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Non‐penetrating deep sclerectomy (NPDS) is a non‐perforating filtration procedure used for the surgical treatment of medically uncontrolled open angle glaucoma. This procedure was developed in an attempt to avoid many of the postoperative complications of trabeculectomy.1 The major advantage of NPDS is that it precludes the sudden hypotony that occurs after trabeculectomy by creating progressive filtration of aqueous humour from the anterior chamber to the subconjunctival space, without perforating the eye.2 Preservation of the thin trabeculo‐Descemet's membrane, however, is technically challenging, particularly before the surgeon gains experience with this procedure.
Previous studies investigated the ability to use the femtosecond laser for photodisruption in the human sclera,3,4 and concluded that complete subsurface photodisruption can be accomplished in human sclera in vitro.
Toyran et al.5 in 2005 published their in‐vitro study that tested the feasibility of using femtosecond laser pulses to fistulise the human trabecular meshwork and concluded that, with appropriate exposure time and pulse energy, femtosecond photodisruption can be employed to create partial and full thickness ablation in the human trabecular meshwork without damaging the surrounding tissues.
In the study described here, we used femtosecond laser technology to perform subsurface photodisruption in the opaque sclera for NPDS surgery in a laboratory model.
Eight NPDS operations were performed in five cadaver eyes for this study.
The donor eye was placed in an eye holder with a suction ring to immobilize it.
Conjunctiva and Tenon's capsule were reflected. A 200 μm thick, limbal‐based, 7 mm diameter round scleral flap was performed using the femtosecond laser (IntraLase Corp Irvine, California, USA). The IntraLase used a raster spot pattern for creating 1–2 micron bubbles with bed energy of 2.6 μJ. The spot separation was 6/7 μm and the firing rate was 60 kHz. The hinge was placed 1–2 mm into clear cornea (fig 11).). The scleral flap separation was started on the corneal side, using a Sibel spatula, and was continued in the same plane into the sclera using corneal dissectors. The flap was lifted and a second circular flap of 400 microns depth, 5 mm diameter, was performed , dissected (fig 2A2A)) and amputated (fig 2B2B),), leaving a thin layer of deep sclera over the choroid posteriorly. Anteriorly, the dissection was into Schlemm's canal, which was unroofed. The corneal stroma was excised to the level of Descemet's membrane and aqueous humour began percolating through the remaining thin trabeculo‐Descemet's membrane. Methylene blue was injected to the anterior chamber, using a 25 gauge needle, in order to demonstrate aqueous humour percolation (fig 2B2B).). The superficial scleral flap could then be repositioned and sutured with two single 10–0 nylon sutures.
The creation of a scleral flap was achieved using an IntraLase without damage to overlying tissue in all eight NPDS operations. A thinned sclera was observed underneath the flaps with percolating aqueous humour (fig 11).
One case had an intraoperative perforation of the sclera with choroid prolapse after performing the second scleral flap with the femtosecond laser.
Despite the advantages of decreased postoperative complications when compared with trabeculectomy in primary open angle glaucoma surgery, deep sclerectomy has not gained wide acceptance partly because of the difficulty of the technique, with a long learning curve, and as a result of studies that indicate that NPDS is less effective than trabeculectomy for intraocular pressure reduction on a medium and long‐term basis.6 When intrascleral collagen implants are used, however, deep sclerectomy yields improved results according to some authors.7 Our preliminary study on cadaver eyes suggests that high precision, subsurface scleral photodisruption can be achieved for NPDS surgery, using the femtosecond laser. This could simplify and facilitate this surgical procedure.
The major known intraoperative complication of this surgery is perforation of the thin trabeculo‐Descemet's membrane during the deep sclerectomy dissection.8,9 No case of intraoperative perforation of the trabeculo‐Descemet's membrane was observed in all eight surgeries performed by us using the femtosecond laser.
Lifting of the scleral portion of the flap was harder than the corneal one. We thus had to use higher bed energies and a sharper corneal dissector to complete the scleral dissection at the same plane as the corneal.
One case of scleral perforation and choroidal prolapse occurred in our series. This could be attributed to a deeper dissection plane in the sclera made by the surgeon using the cornea dissectors during dissection of the first flap (which was thicker than planned). Therefore, when the second circular flap of 400 microns depth, 5 mm diameter was performed, it perforated the sclera.
One limitation of this study is the non‐healing nature of this laboratory model, which does not allow an assessment of the long‐term stability of the wound and efficacy of the procedure in terms of reducing intraocular pressure.
In conclusion, we have shown here that scleral dissection for NPDS surgery can be accurately achieved using a femtosecond laser. Further investigation on an animal model with clinical correlations is needed.
The authors would like to thank Intralase Corp. for support of this study.
Competing interests: None declared.