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DISCUSSION
Histomorphological Change in the Implanted Silicone Membrane and Clinical Implication In our study fibrosis occurred around silicone membrane and resulted in a sac,which is similar to that observed in the study of Rubin et al[6]. Minckler found that the pressure inside the sac was the same as IOP[7]. This implied that IOP achieved after the operation was determined by the size and permeability of the sac around the implant, which in turn, was related to the material, shape and size of the implant[8,9]. Silicone seems to be an ideal material at present because it causes slighter inflammation and has better biocompatibility. Silicone membrane in rectangle helps to form bigger drainage area, facilitate outflow of aqueous humor and form filtering blebs. In our study, the sac wall was formed of 45 layers of collagen fibers, which communicated with blebs.
Fibrosis around silicone membrane is similar in mechanism to that in retinal detachment model[10]. The sac formed is the foundation of filtering blebs. Under microscope, the outer layer of bleb wall was constructed by collagen fibers and small
Figure 1 Bleb ranks of the implant group and control group at different timepoints
blood vessels; in the inner layer, loose connective tissue and capillary vessels can be seen; between the inner and outer layer, debris of inflammatory cells can be seen. Such structure is very important for aqueous humor filtration.
The slender void between the sac and silicone membrane ensures aqueous humor flow, but the speed and amount of the aqueous humor outflow is related to fibroblast proliferation. That is to say, silicone membrane stimulates inflammation to produce adequate collagen to fix silicon membrane; the amount of collagen produced is controlled to allow for the adequate aqueous humor flow to filter blebs. Fibrous tissue grows to the point that it can encapsulate the drainage implant but not result in adhesion or overproliferation. There might be two explanations for this: first, silicone membrane does not support adherence of fibrocytes; second, aqueous humor is inhibitory to fibroblasts[11].
Implant drainage surgery is a special filtering surgery. IOP is the major evaluating index. In the present study, IOP changes were found to be in positive correlation with fibroblast proliferation in the control group, but no such linear correlation was found in the implant group. IOP control in the implant group was much better than that in the control group. However, on the 30th day, IOP ran out of control in 20% eyes in the implant group. This could be the result of subconjunctival fibrosis or shift of silicone membrane so that drainage tract was blocked by fibrous tissue.
AgNoRs Expression in Fibroblast Proliferation The number and shape of AgNors particles indicate activity of fibroblast proliferation. The more AgNors particles there are, the more active proliferation is. In our study, according to the number and shape of AgNoRs particles, fibroblasts in the control group began to proliferate from the 2nd3rdday after operation, reached the peak on the 5th 7th day and then gradually transformed into fibrocytes. This was agreed with Jampels findings[12]. So it can be concluded that silicone membrane does not stimulate constant or excessive proliferation of fibroblasts.
Comparison of Silicone Membrane with Other Implants The silicone membrane used in this study has dual functions of drainage and aqueous diffusion, which avoids postoperative fibrosis in the connection of tube and disk in other implants. In addition, silicone membrane, being soft, can achieve moreadequate anastomosis than other implants and thus avoids diplopia and strabismus resulting from unfit implantation. In the filtering surgery with implants of all other types, the incidence of complications is approximately 50%70% with more than 20 complications. Most of the complications can be avoided because silicone membrane is soft, its fixing site is away from rectus muscle and it does not enter the anterior chamber.
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