Abstract
Abstract: :
Purpose: To present a model applying hydraulic principles to compare the postoperative situation in trabeculectomy (TE) and non-penetrating deep sclerectomy (NPDS) as well as to attempt to explain unanswered aspects of the postoperative outflow. Comparing the data calculated from the model to clinical pre- and postoperative IOP and tonography data. Methods: A primary linear mathematical model based on fluid mechanics and clinical data was used to evaluate possible postoperative outflow pathways after filtration surgery. IOP and tonography data were obtained from 10 patients each pre-and postoperatively undergoing either TE or NPDS. Results: Based on mathematical modeling and using preoperative and postoperative IOP values the amount of postoperative outflow through the fistula (membrane) and the outflow through the residual natural drainage pathway could be estimated and compared. For instance according to the model, an eye with preoperative IOP of 30mmHg and coefficient of outflow facility C=0,09mm3/min*mmHg TE resulting in postoperative IOP of 10mmHg would drain only 8% of aqueous through the residual natural trabecular pathway. If NPDS would be carried out in the same eye with postoperative IOP of 15mmHg, the aqueous outflow through the trabecular route amounts up to 32%. In our mathematical model we demonstrated the decrease of aqueous outflow resistance after NPDS to be smaller than in TE. The model also accounts for differences in filtration blebs after TE and NPDS and demonstrates the inefficiency of extending the scleral lake into the suprachorioidal space as used in modifications of NPDS trying to increase the outflow from the scleral lake. The postoperative coefficient of outflow facility observed in the TE and NPDS patients matched the assumptions of the model. Conclusions: According to theoretical considerations, NPDS provides more aqueous outflow along the natural outflow pathways than TE. Generally, the higher the postoperative IOP and / or the lower the preoperative IOP the larger the amount of aqueous that will postoperatively utilize the natural outflow pathways. From our calculations it seems that the reestablishment of aqueous production postoperatively in addition to factors such as wound healing may be a reason of postoperative IOP increase in some patients. The difference in filtration blebs after TE and NPDS seems to exist because of the higher amount of aqueous exiting through the fistula after TE, not because of the existing hydrostatical pressure inside the bleb.
Keywords: intraocular pressure • aqueous • outflow: trabecular meshwork