Fig. 20.1
Ahmed Glaucoma Valve M4 (porous plate). The device has a porous polyethylene shell, which allows soft tissue growth into pores and integration with surrounding tissue (Photograph courtesy New World Medical, Inc., Rancho Cucamonga, California)
The Model M4 plate is approximately the same size as the Model FP7 plate (191 and 160 mm2 surface area, respectively). However, the theoretical surface area of the M4 is much higher because of the porous nature of the material. The porous polyethylene material is a biocompatible alloplastic material, which is formed by a proprietary process (Porex Corporation, Fairburn, GA). The porous plate material is high-density polyethylene with a long history of use in surgical implants (MEDPOR Biomaterial implants). The pores allow collagen and mature blood vessels to grow into the material [20]. Tissue integration and vascular ingrowth into the pores improve resistance to infection, exposure, and extrusion [21, 22]. In the Model M4, average pore sizes are greater than 100 μm and pore volume is in the 50 % range, with an interconnecting open pore structure that allows for tissue ingrowth.
In experimental models, the Ahmed Glaucoma Valve with a surrounding porous membrane of expanded polytetrafluoroethylene (ePTFE) has thinner and more vascular capsules compared with the unmodified implant [23]. Implants with a ePTFE membrane showed higher resistance at low flow rates and lower resistance at high flow rates [24]. The porous polyethylene used in the Model M4 Ahmed Glaucoma Valve is stiffer and less deformable compared with porous ePTFE, which provided better fluid pressure distribution in the implant approved for human use.
Surgical Procedure
The surgical procedure is similar for the Model FP7 (silicone plate) and the Model M4 (porous plate) Ahmed Glaucoma Valves (Video 20.1). The implant should be examined and primed prior to implantation. Priming is accomplished by injecting balanced salt solution (BSS) through the drainage tube and valve, using a blunt cannula. Because the valve is encased in the porous material, streaming of BSS is not observed during priming. Flow is verified by visualizing oozing of BSS through the porous material.
An incision is made at or near the limbus, through the conjunctiva and Tenon’s capsule. A pocket is formed at the superior quadrant between the medial and lateral rectus muscles by blunt dissection of Tenon’s capsule from the episclera. The valve body is inserted into the pocket between the rectus muscles and sutured to the episclera. The leading edge of the device should be at least 8–10 mm from the limbus. The “rough” surface of the Model M4 helps to hold the implant in position while anchoring the plate to the sclera.
The drainage tube is trimmed to permit 2–3 mm insertion of the tube into the anterior chamber. The tube is cut bevel up to an anterior angle of approximately 30° to facilitate insertion and avoid occlusion by the iris. A paracentesis is performed and the anterior chamber is entered at the limbus with a sharp 23-gauge needle, parallel to the iris. The drainage tube is inserted approximately 2–3 mm into the anterior chamber, through the needle track and parallel to the iris. The leading edge of the device should be 8–10 mm from the limbus.
The exposed drainage tube is covered with a small piece of preserved donor cornea, sclera, or pericardium, which is sutured into place and the conjunctiva is closed. As an alternative, a partial-thickness limbal-based scleral flap may be made. The tube is inserted into the anterior chamber through a 23-gauge needle puncture made under the flap. Postoperatively, the patient is treated with topical corticosteroids and antibiotics, tapered over approximately 6 weeks. After surgery, patients are usually examined at 1 day, 1 week, 3–4 weeks, and at regular 3–4-month intervals thereafter.
Clinical Results
In a retrospective comparative study [25], 40 eyes were implanted with the Model M4 implant with an average of 1.4-year follow-up and compared with 38 eyes treated with the Model S2 (polypropylene plate) and 76 eyes treated with the Model FP7 (silicone plate). After treatment with the Model M4 implant, the mean IOP was reduced from 27.0 ± 12.0 to 15.0 ± 4.0 mmHg at 1 year, which was not statistically significantly different, compared with the groups treated with Models S2 and FP7. The cumulative probability of success was higher, but not statistically significantly different in the M4 group (80 %) compared with the FP7 (70 %) and S2 (66 %) groups (P = 0.99). Blebs formed in the M4 group were low profile. In this small series, complications were similar among groups, although statistically significantly lower mean IOP at 1–3 months in the M4 group suggested less “hypertensive phase” in patients treated with the Model M4 implant. Further studies are needed to determine the long-term safety and efficacy of the Ahmed Glaucoma Valve M4 implant.
Conclusion
The Ahmed Glaucoma Valve Model M4 has a porous plate, which allows integration of vascular tissue with the plate. Early clinical results suggest similar intraocular pressure control compared with other Ahmed Glaucoma Valve plates. Potential advantages include reduction of hypertensive phase and low-profile bleb. It is anticipated that, if needed, explantation of the device would be more difficult compared with other Ahmed Glaucoma Valve plates. The long-term safety and efficacy of this device are under evaluation at this time.
Financial Disclosure
The University of Virginia has received research support from New World Medical for a multicenter randomized clinical trial.
References
1.
Boyle IV JW, Netland PA. Ahmed glaucoma valve drainage implant. In: Shaarawy TM, Sherwood MB, Hitchings RA, Crowston JG, editors. Glaucoma, Surgical management, vol. 2. 1st ed. Edinburgh: Saunders Elsevier; 2009. p. 425–35.
2.
Coleman AL, Hill R, Wilson MR, Choplin N, Kotas-Neumann R, Tam M, Bacharach J, Panek WC. Initial clinical experience with the Ahmed glaucoma valve implant. Am J Ophthalmol. 1995;120:23–31.PubMed
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