| Author: | Wang, Zhaokun |
| Title: | Physical mechanism and control of intraoperative floppy iris syndrome |
| Advisors: | Tang, Hui (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Cataract -- Surgery -- Complications Iris (Eye) -- Diseases Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxxviii, 6, 207 pages : color illustrations |
| Language: | English |
| Abstract: | Cataract is clouding of the crystalline lens of an eye that prevents the light from reaching the retina, and hence impedes clear visual vision and heavily affects people's daily life. It is the leading cause of blindness, and accounts for 33% of the global visual impairment, which is expected to rise further with the population aging and life expectancy increasing. Surgery is the only effective treatment and the most common procedure performed for cataracts, in which the cloudy lens is extracted and replaced with a clear artificial lens. Most cataract surgeries are performed using the phacoemulsification technique, in which an oscillating probe combined with an irrigation and aspiration fluidic system emulsifies and removes the lens. It has become a standard method to remove cataract mainly due to its small incision wounds, less damage to the intraocular tissues, and minimal intraoperative and postoperative complications. Although generally successful, intra-operative complications are not uncommon. One of the major risks is abnormal intra-operative iris behavior, known as intra-operative floppy iris syndrome (IFIS), which may increase the risk of intraoperative complications and degrade the surgery outcomes. However, the physical mechanism of IFIS has still not been fully studied, especially from the mechanics viewpoint. Therefore, the present work aims to address this issue, so that more useful information can be provided for IFIS management. To facilitate the study, a dedicated fluid-structure interaction (FSI) simulation framework is developed, where the intraocular flow is simulated using the lattice Boltzmann method (LBM), the iris structure dynamics is solved using the finite element method (FEM) and the fluid-iris interaction is handled using the immersed boundary (IB) method. The dynamics of the fluid-iris system is studied in detail. With this framework, the iris dynamics in cataract surgery under different probe operating conditions is examined. When the probe is operating in the pure torsional mode (T mode), depending on the probe location, three distinct iris dynamics modes are identified, namely, the repulsion (RP) mode, the attraction (AT) mode and the adhesion (AH) mode. Among them, the RP mode is the most suitable mode for surgery because it ensures the safety of the iris. A parametric study is conducted. It is found that increasing the iris stiffness or reducing the probe input power can relieve the intraoperative iris risk. An optima frequency range (about 100~120Hz) is identified for the probe torsional operation. Meanwhile, dilating pupil is found an effective way in alleviating the risk of iris tissue damage through significantly shrinking the damaging AH mode region. The iris dynamics is then studied with the probe operated in the torsional mode together with irrigation/aspiration (I/A) flows, i.e., in the T-I/A mode. Two types of probes, i.e., the coaxial and bimanual probes, are considered. It shows the I/A flows play a dominant role in the iris dynamics for both types of probes. The coaxial probe seems more suitable for cataract surgery due to its larger safe operation space. Furthermore, it is found that the reduction in the I/A strength can help stabilize the iris and increasing the iris stiffness can alleviate the iris risks by shrinking the damaging AH mode zone. The iris dynamics is also studied with the probe operated in the longitudinal mode together with I/A flows, i.e., in the L-I/A mode. Simulation results show that the probe power greatly affects the iris dynamics. Increasing the probe power expands the safe RP mode zone and hence relieves the iris tissue risk. Stiffening the iris is also beneficial. Compared with the T-I/A mode, the L-I/A mode is better due to the larger RP mode zone for probe operation. To explore the management or control of the IFIS, probe power modulation is investigated. It is found that the power duty cycle in the T-I/A and L-I/A modes exerts an opposite influence on the iris dynamics. On the other hand, the probe's power pulse frequency does not show a significant impact. However, temporal power modulation can considerately strengthen repulsion to the iris in the L-I/A mode and attraction in the T-I/A mode. The lens removal efficiencies in different power-modulation modes are also studied. The results show that nucleus extraction is closely related to the power level, and the T-I/A mode performs better. 3D simulations are conducted to study the effectiveness of some IFIS management strategies, i.e., the use of phenylephrine and Malyugin ring. The results show that both pupil dilation and Malyugin ring implantation can effectively stabilize the iris in cataract surgery, alleviating the IFIS risk. This research greatly improves our understanding of phacoemulsification-based cataract surgery and IFIS from the mechanics point of view. The results have attracted great attention from eye clinicians and hopefully can influence existing surgical protocols for cataract removal. |
| Rights: | All rights reserved |
| Access: | open access |
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