Corneal wound healing after excimer laser keratotomy in a human eye

Corneal opacity after repeated photorefractive keratectomy

Corneal opacity developed in an eye that had photorefractive keratectomy (PRK) with a 193 nm excimer laser 5 times over 3 years. Six months after the last PRK, a partial penetrating keratoplasty was performed. The cornea was stained and immunohistochemically evaluated for collagen types. Light microscopy showed thickening of epithelial layers, proliferation of subepithelial fibroblasts, and the absence of Bowman's membrane. Transmission electron microscopy showed irregular collagen lamellae and electron-dense deposits adjacent to keratocytes. The staining was positive for Alcian blue, and immunohistochemistry was positive for type IV and VI collagen. This case suggests that corneal opacity after repeated PRK is the result of deposits of type IV and VI collagen and acidic mucoprotein in the extracellular matrix.

Laser in Situ Keratomileusis for Myopia and Hyperopia Using the Lasersight 200 Laser in 300 Consecutive Eyes

PURPOSE

To evaluate effectiveness, safety, predictability, and short-term stability of laser in situ keratomileusis (LASIK) using the LaserSight Compac-200 Mini excimer laser with software version 9.0, for all refractive errors.

METHODS

One hundred fifty consecutive patients (300 eyes) that received bilateral LASIK for myopia, hyperopia, and astigmatism were studied prospectively. A new 9.0 software version applying a modified nomogram that takes advantage of bilateral surgery was used. Follow-up at 6 months was available for 267 eyes (89%).

RESULTS

Six months postoperatively, 131 eyes (96.32%) in the low to moderate myopia group (-1.00 to -5.99 D; n=136) had a spherical equivalent refraction within +/-1.00 D, and 123 eyes (90.44%) were within +/-0.50 D of emmetropia. In the high to extreme myopia group (-6.00 to -25.00 D; n=114), 97 eyes (87.08%) had a spherical equivalent refraction within +/-1.00 D and 78 eyes (68.42%) were within +/-0.50 D of emmetropia. In the hyperopia group (+1.00 to +6.00 D; n=50), 44 eyes (88%) had a postoperative spherical equivalent refraction within +/-1.00 D, and 31 eyes (62%) were within +/-0.50 D of emmetropia. Mean change in spherical equivalent refraction at 6 months was less than -0.50 D in the low to high myopia groups and -1.16 +/- 0.55 D in the extreme myopia group. At 6 months follow-up, uncorrected visual acuity was 20/20 or better in 73 eyes (54%) in the low to moderate myopia groups and 21 eyes (18%) in the high to extreme myopia groups. In the hyperopia group at 6 months follow-up, uncorrected visual acuity was 20/20 or better in 31 eyes (62%) and 20/40 or better in 41 eyes (82%). Only two eyes had a temporary loss of two or more lines of spectacle-corrected visual acuity due to corneal folds that were surgically treated. Six months after LASIK, no eye had lost any lines of best spectacle-corrected visual acuity in this series.

CONCLUSIONS

Our modified LASIK nomogram with the 9.0 software of the LaserSight 200 excimer laser (with a larger and smoother ablation pattern) resulted in safe and effective outcomes for the treatment of low to high myopia, astigmatism, and hyperopia.

The relationship between pupil diameter and decentration in myopia

Since optical zone centration is of such importance in refractive surgery, we have studied, with a computer-aided videopupillograph, the problem of identifying it in myopes. The findings of pupil centre shift as a function of pupil diameter and degree of myopia are reported in two groups of myopes (low and medium) and a control group. Results indicate that the amount and direction of decentration vary according to pupil diameter and degree of myopia. No generally valid instructions can be laid down and each case must be treated on its merits.

Excimer Laser Photorefractive Keratectomy for Hyperopia

OBJECTIVE

Excimer laser photorefractive keratectomy (PRK) has been shown to be an effective method in the treatment of refractive errors, especially myopia. We evaluated prospectively the efficacy, predictability, stability, and safety of excimer laser PRK in the treatment of hyperopia.

METHODS

Thirty-four hyperopic eyes were treated with an Aesculap-Meditec (MEL 60) excimer laser. The patients were divided into two groups. In the low-moderate hyperopia group, baseline spherical equivalent refraction was between +1.50 and +6.00 diopters (D) (mean, +4.20 +/- 1.30 D) and in the high hyperopia group between +6.25 and +9.75 D (mean, +7.70 +/- 1.30 D). Follow-up visits occurred 1, 3, 6, and 12 months after surgery.

RESULTS

One-year results were available for a total 27 eyes (79%): 15 eyes with low to moderate hyperopia and 12 eyes with high hyperopia. One year after PRK in the low-moderate group, six eyes (40%) had a refractive error within +/- 1.00 D of emmetropia, but in the high hyperopia group only two eyes (17%) were within +/- 1.00 D of emmetropia; three eyes (20%) and one eye (8%) were within +/- 0.50 D, respectively. The stability of the refractive change was better in the low to moderate hyperopia group; in the high hyperopia group there was still some regression after 6 months. At 12 months, 10 eyes (67%) in the low-moderate and one eye (8%) in the high hyperopia group had postoperative uncorrected visual acuity of 20/40 or better. One eye in the low-moderate hyperopia group saw 20/20 without correction. Only one eye lost two lines of spectacle-corrected visual acuity. Haze was more intense in the high hyperopia group, but it did not reduce visual acuity. No vision-threatening complications were observed.

CONCLUSIONS

When low to moderate hyperopia up to +6.00 D is treated, excimer laser PRK with the Aesculap Meditec MEL60 laser is safe and moderately effective, and refraction stabilizes after 3 months in most eyes. However, PRK is not sufficient to treat high hyperopia in an effective and predictable way.

Ultrastructural and Immunohistochemical Findings after Linear Excimer Laser Keratectomy

BACKGROUND

One human pathology specimen has been studied previously following the use of the excimer laser for the correction of astigmatism. We report histopathologic findings following linear corneal excisions with the excimer laser.

METHODS

A 193 nm excimer laser was used to create symmetrical, transverse excisions in a human eye to correct astigmatism. Three months later, a full-thickness corneal transplant was performed due to unsatisfactory refractive results. The excised corneal button was examined with light microscopy, transmission electron microscopy and immunohistochemistry.

RESULTS

An area 10 to 20-micron wide was observed between the epithelial cells within the keratectomy and the sharply dissected stromal lamellae. This area stained positive for laminin and pro-collagen type III. Some epithelial cells showed processes reaching into this area. Descemet's membrane, immediately underneath the area of the keratectomy, contained atypically striated collagen fibers.

CONCLUSIONS

These findings demonstrate wound healing changes similar to those reported following diamond knife keratotomy and photorefractive keratectomy for myopia. The changes in the posterior cornea are similar to those previously reported when an excimer laser beam approached Descemet's membrane.

An in vivo investigation of the structures responsible for corneal haze after photorefractive keratectomy and their effect on visual function

PURPOSE

To make serial measurements of corneal haze and microscopic anatomy after photorefractive keratectomy (PRK) and compare the results with visual function measured at the same time points in the same single group of human subjects.

METHODS

Ten patients underwent -6.00-diopter, 6-mm PRK. The patients were reviewed frequently for 12 months. Corneal haze was measured objectively in two ways: (1) an opacification index was determined from the variance in digitized retroillumination images; and (2) light reflected and scattered back from the cornea was assessed by gray-scale analysis of video slit images. In vivo confocal microscopy recorded the anatomic changes occurring in the cornea, and computer analysis of the images quantified the keratocytes and subepithelial deposit. Visual performance was assessed by Snellen visual acuity, contrast sensitivity, and glare-induced visual dysfunction.

RESULTS

In the first week, epithelial irregularity resulted in a transient reduction in all aspects of visual function. In the first month, keratocyte disturbances reduced contrast sensitivity at high frequencies and produced glare. Over the next couple of months, the subepithelial deposit resulted in a more prolonged loss of contrast sensitivity at low frequencies and glare-induced visual dysfunction due to the scattering of light. In several patients, these visual defects persisted after 1 year.

CONCLUSIONS

Epithelial and keratocyte disturbances only transiently affect visual function. The subepithelial deposit is more persistent and can have a lasting effect on visual performance. Therefore, attempts to improve the visual outcome of PRK must be aimed at controlling the synthesis of subepithelial material.

Control of Pain After Photorefractive Keratectomy

BACKGROUND

The inadequate control of pain after photorefractive keratectomy remains an important issue for both patients and clinicians.

METHOD

A survey of postoperative pain regimens used in excimer centers worldwide was carried out. A pilot study to assess the temporal basis of pain after photorefractive keratectomy was done by using a visual analog pain score in 30 patients. A comprehensive literature search was undertaken to ascertain the suitability of topical ophthalmic anesthetics in controlling pain after photorefractive keratectomy.

RESULTS

Many drugs, both oral and topical, are used worldwide to control pain after photorefractive keratectomy, but no group has shown satisfactory pain control in all patients. The pilot study revealed that the pain was most severe in the first 24 hours after photorefractive keratectomy and declined to low levels thereafter. Topical anesthetics cause corneal complications when used excessively and for prolonged periods.

CONCLUSION

Pain after photorefractive keratectomy is most severe in the first 24 hours. Topical anesthetics would only be needed for this short time and would guarantee pain relief in all patients. We do not anticipate any corneal complications if the anesthetics are used for short times and under close medical supervision.

Excimer laser photorefractive keratectomy

Excimer laser photorefractive keratectomy has been used for the correction of myopia, hyperopia and astigmatism. This laser removes tissue through a process termed photoablative decomposition, in which incident photon energy is sufficient to break molecular bonds. Selective removal of tissue across the anterior corneal surface results in a change in anterior corneal curvature. The surgical outcome may be influenced also by interindividual variability in wound healing and pharmacologic interventions. The nature of the excimer laser-tissue interaction, and clinical outcomes of predictability, stability and complications of surgery for myopia are discussed in detail.

Do Topical Corticosteroids Have a Role Following Excimer Laser Photorefractive Keratectomy?

The article reviews the evidence for and against the use of topical corticosteroids following PRK. Two significant problems after PRK are the development of corneal haze and unpredictability of the refractive outcome. These arise from changes in the anterior stroma, the deposition of new subepithelial tissue, and individual variations in the wound healing response. In rabbits, corticosteroids reduced corneal haze by limiting the synthesis of subepithelial collagen. However, in humans, controlled trials showed that corticosteroids had no lasting effect on either haze or regression, and were associated with an unacceptably high incidence of unwanted effects. The difference between species probably related to the relative absence of collagen and predominance of glycosaminoglycans during corneal wound healing in humans. Some human studies have shown rapid changes in refraction occurring within days of stopping or restarting corticosteroids. This suggests that their transient hyperopic effect is probably mediated by changes in the hydration of the tissue. Therefore, the role of corticosteroids in PRK is very limited. There is no justification for their routine use after PRK for low or moderate myopia. If corticosteroids have a role in improving haze or refractive outcome in selected patients, a means for their early identification must be found.

Human excimer laser keratectomy. Clinical and histopathologic correlations

PURPOSE

To understand the healing capabilities of the diseased human cornea after excimer laser photoablation by morphologic analysis of laser-treated corneas.

METHODS

Twelve corneal specimens were obtained 5 to 16 months after lamellar or full-thickness keratoplasty following phototherapeutic keratectomy for undercorrected myopic epikeratoplasty (2 eyes), corneal leukomas (2 eyes), herpes zoster corneal scarring (1 eye), band keratopathy (2 eyes), adenoviral subepithelial opacity (1 eye), keratoconus (1 eye), herpes simplex corneal scarring (2 eyes), granular corneal dystrophy (1 eye), and recurrent lattice dystrophy (1 eye). The morphology of the corneas was examined by light and electron microscopy.

RESULTS

Epithelial hyperplasia, abnormal epithelial attachment, and disorganized stromal matrices were observed. Evidence of residual disease frequently observed in these specimens indicated that the pathology either was not excised at the time of laser keratectomy or was recurrent.

CONCLUSIONS

The response of the diseased cornea to excimer laser treatment has similar characteristics to the responses previously observed in animal studies. Incomplete ablation of diseased tissue and/or recurrence of the initial disease was the major reason for failure of the treatment. Possible causes for the inability to remove diseased tissues and superficial scars with the excimer laser include (1) insufficiently achieved ablation depth and/or diameter and (2) decreased laser ablation rates of scarred cornea.

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF-excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200-800 nm. The intensity of secondary radiation in the range of 200-315 nm (UVC and UVB) is approximately 20% of the total intensity at high laser fluences (> 2 J/cm2), and approximately 50% at moderate laser fluences (< 500 mJ/cm2); 10 muJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (< 1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (> 5 J/cm2) are required. As a consequence, laser-induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.

308 nm excimer laser ablation of cartilage

This article reports the investigation of the XeCl excimer laser as a cutting-ablating tool for human fibrocartilage and hyaline cartilage. Quantitative measurements were made of tissue ablation rates as a function of fluence in meniscal fibrocartilage and articular hyaline cartilage. A force of 1.47 Newtons was applied to an 800-microns fiber with the laser delivering a range of fluences (40-190 mJ/mm2) firing at a frequency of 5 Hz. To assess the effect of repetition rate on depth per pulse, a set of measurements was made at a constant fluence of 60 mJ/mm2, with the repetition rate varying from 10 to 40 Hz. Histologic and morphometric analysis of preserved specimens was performed using light microscopy. The results of these studies revealed that the ablation rate was directly proportional to fluence over the range tested. Fibrocartilage was ablated at a rate 2.56 times faster than hyaline cartilage. Repetition rate had no effect on the penetration per pulse. Adjacent tissue damage was noted to be minimal (10-70 microns). The excimer laser achieved ablation rates adequate for arthroscopic applications.

Erb:YAG and Hol:YAG laser osteotomy: the effect of laser ablation on bone healing

Sixty-nine male Sprague Dawley rats were divided into three groups of 23 animals each and osteotomies were performed in group 1 with a power saw, in group 2 with the Erb:Yag laser, and in group 3 with the Hol:YAG laser. Two animals of each group were sacrificed 1 week, 4, 8, and 12 weeks after operation for histologic investigation, and five animals of each group at 4, 8, and 12 weeks after osteotomy for torque testing. Anterior-posterior (AP) radiographs were taken at the same time points and investigated for callus formation and development of pseudoarthrosis. All tibiae osteotomied with the Hol:YAG laser (group 3) developed pseudoarthrosis within 12 weeks and, therefore, torque testing could not be performed for this group. Biomechanical measurements of bone treated by power saw or Erb:YAG laser osteotomies, respectively, showed no significant statistical difference in the stability of bone between the two groups. Histologic examination after 1 week exhibited fibrous tissue at the site of osteotomy in rats of all three groups and additionally carbonization in rats of group 3. Saw osteotomies resulted in more callus formation than Erb:YAG osteotomies, but both techniques provoked a certain reunion within 8 weeks. Hol:YAG laser-treated osteotomies, however, exhibited formation of dense fibrous tissue, carbonization and no callus formation within 12 weeks. Radiographic pictures showed more callus formation for saw osteotomies as compared to those performed with the Erb:YAG laser. For Hol:YAG laser osteotomies pseudoarthrosis was identified also radiologically.

Effect of contact diode laser on the cornea with and without absorbing dye

The semiconductor diode laser is a near-infrared laser; its 810-nm wavelength is maximally absorbed by melanin and has substantial transmissibility through cornea and sclera. Indocyanine green is the best photosensitive dye for the diode laser. The level of corneal damage produced by 810-nm diode laser, with and without absorbing dye (indocyanine green), and photoablative capabilities of this wavelength were studied using albino rabbits. We concluded that the contact application of this wavelength to the cornea in the presence of energy-absorbing dye causes both stromal and endothelial thermal damage. Therefore, 810-nm near-infrared semiconductor diode lasers are not suitable for photorefractive keratectomy or photoablative reprofiling.

Ablation of polymethylmethacrylate by Ho:YAG, Nd:YAG, and Erb:YAG lasers

Ho:YAG, Nd:YAG, and Erb:YAG laser ablation of Polymethylmethacrylate (PMMA) was investigated under in vitro and simulated clinical conditions. Ablation rates were measured for all lasers and after ablation, macroscopic and microscopic appearance of the ablation site was investigated. The mean ablation rates of the Erb:YAG, Ho:YAG, and Nd:YAG laser increased from 8 microns per pulse at 100 mJ to 44 microns per pulse at 300 mJ from 100 microns per pulse at 200 mJ to 222 microns per pulse at 800 mJ and from 28 microns per pulse at 100 mJ to 189 microns per pulse at 800 mJ, respectively. Macroscopic investigation exhibited melting of bone cement for the Ho:YAG and Nd:YAG lasers and pulse-to-pulse vaporization for the Erb:YAG laser. The width of thermal alteration, however, was comparable for all lasers used. Removal of cement from bone specimens under simulated clinical conditions showed good detachment of cement when the fiber was used parallel; in case of perpendicular use, remainders of cement and carbonization of bone could be observed upon histological investigation.

Erb:YAG and Hol:YAG laser ablation of meniscus and intervertebral discs

Erb:YAG and Hol:YAG laser ablation rates of fibrocartilage and nucleus pulposus were measured in vitro simulating clinical conditions. After ablation macroscopic and microscopic appearance of the ablation site was investigated. Hol:YAG and Erb:YAG laser mean ablation rates increased almost linearly with rising energies, showing higher total ablation rates for the Hol:YAG laser due to its higher achievable energy density. At comparable energy densities the Erb:YAG laser appears to be more effective with respect to the corresponding ablation rates. Consequently, the ablational threshold proved to be lower for the Erb:YAG laser. Whereas during Hol:YAG laser ablation, some smoke formation and considerable tissue shrinking occurred, these effects could not be observed during Erb:YAG laser ablation. Consequently macroscopic and microscopic inspection showed some thermal damage after Hol:YAG and only minimal alterations after Erb:YAG laser ablation. Adjacent thermal damage was determined and proved to be lower for the Erb:YAG laser. In our opinion the characteristics of each laser system provide certain advantages for special clinical indications.

Excimer laser ablation of fibrocartilage: an in vitro and in vivo study

To date, lasers have found only limited applications in orthopedics. We employed a 308 nm XeCl excimer laser for ablation of fibrocartilage, in order to investigate the feasibility of excimer laser assisted meniscectomy. Experiments were conducted both in vitro and in vivo. For the in vitro study, human menisci, obtained during surgery and autopsy, were irradiated via a 600 microns core fiber at radiant exposures ranging between 20 mj/mm2 and 80 mj/mm2, at 20 Hz. Ablation rate measurements and histological analysis of the samples were performed. The ablation rates were found to range from 3 microns/pulse to 100 microns/pulse depending on the radiant exposure and/or the applied pressure on the fiber delivery system. Thermographic analysis was also performed during pulsed excimer as well as CW Nd:Yag and CW CO2 laser irradiation. Temperatures were lower for excimer laser (Tmax less than 65 degrees) than CW ND: Yag (Tmax less than 210 degrees) or CW CO2 (Tmax less than 202 degrees) laser. For the in vitro study, medial meniscectomy was performed in 15 rabbits with the excimer laser and a CW Nd:Yag laser in the right and left knee respectively. Excimer laser irradiation was performed at 70 mj/mm2. Nd:Yag irradiation was performed via a 600 microns core fiber at power outputs between 20 to 40 W for 10 and 20 seconds duration. The healing response to injury was investigated by histological analysis of the menisci after 1 day, 1, 2, 4, and 8 weeks following the laser procedure. Excimer laser treated menisci showed less inflammatory reaction and noticeable repair with minimal inflammatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Preliminary study of argon fluoride (193 nm) excimer laser trabeculectomy. Scanning electron microscopy at five months

There is an increasing interest in the use of the excimer laser in ophthalmology, particularly in refractive surgery. The argon fluoride (193 nm) excimer laser ablates tissue with a high degree of precision and without any mechanical or thermal damage to surrounding structures. In this study, the argon fluoride excimer laser was experimentally used in the rabbit model to perform a trabeculectomy. Clinical and scanning electron microscopic evaluations showed a successful and patent filtering procedure. At the five-month follow-up the trabeculectomy was smooth and regular and there was no evidence of inflammation or thermal damage on the surrounding structures.

Radial keratotomy: 500 consecutive cases

A report on 500 consecutive eyes having radial keratotomy for myopia and myopic astigmatism is presented. Surgery was on an out-patient basis under local anaesthetic, with a follow-up of 99.2% of cases, 91.8% of the 294 patients elected to have radial keratotomy on the fellow eye. The surgical protocol was designed to minimise side effects and complications rather than maximise the reduction in myopia. 6/12 unaided vision or better was achieved by 99% of cases in the low myopia group, 91% in the middle and 41% in the high myopia group. Persistent hypermetropia of +1.00D or greater occurred in only 0.4% of cases and secondary astigmatism of 1.00D or greater in 1% of cases. Corneal perforations were minimised by single-pass incisions, but this was at the cost of lesser myopic corrections in the higher myopic group. Centering on the visual axis rather than the centre of the pupil minimised glare sensitivity. No patient developed potentially blinding complications.

Corneal ablation in rabbits using an infrared (2.9-microns) erbium: YAG laser

The authors used a 2.9-microns infrared erbium:YAG (ER:YAG) laser to ablate the central corneas (a 3.5-mm wide, 180-microns deep area) of ten albino rabbits. In group 1 (10 eyes), the central epithelium was removed by mechanical scrubbing to ablate the anterior stroma. In group 2 (10 eyes), a central anterior corneal cap was removed to ablate the midstroma. In group 1, epithelial wound healing occurred in 2 to 3 days. A mild anterior haze was observed with the slit lamp, but it gradually cleared in most eyes of both groups. Histologic specimens were obtained immediately and at 2 and 6 months after ablation for light and transmission and scanning electron microscopy. This acute histopathologic study did not show any endothelial damage even after ablations at 300-microns depth, although there was thermal damage more prominent at the borders of the ablated area for an extension of 40 microns. After 2 months, in the group 1 eyes, the regenerated epithelium appeared normal except for a thickening at the margins of the ablated area. Six months postoperatively there was still epithelial thickening at the margins of the remodeled zone. The basement membrane was present with anchoring filaments and hemidesmosomes. The healing of the anterior (group 1) and midstroma (group 2) occurred with some degree of disorganization of the lamellae and an increase in the population of keratocytes. The density of keratocytes at the ablated areas at 6 months was reduced in comparison with the 2-month observation. By photokeratoscopy, the corneas were smooth and slightly flattened. The authors conclude that corneal ablation using an ER: YAG laser at 2.9 microns may meet the criteria for photorefractive keratectomy (PRK).

Potential use of lasers for penetrating keratoplasty

Experimental corneal trephination has been achieved with the 193 nm argon fluoride excimer and 2.9 microns hydrogen fluoride and Er:YAG laser systems. Compared with metal blades and other lasers, the 193 nm excimer laser creates the best quality corneal excision, but has a relatively slow etch rate through the stroma, and its use requires toxic gas. The mid-infrared laser systems trephine the cornea in less than 10 seconds, but cause a 10 microns to 15 microns zone of adjacent stromal damage and create wounds that are approximately 2.5 times larger than wounds made by metal scalpels. The wavelength and laser pulse duration influence the cutting characteristics of the laser. Optical delivery systems using an axicon lens, a rotating slit, and a computer controlled scanning optical system have been developed for penetrating keratoplasty. Selection of the optimal laser system for penetrating keratoplasty must await further experimental studies. Refinements of the laser cavity and delivery system are necessary before clinical studies can begin. A carefully controlled randomized clinical trial comparing laser trephination with conventional mechanical trephines will be necessary to determine the safety and efficacy of a laser trephination system.

Human excimer laser lamellar keratectomy. A clinical study

The first ten blind human eyes in the United States to receive excimer laser (ArFl 193 nm) lamellar keratectomy (reprofiling) are presented. Seven of these patients were followed 6 to 12 months after ablation. All eyes are grossly clear in the region of ablation. Results of slit-lamp examination of all flattened ablated areas show mild superficial haze at the epithelial/stromal interface. This haze might not interfere significantly with vision in patients 7 to 10. Serial pachymetry and keratometry measurements, refraction, and digital keratoscopy show a progressive filling in of the excavated area by approximately two thirds but a loss of initial diopteric correction of only one third. Histopathologic analysis was obtained for four eyes. Transmission electron microscopy of three eyes enucleated 3 to 12 days after ablation shows 40-microns ablation depths through Bowman's layer and superficial stroma with minimal adjacent tissue damage and no inflammatory cells. The epithelium is increased in thickness by 50%, and firmly attached to the underlying stroma. A 4-month postablation specimen shows keratocyte activation with increased protein synthesis (presumed collagen and ground substance).

Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage

The thermal damage caused by 2.94-micron Er:YAG laser ablation of skin, cornea, aorta, and bone was quantified. The zone of residual thermal damage produced by normal-spiking-mode pulses (pulse duration approximately 200 microseconds) and Q-switched pulses (pulse duration approximately 90 ns) was compared. Normal-spiking-mode pulses typically leave 10-50 microns of collagen damage at the smooth wall of the incisions; however, at the highest fluences (approximately 80J/cm2) tears were produced in cornea and aorta and as much as 100 microns of damaged collagen is found at the incision edge. Q-switched pulses caused less thermal damage, typically 5-10 microns of damage in all tissues.

Excimer ablation of human intervertebral disc at 308 nanometers

Excimer laser energy, which has been shown to photoablate tissue at a precisely controllable rate with minimal thermal damage, was applied to human intervertebral disc in an effort to develop a technique for percutaneous discectomy. Cadaveric samples of human disc were used. Excimer laser energy was produced by a XeCl, magnetically switched, long-pulse laser working at 308 nm, 20 Hz. Annulus tissue of approximately 1 mm thickness was placed in contact with the output tip of a 400 microns core diameter quartz fiber, and measurements of ablation rate were made at different radiant exposures. Ablation rates were found to vary linearly with radiant exposure, from 0.7 micron/pulse at 10 mJ/mm2 to 11.0 microns/pulse at 55 mJ/mm2, with a correlation coefficient of 0.984. Threshold radiant exposure, calculated by extrapolation, was found to be about 7 mJ/mm2. Histologic analysis showed a minimum of thermal damage in these specimens, and when ablated with modification to maintain constant fiber-tissue contact, thermal injury was nearly absent, as compared to samples ablated with Nd:YAG through a contact probe. Thermographic analysis, performed using the AGA 782 Digital Thermography system, showed increasing temperature with increasing radiant exposure, with a maximum temperature of 47.2 degrees C at 55 mJ/mm2. In that precise tissue ablation was demonstrated with minimal generated heat, and excimer energy at 308 nm is transmissible through fiber optics, excimer holds great promise for the development of a percutaneous discectomy technique.

Long-term healing of the central cornea after photorefractive keratectomy using an excimer laser

Using an excimer laser at 193 nm, 3-mm diameter discs were ablated from the optical zone of monkey corneas at a variety of depths up to 130 micron. Tissue was examined by light and electron microscopy 4 and 5 weeks and 6 and 8 months postoperatively. With the exception of the deepest ablation, all corneas were clear immediately postoperatively. At 1 month, ablations of 40 micron remained clear, whereas all others showed some degree of haze. By 3 months, discs at 60 micron depth were clear, and there was less haze in others. Haze was progressively lost over 6 months, but in the deepest discs it could still be discerned on slit-lamp examination. Microscopic observation showed that reepithelialization had occurred within 24 to 48 hours. Over subsequent months, a normal morphology was maintained in this layer with the exception that basal cells were slightly more elevated particularly at the disc margins where the epithelium contained more cell layers. Stromal reorganization was accompanied by an initial phase of vacuolation and invasion by keratocytes, but by 6 months postoperatively almost all vacuolation had disappeared and keratocyte numbers had almost returned to normal. By 8 months, the morphology was near normal with the exception that Bowman's membrane was absent and there was still a degree of disorder in the immediate subepithelial stromal fibers. The basement membrane of epithelial cells was reestablished but slightly more undulant than in nonirradiated areas. This undulation was retained in the 8-month postoperative specimens and at this time a normal thickness of 60 nm was displayed.

Excimer laser trephination in penetrating keratoplasty. Morphologic features and wound healing

The imprecision of trephination of donor and recipient corneas is a major factor in post-keratoplasty astigmatism. In order to improve the quality of trephination, the authors developed a rotating slit delivery system for noncontact penetrating keratoplasty trephination using the excimer laser at 193 nm. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and light microscopy (LM) demonstrated the superior quality of excimer-cut buttons and recipient beds as compared with those obtained by free hand and suction trephines in human cadaver and rabbit eyes. The laser trephined more regularly and precisely without distortion of corneal topography and with less damage to adjacent corneal tissue. The authors morphologically examined wound healing at 6 hours, 12 hours, 3 days, 5 days, 2 weeks, 2 months, and 3 months after penetrating keratoplasty with laser and mechanical trephination in an animal autograft model. The laser did not cause any adverse alteration of wound healing processes including cellular migration, proliferation, and production of new tissue.

Wound healing following excimer laser radial keratotomy

A pulsed excimer laser was used to produce radial keratotomy in vivo in human corneas and the wound healing process was studied. Fluences of 370 mJ/cm2 at a repetition rate of 20 Hz were effective and atraumatic. The wound healing process was remarkably uneventful without an inflammatory or immune reaction apparent at 21 days. Consistent flattening of the cornea was obtained by shallower cuts than with conventional radial keratotomy techniques.