Comparison of Glaucoma Drainage Implants
Comparison of Glaucoma Drainage Implants
The AGV is the most commonly used valved device implanted to reduce IOP. The valve mechanism provides a lower risk for postoperative hypotony while offering immediate IOP lowering. A high proportion of patients achieve initial IOP control with the AGV, but this number declines over time. Long-term studies have shown that at 1 and 4 years, the reported probability of success for AGVs ranges from 63% to 88% and 37% to 54%, respectively. The cause of the late failures is likely the formation of a fibrovascular capsule.
Modulation of the capsule has become the focus of research for improving the success of glaucoma drainage devices. Alteration of the development of this fibrous tissue has been successful in trabeculectomies with the addition of antifibrotic agents such as mitomycin-C and 5-fluorouracil, but it has not shown the same success with glaucoma drainage devices. In contrast, variations in the size, shape, flexibility, and material of the implant have improved its function.
Therefore, the modifications made to develop the new M4 model have a promising physiologic basis. The M4's valve mechanism is identical to that of the AVG S2, but the M4 is also enveloped by a porous high-density polyethylene material. The pores allow for rapid tissue integration and vascular in-growth that imparts resistance to infection, exposure, extrusion, and mechanical deformation. In animal studies, the capsules that were formed were found to be thinner and more vascular and have reduced outflow resistance compared with the AGV S2.
Clinically, and in this study as well, it was reported that on examination of the bleb morphology postoperatively, the blebs from the M4 had a low profile throughout the postoperative course; and in contrast to the typical dome-shaped capsule seen with the FP7 and S2, the adjacent tissue appeared to be integrated into the polyethylene shell surrounding the valve system. Furthermore, there seemed to be a lack of fluid between the plate and capsule of the M4 implant.
Yet, from the results of this study, it appears that although the change in morphology is positive, the results in terms of endpoints of IOP, vision, and complications remain the same for all implants. In fact, although the M4 had the lowest IOP at 3 months, demonstrating an absence of the hypertensive phase, by 18 months, it had a lower rate of success than the FP7 and the S2.
Therefore, with 18-month outcomes from this study, it seems that although the M4 has some promising characteristics, clinically its performance is similar to previously existing devices. Furthermore, the experience of some surgeons has been that it is slightly more difficult to implant given the added resistance of the polyethylene covering. However, this device seems to be a step in the right direction for the continued improvement of these devices. Larger studies with longer follow-up are needed to further delineate the characteristics of the M4 implant.
Abstract
Viewpoint
The AGV is the most commonly used valved device implanted to reduce IOP. The valve mechanism provides a lower risk for postoperative hypotony while offering immediate IOP lowering. A high proportion of patients achieve initial IOP control with the AGV, but this number declines over time. Long-term studies have shown that at 1 and 4 years, the reported probability of success for AGVs ranges from 63% to 88% and 37% to 54%, respectively. The cause of the late failures is likely the formation of a fibrovascular capsule.
Modulation of the capsule has become the focus of research for improving the success of glaucoma drainage devices. Alteration of the development of this fibrous tissue has been successful in trabeculectomies with the addition of antifibrotic agents such as mitomycin-C and 5-fluorouracil, but it has not shown the same success with glaucoma drainage devices. In contrast, variations in the size, shape, flexibility, and material of the implant have improved its function.
Therefore, the modifications made to develop the new M4 model have a promising physiologic basis. The M4's valve mechanism is identical to that of the AVG S2, but the M4 is also enveloped by a porous high-density polyethylene material. The pores allow for rapid tissue integration and vascular in-growth that imparts resistance to infection, exposure, extrusion, and mechanical deformation. In animal studies, the capsules that were formed were found to be thinner and more vascular and have reduced outflow resistance compared with the AGV S2.
Clinically, and in this study as well, it was reported that on examination of the bleb morphology postoperatively, the blebs from the M4 had a low profile throughout the postoperative course; and in contrast to the typical dome-shaped capsule seen with the FP7 and S2, the adjacent tissue appeared to be integrated into the polyethylene shell surrounding the valve system. Furthermore, there seemed to be a lack of fluid between the plate and capsule of the M4 implant.
Yet, from the results of this study, it appears that although the change in morphology is positive, the results in terms of endpoints of IOP, vision, and complications remain the same for all implants. In fact, although the M4 had the lowest IOP at 3 months, demonstrating an absence of the hypertensive phase, by 18 months, it had a lower rate of success than the FP7 and the S2.
Therefore, with 18-month outcomes from this study, it seems that although the M4 has some promising characteristics, clinically its performance is similar to previously existing devices. Furthermore, the experience of some surgeons has been that it is slightly more difficult to implant given the added resistance of the polyethylene covering. However, this device seems to be a step in the right direction for the continued improvement of these devices. Larger studies with longer follow-up are needed to further delineate the characteristics of the M4 implant.
Abstract
