The action of ciliary muscle contraction on accommodation of the lens explored with a 3D model

Finite Element Analysis of Ocular Impact Forces and Potential Complications in Pickleball-Related Eye Injuries

Purpose: Pickleball, the fastest-growing sport in the United States, has seen a rapid increase in participation across all age groups, particularly among older adults. However, the sport introduces specific risks for ocular injuries due to the unique dynamics of gameplay and the physical properties of the pickleball. This study aims to explore the mechanisms of pickleball-related eye injuries, utilizing finite element modeling (FEM) to simulate ocular trauma and better understand injury mechanisms. Methods: A multi-modal approach was employed to investigate pickleball-related ocular injuries. Finite element modeling (FEM) was used to simulate blunt trauma to the eye caused by a pickleball. The FEM incorporated detailed anatomical models of the periorbital structures, cornea, sclera, and vitreous body, using hyperelastic material properties derived from experimental data. The simulations evaluated various impact scenarios, including changes in ball velocity, angle of impact, and material stiffness, to determine the stress distribution, peak strain, and deformation in ocular structures. The FEM outputs were correlated with clinical findings to validate the injury mechanisms. Results: The FE analysis revealed that the rigid, hard-plastic construction of a pickleball results in concentrated stress and strain transfer to ocular structures upon impact. At velocities exceeding 30 mph, simulations showed significant corneal deformation, with peak stresses localized at the limbus and anterior sclera. Moreover, our results show a significant stress applied to lens zonules (as high as 0.35 MPa), leading to potential lens dislocation. Posterior segment deformation was also observed, with high strain levels in the retina and vitreous, consistent with clinical observations of retinal tears and vitreous hemorrhage. Validation against reported injuries confirmed the model's accuracy in predicting both mild injuries (e.g., corneal abrasions) and severe outcomes (e.g., hyphema, globe rupture). Conclusions: Finite element analysis provides critical insights into the biomechanical mechanisms underlying pickleball-related ocular injuries. The findings underscore the need for preventive measures, particularly among older adults, who exhibit age-related vulnerabilities. Education on the importance of wearing protective eyewear and optimizing game rules to minimize high-risk scenarios, such as close-range volleys, is essential. Further refinement of the FEM, including parametric studies and integration of protective eyewear, can guide the development of safety standards and reduce the socio-economic burden of these injuries.

Immediate Effects of Light Mode and Dark Mode Features on Visual Fatigue in Tablet Users

Background: Prolonged tablet use can cause visual fatigue, dry eye, and changes in critical flicker frequency, affecting visual comfort and performance. While the light and dark mode features aim to reduce eye strain, their immediate effects on these issues remain unclear. Objective: To compare the immediate effects of light and dark mode features on visual fatigue in tablet users. Methods: This experimental study involved 30 female tablet users. The participants were randomly assigned using a block randomization method to use both light and dark mode features. Visual fatigue, critical flicker frequency, and dry eye symptoms were measured before and after exposure to each mode. Results: No statistically significant difference in visual fatigue was observed between the two modes (p > 0.05). However, a statistically significant difference was found in critical flicker frequency (p < 0.05) and dry eye symptoms (p < 0.05) between the two modes. Furthermore, visual fatigue, critical flicker frequency, and dry eye symptoms significantly increased after tablet use in both modes (p < 0.05). Conclusions: Both light and dark mode features impact visual fatigue, critical flicker frequency, and dry eye symptoms. However, the dark mode may help reduce the risk of eye fatigue compared to the light mode. Further studies are recommended to explore the long-term effects and optimize screen settings for visual comfort.

Age-Related Changes in Lens Elasticity Contribute More to Accommodative Decline Than Shape Change

Purpose

To determine whether lens biomechanical or geometric changes contribute to the decline in the accommodative capacity of the human eye, and to examine any differences in zonular function between different age groups.

Methods

Eighteen finite element whole eye models were developed to simulate the accommodative process. Six models were constructed in each of the two age cohorts, from the fourth and the sixth decades of life using data from ex vivo human lenses. An additional six models combining the material properties of lenses from the fourth decade with the geometry of those from the sixth decade were included. Optical lens models developed based on the results of mechanical simulations were used to calculate the central optical power (COP).

Results

The change in COP was significantly greater for both the fourth-decade models and the mixed models compared with the sixth-decade models. The rates of the change in geometric parameters relative to the increment of change in COP in the mixed models were greater than those in the fourth-decade models. The distribution of zonular force was consistent across all three groups. However, the sixth-decade models and mixed models exhibited similar distributions of zonular angles, both of which were greater than those in the fourth-decade models.

Conclusions

Both biomechanical and geometric age-related changes contribute to the accommodative decline, with the material property manifesting a more substantial impact. Age-related changes in the lens do not influence the distribution of zonular tension, but do affect the angles that the zonule makes with the lens surface.

The effect of aging on the ciliary muscle and its potential relationship with presbyopia: a literature review

Background

The ciliary muscle is known to play a part in presbyopia, but the mechanism has not received a comprehensive review, which this study aims to achieve. We examined relevant articles published from 1975 through 2022 that explored various properties of the muscle and related tissues in humans and rhesus monkeys. These properties include geometry, elasticity, rigidity, and composition, and were studied using a range of imaging technologies, computer models, and surgical methods. We identified a notable age-related displacement of the ciliary muscle apex that is characterized by anterior and medial shifts, and hypothesized to be primarily attributed to the accrual of connective tissue and tension exerted by the thickening lens. Other factors could also contribute to the movement, particularly the "inward bowing" of the sclera. Another noteworthy observation is that while the ciliary muscle experiences increasing constraint with advancing age due to adjacent anatomical structures, its contractile capacity remains unaltered, alongside the sustained constancy in both the concentration of muscarinic receptors and their binding affinity. Overall, more studies on human ciliary muscle are needed, as it ages differently from that of monkeys' ciliary muscle. These studies should explore other perspectives, including those regarding changes in the physical properties of the tissue and its relationship with other connected tissues.

Methodology

This literature review utilized a systematic methodology to identify and analyze pertinent studies of the presbyopia and ciliary muscles. The approach encompassed a thorough examination of available literature across different academic databases, such as PubMed, Embase, and Cochrane Library.

Results

Many studies have identified age-related thickening in the ciliary muscle and its potential causes, including the heightened deposition of connective tissues and traction exerted by the thickening lens. Notably, these inquiries culminated in the formulation of a geometric theory positing that the morphology of the ciliary muscle and its spatial relationship with adjacent structures exert pivotal influence over the tension exerted on zonular fibers.

Conclusion

The decline in the accommodative response of the muscle is prevalent in advanced age, with reduced mobility likely attributable to the increased stiffness of the Bruch's Membrane-Choroid Complex (BMCC), where the tendons of the ciliary muscle insert, as well as the thickening and stiffening of the lens. Importantly, the ciliary muscle forces do not change with age.

Unveiling the Mysteries of the Chinese lens zonule balance tension: A statistical analysis

Background

This study aims to provide an estimated dataset of lens zonule balance tension (LZBT) measurements in cataractous lenses among Chinese patients and to conduct a statistical analysis of anterior segment parameters.

Methods

This is a cross-sectional study. We included a total of 833 eyes from 833 Chinese participants aged 23-91 years who underwent cataract surgery. Anterior segment parameters were measured using swept-source anterior segment optical coherence tomography (SS-AS OCT) to calculate lens gravity (LG) and crystalline lens volume (VOL). Axial length (AL) was measured using IOL-Master 700. LZBT was calculated using force decomposition and synthesis methods.

Results

The mean LZBT in the horizontal direction was 8.48E-05 ± 3.23E-05 N across all eyes. The LZBT in the subtemporal-supraspinal and supraspinal-temporal directions was2 times greater than in the vertical direction. The balanced force on the suspensory ligament in the horizontal direction was twice as strong as in the vertical direction. Additionally, anterior segment parameters such as lens equivalent diameter (LE-dia), radius of the anterior lens surface curvature (RAL), radius of the posterior lens surface curvature (RPL), anterior chamber depth (ACD), iris area, iris volume, and iris thickness showed positive correlations with AL.

Conclusions

Accounting for gravitational effects, we concluded that zonule force is asymmetrically distributed. Importantly, this study establishes the normal range of LZBT across different meridians using force synthesis and decomposition, offering new insights and feasibility for studying suspensory ligament biomechanics.

BCLA CLEAR presbyopia: Management with scleral techniques, lens softening, pharmaceutical and nutritional therapies

The aging eye undergoes the same progressive crosslinking which occurs throughout the body, resulting in increased rigidity of ocular connective tissues including the lens and the sclera which impact ocular functions. This offers the potential for a scleral treatment that is based on restoring normal biomechanical movements. Laser Scleral Microporation is a laser therapy that evaporates fractional areas of crosslinked tissues in the sclera, reducing ocular rigidity over critical anatomical zones of the accommodation apparatus, restoring the natural dynamic range of focus of the eye. Although controversial and challenged, an alternative theory for presbyopia is Schachar's theory that suggests a reduction in the space between the ciliary processes and the crystalline lens. Widening of this space with expansion bands has been shown to aid near vision in people with presbyopia, a technique that has been used in the past but seems to be obsolete now. The use of drugs has been used in the treatment of presbyopia, either to cause pupil miosis to increase depth of focus, or an alteration in refractive error (to induce myopia in one eye to create monovision). Drugs and laser ablation of the crystalline lens have been used with the aim of softening the hardened lens. Poor nutrition and excess exposure to ultraviolet light have been implicated in the onset of presbyopia. Dietary nutritional supplements, lifestyle changes have also been shown to improve accommodation and the question arises whether these could be harnessed in a treatment for presbyopia as well.

The Effect of the Zonular Fiber Angle of Insertion on Accommodation

Purpose: With age, there is an anterior shift of the ciliary body in the eye, which alters the angle of zonular insertion in older eyes compared with younger eyes. This study aims to simulate lens accommodation with different zonular angles to consider the influence of zonular position on lens accommodative capacity. Methods: Models were constructed based on lenses aged 11, 29, and 45 years using a 2D axisymmetric structure that included a capsule, cortex, nucleus, and zonular fibers. The different zonular fibers were simulated by changing the position of the point where the zonular fibers connect to the ciliary body. The effect of the different zonular fiber insertion angles on the model shape and optical power was analyzed. Results: The models show that smaller angles made by zonular fibers to the surface of the lens lead to larger optical power changes with simulated stretching. When the models were stretched, and when varying the zonule angles, the optical power of the 11-, 29-, and 45-year-old models changed up to 0.17 D, 0.24 D, and 0.30 D, respectively. The effect of zonular angles on the anterior radius of curvature of the anterior surface varied by 0.29 mm, 0.23 mm, and 0.25 mm for the 11-, 29-, and 45-year-old models, respectively. Conclusions: Larger zonular fiber insertion angles cause smaller deformation and less accommodative change, while parallel zonules induce the largest change in lens shape.

The development and the genetic diseases of the ciliary body

The ciliary body, located at the junction of the choroid and iris, is crucial in the development of the embryonic eye. Notch2 signalling, Wnt signalling, transforming growth factor β (TGF-β) signalling, and Pax6 signalling are critical for coordinating the ciliary body formation. These signalling pathways are coordinated with each other and participate in the ciliary body development, ensuring the precise formation and optimal functioning of the eye structure. Although rare, ciliary body hypoplasia, ciliary tumours, and genetic-related iritis indicate the intricate nature of ciliary body development. Given the ciliary body's important biological significance and potential medical relevance, we aim to provide a comprehensive overview of the developmental molecular mechanisms governing ciliary body formation and function. Here, we focus on the intricate signalling pathways governing ciliary body development and corresponding genetic ciliary diseases.

The potential influence of the ligament of Wieger on the crystalline lens shape

This research uses mathematical modelling to evaluate the influence of the ligament of Wieger on the crystalline lens shape at rest, and during accommodation. An axisymmetric model of the anterior segment, including the ligament of Wieger, was created using the finite element method. Different conditions including variations of stiffness and positions of the ligament, with and without the ligament, were tested to see how they affected lens curvature and optical power. Adding the ligament of Wieger to the simulation had a noticeable impact on the optical power of the lens, particularly on the posterior surface power and total power. Ligament stiffness and width significant influenced the accommodative range of the eye by - 0.95D and - 2.39D for ligaments with the same and 3× the stiffness of the capsular bag, respectively. Ligament width and inner diameter had negligible effects on lens thickness but did have significant effects on posterior surface power and accommodation. In this simulation, we found that the ligament of Wieger can significantly affect the lens shape, both at rest and during accommodation, and may need to be considered in lens models.

A new look at an old problem: 3D modeling of accommodation reveals how age-related biomechanical changes contribute to dysfunction in presbyopia

Presbyopia is an age-related ocular disorder where accommodative ability declines so that an individual's focusing range is insufficient to provide visual clarity for near and distance vision tasks without corrective measures. With age, the eye exhibits changes in biomechanical properties of many components involved in accommodation, including the lens, sclera, and ciliary muscle. Changes occur at different rates, affecting accommodative biomechanics differently, but individual contributions to presbyopia are unknown. We used a finite element model (FEM) of the accommodative mechanism to simulate age-related changes in lens stiffness, scleral stiffness, and ciliary contraction to predict differences in accommodative function. The FEM predicts how ciliary muscle action leads to lens displacement by initializing a tensioned unaccommodated lens (Phase 0) then simulating ciliary muscle contraction in accommodation (Phase 1). Model inputs were calibrated to replicate experimentally measured lens and ciliary muscle in 30-year-old eyes. Predictions of accommodative lens deformation were verified with additional imaging studies. Model variations were created with altered lens component stiffnesses, scleral stiffness, or ciliary muscle section activations, representing fifteen-year incremental age-related changes. Model variations predict significant changes in accommodative function with age-related biomechanical property changes. Lens changes only significantly altered lens thickening with advanced age (46% decrease at 75 years old) while sclera changes produced progressive dysfunction with increasing age (23%, 36%, 49% decrease at 45, 60, and 75 years old). Ciliary muscle changes effected lens position modulation. Model predictions identified potential mechanisms of presbyopia that likely work in combination to reduce accommodative function and could indicate effectiveness of treatment strategies and their dependency on patient age or relative ocular mechanical properties.

Implantation of a capsular tension ring during cataract surgery attenuates predicted remodeling of the post-surgical lens capsule along the visual axis

Introduction: Cataract surgery permanently alters the mechanical environment of the lens capsule by placing a hole in the anterior portion and implanting an intraocular lens (IOL) that has a very different geometry from the native lens. We hypothesized that implant configuration and mechanical interactions with the post-surgical lens capsule play a key role in determining long-term fibrotic remodeling. Methods: We developed the first finite element-growth and remodeling (FE-G&R) model of the post-surgical lens capsule to evaluate how implantation of an IOL with and without a capsular tension ring (CTR) impacted evolving lens capsule mechanics and associated fibrosis over time after cataract surgery. Results: Our models predicted that implantation of a CTR with the IOL into the post-surgical lens capsule reduced the mechanical perturbation, thickening, and stiffening along the visual axis in both the remnant anterior and posterior portions compared to implantation of the IOL alone. Discussion: These findings align with patient studies and suggest that implantation of a CTR with the IOL during routine cataract surgery would attenuate the incidence of visually-debilitating capsule fibrosis. Our work demonstrates that use of such modeling techniques has substantial potential to aid in the design of better surgical strategies and implants.

The major influence of anterior and equatorial zonular fibres on the far-to-near accommodation revealed by a 3D pre-stressed model of the anterior eye

PURPOSE

To explore the synergistic function of the ligaments in eye, the zonular fibres, that mediate change in eye lens shape to allow for focussing over different distances.

METHODS

A set of 3D Finite Element models of the anterior eye together with a custom developed pre-stress modelling approach was proposed to simulate vision for distant objects (the unaccommodated state) to vision for near objects (accommodation). One of the five zonular groups was cut off in sequence creating five models with different zonular arrangements, the contribution of each zonular group was analysed by comparing results of each specific zonular-cut model with those from the all-zonules model in terms of lens shape and zonular tensions.

RESULTS

In the all-zonular model, the anterior and equatorial zonules carry the highest tensions. In the anterior zonular-cut model, the equatorial zonular tension increases while the posterior zonular tension decreases, resulting in an increase in the change in Central Optical Power (COP). In the equatorial zonular-cut model, both the anterior and posterior zonular tensions increase, causing a decreasing change in COP. The change in COP decreases only slightly in the other models. For vitreous zonular-cut models, little change was seen in either the zonular tension or the change in COP.

CONCLUSIONS

The anterior and the equatorial zonular fibres have the major influence on the change in lens optical power, with the anterior zonules having a negative effect and the equatorial zonules contributing a positive effect. The contribution to variations in optical power by the equatorial zonules is much larger than by the posterior zonules.

Biomechanical analysis of ocular diseases and its in vitro study methods

Ocular diseases are closely related to the physiological changes in the eye sphere and its contents. Using biomechanical methods to explore the relationship between the structure and function of ocular tissue is beneficial to reveal the pathological processes. Studying the pathogenesis of various ocular diseases will be helpful for the diagnosis and treatment of ocular diseases. We provide a critical review of recent biomechanical analysis of ocular diseases including glaucoma, high myopia, and diabetes. And try to summarize the research about the biomechanical changes in ocular tissues (e.g., optic nerve head, sclera, cornea, etc.) associated with those diseases. The methods of ocular biomechanics research in vitro in recent years are also reviewed, including the measurement of biomechanics by ophthalmic equipment, finite element modeling, and biomechanical analysis methods. And the preparation and application of microfluidic eye chips that emerged in recent years were summarized. It provides new inspiration and opportunity for the pathogenesis of eye diseases and personalized and precise treatment.

Computational Modeling of Ophthalmic Procedures: Computational Modeling of Ophthalmic Procedures

PURPOSE

To explore how finite-element calculations can continue to contribute to diverse problems in ophthalmology and vision science, we describe our recent work on modeling the force on the peripheral retina in intravitreal injections and how that force increases with shorter, smaller gauge needles. We also present a calculation that determines the location and stress on a retinal pigment epithelial detachment during an intravitreal injection, the possibility that stress induced by the injection can lead to a tear of the retinal pigment epithelium.

BACKGROUND

Advanced computational models can provide a critical insight into the underlying physics in many surgical procedures, which may not be intuitive.

METHODS

The simulations were implemented using COMSOL Multiphysics. We compared the monkey retinal adhesive force of 18 Pa with the results of this study to quantify the maximum retinal stress that occurs during intravitreal injections.

CONCLUSIONS

Currently used 30-gauge needles produce stress on the retina during intravitreal injections that is only slightly below the limit that can create retinal tears. As retina specialists attempt to use smaller needles, the risk of complications may increase. In addition, we find that during an intravitreal injection, the stress on the retina in a pigment epithelial detachment occurs at the edge of the detachment (found clinically), and the stress is sufficient to tear the retina. These findings may guide physicians in future clinical research. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Mechanical Load and Piezo1 Channel Regulated Myosin II Activity in Mouse Lenses

The cytoarchitecture and tensile characteristics of ocular lenses play a crucial role in maintaining their transparency and deformability, respectively, which are properties required for the light focusing function of ocular lens. Calcium-dependent myosin-II-regulated contractile characteristics and mechanosensitive ion channel activities are presumed to influence lens shape change and clarity. Here, we investigated the effects of load-induced force and the activity of Piezo channels on mouse lens myosin II activity. Expression of the Piezo1 channel was evident in the mouse lens based on immunoblot and immufluorescence analyses and with the use of a Piezo1-tdT transgenic mouse model. Under ex vivo conditions, change in lens shape induced by the load decreased myosin light chain (MLC) phosphorylation. While the activation of Piezo1 by Yoda1 for one hour led to an increase in the levels of phosphorylated MLC, Yoda1 treatment for an extended period led to opacification in association with increased calpain activity and degradation of membrane proteins in ex vivo mouse lenses. In contrast, inhibition of Piezo1 by GsMTx4 decreased MLC phosphorylation but did not affect the lens tensile properties. This exploratory study reveals a role for the mechanical load and Piezo1 channel activity in the regulation of myosin II activity in lens, which could be relevant to lens shape change during accommodation.

A Biomechanical Model for Evaluating the Performance of Accommodative Intraocular Lenses

Accommodation alters the shape of the eye lens to change focus from distant to near vision. This function declines with age in the development of presbyopia and most people experience a near total loss of accommodative ability by 55 years. Currently, there are no surgical procedures that correct presbyopia, but considerable work has been done in the development of accommodative intraocular lenses (AIOLs) implanted during cataract surgery. Despite these efforts, AIOLs only restore ∼ 20% of youthful accommodative amplitude and they suffer from high rates of visually-debilitating fibrosis. An important design tool that is lacking that could aid in improving AIOL designs is modeling. Herein, we addressed this need through the development of a fully 3-D finite element model that was used to predict the behavior of a dual-optic AIOL implanted within the post-surgical lens capsule. Models of the native human lens were developed to identify the stress-free configuration of the lens capsule needed to accurately predict the accommodated state of the lens and the configuration of the zonular traction needed for the disaccommodated state. The AIOL model demonstrated the functional importance of implant stiffness and predicted an approximately linear relationship between zonular traction magnitude and axial displacement of the optics. To our knowledge, this is the first model that can be used to gain insights into AIOL efficacy. It provides a foundation for continued development of a predictive tool that could ultimately improve AIOL designs that seek to restore youthful accommodative function.