Morphology and accommodative function of the vitreous zonule in human and monkey eyes

BCLA CLEAR presbyopia: Mechanism and optics

With over a billion adults worldwide currently affected, presbyopia remains a ubiquitous, global problem. Despite over a century of study, the precise mechanism of ocular accommodation and presbyopia progression remains a topic of debate. Accordingly, this narrative review outlines the lenticular and extralenticular components of accommodation together with the impact of age on the accommodative apparatus, neural control of accommodation, models of accommodation, the impact of presbyopia on retinal image quality, and both historic and contemporary theories of presbyopia.

Sex as a risk factor regarding presbyopia in the rhesus monkey

PURPOSE

To determine the effect of sex as a risk factor regarding presbyopia.

METHODS

Maximum accommodation was pharmacologically induced (40% cabachol corneal iontophoresis) in 97 rhesus monkeys (49 males and 48 females) ranging in age from 8 to 36 years old. Accommodation was measured by Hartinger coincidence refractometry.

RESULTS

Accommodative amplitude measured refractometrically decreased with age, and the rate of change was not different between males and females (p = 0.827).

CONCLUSIONS

Presbyopia is essentially sex neutral, and no one is spared. There may be modest variations between different populations for various reasons, but essentially it is monotonously predictable. At present there is no biological therapeutic.

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.

Bruch's membrane and Brücke's muscle in the pars plana region

PURPOSE

To examine Bruch's membrane (BM) in association with the longitudinal part of the ciliary muscle (LPCM) in the pars plana region.

METHODS

Using light microscopy, we histomorphometrically assessed BM and the LPCM in the pars plana region.

RESULTS

The histomorphometric study included 51 eyes (51 patients; mean age: 60.8 ± 15.0 years; axial length: 26.0 ± 3.3 mm; range: 21.0-36.0 mm). The LPCM (total length: 4.60 ± 1.10 mm) ended 1.15 ± 0.56 mm anterior to the ora serrata. Within the pars plana region, the LPCM (length: 2.58 ± 0.98 mm) had direct contact with BM for 1.95 ± 0.99 mm (71.1 ± 18.4% of the BM undersurface), while a capillary layer was interposed between the BM and the LPCM for 0.70 ± 0.40 mm (29.0 ± 18.4%). In the pars plana region free of LPCM close to the ora serrata, the percentage of BM covered by the capillary layer was higher than in the pars plana region containing the LPCM (63.0 ± 42.1% vs. 29.0 ± 18.4%; p < 0.001). At the LPCM end, BM was in direct contact with a collagenous tissue from the LPCM and was focally thickened as compared to BM with an underlying capillary layer (9.5 ± 5.3 μm vs. 4.3 ± 1.2 μm; p < 0.001).

CONCLUSIONS

The direct contact of BM with the LPCM in the pars plana in association with focal BM thickening at the LPCM end suggests an insertion of LPCM on the BM. Taking into account the biomechanical strength of BM, it may imply a functional unit of the LPCM with BM in the process of accommodation with a secondary movement of the posterior BM and tertiary thickening of the subfoveal choroidal space.

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.

Research progress of lens zonules

Background

The lens zonule, a circumferential system of fibres connecting the ciliary body to the lens, is responsible for centration of the lens. The structural, functional, and positional abnormalities of the zonular apparatus can lead to the abnormality of the intraocular structure, presenting a significant challenge to cataract surgery.

Main text

The lens zonule is the elaborate system of extracellular fibers, which not only centers the lens in the eye but also plays an important role in accommodation and lens immunity, maintains the shape of the lens, and corrects spherical aberration. The zonules may directly participate in the formation of cataract via the immune mechanism. Abnormal zonular fibers that affect the position and shape of the lens may play an important role in the pathogenesis of angle closure disease and increase the complexity of the surgery. Capsular tension rings and related endocapsular devices are used to provide sufficient capsular bag stabilization and ensure the safety of cataract surgery procedures. Better preoperative and intraoperative evaluation methods for zonules are needed for clinicians.

Conclusions

The microstructure, biomechanical properties, and physiological functions of the lens zonules help us to better understand the pathogenesis of cataract and glaucoma, facilitating the development of safer surgical procedures for cataract. Further studies are needed to carefully analyze the structure-function relationship of the zonular apparatus to explore new treatment strategies for cataract and glaucoma.

Morphometric assessment of the ciliary body in patients with Marfan syndrome and ectopia lentis: A quantitative study using ultrasound biomicroscopy: Ciliary body morphology in Marfan syndrome and ectopia lentis

PURPOSE

To explore the biometric characteristics of the ciliary body in patients with Marfan syndrome (MFS) and ectopia lentis (EL).

DESIGN

Cross-sectional study.

METHODS

Seventy-two consecutive MFS patients with EL and 72 non-disease controls were recruited. Ciliary body biometric parameters such as ciliary muscle cross-sectional area at 2000 μm from the scleral spur (CMA2000), ciliary muscle thickness at 1000 μm from the scleral spur (CMT1000), and maximum ciliary body thickness (CBTmax) were measured from multiple directions with ultrasound biomicroscopy (UBM). The relationship between ciliary body parameters and other ocular characteristics was also evaluated.

RESULTS

Average CMA2000, CMT1000, and CBTmax were 0.692 ± 0.015 mm², 0.405 ± 0.010 mm, and 0.855±0.023 mm in MFS eyes, respectively, and were significantly smaller than controls (all p < 0.001). The prevalence of ciliary body thinning was 22.2% in the MFS group versus 0 in controls (p < 0.001); eyes with more severe EL had smaller CMA2000 (p = 0.050), thinner CMT1000 (p = 0.022) and shorter CBTmax (p = 0.015). Patients with microspherophakia (MSP) had even smaller CMA2000 (p = 0.033) and CMT1000 (p = 0.044) than those without MSP. The most common subluxation direction was in the superonasal quadrant (25, 39.7%), which probably correlates with the thinnest CMT1000 in the inferotemporal quadrant (p = 0.005).

CONCLUSIONS

MFS patients with EL had thinner ciliary muscles, shorter ciliary processes, and a higher prevalence of ciliary body thinning, especially those with MSP. Both the extent and direction of subluxation were associated with ciliary body biometry.

CMS-NET: deep learning algorithm to segment and quantify the ciliary muscle in swept-source optical coherence tomography images

Background

The ciliary muscle plays a role in changing the shape of the crystalline lens to maintain the clear retinal image during near work. Studying the dynamic changes of the ciliary muscle during accommodation is necessary for understanding the mechanism of presbyopia. Optical coherence tomography (OCT) has been frequently used to image the ciliary muscle and its changes during accommodation in vivo. However, the segmentation process is cumbersome and time-consuming due to the large image data sets and the impact of low imaging quality.

Objectives

This study aimed to establish a fully automatic method for segmenting and quantifying the ciliary muscle on the basis of optical coherence tomography (OCT) images.

Design

A perspective cross-sectional study.

Methods

In this study, 3500 signed images were used to develop a deep learning system. A novel deep learning algorithm was created from the widely used U-net and a full-resolution residual network to realize automatic segmentation and quantification of the ciliary muscle. Finally, the algorithm-predicted results and manual annotation were compared.

Results

For segmentation performed by the system, the total mean pixel value difference (PVD) was 1.12, and the Dice coefficient, intersection over union (IoU), and sensitivity values were 93.8%, 88.7%, and 93.9%, respectively. The performance of the system was comparable with that of experienced specialists. The system could also successfully segment ciliary muscle images and quantify ciliary muscle thickness changes during accommodation.

Conclusion

We developed an automatic segmentation framework for the ciliary muscle that can be used to analyze the morphological parameters of the ciliary muscle and its dynamic changes during accommodation.

Anterior Segment Parameter Changes after Cataract Surgery in Open-Angle and Angle-Closure Eyes: A Prospective Study

Background: To investigate the anterior segment parameters before and after cataract surgery in open-angle eyes and different subtypes of primary angle-closure glaucoma (PACG) eyes and to further explore the potential relationship between the anterior rotation of the ciliary process and crystalline lens. Methods: An observational, prospective study was performed on 66 patients who had cataract surgery including 22 chronic PACG patients, 22 acute PACG patients, and 22 open-angle cataract patients. Anterior segment parameters including the trabecular-ciliary process distance, ciliary process area, trabecular-ciliary angle (TCA), maximum ciliary body thickness (CBTmax), and so on, were measured using ultrasound biomicroscopy preoperatively and 3 months postoperatively. Results: After the surgery, there were significant increases in TCA (p < 0.001) and CBTmax (p < 0.05) in all three groups, while there was no significant change in the trabecular-ciliary process distance (p > 0.05) in all three groups. No significant difference in the changes of ciliary process area, TCA, and CBTmax (p > 0.05) pre- and postoperatively among the three groups were identified. Conclusions: Extractions of crystalline lenses played similar roles in terms of decreasing the anterior rotation of ciliary processes in open-angle eyes and angle-closure eyes. A natural anatomical abnormality may be a more important factor in the anterior rotation of ciliary processes in PACG patients.

Novel Discoveries of the Relationship Between the Vitreous Zonule and the Anterior Segment Characteristics in Eyes With Primary Angle-Closure Disease

Purpose

To investigate the presence of the vitreous zonule (VZ) in different subtypes of primary angle-closure disease (PACD) and to explore the relationship between VZ and anterior chamber angle characteristics.

Methods

Patients with clinical diagnoses of acute primary angle-closure (PAC)/PAC glaucoma (APAC[G]) or chronic PAC/PAC glaucoma (CPAC[G]) and healthy subjects were enrolled. A total of 300 eyes of 180 subjects were included. Anterior segment parameters and the presence of the VZ were assessed by ultrasound biomicroscopy. The presence of VZ was compared among different subtypes of PACD. Anterior segment parameters were compared between eyes in vitreous zonule group (VZG) and no vitreous zonule group (NVZG). Logistic regression analysis was conducted to identify factors associated with the presence of VZ.

Results

APAC(G) eyes had lower VZ presence compared to the fellow eyes of APAC(G) (P < 0.001). VZ was more likely to be seen in the eyes of healthy subjects and PAC suspect than in the eyes of PAC and PAC glaucoma (PACG) (P < 0.05). NVZG had shorter angle opening distance 500/750 (P < 0.001), smaller trabecular iris angle 500/750 (P < 0.001), smaller trabecular-iris space area 500/750 (P < 0.001), smaller trabecular-ciliary angle (P = 0.009), smaller iris area (P = 0.010), and greater lens vault (P = 0.004) compared to VZG. Greater lens vault (LV) was independently associated with absence of VZ (odds ratio = 0.253; 95% confidence interval, 0.109-0.586; P = 0.001).

Conclusions

VZ was less likely to be observed in PAC/PACG eyes. PACD eyes with less VZ had narrower angle, more anteriorly rotated ciliary body, and greater LV.

Accommodative movements of the choroid in the optic nerve head region of human eyes, and their relationship to the lens

The ciliary muscle (CM) powers the accommodative response, and during accommodation the CM pulls the choroid forward in the region of the ora serrata. Our goal was to elucidate the accommodative movements of the choroid in the optic nerve region in humans and to determine whether these movements are related to changes in the lens dimensions that occur with aging, in the unaccommodated and accommodated state. Both eyes of 12 human subjects (aged 18-51 yrs) were studied. Homatropine (1 drop/5%) was used to relax the ciliary muscle (unaccommodated or "resting" eye) and pilocarpine was used to induce the maximum accommodative response (2 drops/4%) (accommodated eye). Images of the fundus and choroid were collected in the region of the optic nerve (ON) via Spectralis OCT (infrared and EDI mode), and choroidal thickness was determined. Ultrasound biomicroscopy (UBM; 50 MHz, 35 MHz) images were collected in the region of the lens/capsule and ciliary body. OCT and UBM images were collected in the resting and accommodated state. The unaccommodated choroidal thickness declined significantly with age (p = 0.0073, r = 0.73) over the entire age range of the subjects studied (18-51 years old). The choroidal thickness was significantly negatively correlated with lens thickness in the accommodated (p = 0.01) and the unaccommodated states (p = 0.005); the thicker the lens the thinner the choroid. Choroid movements around the optic nerve during accommodation were statistically significant; during accommodation the choroid both thinned and moved centrifugally (outward/away from the optic nerve head). The accommodative choroid movements did not decline significantly with age and were not correlated with accommodative amplitude. Measurement of the choroidal thickness is possible with the Spectralis OCT instrument using EDI mode and can be used to determine the accommodative changes in choroidal thickness. The choroidal thickness decreased with age and during accommodation. It may be that age-related choroidal thinning is due to changes in the geometry of the accommodative apparatus to which it is attached (i.e., ciliary muscle/lens complex) such that when the lens is thicker, the choroid is thinner. Accommodative decrease in choroidal thickness and stretch of the retina/choroid may indicate stress/strain forces in the region of the optic nerve during accommodation and may have implications for glaucoma.

Intraocular accommodative movements in monkeys; relationship to presbyopia

Our goal was to quantify the age-related changes in the dynamic accommodative movements of the vitreous and aqueous humor in iridic, aniridic, phakic and aphakic primate eyes. Six bilaterally iridic and four bilaterally iridectomized rhesus monkeys, ranging in age from 6 to 25 years, received a stimulating electrode in the midbrain Edinger-Westphal nucleus to induce accommodation, measured by a Hartinger coincidence refractometer. One of the four iridectomized monkeys underwent unilateral extracapsular and another monkey underwent intracapsular lens extraction. Eyes were imaged utilizing specialized techniques and contrast agents to resolve intraocular structures. During accommodation the anterior hyaloid membrane and the posterior lens capsule bowed backward. Central vitreous fluid and structures/strands moved posteriorly toward the optic nerve region as peripheral vitreous, attached to the vitreous zonule, was pulled forward by ciliary muscle contraction. Triamcinolone particles injected intravitreally were also observed in the anterior chamber and moved from the anterior chamber toward the cleft of the anterior hyaloid membrane and then further posteriorly into the vitreous-filled cleft between the vitreous zonule and the ciliary body pars plana. These accommodative movements occurred in all eyes, and declined with age. There are statistically significant accommodative movements of various intravitreal structures. The posterior/anterior fluid flow between the anterior chamber and the vitreous compartments during accommodation/disaccommodation represents fluid displacement to allow/facilitate lens thickening. The posterior accommodative movement of central vitreous fluid may result from centripetal compression of the anterior tips of the cistern-like structure attached to the vitreous zonule, and posterior displacement of the central trunk of the cistern during ciliary muscle contraction and centripetal muscle movement. The findings may have implications for presbyopia.

In vivo analysis of ciliary muscle in myopic Chinese young adults using ArcScan Insight® 100

OBJECTIVE

To analyse the morphological characteristics of the ciliary muscle (CM) and to explore its relationship with different ocular biometric parameters in myopic young Chinese adults.

METHODS

This observational, cross-sectional study included 50 right eyes from 50 myopic adults. The CM area (CMA), CM thickness (CMT) and CM length (CML) were measured using the ArcScan Insight^® 100. CMT was determined at three points: 1.0 mm (CMT-1), 2.0 mm (CMT-2) and 3.0 mm (CMT-3) posterior to the scleral spur. CML was measured on the scleral (CMLs) and vitreous (CMLv) aspects. The spherical equivalent refraction (SER), axial length (AL) and subfoveal choroidal thickness (SFCT) were examined to determine their associations with CM parameters (CMA, CML and CMT).

RESULTS

The mean SER and AL were -4.39 ± 2.29 D and 25.61 ± 1.15 mm, respectively. Compared with the nasal CMA, CML and CMT (CMT-1, CMT-2 and CMT-3) findings, the temporal CM parameters (CMA, CMLs, CMLv, CMT-1, CMT-2 and CMT-3) were found to be significantly thicker (all p < 0.001, except CMLv and CMT-1; p < 0.01). The nasal CMA was associated with the average corneal curvature (r = 0.30, p = 0.03) and SER (r = -0.30, p = 0.04). Nasal and temporal CMT-2 were negatively correlated with SER (r = -0.33 and -0.32, respectively, both p < 0.05). There was no correlation between CM parameters (except nasal CMLs, r = 0.31, p = 0.03) and SFCT, or between CM parameters and either the AL or anterior chamber depth (all p > 0.05).

CONCLUSION

These results suggest that there is temporal versus nasal asymmetry of the CM. CMA, CMT or CML did not vary with axial growth of the eye. The CM is not simply stretched as the eye elongates in myopic young adults.

Future Trends in Presbyopia Correction

Presbyopia affects 1.8 billion people worldwide. This reduction in distance corrected near visual acuity impacts quality of life, which prompts patients to seek treatment. Presbyopia is an early manifestation of the "crystalline lens optical dysfunction through aging," or dysfunctional lens syndrome, and appropriate management of presbyopia is dependent on coexisting factors such as increased higher order aberrations, reduced contrast sensitivity, light scatter, and lenticular opacification. This review of published literature (PubMed and MEDLINE) is presented in narrative format and discusses medical and surgical treatments available to patients who experience presbyopia, while highlighting future therapies. Numerous strategies exist for the management of presbyopia. These strategies include pharmacological therapy, glasses and contact lenses, corneal, scleral, and lenticular procedures. This article discusses the role of several new and existing presbyopia treatments, as well as which patients are candidates for these novel therapies. Although no single treatment is ideal for all patients with presbyopia, new medical and surgical strategies increase the number of options available when addressing different stages of presbyopia and dysfunctional lens syndrome. [J Refract Surg. 2021;37(6 Suppl):S28-S34.].

Quantitative analysis of internal components of the human crystalline lens during accommodation in adults

To quantitatively analyze changes in the inner components of the human crystalline lens during accommodation in adults. Eyes of 23 subjects were sequentially examined using CASIA2 Optical Coherence Tomography under 0D, - 3D and - 6D accommodation states. The anterior chamber depth (ACD), anterior and posterior crystalline lens radius of the curvature (ALRC and PLRC) were obtained using built-in software. The lens thickness (LT), lenticular nucleus thickness (NT), anterior cortex thickness (ACT), posterior cortex thickness (PCT), anterior and posterior lenticular nucleus radius of the curvature (ANRC and PNRC), anterior and posterior lenticular nucleus vertex (ANV and PNV) were quantified manually with the Image-pro plus software. During accommodation, the ACD became significantly shallower and LT significantly increased. For changes in the lens, the ALRC decreased by an average magnitude (related to accommodative stimuli) 0.44 mm/D, and PLRC decreased 0.09 mm/D. There was no difference for the ACT and PCT in different accommodation states. For lenticular nucleus response, NT increased on average by 30 μm/D. Both the ANRC and PNRC decreased on average by 212 μm/D and 115 μm/D respectively. The ANV moved forward on average by 0.07 mm under - 3D accommodative stimuli and 0.16 mm for - 6D. However, there was no statistically significant difference between different accommodation states in the PNV movement. Under accommodation stimulation, lens thickness changed mainly due to the lenticular nucleus, but not the cortex. For the lenticular nucleus, both the ANRC and PNRC decreased and ANRC changed the most. The anterior surface of the nucleus moved forward while the posterior surface of the nucleus moved backward but only slightly.

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

The eye's accommodative mechanism changes optical power for near vision. In accommodation, ciliary muscle excursion relieves lens tension, allowing it to return to its more convex shape. Lens deformation alters its refractive properties, but the mechanics of ciliary muscle actions are difficult to intuit due to the complex architecture of the tissues involved. The muscle itself comprises three sections of dissimilarly oriented cells. These cells contract, transmitting forces through the zonule fibers and extralenticular structures. This study aims to create a finite element model (FEM) to predict how the action of the ciliary muscle sections leads to lens displacement. The FEM incorporates initialization of the disaccommodated lens state and ciliary muscle contraction, with three muscle sections capable of independent activation, to drive accommodative movement. Model inputs were calibrated to replicate experimentally measured disaccommodated lens and accommodated ciliary muscle shape changes. Additional imaging studies were used to validate model predictions of accommodative lens deformation. Models were analyzed to quantify mechanical actions of ciliary muscle sections in lens deformation and position modulation. Analyses revealed that ciliary muscle sections act synergistically: the circular section contributes most to increasing lens thickness, while longitudinal and radial sections can oppose this action. Conversely, longitudinal and radial sections act to translate the lens anteriorly with opposition from the circular section. This FEM demonstrates the complex interplay of the three sections of ciliary muscle in deforming and translating the lens during accommodation, providing a useful framework for future investigations of accommodative dysfunction that occurs with age in presbyopia.

Zinn's zonule

The Zonule of Zinn, or ciliary zonule, is the elaborate system of extracellular fibers that centers the lens in the eye. In humans, the fibers transmit forces that flatten the lens during the process of disaccommodation, thereby bringing distant objects into focus. Zonular fibers are composed almost entirely of 10-12 nm-wide microfibrils, of which polymerized fibrillin is the most abundant component. The thickest fibers have a fascicular organization, where hundreds or thousands of microfibrils are gathered into micrometer-wide bundles. Many such bundles are aggregated to form a fiber. Dozens of proteins comprise the zonule. Most are derived from cells of the non-pigmented ciliary epithelium in the pars plana region, although some are probably contributed by the lens and perhaps other tissues of the anterior segment. Zonular fibers are viscoelastic cables but their component microfibrils are rather stiff structures. Thus, the elastic properties of the fibers likely stem from lateral interactions between microfibrils. Rupture of zonular fibers and subsequent lens dislocation (ectopia lentis) can result from blunt force trauma or be a sequela of other eye diseases, notably exfoliation syndrome. Ectopia lentis is also a feature of syndromic conditions caused typically by mutations in microfibril-associated genes. The resulting ocular phenotypes raise the possibility that the zonule regulates lens size and shape, globe size, and even corneal topology, in addition to its well-recognized role in accommodation.

Vitreous Zonule and its Relation to Anterior Chamber Angle Characteristics in Primary Angle Closure

PRECIS

Primary angle-closure (PAC) eyes with no vitreous zonule (VZ) appear to have a narrower angle despite similar lens vault and iris configuration than eyes with visible VZ.

PURPOSE

To assess the clinical significance of the VZ in PAC.

METHODS

Medical records of 91 eyes of 91 participants with PAC or PAC glaucoma were retrospectively reviewed. Anterior segment parameters were measured using anterior segment optical coherence tomography; presence of the VZ was assessed with ultrasound biomicroscopy. Parameters were compared between eyes with vitreous zonule group (VZG) and no vitreous zonule group (NVZG). Factors associated with VZ presence were determined using logistic regression analysis.

RESULTS

The NVZG was more likely to have PAC glaucoma than PAC (51.4% vs. 25.0%; P=0.010) and use more glaucoma medications (0.77 vs. 0.36; P=0.004) than the VZG. The NVZG had a smaller anterior chamber area than the VZG (13.6 mm vs. 15.1 mm; P=0.020) but there were no significant between-group differences in anterior chamber depth (1.97 vs. 2.08 mm; P=0.119) and lens vault (1.21 vs. 1.13 mm; P=0.337). NVZG had a smaller scleral spur angle (11.5 vs. 17.4 degrees; P<0.001), angle opening distance at 500 μm (AOD500, 105 vs. 168 μm; P<0.001), and trabecular-ciliary process angle (75.7 vs. 81.9 degrees; P=0.029) than VZG. Older age [odds ratio (confidence interval), 1.087 (1.014-1.164); P=0.018], less AOD500 (0.984 (0.975-0.993); P<0.001), and less trabecular-ciliary process angle (0.938 (0.901-0.977); P=0.002) were independently associated with an absence of VZ.

CONCLUSIONS

PAC eyes with no VZ had a narrower angle and required more glaucoma medications than eyes with a VZ.

Optical Coherence Tomography Reveals Sigmoidal Crystalline Lens Changes during Accommodation

This study aimed to quantify biometric modifications of the anterior segment (AS) during accommodation and to compare them against changes in both accommodative demand and response. Thirty adults, aged 18–25 years were rendered functionally emmetropic with contact lenses. AS optical coherence tomography (AS-OCT) images were captured along the 180° meridian (Visante, Zeiss Meditec, Jena, Germany) under stimulated accommodative demands (0–4 D). Images were analysed and lens thickness (LT) was measured, applying a refractive index correction of 1.00. Accommodative responses were also measured sequentially through a Badal optical system fitted to an autorefractor (Shin Nippon NVision-K 5001, Rexxam, Japan). Data were compared with Dubbelman schematic eye calculations. Significant changes occurred in LT, anterior chamber depth (ACD), lens centroid (i.e., ACD + LT/2), and AS length (ASL = ACD + LT) with accommodation (all p < 0.01). There was no significant change in CT with accommodation (p = 0.81). Measured CT, ACD, and lens centroid values were similar to Dubbelman modelled parameters, however AS-OCT overestimated LT and ASL. As expected, the accommodative response was less than the demand. Interestingly, up until approximately 1.5 D of response (2.0 D demand), the anterior crystalline lens surface appears to be the primary correlate. Beyond this point, the posterior lens surface moves posteriorly resulting in an over-all sigmoidal trajectory. he posterior crystalline lens surface demonstrates a sigmoidal response with increasing accommodative effort.

Multiscale Imaging Reveals the Hierarchical Organization of Fibrillin Microfibrils

Fibrillin microfibrils are evolutionarily ancient, structurally complex extracellular polymers found in mammalian elastic tissues where they endow elastic properties, sequester growth factors and mediate cell signalling; thus, knowledge of their structure and organization is essential for a more complete understanding of cell function and tissue morphogenesis. By combining multiple imaging techniques, we visualize three levels of hierarchical organization of fibrillin structure ranging from micro-scale fiber bundles in the ciliary zonule to nano-scale individual microfibrils. Serial block-face scanning electron microscopy imaging suggests that bundles of zonule fibers are bound together by circumferential wrapping fibers, which is mirrored on a shorter-length scale where individual zonule fibers are interwoven by smaller fibers. Electron tomography shows that microfibril directionality varies from highly aligned and parallel, connecting to the basement membrane, to a meshwork at the zonule fiber periphery, and microfibrils within the zonule are connected by short cross-bridges, potentially formed by fibrillin-binding proteins. Three-dimensional reconstructions of negative-stain electron microscopy images of purified microfibrils confirm that fibrillin microfibrils have hollow tubular structures with defined bead and interbead regions, similar to tissue microfibrils imaged in our tomograms. These microfibrils are highly symmetrical, with an outer ring and interwoven core in the bead and four linear prongs, each accommodating a fibrillin dimer, in the interbead region. Together these data show how a single molecular building block is organized into different levels of hierarchy from microfibrils to tissue structures spanning nano- to macro-length scales. Furthermore, the application of these combined imaging approaches has wide applicability to other tissue systems.

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Extracellular matrix fibrillin microfibrils assemble to form ocular ligaments.

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Individual beaded fibrillin microfibrils are highly symmetric biological polymers.

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Zonule fibers are composed of aligned, organized arrays of fibrillin microfibrils.

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Bundles of zonule fibers are wrapped by large fibers providing structural support.

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Fibrillin organization shows how a single building block constructs an elastic tissue.

The Effect of Prostaglandin Analogues on the Ciliary Zonular Fibers of the Rabbit Crystalline Lens

PURPOSE

To evaluate the influence of prostaglandin (PG) analogues on the ciliary zonular fibers of the crystalline lens using scanning electron microscopy (SEM) of rabbit eyes, and to measure the matrix metalloproteinase (MMP) and tissue inhibitor of metalloproteinase (TIMP) levels of the aqueous humor and crystalline lens treated with topical PG analogues Methods: Fifty eyes from 25 New Zealand white rabbits were divided into five groups of five rabbits each. In the control group, balanced salt solution was administered via the topical route once a day to the eyes. The benzalkonium chloride (BAC) group was treated with 0.02% BAC, the Latanoprost group with 0.005% latanoprost, the Travoprost group with 0.004% Travoprost, and the Bimatoprost group with 0.03% Bimatoprost for 10 months. We examined the ciliary zonular fibers using SEM. We also measured the MMP and TIMP levels of the aqueous humor and crystalline lens.

RESULTS

SEM revealed some splitting of zonular fibers in eyes treated with topical PG analogues when compared with the control and BAC groups. The MMP-1 and TIMP-1 levels after treatment with the PG analogues did not differ significantly from the control and BAC groups (P > 0.05). There was no significant difference in MMP-1, MMP-3, TIMP-1, and MMP-1/TIMP-1 levels in the lens among all five groups.

CONCLUSIONS

PG analogues may induce zonular change in rabbits microscopically. There was no association between zonular changes and the levels of certain types of MMP or TIMP in the aqueous humor or crystalline lens after topical treatment with PG analogues.

Long-term visual outcomes of laser anterior ciliary excision

Purpose

To determine the long-term visual outcomes of six eyes of 3 patients up to 13 years following the Laser Anterior Ciliary Excision (LaserACE) procedure.

Methods

Three male patients of ages 59, 59, and 60 presented for evaluation at Storm Eye Institute, Medical University of South Carolina at 8, 10, and 13 years after the LaserACE procedure for presbyopia, respectively. All 3 patients had a history of laser vision correction (LVC) prior to LaserACE treatment. Visual performance was evaluated using ray-tracing aberrometry, specifically higher-order aberrations, visual Strehl of the optical transfer function (VSOTF), depth of focus (DoF), and effective range of focus (EROF). VSOTF was computed as a function of defocus using a through-focus curve. Subjective DoF was overlaid on the VSOTF through-focus curve to establish the best image quality metric threshold value for correlation between subjective and objective DoF. EROF was determined by measuring the difference in diopters between the near and distance DoF curves, at 50% of VSOTF.

Results

Distance-corrected visual acuity, distance-corrected intermediate visual acuity, and distance-corrected near visual acuity for all patients remained at 20/20 or better up to 13 years postoperatively. EROF averaged 1.56 ± 0.36 (D) for all eyes.

Conclusions and Importance

LaserACE provided improvement in near vision functionality in these LVC patients with long-term stability. The LaserACE procedure is not on the visual axis, therefore these patients could still receive correction to their hyperopic regression.

Scleral surgery for the treatment of presbyopia: where are we today?

Presbyopia corrections traditionally have been approached with attempts to exchange power, either at the cornea or the lens planes, inducing multifocality, or altering asphericity to impact the optical system. Treatments that affect the visual axis, such as spectacle and contact lens correction, refractive surgeries, corneal onlays and inlays, and intraocular lenses are typically unable to restore true accommodation to the presbyopic eye. Their aim is instead to enhance 'pseudoaccommodation' by facilitating an extended depth-of-focus for which vision is sufficient. There is a true lack of technology that approaches presbyopia from a treatment based or therapy based solution, rather than a 'vision correction' solution that compromises other components of the optical system. Scleral surgical procedures seek to restore true accommodation combined with pseudoaccommodation and have several advantages over other more invasive options to treat presbyopia. While the theoretical justification of scleral surgical procedures remains controversial, there has nevertheless been increasing interest and evidence to support scleral surgical and therapeutic approaches to treat presbyopia. Enormous progress in scleral surgery techniques and understanding of the mechanisms of action have been achieved since the 1970s, and this remains an active area of research. In this article, we discuss the historic scleral surgical procedures, the two scleral procedures currently available, as well as an outlook of the future for the scleral surgical space for treating presbyopia.

Visual outcomes 24 months after LaserACE

Background

To evaluate the effects on near and intermediate visual performance after bilateral Laser Anterior Ciliary Excision (LaserACE) procedure.

Methods

LaserACE surgery was performed using the VisioLite 2.94 μm erbium: yttrium–aluminum–garnet (Er:YAG) ophthalmic laser system in 4 oblique quadrants on the sclera over the ciliary muscle in 3 critical zones of physiological importance (over the ciliary muscles and posterior zonules) with the aim to improve natural dynamic accommodative forces. LaserACE was performed on 26 patients (52 eyes). Outcomes were analyzed using visual acuity testing, Randot stereopsis, and the CatQuest 9SF patient survey.

Results

Binocular uncorrected near visual acuity (UNVA) improved from +0.20 ± 0.16 logMAR preoperatively, to +0.12 ± 0.14 logMAR at 24 months postoperatively (p = 0.0014). There was no statistically significant loss in distance corrected near visual acuity (DCNVA). Binocular DCNVA improved from +0.21 ± 0.17 logMAR preoperatively, to +0.11 ± 0.12 logMAR at 24 months postoperatively (p = 0.00026). Stereoacuity improved from 74.8 ± 30.3 s of arc preoperatively, to 58.8 ± 22.9 s of arc at 24 months postoperatively (p = 0.012). There were no complications such as persistent hypotony, cystoid macular edema, or loss of best-corrected visual acuity (BCVA). Patients surveyed indicated reduced difficulty in areas of near vision, and were overall satisfied with the procedure.

Conclusions

Preliminary results of the LaserACE procedure show promising results for restoring visual performance for near and intermediate visual tasks without compromising distance vision and without touching the visual axis. The visual function and visual acuity improvements had clinical significance. Patient satisfaction was high postoperatively and sustained over 24 months.

Trial registration

NCT01491360 (https://clinicaltrials.gov/ct2/show/NCT01491360). Registered 22 November 2011.

Age-related posterior ciliary muscle restriction - A link between trabecular meshwork and optic nerve head pathophysiology

The ciliary muscle plays a major role in controlling both accommodation and outflow facility in primates. The ciliary muscle and the choroid functionally form an elastic network that extends from the trabecular meshwork all the way to the back of the eye and ultimately attaches to the elastic fiber ring that surrounds the optic nerve and to the lamina cribrosa through which the nerve passes. The ciliary muscle governs the accommodative movement of the elastic network. With age ciliary muscle mobility is restricted by progressively inelastic posterior attachments and the posterior restriction makes the contraction progressively isometric; placing increased tension on the optic nerve region. In addition, outflow facility also declines with age and limbal corneoscleral contour bows inward. Age-related loss in muscle movement and altered limbal corneoscleral contour could both compromise the basal function of the trabecular meshwork. Further, recent studies in non-human primates show that the central vitreous moves posteriorly all the way back to the optic nerve region, suggesting a fluid current and a pressure gradient toward the optic nerve. Thus, there may be pressure and tension spikes on the optic nerve region during accommodation and these pressure and tension spikes may increase with age. This constellation of events could be relevant to glaucomatous optic neuropathy. In summary, our hypothesis is that glaucoma and presbyopia may be literally linked to each other, via the choroid, and that damage to the optic nerve may be inflicted by accommodative intraocular pressure and choroidal tension "spikes", which may increase with age.

Single ocular injection of a sustained-release anti-VEGF delivers 6months pharmacokinetics and efficacy in a primate laser CNV model

A potent anti-vascular endothelial growth factor (VEGF) biologic and a compatible delivery system were co-evaluated for protection against wet age-related macular degeneration (AMD) over a 6month period following a single intravitreal (IVT) injection. The anti-VEGF molecule is dimeric, containing two different anti-VEGF domain antibodies (dAb) attached to a human IgG1 Fc region: a dual dAb. The delivery system is based on microparticles of PolyActive™ hydrogel co-polymer. The molecule was evaluated both in vitro for potency against VEGF and in ocular VEGF-driven efficacy models in vivo. The dual dAb is highly potent, showing a lower IC50 than aflibercept in VEGF receptor binding assays (RBAs) and retaining activity upon release from microparticles over 12months in vitro. Microparticles released functional dual dAb in rabbit and primate eyes over 6months at sufficient levels to protect Cynomolgus against laser-induced grade IV choroidal neovascularisation (CNV). This demonstrates proof of concept for delivery of an anti-VEGF molecule within a sustained-release system, showing protection in a pre-clinical primate model of wet AMD over 6months. Polymer breakdown and movement of microparticles in the eye may limit development of particle-based approaches for sustained release after IVT injection.

Axial Length of the Eyeball Is Important in Secondary Dislocation of the Intraocular Lens, Capsular Bag, and Capsular Tension Ring Complex

Purpose. To analyze the patients with secondary dislocation of CTR and IOL within 5 years from cataract surgery, to determine predisposing factors. Methods. 16 eyes of 15 patients aged 66.2 ± 6.7 (from 49 to 82) with CTR/IOL complex dislocation within 5 years from cataract surgery were compared with 26 patients aged 67.1 ± 7.2 (from 53 to 85), implanted with CTR during cataract surgery to manage zonule dehiscence and did not dislocate for at least 5 years, in respect of cause, axial length and IOL power, refraction, coexistent pathology, and trauma. Results. Axial length of the eyeball was 23.8 ± 1.3 (from 21 to 29) in the group of patients with CTR/IOL dislocation and 20.7 ± 1.2 (from 19 to 24) in patients with no dislocation present (p = 0.008). Crystalline lens dislocation was diagnosed before surgery in 13 of 16 patients with CTR/IOL complex dislocation as opposed to 7 of 26 eyes in the control group (p = 0.01). Pseudoexfoliation was present in 50% and 58% in both groups, respectively. Traumatic dislocation was present in 8 patients, none of them with CTR/IOL dislocation (p = 0.04). Conclusion. Longer axial length may contribute to the failure of the CTR to prevent in-the-bag IOL dislocation. Traumatic dislocation appears to be well fixed with the CTR.

Accommodative movements of the lens/capsule and the strand that extends between the posterior vitreous zonule insertion zone & the lens equator, in relation to the vitreous face and aging

PURPOSE

To elucidate the dynamic accommodative movements of the lens capsule, posterior lens and the strand that attaches to the posterior vitreous zonule insertion zone and posterior lens equator (PVZ INS-LE), and their age-related changes.

METHODS

Twelve human subjects (ages 19-65 years) and 12 rhesus monkeys (ages 6-27 years) were studied. Accommodation was induced pharmacologically (humans) or by central electrical stimulation (monkeys). Ultrasound biomicroscopy was used to image intraocular structures in both species. Surgical procedures and contrast agents were utilized in the monkey eyes to elucidate function and allow visualization of the intraocular accommodative structures.

RESULTS

Human: The posterior pole of the lens moves posteriorly during accommodation in proportion to accommodative amplitude and ciliary muscle movement. Monkey: Similar accommodative movements of the posterior lens pole were seen in the monkey eyes. Following extracapsular lens extraction (ECLE), the central capsule bows backward during accommodation in proportion to accommodative amplitude and ciliary muscle movement, while the peripheral capsule moves forward. During accommodation the ciliary muscle moved forward by ~1.0 mm, pulling forward the vitreous zonule and the PVZ INS-LE structure. During the accommodative response the PVZ INS-LE structure moved forward when the lens was intact and when the lens substance and capsule were removed. In both the monkey and the human eyes these movements declined with age.

CONCLUSIONS

The accommodative shape change of the central capsule may be due to the elastic properties of the capsule itself. For these capsule/lens accommodative posterior movements to occur, the vitreous face must either allow for it or facilitate it. The PVZ INS-LE structure may act as a 'strut' to the posterior lens equator (pushing the lens equator forward) and thereby facilitate accommodative forward lens equator movement and lens thickening. The age-related posterior restriction of the ciliary muscle, vitreous zonule and the PVZ-INS LE structure dampens the accommodative lens shape change. Future descriptions of the accommodative mechanism, and approaches to presbyopia therapy, may need to incorporate these findings.

Isolated posterior capsular split limited by Weiger's ligament after blunt ocular trauma in a child mimicking posterior lenticonus

A 9-year-old boy presented with a posterior capsular split in the lens following a blunt ocular injury. This split was probably limited by the margins of the Weiger's ligament with opacification of the anterior vitreous face along the patellar fossa producing an appearance of posterior lenticonus.

Stretch-dependent changes in surface profiles of the human crystalline lens during accommodation: a finite element study

BACKGROUND

A non-linear isotropic finite element (FE) model of a 29-year-old human crystalline lens was constructed to study the effects of various geometrical parameters on lens accommodation.

METHODS

The model simulates dis-accommodation by stretching of the lens and predicts the change in surface profiles of the lens capsule, cortex and nucleus at select states of stretching/accommodation. Multiple regression analysis (MRA) is used to develop a stretch-dependent mathematical model relating the lens sagittal height to the radial position of the lens surface as a function of dis-accommodative stretch. A load analysis is performed to compare the finite element results to empirical results from lens stretcher studies. Using the predicted geometrical changes, the optical response of the whole eye during accommodation was analysed by ray-tracing.

RESULTS

Aspects of lens shape change relative to stretch were evaluated, including change in diameter, central thickness and accommodation. Maximum accommodation achieved was 10.29 D. From the multiple regression analysis, the stretch-dependent mathematical model of the lens shape related lens curvatures as a function of lens ciliary stretch well (maximum mean-square residual error 2.5 × 10(-3 ) μm, p < 0.001). The results are compared with those from in vitro studies.

CONCLUSIONS

The finite element and ray-tracing predictions are consistent with Ex Vivo Accommodation Simulator (EVAS) studies in terms of load and power change versus change in thickness. The mathematical stretch-dependent model of accommodation presented may have utility in investigating lens behaviour at states other than the relaxed or fully accommodated states.

Changes of Accommodative Power in Vitrectomized Eyes with Crystalline Lenses

PURPOSE

To evaluate changes of accommodative power in phakic eyes after uneventful pars plana vitrectomy (PPV) in patients aged younger than 45 years without presbyopia.

METHODS

We investigated patients aged younger than 45 years who underwent PPV without crystalline lens extraction because of vitreoretinal disorders. Twelve weeks after vitrectomy, the near point of accommodation, high-frequency component of accommodative microfluctuation, axial length, and anterior chamber depth of vitrectomized and contralateral nonvitrectomized eyes were examined.

RESULTS

Ten eyes of 10 patients were included. The average patient age was 39.8 (±4.3) years. None experienced cataract progression in the vitrectomized eye up to 12 weeks after surgery. Near point of accommodation was significantly lower in the vitrectomized eye than in the opposite eye at 12 weeks after vitrectomy (5.23 [±1.39] diopters vs. 5.91 [±1.83] diopters, p < 0.001). The high-frequency components in the vitrectomized eyes were significantly greater than those in the contralateral eyes (p = 0.01). However, anterior chamber depth and axial length were similar in value to the preoperative observations.

CONCLUSIONS

Uneventful PPV in relatively young patients without presbyopia reduced accommodative power during the early postoperative period with no cataract progression.

Anteriorly located zonular fibres as a tool for fine regulation in accommodation

PURPOSE

To describe an anteriorly located system of zonular fibres that could be involved in fine-tuning of accommodation.

METHODS

Forty-six human and 28 rhesus monkey eyes were dissected and special preparations were processed for scanning electron microscopy and reflected-light microscopy. Additional series of frontal and sagittal histological and ultrathin sections were analysed in respect to the origin and insertion of anteriorly located zonules. The presence of sensory terminals at the site of the originating zonules within the connective tissue of the ciliary body was studied by immunohistochemistry. For in-vivo visualization ultrasound biomicroscopy (UBM) was performed on 12 human subjects.

RESULTS

Fine zonular fibres originated from the valleys and lateral walls of the most anterior pars plicata that covers the anterior and inner circular ciliary muscle portion. These most anterior zonules (MAZ) showed attachments either to the anterior or posterior tines or they inserted directly onto the surface of the lens. At the site of origin, the course of the MAZ merged into the connective tissue fibres connecting the adjacent pigmented epithelium to the ciliary muscle. Numerous afferent terminals directly at the site of this MAZ-origin were connected to the intrinsic nervous network of the ciliary muscle.

CONCLUSIONS

A newly described set of zonular fibres features the capabilities to register the tensions of the zonular fork and lens capsule. The close location and neural connection towards the circular ciliary muscle portion could provide the basis for stabilization and readjustment of focusing that serves fast and fine-tuned accommodation and disaccommodation.

The zonules selectively alter the shape of the lens during accommodation based on the location of their anchorage points

PURPOSE

To determine the role of anterior and posterior zonular tension on the optomechanical lens response during accommodation simulation.

METHODS

Ten eyes from nine hamadryas baboons (4.9 ± 0.7 years) and 20 eyes from 18 cynomolgus monkeys (5.4 ± 0.3 years) were dissected, leaving the lens, zonules, ciliary body, hyaloid membrane, anterior vitreous, and a segmented scleral rim intact. The lens preparation was mounted in a lens stretcher, and the outer scleral shell was displaced radially in a stepwise fashion. The load, lens, and ciliary body diameters, lens power, lens thickness, and the anterior and posterior radius of curvature were measured during stretching. The zonular fibers attached to either the posterior or anterior lens surface were then carefully transected and the experiment was repeated. Zonular transection was confirmed in four eyes via laser scanning confocal microscopy after immunostaining. The effect of zonular transection on the tissue response to stretching was quantified.

RESULTS

Without anterior zonules, 48% and 97% of the changes in anterior and posterior radii are retained. Without posterior zonules, 81% and 67% of the changes in anterior and posterior radii are retained. The changes in lens shape were reduced after transecting either the anterior or posterior zonules; however, both surfaces still changed shape.

CONCLUSIONS

While either the anterior or posterior zonules alone are capable of changing the shape of both lens surfaces, the anterior zonules have a greater effect on the anterior lens surface, and the posterior zonules have a greater effect on the posterior lens surface.

Morphological study of the accommodative apparatus in the monkey eye

For more than a century there has been debate concerning the mechanism of accommodation—whether the lens capsule or lens material itself determines the functional relationship between ciliary muscle contractility and lens deformation during refractive adaptation. This morphological study in monkey eyes investigates the composition and distribution of several connective tissue components in the accommodative apparatus relaying muscle force to lens organization. Elastin distributes on the marginal surface of the ciliary process. A zonule is composed of fibrillin produced by epithelial cells of the process. In the progress of extension over the posterior chamber, fibrils unite into strands and possess longitudinal plasticity. By induction of the elastin network, strands extend in a concentric direction covering the equatorial region of the capsule. Upon tethering to the lens, the strand ramifies into fibrils, penetrating deeply close to the epithelial layer of the lens and binding with the collagen of the intercellular spaces. Tight linkage of the zonule with the capsule transmits precise contractility. Inside the lens, the cortical layer's elastic connective tissue network forms widely spaced lamellae of crystalline fibers. In contrast, the central nuclear lamellae are tightly opposed. The accumulation of lamellae is greater in the anterior cortex than in the posterior, yielding a more variable anterior chamber depth in the visual axis. The plasticity of the zonule and connective tissue distribution inside the lens produces an adjustable configuration. Thus, tight linkage between the dynamism of the capsule with interaction of the lenticular flexibility provides a novel understanding of accommodation. Anat Rec, 298:630–636, 2015.

Evaluation of the mechanical behaviour and estimation of the elastic properties of porcine zonular fibres

The mechanical behaviour of zonular fibres greatly affects the accommodation process in mammalian eyes. This paper introduces a detailed measurement procedure for the purpose of obtaining the force-displacement diagram necessary to evaluate the mechanical properties of porcine zonular fibres in situ. It is a complex technique, keeping the integrity of the zonular bundles between the crystalline lens and the ciliary muscle cells. We present a brief description of the measurement procedure both in theory and in practice, along with the force-displacement diagrams acquired from a porcine sample group. The strengths of this newly developed method are the unequivocal force transmission between the sample and the transducer, and the intact connection between the ciliary body and the crystalline lens via zonular fibres. With the aid of these measurements, we define an estimated material model for the zonular apparatus both analytically and using the finite element method. The two different evaluation methods show close agreement in the calculated Young's modulus for the zonular fibres. The range of the calculated elastic modulus is 200-250 kPa. This new measuring method is adaptable to human specimens. Despite its complexity, the entire procedure and the evaluation part are reproducible. The constitutive model aims to shed light on the mechanics of the accommodation process.

The Ophthalmic Examination as It Pertains to General Ocular Toxicology: Basic and Advanced Techniques and Species-Associated Findings

Ocular toxicology pertains to toxicologic effects of drugs administered topically, intraocularly, or systemically. It should also include evaluation of adverse effects of ophthalmic devices such as contact lenses, intraocular lenses, and glaucoma implants. The ophthalmic examination is able to provide detailed in-life information and is used in combination with clinical observations, clinical pathology, and histopathology to assess potential toxicologic effects. The ophthalmologist must be familiar with the wide range of species used in the field of toxicology, be familiar with the anatomic variations associated with these species, be able to determine what is an inherited or a breed-related finding from a study-related effect, be competent with the required ophthalmic equipment, and be capable of examining this wide range of animals.

Extralenticular and lenticular aspects of accommodation and presbyopia in human versus monkey eyes

PURPOSE

To determine if the accommodative forward movements of the vitreous zonule and lens equator occur in the human eye, as they do in the rhesus monkey eye; to investigate the connection between the vitreous zonule posterior insertion zone and the posterior lens equator; and to determine which components-muscle apex width, lens thickness, lens equator position, vitreous zonule, circumlental space, and/or other intraocular dimensions, including those stated in the objectives above-are most important in predicting accommodative amplitude and presbyopia.

METHODS

Accommodation was induced pharmacologically in 12 visually normal human subjects (ages 19-65 years) and by midbrain electrical stimulation in 11 rhesus monkeys (ages 6-27 years). Ultrasound biomicroscopy imaged the entire ciliary body, anterior and posterior lens surfaces, and the zonule. Relevant distances were measured in the resting and accommodated eyes. Stepwise regression analysis determined which variables were the most important predictors.

RESULTS

The human vitreous zonule and lens equator move forward (anteriorly) during accommodation, and their movements decline with age, as in the monkey. Over all ages studied, age could explain accommodative amplitude, but not as well as accommodative lens thickening and resting muscle apex thickness did together. Accommodative change in distances between the vitreous zonule insertion zone and the posterior lens equator or muscle apex were important for predicting accommodative lens thickening.

CONCLUSIONS

Our findings quantify the movements of the zonule and ciliary muscle during accommodation, and identify their age-related changes that could impact the optical change that occurs during accommodation and IOL function.

Accommodative movements of the vitreous membrane, choroid, and sclera in young and presbyopic human and nonhuman primate eyes

PURPOSE

We report, for the first time to our knowledge, dynamic movements of the vitreous membrane and peripheral choroid during accommodation, and age-related changes in the anterior sclera.

METHODS

We studied 11 rhesus monkeys (ages 6-27 years) and 12 human subjects (ages 19-65 years). Accommodation was induced pharmacologically in human subjects and by central electrical stimulation in the monkeys. Ultrasound biomicroscopy, endoscopy, and contrast agents were used to image various intraocular structures.

RESULTS

In the monkey, the anterior hyaloid membrane bows backward during accommodation in proportion to accommodative amplitude and lens thickening. A cleft exists between the pars plicata region and the anterior hyaloid membrane, and the cleft width increases during accommodation from 0.79 ± 0.01 mm to 1.01 ± 0.02 mm in young eyes (n = 2, P < 0.005), as fluid from the anterior chamber flows around the lens equator toward the cleft. In the older eyes the cleft width was 0.30 ± 0.19 mm, which during accommodation increased to 0.45 ± 0.20 mm (n = 2). During accommodation the ciliary muscle moved forward by approximately 1.0 mm, pulling forward the choroid, retina, vitreous zonule, and the neighboring vitreous interconnected with the vitreous zonule. Among the humans, in the older eyes the scleral contour bowed inward in the region of the limbus, compared to the young eyes.

CONCLUSIONS

The monkey anterior hyaloid bends posteriorly during accommodation in proportion to accommodative amplitude and the sclera bows inward with increasing age in both species. Future descriptions of the accommodative mechanism, and approaches to presbyopia therapy, may need to incorporate these findings.

Development, composition, and structural arrangements of the ciliary zonule of the mouse

PURPOSE

Here, we examined the development, composition, and structural organization of the ciliary zonule of the mouse. Fibrillin 1, a large glycoprotein enriched in force-bearing tissues, is a prominent constituent of the mouse zonule. In humans, mutations in the gene for fibrillin 1 (FBN1) underlie Marfan syndrome (MS), a disorder characterized by lens dislocation and other ocular symptoms.

METHODS

Fibrillin expression was analyzed by in situ hybridization. The organization of the zonule was visualized using antibodies to Fbn1, Fbn2, and microfibril-associated glycoprotein-1 (Magp1) in conjunction with 5-ethynyl-2'-deoxyuridine (EdU), an S-phase marker.

RESULTS

Microfibrils, enriched in Fbn2 and Magp1, were prominent components of the temporary vascular tunic of the embryonic lens. Fbn2 expression by nonpigmented ciliary epithelial cells diminished postnatally and there was a concomitant increase in Fbn1 expression, especially in cells located in valleys between the ciliary folds. Zonular fibers projected from the posterior pars plicata to the lens in anterior, equatorial, and posterior groupings. The attachment point of the posterior zonular fibers consisted of a dense meshwork of radially oriented microfibrils that we termed the fibrillar girdle. The fibrillar girdle was located directly above the transition zone, a region of the lens epithelium in which cells commit to terminal differentiation.

CONCLUSIONS

The development and arrangement of the murine ciliary zonule are similar to those of humans, and consequently the mouse eye may be a useful model in which to study ocular complications of MS.

Nondestructive volumetric imaging of tissue microstructure with benchtop x-ray phase-contrast tomography and critical point drying

The in vitro investigation of many optically opaque biological microstructures requires 3D analysis at high resolution over a large field of view. We demonstrate a new nondestructive volumetric imaging technique that eliminates the structural and computational limitations of conventional 2D optical microscopy by combining x-ray phase-contrast tomography with critical point drying sample preparation. We experimentally demonstrate the enhancement of small features afforded by phase-contrast imaging and show the contrast improvement afforded by the drying of a hydrated specimen. We further demonstrate the biological application of this technique by imaging the microstructure of the accommodative apparatus in a primate eye using a benchtop phase-contrast tomography system.

Computer-animated model of accommodation and theory of reciprocal zonular action

This report presents a computer-animated model of the structures of accommodation based on new understanding of the anatomy of the zonular apparatus integrated with current understanding of the mechanism of accommodation. Analysis of this model suggests a new, consolidated theory of the mechanism of accommodation including a new theory of reciprocal zonular action. A three-dimensional animated model of the eye in accommodation and disaccommodation was produced in collaboration with an experienced medical animator. Current understanding of the anatomy of the zonule and the attachments of the vitreous zonule to the anterior hyaloid membrane is incomplete. Recent studies have demonstrated three components of the vitreous zonule: (1) anterior vitreous zonule (previously “hyalocapsular” zonule), which attaches the ciliary plexus in the valleys of the ciliary processes to the anterior hyaloid membrane in the region medial to the ciliary body and Weiger’s ligament; (2) intermediate vitreous zonule, which attaches the ciliary plexus to the anterior hyaloid peripherally; and (3) posterior vitreous zonule, which creates a sponge-like ring at the attachment zone that anchors the pars plana zonules. The pars plana zonules attach posteriorly to the elastic choroid above the ora serrata. Analysis of the computer-animated model demonstrates the synchronized movements of the accommodative structures in accommodation and disaccommodation. Utilizing model-based reasoning, it is shown that the posterior zonules attach to and provide traction to the anterior vitreous membrane and Weiger’s ligament. This model supports the concept that the ciliary body/zonule/anterior hyaloid complex contributes to the changes in the posterior lens capsule during accommodation, supporting an extralenticular component to accommodation and demonstrating an alternative to the “vitreous support” theories. There is a reciprocal action of the anterior zonules and the posterior zonules. During ciliary body contraction, the anterior zonules lose tension while the posterior zonules stretch and exert force on the posterior lens capsule playing a role in shaping the posterior lens thickness and curvature. During ciliary body relaxation, the posterior zonules lose tension as the lens flattens and is pulled back by the increasing tension of the anterior zonules.