Erbium:YAG laser trabecular ablation

Background

Numerous morphological and functional investigations on the human trabecular meshwork have produced evidence to support the notion that the increased aqueous outflow resistance in primary open-angle glaucoma is caused by specific changes within the inner wall of Schlemm’s canal and the adjacent juxtacanalicular trabecular meshwork.1-3 By performing selective trabecular surgery, the site of surgical intervention can be limited to the original ‘locus causae’ and postoperative over-filtra- tion with subsequent problems such as choroidal detachment and anterior chamber loss can thus be avoided. A further conceptual advantage of ab interno trabecular surgery over conventional filtration surgery4 is the untouched conjunctiva, allowing future ab externo interventions in case of failure. Theoretical calculations suggested that just a few 10-20 µm openings within the trabecular meshwork would be sufficient to re-establish a normal outflow facility in glaucomatous eyes.5

Simple incision of the trabecular meshwork was described back in the 19th century,6 but another 50 years were to pass before Otto Barkan developed the well-known technique of goniotomy, based on this approach.7 Today, goniotomy is performed with success in congenital and developmental glaucomas. Using the ab externo approach with the disadvantage of conjunctival lesions, trabeculotomy also selectively targets the trabecular meshwork by rupturing the meshwork rather than incising it.8,9 In neither procedure is the trabecular meshwork removed; it is only ruptured or incised. Histopathological studies in animals have proven that the tissue cleft after trabeculotomy tends to be reclosed

by secondary repair mechanisms.10 Ideal trabecular surgery should therefore actually remove the diseased tissue and guarantee a broad direct opening into the lumen of Schlemm’s canal.

Different surgical methods to remove trabecular tissue mechanically by an ab interno approach are currently under investigation.11,12 These manual techniques require great surgical precision; the reproducibility of the mechanical effects is totally dependent on the skill of the individual surgeon. Laser surgery of the trabecular meshwork could offer the advantage of better reproducibility, owing to fixed energy parameters.

Krasnov was one of the first to try to perforate the trabecular meshwork with the aid of a laser.13,14 The collateral thermal damage with these early laser systems was extensive, limiting the hope of achieving long-term effectiveness with these procedures. The use of the Nd:YAG laser via a gonioscope presented a means of selectively treating the trabecular meshwork without penetrating the eye, although this photodisruptive laser does not actually remove trabecular tissue, but only ruptures the meshwork. By perforating the trabecular tissue in several places, it was possible to lower the intraocular pressure (IOP) and to produce increased outflow facility. Despite promising results in the early postoperative phase, IOP normalization in the treated eyes and ’tissue-filling-in’ of laser-induced openings within the trabecular meshwork was noted after a few weeks.15 Clinical results were more promising in juvenile glaucoma than in adult open-angle glaucoma.16,17 Today, Nd:YAG laser goniopuncture is experiencing a revival in the postoperative management of non-penetrating glaucoma surgery. In this type of surgery, a deep scleral

flap is dissected and removed while the so-called trabeculo-descemetic membrane remains intact. If the IOP increases again postoperatively, this tra- beculo-descemetic membrane can be perforated by Nd:YAG goniopuncture, with relatively good results.18

Erbium:YAG laser trabecular ablation

Pre-clinical studies

In contrast to the photodisruptive Nd:YAG laser, the Erbium:YAG (Er:YAG) laser can potentially evaporate the targeted tissue and achieve a broader opening to Schlemm’s canal. The infrared Er:YAG laser with a wavelength at 2.94 µm, corresponding to an absorption peak of water, had already been proposed for laser sclerostomy, since reduction of collateral tissue necrosis raised hope of a limited repair response after photoablative filtrating glaucoma surgery. However, being an unguarded filtration procedure, laser sclerostomy could never ultimately succeed, regardless of the wavelength used.19-21

Hill et al.22 were the first to perform photoablation of the trabecular meshwork by applying the Er:YAG laser in direct tissue contact with the trabecular meshwork of donor eyes. In a consecutive animal study, Hill et al.23 demonstrated that Er:YAG laser trabecular ablation (LTA) was capable of increasing the outflow facility and lowering the IOP for at least a number of weeks following surgical intervention. They observed a filling-in of the laser-induced craters in the trabecular meshwork with tissue when the IOP and tonographic curve had normalized again.22,23

The Er:YAG laser (MCL 29, Fa. Aesculap Meditec, Heroldsberg, Germany) that we initially used for LTA acted in the spiking mode of a 200-µsec macropulse. The laser energy was delivered through an articulated system of a zirconium fluoride fiber (350 µm diameter) and an exchangeable quartz fiber endoprobe. The quartz fiber endoprobe for LTA is longer than the conventional tip for laser sclerostomy, because the instrument is applied transcamerally in direct contact with the trabecular meshwork.

In an experimental in-vitro study, we investigated the changes of outflow facility24 in the anterior segment cups of porcine cadaver eyes following LTA and in the anterior segment cups of untreated control eyes. While the mean outflow facility in the control eyes was 0.128+0.041 µl/min × mmHg, the application of 20 pulses (4 mJ pulse energy) to an 90-120° area of the trabecular meshwork yielded an increased outflow facility of 0.308 +0.093 µl/min × mmHg.25 In a similar study with human donor eyes, McHam and co-workers used sapphire tips for Er:YAG LTA.26,27 Their laser system (Candela Laser Corporation, Wayland, MA)

worked with pulse energies of between 10 and 20 mJ, a pulse length of 250 µsec and an endoprobe core diameter of 300 µm. They also observed a significant increase in outflow facility of from 0.283+0.08 µl/min × mmHg to 0.62+0.15 µl/min × mmHg with Er:YAG LTA. Both studies underlined the functional efficacy and clinical potential of LTA.

In another experimental in-vitro study, we investigated the photoablative profile of different quartz fiber endoprobes in human donor eyes.28 Using the Er:YAG laser, the pulse energy can be delivered by an endor a side-firing tip and the core diameter of the endoprobe can also be varied. Full-thickness Er:YAG photoablation of the trabecular meshwork succeeded with each of the different fiber tips used (Figs 1-3). Opening of Schlemm’s canal could be reproducibly achieved at a specific pulse energy characteristic for each endoprobe. Laser-induced apertures in the trabecular meshwork revealed an elliptic inner surface when using a side-firing bevelled tip.29 Already in this in-vitro study, two major problem became obvious: firstly, there was the mechanical fragility of the quartz fiber tips after frequent use. Microscopic examination of aged quartz fiber endoprobes revealed mechanical damage such as splitting and cracking of the end of the tip. Whether sapphire tips are less fragile and hence more reliable in clinical practice still has to be established. Secondly, the manual pressure along the fiber axis has a critical influence on ablation depth. As already shown by Fankhauser et al., in laser sclerostomy, total energy can be massively reduced if the surgeon increases the mechanical pressure on the fiber tip.30 This ‘human’ factor – together with the fragility of the quartz fiber tips – introduces a degree of unpredictability and variability to the intended full-thickness ablation of the trabecular meshwork. According to laser physics, the larger endoprobes (> 300 µm core diameter) require higher single pulse energies for opening of Schlemm’s canal and complete removal of the trabecular meshwork, but the collateral tissue damage of around 30 µm was relatively similar for all endoprobes at the optimal energy level.31,32

In order to investigate the safety and scarring potential of this procedure, we performed Er:YAG LTA in rabbits as part of a preclinical study.32 Twenty healthy pigmented rabbits underwent surgery of the right eye, while their left eye served as a control. IOP measurements and anterior chamber inspection were performed at intervals, and the animals were sacrificed on Days 2, 10, 30, and 60 following surgery in order to document the morphological changes induced by LTA. Between ten and 15 ablation craters (2 mJ single pulse energy) were created with a 200-µm-core-diameter quartz fiber endoprobe under gonioscopic control. Although this procedure is relatively difficult in rabbits, light-microscopic morphology revealed

Fig. 1. Scanning electron microscopy of the trabecular meshwork after Er:YAG LTA in a human donor eye. (Reproduced from Dietlein et al.28 by courtesy of the publisher.)

Fig. 2. Light microscopy of Er:YAG LTA (see arrow) in a human donor eye using a 320-µm-core-diameter quartz fiber endoprobe at 6 mJ single pulse energy. The outer wall of Schlemm’s canal shows only minimal thermal damage. (Reproduced from Dietlein et al.28 by courtesy of the publisher.)

Fig. 3. Different quartz fiber tips after frequent use. The ends of the tips reveal microscopic lesions, such as splitting and cracking. (Reproduced from Dietlein et al.28 by courtesy of the publisher.)

successful photoablation of the trabecular meshwork on Day 2 with blood-filled craters in the meshwork. As in a previous animal study of laser sclerostomy in rabbits,33,34 collateral thermal tissue

Fig. 4. Light microscopy of the anterior chamber angle after Er:YAG LTA in rabbits: already ten days after surgery, massive invasion of fibroblasts into the craters has taken place; no collateral thermal damage is visible any longer.

damage was already undetectable ten days after LTA (Fig. 4), although massive secondary repair mechanisms continued for a number of weeks. After two months, dense occlusion of the laserinduced craters and even anterior synechiae in the chamber angle were seen. IOP in these rabbits slightly decreased from 9.1 mmHg preoperatively to 7.2 mmHg after two months, but was not significantly lower than that of the untreated control eyes (8.5 and 8.6 mmHg). This rabbit study appeared to confirm Hill’s experience of early tissue refilling after LTA.

Clinical studies

Although animal studies had indicated some limitations with LTA in vivo with regard to wound healing, we started a pilot study in patients with advanced open-angle glaucoma in order to evaluate the potential of Er:YAG LTA for glaucoma management. We were encouraged by the differences between the secondary tissue repair observed in young, healthy animals and that seen in elderly glaucoma patients. These are well-documented in trabeculectomy and laser sclerostomy studies.33,34

In one pilot study, 11 patients with open-angle glaucoma were treated by LTA. The mean preoperative maximum IOP was 36.0+8.8 mmHg. The procedure was performed using retroor parabulbar anesthesia. After injection of high-viscosity viscoelastics into the anterior chamber via a small temporal corneal incision, the quartz endoprobe was transcamerally inserted up to the opposite trabecular meshwork (Fig. 5). Pulse energy was delivered via an end-firing quartz fiber endoprobe (core diameter 320 µm, coating diameter 385 µm). The pulse energy emitted at the fiber tip ranged between 5 and 7 mJ, and was preoperatively checked by an external joulemeter. Between ten and 30 neighbouring single laser pulses were applied under gonioscopic guidance, while exerting gentle pressure along the fiber tip, which was in direct contact with the trabecular meshwork. After

Fig. 5. Gonioscopic view during Er:YAG LTA in a glaucoma patient: the endoprobe has been introduced into the viscoelasticfilled anterior chamber via a temporal corneal incision. The end of the tip has to be in direct contact with the opposite trabecular meshwork before laser energy will be emitted.

the procedure, the viscoelastics, blood, and debris were rinsed out by irrigation-aspiration maneuvers.

The mean number of applied laser pulses in this first study was 18+5.5. The mean maximum IOP at the end of a one-year follow-up was 22.0+7.2 mmHg, representing a percentage decrease from baseline of 38.9% (p = 0.008). The average medication score per eye dropped from 2.82 to 1.46 at the end of the follow-up period, representing a reduction of 48% (p = 0.021). At the end of the follow-up period, seven of 11 eyes (64%) constantly maintained their IOP below 21 mmHg. In these successfully treated patients, the mean maximum IOP was 17.6+1.6 mmHg and the mean number of topical medications used 1.43+0.54. No serious intraor postoperative complications occurred in the treated eyes. The laser-induced craters were gonioscopically visible, especially in the pigmented trabecular meshwork. However, visibility slightly decreased during follow-up (Figs 6-7), but without any correlation with the IOP level.35

In recent studies, Funk et al.36,37 investigated the clinical efficacy of Er:YAG LTA in combination with modern cataract surgery in patients with openangle glaucoma. After performing phacoemulsification with implantation of a foldable intraocular lens, they used an endoscope with an integrated Er:YAG laser. Owing to the larger core diameter of the endoprobe used, 15-18 single pulses of around 14 mJ each had to be applied. In 21 eyes, the mean preoperative IOP of 23.4 mmHg could be reduced to 16.4 mmHg more than one year

Fig. 6. Gonioscopic view two months after Er:YAG LTA in a glaucoma patient: the ablation craters can be clearly identified within the trabecular meshwork by their lack of pigmentation.

Fig. 7. Gonioscopic view three years after Er:YAG LTA in a glaucoma patient: there are only a few irregularities of pigmentation in the area of laser trabecular ablation, but no clearly identifiable laser craters. IOP is still under control.

after Er:YAG LTA. Topical medication could also be significantly reduced. In terms of pressure reduction, combined Phako-LTA was as effective as combined phacoemulsification-trabeculectomy (Phako-TE) in glaucoma patients, but brought significantly fewer postoperative complications, required less re-surgery, and achieved significantly faster postoperative rehabilitation.38

Conclusively, it should be taken into account that the repair mechanisms in the relatively old glaucoma patients are very slow and incomplete in comparison to those observed in animal models. Moreover, the new scar tissue replacing the glaucomatous trabecular tissue in the treatment area could also contribute to the improved outflow facility after surgery, and newly formed vessels have indeed been found within the scar area after pressurereducing laser treatment in animals.

In one of our patients, who received LTA for silicone-oil glaucoma in his blind eye, enucleation was performed three years following LTA and histological investigation of the treatment area could be performed.39 Morphological proof of laser craters without significant scarification three years following Er:YAG LTA also demonstrated the massive differences in healing processes between young healthy animals and elderly glaucoma patients. Even in treatment areas where the laser-induced craters were localized adjacent to

Schwalbe’s line, no endothelial covering of the anterior meshwork was observed, as previously reported in animal studies and after argon laser trabeculoplasty in glaucoma patients.32,40 Interestingly, only two laser-induced craters fully perforated the trabecular meshwork and opened the lumen of Schlemm’s canal. This highlighted another major problem of gonioscopically-guided LTA. Possible difficulties in identifying the target tissue and the lack of any means of monitoring the intraoperative success raise the risk of missing Schlemm’s canal in a relatively high percentage of cases. Especially in secondary glaucomas and after previous filtering surgery, the lumen of Schlemm’s canal may have collapsed or be obliterated and is therefore more likely to be missed by the surgeon performing LTA.

Similar approaches

Excimer laser trabecular ablation

Clinical studies on trabecular ablation have also been reported for the excimer laser with a wavelength of 308 nm.41 Vogel and co-workers42 performed punctual excimer laser photoablation of the trabecular meshwork in 27 patients with openangle glaucoma and in seven patients with nor- mal-pressure glaucoma. They achieved a mean IOP decrease of 7 mmHg by creating three to six laserinduced perforations of the meshwork. The energy fluence applied ranged between 3.5 and 5.5 J/cm². The most frequent postoperative complications in this study were temporary pressure spikes produced by retained intraocular viscoelastics in 16 patients. Reflux bleeding was only observed in ten of the treated patients during surgery. In one patient, anterior chamber bleeding was so massive that a

second intervention was necessary to clear the blood from the anterior chamber. In another patient, transient opacity of the corneal periphery near the treatment area was described, but without disturbance of visual function. No other essential complications were observed.42

Neuhann et al. also used the excimer laser for ab interno trabecular ablation in 14 patients. After six months, the mean IOP was 15.5 mmHg compared to 26.8 mmHg before surgery.43

The surgical concept is identical to that of Er: YAG LTA. Light microscopy of the laser-induced craters after excimer ablation of the trabecular tissue in human donor eyes, reveals a morphology similar to that produced by the Er:YAG laser (Fig. 8). The collateral thermal damage seems to be a little less, but transmission variability and loss after frequent use also occur with the excimer laser and may lead to insufficient ablation of the trabecular meshwork.

Intracanalicular trabeculostomy

Kampmeier et al. proposed a new concept for photoablation of the trabecular meshwork with the Er: YAG laser, using an ab externo approach similar to conventional trabeculotomy. After dissecting a scleral flap and searching for Schlemm’s canal, a side-firing laser endoprobe is introduced into the lumen of Schlemm’s canal and the meshwork can be photovaporized. In-vitro studies have been performed and clinical studies are in progress.44

The main conceptual problem of this procedure is the ab externo approach, which causes scarification on the conjunctival side. Another point of concern is the difficulty of introducing a rigid instrument into Schlemm’s canal without damaging surrounding structures such as the drainage vessels.

Conclusions

At the present time, Er:YAG LTA represents a promising ab interno approach for the treatment of open-angle glaucoma. The major conceptual advantages of this procedure are the lack of conjunctival lesions and of filtration-related problems (e.g., overfiltration, early and late bleb leakage, etc.).

The main problems are the relatively expensive and complicated equipment required, difficulties in visualization, and the lack of a means of monitoring intraoperative success. Reflux bleeding during LTA may be an indicator for opening Schlemm’s canal, but it is far from reliable as a sign that sufficient ablation of the meshwork has been achieved for the opening of Schlemm’s canal.

Improvements in intraocular endoscopy may help to facilitate visualization of the chamber angle, making the intervention easier than surgery under the gonioscope.45-47 The ultrasound biomicroscope could provide another viable means of controlling the intraoperative position of the fiber tip end.

Even more promising is the development of ul- trashort-pulsed lasers,48 including the femtosecond lasers,49 which, in the future, may allow ablation of the trabecular tissue without direct tissue contact, thus avoiding all the risks of intraocular surgery.

References

1.Bill A, Maepea O, Hamanaka T: Aspekte der Kammerwasserdrainage Über den Schlemmschen Kanal. Klin Mbl Augenheilk 195:277-280, 1989

2.Grant WM: Further studies on facility of flow through the trabecular meshwork. Arch Ophthalmol 60:523-533, 1958

3.Grant WM: Experimental aqueous perfusion in enucleated human eyes. Arch Ophthalmol 69:783-801, 1963

4.Cairns JE: Trabeculectomy: preliminary report of a new method. Am J Ophthalmol 66:673-679, 1968

5.Goldschmidt CR, Ticho U: Theoretical approach to laser trabeculotomy. Med Phys 5:92-98, 1978

6.DeVincentiis C: Incisione dell angolo irideo nel glaucoma. Ann Ottalmol 22:540-542, 1893

7.Barkan O: Operation for congenital glaucoma. Am J Ophthalmol 25:552-568, 1942

8.Grehn F: The value of trabeculotomy in glaucoma surgery. Curr Opin Ophthalmol 6:52-60, 1995

9.Harms H, Dannheim R: Trabeculotomy: results and problems. Adv Ophthalmol 22:121-131, 1970

10.Ito S, Nishikawa M, Tokura T et al: Histopathological study of trabecular meshwork after trabeculotomy in monkeys. Nippon Ganka Gakkai Zasshi 98:811-819, 1994

11.Jacobi PC, Dietlein TS, Krieglstein GK: Goniocurettage for removing trabecular meshwork: clinical results of a new surgical technique in advanced chronic open-angle glaucoma. Am J Ophthalmol 127:505-510, 1999

12.Quaranta L, Hitchings RA, Quaranta CA: Ab-interno goniotrabeculotomy versus mitomycin C trabeculectomy for adult open-angle glaucoma: a 2-year randomized clinical trial. Ophthalmology 106:1357-1362, 1999

13.Krasnov MM: Laser puncture of anterior chamber angle in glaucoma. Am J Ophthalmol 75:674-678, 1973

14.Krasnov MM: Q-switched laser goniopuncture. Arch Ophthalmol 92:37-41, 1974

15.Melamed S, Pei J, Puliafito CA, Epstein DL: Q-switched neodymium-YAG laser trabeculopuncture in monkeys. Arch Ophthalmol 103:129-133, 1985

16.Epstein DL, Melamed S, Puliafito CA, Steinert RF: Neodymium:YAG laser trabeculopuncture in open-angle glaucoma. Ophthalmology 92:931-937, 1985

17.Melamed S, Latina MA, Epstein DL: Neodymium:YAG laser trabeculopuncture in juvenile open-angle glaucoma. Ophthalmology 94:163-170, 1987

18.Mermoud A, Karlen ME, Schnyder CC, Sickenberg M, Chiou AG, Hediguer SE, Sanchez E: Nd:YAG goniopuncture after deep sclerectomy with collagen implant. Ophthalmic Surg Lasers 30:120-125, 1999

19.Berlin MS, Yoo PH, Roy JH, Ahn BA: The role of laser sclerostomy in glaucoma surgery. Curr Opin Ophthalmol 6:102-114, 1995

20.Jacobi PC, Dietlein TS, Krieglstein GK: Prospective study of ab externo Erbium:YAG laser sclerostomy in humans. Am J Ophthalmol 123:478-486, 1997

21.Kampmeier J, Klafke M, Hibst R, Wierschien S: Ab externo Sklerostomie mit dem Er:YAG Laser: Ergebnisbericht nach 2 Jahren. Klin Mbl Augenheilk 208:218-223, 1996

22.Hill RA, Baerveldt G, Ozler SA, Pickford M, Profeta GA, Berns MW: Laser trabecular ablation (LTA). Lasers Surg Med 11:341-346, 1991

23.Hill RA, Stern D, Lesiecki ML, Hsia J, Liaw LH, Baerveldt G, Berns MW: Primate experience with erbium (Er):YAG laser trabecular ablation. ARVO abstracts. Invest Ophthalmol Vis Sci 33(Suppl):1018, 1992

24.Becker B, Constant MA: Species variation in facility of aqueous outflow. Am J Ophtalmol 42:189-194, 1956

25.Jacobi PC, Dietlein TS, Krieglstein GK: Effects of Er:YAG laser trabecular ablation on outflow facility in cadaver porcine eyes. Graefe’s Arch Clin Exp Ophthalmol 234:S204208, 1996

26.Fitzgibbon JJ, Bates HE, Pryshlak AP, Phickbrick MJ: Sapphire optical fibers for the delivery of Erbium:YAG laser energy. Proc Biomed Opt Soc SPIE 2131:50-55, 1994

27.McHam ML, Eisenberg DL, Schuman JS, Wang N: Erbium: YAG laser trabecular ablation with a sapphire optical fiber. Exp Eye Res 65:151-155, 1997

28.Dietlein TS, Jacobi PC, Krieglstein GK: Erbium:YAG laser ablation on human trabecular meshwork by contact delivery endoprobes. Ophthalmic Surg Lasers 27:939-945, 1996

29.Melnik I, Krivokhizha A, Ptashnik O: Multipurpose fiber optic sensors with slope tips. SPIE 1572:118-122, 1991

30.Fankhauser F, Dürr U, England C, Kwasniewska S, Van der Zypen E, Henchoz PD: Optical principles related to optimizing sclerostomy procedures. Ophthalmic Surg 23:752-761, 1992

31.Kramer TR, Noecker RJ, Ellsworth LG, Yarborough JM: Laser trabecular ablation of human eyes with the Erbium: YAG laser: a histopathologic study. SPIE (Ophthalm Technologies IV) 2126:242-250, 1994

32.Dietlein TS, Jacobi PC, Schröder R, Krieglstein GK: Experimental Erbium:YAG laser photoablation of trabecular meshwork in rabbits: an in-vivo study. Exp Eye Res 64:701706, 1997

33.Iliev M, Van der Zypen E, Fankhauser F, England C: The repair response following Nd:YAG laser sclerostomy ab interno in rabbits. Exp Eye Res 61:311-321, 1995

34.Iliev M, Van der Zypen E, Fankhauser F: Vernarbung von Laser-erzeugten Sklerostomie-Kanälen beim Kaninchen. Klin Mbl Augenheilk 206:376-379, 1995

35.Dietlein TS, Jacobi PC, Krieglstein GK: Erbium:YAG laser trabecular ablation (LTA) in the surgical treatment of glaucoma. Lasers Surg Med 23:104-110, 1998

36.Funk J, Schlunck G: Endoscopically controlled Erbium:YAG

laser goniotomy: initial preclinical trials. Ophthalmologe 95:33-36, 1998

37.Funk J, Feltgen N, Asbeck D: Augendrucksenkung durch endoskopisch kontrollierte Erb:YAG Goniotomie. Ophthalmologe (Suppl):S27, 1999

38.Feltgen N, Ott B, Frenz M, Funk J: Endoskopisch kontrollierte Erbium:YAG Goniotomie versus Trabekulektomie: Drucksenkung bei der Kombination mit einer Kataraktoperation. Ophthalmologe (Suppl 1):S33, 2001

39.Dietlein TS, Jacobi PC, Krieglstein GK: Morphology of the trabecular meshwork three years after laser trabecular ablation (LTA). Ophthalmic Surg Lasers 32:483-485, 2001

40.Alexander RA, Grierson I, Church WH: The effect of argon laser trabeculoplasty upon the normal human trabecular meshwork. Graefe’s Arch Clin Exp Ophthalmol 227:7277, 1989

41.Vogel M, Scheurer G, Neu W, Dressel M, Gerhardt H.:Die Ablation des Trabekelwerks. Klin Mbl Augenheilk 197:250253, 1990

42.Vogel M, Lauritzen K, Quentin CD: Punktuelle Ablation des Trabekelwerks mit dem Exzimerlaser beim primären Offenwinkelglaukom. Ophthalmologe 93:565-568, 1996

43.Neuhann T, Scharrer A, Haefliger E: Excimer Laser-

Trabekelablation ab interno beim chronischen Offenwinkelglaukom. Ophthalmo-Chirurgie 13:53-58, 2001

44.Kampmeier J, Stock K, Hibst R, Lang GE, Steiner R, Lang GK: Intrakanalicular trabeculostomy: a new approach to glaucoma surgery. Klin Mbl Augenheilk 212:159-162, 1998

45.Jacobi PC, Dietlein TS, Krieglstein GK: Microendoscopic trabecular surgery in glaucoma management. Ophthalmology 106:538-544, 1999

46.Joos KM, Alward WLM, Folberg R: Experimental endoscopic goniotomy. A potential treatment for primary infantile glaucoma. Ophthalmology 100:1066-1070, 1993

47.Rivera BK, Joos KM, Shen JH, Hernandez E, Ren Q: Endoscopic goniotomy with the Erbium:YAG laser. Invest Ophthalmol Vis Sci 35:2066, 1994

48.Rohrschneider K, Kruse FE, Kessler R, Gölz S, Bille JF, Völcker HE: Ab-interno-Trabekulotomie mit dem Nd:YLFPikosekundenlaser. Ophthalmologe 97:748-752, 2000

49.Juhasz T, Kastis GA, Suarez C, Bor Z, Bron WE: Timeresolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in cornea tissue and water. Lasers Surg Med 19:23-31, 1996