[Retinal photoablation with the Erbium:YAG laser. Initial experimental results for traction-free removal of tissue]. / Retinale Photoablation mit dem Erbium:YAG-Laser. Erste experimentelle Ergebnisse zur traktionsfreien Gewebeabtragung.

BACKGROUND: To investigate the potential of an Er:YAG laser for precise and traction-free removal of retinal layers in vitro. MATERIAL AND METHODS: Retinal ablation in porcine retinal explants was performed using a free running Er:YAG laser focused either into a low-OH quartz fiber or a sapphire fiber. The explants were treated under air or perfluorodecaline (PFD). The ablation depth was evaluated by optical coherence tomography (OCT) and histology sections. RESULTS: A radiant exposure of 5.0 J/cm(2) under air and PFD resulted in complete transsection of the neurosensory retina. Between 3.5 and 2.0 J/cm(2) the ablation depth and the defect patterns varied markedly and adjacent thermal zones areas were seen. Below 2.0 J/cm(2) no defects could be created in air, whereas under PFD the ablation extended into the ganglion cell layer. Ablations using a sapphire fiber and 2.0 J/cm(2) showed a significantly higher reproducibility of ablation depth, and homogeneous defect patterns limited to the nerve fiber layer could be produced without thermal damage. CONCLUSIONS: The Er:YAG laser system with a low-OH quartz fiber allowed ablation of inner retinal layers in vitro, but revealed a variable ablation depth and low reproducibility.However, a sapphire fiber showed markedly improved results. Therefore its use during vitreoretinal surgery seems possible not only as a cutting device but also as a tool for the ablation of fine retinal structures.

Ablation dynamics in laser sclerostomy Ab externo by means of pulsed lasers in the mid-infrared spectral range.

BACKGROUND AND OBJECTIVE: Sclerostomy ab externo with pulsed laser systems is currently in phase II clinical trials. The authors investigated the ablation dynamics of tissue treated with pulsed laser systems in the mid-infrared range to estimate the extent of thermo-mechanical damage to the sclera and the anterior chamber. MATERIALS AND METHODS: Freshly harvested porcine eyes were used. A bare 400-micron fiber in direct contact with tissue was used for fistulization. Polarization light microscopy, fast-flash photography, as well as optical and acoustic transients were performed for analysis. RESULTS: Substantial mechanical tissue deformation and dissections were found during pulsed laser ablation. The mechanical damage range within tissue far exceeds the pure thermal damage zone. Aspheric cavitation bubbles of up to 3 mm in length penetrate the anterior chamber after perforation. The cavitation demonstrates a significantly larger time constant in tissue than in water. CONCLUSIONS: Early fistula occlusions due to iris adherences may be attributed to iris trauma caused by cavitation. In response to the findings of this study, the authors propose an automatic feedback system to control the ablation process and minimize secondary ocular tissue effects. With respect to the overall damage zones, a new continuous-wave, mid-infrared diode laser system seems to be superior to pulsed laser systems.

Mitomycin-C in laser sclerostomy: benefit and complications.

BACKGROUND AND OBJECTIVE: The authors studied the effect of topical mitomycin-C administration on the maintenance of filter function and intraocular pressure (IOP) following laser sclerostomy. PATIENTS AND METHODS: Twenty-six patients with a presumed high risk of episcleral scarring were treated intraoperatively with topical mitomycin-C (0.5 mg/ml) for 3 to 5 minutes. Their IOPs were monitored for at least 2 years. The IOP data of these patients were compared with the results for 46 patients who underwent the identical procedure without antimetabolite. RESULTS: IOPs below 23 mm Hg were achieved in 70% of the mitomycin-C-treated patients. Compared with the non-mitomycin-C group, the rate and duration of early postoperative hypotony was significantly increased in the mitomycin-C-treated group. CONCLUSION: Mitomycin-C is useful for maintaining successful filter function in patients with unfavorable prognoses. However, severe and persistent hypotony may occur.

Morphological and biomorphometrical observations on laser thermal keratoplasty. Histological and biomorphometrical examination of the relationship between refractive change and the volume following Cr:Tm:Ho:YAG laser treatment.

Laser thermal keratoplasty (LTK) is currently under clinical trial for the correction of hyperopia and hyperopic astigmatism by means of collagen coagulation in the peripheral cornea. The purpose of our study was to optimize the ratio between the volume of damaged corneal stroma and the refractive effect so as to minimize potential side effects such as endothelial damage or induction of glare phenomena. We therefore performed histological and morphometrical examinations of enucleated pig eyes to determine the relationship between the coagulated stromal volume and the refractive change after LTK using a pulsed Cr:Tm:Ho:YAG laser (wavelength 2.12 microns) on enucleated pig eyes. The refractive change was documented with a Littman ophthalmometer. Morphometrical analysis was performed using polarized light microscopy of sirius red-stained specimens. This special stain separated the thermally changed stroma into a dark nonbirefringent center and a birefringent peripheral zone. The volume of both zones was positively correlated with the refractive change induced. The volume was in turn influenced by the choice of laser parameters. From the ratio of the volume to the refractive change it was found that pulse energies above 30 mJ let to an enlargement of the coagulation volume without increasing the refractive change effectively. The use of high pulse energies did not improve the effect of LTK but only increased the risk of unwanted side effects. However, an increase in the laser repetition rate at a constant pulse number per spot led to refractive changes with minimal coagulation volume. The highest relative refractive change was achieved with a dark central zone and a birefringent zone, each having a volume of about 50-80 x 10(-3) mm3.

[Progress in laser sclerotomy ab externo] Enlarging the sclerostomy channel and local mitomycin administration]. / Fortschritte bei der Lasersklerostomie ab externo. Erweiterung des Skerostomiekanals und lokale Mitomycingabe.

In a previous study we presented our first results after sclerostomy using a pulsed erbium/holmium: YAG laser. At a fiber diameter of 300 microns a success rate of 20-30% was found after a 1-year follow-up. This study concentrates on the improvement of the parameters for filter survival. METHODS. The fiber diameter and thus the size of the fistula increased to 400 microns. Patients who had undergone fibrosis for a previous fistula were treated intraoperatively with topical mitomycin administration. RESULTS. After a follow-up of 6 months, filter function in the 300 microns group was maintained in 26% of the treated eyes (n = 23), while 400 microns fistulas were successful in 48% (n = 26). In the mitomycin-treated group (n = 6), sclerostomy achieved an IOP regulation in four patients. Postoperative hypotony was more frequent, but did not exceed 2 weeks. CONCLUSION. An increase in fistula diameter improves the long-term results of laser sclerostomy. Mitomycin is useful in maintaining filter function in patients with an unfavorable prognosis.

Laser sclerostomy ab externo using two different infrared lasers: a clinical comparison.

This study was designed to compare the clinical results of an ab externo approach to laser sclerostomy using two different laser sources under identical conditions. A pulsed (200 microseconds) erbium-YAG laser (lambda = 2940 nm) and a pulsed (200 microseconds) holmium:YAG laser (lambda = 2120 nm) were used. The energy of each laser was transmitted via a fiber (300 microns in diameter) and applied near the limbus, with the fiber tip remaining in steady contact with the sclera. According to the higher tissue absorption coefficient, a shorter penetration depth and, therefore, fewer complications were expected for the Er:YAG laser. A total of 26 patients with advanced open-angle glaucoma were treated and followed for up to 6 months. In all cases a functioning fistula with a prominent filtering bleb and a marked reduction in the intraocular pressure (from up to 35 mm Hg to < 10 mm Hg) could be achieved primarily. The total energy required was about 4 times lower for erbium:YAG laser was compared with holmium:YAG laser sclerostomies. No complication occurred intraoperatively. Postoperatively, reversible adherence of the iris to the internal ostium was more frequently observed in Ho:YAG laser sclerostomies (60%) and small hyphemas were more often seen in Er:YAG laser sclerostomies (30%). After 6 months of follow-up, about 40% of the fistula remained patent in both groups. In principle, both lasers are suited to serve as energy sources for the described approach. Under the conditions of the present study, different levels of thermal effects did not cause any significant difference in the clinical success rate.(ABSTRACT TRUNCATED AT 250 WORDS)

[Experimental studies on the effect of the Er:glass and Cr:Tm:Ho:YAG laser in thermokeratoplasty]. / Experimentelle Untersuchungen zur Wirkung des Er:Glas- und Cr:Tm:Ho: YAG-Lasers bei der Thermokeratoplastik.

So far the dose-effect ratio of the Holmium laser (wavelength 2.12 microns) and the erbium laser (1.54 microns) for laser thermokeratoplasty (LTK) are not defined in detail. Our study was designed not only to compare the erbium contact and the holmium non-contact applications but also to throw light on the influence of different geometrical application patterns, pulse energies, pulses per coagulation site and repetition rates under experimental conditions. Enucleated sheep and pig eyes were used 2-6 h post mortem, pressurized to 25 mmHg and moisturized with saline solution. Before and after LTK, pachymetry and keratometry were performed. Some specimens were prepared for light and scanning microscopy. The coagulation threshold for the erbium laser in a contact mode with a 200-microns fibre was 25 J/cm2 (ca. 8 mJ/pulse) and for the holmium laser 8 J/cm2 (ca. 2.5 mJ/pulse). The erbium laser was used in a single shot per spot mode, the holmium laser in repeated pulse per spot mode. With the single shot per spot mode, we were able to induce a promising hyperopic shift of up to -3.47 +/- 0.61 D, while myopic changes could only be induced up to +1.89 +/- 0.74 D. Higher changes of up to +8.27 +/- 1.3 D could be achieved by means of repeated pulses per spot (20 pulses, 45 mJ, 10 Hz). Our experiments showed an obvious increase of dioptric changes when using a higher repetition rate while pulse energy and number were kept constant.(ABSTRACT TRUNCATED AT 250 WORDS)