Mechanical properties of the human lens capsule

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.

Femtosecond laser-assisted cataract surgery--current status and future directions

Femtosecond laser-assisted cataract surgery (FLACS) putatively offers several advantages over conventional phacoemulsification. We review the current status of FLACS and discuss the evolution of femtosecond lasers in cataract surgery and the currently available femtosecond laser platforms. We summarize the outcomes of FLACS for corneal wound creation, limbal relaxing incisions, capsulotomy, and lens fragmentation. We discuss surgical planning, preoperative considerations, clinical experiences including the learning curve and postoperative outcomes with FLACS, and also the cost effectiveness of FLACS. We present the intraoperative complications and management of challenging cases where FLACS offers an advantage and also speculate on the future directions with FLACS. Further advancements in laser technology to refine its efficacy, advancement in intraocular lens design to harness the potential benefits of FLACS, and a reduction in cost are needed to establish a clear superiority over conventional phacoemulsification.

New concepts in basement membrane biology

Basement membranes (BMs) are thin sheets of extracellular matrix that outline epithelia, muscle fibers, blood vessels and peripheral nerves. The current view of BM structure and functions is based mainly on transmission electron microscopy imaging, in vitro protein binding assays, and phenotype analysis of human patients, mutant mice and invertebrata. Recently, MS-based protein analysis, biomechanical testing and cell adhesion assays with in vivo derived BMs have led to new and unexpected insights. Proteomic analysis combined with ultrastructural studies showed that many BMs undergo compositional and structural changes with advancing age. Atomic force microscopy measurements in combination with phenotype analysis have revealed an altered mechanical stiffness that correlates with specific BM pathologies in mutant mice and human patients. Atomic force microscopy-based height measurements strongly suggest that BMs are more than two-fold thicker than previously estimated, providing greater freedom for modelling the large protein polymers within BMs. In addition, data gathered using BMs extracted from mutant mice showed that laminin has a crucial role in BM stability. Finally, recent evidence demonstrate that BMs are bi-functionally organized, leading to the proposition that BM-sidedness contributes to the alternating epithelial and stromal tissue arrangements that are found in all metazoan species. We propose that BMs are ancient structures with tissue-organizing functions and were essential in the evolution of metazoan species.

Multiscale Mechanical Model of the Pacinian Corpuscle Shows Depth and Anisotropy Contribute to the Receptor's Characteristic Response to Indentation

Cutaneous mechanoreceptors transduce different tactile stimuli into neural signals that produce distinct sensations of touch. The Pacinian corpuscle (PC), a cutaneous mechanoreceptor located deep within the dermis of the skin, detects high frequency vibrations that occur within its large receptive field. The PC is comprised of lamellae that surround the nerve fiber at its core. We hypothesized that a layered, anisotropic structure, embedded deep within the skin, would produce the nonlinear strain transmission and low spatial sensitivity characteristic of the PC. A multiscale finite-element model was used to model the equilibrium response of the PC to indentation. The first simulation considered an isolated PC with fiber networks aligned with the PC's surface. The PC was subjected to a 10 μm indentation by a 250 μm diameter indenter. The multiscale model captured the nonlinear strain transmission through the PC, predicting decreased compressive strain with proximity to the receptor's core, as seen experimentally by others. The second set of simulations considered a single PC embedded epidermally (shallow) or dermally (deep) to model the PC's location within the skin. The embedded models were subjected to 10 μm indentations at a series of locations on the surface of the skin. Strain along the long axis of the PC was calculated after indentation to simulate stretch along the nerve fiber at the center of the PC. Receptive fields for the epidermis and dermis models were constructed by mapping the long-axis strain after indentation at each point on the surface of the skin mesh. The dermis model resulted in a larger receptive field, as the calculated strain showed less indenter location dependence than in the epidermis model.

Finite element implementation of a multiscale model of the human lens capsule

An axisymmetric finite element implementation of a previously described structural constitutive model for the human lens capsule (Burd in Biomech Model Mechanobiol 8(3):217-231, 2009) is presented. This constitutive model is based on a hyperelastic approach in which the network of collagen IV within the capsule is represented by an irregular hexagonal planar network of hyperelastic bars, embedded in a hyperelastic matrix. The paper gives a detailed specification of the model and the periodic boundary conditions adopted for the network component. Momentum balance equations for the network are derived in variational form. These balance equations are used to develop a nonlinear solution scheme to enable the equilibrium configuration of the network to be computed. The constitutive model is implemented within a macroscopic finite element framework to give a multiscale model of the lens capsule. The possibility of capsule wrinkling is included in the formulation. To achieve this implementation, values of the first and second derivatives of the strain energy density with respect to the in-plane stretch ratios need to be computed at the local, constitutive model, level. Procedures to determine these strain energy derivatives at equilibrium configurations of the network are described. The multiscale model is calibrated against previously published experimental data on isolated inflation and uniaxial stretching of ex vivo human capsule samples. Two independent example lens capsule inflation analyses are presented.

Analysis of femtosecond laser assisted capsulotomy cutting edges and manual capsulorhexis using environmental scanning electron microscopy

Purpose. To investigate the structure and irregularity of the capsulotomy cutting edges created by two femtosecond (FS) laser platforms in comparison with manual continuous circular capsulorhexis (CCC) using environmental scanning electron microscopy (eSEM). Methods. Ten anterior capsulotomies were obtained using two different FS laser cataract platforms (LenSx, n = 5, and Victus, n = 5). In addition, five manual CCC (n = 5) were obtained using a rhexis forceps. The specimens were imaged by eSEM (FEI Quanta 400, OR, USA). Objective metrics, which included the arithmetic mean deviation of the surface (Sa) and the root-mean-square deviation of the surface (Sq), were used to evaluate the irregularity of both the FS laser capsulotomies and the manual CCC cutting edges. Results. Several microirregularities were shown across the FS laser capsulotomy cutting edges. The edges of manually torn capsules were shown, by comparison of Sa and Sq values, to be smoother (P < 0.05) than the FS laser capsulotomy edges. Conclusions. Work is needed to understand whether the FS laser capsulotomy edge microirregularities, not seen in manual CCC, may act as focal points for the concentration of stress that would increase the risk of capsular tear during phacoemulsification as recently reported in the literature.

Effect of Supercoiling on the Mechanical and Permeability Properties of Model Collagen IV Networks

Collagen IV networks in the glomerular basement membrane (GBM) are essential for the maintenance and regulation of blood filtration in the kidneys. The GBM contains two different types of collagen IV networks: [α1(IV)]2α2(IV) and α3(IV)α4(IV)α5(IV), the latter of which has a higher number of supercoils (two or more collagens coiling around each other). To investigate the effects of supercoiling on the mechanical and permeability properties of collagen IV networks, we generated model collagen IV networks in the GBM and reconnected them to create different levels of supercoiling. We found that supercoiling greatly increases the stiffness of collagen IV networks but only minimally decreases the permeability. Also, doubling the amount of supercoils in a network had a bigger effect than doubling the stiffness of the supercoils. Our results suggest that the formation of supercoils is a specialized mechanism by the GBM that provides with a network stiff and strong enough to withstand the high hydrostatic pressures of filtration, yet porous enough that filtration is not hindered. Clinically, understanding the effects of supercoiling gives us insight into the mechanisms of GBM failure in some disease states where the normal collagen IV structure is disrupted.

A quantitative geometric mechanics lens model: insights into the mechanisms of accommodation and presbyopia

This study expands on a geometric model of ocular accommodation (Reilly and Ravi, Vision Res. 50:330-336; 2010) by relaxing assumptions regarding lens symmetry about the equator. A method for predicting stretching force was derived. Two models were then developed: Model 1 held the equatorial geometry constant at all stages of accommodation, while Model 2 allowed localized deformation at the equator. Both models were compared to recent data for axial thickness, anterior and posterior radii of curvature, surface area, cross-sectional area, volume, and stretching force for the 29-year-old lens. Age-related changes in accommodation were also simulated. Model 1 gave predictions which agreed with the Helmholtz theory of accommodation, while Model 2's predictions agreed with the Schachar mechanism of accommodation. Trends predicted by Model 1 agreed with all available experimental data, while Model 2 disagreed with recent surface area measurements. Further analysis indicated that Model 1 was fundamentally more efficient in that it required less force per diopter change in optical power than Model 2. Model 1 more accurately predicted age-related changes in accommodation amplitude. This implies that the zero-force (fully accommodated) state geometry changes with age due to a shifting balance in residual stresses between the lens and capsule.

Effects of pressure and electrical charge on macromolecular transport across bovine lens basement membrane

Molecular transport through the basement membrane is important for a number of physiological functions, and dysregulation of basement membrane architecture can have serious pathological consequences. The structure-function relationships that govern molecular transport in basement membranes are not fully understood. The basement membrane from the lens capsule of the eye is a collagen IV-rich matrix that can easily be extracted and manipulated in vitro. As such, it provides a convenient model for studying the functional relationships that govern molecular transport in basement membranes. Here we investigate the effects of increased transmembrane pressure and solute electrical charge on the transport properties of the lens basement membrane (LBM) from the bovine eye. Pressure-permeability relationships in LBM transport were governed primarily by changes in diffusive and convective contributions to solute flux and not by pressure-dependent changes in intrinsic membrane properties. The solute electrical charge had a minimal but statistically significant effect on solute transport through the LBM that was opposite of the expected electrokinetic behavior. The observed transport characteristics of the LBM are discussed in the context of established membrane transport modeling and previous work on the effects of pressure and electrical charge in other basement membrane systems.

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.

Mechanical response of wild-type and Alport murine lens capsules during osmotic swelling

The mechanical support of basement membranes, such as the lens capsule, is believed to arise from one of their main constituents - collagen IV. The basement membranes of the lens, kidney, and ear normally contain two different types of collagen IV networks, referred to as the major and minor chain networks. In Alport syndrome, a mutation in one of the minor chain COL4 genes leads to the absence of the minor chain network, causing life-threatening disturbances. We hypothesized that the absence of the minor chain network increases basement membrane distensibility, as measured in wild-type (n = 25) and Alport syndrome (n = 21) mice using the lens capsule as a model. Osmotic swelling experiments revealed direction-dependent changes. As a reflection of lens capsule properties, Alport lenses strained significantly more than wild-type lenses in the anterior-posterior direction, i.e. along their thickness, but not in the equatorial direction (p = 0.03 and p = 0.08, respectively). This is consistent with clinical data: Alport patients develop conical protrusions on the anterior and posterior lenticular poles. There was no evidence of significant change in total amount of collagen between Alport and wild-type lenses (p = 0.6). The observed differences in distensibility could indicate that the major chain network alone cannot fully compensate for the absence of the more highly cross-linked minor chain network, which is believed to be stronger, more stable, and resistant to deformation. The addition of mechanical information on Alport syndrome to the currently available biological data provides a fuller picture into the progression of the disease.

Materials characterization and mechanobiology of the eye

The eye responds to a great deal of internal and external stimuli throughout its normal function. Due to this, a mechanical or chemical analysis alone is insufficient. A systematic materials characterization is needed. A mechanobiological approach is required for a full understanding of the unique properties and function of the eye. This review compiles the mechanical properties of select eye components, summarizes mechanical and chemical testing platforms, and overviews modeling approaches. Analysis is done across studies, experimental methods, and between species in order to summarize what is known about the mechanobiology of the eye. Several opportunities for future research are identified.

Intraocular electro-optic lens with ciliary muscle controlled accommodation

In this paper a concept is proposed of an intraocular lens implant with electro-optic accommodation of a variable-focus hybrid liquid-crystal-based lens. The dioptric strength of the lens is electronically controlled by a signal that is derived from the change of inductance of a sensing coil due to a marker implanted in the nearby contracting or decontracting ciliary muscle. Analytical, numerical and experimental results are reported on the dependency of the frequency of a Colpitts oscillator circuit on the location of a nearby conductive marker. A concept is also reported on the use as an electro-optic lens of a device based on a liquid crystal in planar alignment, which is held between a flat and a curved window coated with optically transparent and electrically conductive layers.

Protein composition and biomechanical properties of in vivo-derived basement membranes

Basement membranes (BMs) evolved together with the first metazoan species approximately 500 million years ago. Main functions of BMs are stabilizing epithelial cell layers and connecting different types of tissues to functional, multicellular organisms. Mutations of BM proteins from worms to humans are either embryonic lethal or result in severe diseases, including muscular dystrophy, blindness, deafness, kidney defects, cardio-vascular abnormalities or retinal and cortical malformations. In vivo-derived BMs are difficult to come by; they are very thin and sticky and, therefore, difficult to handle and probe. In addition, BMs are difficult to solubilize complicating their biochemical analysis. For these reasons, most of our knowledge of BM biology is based on studies of the BM-like extracellular matrix (ECM) of mouse yolk sac tumors or from studies of the lens capsule, an unusually thick BM. Recently, isolation procedures for a variety of BMs have been described, and new techniques have been developed to directly analyze the protein compositions, the biomechanical properties and the biological functions of BMs. New findings show that native BMs consist of approximately 20 proteins. BMs are four times thicker than previously recorded, and proteoglycans are mainly responsible to determine the thickness of BMs by binding large quantities of water to the matrix. The mechanical stiffness of BMs is similar to that of articular cartilage. In mice with mutation of BM proteins, the stiffness of BMs is often reduced. As a consequence, these BMs rupture due to mechanical instability explaining many of the pathological phenotypes. Finally, the morphology and protein composition of human BMs changes with age, thus BMs are dynamic in their structure, composition and biomechanical properties.

Hyperelastic modelling of the crystalline lens: Accommodation and presbyopia

Purpose

The modification of the mechanical properties of the human crystalline lens with age can be a major cause of presbyopia. Since these properties cannot be measured in vivo, numerical simulation can be used to estimate them. We propose an inverse method to determine age-dependent change in the material properties of the tissues composing the human crystalline lens.

Methods

A finite element model of a 30-year-old lens in the accommodated state was developed. The force necessary to achieve full accommodation in a 30-year-old lens of known external geometry was computed using this model. Two additional numerical models of the lens corresponding to the ages of 40 and 50 years were then built. Assuming that the accommodative force applied to the lens remains constant with age, the material properties of nucleus and cortex were estimated by inverse analysis.

Results

The zonular force necessary to reshape the model of a 30-year-old lens from the accommodated to the unaccommodated geometry was 0.078 newton (N). Both nucleus and cortex became stiffer with age. The stiffness of the nucleus increased with age at a higher rate than the cortex.

Conclusions

In agreement with the classical theory of Helmholtz, on which we based our model, our results indicate that a major cause of presbyopia is that both nucleus and cortex become stiffer with age; therefore, a constant value of the zonular forces with aging does not achieve full accommodation, that is, the accommodation capability decreases.

Shear modulus data for the human lens determined from a spinning lens test

The paper describes a program of mechanical testing on donated human eye bank lenses. The principal purpose of the tests was to obtain experimental data on the shear modulus of the lens for use in future computational models of the accommodation process. Testing was conducted using a procedure in which deformations are induced in the lens by spinning it about its polar axis. Shear modulus data were inferred from these observed deformations by means of a finite element inverse analysis procedure in which the spatial variation of the shear modulus within the lens is represented by an appropriate function (see Burd et al., 2011 for a detailed specification of the design of the spinning lens test rig, experimental protocols and associated data analysis procedures that were employed in the tests). Inferred data on lens shear modulus are presented for a set of twenty-nine lenses in the age range 12 years to 58 years. The lenses were tested between 47 h and 110 h from the time of death (average post-mortem time 74 h). Care was taken to exclude any lenses that had been affected by excessive post-mortem swelling, or any lenses that had suffered mechanical damage during storage, transit or the testing process. The experimental data on shear modulus indicate that, for young lenses, the cortex is stiffer than the nucleus. The shear modulus of the nucleus and cortex both increase with increasing age. The shear modulus of the nucleus increases more rapidly than the cortex with the consequence that from an age of about 45 years onwards the nucleus is stiffer than the cortex. The principal shear modulus data presented in the paper were obtained by testing at a rotational speed of 1000 rpm. Supplementary tests were conducted at rotational speeds of 700 rpm and 1400 rpm. The results from these supplementary tests are in good agreement with the data obtained from the principal 1000 rpm tests. Studies on the possible effects of lens drying during the test suggested that this factor is unlikely to have led to significant errors in the experimental determination of the shear modulus. The shear modulus data presented in the paper are used to develop ‘age-stiffness’ models to represent the shear modulus of the lens as a function of age. These models are in a form that may be readily incorporated in a finite element model of the accommodation process. A comparison is attempted between the shear modulus data presented in the current paper and equivalent data published by previous authors. This comparison highlights various limitations and inconsistencies in the data sets.

A comparative study on the viscoelastic properties of human and animal lenses

A new method of compression between two parallel plates is used to measure the viscoelastic properties of whole and decapsulated human lenses and compare them with other animal species. Compressive load relaxation was performed by deforming the lens by 10% and measuring the force relaxation response for 100 s to obtain thickness, stiffness and relaxation of the induced loading force and Maxwell parameters for human, monkey, porcine and leporine whole and decapsulated lenses. Thickness and percentage loading force relaxation increased linearly with lens age, whereas stiffness and induced loading force increased exponentially. Human and monkey lenses aged at different rates. Loading force relaxation in a generalized Maxwell model was described by three time constants ranging from 1 to 1000 s. Compressive load relaxation is a very versatile method to study the viscoelastic properties of whole and decapsulated lenses and potentially also artificial accommodating lenses. The data presented in the study will help researchers choose the most suitable animal lenses based on the desired properties and age to be mimicked from the human lenses.

Anticipation of presbyopia in Portuguese familial amyloidosis ATTR V30M

The aim of this study was to evaluate if Portuguese patients with familial amyloidosis, liver transplanted and not, have an earlier development of presbyopia compared with a normal population and its relation with the presence or the absence of anterior capsule opacification of the lens. This study was performed to evaluate if Portuguese patients with familial amyloidosis and in a blood donors population (control group). Three hundred and fifty-six subjects, 144 amyloidotic patients and 212 healthy individuals, were evaluated for the need of plus lenses for normal near reading (Jaeger chart 1 at 33 cm). In familial amyloidosis patients, the value of the add-power was related to age, liver transplantation status, and presence of visible anterior capsule opacification of the lens. In both groups, the value of add-power was positively correlated with age (r=0.91; P<0.005). Familial amyloidosis patients require more add-power than control individuals of similar age, and need to use reading glasses at earlier ages. The age of onset of presbyopia in familial amyloidosis patients was significantly lower than in control individuals (32 years vs. 42 years). Adjusting for age, no significant difference was observed in add-power values between liver transplanted and not transplanted amyloidotic patients, suggesting that liver transplantation has no influence on presbyopia evolution in these patients. Familial amyloidosis patients had an earlier onset of presbyopia, probably related to amyloid deposition on the anterior capsule of the lens, which is not halted by liver transplantation.

Impact of heparan sulfate chains and sulfur-mediated bonds on the mechanical properties of bovine lens capsule

We assessed the importance of glycosaminoglycans and sulfur-mediated bonds for the mechanical properties of lens capsules by comparing the stress-strain responses from control and treated pairs of bovine source. No significant change in mechanical properties was observed upon reduction of disulfide bonds. However, removal of glycosaminoglycan chains resulted in a significantly stiffer lens capsule, whereas high concentrations of reducing agent, which is expected to reduce the recently reported sulfilimine bond of collagen IV, resulted in a significantly less stiff lens capsule. A comparison of the diffraction patterns of the control and strongly reduced lens capsules indicated structural rearrangements on a nanometer scale.

Characterization of ocular tissues using microindentation and hertzian viscoelastic models

PURPOSE

The authors applied a novel microindentation technique to characterize biomechanical properties of small ocular and orbital tissue specimens using the hertzian viscoelastic formulation, which defines material viscoelasticity in terms of the contact pressure required to maintain deformation by a harder body.

METHODS

They used a hard spherical indenter having 100 nm displacement and 100 μg force precision to impose small deformations on fresh bovine sclera, iris, crystalline lens, kidney fat, orbital pulley tissue, and orbital fatty tissue; normal human orbital fat, eyelid fat, and dermal fat; and orbital fat associated with thyroid eye disease. For each tissue, stress relaxation testing was performed using a range of ramp displacements. Results for single displacements were used to build quantitative hertzian models that were, in turn, compared with behavior for other displacements. Findings in orbital tissues were correlated with quantitative histology.

RESULTS

Viscoelastic properties of small specimens of orbital and ocular tissues were reliably characterized over a wide range of rates and displacements by microindentation using the hertzian formulation. Bovine and human orbital fatty tissues exhibited highly similar elastic and viscous behaviors, but all other orbital tissues exhibited a wide range of biomechanical properties. Stiffness of fatty tissues tissue depended strongly on the connective tissue content.

CONCLUSIONS

Relaxation testing by microindentation is a powerful method for characterization of time-dependent behaviors of a wide range of ocular and orbital tissues using small specimens, and provides data suitable to define finite element models of a wide range of tissue interactions.

Elasticity of the porcine lens capsule as measured by osmotic swelling

As an alternative to purely mechanical methods, optical tracking of passive osmotic swelling was used to assess mechanical properties of the porcine lens capsule. A simple model was developed accounting for the permeability of the lens fiber cells and capsule to water, the concentration of fixed charges in the fiber cells, and the capsule's resistance to the swelling of fiber cells. Fitting the model solution to experimental data provided an estimate of the elastic modulus of the lens capsule under the assumption of linear isotropic elasticity. The calculated elastic modulus at a fixed charge density of 20 mol m(-3) was 2.0+/-0.5 MPa (mean+/-95% confidence interval; n=15) for 0.1% saline solution, 0.64+/-0.3 MPa (n=10) for 0.2% saline solution, and 0.28+/-0.5 MPa (n=6) for 0.5% saline solution. These values are comparable to previously reported moduli of elasticity for the porcine lens capsule at small strains (<10%), and the slight increase with hypotonicity is consistent with the nonlinear mechanical behavior of the lens capsule. Although limited by being a single measurement on a heterogeneous tissue, osmotic swelling provides a quantitative assessment of the stiffness of the lens capsule without requiring dissection or manipulation of the lens. Thus, the new method could be useful for small animal models.

Significance of the lacunar hydration pattern after corneal cross linking

PURPOSE

The aim of the present study was to investigate the characteristic honeycomb hydration pattern after corneal cross linking using in vivo rabbit cornea.

METHODS

After removal of the central epithelium, the right corneas of 4 New Zealand white rabbits were cross-linked applying a photosensitizing 0.1% riboflavin-dextran solution and UV-A light of 370 nm wavelength with a surface irradiance of 3 mW/cm for 30 minutes. Two animals were euthanized 3 days postoperatively and another 2 were euthanized 6 weeks postoperatively. The corneas of the enucleated eyes were evaluated using 4-mum light microscopic sections with tangential en face and cross-sectional orientation.

RESULTS

By day 3 after treatment, complete apoptotic damage and loss of the stromal keratocytes and endothelial cells were found in the central irradiated area through the entire thickness of the stroma. There was marked lacunar edema in the former positions of the apoptotic keratocytes in the anterior 250 microm of the stroma and diffuse edema in the adjacent posterior and lateral zones. Lacunar edema was identified best on tangential sections. By week 6, the cytoarchitecture of the cornea appeared normal again, and complete resolution of both lacunar and diffuse corneal edema had occurred.

CONCLUSIONS

After riboflavin/UV-A cross linking of in vivo rabbit cornea, a characteristic lacunar hydration pattern can be observed in the anterior stroma with maximum cross linking, whereas diffuse edema is present in the adjacent areas without significant cross linking. The lacunar edema may explain the temporary demarcation of the anterior stroma after cross linking on biomicroscopy because of increased light scattering. The network pattern of cross linking may contribute to the elasticity of the cornea after cross linking.

[Lens refilling]

Cataract surgery has developed into a very safe and highly reproducible procedure but the ultimate goal to also restore physiological accommodation has not yet been achieved. A variety of accommodative intraocular lenses (IOLs) and surgical techniques have been suggested to cure presbyopia but all showed only poor accommodative effects by objective measurements. Complete lens refilling with flexible polymers might be an interesting alternative. Recent investigations on accommodation and presbyopia have given support to the lens refilling method. After development of suitable surgical techniques and filling materials only secondary cataract formation and the unsolved intraoperative control of refraction restrict the clinical use of this technique.

Light-scattering study of the normal human eye lens: elastic properties and age dependence

The human ocular lens is a tissue capable of changing its shape to dynamically adjust the optical power of the eye, a function known as accommodation, which gradually declines with age. This capability is the response of the lens tissue to external forces, which, in turn, is modulated by the biomechanical characteristics of lens tissues. In order to investigate the contributions of lens sclerosis to loss of accommodation, we report on in vitro confocal Brillouin light scattering studies of human ocular lenses spanning over a 30-70 year age range. Using this nondestructive measurement method, we determined that the longitudinal bulk modulus (average ± SD) of the lens nucleus (2.79 ± 0.14 GPa) was consistently greater than the bulk modulus of the lens cortex (2.36 ± 0.09 GPa). Moreover, our results showed that these differences were not age dependent over the 40 year age range that we evaluated using healthy lens tissues. Our results are consistent with the hypothesis that an age-dependent change in the bulk modulus of lens tissues does not fully account for the natural decline of accommodation.

Characterizing molecular diffusion in the lens capsule

The lens capsule compartmentalizes the cells of the avascular lens from other ocular tissues. Small molecules required for lens cell metabolism, such as glucose, salts, and waste products, freely pass through the capsule. However, the lens capsule is selectively permeable to proteins such as growth hormones and substrate carriers which are required for proper lens growth and development. We used fluorescence recovery after photobleaching (FRAP) to characterize the diffusional behavior of various sized dextrans (3, 10, 40, 150, and 250 kDa) and proteins endogenous to the lens environment (EGF, gammaD-crystallin, BSA, transferrin, ceruloplasmin, and IgG) within the capsules of whole living lenses. We found that proteins had dramatically different diffusion and partition coefficients as well as capsule matrix binding affinities than similar sized dextrans, but they had comparable permeabilities. We also found ionic interactions between proteins and the capsule matrix significantly influence permeability and binding affinity, while hydrophobic interactions had less of an effect. The removal of a single anionic residue from the surface of a protein, gammaD-crystallin [E107A], significantly altered its permeability and matrix binding affinity in the capsule. Our data indicated that permeabilities and binding affinities in the lens capsule varied between individual proteins and cannot be predicted by isoelectric points or molecular size alone.

Femtosecond laser induced flexibility change of human donor lenses

BACKGROUND

According to the Helmholtz theory of accommodation the loss of accommodation amplitude is caused by the growing sclerosis of the crystalline lens, whereas the ciliary muscle and the lens capsule are mainly uneffected by age. A permanent treatment method for presbyopia which offers a dynamic accommodation ability is a recent field of study. The concept followed in this paper uses femtosecond laser pulses to potentially overcome the loss of deformation ability of the crystalline lens by creating gliding planes inside the lens tissue to improve its flexibility.

METHODS

The aim of the study is to show that the flexibility of human donor lenses can be increased by applying tightly focused near infrared femtosecond laser pulses into the lens tissue. Thereby the tissue is separated by the photodisruption effect. A certain pattern of gliding planes is cut inside the tissue of 41 human donor lenses and the deformation ability of the lenses are compared using the Fisher spinning test before and after laser treatment.

RESULTS

The laser treatment results in an increased deformation ability of the crystalline lens. The lens a-p thickness increases on average by 97 microm+/-14 microm after the treatment. The Fisher spinning test shows an increase of 16% in deformation ability of the lens at a rotational speed of 1620 rpm.

CONCLUSION

The creation of gliding planes with a fs laser inside the crystalline lens tissue can change the deformation ability of the lens. This might be an indication for a possible method to treat presbyopia in future.

Microindentation of the Young Porcine Ocular Lens

Debate regarding the mechanisms of how the eye changes focus (accommodation) and why this ability is lost with age (presbyopia) has recently been rejoined due to the advent of surgical procedures for the correction of presbyopia. Due to inherent confounding factors in both in vivo and in vitro measurement techniques, mechanical modeling of the behavior of the ocular lens in accommodation has been attempted to settle the debate. However, a paucity of reliable mechanical property measurements has proven problematic in the development of a successful mechanical model of accommodation. Instrumented microindentation was utilized to directly measure the local elastic modulus and dynamic response at various locations in the lens. The young porcine lens exhibits a large modulus gradient with the highest modulus appearing at the center of the nucleus and exponentially decreasing with distance. The loss tangent was significantly higher in the decapsulated lens and the force waveform amplitude decreased significantly upon removal of the lens capsule. The findings indicate that localized measurements of the lens’ mechanical properties are necessary to achieve accurate quantitative parameters suitable for mechanical modeling efforts. The results also indicate that the lens behaves as a crosslinked gel rather than as a collection of individual arched fiber cells.

Contributions of mouse genetic background and age on anterior lens capsule thickness

Accurate lens capsule thickness measurements are necessary for studies investigating mechanical characteristics of the capsule. Confocal Z-axis imaging was used to measure the anterior lens capsule thickness of living intact lenses with minimal tissue manipulation. Measurements of the anterior capsule thickness is reported for the first time in young and old mice from four inbred strains, BALB/c, FVB/N, C57BL/6, and 129X1, and the outbred strain ICR. Our data demonstrates that the mouse anterior lens capsule continues to grow postnatally similar to that described in other mammals. It is also shown there is a significant difference in anterior lens capsule thickness between unrelated mouse strains, suggesting that capsule thickness is a quantitative trait shared by strains with common ancestry. Measurements, taken from other regions of FVB/N capsules revealed the anterior pole to be the thickest, followed by the equatorial region and posterior pole. In addition to mouse, anterior capsule measurements taken from intact cattle, rabbit, rat lenses, and human capsulotomy specimens correlated with the overall size of the animal.

Comparison of the behavior of natural and refilled porcine lenses in a robotic lens stretcher

The mechanism by which the eye dynamically changes focal distance (accommodation), and the mechanism by which this ability is lost with age (presbyopia), are still contested. Due to inherent confounding factors in vivo, in vitro measurements have been undertaken using a robotic lens stretcher to examine these mechanisms as well as the efficacy of lens refilling - a proposed treatment for presbyopia. Dynamic forces, anterior and posterior curvatures, and lens thickness are all correlated for young natural and refilled porcine lenses. Comparisons are made to lenses refilled with a homogeneous polymer system. The amplitude of accommodation of the young porcine lens is very small such that it may be a suitable model for presbyopia. The behavior of refilled lenses was highly dependent on the refill volume. The volume could be tuned to maximize accommodative amplitude in the refilled lens.

The lens capsule

The lens capsule is a modified basement membrane that completely surrounds the ocular lens. It is known that this extracellular matrix is important for both the structure and biomechanics of the lens in addition to providing informational cues to maintain lens cell phenotype. This review covers the development and structure of the lens capsule, lens diseases associated with mutations in extracellular matrix genes and the role of the capsule in lens function including those proposed for visual accommodation, selective permeability to infectious agents, and cell signaling.

Restoration of accommodation: surgical options for correction of presbyopia

Accommodation is a dioptric change in the power of the eye to see clearly at near. Ciliary muscle contraction causes a release in zonular tension at the lens equator, which permits the elastic capsule to mould the young lens into an accommodated form. Presbyopia, the gradual age-related loss of accommodation, occurs primarily through a gradual age-related stiffening of the lens. While there are many possible options for relieving the symptoms of presbyopia, only relatively recently has consideration been given to surgical restoration of accommodation to the presbyopic eye. To understand how this might be achieved, it is necessary to understand the accommodative anatomy, the mechanism of accommodation and the causes of presbyopia. A variety of different kinds of surgical procedures has been considered for restoring accommodation to the presbyopic eye, including surgical expansion of the sclera, using femtosecond lasers to treat the lens or with so-called accommodative intraocular lenses (IOLs). Evidence suggests that scleral expansion cannot and does not restore accommodation. Laser treatments of the lens are in their early infancy. Development and testing of accommodative IOLs are proliferating. They are designed to produce a myopic refractive change in the eye in response to ciliary muscle contraction either through a movement of an optic or through a change in surface curvature. Three general design principles are being considered. These are single optic IOLs that rely on a forward shift of the optic, dual optic IOLs that rely on an increased separation between the two optics, or IOLs that permit a change in surface curvature to produce an increase in optical power in response to ciliary muscle contraction. Several of these different IOLs are available and being used clinically, while many are still in research and development.

A structural constitutive model for the human lens capsule

Published data on the mechanical performance of the human lens capsule when tested under uniaxial and biaxial conditions are reviewed. It is concluded that two simple phenomenological constitutive models (namely a linear elastic model and a Fung-type hyperelastic model) are unable to provide satisfactory representations of the mechanical behaviour of the capsule for both of these loading conditions. The possibility of resolving these difficulties using a structural constitutive model for the capsule, of a form that is inspired by the network of collagen IV filaments that exist within the lens capsule, is explored. The model is implemented within a rectangular periodic cell. Prescribed stretches are imposed on the periodic cell and the network is allowed to deform in a non-affine manner. The performance of the constitutive model correlates well with previously published test data. One possible application of the model is in the development of a multi-scale analysis of the mechanics of the human lens capsule.

Fluid transport phenomena in ocular epithelia

This article discusses three largely unrecognized aspects related to fluid movement in ocular tissues; namely, (a) the dynamic changes in water permeability observed in corneal and conjunctival epithelia under anisotonic conditions, (b) the indications that the fluid transport rate exhibited by the ciliary epithelium is insufficient to explain aqueous humor production, and (c) the evidence for fluid movement into and out of the lens during accommodation. We have studied each of these subjects in recent years and present an evaluation of our data within the context of the results of others who have also worked on electrolyte and fluid transport in ocular tissues. We propose that (1) the corneal and conjunctival epithelia, with apical aspects naturally exposed to variable tonicities, are capable of regulating their water permeabilities as part of the cell-volume regulatory process, (2) fluid may directly enter the anterior chamber of the eye across the anterior surface of the iris, thereby representing an additional entry pathway for aqueous humor production, and (3) changes in lens volume occur during accommodation, and such changes are best explained by a net influx and efflux of fluid.