Analysis of Higher-Order Wavefront Aberrations in Standard PRK

PURPOSE: To investigate the preoperative characteristics and postoperative change of the higher-order wavefront aberrations (HOAs) in myopic photorefractive keratectomy (PRK). METHODS: Standard PRK was performed in 39 eyes (spherical equivalent -4.71+/-1.56D) using the VISX(R) STAR S4(TM) excimer laser system. Wavefront analysis was performed preoperatively and 6 months postoperatively using the VISX(R) WaveScan(TM) aberrometer. Statistical analysis was performed to assess the preoperative characteristics of the HOAs and the PRK-induced aberrations. RESULTS: The preoperative root mean square (RMS) value of the total HOAs was 0.291 micrometer: coma 0.159 micrometer, trefoil 0.157 micrometer, spherical aberration 0.093 micrometer. Among the 3 HOAs, only the spherical aberration showed moderate magnitude negative correlation with the spherical equivalent (p=0.012) and sphere (p=0.005). The magnitude of all of the HOAs, except trefoil, were significantly increased at postoperative 6 months: total HOAs 183% (0.533 micrometer, p<0.001), spherical aberration 341% (0.317 micrometer, p<0.001), coma 185% (0.294 micrometer, p<0.001), trefoil 104% (0.163 micrometer, p=0.681). In the postoperative correlation analysis, only the PRK-induced spherical aberration showed the increase of magnitude with increasing preoperative spherical equivalent (p=0.036) and sphere (p=0.019). CONCLUSIONS: The HOAs, especially spherical aberration and coma, were increased after PRK. Among the 3 HOAs, the spherical aberration showed the greatest increase of magnitude and percent change; it also showed a positive correlation with increasing preoperative spherical equivalent and sphere. Further software improvements in the wavefront-guided surface ablations need to have a diminution of PRK-induced spherical aberration.

Clinical Outcome of PRK with the Use of a Rotary Brush, Corneal Chilling, and Bandage Contact Lens

PURPOSE: To evaluate the effect of rotary epithelial scrubber, cold BSS(R) irrigation, and bandage contact lens on the clinical outcome of excimer laser PRK. METHODS: This updated PRK was performed in 90 eyes (Group I: -4.54 +/- 0.91 D). Conventional PRK with mechanical epithelial removal using Beaver(R) blade and patch dressing was performed in 181 eyes (Group II: -4.30 +/- 0.97 D). Clinical outcome was compared between group I and group II retrospectively. RESULTS: Mean epithelial removal time was significantly shorter in group I (6.6 +/- 0.9 sec) than in group II (33.2 +/- 11.2 sec) (p<0.05). Mean epithelial healing time showed more rapid epithelial healing in group I (2.0 +/- 0.3 days) than in group II (3.0 +/- 1.0 days) (p<0.05). At 1 week and 24 months, UCVA of 0.7 or better was achieved in 91.1%, 97.8% in group I and in 71.8%, 88.2% in group II (p<0.05). At 6 and 24 months, spherical equivalent within 0.5 D of emmetropia was in 91.1%, 91.3% in group I and in 80.7%, 56.9% in group II (p<0.05). After 1 year follow-up, no eye in group I showed either corneal haze worse than grade 1 or loss of 2 or more lines of BSCVA. CONCLUSIONS: Updated PRK provided more rapid epithelialization than conventional PRK. And there was significant improvement in clinical outcome and safety in group I compared with group II. We suggested that updated PRK should be used for the correction of mild to moderate myopia.

Excimer Laser Photorefractive Keratectomy with a Rotary Epithelial Scrubber and Post-laser Corneal Chilling

PURPOSE: To evaluate the effect of rotary epithelial scrubber and post-laser corneal chilling on clinical outcomes of excimer laser PRK (VISX 20/20B VisionKeyTM) METHODS: PRK with a rotary epithelial scrubber and post-laser chilled BSS irrigation was performed in 231 eyes of 141 patients (Group I: -4.33+/-0.98 D, July 1997~July 1999). Conventional PRK with mechanical epithelial removal was performed in 282 eyes of 181 patients (Group II: -4.33+/-0.99 D, September 1994~April 1997). A retrospective clinical data of two different PRK procedures was compared. RESULTS: Mean epithelial removal time was significantly short in group I (8.6+/-3.4 sec) than in group II (29.8+/-12.8 sec) (p<0.05). More rapid epithelial healing was achieved in group I (2.6+/-0.7 days) than in group II (2.8+/-0.7 days) (p<0.05). At 12 months, UCVA was 0.7 or better in 99.4% (162/163 eyes) of group I and 92.8% (77/88 eyes) of group II (p<0.05). At 12 months, 98.8% (161/163 eyes) in group I and 95.2% (79/83 eyes) in group II were within +/-1.0 D of emmetropia (p<0.05). After 2 years follow-up, there were no cases showing corneal haze worse than grade 1 in group I. CONCLUSIONS: Excimer laser PRK after precise removal of the epithelium with a rotary scrubber and post-laser corneal chilling provided more rapid epithelialization and visual recovery than conventional PRK. There was statistically significant improvement in clinical outcomes and occurrence of corneal haze in group I comparing with group II. We think that this excimer laser PRK technique be more ideal for the correction of mild to moderate myopia to avoid the serious flap complication of LASIK.

The Reduction of the Postkeratoplasty Astigmatism by Excimer Laser and Diamond Knife Procedure

PURPOSE: To evaluate the efficacy of excimer laser PRK (PAK) and arcuate keratotomy (AK) for correction of post-keratoplasty astigmatism. METHODS: We performed AK in 13 patients and PAK in 8 patients who had astigmatism over 3.5 D, and who could not wear contact lens or spectacles. We follow up patients in early phase (1~3 months) and late phase (6~12 months) after operation. Net change in astigmatic magnitude , uncorrected visual acuity, and best corrected visual acuity were examined. Surgery-induced astigmatism was calculated by bector anaysis method. RESULTS: Mean preoperative astigmatism in AK patients was 8.23+/-3.25 D and in PAK patients was 7.84+/-1.58 D. Astigmatic magnitude was decreased 61.2% in early phase, 58.3% in late phase in AK patients and decreased 51.4% in early phase, 41.2% in late phase in PAK patients. Post operative astigmatic axis was changed within 30 degrees 76.9% in early phase, 61.5% in late phase in AK, and 50% in early phase, 37.5% in late phase in PAK. Postoperative uncorrected visual acuity showed stastically significant improvement in early and late phase in AK patients and late phase in PAK patients. Postoprative best corrected visual acuity improved 53.8% in AK patients and 50% in PAK patients above 2 Snellen chart lines, but both was not stastically significant. Postoperative complication was mild haze not interfering refraction and visual acuity in 5 eyes of PAK. CONCLUSIONS: AK corrected astigmatsm and visual acuity more rapidly than PAK in early phase, but more undercorrected in late phase. Mild haze was developed in PAK. But AK and PAK show nearly same astigmatic and visual correction results in conclusion.

Intraocular Lens Power Calculation in Cataract Surgery after Excimer Laser Photorefractive Keratectomy

To evaluate the accuracy of a new IOL power calculation method, the intraocular lens[IOL]power using a new method [refraction corrected method]was compared with the IOL power calculated using refraction derived keratometric values and conventional keratometry values in 4 eyes of 4 patients received cataract surgery after Excimer PRK.We performed phacoemulsifications and implanted intraocular lens, the power of which was calculated using the new method, and checked refractive errors 2 months after cataract surgery.The intraocular lens powers calculated using refraction derived keratometric values were lower than those calculated using the new method in the range of 0.52 diopter to 2.48 diopter.The intraocular lens powers calculated using conventional keratometry values were much lower than those calculated using the new method and refrac-tion derived keratometric value.Conclusively, the conventional method should not be used in IOL power calculation in catarct surgery after Excimer laser PRK, and in that case our new method is more accurate than a method using refraction derived keratometry value.

The Change of Interleukin-1beta & Nitric Oxide in Tear after Excimer Laser Photorefractive Keratectomy

This study was performed to investigate the amount of IL-1beta & NO in tear film after Excimer laser PRK and to determine whether NO induces cytotoxicity associated with apoptosis of keratocytes. Twelve myopes underwent PRK and were divided into 3 groups according to spherical equivalent. IL-1beta and NO production were measured at preop and postop 1, 4, 7 days, and expression of NOS and apoptosis of keratocytes were evaluated in the corneas of rats at 2 days after PRK. IL-1beta and NO production were significantly increased at postop 1 day(p<0.01) and then gradually decreased to preop levels at postop 7 days. The corneas of rats at 2 days after PTK revealed increased expression of NOS(iNOS, bNOS) and increased apoptosis of keratocytes in stroma of cornea. The degree of corneal opacity and NO production at postop 1 days were increased, positively correlated to ablation depth. It is suggested that NO production induced by IL-1beta may be related with apoptosis of keratocyte, which can influence on the corneal opacity after PRK.

A Case of Ocular Deviation after Excinier Laser Photorefractive Keratectomy

Patients undergoing monocular excimer laser photorefractive keratectomy (PRK) for myopic correction can develop strabismus secondary to decompensated heterophorias. We experienced a case of ocular esodeviation and diplopia that developed 8 nionths after monocular excimer laser phtorefractive keratectomy. So, we report this case with 8 review of the literature.

The Relationship between Nitric Oxide and CAtaractohenesis after Eximer Laser Photorefractive Keratectomy

To assess the change of nitric oxide(NO) concentration according to photorefractive keratectomy(PRK) depth and the role of NO in cataractohenesis after excimer laser PRK. The treated 36 rabbits were divided into 12 groups according to PRK depth(-3.0, -7.0, -11.0 diopter [0]), the dates of sampling(the 1st day and the 7th day after PRK), and reatment of NOS inhibitor (NG-nitro-L-arginine methylester, L-NAME). NO concentration and superoxide dismutase(SOD) activity in the aqueous humor and malondialdehyde(MDA) concentration in the lens were measured. Changes of lens epithelium were examined by eletron microscopy on the 3rd day and the 6th day after -11.0D of PRK. The NO and MDA concentration in the group without L-NAME treatment on the 1st day after -11.0D of PRK were significantly higher than those in the L-NAME treated group and control group. The activity of SOD increased in all of the PRK treated group on the 1st day and was even higher on the 7th day. The histopathological changes of lens epithelium on the 3rd day after PRK were compatible with cellular necrosis and those of the 6th day after PRK were compatible with cellular reparing. The results suggested that the toxicity of NO after PRK correcting over -11.0D may a role in cataractogenesis, but the progress could be suppressed by treatment of NOS inhibitor.

Change of Epithelial Barrier Function after Excimer Laser Photorefractive Keratectomy

We used the fluorophotometry to investigate the corneal epithelial barrier function after excimer laser photorefractive keratectomy (PRK). Twenty-five eyes of 21 subjects (13 women, 8 men) underwent PRK to correct rnyopia. Corneal epithelial healing time was measured and corneal epithelial permeability to sodiurn fluorescein was evaluated by fluorophotoinetry at I, 2, and 3 weeks after surgery. The corneal epithelial permeability increased significantly 1 week after surgery and returned to preoperative level 2 weeks after surgery. The permeability differences according to epithelial healing days and corrected diopters were not statistically significant(p>0. 05). These results suggest that PRK delays complete reconstruction of corneal epithelial barrier function. The corneal epithelium regained its functional barrier 2 weeks after PRK in patients, so, at least, during the first 2 weeks, care should be taken to miniinize further epithelial trauma from topical inedication or surgical manipulation.

The Effect of Topical Prostaglandin Synthesis Inhibitor and Therapeutic Contact Lens on the Pain and Corneal Reepithelialization after Excimer Laser PRK

In order to evaluate the effect of topical prostaglanduin synthesis inhibitor(Diclod) and therapeutic contact lens(T-lens) on pain control and epithelial wound healing following excimer laser photorefractive keratectomy(PRK) we examined the score of subjective pain & the degree of corneal epithelial defect on 40 consecutive patients with or without above treatment. The patients were subdivided into 4 groups : 10 patients treated with only pressure patch(Group 1), 10 patients with pressure patch & Diclod(Group 2), 10 patients with only T-lens(Group 3), and 10 patients with T-lens & Diclod(Group 4). Diclod(Group 2&4) was effective on pain control during three days after excimer laser PRK, while T-lens treatment was not effective. Subjective pain score between the patient with Diclod(Group 2&4) and those without Diclod(Group 1&3) was significantly different(p< 0.05). In the point of epithelial wound healing after excimer laser PRK, 95% of patients treated with T-lens(Group 3&4) revealed no epithelial defect three days after excimer laser PRK, while only 65% of the patients treated with pressure patch revealed that. The difference was statistially significant(p< 0.05). Diclod, however, did not influence the epithelial wound healing in both pressure group and T-lens group. In this study we could find that the combination of Diclod & T-lens was a recommendable method to decrease the pain & to increase the epithelial wound healing after excimer laser PRK.

Corneal Endothelial Damage after Deep Excimer Laser Ablation

To investigate the effect of deep excimer laser ablation on the corneal endothelium, excimer laser photo refractive keratectomy(PRK) with different ablation depths were performed to obtain various residual corneal thickness(range: 90-250 micrometer) in white rabbit corneas(N=50). Corneal endothelium was stained with Alizarin red S(pH 4.2) for 2 minutes three days after excimer laser ablation and was analyzed morphometrically using digitizer the morphometric parameters according to residual corneal thickness. In the corneas of residual corneal thickness(RT) over 200 micrometer and untreated controls(N=6), endothelial damages were rarely seen. With the decrease of residual corneal thickness, hexagonality and shape factor decreased, and coefficient of variation of the cell area(CV) increased(p<0.01). In corneas of RT less than 200 micrometer, endothelial damages were found and become more severe in corneas of RT between 90~149 micrometer. Hexagonal shaped cells were rarely observed, and the shapes of most cells were changed. Deep excimer laser PRK might affect corneal endothelial cells if the RT is less than 200 micrometer, and these findings suggest that care is recommended when doing deep excimer laser corneal ablation, especially excimer laser assisted in situ keratomileusis.

Corneal Topographic Evaluation of Centration of Excimer Laser Myopic Photorefractive Keratectomy

In order to get a good visual outcome following excimer laser photorefractive keratectomy(PRK) the centration of ablation is very important. In this study, the ablation center of excimer laser PRK using VISX 20/20 B(R) with VisionKeyTM(VISX, Inc. Santa Clara, CA) was evaluated by using corneal topography. Corneal topography was performed before operation and one month after operation to 190 eyes of 123 patients. The location and distance of center of ablation from center of the entrance pupil were calculated by using EyeSys Corneal Analysis System(R) with pupil-finding software(EyeSys Laboratories, Inc. Houston, TX). Supero-nasal displacement of ablation center occurred in 84 eyes(44%) after excimer laser PRK. The mean distance was 0.33 +/- 0.21mm(range: 0.02~1.36mm). The distance less than 0.5mm was found in 157 eyes(83%) and that between 0.5mm and 0.75mm was found in 26 eyes(14%). The decentering amount following excimer laser PRK was not depending on laterality(OD vs. OS), sex, age, amount of attempted correction(6D down vs. 6D up), type of software(ver 3.20 vs. 4.01) or type of procedure(PRK vs. PARK). In contrast, the second eye receiving excimer laser PRK showed a better centration of ablation than the first eye. And the centration results improved with surgeon's experience in the most recent 50 eyes(0.28 +/- 0.16mm). However, there was no relation between amount of decentration and visual improvement. In this study, it was found that decentering of ablation center less than 0.75mm did not influence the visual outcome in excimer laser PRK.

Difference between Goldmann Applanation and Topcon Noncontact Tonometer Measurements after Excimer Laser Photorefractive Keratectomy

The authors' clinical experience has suggested that the noncontact tonometer (NCT) has a tendency to record a lower intraocular pressure(IOP) than the Goldmann applanation tonometer(GAT) after excimer laser photo refractive keratectomy(PRK). This study was done to verify this tendency. We performed PRK on 177 eyes of 122 patients using a VISX 20/20 excimer laser, and topical fluorometholone was used after PRK. The eyes were divided into three groups according to their ablation depths, and the IOP was measured with the GAT and the Topcon NCT before surgery and 1, 3, 6, 9, and 12 months postoperatively. The mean postoperative IOP measured with the GAT or the NCT decreased as compared with the mean preoperative IOP(GAT: 1.67 ~ 2.90 mmHg, NCT:5.27 ~ 6.54 mmHg). The mean NCT reading was 1.12 mmHg higher than the mean GAT reading preoperatively, but was 2.48 ~ 2.88 mmHg lower than the mean GAT reading postoperatively. Difference between GAT and NCT measurements postoperatively was more prominent in the group of deeper ablation. These results suggest that we should be cautious of evaluating the IOP measured with the GAT or the NCT after PRK.

Effect of Excimer Laser Photorefractive Keratectomy on Goldmann Applanation Tonometry

It can be speculated that the thinned and flattened central cornea after excimer laser photorefractive keratectomy (PRK) might give a falsely low Goldmann applanation tonometer(GAT) reading. We studied 353 eyes of 264 patients who underwent PRK to determine the effect of excimer laser PRK on the accuracy of Goldmann applanation tonometry. PRK was done with a VISX 20/20 excimer laser and topical fluorometholone was used after PRK. The intraocular pressure(IOP) was measured with the GAT before surgery and 1, 3, 6, 9, and 12 months postoperatively. The mean ablation depth of the cornea was 71.4 +/- 22.0 micrometer, and the eyes were divided into three groups according to their ablation depths. The mean postoperative IOP measured with the GAT decreased as compared with the mean preoperative IOP(P<0.01). The mean postoperative IOP at 9 or 12 months was lower than that at 1, 3, or 6 months(p<0.05). The mean IOP(11.9 +/- 2.5 mmHg) at 12 months was 2.8 mmHg lower than the mean preoperative IOP(14.7 +/- 2.7 mmHg). There was no statistically significant difference in the amount of postoperative IOP decrease among the three groups.