Finite element modelling of intraocular lens positions inside the capsular bag

This thesis presents finite element models to investigate the mechanical stability of intraocular lenses (IOLs) after cataract surgery, specifically focusing on their decentration, tilt, and rotation. The research examined three key areas: (1) the effect of zonular insertion angles on lens accommodation, (2) the impact of capsulorhexis size and location, and (3) the influence of capsular contraction on IOL stability. The first study served as an exercise to establish and verify the finite element methodology by comparing simulation results with existing literature, thereby ensuring that the models used for analysing IOL stability and accommodation were robust and reliable. The first study focused on how zonular fibre insertion angles affect the accommodative capacity of the crystalline lens as a function of age. Models based on lenses aged 11, 29, and 45 years were simulated with various zonular insertion angles. The study found that smaller zonular angles allowed for greater optical power changes, with the 45-year-old model showing the largest change (0.30 D). This confirms that age-related changes in zonular fibre positioning affect accommodation. In the second study, finite element models were created to explore how varying capsulorhexis sizes (4.5 mm and 5.5 mm) and decentrations (0.5 mm and 1 mm) influenced IOL decentration and tilt. Different IOL powers were also compared (5 D, 29 D, 34 D). The results showed minimal impact of capsulorhexis size and decentration on IOL positioning, with decentration ranging between 0.43 µm and 8.3 µm and tilt between 0.02 and 0.09. These findings suggest that variations in capsulorhexis size and location are unlikely to cause noticeable clinical differences in IOL stability. The results were smaller than the clinical values, however. The third study investigated how IOL stability is affected by capsular contraction, which can occur post-surgery due to fibrosis. Finite element models simulated capsular contraction in different regions of the capsular bag, as well as the resulting IOL decentration, tilt, and rotation. Contractions on the anterior and posterior sides of the capsule caused the greatest decentration (up to 0.51 mm), while distant regions from the haptic led to the most significant tilt (up to 3.5). These results provide insights into how localized capsular contraction can impact IOL stability, which is particularly important for patients with Toric IOLs where rotation can affect astigmatic correction. In conclusion, this thesis contributes to understanding the mechanical behaviour of IOLs under various post-surgical conditions. It suggests that while capsulorhexis size and location do not critically affect IOL stability, capsular contraction can impact the IOL positioning significantly. These findings could inform surgical practices and postoperative care, particularly in managing cases of fibrosis and astigmatism correction. Furthermore, the simulations have shown that zonular insertion angles influence the accommodation, which complements our current understanding of accommodation and presbyopia. These findings could inform surgical practices and postoperative care, particularly in managing cases of fibrosis and astigmatism correction.