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Milling trabeculoplasty a new technique for non-penetrating glaucoma surgery
A motorized drill can be used securely to mill and refine the remaining scleral thickness.
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In the past 10 years non-penetrating surgery has aroused great interest as a possible alternative to trabeculectomy for glaucoma surgery. Krasnov and Zimmerman originated this class of procedures, which is now mainly represented by deep sclerotomy and viscocanalostomy, with their original studies on non-penetrating trabeculectomy.
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The new procedures aim to allow drainage of the aqueous humor from the anterior chamber by slow percolation through the inner trabecular meshwork and/or Descemet’s membrane (the trabeculo-Descemetic membrane) rather than through a patent scleral opening, as in standard trabeculectomy. This avoids sudden reductions in intraocular pressure (IOP), hypotony and flat chambers. The absence of anterior chamber opening and iridectomy limits the risk of cataract and infection.
The original idea of the glaucoma department at the Alicante Institute was to mill and refine the remaining scleral thickness using a safer, more secure and faster technique that eliminates the rupture of the trabeculo-Descemetic membrane. We searched for alternative surgical solutions that could provide lower complication rates without affecting the overall success, taking into account the quality of life of the patient postoperatively.
One technique, milling trabeculoplasty, provides the opportunity to shed additional light on the role of intraocular pressure reduction in non-penetrating trabeculoplasty procedures, as it does not require any specific accuracy in the dissection of the deep sclera for the deroofing of Schlemm’s canal. Further, it reduces surgery time.
Mechanism, specifications
The mode of action of the drill is similar to the drill used to burr the nasal bone in dacryocystorhinostomy or to that used to polish the bed after removing corneal foreign bodies.
Refining tissue removal has three goals: assuming that the high frequency and velocity of the drill tip polishes and does not cut the remaining scleral thickness; keeping the cutting, spreading and tearing of tissue to a minimum; and making scars as small and localized as possible.
The high-velocity milling drill (6,000 and 10,000 rpm) allows easy, quick and more controlled refining of the remaining scleral thickness by using the notched hemispherical metallic tip to polish and not cut the remaining scleral thickness. Another tip covered with diamond powder is available for more delicate maneuvers such as removing debris, making the technique ideal for producing an extremely smooth bed.
Surgical technique
The initial steps in the operation are the same as for a basic trabeculectomy. A fornix-based conjunctival flap is fashioned. A small amount of cautery is then applied to the limbal area to achieve homeostasis. The bipolar cautery can be used to gently outline the area for the subsequent preparation of the split-thickness scleral flap.
A rectangular flap of 4 mm on each side and hinged at the limbus is made, through a flap half the scleral thickness and 4 mm in width at the base. The site of the split-thickness scleral flap should be free of all vessels. An ultra-sharp mini-blade such as the No. 7511 Beaver is used to outline the trabeculectomy flap by cutting through 50% of the scleral thickness, creating a rectangle of the dimensions given above. The incision should be made definitively, without multiple tentative strokes. This will achieve a sharp-edged flap that will lie neatly in its scleral bed at the conclusion of the procedure.
The edge of the scleral flap is then grasped with Hoskins forceps and gently retracted. A spatula type of blade, such as the No. 6400 Beaver or the No. 681.28 is used. The lamellar dissection is brought well anterior to the blue limbal zone until a clear cornea is reached and the iris details can be seen through the deep layer of tissue.
The scleral bed is then refined and grazed using the milling motorized drill in dry field through the remaining scleral thickness in a linear pattern to leave a thin layer of sclera below and a width of 3.5 mm. The refining is carried anteriorly and down until the roof of Schlemm’s canal is barely visible.
Deroofing of the canal can be done either by continued use of the motorized milling drill but with applying less pressure and by using the same high velocity mentioned above, or it can be done by using Vannas scissors, leaving two patent openings on the lateral edges of the cut to provide sharp lateral canal openings, exposing 1 to 2 mm of Descemet’s membrane.
At this stage of the procedure, aqueous humor should be seen percolating through the trabeculo-Descemetic membrane. If there is reduced outflow, the inner wall of Schlemm’s canal can be stripped to increase percolation by delicately depressing the floor and Descemet’s membrane with the tip of a cotton swab. Ultimately, only the trabeculo-Descemetic membrane remains intact. At this moment a visible filtration of aqueous humor is obtained through this thin trabeculo-Descemetic membrane, which is responsible for the IOP-lowering mechanism.
To make cannulation of Schlemm’s canal easier and to reduce bulging of Descemet’s membrane during cleavage from the corneal stroma, the IOP can be reduced by paracentesis.
The flap is then gently laid in its normal anatomic position. It is preferable to leave the flap quite loose at this stage because many failures result from too-tight closure of the scleral flap. A single 10-0 nylon suture is placed in the middle of the apical edge and tied loosely. The suture ends cut even with the knot, and the knot entry is then rotated into the sclera to prevent the suture tips from eroding the conjunctiva.
After closing the sclera, repositioning the conjunctiva is done with two lateral stitches.
The authors are developing a special drill, simple to use and with disposable tips, with Katena Instruments.
Potential advantages
Milling trabeculoplasty seems to be a promising technique for glaucoma surgery. Although deep sclerectomy operation is more difficult and time-consuming, in both techniques the postop management is easier and fewer follow-up outpatient appointments are required when compared to trabeculectomy.
Although the postop results were satisfactory in our patients after being operated without the use of a collagen device or mitomycin-C, another series of patients are now under investigation using the same technique accompanied by mitomycin-C over the sclera, in addition to the use of absorbable porcine collagen implants. This was to compare the results of both series in a trial to approach the maximum success rates (Prats JL, unpublished data, 2001).
Following are the potential advantages of the Milling trabeculoplasty over the deep non-penetrating sclerectomy:
It facilitates the surgical procedure of non-penetrating technique, as the refining is carried anteriorly and down until the roof of Schlemm’s canal is barely visible. Unlike deep non-penetrating sclerectomy, this technique does not require any specific accuracy in the dissection of the deep sclera.
It reduces wound healing as a determinating factor for the IOP outcome, since the blunt diamond tip is available for more delicate maneuvers such as removing debris, making the technique ideal and leaving an extremely smooth bed.
It carries away fewer complications than deep sclerectomy and it prevents the “double-cut” sclera and hazards.
The economic cost of the drill.
It saves time, as this technique has an easier approach to the trabeculo-Descemetic membrane.
There is no need for the use of a specific surgical set.
Conclusion
Milling trabeculoplasty is a potential alternative to deep sclerectomy. This technique does not require any specific accuracy in the dissection of the deep sclera. The procedure aims not only to profit from the advantages of being non-penetrating, like deep sclerotomy, but most importantly, with its easier approach to the trabeculo-Descemetic membrane, it seems to be a step forward and a new approach in non-penetrating procedures.
For Your Information:
- Jorge L. Alió, MD, PhD, Jose L. Rodriguez Prats, MD, and Tamer F. Salem, MD, can be reached at the Department of Glaucoma, Instituto Oftalmológico de Alicante, Miguel Hernandez University School of Medicine, Adva, Dénia 111, 03015 Alicante, Spain; (34) 96-515-4062; fax: (34) 96-515-1501; e-mail: jlalió@oftalió.com. Dr. Salem is also at the Research Institute of Ophthalmology, Giza, Egypt. The authors have no direct financial interest in the products mentioned in this article, nor are they paid consultants for any companies mentioned.