Scleral growth stunting via sub-Tenon injection of cross-linking solutions in live rabbits

BACKGROUND

Scleral cross-linking is a potential method to inhibit axial elongation of the eye, preventing the progression of pathological myopia. Formaldehyde releasers, which are common preservatives found in cosmetics and ophthalmic solutions, have been shown to be not only effective in cross-linking corneal collagen in vitro and in vivo, but also have minimal toxicity effects on the eye. The present study aims to evaluate the efficacy of scleral cross-linking using sodium hydroxymethylglycinate (SMG) to inhibit eye growth using an in vivo rabbit model.

METHODS

A cross-linking solution containing 40 mM SMG was delivered to the sub-Tenon's space behind the equator. The application regimen included a two-quadrant injection performed five times over 2 weeks on New Zealand White rabbits (n=5, group 1), and one-time injection followed for up to 5 days on Dutch-Belted rabbits (n=6, group 2). Group 1 was monitored serially for axial length changes using B-scan ultrasound for 5-6 weeks. Group 2 was injected with a higher viscosity solution formulation. Both groups were evaluated for thermal denaturation temperature changes of the sclera postmortem.

RESULTS

Axial growth was limited by 10%-20% following SMG treatment as compared with the untreated eye. Thermal denaturation analysis showed increased heat resistance of the treated eyes in the areas of injection. Overall, the SMG treatment inhibited eye growth with few side effects from the injections.

CONCLUSIONS

Cross-linking solutions delivered via sub-Tenon injection provide a potential method for limiting axial length growth in progressive myopia and could be used as a potential treatment for myopia.

Topical Corneal Cross-Linking Solution Delivered Via Corneal Reservoir in Dutch-Belted Rabbits

Purpose

A topical corneal cross-linking solution that can be used as an adjunct or replacement to standard photochemical cross-linking (UV-riboflavin) methods remain an attractive possibility. Optimal concentration and delivery method for such topical corneal stabilization in the living rabbit eye were developed.

Methods

A series of experiments were carried out using Dutch-belted rabbits (3 months old, weighing 1.0–1.5 kg) and topical cross-linking solutions (sodium hydroxymethylglycinate) (10–250 mM) delivered via corneal reservoir. The application regimen included a one-time 30-minute application (10–40 mM sodium hydroxymethylglycinate) as well as a once per week 5-minute application (250 mM sodium hydroxymethylglycinate) for 7 weeks. Animals were evaluated serially for changes in IOP, pachymetry, epithelial integrity, and endothelial cell counts. Keratocyte changes were identified using intravital laser scanning confocal microscopy. Post mortem efficacy was evaluated by mechanical inflation testing.

Results

Overall, there were very few differences observed in right eye treated versus left eye controls with respect to intraocular pressure, pachymetry, and endothelial cell counts, although 30-minute cross-linking techniques did cause transient increases in thickness resolving within 7 days. Epithelial damage was noted in all of the 30-minute applications and fully resolved within 72 hours. Keratocyte changes were significant, showing a wound healing pattern similar to that after riboflavin UVA photochemical cross-linking in rabbits and humans. Surprisingly, post mortem inflation testing showed that the lower concentration of 20 mM delivered over 30 minutes showed the most profound stiffening/strengthening effect.

Conclusions

Topical cross-linking conditions that are safe and can increase corneal stiffness/strength in the living rabbit eye have been identified.

Translational Relevance

A topical corneal cross-linking solution delivered via corneal reservoir is shown to be both safe and effective at increasing tissue strength in living rabbit eyes and could now be tested in patients suffering from keratoconus and other conditions marked by corneal tissue weakness.

Ex vivo anti-microbial efficacy of various formaldehyde releasers against antibiotic resistant and antibiotic sensitive microorganisms involved in infectious keratitis

BACKGROUND

Corneal infections with antibiotic-resistant microorganisms are an increasingly difficult management challenge and chemically or photochemically cross-linking the cornea for therapy presents a unique approach to managing such infections since both direct microbial pathogens killing and matrix stabilization can occur simultaneously. The present study was undertaken in order to compare the anti-microbial efficacy, in vitro, of 5 candidate cross-linking solutions against 5 different microbial pathogens with relevance to infectious keratitis.

METHODS

In vitro bactericidal efficacy studies were carried out using 5 different FARs [diazolidinyl urea (DAU), 1,3-bis(hydroxymethyl)-5,5-dimethylimidazolidine-2,4-dione (DMDM), sodium hydroxymethylglycinate (SMG), 2-(hydroxymethyl)-2-nitro-1,3-propanediol (NT = nitrotriol), 2-nitro-1-propanol (NP)] against 5 different microbial pathogens including two antibiotic-resistant species [methicillin-sensitive Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), Pseudomonas aeruginosa (PA), and Candida albicans (CA)]. Standard in vitro antimicrobial testing methods were used.

RESULTS

The results for MSSA were similar to those for MRSA. DAU, DMDM, and SMG all showed effectiveness with greater effects generally observed with longer incubation times and higher concentrations. Against MRSA, 40 mM SMG at 120 min showed a > 95% kill rate, p < 0.02. Against VRE, 40 mM DAU for 120 min showed a > 94% kill rate, p < 0.001. All FARs showed bactericidal effect against Pseudomonas aeruginosa, making PA the most susceptible of the strains tested. Candida showed relative resistance to these compounds, requiring high concentrations (100 mM) to achieve kill rates greater than 50%.

CONCLUSION

Our results show that each FAR compound has different effects against different cultures. Our antimicrobial armamentarium could potentially be broadened by DAU, DMDM, SMG and other FARs for antibiotic-resistant keratitis. Further testing in live animal models are indicated.

Corneal collagen crosslinking in patients treated with dextran versus isotonic hydroxypropyl methylcellulose (HPMC) riboflavin solution: a retrospective analysis

Background

Corneal collagen crosslinking (CXL) is a widely used treatment for halting the progression of keratoconus. Although initial studies of CXL were performed with a riboflavin solution containing dextran, recent protocols for CXL have indicated the use of a riboflavin solution containing isotonic hydroxypropyl methylcellulose (HPMC). This study was performed to investigate differences in visual outcomes and Scheimpflug (Pentacam) analysis in patients who have undergone epithelium-off CXL with riboflavin solution containing either 20% dextran versus 1.1% HPMC.

Methods

All patients in this non-randomized, non-masked, retrospective cohort analysis were treated at Edward S. Harkness Eye Institute, Columbia University Medical Center, New York, NY, USA. Thirty-seven eyes of 33 patients were crosslinked with a dextran solution and 19 eyes of 19 patients crosslinked with an isotonic HPMC solution, both using an epithelium-off 30-min, 3 mW/cm² protocol. All patients had a diagnosis of keratoconus or post-refractive surgery ectasia. Best spectacle corrected visual acuity (BSCVA) and Pentacam parameters were compared at all follow up visits (1, 6, 12, and 24 months). Differences between groups treated with HPMC and dextran were compared using student’s t-test. Differences between treated eye and fellow eye were calculated and compared between HPMC and dextran groups using paired t-test.

Results

Patients treated with a dextran solution had significantly greater improvement in BSCVA at 1, 6, and 24 months (p < 0.05) compared to the isotonic HPMC-treated group. Kmax increased in both groups at 1 month; however, HPMC-treated patients had a greater increase compared to dextran-treated patients (p = 0.01). Kmax decreased in both groups at 6 and 12 months, although this finding was only significant in the HPMC-treated group at 12 months.

Conclusions

Our data suggest that crosslinking with the dextran solution may result in significantly better visual outcomes (demonstrated by visual acuity) compared to the isotonic HPMC riboflavin solution. Dextran solutions may have other potential advantages intrinsic to its biochemical properties facilitating more efficient crosslinking. Further research and long-term evidence regarding the use of dextran versus HPMC riboflavin solutions in collagen crosslinking is necessary.

Antimicrobial Studies Using the Therapeutic Tissue Cross-Linking Agent, Sodium Hydroxymethylglycinate: Implication for Treating Infectious Keratitis

Purpose

Our recent studies raise the possibility of using sodium hydroxymethylglycinate (SMG), for pharmacologic therapeutic tissue cross-linking (TXL) of the cornea. The present study was performed to evaluate the antimicrobial effects of SMG for potential use in treating infectious keratitis.

Methods

In initial (group 1) experiments, methicillin-sensitive Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus (MRSA), and Pseudomonas aeruginosa (PA) were treated with SMG (10–40 mM) for 10 to 120 minutes. In group 2 experiments, MRSA, PA, Candida albicans (CA), and vancomycin-resistant Enterococcus (VRE) were treated with SMG (20–200 mM) for 30 minutes. In group 2 experiments, BSA and neutralizing buffer were added to provide a proteinaceous medium, and to ensure precise control of SMG exposure times, respectively. SMG effectiveness was quantitated based on pathogen growth following a 24- to 48-hour incubation period.

Results

In group 1 experiments, as expected, time- and concentration-dependent bactericidal effects were noted using MSSA. In addition, the effect of SMG (40 mM) was greatest against MSSA (99.3%), MRSA (96.0%), and PA (97.4%) following a 2-hour exposure with lesser effects following 30- and 10-minute exposures. In group 2 experiments, concentration-dependent bactericidal effects were confirmed for MRSA (91%), PA (99%), and VRE (55%) for 200-mM SMG with 30-minute treatment. SMG was not as effective against CA, with a maximum kill rate of 37% at 80 mM SMG.

Conclusions

SMG solution exhibits a dose-dependent bactericidal effect on MSSA, MRSA, and PA, with milder effects on VRE and CA. These studies raise the possibility of using SMG TXL for the treatment of infectious keratitis.

Evaluation of Therapeutic Tissue Crosslinking (TXL) for Myopia Using Second Harmonic Generation Signal Microscopy in Rabbit Sclera

Purpose

Second harmonic generation signals (SHG) are emitted preferentially from collagenous tissue structures and have been used to evaluate photochemically-induced (CXL) crosslinking changes in the cornea. Since therapeutic tissue crosslinking (TXL) using sodium hydroxymethylglycinate (SMG) of the sclera is a potential treatment for high myopia, we explored the use of SHG microscopy to evaluate the effects.

Methods

Single sub-Tenon's (sT) injections (400 μL) using SMG (40-400 mM) were made at the equatorial 12 o'clock position of the right eye of cadaveric rabbit heads (n = 16 pairs). After 3.5 hours, confocal microscopy (CM) was performed using 860 nm two-photon excitation and 400 to 450 nm emission. Pixel density and fiber bundle "waviness" analyses were performed on the images. Crosslinking effects were confirmed using thermal denaturation (Tm) temperature. Comparison experiments with riboflavin photochemical crosslinking were done.

Results

Therapeutic tissue crosslinking localization studies indicated that crosslinking changes occurred at the site of injection and in adjacent sectors. Second harmonic generation signals revealed large fibrous collagenous bundled structures that displayed various degrees of waviness. Histogram analysis showed a nearly 6-fold signal increase in 400 mM SMG over 40 mM. This corresponded to a ΔTm = 13°C for 400 mM versus ΔTm = 4°C for 40 mM. Waviness analysis indicated increased fiber straightening as a result of SMG CXL.

Conclusions

Second harmonic generation signal intensity and fiber bundle waviness is altered by scleral tissue crosslinking using SMG. These changes provide insights into the macromolecular changes that are induced by therapeutic crosslinking technology and may provide a method to evaluate connective tissue protein changes induced by scleral crosslinking therapies.

Laser-Induced Photic Injury Phenocopies Macular Dystrophy

OBJECTIVE

To describe the phenotypes associated with laser-induced retinal damage in children.

METHODS

Five patients with maculopathy and reduced visual acuity associated with laser pointer use were evaluated. Best-corrected visual acuity, retinal structure, and function were monitored with color fundus, infrared (IR), and red-free images, fundus autofluorescence (AF), spectral domain-optical coherence tomography (SD-OCT), and full-field electroretinography (ERG).

RESULTS

All five laser pointer injury patients had retinal lesions resembling a macular dystrophy (one bilateral and four unilateral). These lesions were irregular in shape but all had a characteristic dendritic appearance with linear streaks radiating from the lesion. Photoreceptor damage was present in all patients, but serial OCT monitoring showed that subsequent photoreceptor recovery occurred over time in the eyes of at least four patients. One patient also had bilateral pigment epithelial detachments (PED). Both hyper- and hypoautofluorecence were observed in the laser damage area.

CONCLUSIONS

In general, OCT and IR images are quite useful to diagnose laser damage, but AF is not as sensitive. Laser pointer damage in children can occasionally be misdiagnosed as a macular dystrophy disease, but the distinctive lesions and OCT features are helpful for differentiating laser damage from other conditions.

Vitrectomy for macular hole following Nd:YAG laser injury

The Q-switched Nd:YAG laser can cause significant ocular injury, because it can emit an invisible and powerful load of energy in a short period of time. One of these injuries is macular hole formation. We report the case of a 21-year-old woman who presented with acute floaters in her left eye after exposure to a Q-switched Nd:YAG laser. Her initial best-corrected visual acuity (BCVA) in the left eye was 20/80. Examination demonstrated an acute vitreous hemorrhage, and spectral-domain optical coherence tomography (SD-OCT) confirmed a full-thickness macular hole (FTMH). Four months after her injury, her BCVA deteriorated to 20/400, and she underwent vitrectomy, internal limiting membrane peeling, and gas injection. Three weeks following the procedure, her FTMH was closed, however, her BCVA remained 20/150. At her last office visit, 19 months after the surgery, the examination was unchanged. There are few reports of vitrectomy to close an FTMH after Nd:YAG laser exposure, and the factors that contribute to visual recovery remain unclear. The aim of this case report is to emphasize the importance of early diagnosis and surgical repair, and to review the literature and surgical outcomes of cases of Nd:YAG laser-induced macular holes. Imaging with SD-OCT was essential in evaluating the visual outcome, as it was dependent on the degree of photoreceptor and retinal pigment epithelium injury.

An Evaluation of Lysyl Oxidase-Derived Cross-Linking in Keratoconus by Liquid Chromatography/Mass Spectrometry

Purpose

Current literature contains scant information regarding the extent of enzymatic collagen cross-linking in the keratoconus (KC) cornea. The aim of the present study was to examine levels of enzymatic lysyl oxidase–derived cross-links in stromal collagen in KC tissue, and to correlate the cross-link levels with collagen fibril stability as determined by thermal denaturation temperature (T_m).

Methods

Surgical KC samples (n = 17) and Eye-Bank control (n = 11) corneas of age 18 to 68 years were analyzed. The samples were defatted, reduced (NaBH₄), hydrolyzed (6N HCl at 110°C for 18 hours), and cellulose enriched before analysis by C8 high-performance liquid chromatography equipped with parallel fluorescent and mass detectors in selective ion monitoring mode (20 mM heptafluorobutyric acid/methanol 70:30 isocratic at 1 mL/min). Nine different cross-links were measured, and the cross-link density was determined relative to collagen content (determined colorimetrically). The T_m was determined by differential scanning calorimetry.

Results

Cross-links detected were dihydroxylysinonorleucine (DHLNL), hydroxylysinonorleucine, lysinonorleucine (LNL), and histidinohydroxylysinonorleucine in both control and KC samples. Higher DHLNL levels were detected in KC, whereas the dominant cross-link, LNL, was decreased in KC samples. Decreased LNL levels were observed among KC ≤ 40 corneas. There was no difference in total cross-link density between KC samples and the controls. Pyridinolines, desmosines, and pentosidine were not detected. There was no notable correlation between cross-link levels with fibril instability as determined by T_m.

Conclusions

Lower levels of LNL in the KC cornea suggest that there might be a cross-linking defect either in fibrillar collagen or the microfibrillar elastic network composed of fibrillin.

Cosmetic preservatives as therapeutic corneal and scleral tissue cross-linking agents

PURPOSE

Previously, aliphatic β-nitroalcohols (BNAs) have been studied as a means to chemically induce tissue cross-linking (TXL) of cornea and sclera. There are a number of related and possibly more potent agents, known as formaldehyde releasers (FARs), that are in commercial use as preservatives in cosmetics and other personal care products. The present study was undertaken in order to screen such compounds for potential clinical utility as therapeutic TXL agents.

METHODS

A chemical registry of 62 FARs was created from a literature review and included characteristics relevant to TXL such as molecular weight, carcinogenicity/mutagenicity, toxicity, hydrophobicity, and commercial availability. From this registry, five compounds [diazolidinyl urea (DAU), imidazolidinyl urea (IMU), sodium hydroxymethylglycinate (SMG), DMDM hydantoin (DMDM), 5-Ethyl-3,7-dioxa-1-azabicyclo [3.3.0] octane (OCT)] were selected for efficacy screening using two independent systems, an ex vivo rabbit corneal cross-linking simulation setup and incubation of cut scleral tissue pieces. Treatments were conducted at pH 7.4 or 8.5 for 30 minutes. Efficacy was evaluated using thermal denaturation temperature (Tm), and cell toxicity was studied using the trypan blue exclusion method.

RESULTS

Cross-linking effects in the five selected FARs were pH and concentration dependent. Overall, the Tm shifts were in agreement with both cornea and sclera. By comparison with BNAs previously reported upon, the FARs identified in this study were significantly more potent but with similar or better cytotoxicity.

CONCLUSIONS

The FARs, a class of compounds well known to the cosmetic industry, may have utility as therapeutic TXL agents. The compounds studied thus far show promise and will be further tested.

Pharmacologic alternatives to riboflavin photochemical corneal cross-linking: a comparison study of cell toxicity thresholds

PURPOSE

The efficacy of therapeutic cross-linking of the cornea using riboflavin photochemistry (commonly abbreviated as CXL) has caused its use to become widespread. Because there are known chemical agents that cross-link collagenous tissues, it may be possible to cross-link tissue pharmacologically. The present study was undertaken to compare the cell toxicity of such agents.

METHODS

Nine topical cross-linking agents (five nitroalcohols, glyceraldehyde [GLYC], genipin [GP], paraformaldehyde [FA], and glutaraldehyde [GLUT]) were tested with four different cell lines (immortalized human corneal epithelial cells, human skin fibroblasts, primary bovine corneal endothelial cells, and immortalized human retinal pigment epithelial cells [ARPE-19]). The cells were grown in planar culture and exposed to each agent in a range of concentrations (0.001 mM to 10 mM) for 24 hours followed by a 48-hour recovery phase. Toxicity thresholds were determined by using the trypan blue exclusion method.

RESULTS

A semiquantitative analysis using five categories of toxicity/fixation was carried out, based on plate attachment, uptake of trypan blue stain, and cellular fixation. The toxicity levels varied by a factor of 10(3) with the least toxic being mononitroalcohols and GLYC, intermediate toxicity for a nitrodiol and nitrotriol, and the most toxic being GLUT, FA, GP, and bronopol, a brominated nitrodiol. When comparing toxicity between different cell lines, the levels were generally in agreement.

CONCLUSIONS

There are significant differences in cell toxicity among potential topical cross-linking compounds. The balance between cross-linking of tissue and cell toxicity should be borne in mind as compounds and strategies to improve mechanical tissue properties through therapeutic tissue cross-linking continue to develop.

Aliphatic β-nitroalcohols for therapeutic corneoscleral cross-linking: chemical stability studies using 1H-NMR spectroscopy

Recent studies suggest that aliphatic β-nitro alcohols may represent a useful class of compounds for use as in vivo therapeutic corneoscleral cross-linking agents with higher order nitroalcohols (HONAs) showing enhanced efficacy over the mono-nitroalcohols. The current study was undertaken in order to evaluate the chemical stability of these compounds during storage conditions. Two mono-nitroalcohols (2-nitroethanol=2ne and 2-nitro-1-propanol = 2 nprop) and two HONAs, a nitrodiol (2-methyl-2-nitro-1,3-propanediol = MNPD), and a nitrotriol (2-hydroxymethyl-2-nitro-1,3-propanediol = HNPD) were monitored for chemical stability by (1)H-NMR for up to 7 months. Each compound was studied at two concentrations (1% and 10%) either in unbuffered H2O or 0.2 m NaH2 PO4/Na2 HPO4 (pH = 5), and at 0 °C and room temperature (RT) for a total of eight conditions for each compound. The (1)H-NMR spectra for the starting material were compared to subsequent spectra. Under all four of the conditions studied, both the nitrodiol (MNPD) and nitrotriol (HNPD) were stable for the duration of 7 months. 2nprop became unstable under all conditions at 3 months. 2ne was the most unstable of all the compounds tested. HONAs exhibit excellent chemical stability under long-term storage conditions. In contrast, the nitromonols tested are significantly less stable. These findings are relevant to the translation of this technology into clinical use.

Aliphatic beta-nitroalcohols for therapeutic corneoscleral cross-linking: chemical mechanisms and higher order nitroalcohols

PURPOSE

The recent tissue cross-linking studies indicate that aliphatic beta-nitroalcohols (BNAs) may be useful as pharmacologic corneoscleral cross-linking agents. The present study was performed to identify the specific chemistry involved under physiologic conditions, with the intent of identifying more effective agents.

METHODS

The mechanism of chemical cross-linking at pH 7.4 and 37 degrees C was studied using three techniques. The colorimetric Griess assay was used to follow the release of nitrite from three mono-nitroalcohols (2-nitroethanol [2ne], 2-nitro-1-propanol [2nprop]), and 3-nitro-2-pentanol [3n2pent]). Second, the evolution of 2nprop in 0.2 M NaH(2)PO(4)/Na(2)HPO(4)/D(2)O was studied using (1)H-NMR. Third, thermal shrinkage temperature analysis (T(s)), a measure of tissue cross-linking, was used to support information from (1)the H-NMR studies.

RESULTS

A time-dependent release of nitrite was observed for all three mono-nitroalcohols studied. The maximum levels were comparable using either 2ne or 2nprop (approximately 30%). However, much less (approximately 10%) was observed from 3n2pent. Using (1)H-NMR, 2nprop evolved into a unique splitting pattern. No match was observed with reference spectra from three possible products of denitration. In contrast, 2-methyl-2-nitro-1,3-propanediol (MNPD), a nitro-diol, was identified, implying the formation of formaldehyde from a retro-nitroaldol (i.e., reverse Henry) reaction. In support of this mechanism, T(s) shifts induced by the nitro-triol 2-hydroxymethyl-2-nitro-1,3-propanediol (HNPD) were superior to the nitro-diol MNPD which were superior to the mono nitroalcohol 2nprop.

CONCLUSIONS

BNAs function as both formaldehyde and nitrite donors under physiologic conditions to cross-link collagenous tissue. Higher order BNAs are more effective than mono nitroalcohols, raising the possibility of using these agents for therapeutic corneoscleral cross-linking.

Short chain aliphatic beta-nitro alcohols for corneoscleral cross-linking: corneal endothelial toxicity studies

PURPOSE

Recent studies suggest that short chain aliphatic beta-nitro alcohols can cross-link corneoscleral tissue under physiologic conditions. The present study was conducted to determine the cytotoxic threshold for these agents in vitro and to draw comparisons to commonly used topical ophthalmic agents.

METHODS

Primary cultures of bovine corneal endothelial cells were grown to confluence in 24- and 96-well plates using standard protocol. The cells were exposed to three beta-nitro alcohols, 2-nitroethanol, 2-nitro-1-propanol, and 3-nitro-2-pentanol in a range of concentrations from 0.1 to 10 mM. After a 48-hour exposure, cell necrosis and apoptosis were evaluated using trypan blue, propidium iodide, and annexin V staining. In addition, a review of the ophthalmic literature was conducted to derive comparisons with agents commonly used in clinical practice.

RESULTS

An all-or-none response was observed for each compound. Positive staining with trypan blue, propidium iodide, and annexin V occurred at identical concentrations. The most toxic of the group was 2-nitro-1-propanol. The cytotoxic level for 2-nitroethanol, 2-nitro-1-propanol, and 3-nitro-2-pentanol was 3 mM (0.0273%), 1 mM (0.0105%), and 3 mM (0.0399%), respectively. Furthermore, by comparison with several agents used in ophthalmic practice such as fluoroquinolone antibiotics, anti-proliferative agents, and benzalkonium chloride, the beta-nitro alcohols exhibit less toxicity in vitro.

CONCLUSIONS

Short chain aliphatic beta-nitro alcohols exhibit favorable in vitro toxicity thresholds. The result of this study encourages further evaluation of these compounds as potential pharmacologic topical stiffening agents for corneoscleral disorders.

Initial studies using aliphatic beta-nitro alcohols for therapeutic corneal cross-linking

PURPOSE

Corneal collagen cross-linking through UVA-riboflavin photochemistry (UVAR) has been shown to be an effective treatment for keratoconus and related keratectasias. In recent studies using sclera, the authors observed that short-chain aliphatic beta-nitro alcohols can cross-link collagenous tissue under physiologic conditions. Thus, this study was undertaken to evaluate these agents as potential pharmacologic alternatives to UVAR.

METHODS

Porcine corneal strips (8 x 4 mm) and corneoscleral complexes were cross-linked using 1 to 100 mM 2-nitroethanol (2ne), 2-nitro-1-propanol (2nprop), and 3-nitro-2-pentanol (3n2pent) at pH 7.4, 34 degrees C. Cross-linking by UVAR was carried out for comparison. Thermal shrinkage temperature analysis was used to evaluate cross-linking effects, and changes in corneal light transmission were determined with a fiber-optic spectrophotometer.

RESULTS

At 10 and 100 mM for 96 hours, initial shrinkage temperature (T(i)) was shifted by 3.3 degrees C (P < 0.001) and 9.8 degrees C (P < 0.001) for 2ne, 2.9 degrees C (P = 0.008) and 4.9 degrees C (P < 0.001) for 2nprop, and 3.8 degrees C (P = 0.003) and 4.9 degrees C (P < 0.001) for 3n2pent. Reacting at 1 mM through daily exchange of fluid over 7 days shifted T(i) by 3.8 degrees C (P < 0.001), 4.4 degrees C (P = 0.002), and 3.2 degrees C (P = 0.005), for 2ne, 2nprop, and 3n2pent, respectively. These shifts were greater than cross-linking using UVAR (T(i) = 1.9 degrees C; P = 0.012). In the blue light region (400-500 nm), transmission was decreased by 5.6% (P = 0.003), 2.1% (P = 0.260), and 0% (P = 0.428) for 2ne, 2nprop, and 3n2pent, respectively.

CONCLUSIONS

beta-Nitro alcohols can induce corneal cross-linking in vitro better than the UVAR technique and can induce negligible effects on light transmission. These early results suggest that such compounds could be used as topical stiffening agents for keratoconus and related disorders.

Evaluating central corneal thickness measurements with noncontact optical low-coherence reflectometry and contact ultrasound pachymetry

PURPOSE

To compare the central corneal thickness (CCT) measurements obtained with noncontact optical low-coherence reflectometry (OLCR) and ultrasound (US) pachymetry.

DESIGN

Prospective, comparative observational study.

METHODS

Three sequential ultrasonic measurements and a set of five OLCR scans of 52 eyes of 26 healthy subjects were recorded. Noncontact measurement was repeated five minutes after anesthetic drop instillation.

RESULTS

Mean CCT values for noncontact OLCR and US pachymetry were 544.03 microm and 548.66 microm, respectively, with mean SDs of 0.97 microm and 4.63 microm, respectively. Noncontact OLCR measured on average 4.64 microm less than US pachymetry (95% confidence interval -7.56 to -1.72; P = .003). The OLCR measured 1.68 microm less than US pachymetry in the thinner cornea group (< or =548.7 microm, n = 24) and 7.48 mum less in the thicker group (n = 25). This difference was statistically significant (P = .04).

CONCLUSIONS

There was agreement between the two pachymetric measurements. Noncontact OLCR appeared to measure slightly smaller than US pachymetry.

Clinical Significance of Central Corneal Thickness in the Managementof Glaucoma

OBJECTIVE

To evaluate the effect of central corneal thickness determination on the clinical management of patients with glaucoma and glaucoma suspect.

METHODS

A cross-sectional retrospective study was performed on 188 consecutive patients. Mean ultrasound pachymetry measurements of central corneal thickness and corresponding Goldmann applanation tonometry measurements were obtained. Intraocular pressures (IOPs) were corrected using linear and mathematical (Orssengo-Pye) algorithms. Measurement-significant outcomes were defined as an IOP adjustment of 1.5 mm Hg or greater and outcomes-significant results as an IOP adjustment of 3.0 mm Hg or greater. Changes in therapy such as the use of eyedrops and addition or cancellation of laser therapy or surgery were then noted for those individuals with measurement- or outcomes-significant changes.

RESULTS

Using the linear correction scale, 105 (55.9%) of 188 patients had at least a measurement-significant adjustment in their IOP measurements: 67 (35.6%) had adjustments between 1.5 and 3.0 mm Hg, while 38 (20.2%) had an outcomes-significant IOP adjustment (> or =3.0 mm Hg). Among the 188 patients, 16 (8.5%) had a change in eyedrop therapy, 4 (2.1%) had a change regarding laser therapy, and 6 (3.2%) had a change in the decision regarding glaucoma surgery. Using the exponential correction (Orssengo-Pye) scale, similar percentages were obtained.

CONCLUSION

Pachymetry-measured central corneal thickness has a significant effect on the clinical management of patients with glaucoma and glaucoma suspect.

Isolation of human fetal cones

PURPOSE

To describe a method for isolating a monolayer of human fetal cone photoreceptors and to compare their structure and ultrastructure before and after preparation.

METHODS

Eyes from human fetuses (fetal week 20 to 24) were dissected and the neural retina of the developing fovea identified, cut out and placed on 10% gelatin. A VISX Star excimer laser was used to remove the inner retinal layers. The isolated cone monolayers were cultured for 18 hours and compared with untreated retinas by light microscopy and transmission electron microscopy.

RESULTS

Excimer laser ablation removed the inner nuclear and ganglion cell layers leaving a monolayer of fetal cones. These cones survive in culture for at least 18 hours. The laser ablation disorganized the ultrastructure of the synaptic pedicles of these cones, left their plasma membranes intact.

CONCLUSIONS

The developing central retina of human fetal eyes provides a source of fetal cones, which can be isolated from inner retinal cells using the excimer laser. Such a monolayer of human fetal cones may be useful for transplantation or biochemical studies.

Reversal of dysthyroid optic neuropathy following orbital fat decompression

AIMS

To document the successful treatment of five patients with dysthyroid optic neuropathy by orbital fat decompression instead of orbital bone decompression after failed medical therapy.

METHODS

Eight orbits of five patients with dysthyroid optic neuropathy were selected for orbital fat decompression as an alternative to bone removal decompression. Treatment with systemic corticosteroids and/or orbital radiotherapy was either unsuccessful or contraindicated in each case. All patients satisfied clinical indications for orbital bone decompression to reverse the optic neuropathy. High resolution computerised tomographic (CT) scans were performed in all cases and in each case showed signs of enlargement of the orbital fat compartment. As an alternative to bone decompression, orbital fat decompression was performed on all eight orbits.

RESULTS

Orbital fat decompression was performed on five patients (eight orbits) with optic neuropathy. Optic neuropathy was reversed in all cases. There were no cases of postoperative diplopia, enophthalmos, globe ptosis, or anaesthesia. All patients were followed for a minimum of 1 year.

CONCLUSIONS

In a subset of patients with an enlarged orbital fat compartment and in whom extraocular muscle enlargement is not the solitary cause of optic neuropathy, fat decompression is a surgical alternative to bony decompression.

Corneal Ablation Profilometry and Steep Central Islands

BACKGROUND

Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography.

METHODS

Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea.

RESULTS

The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity.

CONCLUSIONS

Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

Enlargement of the photorefractive keratectomy optical zone

PURPOSE

To evaluate the safety and efficacy of a photorefractive keratectomy (PRK) re-treatment procedure that enlarges the optical zone and treats undercorrection.

SETTING

Rotterdam Eye Hospital and Medisch Centrum Alkmaar, The Netherlands.

METHODS

This study evaluated 16 eyes that had PRK for myopia with the Summit excimer laser that resulted in a mean undercorrection of -2.82 diopters (D). Patients also reported impaired night vision including difficulty in driving, halos, and stray light and ghost images. These phenomena persisted after spectacle correction of residual refractive error, necessitating further treatment with a larger ablation zone. With a VISX 20/20 excimer laser, the optical zone was enlarged to 6.0 mm and undercorrection treated with a 6.0 mm ablation.

RESULTS

At 13.5 months after re-treatment, mean reduction in myopia was 1.70 D, resulting in a residual undercorrection of -1.08 D. In seven eyes, final refraction was within 1.00 D of emmetropia. Only two patients continued to report night-driving problems.

CONCLUSIONS

Re-treating undercorrections combined with enlarging the ablation zone resulted in a reduction in myopia from a mean of 2.82 to 1.08 D. Subjective reports of halos and stray light images were decreased in all cases.

Night Vision Testing in Unoperated Eyes

BACKGROUND

We previously described a simple test which evaluates image degradation in post-excimer laser (PRK) patients under scotopic conditions. After refractive surgery, corneal haze, ablation zone decentration, ablation zone/pupillary diameter disparity, and under-correction each result in a characteristic pattern on the Night Vision Recording Chart.

METHODS

Using the same method, further studies evaluated night vision image degradation in 118 un-operated emmetropic, myopic, hyperopic, and astigmatic eyes and in 26 contact lens wearers.

RESULTS

Scotopic image degradation increases with myopic refractive error, image displacement increases with astigmatism, and contact lens wearers have more image degradation that with spectacle correction.

CONCLUSION

Our Night Vision Recording Chart offers a simple, reproducible method to characterize image degradation under scotopic conditions.

Efficiency of disc ablation with lasers of various wavelengths

In testing the CO2 lasers in CW and pulse mode, the erbium:YAG, the Nd:YAG 1318 microns and 1064 microns, the argon, the holmium:YAG, and the excimer, we found the greatest efficiency in the CO2 CW and pulse mode, and the lowest efficiency in the argon. Data with the holmium:YAG were unreliable because of the early generation laser tested. The Nd:YAG was second only to the CO2 laser, and because the latter has no waveguide, we deemed the Nd:YAG the laser of choice for PLDD.

Transconjunctival sinusotomy using the 193-nm excimer laser

Successful surgical management of glaucoma is limited as a consequence of undesired fibroblastic response of episcleral tissues. In 10 rabbit and three human eye bank eyes, we have performed a modified sinusotomy procedure using the 193-nm excimer laser. The technique is minimally invasive. Conjunctiva is pulled over the corneoscleral limbus, and a slit-shaped (1 x 2 mm) excimer beam ablates through conjunctiva and external sclera until penetrating the outer wall of Schlemm's canal, at which time brisk egress of fluid is noted. The conjunctiva is allowed to retract, and a conjunctival bleb is formed. A single suture is used to close the conjunctival defect. Intraocular pressure in the rabbit eye was decreased from 10.0 +/- 1.8 to 4.9 +/- 1.8 mmHg immediately after surgery (p < 0.0001). Histology revealed a partial thickness, smooth-walled ablation consistent with a sinusotomy. This procedure is minimally invasive and may be useful for management of glaucoma, particularly when a substantial component of the impediment to aqueous outflow is external to the trabecular meshwork.

Correction of irregular astigmatism with the excimer laser

BACKGROUND

Correction of irregular astigmatism has not been possible using available keratorefractive technology.

METHODS

The authors used a topographic map as a guide and created a custom excimer ablation program, designed to create a more regular surface. The program consisted of a combination of phototherapeutic and photorefractive ablation patterns. The amount of tissue to be removed was calculated on the basis of the diameter and steepness of the irregular areas of the corneal surface.

RESULTS

A more regular surface, as evidenced by topographic analysis, reduced astigmatism, and improved uncorrected visual acuity, was produced.

CONCLUSION

Using the corneal topographical map as a guide, excimer laser ablation can be used to create a more regular optical surface with improved visual function.

Evaluation of night vision disturbances

BACKGROUND

Evaluation of night vision disturbances has relied on subjective responses. We designed a test to more objectively measure night vision disturbances.

METHODS

The test consisted of projecting a small circle onto a visual acuity screen. The patient is asked to draw exactly what he sees on an Amsler grid. We evaluated 118 eyes in photopic and scotopic conditions and under different conditions of refractive correction.

RESULTS

Image degradation increased in scotopic conditions for myopes (p = .0001), hyperopes (p = .005), and emmetropes (p = .01). Myopic refractive error correlated with size of glare response (p = .001). Astigmatism correlated with decentration of glare response (p = .0001). Decentration increased in scotopic compared to photopic conditions (p = .002).

CONCLUSION

Our test offers a simple, convenient way to evaluate night vision disturbances and may offer a means of assessing night vision disturbances in patients considering refractive surgery.

Corneal pachymetric topography

PURPOSE

The authors developed a system for producing topographic pachymetric maps of the corneal epithelium and anterior scar tissue.

METHOD

The system uses high-frequency ultrasound scanning enhanced by digital signal processing. Ultrasonic echo data from consecutive parallel B-scans of the cornea spaced at 250-microns intervals are digitized and stored. Using the I-scan (obtained by computing the analytic signal magnitude of the deconvolved ultrasound signal), layer thickness measurements are made with a precision of 2 microns (standard deviation) at 120-microns intervals along each scan plane. The data are stored as an array, z(x,y), mapping thickness, z, onto horizontal and vertical (x,y) spatial coordinates. Pachymetric maps are then constructed by plotting local thickness, represented by a color scale, against measurement point position.

RESULTS

Examples of a normal cornea, a contact lens-wearing cornea, Reis-Bückler dystrophy, and postphotorefractive keratectomy are presented. Areas with significant subepithelial scarring and general epithelial thickening in a subject with Reis-Bückler dystrophy are mapped. Unevenness in the epithelial thickness profile of the cornea in a subject after photorefractive keratectomy is shown, relative to the fellow (untreated) cornea.

CONCLUSION

This technique provides the corneal surgeon with a new tool for the topographic evaluation of the thickness of anterior corneal layers in normal and pathologic corneas with high precision. In addition, the technique is not limited to optically transparent tissue.

Diffractive smoothing of excimer laser ablation using a defocused beam

OBJECTIVE

To determine if excimer laser myopic ablation with a defocused laser image produces a smoother ablation profile than does focused laser light.

METHODS

An ArF excimer laser was used to ablate a 5.00-diopter myopic correction in test blocks using both a contracting and expanding iris aperture. Defocused ablation was performed using a contracting iris aperture by translating the target away from the laser source. A confocal laser scanning microscope was used to analyze the surface smoothness at 55x and 275x magnifications.

RESULTS

The confocal laser scanning micrographs revealed a series of sharply demarcated concentric ridges in the focused ablation, and less prominent, slightly wavy lines in the defocused ablation performed with a contracting aperture. The focused ablation with an expanding aperture also created concentric ridges toward the periphery, but with slightly smoother edges.

Surface morphology following photorefractive keratectomy

To examine the effects of surface dehydration on corneal surface morphology before and after photorefractive keratectomy, corneas of fresh porcine eyes were deepithelialized and then immediately fixed or treated according to one of six protocols (four eyes per protocol): dehydrated under the illumination of an operating microscope; dehydrated and then rehydrated with topically applied balanced salt solution; photorefractive keratectomy; photorefractive keratectomy followed by surface dehydration; or photorefractive keratectomy followed by dehydration and subsequent rehydration. Surface smoothness (S) was measured with an image analysis system. The surfaces of the unablated corneas were smooth (S = 0.992 +/- 0.002), and dehydration did not significantly (P = .7) influence the morphology. After ablation, dehydration resulted in apparent fragmentation of the superficial lamellae, with consequent roughening of the surface (P = .0001). Rehydration was only partially successful in reducing surface irregularity. We conclude that corneal dehydration during and following photorefractive keratectomy roughens the corneal surface and should be carefully avoided.

Ablation rates and surface ultrastructure of 193 nm excimer laser keratectomies

PURPOSE

To determine whether photorefractive keratectomy can be performed at lower energies than are currently employed in clinical trials.

METHODS

Fresh pig corneas were ablated using a clinical excimer laser to study the effects of various energy densities (100-200 mJ/cm2) and beam diameters on ablation rates and on the surface ultrastructure of the ablated cornea.

RESULTS

A 20-mJ increase in energy density was associated with a 0.03 micron per pulse increase in the ablation rate. A nearly linear increase in the pseudomembrane thickness occurred with increasing energy densities (r2 = 0.83) or decreasing ablation area diameter (r2 = 0.86).

CONCLUSIONS

Our findings suggest that fluences less than those currently used in clinical trials (160-180 mJ) are capable of ablating tissue while producing thinner electron-dense pseudomembranes on the corneal surface. The relationship between pseudomembrane thickness and clinical factors such as reepithelialization and postoperative haze remains to be determined. Operating at lower fluences does have the advantages of allowing larger diameter ablations, reducing possible shockwave damage, and reducing the maintenance requirements for the laser.

Soft tissue effects of the THC:YAG laser on canine vocal cords

Recently, a laser based on a thulium-holmium-chromium (THC) doped Yttrium-aluminum-garnet (YAG) rod has been developed that produces light of 2.15 microns wavelength and can be transmitted through a low OH- silica fiberoptic cable. This wavelength falls on one of the peaks of the energy absorption spectrum of water. Thus, the THC:YAG laser eliminates the disadvantage of a cumbersome delivery system found in the CO2 laser while still providing precise cutting and minimal tissue injury inherent in lasers emitting light absorbed by water. We evaluated the soft tissue effects of this laser on canine vocal cords. Ablative lesions were produced by the THC:YAG laser and histologically examined on postoperative days 1, 7, and 28. Results indicate that the depth of tissue penetration is easily controlled and the healing response to tissue injury is comparable to that of the CO2 laser. The THC:YAG laser should prove to be a superior laser for use in otorhinolaryngology, especially when adapted to a flexible endoscope.

In situ collagen gel mold as an aid in excimer laser superficial keratectomy

PURPOSE

The aim of this study is to evaluate the potential use of bovine type I collagen as an adjuvant to excimer laser keratectomy.

METHOD

A suspension of collagen with the capability to polymerize into a gel was applied to the anterior corneal surface of freshly enucleated porcine eyes, using 35.0 diopter (D), 45.5 D, or 52.0 D contact lenses as molds. Keratometry, photokeratoscopy, slit-lamp photography, scanning electron microscopy, and light microscopy were performed on the new surfaces. Furthermore, an irregular corneal surface was created and a suspension of collagen was applied to mask protruding irregularities, therefore creating a smooth surface that was subjected to excimer laser keratectomy. Ablation rates for both collagen and cornea were measured.

RESULTS

Collagen suspension placed on a cornea and molded with contact lenses created a smooth-surfaced gel that conformed to the shape of the contact lens and adhered to the anterior cornea; it was optically smooth and regular as shown by photokeratoscopy, keratometry, and scanning electron microscopy. The corneal curvature was altered in accordance with the base curvature of the contact lens used. Results of keratometry showed resolution of pre-existing astigmatism without induction of new astigmatism. The ablation rate of the gel was not measurably different than that of cornea; hence, when applied to an irregular corneal surface, a smooth surface was created after excimer laser ablation.

CONCLUSION

This study supports the potential value of collagen gel as an adjuvant to excimer laser keratectomy for removal of corneal irregularities as well as for correction of myopia or hyperopia with or without astigmatism.

Pupil diameter and the principal ray

Placement of the surgical zone is critical in refractive procedures that alter a portion of the corneal curve. An improperly centered optical zone may produce glare, decrease best corrected visual acuity, and decrease contrast sensitivity. For proper placement, the new surface should be centered around the line of sight, which is the principal ray from the object of regard that passes through the image of the patient's pupil as projected on the cornea. This point is not necessarily at the geometric center of the cornea and is found by locating the center of the pupil while the patient is maintaining fixation coaxially with the surgeon. However, the pupil does not dilate concentrically and its geometric center moves as the pupil diameter changes. We have found a shift up to 0.7 mm in the geometric center of the pupil as it dilates. Therefore, centration of an ablated or a radial keratotomy zone is most efficiently done when the diameter of the modified corneal optical zone is centered around the line of sight and is superimposed upon the entrance pupil. This will minimize extension of the edge of the large pupil beyond the ablated zone and reduce unwanted secondary optical effects from degrading vision.

Photokeratitis from subablative 193-nanometer excimer laser radiation

BACKGROUND

Photokeratitis is a side effect of UV light exposure whereby the corneal epithelium is photochemically injured in a time delayed fashion. UV light exposure in the far ultraviolet wavelength range has not previously been observed. This study addresses the concern of photokeratitis from subablative 193-nanometer excimer laser light.

METHODS

Dutch belted rabbit corneas were irradiated with subablative 193-nanometer excimer laser light over a wide range of total energy exposures, and examined by slit lamp biomicroscope for signs of photokeratitis. Photokeratitis was identified by epithelial haze and stippling, and rose bengal and fluorescein staining at varied time intervals between 1/2 to 26 hours post exposure.

RESULTS

At threshold energy exposures of 1.0 to 1.5 J/cm2, an immediate superficial epithelial haze was seen which disappeared within several hours. At higher energy exposures of 10 J/cm2, a delayed photokeratitis with deep rose bengal and even fluorescein staining was seen. This latter delayed photokeratitis resembles that of longer UV wavelengths and is due to excimer laser fluorescence, whereas the former is a direct response of the 193-nanometer light. The percentage of excimer laser light undergoing fluorescence is calculated as less than 1%.

CONCLUSIONS

The potential side effects and hazards of scattered 193-nanometer radiation during excimer laser surgery are extremely limited because of the shorter penetration depth of direct excimer radiation and the minimal fluorescent emission of longer UV wavelengths for energy exposures within the realm of clinical use.

Evidence of genotoxic damage in human cataractous lenses

Lens epithelial fragments (tags) recovered from individuals during routine cataract extraction have been assessed for cellular changes reflective of genotoxic damage. A high percentage of tags exhibited a population of micronucleated and polyploid cells. The presence and number of micronuclei (MN) in the epithelia of cataract patients appears to be independent of age and sex. However, a large number of MN in the epithelial cells of some individuals strongly suggests a history of compromised genomic integrity. While the study was not designed to define the role of DNA damage in the development of cataracts or to monitor human populations at risk of exposure to exogenous mutagens/cataractogens, the potential of the methodology to address each is demonstrated.

Laser radiation at various wavelengths for decompression of intervertebral disk. Experimental observations on human autopsy specimens

The interaction of laser radiation with the nucleus pulposus from autopsy specimens of human intervertebral disks was evaluated at different wavelengths (193 nm, 488 nm & 514 nm, 1064 nm, 1318 nm, 2150 nm, 2940 nm, and 10600 nm). A significant correlation of linear least squares fit of the mass ablated as a function of incident energy was found for all lasers used except the Excimer at 193 nm. The 2940-nm Erbium:YAG laser was most efficient in terms of mass of disk ablated per joule in the limited lower range where this wavelength was observed. At higher energy levels, the CO2 laser in the pulsed mode was most efficient. However, the Nd:YAG 1064-nm and 1318-nm lasers are currently best suited for percutaneous laser disk decompression because of the availability of usable waveguides. Carbonization of tissue with the more penetrating Nd:YAG 1064-nm laser increases the efficiency of tissue ablation and makes it comparable to the Nd:YAG 1318-nm laser.

Photokeratitis from 193 nm argon-fluoride laser radiation

The threshold for photokeratitis at 193 nm was obtained for the rabbit cornea using an ArF excimer laser. Because ablation occurs at a level below that for photokeratitis, it was necessary to expose the cornea to a lengthy series of low-energy exposures. It is concluded that the 193 nm photons have such a shallow penetration depth, being limited to the outermost epithelial cells, that classical photokeratitis occurs from the fluorescence emitted at the corneal epithelial absorption site. An intact tear film may help to protect the cornea from low-level, scattered 193 nm laser radiation.

Alternative lasers for endoscopic surgery: comparison of pulsed thulium-holmium-chromium:YAG with continuous-wave neodymium:YAG laser for ablation of colonic mucosa

Precise and controllable tissue vaporization is essential for minimizing risk in removal of sessile polyps from the lumen of thin walled gastrointestinal organs such as the colon. We compared the ablative efficiency on canine colonic mucosa of the THC:YAG laser with the clinically employed cw Nd:YAG laser. Fresh canine colon was treated with a progressive dose schedule using each laser at several energy/power densities. Ablation depth was measured on fresh tissue and thermal (non-ablation or coagulative) damage examined histologically. The THC:YAG ablation rates were 13.7 +/- 0.8 and 10.2 +/- 0.4 microns/J at 55 and 85 J/cm2, respectively. The Nd:YAG laser generated 3.7 +/- 0.3, 2.8 +/- 0.1, and 3.6 +/- 0.2 microns/J at 4,460, 5,095, and 5,730 W/cm2, respectively. There was a significant (P less than 0.001) difference among the THC:YAG ablation rates and between the THC:YAG and Nd:YAG ablation rates (ANOVA). The THC:YAG laser craters had significantly less collateral thermal damage than Nd:YAG. The pulsed THC:YAG laser should have an important clinical role since its use could reduce the risk of perforation in endoscopic laser procedures such as the removal of sessile polyps.

Effects of a 2.15-micron laser on human atherosclerotic xenografts in vivo

The thulium-holmium-chromium:yttrium-aluminum-garnet (THC:YAG) laser has a tissue effect similar to that of the CO2 laser, with the advantage of transmissibility through flexible fibers. The authors used a human-rabbit xenograft model to evaluate the thrombotic and healing responses of atherosclerotic vessels subjected to laser energy. Occluded atherosclerotic human coronary artery segments were recanalized in vitro by use of the THC:YAG laser. Destruction of plaque by the laser was achieved with minimal collateral thermal damage. These vascular segments were then transplanted into the rabbit abdominal aorta. The authors observed that the luminal surface of the lased vessels was more thrombogenic than that of the nonlased control vessels. However, occlusion of the lased vessels did not occur. Repair of laser-treated tissue progressed until a mature, nonthrombogenic fibrin-platelet aggregate was adherent to the luminal wall. Overall, the lased vessels behaved in a fashion similar to the nonlased control vessels. On the basis of these results, the authors believe that the THC:YAG laser may have use in human angioplasty.

Corneal healing following laser refractive keratectomy

A 193-nm excimer laser system was used to ablate 4.5-mm optically contoured zones in the corneal stroma of rabbits to achieve optical flattening of 2, 4, 8, and 16 diopters. We studied the effect of edge profile and wound depth on reepithelialization and stromal remodeling using dichlorotriazinyl aminofluorescein, a vital dye that covalently binds to the stromal bed and delineates the boundaries of new collagen synthesis. All the corneas reepithelialized; no subsequent recurrent erosions occurred. All seven corneas that received an ablation of less than 50 microns were clear centrally at 8 weeks. At an ablation depth of approximately 100 microns, opacification and scarring were observed biomicroscopically and histopathologically in two specimens. No evidence of new collagen formation or epithelial hyperplasia was found in any of the seven corneas that remained clear. Stromal remodeling was observed in the two corneas that exhibited scarring.