Computational fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow and allows for the simulation of complex fluid systems. It has been applied to various biomedical systems, such as those involved in heart pumping, cardiac valve design, blood flow analysis, vessel graft evaluation, nose or sinus flow analysis, and lung airflow analysis. The separation of inflow and outflow in BMICS allows for smaller incision sizes, which in turn reduces the risk of surgically induced astigmatism, improves intraoperative visibility, and reduces the risk of endophthalmitis. The separation of inflow and outflow also increases the followability of nuclear fragments and flexibility of the two incisions. However, the main disadvantage of BMICS is an increase in anterior chamber instability that results from the limited irrigation. In the present study, we used computational fluid dynamics to further compare the advantages and disadvantages of BMICS and CMICS. Variables of interest included the flow velocity, pressure distribution of the anterior and posterior chambers, turbulence intensity, stability of the posterior capsule, and predicted anterior chamber pressure; all variables were investigated using different flow rates of the irrigating solution. We were particularly interested in measuring the followability of nuclear fragments, posterior capsule fluctuation and the anterior chamber instability during BMICS.
Figures 3, 4 show the pressure contours around the bimanual cannula outlet for flow rates of 18, 23, and 28 ml/min, respectively. For these flow rates during BMICS, the pressure differences between the outside and inside of the cannula were 0.512 Pascal, 1.509 Pascal, and 2.219 Pascal, respectively. For these same flow rates during CMICS, the pressure differences between the outside and inside of the cannula were 0.301 Pascal, 1.266 Pascal, and 2.219 Pascal, respectively (Figures 5,6; Table 2). For both types of cataract surgeries, the absolute pressure difference between the outside and inside of the cannula increased as the flow rate increased. At the same flow rate, the pressure difference between the outside and inside of the cannula was always larger in BMICS than in CMICS. This pressure difference between the outside and inside of the cannula is the measure of followability. The larger pressure difference between the outside and inside of the cannula has higher followability of nuclear fragments. Therefore, in our simulation, the followability measured during BMICS was found to be greater than that measured during CMICS. These results are consistent with those of previous reports. Agarwal et al. also reported that the separation of irrigation and aspiration increased the followability of nuclear fragments. The inflow and outflow of fluids occurred at the same spot of cannula simultaneously in CMICS. However, in the BMICS, the nucleus is not pushed away by the infusion fluid flow. And there are no published evidence reporting such a result. Overall, based on our simulation, bimanual cataract surgery was shown to be superior in terms of followability.
One of the worst complications that can occur during cataract surgery procedures is posterior capsule rupture(PCR). To compare the probability of PCR occurring in either type of microincision cataract surgery, we evaluated the turbulence intensity measured by turbulence kinetic energy and the pressure gradient of the posterior capsule. The turbulence kinetic energy means the intensity of the fluctuation and unsteadiness of the flow velocity. The turbulence kinetic energy measured during BMICS was found to be higher than that during CMICS(Table3). As the flow rate increased, the turbulence kinetic energy for both cataract surgeries decreased. The pressure gradient of the posterior capsule in BMICS was more diffuse than that observed in CMICS(Figures 3–6). In the BMICS, the difference in the posterior capsule pressure gradient decreased as the flow rate increased. In BMICS, with the flow rate increasing, the highest posterior capsule pressure gradient areas become smaller(Fig. 3-5). For the same pressure gradient, smaller areas are more easily influenced by pressure gradients, which increases the fluctuations of the posterior capsule and the probability of PCR. Overall, in the analysis of the turbulence intensity and pressure gradient of posterior capsule, the posterior capsule instability were found to be higher in BMICS than in CMICS. The limited irrigation that occurs during BMICS has also been previously shown to cause anterior chamber instability. Therefore, the bimanual irrigation and aspiration procedure required to have a lower flow resistance for the irrigation system than for the aspiration system can be achieved using cannulas with larger lumen diameters and shorter lengths.
In the present study, the anterior chamber(AC) pressure decreased linearly as the flow rate increased. Typically, below a critical flow rate, when the AC pressure becomes less than the ambient atmospheric pressure, it results in collapse of the AC. Our measured critical values of the AC pressure less than the ambient atmospheric pressure are 23.148cc/min in BMICS and 29.419 cc/min in CMICS. This simulations are similar results of existing report.
This study has some limitations. While we modeled the posterior capsule as a rigid structure, it is actually a thin membrane whose various movements are influenced by fluid flow. Therefore, the present study does not account for the various sequential movements of the posterior capsule. Other factors, including surgical setting, location of the cannula tips, and surgical method used, can also influence the movement of the posterior capsule. This simulations do not present shape of remnant cataract lens and ultrasonic energy. This simulations cannot present real fluid flow of chamber and movement of posterior capsule perfectly but also can be a clue of prevalent convention which is inferred by previous studies.
Despite the limitations of the present study, our findings show that computational fluid dynamics can provide a better understanding of the fluid flow within the anterior chamber and lens cavity during phacoemulsification procedures. This is critical given that a similarly comprehensive analysis of the fluid flow cannot be obtained solely through experimental methods. Here we have shown that BMICS produces better followability of nuclear fragments. However, BMICS also increases the fluctuation of the posterior capsule and increases the risk of anterior chamber collapse. It can be a scientific clue of prevalent convention of superior in terms of followability which is inferred by previous studies. The numerical values obtained by fluid flow analysis also can be applied in the clinical field to help prevent PCR and chamber collapse.