Clinical Ocular Biomechanics: Where Are We after 20 Years of Progress?

Purpose: Ocular biomechanics is an assessment of the response of the structures of the eye to forces that may lead to disease development and progression, or influence the response to surgical intervention. The goals of this review are (1) to introduce basic biomechanical principles and terminology, (2) to provide perspective on the progress made in the clinical study and assessment of ocular biomechanics, and (3) to highlight critical studies conducted in keratoconus, laser refractive surgery, and glaucoma in order to aid interpretation of biomechanical parameters in the laboratory and in the clinic.Methods: A literature review was first conducted of basic biomechanical studies related to ocular tissue. The subsequent review of ocular biomechanical studies was limited to those focusing on keratoconus, laser refractive surgery, or glaucoma using the only two commercially available devices that allow rapid assessment of biomechanical response in the clinic.Results: Foundational studies on ocular biomechanics used a combination of computer modeling and destructive forces on ex-vivo tissues. The knowledge gained from these studies could not be directly translated to clinical research and practice until the introduction of non-contact tonometers that quantified the deformation response of the cornea to an air puff, which represents a non-destructive, clinically appropriate load. The corneal response includes a contribution from the sclera which may limit corneal deformation. Two commercial devices are available, the Ocular Response Analyzer which produces viscoelastic parameters with a customized load for each eye, and the Corvis ST which produces elastic parameters with a consistent load for every eye. Neither device produces the classic biomechanical properties reported in basic studies, but rather biomechanical deformation response parameters which require careful interpretation.Conclusions: Research using clinical tools has enriched our understanding of how ocular disease alters ocular biomechanics, as well as how ocular biomechanics may influence the pathophysiology of ocular disease and response to surgical intervention.

Safety and Long-term Scleral Biomechanical Stability of Rhesus Eyes after Scleral Cross-linking by Blue Light.

Purpose: To assess the safety and long-term scleral biomechanical stability of rhesus eyes after blue light scleral CXL by investigating the biomechanical and microstructural changes.Methods: Seven rhesus monkeys (14 eyes) were observed in this study. All right eyes received blue light scleral CXL at the superior temporal equatorial sclera, and the left eyes served as controls. Biological ocular parameters were followed up to 1 year after scleral CXL. Stress-strain measurements of three rhesus sclera were measured, three rhesus retinas were examined histologically by H&E and TUNEL staining. And the microstructure of both the sclera and retina were observed by transmission electron microscopy at 1 year.Results: As for the retinal thickness, choroidal thickness, flow density of retinal superficial vascular networks and flash electroretinography (f-ERG) results, no significant differences were observed between the paired eyes at 1 year (P >.05). At the same time, the scleral collagen fibril distribution was much tighter, and the scleral biomechanical properties were significantly increased in the experimental eyes. However, apoptotic cells and retinal ultrastructural changes could still be found in the retina of the experimental eyes.Conclusion: This study demonstrates that blue light scleral CXL could effectively increase the scleral stiffness of the rhesus eye for at least 1 year, but ultrastructural change was still observed in the retina of scleral CXL eye. Therefore, the long-term intraocular safety of the blue light scleral CXL technique for preventing myopia progression should be investigated further.

Differential Regional Stiffening of Sclera by Collagen Cross-linking.

Purpose: Corneal collagen cross-linking by ultraviolet light activation of riboflavin has been used clinically to enhance corneal stiffness. We sought to determine if cross-linking differentially affects scleral regions.Methods: Adjacent, parallel strips of sclera were cut from superolateral, superomedial, inferolateral, and inferomedial quadrants of posterior and equatorial sclera of 12 human cadaver eyes. One of each pair served as control while the other was cross-linked by immersion in 0.1% riboflavin and 365 nm exposure at 6 mW/cm2 irradiance for 30 min. Behavior of strips was characterized using a microtensile load cell. Preloaded strips were imaged using orthogonally mounted cameras and optical coherence tomography to determine specimen dimensions including cross-sectional area. Tension was measured during 0.1 mm/s constant rate elongation.Results: Young's modulus (YM), the slope of the relationship relating tensile stress to strain, was calculated at 8% strain, and increased significantly after cross-linking (P < .001). In posterior sclera, mean (± standard error of mean, SEM) YM is increased in the superolateral, superomedial, inferolateral, and inferomedial quadrants by 46 ± 15%, 32 ± 11%, 67 ± 20%, and 53 ± 11%, respectively. In equatorial sclera, YM is increased by 139 ± 43%, 68 ± 27%, 143 ± 92%, and 68 ± 14%, respectively. The YM of pooled equatorial quadrants increased significantly more than that of the pooled posterior quadrants.Conclusions: Scleral collagen cross-linking by ultraviolet activation of riboflavin differentially increases scleral YM more in the equatorial than posterior sclera, and most in the lateral, equatorial sclera. Cross-linking might be used to arrest progressive myopia or to prevent staphyloma formation.

Correlation of Corneal and Scleral Pneumatonometry in Pediatric Patients.

PURPOSE: To study the correlation between corneal and scleral intraocular pressure (IOP) by pneumatonometry in pediatric patients. DESIGN: Cross-sectional study. PARTICIPANTS: Patients (age range, 0-15 years) undergoing an eye examination under anesthesia or eye surgery were recruited at the University of California, San Francisco, Benioff Children's Hospital between July 2015 and April 2016. METHODS: Intraocular pressure measurements were obtained by pneumatonometry on the central cornea and the inferonasal and inferotemporal sclera in a random order. Spearman correlations between corneal versus inferonasal scleral IOP and corneal versus inferotemporal scleral IOP were calculated. A linear mixed-effect model was used to derive a predictive equation for corneal IOP from scleral IOP and to perform covariate analysis for age, axial length, central corneal thickness, and lens status. The standard deviation of the predicted corneal IOP was determined by bootstrap mixed-effect regression analysis. MAIN OUTCOME MEASURES: The predictive model of corneal IOP from scleral IOP. RESULTS: Seventy-five eyes from 40 patients were included in the study. Spearman correlation coefficient for corneal versus inferotemporal scleral IOP was 0.79 (P < 0.01) and 0.48 for corneal versus inferonasal scleral IOP (P < 0.01). Corneal IOP may be predicted from scleral IOP via the following equations: corneal IOP = 0.73 × inferotemporal scleral IOP + 7.45 and corneal IOP = 0.21 × inferonasal scleral IOP + 17.83. Central corneal thickness (P = 0.07), lens status (P = 0.4), age (P = 0.33), and axial length (P = 0.15) did not affect significantly the relationship between corneal and scleral IOP in the multivariate regression analysis. The standard deviation of predicted corneal IOP was less than 1.2 mmHg within an inferotemporal scleral IOP range of 10 to 35 mmHg. CONCLUSIONS: In children, corneal and scleral IOP are correlated significantly when measured by pneumatonometry. Measurements obtained from the inferotemporal sclera are better predictors of corneal IOP than those obtained from inferonasal sclera. Pneumatonometry on the inferotemporal sclera may be an alternative method to estimate IOP for pediatric patients from whom corneal IOP measurement is difficult to obtain.

Suprachoroidal shunts.

PURPOSE OF REVIEW: This article reviews recent studies evaluating the efficacy and safety of ab externo and ab interno suprachoroidal shunts designed to augment uveoscleral outflow in patients with open angle glaucoma. RECENT FINDINGS: The Gold Shunt and STARflo are placed into the suprachoroidal space via an ab externo approach, while the CyPass Micro-Stent and iStent Supra are inserted ab interno through a clear corneal incision. A small randomized clinical trial suggested similar efficacy among two different models of the Gold Shunt and the Ahmed glaucoma valve. The pivotal multicenter, randomized COMPASS trial showed that the CyPass Micro-Stent combined with cataract surgery is more effective than cataract surgery alone through 2 years. Study results suggest a favorable safety profile for each suprachoroidal device and intraocular pressure-lowering to the mid-to-high teens with an associated reduction in medication burden. Given the relative novelty of these procedures, it is currently unknown whether fibroblastic proliferation within the suprachoroidal space may limit their long-term success. SUMMARY: Renewed interest in the suprachoroidal space has led to the development of new biocompatible glaucoma implants. Ab interno devices are easily combined with cataract surgery and preserve conjunctiva for future incisional glaucoma surgery. Additional data regarding the long-term efficacy and late complications of suprachoroidal shunts are needed.

Location and pressure dependent transmission of human and porcine sclera: an anterior to posterior examination.

PURPOSE: For diaphanoscopy or transscleral laser applications, the transmission of the sclera is an essential property. The study aimed to determine the pressure dependent transmission of human sclera from anterior to posterior. METHODS: Pressure dependent transmission measurements were performed by a pressure inducing setup at the range of 60-2058 kPa. The transmissions were measured within spectral range of 350-1100 nm. Specimens of human sclera were taken from corneo-scleral transplants. Those compounds were obtained at pars plicata residual sclera tissue. For an anterior to posterior examination of transmission, samples were taken from halved eye globes, which were formerly fixed in formalin. RESULTS: The pressure dependent transmission increased with rising load at all measured wavelengths for human sclera samples. The highest increase was observed for short wavelengths. With rising pressure, the increase of transmission aimed for a steady state. This behavior was fitted by a limited growing function. With an inducing burden of 2058 kPa, the steady state was already reached and exhibited an increase in transmission factor of 4.1 at 400 nm and 1.8 at 1000 nm. The anterior to posterior measurements of human sclera fixed in formalin were not corresponding to the results of the other human samples. For the porcine samples, the transmission increased from anterior to the equator of the eye globe. Further posterior the transmission decreased and rose again to N. opticus. With rising pressure, the transmission increased at all wavelengths and all locations. Posterior from the equator, with higher pressure the transmission became superior compared to anterior. CONCLUSIONS: The results of human sclera fixed in formalin could be related to formalin-induced cross-linking between the collagen fibers. Because of doubt about the physiological behavior of formalin-fixed samples, formalin-free porcine postmortem eye globes were also probed having a very similar thickness and histological structure as human sclera, so the results could be set in relation to human probes. These results can now be used to create an eye-map to determine maximum possible retina irradiation or illumination durations for transscleral applications in eye surgery.

Importance of accurately assessing biomechanics of the cornea.

PURPOSE OF REVIEW: This article summarizes the state-of-the-art in clinical corneal biomechanics, including procedures in which biomechanics play a role, and the clinical consequences in terms of error in estimating intraocular pressure (IOP). RECENT FINDINGS: Corneal biomechanical response to refractive surgery can be categorized into either stable alteration of surface shape and thus visual outcome, or unstable biomechanical decompensation. The stable response is characterized by central flattening and peripheral steepening that is potentiated in a stiffer cornea. Two clinical devices for assessing corneal biomechanics do not yet measure classic biomechanical properties, but rather provide assessment of corneal deformation response. Biomechanical parameters are a function of IOP, and both the cornea and sclera become stiffer as IOP increases. Any assessment of biomechanical parameters must include IOP, and one value of stiffness does not adequately characterize a cornea. SUMMARY: Corneal biomechanics plays a role in the outcomes of any procedure in which lamellae are transected. Once the corneal structure has been altered in a manner that includes central thinning, IOP measurements with applanation tonometry are likely not valid, and other technologies should be used.

The efficacy and safety of a novel posterior scleral reinforcement device in rabbits.

PURPOSE: To evaluate the efficacy and safety of posterior scleral reinforcement (PSR) device for myopia suppression in rabbits' eyes. METHODS: PSR surgery was performed on the normal 12 8-week-old New Zealand white rabbits' right eyes. To determine efficacy of the device, ophthalmic examination would be taken at pre-operation and post-operation (1 week, 1 month, 3 months, 6 months, and 1 year), such as A-ultrasound, diopter and B-ultrasound. Evaluation of safety were based on the following indicators: intraocular pressure (IOP), slit lamp, fundus photography, fundus fluorescein angiography and pathological examination after surgery. The efficacy and safety of PSR device were evaluated by comparison (treated eyes and contralateral eyes) of pre and post-operation. RESULTS: The novel PSR device could significantly shorten axial length (preoperative axial length: 16.36 ± 0.14 mm, postoperative 1 week, 1 month, 3 months, 6 months and 1 year axial lengths: 15.03 ± 0.28 mm, 15.23 ± 0.32 mm, 15.39 ± 0.31 mm, 15.45 ± 0.22 mm and 15.45 ± 0.22 mm; P=0.00037<0.001) in the treated eyes (right eyes) after surgery. At different postoperative time points, the B-ultrasound images showed that the PSR located in appropriate position and supported the posterior sclera very well. At the same time, IOP of treated eyes kept a relatively stable level (preoperative IOP: 12.56 ± 2.01 mmHg, postoperative IOP: ranging from 11.33 ± 1.23 mmHg to 13.44 ± 2.19 mmHg, P>0.05) post-operation 1 year. During observation period, there was no significant inflammatory reaction and complications such as anterior chamber flare, empyema, endophthalmitis, vitreous hemorrhage, retina detachment and retinal choroid neovascularization by slit lamp, fundus photography and fundus fluorescein angiography. In addition, there were no pathologic changes be found by comparison treated eyes group and contralateral group eyes based on pathological examinations. CONCLUSIONS: In vivo study, effectively and safely, the novel PSR device can inhibit rabbits' axial length elongation during postoperative 1 year. This study demonstrates that this novel PSR could be a potential treatment approach for myopia.

Technical note: Digital quantification of eye pigmentation of cattle with white faces.

Cancer of the eye in cattle with white faces occurs less frequently in cattle with pigmented eyelids. Corneoscleral pigmentation is related to eyelid pigmentation and occurrence of lesions that may precede cancer. Objectives of this study were to assess 1) variation in the proportion of eyelid and corneoscleral pigmentation in Hereford, Bos taurus, and Bos indicus crossbreds and 2) the occurrence of lesions with the presence of pigmentation in those areas. Hereford and Bos indicus crosses (Brahman or Nellore with Angus and Hereford and straightbred Brafords) and Bos taurus crosses (Angus-Hereford) were included in the study (n = 1,083). Eyelid pigmentation proportions were estimated by pixel quantification and were evaluated as total proportions and for upper and lower eyelids distinctly for each eye. Fixed effects included breed type, age categories, and sex of the animal. Lesion presence (1) or absence (0) was obtained by visual appraisal of image and was assumed to be binomially distributed. Eyelid pigmentation proportions (overall, upper, and lower eyelids) for Hereford ranged from 0.65 ± 0.03 to 0.68 ± 0.03 and were significantly lower than Bos indicus (range from 0.93 ± 0.02 to 0.95 ± 0.02) or Bos taurus (ranged from 0.88 ± 0.02 to 0.92 ± 0.02) crosses. Corneoscleral pigmentation in Hereford cows (0.17 ± 0.06) did not differ (P = 0.91) from Hereford calves and yearlings (0.16 ± 0.07). Bos indicus and Bos taurus crossbred cows had larger corneoscleral pigmentation (0.38 ± 0.05 and 0.48 ± 0.04 for left eyes and 0.37 ± 0.05 and 0.53 ± 0.04 for right eyes, respectively) than all calves (P < 0.001), and their corneoscleral pigmentations were greater than that of Hereford cows (P < 0.003). Bos indicus and Bos taurus cows had greater proportions of left eye corneoscleral pigmentation (0.38 ± 0.05 and 0.48 ± 0.04, respectively) than Hereford cows (0.17 ± 0.06) and all young animal breed types (P < 0.05). Right eye proportions differed for all cow groups (P < 0.05; 0.53 ± 0.04, 0.37 ± 0.05, and 0.17 ± 0.06). Among calves and yearlings, Hereford had a lower right eye corneoscleral pigmentation proportion (0.16 ± 0.07) than Bos taurus (P = 0.02). The lesion proportion for Hereford (0.08 ± 0.03) was significantly greater than that of either Bos indicus (0.01 ± 0.005) or Bos taurus (0.01 ± 0.003). Crossbreeding with Bos taurus or Bos indicus animals appears to increase eye pigmentation, which may help reduce the occurrence of cancer in eyes of cattle with white faces.

Correlation of Serial Scleral and Corneal Pneumatonometry.

PURPOSE: To evaluate the usefulness of scleral pneumatonometry as an alternative for corneal measurements of intraocular pressure (IOP) over a broad range of IOPs. DESIGN: Prospective, observational cohort study. PARTICIPANTS: The study was conducted in the University of California, San Francisco, Retina Clinic between August and November 2013 in 33 adult patients (age range, 34-94 years; mean ± standard deviation, 74.1±13.4 years) receiving anti-vascular endothelial growth factor intravitreal injections, which transiently increase IOP. METHODS: Corneal pachymetry and serial corneal and temporal scleral pneumatonometry (baseline, immediately after, and 10, 20, and 30 minutes after injection) were collected. One-time baseline corneal and scleral pneumatonometry readings were obtained in the noninjected eye. MAIN OUTCOME MEASURES: Correlation analysis and a Bland-Altman plot were used to evaluate reliability and agreement between scleral and corneal measurements of IOP. A linear mixed model was used to determine the relationship between measurements and to perform covariate analyses. RESULTS: Scleral and corneal pneumatonometry showed nearly 1:1 linear correlation, although scleral pneumatonometry was biased toward higher values (r = 0.94; P < 0.001). Scleral pneumatonometry averaged 9.0 mmHg higher than corneal pneumatonometry (95% limits of agreement, -1.5 to 19.5 mmHg). A linear mixed model resulted in the following equation: corneal IOP = 1.04 × scleral IOP - 10.37. Age, central corneal thickness, laterality, and glaucoma and lens status did not impact this relationship. The difference between corneal and scleral pneumotonometry was correlated between the two eyes of individual patients (r = 0.75; P < 0.001). CONCLUSIONS: Differences between serial scleral measurements reflect differences between serial corneal measurements. Scleral pneumatonometry should be considered as an alternative to corneal pneumatonometry for following patients in whom corneal measurements are unreliable or unobtainable.

Novel characterization and live imaging of Schlemm's canal expressing Prox-1.

Schlemm's canal is an important structure of the conventional aqueous humor outflow pathway and is critically involved in regulating the intraocular pressure. In this study, we report a novel finding that prospero homeobox protein 1 (Prox-1), the master control gene for lymphatic development, is expressed in Schlemm's canal. Moreover, we provide a novel in vivo method of visualizing Schlemm's canal using a transgenic mouse model of Prox-1-green fluorescent protein (GFP). The anatomical location of Prox-1⁺ Schlemm's canal was further confirmed by in vivo gonioscopic examination and ex vivo immunohistochemical analysis. Additionally, we show that the Schlemm's canal is distinguishable from typical lymphatic vessels by lack of lymphatic vessel endothelial hyaluronan receptor (LYVE-1) expression and absence of apparent sprouting reaction when inflammatory lymphangiogenesis occurred in the cornea. Taken together, our findings offer new insights into Schlemm's canal and provide a new experimental model for live imaging of this critical structure to help further our understanding of the aqueous humor outflow. This may lead to new avenues toward the development of novel therapeutic intervention for relevant diseases, most notably glaucoma.

Human eye phantom for developing computer and robot-assisted epiretinal membrane peeling.

A number of technologies are being developed to facilitate key intraoperative actions in vitreoretinal microsurgery. There is a need for cost-effective, reusable benchtop eye phantoms to enable frequent evaluation of these developments. In this study, we describe an artificial eye phantom for developing intraocular imaging and force-sensing tools. We test four candidate materials for simulating epiretinal membranes using a handheld tremor-canceling micromanipulator with force-sensing micro-forceps tip and demonstrate peeling forces comparable to those encountered in clinical practice.

Microstructural changes in sclera under thermo-mechanical effect of 1.56 µm laser radiation increasing transscleral humor outflow.

BACKGROUND AND OBJECTIVES: Pores in the sclera are a candidate pathway for aqueous transport and therefore can be utilized to decrease the intraocular pressure (IOP) in glaucomatous eyes. Since pore formation is a well-known mechanism for stress relaxation in solids, laser-induced creation of pores in cartilage increases hydraulic permeability and promotes tissue regeneration. The aim of this paper is to demonstrate the thermo-mechanical effect of non-destructive laser irradiation on microstructural changes in sclera, in particular pore formation, resulting in substantial increase of water permeability of eye tissues that can be a novel approach to normalize the IOP. MATERIALS AND METHODS: Experiments were performed ex vivo on eight eyes of four mini-pigs and in vivo on eight eyes of four rabbits using pulse repetitive laser radiation of 1.56 µm in wavelength. Twenty laser spots of 0.6 mm in diameter with laser settings (power 0.9 W, pulse duration of 200 milliseconds, pulse repetition rate of 2 Hz) resulting in substantial increase of sclera hydraulic permeability were applied on the sclera at 1-2 mm from the eye limb. Sclera and underlying structures (choroid and ciliary body) of the rabbits' eyes were examined histologically in 1 and 45 days after laser irradiation, atomic force microscope (AFM) was applied before and after laser irradiation. RESULTS: Histological and AFM examinations have clearly recognized laser-assisted stable structural alterations: rarefication of the collagen structure in the laser irradiated zone and formation of sub-micron pores. Laser-induced alterations in the structure of ciliary bodies were small in size and mainly reversible. We have proposed a possible mechanism of the arising pores stabilization due to formation of small stable gas bubbles in sclera tissue. CONCLUSIONS: It is shown, for the first time, that thermo-mechanical effect of pulse repetitive laser irradiation results in pores formation in sclera. That can be a basis of a novel, safe, and effective technique for IOP normalization due to enhancing of uveoscleral outflow under non-destructive laser irradiation of the sclera.

The effects of tenascin C knockdown on trabecular meshwork outflow resistance.

PURPOSE: Tenascin C (TNC) is a matricellular glycoprotein whose expression in adult tissue is indicative of tissue remodeling. The purpose of the current study was to determine the localization of TNC in trabecular meshwork (TM) tissue and to analyze the effects of TNC on intraocular pressure (IOP). METHODS: Human TM frontal sections were immunostained with anti-TNC and imaged by confocal microscopy. TNC mRNA and protein levels were quantitated in anterior segments perfused at physiological and elevated pressure. Short, hairpin RNA (shRNA) silencing lentivirus targeting full-length TNC (shTNC) was applied to anterior segment perfusion organ cultures. The IOPs and central corneal thickness (CCT) of wild-type, TNC(-/-), and tenascin X (TNX(-/-)) knockout mice were measured. RESULTS: TNC was distributed in the juxtacanalicular (JCT) region of adult human TM, predominantly in the basement membrane underlying the inner wall of Schlemm's canal. Application of shTNC lentivirus to human and porcine anterior segments in perfusion culture did not significantly affect outflow rate. Although TNC was upregulated in response to pressure, there was no difference in outflow rate when shTNC-silenced anterior segments were subjected to elevated pressure. Furthermore, IOPs and CCTs were not significantly different between TNC(-/-) or TNX(-/-) and wild-type mice. CONCLUSIONS: TNC does not appear to contribute directly to outflow resistance. However, TNC immunolocalization in the JCT of adult human eyes suggests that certain areas of the TM are being continuously remodeled with or without an IOP increase.

Effect of irradiation time on riboflavin-ultraviolet-A collagen crosslinking in rabbit sclera.

PURPOSE: To determine the effect of the duration of irradiation on the biomechanical parameters of combined riboflavin-ultraviolet-A (UVA) collagen crosslinking (CXL) in rabbit sclera. SETTING: Department of Ophthalmology, Provincial Hospital affiliated with Shandong University, Shandong, China. DESIGN: Experimental study. METHODS: Thirty-six New Zealand rabbits were divided into 6 groups based on the duration of irradiation (10, 20, 30, 40, 50, or 60 minutes). After the application of riboflavin 0.1% drops (without dextran) as a photosensitizer, the animals were irradiated with 3 mW/cm(2) UVA at 365 nm. Only the left eye of each rabbit was treated. All the animals were humanely killed 24 hours postoperatively. One eye in each treated group was used for light microscopy. The other treated eye and all control eyes were prepared for biomechanical testing. The biomechanical parameters were ultimate stress, Young modulus, and the physiological modulus. RESULTS: The eyes irradiated for 10 or 20 minutes did not differ significantly from the control eyes. Stress-strain measurement of scleral strips irradiated for 40 minutes or longer showed a significant increase in the ultimate stress, Young modulus, and the physiological modulus. There was a significant increase in the physiological modulus of scleral strips irradiated for 30 minutes or longer. Eyes that were irradiated for 50 minutes and 60 minutes had retinal damage. CONCLUSIONS: Riboflavin-UVA CXL can lead to a noticeable increase in the biomechanical stiffness of the sclera. The physiological modulus is the most sensitive tool to measure stiffness. In this study, the optimum duration of irradiation was 40 minutes.

[The application of computer modeling in ophthalmologic investigation].

It is very difficult to measure the human eye properties directly, such as the accommodation mechanism, intraocular pressure distribution, the dynamics of aqueous humor flow and the bio-heat transfer in human eyes. Modeling and simulation may, therefore, play an increasingly important role in the ophthalmologic investigation. The major computer modeling methods, including geometric modeling, physical modeling and mathematical modeling, are introduced in this paper. Modeling and simulation anatomy properties and physiological properties of eye tissues, such as the cornea, aqueous humor and crystalline lens, vitreous, optic nerve head, sclera, are analyzed in the order from global to local, from front to back, from outside to inside. Finally, the problems of computer modeling in ophthalmologic investigation are discussed, and the development trends of the future are pointed out.

Regional Biomechanical properties of human sclera after cross-linking by riboflavin/ultraviolet A.

PURPOSE: To evaluate the biomechanical difference of human scleral collagen cross-linking (CXL) by comparing different riboflavin-instilling methods and different cross-linked regions (equatorial and posterior sclera). METHODS: Fifteen donor human eyes were randomly divided into five groups. One group, in which CXL was not applied, was designated as the control group. In the remaining four groups, 0.1% riboflavin solution was instilled on the scleral surface for 5, 10, 20, or 30 minutes, respectively, followed by 30 minutes of ultraviolet A irradiation. The equatorial and posterior scleral strips in each eye were dissected. Stress-strain measurements of all scleral strips were performed by a biomaterial tester. Young modulus was calculated at 8% strain. Data of the stress and Young modulus in different regions and groups were compared using one-way analysis of variance. RESULTS: Under a 1 mm/minute stretching, the sclera exhibited an exponential stress-strain behavior. The stress and modulus of equatorial and posterior sclera after CXL gradually increased with riboflavin instillation before surgery. No statistical difference was noted in the modulus between 20 and 30 minutes riboflavin infiltration after CXL (P>.05). At the same strain levels, equatorial sclera with and without CXL exhibited higher stress and Young modulus than that of posterior sclera. CONCLUSIONS: Equatorial and posterior human sclera may be enhanced by CXL with riboflavin/ultraviolet A irradiation. Equatorial scleral CXL may be a good choice for the treatment of progressive myopia. Because of its safety and efficacy, 20 minutes of riboflavin infiltration before CXL is recommended.

Computational simulation of altitude change-induced intraocular pressure alteration in patients with intravitreal gas bubbles.

PURPOSE: To study the impact of altitude on the intraocular pressure (IOP) in an eye with an intravitreal gas bubble. METHODS: A mathematical model was developed to simulate intravitreal gas bubble expansion caused by change in altitude. Mechanical deformation of the eye was simulated using a finite-element model. Intraocular pressure-driven changes in aqueous humor flow were also considered. Two cases were studied: 1) ascent from sea level to 3,000 ft followed by immediate return to sea level and 2) ascent to 3,000 ft followed by prolonged exposure to 3,000 ft. The effect of IOP-lowering medications was studied by changing the model parameters. RESULTS: The IOP increase was directly related to the initial bubble size when ascent to 3,000 ft was simulated. When prolonged exposure to high altitude was modeled, loss of aqueous humor led to a less elevated value of IOP. In a typical simulated case, when the outflow facility was increased, the predicted IOP rise was reduced by 28%. CONCLUSION: Theoretical modeling of an eye with an intravitreal gas bubble can help an ophthalmologist evaluate the impact of altitude-induced IOP changes. Our model suggests that IOP-lowering drugs could help manage altitude-induced IOP changes in the presence of intravitreal gas bubbles.

Biomechanical considerations: evaluating scleral reinforcement materials for pathological myopia.

OBJECTIVE: Scleral biomechanical weakness and thinness are known to be one of the main factors in the pathogenesis of progressive myopia. Posterior scleral reinforcement surgery remains the rational treatment for pathological myopia. We tested the biomechanical properties of 3 types of scleral reinforcement materials (artificial pericardium, human sclera, and all-dermal matrix) in an attempt to select the ideal material for the reinforcement of pathologic sclera. DESIGN: Experimental study. PARTICIPANTS: Forty-five adult Japanese white rabbits. METHODS: Animals were equally divided into 3 groups. For each group, 1 type of material was surgically implanted at the back of the globe. We harvested samples after 10 months of implantation, tested the elasticity modulus for both reinforced sclera and unreinforced control sclera, and assessed data by t test methods. Statistically significant differences were considered when p < 0.05. RESULTS: Rabbit sclera reinforced by artificial pericardium or human sclera showed significant increases in the elasticity modulus compared with control eyes. However, the rabbit sclera reinforced by all-dermal matrix showed no significant difference in the elasticity modulus compared with normal controls. CONCLUSIONS: The analysis of biomechanical considerations in scleral reinforcement materials presented here is a very helpful method to choose the best materials for treatment of myopia. Of all materials tested, the artificial pericardium and human foreign-body sclera provided the best biomechanical characteristics.

Anatomy of the scleral ossicles in brazilian bird

The sclera is the outermost layer of the eye. In some vertebrates, it consists of a scleral ring of osseous plates that afford protection against pressure and help maintain the shape of the eyeball. The morphology, number, development and position of the scleral ossicles differ in distinct groups of vertebrates. Therefore, the objective here was to examine the number, shape and arrangement of the scleral ossicles in Brazilian birds of different orders. The study involved 208 birds of 18 orders, which died from causes unrelated to eye infections. The birds’ eyeballs were removed and subjected to diaphanization and staining of the ossicles with Alizarin red S. All the eyeballs analyzed presented scleral ossicles in a fixed position in the central portion of the eyeball, but which varied in size, shape and number. The number of bony plates in the various bird species varied from 11 to 16, with a modal number of 14, and their shape was predominantly quadrangular, showing a similar pattern in species of the same order. Some specimens presented differences in the number of ossicles in the right and left eyeball, as in Spheniscus magellanicus, Elanus leucurus, Ramphastos toco and others and Leptodon cayanensisalso presented a scleral sesamoid bone. The distribution and morphology of the ossicles vary according to the taxonomic group, although species of the same order present a similar pattern.