akorn Inc, Metairie, La); group 2, Optisol drops (Chiron IntraOptics, Irvine, Calif); group 3, a collagen shield soaked in sterile BSS plus BSS drops; group 4, the same collagen shield soaked in Optisol plus Optisol drops; group 5, a different collagen shield soaked in sterile BSS plus BSS drops; and group 6, the same collagen shield soaked in Optisol plus Optisol drops. The animals were killed at 24 hours after surgery, the corneas were fixed, and keratocytes within the anterior and posterior cornea, beneath the epithelial defect, were quantitated. Four untreated rabbits served as controls. RESULTS: Optisol drops alone, applied every 4 hours after deepithelialization, did not prevent keratocyte loss to a greater extent than did BSS drops alone (P = .96). Both collagen shields soaked in sterile BSS plus BSS drops every 4 hours were associated with less keratocyte loss than were BSS or Optisol drops alone. Both collagen shields soaked in Optisol plus Optisol drops every 4 hours were most successful at minimizing keratocyte loss (P = .0002 and P = .001). CONCLUSION: After corneal deepithelialization, use of a collagen shield in combination with topical application of a corneal storage medium may minimize keratocyte loss and may thus be beneficial after refractive surgery.

Autorefraction/autokeratometry and subjective refraction in untreated and photorefractive keratectomy-treated eyes.

OBJECTIVE: To examine the relative accuracy of autorefraction and autokeratometry (autorefraction/autokeratometry) and subjective refraction in untreated and photorefractive keratectomy (PRK)-treated eyes. DESIGN: Results of autorefraction/autokeratometry obtained by means of 2 contemporary devices (Nidek ARK 2000 and Canon Rk-3) were compared with those of subjective refraction in 48 untreated eyes and in 78 eyes treated with (PRK). RESULTS: A trend toward greater differences between subjective and objective methods in PRK-treated eyes was observed. Both autorefraction devices showed similarly higher differences from subjective readings in PRK-treated eyes with regard to sphere (P < .001) and cylinder (P = .02), but not axis (P =.4). For autokeratometry, the mean difference from manual readings was highly significant when all K readings were considered together, but not so for the individual differences in keratometric astigmatism (Canon RK-3 and Nidek ARK 2000 vs manual results: P =.03 and P =.93, respectively) and keratometric axis (Canon RK-3 and Nidek ARK 2000 vs manual results: P =.88 and P =.19, respectively). CONCLUSIONS: Refractive and keratometric readings obtained with the automated devices tested in this study varied more from those obtained with subjective methods when the eyes examined had been treated with PRK. Corneal topographical changes inherent in the wound-healing process may influence objective measurement with automated devices.

Keratitis as a complication of bilateral, simultaneous radial keratotomy.

During a one-month period, we examined four patients referred for evaluation of probable microbial keratitis after bilateral, simultaneous radial keratotomy. Each patient had midstromal infiltrates compatible with microbial keratitis that involved one or more of the radial incisions. In two patients the keratitis was bilateral. All patients had been treated empirically with antibiotic agents; superficial cultures with cotton-tipped applicators and corneal scraping by inserting a platinum spatula into the radial incisions were negative. Corneal biopsy of one patient disclosed gram-positive rods and culture of the biopsy specimen grew diphtheroids. The infiltrates gradually resolved over a period of several months with intensive antibiotic therapy. Sight-threatening infectious keratitis can occur after radial keratotomy, and we believe that simultaneous bilateral ocular surgery of any kind should be discouraged.

Decrease in normal human corneal sensitivity with topical diclofenac sodium.

We tested the effect of topical diclofenac sodium on corneal sensitivity in the human eye. Corneal sensitivity was measured in ten adult subjects with the Cochet-Bonnet esthesiometer before, and immediately after, applying one drop of diclofenac sodium 0.1% in one eye and one drop of diclofenac vehicle in the other eye. Application was repeated every five minutes for 20 minutes; then no more drops were applied, and corneal sensitivity was measured every 15 minutes until sensitivity measurements returned to baseline levels. Diclofenac sodium decreased corneal sensitivity significantly (P = .0001) in all ten subjects, compared with eyes treated with the vehicle. The effect of diclofenac sodium increased as additional drops were administered. After the drug instillation was stopped, corneal sensitivity returned to baseline measurements within less than an hour in all the subjects. Diclofenac sodium substantially lowers sensitivity in normal, unoperated-on human corneas; the vehicle has no measurable effect on sensitivity.

Clinical follow-up of phototherapeutic keratectomy for treatment of corneal opacities.

We performed phototherapeutic keratectomy with a 193-nm excimer laser on 18 sighted patients (18 eyes) to treat corneal opacities. The corneal opacities were caused by corneal dystrophies in five patients; corneal scars secondary to corneal ulcers in six patients; corneal scar secondary to trauma in four patients; and band keratopathy, atopy, or corneal calcification in three patients. Mean follow-up was eight months (range, two to 18 months). Corneal clarity improved in 14 of the 18 eyes (77.7%). Four patients, three with band keratopathy or calcification and one with postinfectious corneal scar, did not improve. Uncorrected visual acuity improved in 11 patients, did not improve in five patients (including the four patients in whom treatment failed), and decreased in another two patients, apparently because of an increase in irregular astigmatism. A hyperopic shift was observed in ten patients. None of the successfully treated eyes developed surface problems or recurrence of the disease during the follow-up. Phototherapeutic keratectomy thus appears to be a safe and effective alternative to penetrating keratoplasty in some patients with selected anterior stromal opacities.

Photoastigmatic refractive keratectomy for primary treatment and revision of myopic astigmatism.

PURPOSE: To evaluate the results in 43 eyes treated with a rotating mask for myopic astigmatism and followed for up to 1 year. SETTING: Lucerne Eye Clinic, Cantonal Hospital, Lucerne, Switzerland. METHODS: Thirty-two patients (42 eyes) were selected to have photoastigmatic refractive keratectomy (PARK). In 33% (14 eyes), this treatment was the second or third ablation. The Aesculap Meditec MEL 60 excimer laser was operated in toe scanning slit mode, and a rotating mask was used. To evaluate cylindrical shaping, vector analysis was performed. RESULTS: One year after PARK, mean uncorrected visual acuity in all patients (26 eyes) improved from 20/160 preoperatively to 20/40. Surgically induced astigmatism in 20 of 26 eyes (77%) was within +/- 1.00 diopter (D) of the targeted induced astigmatism. At 1 year, 81% of patients who had primary excimer laser treatment for myopic astigmatism equivalent to -10.00 D or less were within +/- 1.00 D of target refraction compared with 44% of re-treated eyes. CONCLUSION: The result of PARK in eyes with low to moderate degrees of myopic astigmatism was satisfactory. However, in eyes with extensive scarring and wound healing activity after the first ablation, re-treatment was less predictable.

Central and peripheral pressure measurements with the Goldmann tonometer and Tono-Pen after photorefractive keratectomy for myopia.

To compare the accuracy of Goldmann tonometry with that of the Tono-Pen(R) in measuring intraocular pressure (IOP) after photorefractive keratectomy (PRK). Eye Clinic, Cantonal Hospital, Lucerne, Switzerland.Thirty-five eyes (25 patients) had PRK for a mean myopia of -6.2 diopters +/- 2.6 (SD). Intraocular pressure (IOP) measurements were performed first with the Goldmann tonometer and then with the Tono-Pen in 2 corneal locations: centrally in the usual manner and temporally. For the temporal measurements, the patient was directed to gaze nasally as the tonometer was placed perpendicular to the temporal portion of the cornea and with the rim of the tonometer positioned just inside the limbus of the cornea. Subsequently, similar measurements were made with the Tono-Pen. Measurements were performed before and 1 and 3 months after PRK. Preoperative IOPs measured centrally and temporally were similar. After PRK, the central pressure readings with the Goldmann tonometer and the Tono-Pen were between 1.8 and 2.3 mm Hg lower than those measured temporally. The differences were statistically significant (P <.0001). Central corneal pressure readings obtained with either the Goldmann tonometer or the Tono-Pen after myopic PRK are inaccurate. Measurements over the temporal part of the cornea are likely more reliable.

Immunochemistry with 5-bromo-2-deoxyuridine for visualization of mitotic cells in the corneal epithelium.

The purpose of this study was to determine whether the DNA of mitotic cells in the corneal and conjunctival epithelium can be labeled with 5-bromo-2-deoxyuridine (BrdU) immunostaining. Both corneas of four New Zealand white rabbits were deepithelialized in the center and the regenerated epithelium was evaluated for mitosis at 1, 3, 6, and 10 days. Unwounded corneas of three rabbits were labeled for baseline measurements. We administered the marker intravenously to all seven rabbits 15 h before scheduled killing. Immediately after killing, all of the globes were enucleated and histologic sections were prepared. In unwounded corneas, labeled cells were quantitated and the fraction of mitotic cells in the center of the cornea, in the periphery of the cornea, and in the conjunctiva were compared. In deepithelialized corneas, increase in mitosis in the central epithelium was quantitated. All of the unwounded eyes showed mitosis in the basal layer of both the corneal and conjunctival epithelium. In the center of the cornea 4.1 +/- 2.9% of the epithelial cells were labeled, in the corneal periphery 4.3 +/- 1.7% of the cells were labeled, and in the conjunctiva 4.1 +/- 1.9% of the epithelial cells were labeled, with a p value ranging from 0.84 to 0.99. In wounded corneas, when compared with cell counts in unwounded eyes, 52.6% of the epithelial cells were in mitosis on day 3, 13.9% were in mitosis on day 6, and by day 10 baseline values of 4.2% were obtained. We conclude that BrdU immunostaining is a safe, efficient, and less costly alternative to autoradiography for visualization of dividing corneal and conjunctival epithelium.

Prostaglandin E2 production after lamellar keratectomy and photorefractive keratectomy.

PURPOSE: To compare the levels of prostaglandin E2 (PGE2) in corneal tissue after 193-nanometer excimer laser keratectomy and mechanical keratectomy with a microkeratome. METHODS: Four rabbits underwent 193-nanometer excimer laser phototherapeutic keratectomy on one eye, and lamellar keratectomy with the microkeratome on the fellow eye. The corneas were harvested at 10 hours after the treatment and quantitated for PGE2 levels using an enzyme-linked immune assay. Control levels of PGE2 in untreated corneas were obtained from a previous study. RESULTS: Unoperated control corneas had low levels of PGE2 (1.79 +/- 1.0 pg/mL). Both surgical techniques resulted in a significant (p < .01) increase in PGE2. Corneas ablated mechanically with the microkeratome had an average PGE2 level of 15.48 +/-5.36 pg/mL, which represented an 8.6-fold increase compared to control; there was an additional 330% mean increase in PGE2 concentration in the laser-ablated corneas (51.29 +/- 36.08 pg/mL) compared to the corneas treated with mechanical lamellar keratectomy (p = .014). CONCLUSIONS: Mechanical and photochemical superficial keratectomies induce production of an inflammatory mediator, PGE2. The 193-nanometer excimer laser irradiation causes a greater increase of PGE2 production in the corneal tissue than does keratectomy with the microkeratome; this observation may support a role for cyclo-oxygenase inhibitors in postoperative therapy.

Topical diclofenac treatment prior to excimer laser photorefractive keratectomy in rabbits.

PURPOSE: This study sought to determine whether pre- and posttreatment with topical diclofenac sodium 0.1% eye drops suppresses corneal inflammation after 193-nanometer excimer laser corneal ablation more effectively than does posttreatment alone. METHODS: Eight rabbits were divided into four groups. Animals in group I were treated with topical diclofenac every half hour for 2 hours prior to photorefractive keratectomy; treatment was continued every hour for 3 hours after the ablation. Group II animals, used as controls, were treated with the diclofenac vehicle according to the same schedule. The third group (III) received diclofenac topically only after the excimer laser ablation. The fourth group (IV) consisted of normal corneas from these same animals. At 3 hours after ablation, prostaglandin E2 (PGE2) levels were measured in the corneas and leukocytes were quantified. RESULTS: Treatment with topical diclofenac significantly reduced levels of PGE2 compared to treatment with the vehicle (p = .024). Presurgical treatment with topical diclofenac did not result in greater suppression of PGE2 than did posttreatment alone (5.72 +/- 0.91 pg/mL versus 5.79 +/- 1.29 pg/mL). Similarly, there was a significant inhibition of leukocyte invasion in the diclofenac treated corneas (I vs II: p = .019, III vs II: p = .024), but no statistically significant difference between pretreatment and posttreatment alone groups (I vs III: p = .72). CONCLUSIONS: Topical administration of diclofenac reduces the release of PGE2 and the migration of polymorphonuclear leukocytes in the rabbit cornea 3 hours after 193-nanometer excimer laser ablation. However, pretreatment of the cornea, starting 2 hours prior to laser surgery, does not seem to offer advantages over postablation treatment in this animal model.

Effect of diclofenac on corneal haze after photorefractive keratectomy in rabbits.

PURPOSE: To evaluate the ability of topical diclofenac to decrease corneal opacity after excimer laser photorefractive keratectomy (PRK). METHODS: Twenty New Zealand white rabbits underwent a unilateral 193-nm excimer laser myopic photorefractive keratectomy to correct 5 diopters. There were four groups of five rabbits each. The first group of rabbits received postoperative topical treatment with placebo (Voltaren Ophthalmic vehicle), the second group received topical 0.1% diclofenac, the third group received topical corticosteroid (0.1% fluorometholone), and the fourth group received diclofenac and fluorometholone. In the first month, the topical drugs were applied four times daily, and in the second month twice daily. Corneal haze was graded from 0 (totally clear) to 4 (completely opaque cornea). Slit-lamp pictures were obtained at weeks 2, 4, and 8, and keratometry readings was performed at weeks 4 and 8. At week 8, the rabbits were killed, and the eyes were submitted for histologic examination. RESULTS: At week 8, there was less corneal haze in the diclofenac-treated animals and in the fluorometholone-treated animals than in the control group, but only in the diclofenac-treated group was the difference statistically significant. Combination treatment with diclofenac and fluorometholone did not result in a further decrease in haze. CONCLUSIONS: These data suggest that diclofenac and fluorometholone may influence corneal wound healing in rabbits after excimer laser PRK and support a potential role for using topical nonsteroidal anti-inflammatory drugs in preventing the development of excessive corneal haze after excimer laser surgery.