Effect of varying the optical zone diameter on the results of hyperopic laser in situ keratomileusis

PURPOSE

To compare the predictability and safety of laser in situ keratomileusis (LASIK) for low to moderate spherical hyperopia using different ablation zone diameters.

DESIGN

Retrospective, nonrandomized, comparative trial.

PARTICIPANTS

Forty-nine eyes that underwent hyperopic LASIK.

INTERVENTION

Two surgeons (JMD, RKM) performed hyperopic LASIK using the VISX STAR S2 excimer laser (VISX, Inc., Sunnyvale, CA) and the Bausch & Lomb Hansatome microkeratome (Chiron Vision, Irvine, CA) using ablation zone diameters of 5 x 9 mm, 5.5 x 8.5 mm, or 6 x 9 mm (the first number represents the optical zone diameter and the second number represents the diameter of the outer border of the ablation zone).

MAIN OUTCOME MEASURES

Refractive and visual outcomes at 3 to 6 months after surgery were analyzed. Groups were compared for deviations from targeted spherical equivalent, uncorrected visual acuity, and loss of best spectacle-corrected visual acuity (BSCVA).

RESULTS

The mean intended hyperopic correction was +2.48 +/- 1.13 diopters (D; 0.63-5.50 D). There were 16 eyes in the 5 x 9-mm group, 15 eyes in the 5.5 x 8.5-mm group, and 18 eyes in the 6 x 9-mm group. On average, the 5 x 9-mm group achieved 97% of the programmed correction, the 5.5 x 8.5-mm group achieved 104%, and the 6 x 9-mm group achieved 112% of the programmed correction. The tendency toward overcorrection in the 6 x 9-mm group compared with the 5 x 9-mm group was statistically significant (P < 0.05). The incidence of one line loss of BSCVA was greatest in the 5 x 9-mm group (19%) and lowest in the 6 x 9-mm group (6%). These differences were not statistically significant. No eyes experienced a loss of two or more lines of BSCVA at last examination.

CONCLUSIONS

Hyperopic LASIK using the VISX STAR is safe and effective using different ablation zone diameters. There appears to be an increased tendency toward overcorrection with progressively larger optical zone diameters.

Symptoms of dry eye and recurrent erosion syndrome after refractive surgery

PURPOSE

To determine the incidence and severity of patient complaints typical of dry eye and recurrent erosion syndrome after excimer laser refractive surgery and to compare the incidence of these symptoms after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

SETTING

Outpatient university practice.

METHODS

A questionnaire was mailed to 1731 patients who had had primary myopic PRK or LASIK at least 6 months previously. Questions were designed to determine the incidence and character of ocular dryness and recurrent erosion symptoms and their impact on patient satisfaction and willingness to have surgery again. Responses from PRK and LASIK patients were compared.

RESULTS

Responses from 231 PRK patients and 550 LASIK patients revealed an incidence of dryness symptoms in 43% and 48%, respectively (P >.05). Soreness of the eye to touch was reported by 26.8% and 6.7%, respectively (P <.0001). Sharp pains occurred in 20.4% of PRK patients and 8.0% of LASIK patients (P =.0001). Complaints of the eyelid sticking to the eyeball occurred in 14.7% and 5.6%, respectively (P =.0001). All symptoms occurred predominantly on waking. Frequency of eyelid sticking (P <.0005) and sharp pain (P <.005) symptoms, as well as severity of sharp pain symptoms (P <.0001), were significantly greater in PRK patients than in LASIK patients. On a scale of 0 to 10 (10 high), median overall patient satisfaction with surgery was 9 in both groups. Soreness of the eyelid to touch occurred significantly more frequently among patients with symptoms of sharp pains on waking (P <.001) and the sensation of the eyelid sticking to the eyeball (P <.001). Patients with 1 or more symptoms were twice as likely as asymptomatic patients to have a satisfaction score of less than 8 (P <.001).

CONCLUSIONS

Ocular dryness symptoms occurred commonly after PRK and LASIK. Symptoms suggestive of mild recurrent erosions included sharp pains, the sensation of the eyelid sticking to the eyeball, and soreness of the eyelid to touch, a previously unrecognized symptom of this condition. These symptoms occurred commonly after excimer laser procedures but were significantly more common, more severe, and more prolonged after PRK. The presence of these symptoms had a significant effect on patient satisfaction.

Conductive keratoplasty for the correction of hyperopia

BACKGROUND/PURPOSE

Conductive keratoplasty (CK) is a surgical technique that delivers radio frequency (350 kHz) current directly into the corneal stroma through a Keratoplasty tip inserted into the peripheral cornea at 8 to 32 treatment points. A full circle of CK spots produces a cinching effect that increases the curvature of the central cornea, thereby decreasing hyperopia. We report here the 12-month results of a 2-year, prospective, multicenter US clinical trial conducted to evaluate the efficacy, safety, and stability of CK.

METHODS

A total of 233 patients (401 eyes) with preoperative hyperopia of +0.75 to +3.00 D and < or = 0.75 D of astigmatism (mean preoperative manifest refractive spherical equivalent = +1.76 D +/- 0.60) were enrolled into the study at 13 centers and underwent CK treatment.

RESULTS

Twelve-month postoperative data are available on 203 eyes for safety and stability and 171 eyes for safety, stability, and efficacy. A total of 91% had uncorrected visual acuity (UCVA) of 20/40 or better, and 51% had UCVA of 20/20 or better. Manifest refractive spherical equivalent was within +/- 0.50 D in 58%, within +/- 1.00 D in 91%, and within +/- 2.00 D in 99%. The mean change in residual refraction was 0.26 D +/- 0.49 between 3 and 6 months, 0.09 D +/- 0.37 between 6 and 9 months, and 0.13 D +/- 0.39 between 9 and 12 months.

CONCLUSIONS

One-year data show safety and efficacy of CK in the treatment of hyperopia. Changes in residual refractive error after CK appeared to be small, suggesting that a stable refraction could be achieved by 6 months.

Postoperative Inflammation, Microbial Complications, and Wound Healing Following Laser in situ Keratomileusis

Although the biology of corneal wound healing is only partly understood, healing after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) differs in many respects, and the mechanisms appear to be differently controlled. There is less of an inflammatory and healing response after LASIK, but a longer period of sensory denervation. The cellular, molecular, and neural regulatory phenomena associated with postoperative inflammation and wound healing are likely to be involved in the adverse effects after LASIK, such as flap melt, epithelial ingrowth, and regression. Interface opacities in the early postoperative period include diffuse lamellar keratitis (DLK), microbial keratitis, epithelial cells, and interface opacities. Diffuse lamellar keratitis (sands of the Sahara syndrome) describes an apparently noninfectious diffuse interface inflammation after lamellar corneal surgery probably caused by an allergic or a toxic inflammatory reaction. Noninfectious keratitis must be distinguished from microbial keratitis to avoid aggressive management and treatment with antimicrobial drugs. Microbial keratitis is a serious complication after LASIK, but a good visual outcome can be achieved following prompt and appropriate treatment.

Epithelial ingrowth after laser in situ keratomileusis

PURPOSE

To report the incidence and risk factors for clinically significant epithelial ingrowth after laser in situ keratomileusis as well as the recurrence rate and visual outcomes after its treatment.

METHODS

We defined clinically significant epithelial ingrowth as that which required surgical removal. From a cohort of 3, 786 eyes that underwent primary laser in situ keratomileusis from February 1996 to August 1998 and its derivative of 480 eyes that later underwent enhancement laser in situ keratomileusis by one surgeon (R.K.M.), we identified all eyes with clinically significant epithelial ingrowth.

RESULTS

The incidence of significant epithelial ingrowth after primary treatment was 35 (0.92%) of 3,786 eyes. The incidence after enhancement treatment was eight (1.7%) of 480 eyes (p = NS). Fourteen of 43 eyes had a postoperative epithelial defect associated with subsequent development of epithelial ingrowth. Six of 43 eyes had loose epithelium intraoperatively, suggesting epithelial basement membrane dystrophy. Epithelial ingrowth was treated by lifting the flap, scraping the bed and the posterior surface of the flap, and replacing the flap without the use of caustic agents. In 42 of 43 eyes, the epithelial ingrowth under the flap was continuous with the surface epithelium. Clinically significant ingrowth recurred in 10 of 43 eyes after the initial surgical removal.

CONCLUSIONS

Clinically significant epithelial ingrowth is an infrequent complication of laser in situ keratomileusis. We hypothesize that epithelial ingrowth is secondary to postoperative invasion under the flap by surface epithelial cells rather than intraoperative implantation of epithelial cells. Treatment should consist of complete mechanical removal of epithelium from the posterior surface of the corneal flap and keratectomy bed and ensuring tight apposition of the flap with the bed.

Quantitative topographic irregularity as a predictor of spectacle-corrected visual acuity after refractive surgery

PURPOSE

To evaluate a new topographic index called topographic irregularity as a quantitative predictor of corrected vision after refractive surgery.

METHODS

We defined topographic irregularity as the summed difference at all points between a topographic refractive corneal power map and its best-fit spherocylinder. We prospectively studied 107 eyes of 107 patients 3 months after a variety of refractive procedures. Topographic irregularity was calculated from topographic maps, and the correlation between topographic irregularity and spectacle-corrected visual acuity was determined using both high-contrast and low-contrast acuity charts. This correlation was compared with correlations for the surface regularity index and the surface asymmetry index. Next, we analyzed 54 of these topographic maps to create a regression scale relating surface regularity index, surface asymmetry index, and topographic irregularity to predict spectacle-corrected visual acuity. This scale was then used to predict spectacle-corrected visual acuity on the remaining 53 postoperative patients.

RESULTS

The correlation of topographic irregularity with spectacle-corrected visual acuity (R(2) =.36) was comparable to the correlation for the surface regularity index (R(2) =.36) and stronger than for the surface asymmetry index (R(2) =.11) when spectacle-corrected visual acuity was measured with high-contrast eye charts. Topographic irregularity correlated more strongly with spectacle-corrected visual acuity (R(2) =.42) than either the surface regularity index (R(2) =.28) or the surface asymmetry index (R(2) =.14) when spectacle-corrected visual acuity was measured with low-contrast eye charts. Using the regression scale, prediction of high-contrast and low-contrast spectacle-corrected visual acuity from topographic irregularity was superior to or comparable to predictions using the surface regularity index and the surface asymmetry index.

CONCLUSIONS

Topographic irregularity has a closer correlation with spectacle-corrected visual acuity than existing topographic indexes. Topographic irregularity is also an accurate predictor of spectacle-corrected visual acuity and may be a more sensitive tool for evaluating postoperative visual performance than current topographic measures.

Microkeratome complications of laser in situ keratomileusis

PURPOSE

To evaluate the incidence, types, and outcome of microkeratome complications that occur during laser in situ keratomileusis (LASIK).

DESIGN

Retrospective, noncomparative, case series.

PARTICIPANTS

Three thousand nine hundred ninety-eight eyes that underwent primary LASIK by four surgeons between November 1996 and August 1998 at a university-based refractive center.

METHODS

All cases with significant microkeratome complications leading to abandonment of the LASIK procedure were identified and reviewed.

MAIN OUTCOME MEASURES

Incidence of complications, change in best corrected visual acuity (BCVA), change in refractive error, and types of complication.

RESULTS

There were 27 complications leading to abandonment of the LASIK procedure of 3998 eyes. The overall rate of microkeratome complication was 1 in 150 (0.68%), but it was 1 in 77 (1.3%) in the surgeons' first 1000 eyes, decreasing to 1 in 250 (0.4%) in the last 1000 eyes. Of the 24 planned bilateral cases, 15 complications (63%) happened on the first operated eye. Twenty-six of 27 eyes (96%) recovered to within one line of preoperative BCVA, and one eye lost two lines. At last examination before any repeat refractive procedures, spherical equivalent manifest refraction returned to within 1 diopter (D) of its preoperative value in 18 of 19 eyes (95%), and astigmatism in 16 of 19 eyes (84%) returned to within 1 D of its preoperative value. Sixteen of 27 eyes (59%) had repeat LASIK. Two eyes had complications at repeat LASIK, one of which led to abandonment of the LASIK procedure for a second time.

CONCLUSIONS

There is a significant learning curve in the use of the microkeratome. If ablation is not performed, flap complications rarely lead to significant visual loss and generally do not result in a change in refractive error.

Deep lamellar keratoplasty using viscoelastic dissection

We describe a technique for performing deep lamellar keratoplasty using viscoelastic dissection. Deep lamellar dissections of the cornea using viscoelastic substances (sodium hyaluronate) were performed on 4 eyes of 4 patients. One patient with keratoconus and another with corneal scarring underwent lamellar keratoplasty using the technique as the sole procedure for visual rehabilitation. Two patients (2 eyes) with opaque corneas underwent deep lamellar dissection with removal of stromal tissue to allow visualization of the anterior segment structures prior to penetrating keratoplasty, thereby facilitating separation of iridocorneal adhesions as the Descemet membrane was incised. Deep lamellar dissection was performed without complications related to the procedure in all 4 eyes. The 2 lamellar grafts cleared completely, and both eyes achieved excellent visual acuity with spectacle correction. In the other 2 eyes, deep lamellar dissection provided clear visualization of anterior segment structures during incision of the Descemet membrane. Deep lamellar dissection using viscoelastic substances is a useful technique during lamellar keratoplasty.

Growth factor expression in corneal wound healing after excimer laser keratectomy

PURPOSE

To develop a rabbit model of reproducible corneal haze after excimer laser keratectomy and to characterize expression of transforming growth factor beta (TGFbeta) and basic fibroblast growth factor (bFGF) in rabbit corneas during haze formation.

METHODS

Seven rabbits underwent a 100 microm deep phototherapeutic keratectomy (PTK) in one eye and a 15-microm shallow PTK in the contralateral eye. Corneal haze was compared at 1-20 weeks after surgery. Subsequently, 16 rabbits underwent 100-microm PTK in one eye and 15-microm PTK in the contralateral eye. Four rabbits were killed at 1, 2, 3, and 4 weeks, respectively, after surgery. Immunohistochemistry was performed on the corneas to localize the expression of TGFbeta and bFGF. Control subjects were rabbits that underwent either epithelial debridement alone or no surgery.

RESULTS

A 100-microm PTK resulted in significantly more corneal haze than a 15-microm PTK at every postoperative examination (p < 0.05). Both TGFbeta and bFGF were expressed in the scars at 1-4 weeks after deep and shallow excimer ablations. bFGF was expressed in the keratocytes of both treated and control corneas. Minimal TGFbeta was detected in the keratocytes of the control corneas, whereas prominent TGFbeta expression was noted in the keratocyte-like cells adjacent to the postkeratectomy scars.

CONCLUSIONS

The 100-microm PTK ablation resulted in significantly more corneal scarring than the 15-microm PTK ablation. Even though there was no immunohistochemical difference in the pattern of TGFbeta and bFGF expression after deep and shallow ablations, there was an association between the expression of the growth factors and corneal scarring after excimer laser keratectomy.

Visual performance after photorefractive keratectomy with a 6-mm ablation zone

PURPOSE

To prospectively examine the effect of photorefractive keratectomy with a 6-mm ablation zone on best-spectacle-corrected visual performance.

METHODS

A prospective study was conducted of 164 eyes of 164 patients with an average (+/-SD) of -4.02 +/- 1.74 diopters (range, -0.63 to -8.38 diopters spherical equivalent). Best-spectacle-corrected high-contrast and low-contrast visual acuity (18% Weber contrast) was measured with both natural and dilated pupils. Patients were tested preoperatively and at 3, 6, and 12 months after photorefractive keratectomy. Photorefractive keratectomy was performed with an argon fluoride excimer laser. Fifty-five eyes of 55 patients also underwent astigmatic keratotomy.

RESULTS

Twelve months after photorefractive keratectomy, best-spectacle-corrected high-contrast visual acuity with natural pupils showed no significant change from preoperative values; mean (+/-SD) change was 0.004 +/- 0.10 logMAR (t = 0.45, P = .65). Best-spectacle-corrected low-contrast visual acuity with natural pupils was significantly reduced compared to baseline; mean (+/-SD) change was 0.04 +/- 0.13 logMAR (t = 3.3, P = .001). The low-contrast loss was larger (1.5 lines) with dilated pupils; mean (+/-SD) change was 0.13 +/- 0.15 logMAR (t = 9.31, P < .001). Greater losses in dilated low-contrast visual acuity were associated with concurrent astigmatic ketatotomy (t = 2.28, P = .025) and corneal haze of grade 1 or greater (t = 2.71, P = .005).

CONCLUSIONS

Reductions in visual performance occur after photorefractive keratectomy with a 6-mm zone. These changes are greatest for low-contrast visual acuity with dilated pupils. Corneal haze and concurrent astigmatic keratotomy are associated with greater losses in low-contrast visual acuity. Best-spectacle-corrected low-contrast visual acuity is a sensitive measure for evaluating visual performance after refractive surgery.

Effect of astigmatic keratotomy on spherical equivalent: results of the Astigmatism Reduction Clinical Trial

PURPOSE

To determine the effect of astigmatic keratotomy on spherical equivalent, as measured by the coupling ratio and a new quantity, coupling constant.

METHODS

In a prospective multicenter study, subjects underwent arcuate keratotomy at a 7-mm optical zone by means of the Lindstrom nomogram for correction of astigmatism. One hundred fifty-seven eyes of 95 patients who had a follow-up examination 1 month postoperatively were studied. Mean preoperative refractive cylinder +/- SEM was 2.82 +/- 1.17 diopters. Coupling ratio was defined as the ratio of the flattening of the incised meridian to the steepening of the opposite meridian. Coupling constant was defined as the ratio of the change in spherical equivalent to the magnitude of the vector change in astigmatism. Coupling ratio, coupling constant, and change in spherical equivalent were calculated on the basis of change in refraction and keratometry.

RESULTS

On the basis of change in refraction, coupling ratio was 0.95 +/- 0.10 (mean +/- SEM) and coupling constant was -0.01 +/- 0.03, consistent with a minor shift in the spherical equivalent of -0.03 +/- 0.07 diopter. On the basis of change in keratometry, coupling ratio was 0.84 +/- 0.05 and coupling constant was -0.04 +/- 0.02, consistent with minor postoperative keratometric steepening of -0.10 +/- 0.04 diopter. Coupling ratio based on change in refraction was not statistically different from the coupling ratio predicted by the Gauss' law for inelastic domes (P = .370). Incision length and number, amount of achieved cylinder correction, age, and sex had no statistically significant effect on coupling ratio, coupling constant, and change in spherical equivalent.

CONCLUSIONS

Cornea behaved as an inelastic surface in response to arcuate keratotomy performed with the Astigmatism Reduction Clinical Trial study nomogram. On average, astigmatic keratotomy had a minimal effect on spherical equivalent refraction. There was variability, however, in coupling ratio, coupling constant, and change in spherical equivalent from eye to eye after astigmatic keratotomy. Caution is therefore advised when simultaneous correction of cylinder and spherical equivalent is planned.

Flap complications associated with lamellar refractive surgery

PURPOSE

Corneal lamellar refractive surgery for myopia reduces the risk of corneal haze but adds to the risk of flap complications. We retrospectively determined the incidence of flap complications in the initial series of eyes undergoing lamellar refractive surgery by one surgeon. We assessed the incidence of flap complications overall, the trend in these complications during the surgeon's learning curve, and the impact of the complications on best spectacle-corrected visual acuity.

METHODS

Charts of the first 1,019 eyes that underwent myopic keratomileusis in situ or laser in situ keratomileusis were reviewed for flap complications and visual outcome.

RESULTS

Of the 1,019 eyes, 490 eyes underwent myopic keratomileusis in situ, and 529 eyes underwent laser in situ keratomileusis. Eighty-eight (8.6%) of 1,019 eyes had flap-related complications. Six eyes had two complications. Intraoperative complications included irregular keratectomy in nine eyes (0.9%), incomplete keratectomy in three eyes (0.3%), and a free cap in 10 eyes (1.0%). The incidence of intraoperative complications was six (6.0%) in the first 100 consecutive eyes, 14 (2.3%) in the next 600 consecutive eyes (P = .04, chi-square test), and one (0.3%) in the last 300 eyes (P = .03, chi-square test). Postoperative complications included displaced flaps that required repositioning in 20 eyes (2.0%), folds in the flap that required repositioning in 11 eyes (1.1%), diffuse lamellar keratitis in 18 eyes (1.8%), infectious keratitis in one eye (0.1%), and epithelial ingrowth that required removal in 22 eyes (2.2%). The incidence of flap displacement and folds in 200 eyes in which we irrigated under the flap and allowed it to settle without further manipulation averaged 8.5%, whereas the incidence in other groups of 100 consecutive eyes averaged 0.8% (P < .00001, chi-square test). The incidence of diffuse lamellar keratitis was 0.2% in eyes that had undergone myopic keratomileusis in situ and 3.2% in eyes treated by laser in situ keratomileusis (P = .0003, chi-square test). No eye lost 2 or more lines of best spectacle-corrected visual acuity because of flap complications.

CONCLUSION

Flap complications after lamellar refractive surgery are relatively common but rarely lead to a permanent decrease in visual acuity. Physician experience with the microkeratome and with the handling of the corneal flap decreases the incidence of flap complications.

Induced astigmatism after photorefractive keratectomy

PURPOSE

To retrospectively analyze a group of patients to determine whether their induced astigmatism was caused by asymmetry in the laser beam, asymmetry in ablation rates, or wound healing in different corneal meridians.

SETTING

Single-center physician office.

METHODS

In this study, 146 eyes of 116 patients who had photorefractive keratectomy (PRK) for myopia with the Apex laser (Summit Technology) were retrospectively identified. In 28 eyes, the patient's chair had been rotated 90 degrees from its usual position under the laser. The vector-summated mean change in astigmatism in eyes with the chair rotated 90 degrees was compared with that in a group of control eyes in which the chair was in the usual position.

RESULTS

The vector-summated mean change in the control eyes was 0.30 diopter (D) at 83 degrees. Forty-eight of 113 eyes (42.5%) had induced with-the-rule (WTR) astigmatism, and 14 of 113 eyes (12.4%) had induced against-the-rule (ATR) astigmatism. In the eyes in which the chair was rotated 90 degrees, vector-summated mean change was 0.10 D at 13 degrees (P < .0005). One of 27 eyes (3.7%) had induced WTR astigmatism, and 13 of 27 eyes (48.1%) had induced ATR astigmatism (P < .001, chi-square).

CONCLUSION

Astigmatism induced by myopic PRK with the Apex laser was small. The axis of induced astigmatism rotated 90 degrees when the patient's chair was rotated, implying that it is inhomogeneities in the beam rather than meridional asymmetry in ablation rates or wound healing that are responsible for induced astigmatism.

Diffuse lamellar keratitis. A new syndrome in lamellar refractive surgery

OBJECTIVE

This study aimed to describe a syndrome that the authors call diffuse lamellar keratitis that follows laser in situ keratomileusis (LASIK) and related lamellar corneal surgery.

DESIGN

Noncomparative case series and record review.

PARTICIPANTS

Thirteen eyes of 12 patients in whom infiltrates developed in the interface after lamellar refractive surgery were studied.

INTERVENTION

Topical antibiotics or corticosteroids or both were administered.

MAIN OUTCOME MEASURES

Corneal infiltrate appearance, focality, location, and clinical course were measured.

RESULTS

Patients presented between 2 and 6 days after surgery with pain, photophobia, redness, or tearing. Ten cases directly followed either myopic keratomileusis or LASIK. Three cases followed enhancement surgery without the use of a microkeratome. All 13 cases had infiltrates that were diffuse, multifocal, and confined to the flap interface with no posterior or anterior extension. The overlying epithelium was intact in each case. Cultures were negative in the two cases cultured. Ten eyes were treated with antibacterial agents; two eyes had fluorometholone four times daily added to the routine postoperative antibacterial regimen, and one eye had the antibacterial agent discontinued and was treated with topical fluorometholone alone. All infiltrates resolved without sequelae.

CONCLUSIONS

A distinct syndrome of unknown cause of noninfectious diffuse infiltrates in the lamellar interface is described. It can be distinguished from infectious infiltrates by clinical presentation and close follow-up. Patients with the syndrome should be spared the more invasive treatment of infectious keratitis.

Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. A randomized prospective study

OBJECTIVE

This report presents the results of a randomized clinical trial of photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK).

DESIGN

A randomized, prospective multicenter clinical trial.

PARTICIPANTS

A total of 220 eyes of 220 patients entered the study cohort: 105 randomized to PRK and 115 to LASIK. The mean preoperative manifest refraction spherical equivalent was -9.23 diopters (D) in the PRK group and -9.30 D in the LASIK group.

INTERVENTION

All patients received a one-pass, multizone excimer laser ablation as part of either a PRK or LASIK procedure using the Summit Apex excimer laser. Attempted corrections ranged from 6.00 to 15.00 D.

MAIN OUTCOME MEASURES

Data on uncorrected and spectacle-corrected visual acuity, predictability,and stability of refraction, corneal haze, and flap complications were analyzed. Patients were observed for up to 6 months.

RESULTS

One day after surgery, 0 (0.0%) and 3 (4.5%) eyes in the PRK group saw 20/20 and 20/40 or better uncorrected, respectively, while 7 (10%) and 48 (68.6%) eyes in the LASIK group saw 20/20 and 20/40 or better, respectively. At 6 months after PRK, 13 (19.1%) and 45 (66.2%) eyes saw 20/20 and 20/40 or better, respectively, while after LASIK, 16 (26.2%) and 34 (55.7%) eyes saw 20/20 and 20/40 or better, respectively (odds ratio = 0.56 for likelihood of uncorrected visual acuity < 20/40 for PRK vs. LASIK, 95% confidence interval [CI] = 0.31-1.19). After PRK, 39 eyes (57.4%) were within 1.0 D of attempted correction compared with 24 eyes (40.7%) in the LASIK group (odds ratio = 0.50 for likelihood fo undercorrection 1.0 D for PRK vs. LASIK, 95% CI = 0.24-1.04); however, the standard deviation of the predictability was similar between groups: 1.01 D for PRK and 1.22 D for LASIK. From months 1 to 6, there was an average regression of 0.89 D in the PRK group and 0.55 D in the LASIK group. After PRK, eight eyes (11.8%) had a decrease in spectacle-corrected visual acuity of two Snellen lines or more; after LASIK, two eyes (3.2%) had a decrease of two lines or more (odds ratio = 3.89 for risk of loss of spectacle-corrected visual acuity for PRK vs. LASIK, 95% CI = 0.71-21.30). Only two eyes had postoperative spectacle-corrected visual acuity less than 20/32, however.

CONCLUSIONS

Although improvement in uncorrected visual acuity is more rapid in LASIK than in PRK, efficacy outcomes in the longer term generally are similar between the two procedures. There is a greater tendency toward undercorrection in LASIK eyes using the specific laser and nomogram in this study, but the scatter in achieved versus attempted correction is similar, suggesting little difference in the accuracy of the two procedures. A suggestion of decreased propensity for loss of spectacle-corrected visual acuity in LASIK eyes requires further investigation.

Treatment of topographic central islands following refractive surgery

PURPOSE

To evaluate the safety and efficacy of using central reablation to treat topographic central islands following photorefractive keratectomy (PRK), myopic keratomileusis in situ, and laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Stanford University School of Medicine, Stanford, and Jules Stein Eye Institute, Los Angeles, California, USA.

METHODS

Central reablation was performed on eight eyes with clinically significant topographic central islands after refractive surgery. Two eyes developed central islands after PRK, five eyes after LASIK, and one eye after myopic keratomileusis in situ. A clinically significant topographic central island was defined as an area of steepening of at least 3.0 diopters by at least 1.5 mm in diameter documented by computerized videokeratography. Reablation was tailored to each eye based on the diameter and power of the topographic central island using the Munnerlyn formula.

RESULTS

All eyes experienced a reduction or elimination of the topographic central islands following central reablation. Six eyes experienced an improvement in uncorrected visual acuity, and all eyes returned to within one line of their preoperative level of best spectacle-corrected visual acuity 1 month after the procedure.

CONCLUSION

Topographic central islands following PRK, myopic keratomileusis in situ, and LASIK can be effectively treated using the excimer laser. Poor predictability of the refractive effect of central reablation may be the limitation of this treatment modality.

The prediction of surgically induced refractive change from corneal topography

PURPOSE

To develop a method to predict the refractive power of the cornea from corneal topography.

METHODS

We reviewed preoperative and postoperative cycloplegic refraction, keratometry, and corneal topography in 40 eyes of 40 patients who had undergone photorefractive keratectomy, radial keratotomy, myopic keratomileusis in situ, or hyperopic lamellar keratoplasty. For each axial dioptric power map, we calculated the aspheric ellipsoid that best fit that map. Central corneal points were weighted more heavily than peripheral points, based on the Stiles-Crawford effect. The equation of the best-fit ellipsoid yielded the spherical and astigmatic power and axis for each cornea preoperatively and postoperatively.

RESULTS

The preoperative corneal spherical and astigmatic powers measured by the best-fit method were consistent with the spherical and astigmatic powers measured by keratometry and simulated keratometry. The change in corneal spherical power predicted by the best-fit method was significantly (P < .05) more accurate at predicting the change in spherical equivalent refraction than change either in spherical equivalent keratometry or in spherical equivalent simulated keratometry. The prediction of the astigmatic change was less precise than that of the spherical, but the best-fit method was the most accurate.

CONCLUSIONS

The best-fit method is more accurate than simulated keratometry and standard keratometry are in evaluating corneal refractive power after refractive surgery. An improved method of calculating corneal refractive power may facilitate subjective refraction after refractive surgery, improve the accuracy of intraocular lens power calculation for eyes that have had previous refractive surgery, and improve ablation profiles for excimer laser refractive surgery.

Factors influencing the outcome of hyperopic lamellar keratoplasty

PURPOSE

To determine which factors affect the refractive outcome of hyperopic lamellar keratoplasty.

SETTING

Jules Stein Eye Institute and Department of Ophthalmology, UCLA, Los Angeles, California, USA.

METHODS

This retrospective study comprised 38 consecutive eyes of 25 patients with naturally occurring hyperopia who had automated lamellar keratoplasty by one surgeon. Mean attempted correction was 3.80 diopters (D) (range of 1.50 to 6.00 D); 34 eyes were followed for 3 months. The effect of applanation lens diameter, keratometry, age, corneal thickness, absolute flap thickness in microns, thickness of the posterior lamellae in microns, and flap thickness as a percentage of corneal thickness were determined using multivariate linear regression.

RESULTS

With current nomograms, mean undercorrection 3 months after hyperopic lamellar keratoplasty was 1.26 D +/- 0.91 (SD); 14 of 34 eyes were within +/- 1.00 D of the attempted correction. The best predictive factors of achieved correction were applanation lens diameter and absolute flap thickness in microns, which accounted for 54% of the variability in outcome. Keratometry, corneal thickness, and flap thickness as a percentage of corneal thickness had no significant additional predictive value.

CONCLUSION

The refractive outcome of hyperopic lamellar keratoplasty was primarily determined by applanation lens diameter and absolute flap thickness in microns. Current nomograms suggest that flap thickness as a percentage of corneal thickness is a major determinant of effect, but this factor appears unimportant to the refractive effect of hyperopic lamellar keratoplasty. We hypothesize that it is slight swelling of the anterior corneal stroma rather than the bulging of the posterior cornea that causes the hyperopic correction in hyperopic lamellar keratoplasty.

Lack of virus transmission by the excimer laser plume

PURPOSE

To test the possibility of pathogenic virus transmission into the operating suite during excimer laser treatment of corneal tissue. Such treatment vaporizes corneal tissue, which may put the surgeon at risk of infection from human immunodeficiency virus, hepatitis virus, or other viruses. We developed a model system to test the possibility of such virus transmission.

METHODS

Pseudorabies virus is a porcine enveloped herpesvirus similar in structure and life cycle to human immunodeficiency virus and herpes simplex virus. An excimer laser was used to ablate a virus-infected tissue culture plate while an uninfected tissue culture plate was in an inverted position over the infected plate. Six hundred laser pulses were applied. Pseudorabies virus in the excimer laser plume would, potentially, contact and infect the uninfected cells. The experiment was repeated 20 times with appropriate positive and negative controls.

RESULTS

None of the 20 uninfected plates was infected by the laser plume rising from ablation of infected tissue culture plates. Positive and negative controls performed as expected.

CONCLUSIONS

Even under conditions designed to maximize the likelihood of virus transmission, the excimer laser ablation plume does not appear capable of transmitting this particular live enveloped virus. Excimer laser ablation of the cornea of a human immunodeficiency virus (HIV)-infected or herpesvirus-infected patient is unlikely to pose a health hazard to the surgeon.

Combination intravenous ceftazidime and aminoglycosides in the treatment of pseudomonal scleritis

BACKGROUND

Pseudomonal scleritis is a serious and potentially blinding infection that usually is resistant to medical management.

METHODS

Results for three patients with pseudomonal scleritis who were treated with both topical anti-infectives and a combination of intravenous ceftazidime and aminoglycoside are presented in this case series.

RESULTS

All three patients had a rapid response to the addition of combination intravenous drug therapy to topical therapy; eradication of the infection and healing of the ocular surface occurred within 8 weeks. Only one patient, in whom cystoid macular edema developed, lost useful vision as a result of the infection.

CONCLUSIONS

Combination therapy with intravenous ceftazidime and aminoglycoside may be more effective than single-intravenous agents when used in addition to topical antibiotics and may obviate the need for adjunctive surgical procedures, such ascryotherapy, surgical extirpation, or conjunctival recession.

Refractive outcome of radial keratotomy: does the result of the first eye predict outcome in the second eye?

PURPOSE

To present a method of quantifying variability in the outcome of a refractive surgical procedure using the SD of the difference between achieved and expected refractive changes. We used this method to determine whether the refractive outcome of radial keratotomy in a first eye is predictive of outcome in the second eye.

METHODS

We retrospectively identified patients who underwent eight-incision radial keratotomy in the first eye from February 1993 through April 1994, with follow-up refraction 2.5 to 5 months postoperatively. This group consisted of 129 eyes of 81 patients. Thirty-nine patients had bilateral surgery with appropriate follow-up. Achieved refractive change was analyzed by multivariate linear and nonlinear regression to yield an expected refractive change for each eye based on patient age and optical zone size.

RESULTS

Residuals, defined as the difference between the achieved and expected refractive change, were normally distributed. The SD of the residuals was 0.68 diopter and was independent of the expected correction. The prediction of second-eye refractive change was not significantly improved by incorporating the residual from the first eye into the regression prediction.

CONCLUSIONS

The SD of the difference between the achieved and expected refractive change is an appropriate measure of the variability in refractive outcome following a refractive surgical procedure. Surgeons who perform bilateral simultaneous radial keratotomy do not sacrifice refractive accuracy in the second eye.

Keratomileusis in situ for high myopia

PURPOSE

To examine the safety and efficacy of myopic keratomileusis in situ (automated lamellar keratoplasty) using an automated microkeratome.

SETTING

Jules Stein Eye Institute and the Department of Ophthalmology, UCLA School of Medicine, Los Angeles, California.

METHODS

A retrospective study was done on 135 consecutive eyes of 91 patients who had keratomileusis in situ for myopia by one surgeon between October 4, 1993, and February 23, 1995. Mean preoperative myopia was 8.30 +/- 2.50 diopters (D) (range 4.50 to 20.50). Follow-up ranged from 1 (108 eyes) to 6 months (52 eyes). No eye had enhancement surgery before 3 months; eyes were dropped from the study at the time of enhancement.

RESULTS

Three months after myopic keratomileusis in situ, the mean difference between attempted and achieved correction was an undercorrection of 0.90 +/- 1.50 D; 32 of 83 eyes were within 1.00 D of attempted correction. Forty eyes were undercorrected by more than 1.00 D, and 11 were overcorrected by more than 1.00 D. Uncorrected visual acuity improved to 20/40 or better in 42 of 83 eyes at 3 months. Best spectacle-corrected visual acuity (BSCVA) declined by two or more lines in 8 of 83 eyes at 3 months and 1 of 52 eyes at 6 months. In eyes with 6 months of follow-up, a mean myopic shift of 0.40 D occurred between 3 and 6 months. Two eyes had clinically significant ingrowth into the interface. The variability in outcome (standard deviation in achieved less attempted correction) at 3 months was 1.70 D in the first 45 cases, 1.20 D in the second 45 cases, and 1.10 D in the last 45 cases.

CONCLUSION

Myopic keratomileusis in situ is relatively effective and safe for the correction of high myopia; however, a mild loss of BSCVA is not uncommon in the early postoperative period. Surgeon experience may be important for refractive accuracy.

Visual performance after photorefractive keratectomy. A prospective study

OBJECTIVE

To prospectively examine the effect of excimer laser photorefractive keratectomy (PRK) on best-corrected visual performance using psychophysical tests that were likely to be more sensitive to image degradation than high-contrast Snellen visual acuity.

DESIGN

Prospective cases series.

PATIENTS

A cohort of 18 subjects with an average of -5.08 diopters (D) of myopia (SD = +/- 1.63 D) was tested before PRK and at 3, 6, and 12 months after PRK.

INTERVENTION

Photorefractive keratectomy was performed using a laser (Excimed UV200, Summit Technology, Waltham, Mass) and a polymethylmethacrylate mask; a 5-mm ablation zone was used.

MAIN OUTCOME MEASURES

Best-corrected high-contrast visual acuity, best-corrected low-contrast visual acuity (18% Weber contrast), and best-corrected letter-contrast sensitivity. Measurements were repeated with dilated pupils and in the presence of a glare source.

RESULTS

One year after PRK, the mean best-corrected high-contrast visual acuity was reduced by half a line (P = .01), and the mean best-corrected low-contrast visual acuity was reduced by 1 1/2 lines (P = .002). The losses were somewhat greater when the subject's pupils were dilated and a glare source was used. The reduction in dilated low-contrast visual acuity was positively correlated with the decentration of the ablation zone (r = 0.47), providing evidence of an association between corneal topography and the functional outcome of PRK.

CONCLUSION

Low-contrast visual acuity losses after PRK are notably greater than high-contrast visual acuity losses for best-corrected vision. Low-contrast visual acuity is a sensitive measure for gauging the outcome success and safety of refractive surgery.

A prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal vision loss. The Summit Phototherapeutic Keratectomy Study Group

PURPOSE

The 193-nm argon fluoride excimer laser can remove corneal scars and smooth corneal irregularities, obviating corneal transplantation. We conducted a prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal vision loss as a basis for Food and Drug Administration premarket approval.

METHODS

We treated 232 eyes of 211 patients with corneal vision loss. All had corneal pathology in the anterior 100 microns of the stroma. Mean postoperative follow-up was 10 +/- 8 months. The primary outcome variable was change in best spectacle-corrected visual acuity.

RESULTS

At postoperative month 12, best spectacle-corrected visual acuity improved in 46 (45%) of 103 eyes and worsened in nine (9%) of 103 eyes by 2 or more Snellen lines. Best spectacle-corrected visual acuity improved by a mean of 1.6 +/- 2.8 Snellen lines (95% confidence interval, 1.1 to 2.1 lines). Every postoperative visit confirmed statistically significant improvement of mean best spectacle-corrected acuity. At month 12, treated eyes had a mean hyperopic shift in refraction of 0.87 diopter and a mean reduction in astigmatism of 0.36 diopter. Treatment appeared most effective in eyes with hereditary corneal dystrophies, Salzmann's nodular degeneration, and corneal scars, and least effective in eyes with calcific band keratopathy. Complications included recurrence of underlying pathology, corneal graft rejection, and bacterial keratitis.

CONCLUSIONS

Argon fluoride excimer laser phototherapeutic keratectomy is effective, with relatively few complications, for treating vision loss from corneal opacification or irregularity. Efficacy, however, varies widely depending upon individual eyes and underlying diagnoses.

Astigmatic keratotomy combined with myopic keratomileusis in situ for compound myopic astigmatism

PURPOSE

Myopic keratomileusis in situ by an automated microkeratome corrects myopia but not astigmatism, which is traditionally corrected by astigmatic keratotomy months after keratomileusis. We developed a technique for simultaneously correcting astigmatism and severe myopia, and examined its effectiveness in a retrospective case-control study.

METHODS

Thirty-four eyes (23 patients) underwent myopic keratomileusis in situ combined with one or two arcuate keratotomy incisions performed after the refractive cut, in the bed of the primary keratectomy flap. The myopic keratomileusis control group consisted of 34 matched eyes (30 patients) undergoing keratomileusis without astigmatic keratotomy. The astigmatic control group consisted of 117 unmatched eyes (85 patients) undergoing astigmatic keratotomy combined with radial keratotomy.

RESULTS

Mean refractive astigmatism in the study group decreased from 2.4 diopters (range, 1.0 to 4.0 diopters) preoperatively to 1.7 diopters (range, 1.0 to 4.0 diopters) at three months postoperatively, and increased by 0.4 diopter in the myopic keratomileusis control group at three months postoperatively (P < .005). Eighteen of 27 eyes in the study group showed decreased refractive astigmatism compared with ten of 34 eyes in the myopic keratomileusis control group (P < .0001). Combining astigmatic keratotomy with myopic keratomileusis produced 0.2 +/- 0.9 diopter less astigmatic correction than that expected from the astigmatic control group. One of 27 eyes lost two or more lines of best spectacle-corrected visual acuity at the three-month postoperative visit. No eye lost two or more lines of best spectacle-corrected visual acuity at postoperative month 6.

CONCLUSION

Eyes with substantial preoperative refractive astigmatism that undergo myopic keratomileusis in situ may benefit from simultaneous astigmatic keratotomy to reduce residual post-operative refractive astigmatism.

Autologous Keratophakia for the Correction of Consecutive Hyperopia After Automated Lamellar Keratoplasty for Myopia

PURPOSE

Consecutive hyperopia occurs if too much corneal tissue is resected during automated lamellar keratoplasty for myopia. We report what are, to our knowledge, the first two cases of consecutive hyperopia after automated lamellar keratoplasty that were treated by keratophakia with autologous corneal tissue.

METHODS

The patient in case 1 had a spherical equivalent refraction of +3.38 diopters (D) and the patient in case 2 a refraction of +3.63 D in each eye after automated lamellar keratoplasty for myopia. Corneal tissue from the contralateral eye of each patient was obtained with an automated microkeratome and transferred to the overcorrected eye in an autologous keratophakia procedure.

RESULTS

The patient in case 1 had an unaided visual acuity of 20/20, with a spherical equivalent refraction of +0.63 D 4 months after the autologous keratophakia. The patient in case 2 had an unaided visual acuity of 20/60, with a spherical equivalent refraction of -2.25 D 2.5 months postoperatively.

CONCLUSION

These two cases illustrate the use of simultaneous contralateral myopic automated lamellar keratoplasty with autologous keratophakia to treat eyes overcorrected following previous automated lamellar keratoplasty for myopia.

Lamellar Keratoplasty for Hyperopia

PURPOSE

Lamellar keratoplasty for hyperopia (automated lamellar keratoplasty for hyperopia) can correct hyperopia by weakening the cornea with a deep lamellar resection. The safety and efficacy of the procedure is uncertain.

METHODS

Twenty-four eyes of 17 patients underwent hyperopic lamellar keratoplasty using the automated corneal shaper (Chiron Corp, Irvine, Calif) by one surgeon using a flap technique. The mean attempted correction was +3.90 +/- 0.90 D (range, +2.00 to +6.00). The eyes were followed for 1 month (23 eyes) to 6 months (17 eyes).

RESULTS

Six months after hyperopic lamellar keratoplasty, the mean difference between attempted and achieved correction was an undercorrection of +1.40 +/- 0.80 diopters (D) with 7 of 17 eyes within 1.00 D of the attempted correction. No eyes were overcorrected, and 15 eyes were undercorrected. In eyes with a refractive goal of emmetropia, uncorrected visual acuity was 20/40 or better in 13 of 15 eyes and 20/20 or better in 8 of 15 eyes. No eye lost two or more lines of spectacle-corrected visual acuity at 3 or 6 months postoperatively. Between 1 and 6 months, there was a mean hyperopic shift of 0.20 D. There was clinically significant epithelial ingrowth into the interface in two eyes.

CONCLUSION

Hyperopic lamellar keratoplasty is an effective method of reducing hyperopia and induces little irregular astigmatism. The nomogram we used produces a consistent undercorrection. Refraction appears to stabilize at 1 month, but longer follow up is necessary to assess stability.

A multicenter trial of photorefractive keratectomy for residual myopia after previous ocular surgery. Summit Therapeutic Refractive Study Group

PURPOSE

The Summit Therapeutic Refractive Clinical Trial is a nine-center prospective, nonrandomized, self-controlled trial to assess the efficacy, stability, and safety of using a standardized technique of excimer laser photorefractive keratectomy (PRK) to correct residual myopia in eyes with previous refractive surgery or cataract surgery.

PATIENTS AND METHODS

Eligible eyes with a mean residual myopia of -3.7 +/- 1.8 diopters (D) (range, -0.63 to -11.00 D) underwent PRK with a 193-nm excimer laser for myopic corrections between -1.50 and -7.50 D. Standardized settings were used for the ablation zone, ablation rate, repetition rate, and fluence. One hundred seven of the first 114 treated eyes were examined 1 year after PRK, with 98% of eyes having had refractive keratotomy and 2% having had cataract surgery.

RESULTS

One year postoperatively, the mean manifest spherical equivalent refraction was -0.6 +/- 1.4 D (range, -6.50 to 2.50 D); 63% of eyes were within +/-1.00 D of the attempted correction; and uncorrected visual acuity was 20/40 or better in 74% of eyes. Twenty-nine percent of eyes lost two or more Snellen lines of best-corrected visual acuity, and central corneal haze was moderate or severe in 8% of eyes.

CONCLUSIONS

Excimer laser PRK is effective in reducing residual myopia after previous refractive and cataract surgery. However, it is less accurate than PRK in eyes that did not undergo surgery and is more likely to cause a loss of best-corrected visual acuity 1 year after treatment.

Photorefractive keratectomy for myopia using a 4.5-millimeter ablation zone

BACKGROUND

Argon fluoride (193 nm) excimer laser photorefractive keratectomy for myopia is under evaluation by the United States Food and Drug Administration.

METHODS

We report a consecutive prospective series of 100 patients (one eye per patient) treated as part of the Phase IIB FDA-approved protocol, with 80 patients followed for 1 year. Patients' ages ranged from 21 to 62 years (mean, 35 years). The Summit Technology, Inc ExciMed UV200LA with a 4.5-mm diameter ablation was used.

RESULTS

Baseline spherical equivalent refraction ranged from -2.00 to -6.90 diopters (D) (mean -4.60 D). Ninety-five percent of eyes reepithelialized by 72 hours. At 1 year, the difference between attempted and achieved correction was +/- 0.50 D for 42 eyes (53%) and +/- 1.00 D for 60 eyes (75%). During the first 6 months, there was a trend toward overcorrection and the majority of eyes showed some loss of initial refractive correction; 10 eyes (14%) changed by 1.00 D or more between 6 and 12 months. An uncorrected visual acuity of 20/25 or better was achieved by 50 eyes (63%) and 20/40 or better by 61 eyes (77%). Of the 10 eyes (12%) that lost two or more Snellen lines of spectacle-corrected or glare visual acuity, two had visual acuity of worse than 20/25. Central subepithelial corneal haze was absent to mild in 77 (96%) eyes at 12 months.

CONCLUSIONS

Excimer laser photorefractive keratectomy as performed in this study was generally effective and safe in reducing simple spherical myopia. Further studies of the effect of a larger diameter ablation zone, smoother transitional corneal contours, and the effect of postoperative topical corticosteroids may lead to further improvements in outcome.

The effect of globe fixation on ablation zone centration in photorefractive keratectomy

PURPOSE

For optimal vision, the ablation zone in photorefractive keratectomy should be centered over the entrance pupil. During ablation, the globe can be immobilized by the surgeon, with a suction ring around the corneoscleral limbus. Alternatively, the globe can be immobilized by patient fixation on a target, unassisted by the surgeon. We investigated which method results in better centration of the ablation zone over the entrance pupil, by using an objective, mathematical method to determine the ablation zone center.

METHODS

Forty-eight eyes from 48 patients who underwent photorefractive keratectomy by the two techniques were studied retrospectively. The centers of the ablation zones were objectively determined by a weighted center of mass algorithm applied to the preoperative minus postoperative difference maps. The validity of the objective method was confirmed by comparison to subjective estimates of ablation zone centers made by independent human observers.

RESULTS

The 19 eyes treated by surgeon fixation had an average decentration of the ablation zone of 0.63 +/- 0.31 mm (range, 0.01 to 1.00 mm), and the 29 eyes treated by patient fixation had an average decentration of 0.41 +/- 0.23 mm (range, 0.11 to 1.18 mm) (P = .027).

CONCLUSIONS

The center of the ablation zone can be determined mathematically from the topographic map, to avoid observer bias. In this study, unassisted patient fixation during photorefractive keratectomy produced more accurate centration of the ablation zone than did surgeon fixation and has the potential for maximizing the quality of vision postoperatively.

Topographic determination of corneal asphericity and its lack of effect on the refractive outcome of radial keratotomy

PURPOSE

The normal human cornea flattens peripherally. The amount of flattening, or asphericity, has traditionally been calculated from multiple keratometric measurements. We devised a mathematical technique for determining asphericity from computed corneal topography. We then determined whether asphericity affects the refractive outcome of radial keratotomy.

METHODS

One eye each of 41 patients who underwent four- or eight-incision radial keratotomy and preoperative computed corneal topography was identified retrospectively and analyzed. The asphericity, P, of each cornea was calculated by fitting Baker's equation (y2 = 2r0x-Px2) to each meridian of the topographic map. For each patient, we calculated the difference between the refractive outcome in diopters for radial keratotomy and the prediction of a quadratic least-squares best-fit model involving optical zone size and age.

RESULTS

Aspericity could be calculated from the topographic maps in all 41 patients and ranged from 0.33 to 1.28, with mean +/- S.D. of 0.82 +/- 0.21. Aphericity varied among the meridians of a cornea, with an average standard deviation among meridians of 0.17. No statistical correlation was found between calculated asphericity and refractive outcome.

CONCLUSIONS

Corneal asphericity can be calculated from corneal topographic maps. Asphericity is not constant in the different meridians of a normal cornea. Corneal asphericity is not useful in predicting the refractive outcome of radial keratotomy.

Myopic shift after removal of Salzmann's nodular degeneration

A patient with bilateral Salzmann's nodular degeneration developed a myopic shift of 9 diopters in the right eye and 5 diopters in the left eye after removal of his nodules. This previously unreported complication of removal of peripheral corneal pathology is discussed in light of our current understanding of keratorefractive surgery.

Six-month results of the multicenter phase I study of excimer laser myopic keratomileusis

We report six-month results of the Summit Technology Myopic Keratomileusis Phase I multicenter study. Fifty-seven eyes of 57 patients had keratomileusis to correct high myopia. A microkeratome was used for the primary keratectomy and the excimer laser was used to ablate the stroma of the resected lenticle (cap) or the stromal bed (in situ). At six months, 31 of the 47 eyes available for follow-up (65.9%) had uncorrected visual acuity of 20/40 or better; 16 (34.0%) had uncorrected acuity of 20/25 or better. Thirty-seven eyes (78.7%) maintained the same (+/- one Snellen line) best corrected visual acuity as before surgery; seven (14.9%) lost two lines and three (6.4%) lost more than two lines. In addition to the six-month multicenter study results, we report two year results in a subset of 28 eyes (22 from the multicenter study and six fellow eyes). At six months, 17 of the 24 eyes available for follow-up (70.9%) had uncorrected visual acuity of 20/40 or better and nine (37.5%) had uncorrected acuity of 20/25 or better, including eyes that had worse than 20/80 best corrected visual acuity preoperatively. At 24 months, five of the seven eyes available for follow-up (71.4%) had uncorrected acuity of 20/25 or better. Only one patient lost two lines of best corrected vision at six months and no patient lost more than two lines; at 24 months, all patients maintained (+/- one line) best corrected vision.(ABSTRACT TRUNCATED AT 250 WORDS)

Excimer laser ablation rate and corneal hydration

During excimer laser photorefractive keratectomy, dehydration of the cornea begins as soon as the epithelium is removed. Corneal hydration might affect the excimer laser ablation rate, which could affect the accuracy of correction. We studied the effect of corneal hydration on the excimer laser ablation rate in bovine eyes. To control hydration, bovine corneoscleral rims were equilibrated in dextran solutions of varying concentrations. One button trephined from each rim underwent laser ablation. Hydrated tissue ablation rates (amount of collagen, ground substance, and water removed per pulse) and dry component ablation rates (amount of collagen and ground substance removed per pulse) were calculated from mass removed. The hydrated tissue ablation rate at physiologic hydration was 0.40 micron/pulse. As corneal hydration increased, the hydrated tissue ablation rate increased by 5.6 micrograms/cm2/pulse per increase in unit corneal hydration (simple linear regression analysis, P = .0001). The dry component ablation rate decreased linearly by 0.82 microgram/cm2/pulse per unit increase in corneal hydration (simple linear regression analysis, P = .0001). Both clinical data and theoretical arguments imply that dry component ablation rate determines refractive outcome after photorefractive keratectomy. Since the dry component ablation rate increases as the cornea dries, significant dehydration of the cornea before ablation might lead to relative overcorrections of myopia. Surgeons should use a technique that minimizes changes in hydration to maximize the predictability of excimer laser photorefractive keratectomy.

Corneal ectasia as a complication of repeated keratotomy surgery

BACKGROUND

Staged keratotomy surgery, or "enhancement surgery," may allow a more predictable outcome, but also subjects the patient to additional surgical risks.

METHODS

A 39-year-old man underwent astigmatic keratotomy for myopic astigmatism, followed by 12 enhancement procedures for residual astigmatism.

RESULTS

These procedures effectively resulted in a double hexagonal keratotomy. The patient's best spectacle-corrected acuity deteriorated to counting fingers. Clinically, a conically-shaped protrusion of the central cornea, Munson's sign, diffuse subepithelial scarring, and central corneal thinning were noted. Penetrating keratoplasty was performed. Histopathologic examination showed central thinning, epithelial edema, disruption of Bowman's layer, marked stromal scarring, and focal areas of endothelial attenuation--findings consistent with keratoconus.

CONCLUSION

This case illustrates that multiple keratotomy procedures may result in corneal ectasia in apparently normal eyes and suggests that hexagonal keratotomy may be more likely to cause iatrogenic keratoconus.

Results of radial and astigmatic keratotomy by beginning refractive surgeons

BACKGROUND

There is little information available on the results of radial and astigmatic keratotomy surgery that is performed by beginning refractive surgeons.

METHODS

A retrospective review of all refractive keratotomies performed by Corneal Fellows in the University of California, Los Angeles, Department of Ophthalmology between October 1985 and October 1991 was performed. Data from all eyes with at least 3 months of follow-up were analyzed. Visual acuity, refractive error, and complication rates were compared with published case series.

RESULTS

The mean preoperative spherical equivalent for the 79 eyes analyzed was -3.97 diopters (D) (range, -0.75 to -8.50 D). The mean postoperative spherical equivalent was -0.44 D (range, +1.50 to -3.88 D). The postoperative spherical equivalent was within 1.00 D of emmetropia in 85% of eyes, and uncorrected visual acuity was 20/40 or better in 94% of eyes. There were no vision-threatening complications. No patient lost more than one line of best-corrected visual acuity.

CONCLUSION

Radial and astigmatic keratotomies that are performed by beginning refractive surgeons in a supervised setting can be safe and effective procedures with results comparable with those obtained by experienced refractive surgeons.

A prototype erodible mask delivery system for the excimer laser

PURPOSE

The authors developed an erodible mask delivery system for the argon-fluoride 193-nm excimer laser, which offers the possibility of correcting hyperopia and astigmatism as well as myopia.

METHOD

Masks were made of polymethylmethacrylate on a quartz window, with intended corrections for myopia and hyperopia of 2.5 and 5 diopters (D). Ablations using the mask and control ablations using an expanding diaphragm were performed in 30 eyes of 15 pigmented rabbits with an Excimed UV200 laser (Summit Technology, Inc, Waltham, MA). The rabbits were followed for 134 days with regular biomicroscopy and retinoscopic examination by two observers.

RESULTS

Ablations with the mask to correct myopia were successful and produced stable corrections, although the higher-power mask produced undercorrections. Hyperopic masks produced paradoxic myopic corrections, possibly due to the lack of a transition zone at the edge of the mask. Corneas ablated with the mask had less sub-epithelial haze than those ablated with the diaphragm at all examinations. Results of histopathologic examination showed epithelial hyperplasia over the ablation zone in all eyes. Dichlorotriazinyl aminofluorescein collagen staining showed subepithelial new collagen in all eyes, but there was no relation between the depth of ablation at any point on the cornea and the amount of new collagen deposited there.

CONCLUSIONS

Myopic ablations are feasible with the erodible mask, although additional calibration is needed. Hyperopic ablations were unsuccessful with the current design. Corneas ablated with the mask may be clearer than corneas ablated with the diaphragm, possibly due to a smoother ablated surface. Regression of effect after laser ablation in the rabbit model is likely due more to epithelial hyperplasia than to stromal remodeling.

Effective spherical aberration of the cornea as a quantitative descriptor in corneal topography

Following excimer laser photorefractive keratectomy and other refractive surgical procedures, complaints of halos, glare, and monocular diplopia are common. These procedures increase the asphericity of the cornea, which may cause the optical distortions. We used ray tracing techniques to estimate the longitudinal monochromatic aberration of the cornea from the measured corneal topography (effective spherical aberration) in 15 normal eyes with varying degrees of astigmatism and in ten eyes after photorefractive keratectomy. Best spherical corrected visual acuity in the astigmatic eyes was highly correlated with effective spherical aberration (r = -0.9527, P < .001). In the eyes that had photorefractive keratectomy, the effective spherical aberration was highly correlated with measured glare visual acuity (r = 0.875, P < .002). These results suggest that effective spherical aberration is a valuable topographic measure that provides information about the optical performance of aspheric corneas.

Determination of corneal image-forming properties from corneal topography

Keratometry provides useful information about the cornea's image-forming properties, such as corneal astigmatism, but is inaccurate on irregular corneas. Quantitative corneal topographic information is now obtainable on irregular corneas, but is difficult for the clinician to interpret. We developed a method to determine the spherical power, astigmatism, and topographic irregularity of a cornea by finding the best-fit spherocylinder that was closest to its measured topography. Keratometric measurements and two videokeratographs were gathered prospectively on 262 normal and abnormal corneas. The best-fit measurements of spherical power, astigmatism, and topographic irregularity were reproducible with one standard deviation of 0.75 diopter or better; agreement with keratometric measurements in normal eyes was good (0.60 diopter or better). Topographic irregularity averaged 0.1 diopter on precision spheres, 0.4 diopter on 146 normal eyes, 0.8 diopter on 29 eyes after radial keratotomy, 2.0 diopters on 58 eyes after penetrating keratoplasty, and 3.0 diopters on 29 eyes with advanced keratoconus. We conclude the following: basic corneal image-forming properties can be measured from videokeratographs; the properties can be determined, by our methods, on irregular corneas in which keratometry is unreliable; and topographic irregularity provides a measure of irregular astigmatism.

Subregions of differing refractive power within the clear zone after experimental radial keratotomy

After radial keratotomy (RK), some patients experience a mild decrease in best corrected visual acuity, visual distortion, or monocular diplopia. These optical effects of radial keratotomy are best explained by subregions of different refractive powers within the surgery-free clear zone. To investigate the topography of the clear zone, we performed four- and eight-incision radial keratotomy in eight cadaver eyes. After radial keratotomy, we found subregions within the clear zone of two types: 1) small, very flat regions at the ends of the radial incisions (seven of eight eyes), and 2) a series of concentric rings centered on the visual axis with a continuously progressive decline in refractive power toward the periphery of the clear zone (all eyes). The clear zone after radial keratotomy is often nipple-shaped, with a more myopic segment centrally and a more hyperopic region near the periphery of the clear zone.

Computer-assisted videokeratography of corneal topography after radial keratotomy

We used computer-assisted videokeratography to compare the topographies of 32 corneas from 23 subjects after radial keratotomy with those of 47 normal corneas from 47 subjects controlled for age and preoperative keratometric and refractive power. Three ophthalmologists independently classified color-coded videokeratographs based on the color-coded pattern of dioptric power distribution and the cross-sectional shape. Corneas that had radial keratotomy exhibited a polygonal pattern not seen in normal eyes; this occurred in 59% of corneas. All normal corneas demonstrated a cross-sectional shape configuration that was steeper centrally than peripherally; 79% of corneas after radial keratotomy had a shape that was flatter centrally than peripherally. After radial keratotomy, the dioptric power increased from the center to the periphery (radius of approximately 4.6 mm) by 2.8 +/- 2.2 diopters (mean +/- SD), with a sharp inflection zone ("paracentral knee") 2.7 mm from the center; normal corneas showed a smooth decrease in power from the center to the periphery of 1.9 +/- 0.5 diopters.

Corneal topography and optical zone location in photorefractive keratectomy

Optical zone diameters in photorefractive keratectomy are small in order to minimize the depth of stromal tissue excision and, possible, the resultant stromal scarring. A small optical zone demands accurate placement on the corneal surface. This article reviews the principles that are important to location and measurement of the optical zone in photorefractive keratectomy. The ablated area should be centered on the cornea overlying the entrance pupil while the patient is fixating coaxially with the surgeon. An optical zone that is too small or decentered may decrease acuity, lessen contrast sensitivity, or produce glare, either on the fovea or on the perifoveal retina. The relationship between optical zone size and location and quality of vision is not yet known. Studies of this relationship will require videokeratography to locate the optical zone and its relationship to the entrance pupil, a capability that current videokeratoscopes do not possess.