Surface ablation techniques

The ongoing quest for a safe, simple, effective, minimally invasive, and stable refractive surgical procedure to correct refractive errors has stimulated the development of surface ablation techniques and laser in situ keratomileusis. In this review, we describe the history, patient assessment, techniques, outcomes, and complications of surface ablation (photorefractive keratectomy, laser-assisted sub-epithelial keratectomy, epithelial laser-assisted in situ keratomileusis) and compare the results of various surface techniques. Surface ablation procedures will continue to evolve, with potential improvements in outcomes accompanying future sophisticated ablation profiles and laser technology.     

Excimer laser surface ablation: a review of recent literature

The aim was to review the recently published literature on excimer laser surface ablation procedures, including photorefractive keratectomy (PRK), laser sub‐epithelial keratomileusis (LASEK), microkeratome‐assisted PRK (epi‐LASIK) and trans‐epithelial (laser‐assisted) PRK, to help elucidate where and how surface ablation may best fit into current refractive surgical practice. The emphasis was on publications within the last three years and included systemic reviews, meta‐analyses and randomised controlled trials. Where such evidence did not exist, selective large series cohort studies, case‐controlled studies and case series with follow‐up preferably greater than six months were examined and included. Refractive and visual outcomes are excellent and comparable to those after LASIK even in complex cases after previous corneal surgery. Indeed, surface ablation combined with corneal collagen cross‐linking may be used in selected eyes with biomechanical instability, where LASIK is contraindicated. In addition, there is evidence to suggest that there may be less induction of higher order aberrations with surface techniques. Long‐term stability and safety appear to be extremely satisfactory. The literature supports the use of modern excimer laser surface treatments, with outcomes comparable to those after LASIK and evidence of less induction of higher‐order aberrations. Follow‐up studies at 10 to 20 years indicate excellent stability and safety.     

Evaluation of the difference between intended and measured ablation and its impact on refractive outcomes of the wavefront optimize profile and the S001 Wellington nomogram in myopic spherocylindrical corrections

Background: To evaluate the level of agreement between intended and measured ablation of the wavefront optimize profile corrected by the Wellington nomogram in myopic spherocylindrical corrections and assess its impact on refractive outcomes. Design: Prospective, university–institute setting. Participants: Eighty‐six participants (86 eyes for photorefractive keratectomy group) and 86 participants (86 eyes for laser in situ keratomileusis group) recruited in a consecutive‐if –eligible basis. Methods: Differences between intended and measured ablation were evaluated with Scheimpflug camera. Refractive outcomes were evaluated by means of postoperative spherical equivalent, postoperative defocus equivalent, contrast sensitivity, correction index, difference vector and index of success. Main Outcome Measures: Correlation of visual outcomes with intended ablation. Results: Both groups demonstrated significant over‐ablations (16.7 ± 8.5, P < 0.001 & 11.8 ± 18.5, P < 0.001, respectively). Intended ablation was the primary determinant of the measured difference (r‐square 0.769 & 0.765, respectively). Photorefractive keratectomy corrections over 100 µ had significant impact on postoperative spherical equivalent, postoperative defocus equivalent, correction index, difference vector and index of success (P = 0.044, P = 0.05, P = 0.019, P = 0.016, P = 0.006, respectively), but laser in situ keratomileusis corrections over 100 µ had significant impact only on postoperative defocus equivalent, difference vector and index of success (P = 0.04, P = 0.05). Conclusions: The results suggest that the wavefront optimize profile seems to over‐ablate corneal tissue both in photorefractive keratectomy and laser in situ keratomileusis. Over‐ablation exerts significant impact on refractive outcomes only in high spherocylindrical corrections.     

Transepithelial photorefractive keratectomy for treatment of thin flaps or caps after complicated laser in situ keratomileusis

PURPOSE: To report transepithelial photorefractive keratectomy treatment of corneal irregularities produced during laser in situ keratomileusis (LASIK) in which there is a thin flap or cap associated with central corneal scarring or epithelial ingrowth that threatens vision.METHODS: Case reports. The thickness of the abnormal corneal flap or cap and associated scarring or epithelial ingrowth is estimated at the slit lamp or measured with an optical pachymeter. If residual myopia is sufficiently high to allow complete ablation of the flap or cap in the central cornea, a transepithelial photorefractive keratectomy is performed in which the epithelium is completely ablated with the excimer laser in phototherapeutic keratectomy mode; residual myopia is corrected using photorefractive keratectomy.RESULTS: This method was used successfully in two eyes of two patients in which a thin cap was associated with a transverse cut through the central cornea or a donut-shaped flap associated with epithelial ingrowth in the central cornea. In both cases, the abnormal cap or flap was ablated, central corneal clarity restored, and visual function improved.CONCLUSION: Transepithelial photorefractive keratectomy may be effective in treating central corneal thin cap or flap abnormalities associated with LASIK.     

全激光表层切削术研究进展:经上皮准分子激光屈光性角膜切削术

经上皮准分子激光屈光性角膜切削术(transepithelial photorefractive keratectomy,TransPRK)是运用模式去除角膜上皮,随即附加一定度数的激光切削基质来完成的表层手术.另外一种新型的TransPRK手术运用特殊的切削模式,去上皮和切削基质一步完成.两种方法所行的TransPRK手术,与机械性的去上皮手术比较,均可减少细胞凋亡、降低角膜上皮下混浊(haze)的发生,改善患者术后视力.     

Advances in subepithelial excimer refractive surgery techniques: Epi-LASIK

The reports of an increasing incidence of iatrogenic ectasia, the evolution of wavefront aberrometry, and the suggestion that the laser in situ keratomileusis flap could lead to unpredictable biomechanical corneal changes have renewed interest in surface ablation and have set the stage for the introduction of alternative photorefractive treatment modalities. The theoretical advantage of surface procedures, such as laser epithelial keratomileusis that preserve the epithelial button, stems from the repositioning of the epithelial flap over the laser-ablated corneal surface. This epithelial sheet is thought to act as a natural contact lens that decreases postoperative pain and haze formation. Epipolis laser in situ keratomileusis is an alternative modality of epithelial separation with the use of a device that features a blunt oscillating blade. This surgical approach does not require the use of alcohol for epithelial loosening.     

Macular hole after laser in situ keratomileusis and photorefractive keratectomy

PURPOSE: To describe three myopic patients who developed unilateral macular hole after undergoing bilateral laser in situ keratomileusis or photorefractive keratectomy. METHODS: Case reports. RESULTS: Three eyes of three myopic patients developed a macular hole in one eye after bilateral laser in situ keratomileusis or photorefractive keratectomy. The macular hole formed between 4 to 7 weeks after laser in situ keratomileusis in case 1 (a 48-year-old woman), and within 2 months after laser in situ keratomileusis in case 2 (a 36-year-old woman). In case 3 (a 45-year-old man), the macular hole was found 9 months after photorefractive keratectomy. A vitrectomy closed the macular hole of case 1 with final best-corrected visual acuity of 20/25 and case 2 with 20/30, whereas case 3 declined further surgery. CONCLUSION: A macular hole may develop in myopic eyes after laser in situ keratomileusis or photorefractive keratectomy. Vitreoretinal interface changes may play a role.     

Laser in situ keratomileusis for treatment of residual myopia after photorefractive keratectomy

PURPOSE: To evaluate the results of laser in situ keratomileusis after photorefractive keratectomy. METHODS: Eighty eyes of 80 patients with residual myopia after photorefractive keratectomy were reoperated with laser in situ keratomileusis. The study was retrospective. Laser in situ keratomileusis was performed using the automated corneal shaper microkeratome and Chiron Technolas 217-C d1 excimer laser. Data measured after laser in situ keratomileusis included uncorrected visual acuity, best-corrected visual acuity, refraction, haze, pachymetry, and keratometry. The follow-up was at least 12 +/- 1.6 months (range, 12 to 15 months). RESULTS: After laser in situ keratomileusis the mean spherical equivalent was -0.24 diopters +/- 0.78. (range, -3 to +1.5) at 12 months, and the mean uncorrected visual acuity was 0.76 diopters +/- 0.24 (range, 0.1 to 1). Sixty-five eyes (81.3%) had various degrees of haze after laser in situ keratomileusis. One eye (1.2%) lost 2 lines of best-corrected visual acuity. CONCLUSIONS: Laser in situ keratomileusis enhancement may be a good alternative to correct residual myopia and astigmatism after primary photorefractive keratectomy. Corneal haze is a common problem in these eyes, and the treatment after laser in situ keratomileusis enhancement should be the same as the treatment after primary photorefractive keratectomy.     

A standardized drawing scheme to document corneal changes following refractive corneal surgery

PURPOSE: To present a systematic and standardized drawing scheme for unambiguous and reproducible documentation of corneal changes after incisional techniques, coagulative procedures, and lamellar and surface ablation excimer surgery. METHODS: Standardized symbols in five colors representing specific corneal conditions after incisional surgery (eg, radial keratotomy, astigmatic keratotomy), coagulative procedures (eg, laser thermokeratoplasty, conductive keratoplasty), surface ablation (eg, photorefractive keratectomy, laser subepithelial keratomileusis, epi-LASIK), and LASIK are used to record corneal changes in frontal and sectional views. RESULTS: Corneal changes following refractive corneal surgery were documented. CONCLUSIONS: The drawing scheme permits specific features to be followed in the clinic in a clear and unambiguous manner.     

Corneal sensitivity after photorefractive keratectomy and laser in situ keratomileusis for low myopia.

PURPOSE: To evaluate and compare the recovery of postoperative corneal sensitivity after laser in situ keratomileusis and photorefractive keratectomy for the correction of low myopia. METHODS: In a prospective study, 17 consecutive eyes (17 patients) underwent laser in situ keratomileusis to correct myopia ranging from -3.25 to -6.75 diopters, and another 18 consecutive eyes (18 patients) underwent photorefractive keratectomy to correct myopia from -3.12 to -7.00 diopters. Corneal sensitivity was tested preoperatively and 1 week and 1, 3, and 6 months postoperatively using the Cochet-Bonnet esthesiometer. Corneal sensitivity was tested at the center of the cornea, and in four additional central points 2 mm from the corneal center (nasal, inferior, temporal, and superior). RESULTS: Corneal sensitivity after laser in situ keratomileusis was reduced at the ablated zone during the first 3 months after surgery (Wilcoxon rank sum test, P < .05), and only after 6 months it returned to its preoperative values. However, corneal sensitivity recovered its preoperative values 1 month after photorefractive keratectomy (Wilcoxon rank sum test, P > .05), except for the central corneal point, where 3 months were required. Comparing both groups, corneal sensitivity was more depressed after laser in situ keratomileusis than after photorefractive keratectomy during the first 3 months (Mann-Whitney test, P < .05), except for the nasal central point, although no differences were found between both groups at 6 months (P > .05). CONCLUSIONS: In the correction of low myopia, corneal sensitivity at the ablated zone was more depressed after laser in situ keratomileusis than after photorefractive keratectomy during the first 3 months after surgery. Only after 6 months were corneal sensitivity values similar in both groups.     

Laser subepithelial keratomileusis: electron microscopy and visual outcomes of flap photorefractive keratectomy

Laser epithelial keratomileusis (LASEK) is a surgical technique that may be performed in patients with low myopia who are considering conventional photorefractive keratectomy (PRK), in patients with thin corneas, and in patients with professions or lifestyles that predispose them to trauma. Ethanol (18%) is applied for 25 seconds and a hinged epithelial flap is created. After laser ablation, the flap is repositioned over the ablated stroma. Our data showed that epithelial defects occurred in 63% eyes on Day 1. Postoperative pain was absent in 47%. Uncorrected visual acuity of 20/40 or better was achieved in all patients at 1 week. Data from several studies suggest that LASEK may reduce postoperative pain and corneal haze associated with PRK. LASEK offers the potential advantage of avoiding flap-related complications associated with laser in situ keratomileusis and decreasing postoperative pain and corneal scarring associated with PRK. Long-term studies are needed to confirm these potential advantages and to determine the safety, efficacy, and predictability of this surgical technique.     

The wound healing response after laser in situ keratomileusis and photorefractive keratectomy: elusive control of biological variability and effect on custom laser vision correction

Biological diversity in the wound healing response is thought to be a major factor limiting the predictability of the outcome of refractive surgical procedures such as laser in situ keratomileusis and photorefractive keratectomy. Corneal wound healing is critical to the success of topography-linked or wave front-linked excimer laser ablation to optimize visual performance. This is because of the importance of retaining subtle features of custom ablation and the tendency of epithelial hyperplasia and stromal remodeling to obscure these features following either procedure. The corneal wound healing response is exceedingly complex. Keratocyte apoptosis, which occurs in response to epithelial injury, is the earliest observable event in the wound healing cascades and is therefore an excellent target for pharmacological intervention. Alterations of surgical technique can be designed to limit keratocyte apoptosis and the subsequent events in corneal wound healing. Abnormalities of the cascades could contribute to the pathogenesis of corneal diseases. For example, recent data have suggested that perturbation of the keratocyte apoptosis/mitosis balance could underlie the development of keratoconus in a proportion of patients.     

Inadvertent stromal dissection during mechanical separation of the corneal epithelium using an epikeratome

Epithelial flap complications occurred in 2 patients during epithelial separation using a Centurion SES epikeratome (Norwood Eye Care) in epi-laser in situ keratomileusis (LASIK). The complications consisted of stromal dissection at the margin of the pupil and an epithelial free cap including the superficial stroma. The epithelial flaps were repositioned without laser ablation. Three months postoperatively, the best corrected visual acuity in both patients was 20/20 and neither complained of visual discomfort. Slitlamp biomicroscopic examination showed that both corneas were completely healed with trace opacity, and topographic examinations revealed that irregularities in the stromal cutting sites were decreased. The patients had successful photorefractive keratectomy after complete healing of the dissected stroma. Stromal dissection during mechanical separation of the epithelium with an epikeratome is a potential complication of the epi-LASIK procedure, but proper management can result in good recovery without severe visual impairment.     

Sloughing of corneal epithelium and wound healing complications associated with laser in situ keratomileusis in patients with epithelial basement membrane dystrophy

PURPOSE: To report sloughing of corneal epithelium during laser in situ keratomileusis and subsequent wound healing complications in patients with epithelial basement membrane dystrophy. METHODS: In a retrospective study, the surgical procedures, postoperative course, and visual acuities of 16 eyes of nine patients with epithelial basement membrane dystrophy who underwent laser in situ keratomileusis complicated with epithelial sloughing at three centers were reviewed. The mean follow-up period was 23 weeks (range, 4 to 52 weeks). RESULTS: In 13 (81%) of 16 eyes with epithelial basement membrane dystrophy, epithelial sloughing occurred during laser in situ keratomileusis. In eight of the 13 eyes, epithelial growth beneath the flap was observed. The flap was lifted and the interface epithelium scraped in six eyes. Flap melt or keratolysis occurred in four eyes. At the last follow-up visit, 13 of 16 eyes had an uncorrected visual acuity of 20/30 or better, and all eyes had a best-corrected visual acuity of 20/30 or better. CONCLUSIONS: Patients with epithelial basement membrane dystrophy have poorly adherent corneal epithelium and are predisposed to epithelial sloughing during the microkeratome pass of laser in situ keratomileusis. This may lead to flap distortion, interface epithelial growth, flap keratolysis, and corneal scarring. It is not recommended that laser in situ keratomileusis be performed in patients with classic, symptomatic epithelial basement membrane dystrophy. In patients who present with mild and asymptomatic epithelial basement membrane dystrophy, laser in situ keratomileusis should be performed with caution, or photorefractive keratectomy may be the preferred refractive procedure.     

准分子激光角膜表层切削术后的止痛措施

准分子激光角膜表层切削术因降低角膜膨隆的风险和避免准分子激光原位角膜磨镶术(LASIK)角膜瓣相关的并发症而受到青睐。但术后严重疼痛不适为表层切削的主要缺陷,因此表层切削术后疼痛不适的控制显得尤其重要。我们总结了表层切削术后疼痛的机制以及降低术后疼痛的措施的进展。     

PRK for epithelial ingrowth in buttonhole after LASIK

PURPOSE: To evaluate the efficacy of photorefractive keratectomy (PRK) in progressive central epithelial ingrowth after buttonholes after laser in situ keratomileusis (LASIK). METHODS: Two eyes of two patients with progressive central epithelial ingrowth and fibrosis in flap buttonholes after LASIK underwent PRK 6 months following primary surgery. RESULTS: Epithelial ingrowth was treated successfully in both eyes. The uncorrected visual acuities were 20/20, and there was no evidence of recurrent epithelial ingrowth after 6 months. CONCLUSIONS: Photorefractive keratectomy is a useful modality in the treatment of central epithelial ingrowth in buttonholes following LASIK. As a single-step surgery, it offers both therapeutic and optical advantages by simultaneously clearing the corneal opacity and correcting the refractive error in selected cases.     

The excimer laser for corneal refractive surgery—recent developments and evolutionary directions

This article reviews recent developments in excimer laser refractive corneal surgery. The excimer laser is now used to treat myopia, hyperopia and astigmatism by either surface (photorefractive keratectomy or PRK) or stromal (laser in situ keratomileusis or LASIK) ablation. The refractive outcome of PRK is excellent for low (less than six dioptres) myopia, but it is not without complications. These include initial severe pain, myopic regression, haze, glare, halos and difficulty with vision in reduced light. The refractive outcome for LASIK is similar to that for PRK, but there is negligible pain, much less haze, less regression among highly myopic patients and a more rapid establishment of final unaided visual acuity. Although excimer laser treatment is currently available, some important advances are necessary before the spectacle and contact lens markets are threatened. Control of the healing process is crucial to improve the accuracy of the outcome, reduce regression and eliminate haze. More attention must be paid to the visual outcome and ablation design so that visual functions, such as best corrected visual acuity and contrast sensitivity, are not reduced. It even may be possible, in the future, to custom design aspheric ablations to yield improved best corrected visual acuity.     

Corneal optical coherence tomography before and immediately after excimer laser photorefractive keratectomy

PURPOSE: To investigate the representation of the corneal structure with optical coherence tomography before and immediately after excimer laser photorefractive keratectomy. METHODS: Twenty-four eyes of 24 patients with myopia and myopic astigmatism were prospectively studied. The corneal thickness and the corneal profile were assessed with slit-lamp-adapted optical coherence tomography preoperatively and immediately after excimer laser photorefractive keratectomy. RESULTS: The attempted mean spherical equivalent of the refractive corrections was -6.7 +/- 3.6 (mean +/- SD) diopters with a mean calculated stromal ablation depth of 91 +/- 38 microm. The corneal optical coherence tomography was reproducible in all patients, demonstrating a mean decrease of central corneal thickness after epithelial debridement and excimer laser photorefractive keratectomy of 118 +/- 45 microm. The comparison of the calculated stromal ablation depth and the corneal thickness changes determined by corneal optical coherence tomography revealed a significant linear relationship with a correlation coefficient of 0.88 (P <.001). The flattening of the corneal curvature was confirmed in all patients with the optical coherence tomography system and correlated with the attempted refractive correction (r =.82, P <.001). CONCLUSIONS: The slit-lamp-adapted optical coherence tomography system presented in this study allowed noncontact, cross-sectional, and high-resolution imaging of the corneal configuration. This initial clinical evaluation demonstrated that corneal optical coherence tomography could be a promising diagnostic modality to monitor corneal changes of thickness and curvature before and after excimer laser photorefractive keratectomy.     

Diametral ablation--a technique to manage decentered photorefractive keratectomy for myopia.

PURPOSE: Decentered ablation after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) is an intraoperative complication that may significantly influence the visual outcome. Currently, there is no accepted technique available to manage this problem. METHODS: A technique of reoperation for eccentric ablation after PRK called diametral ablation is presented that uses a transepithelial phototherapeutic keratectomy (PTK) that leaves epithelium over the area of primary PRK. This residual epithelium acts as a shield for the previously treated stroma during a second PRK located opposite to the primary PRK with reference to the pupil center. The results of six such retreatments (6 eyes of 6 patients) are presented. RESULTS: In all eyes, a significant improvement regarding centration and subjective complaints was achieved. The eccentricity was reduced from 1.43+/-0.66 mm to 0.36+/-0.15 mm (P = .003). CONCLUSION: Diametral ablation is a promising alternative treatment of eccentric ablation.     

准分子激光术后近视回退和欠矫原因分析

目的：探讨准分子激光近视治疗术后屈光回退和欠矫的原因,为临床提供理论依据。方法：将我院近视中心准分子激光术后发生屈光回退并行二次手术的45例70眼,按〈-6．00D为Ⅰ组,〉-6．00D度为Ⅱ组,将两组患者两次治疗前后屈光度、术后不同时间的视力、术式、切削光斑直径、角膜切削量及残留厚度等相关资料进行了对比分析。结果：两组未手术时屈光度差别显著（P〈0．05）。屈光回退后两组屈光度差别无意义（P〉0．05）。Ⅰ,Ⅱ两组首次激光术后6mo内,不同时间点裸眼视力与术前矫正视力有明显差别（P〈0．05）。二次手术后,1wk内裸眼视力与矫正视力差别明显（P〈0．05）,1～3mo无差别（P〉0．05）;Ⅰ组于6mo时裸眼视力与矫正视力出现差别（P〈0．05）,Ⅱ组差别不显著（P〉0．05）;两组激光切削光斑的直径及角膜切削量有明显差别（P〈0．05）。结论：准分子激光近视治疗术后,屈光度回退程度与术前屈光状态无关。不同屈光度屈光回退程度相同。高度近视、LASIK多区切削和切削光斑直径小是发生屈光回退和欠矫的主要原因。