Determination of optical parameters of the porcine eye and development of a simulated model

Storage-induced mechanical changes of porcine lenses assessed with optical coherence elastography and inverse finite element modeling

Introduction

In an effort of gaining a better understanding of the lens mechanics, ex vivo lenses samples are often used. Yet, ex vivo tissue might undergo important postmortem changes depending on the unavoidable preservation method employed. The purpose of this study was to assess how various storage conditions and the removal of the lens capsule affect the mechanical properties of ex vivo porcine lens samples.

Methods

A total of 81 freshly enucleated porcine eyes were obtained and divided into six groups and preserved differently. In the first three groups, the lens within the intact eye was preserved for 24 h by: (i) freezing at -80°C (n = 12), (ii) freezing at -20°C (n = 12), and (iii) refrigeration at +8°C (n = 12). In the remaining groups, the lenses were immediately extracted and treated as follows: (iv) kept intact, no storage (n = 12), (v) decapsulated, no storage (n = 21), and (vi) immersed in Minimum Essential Medium (MEM) at +8°C (n = 12) for 24 h. Frozen lenses were thawed at room temperature. Each lens was compressed between two glass lamella and subjected, first to a period of relaxation during which the compression force was recorded and second to an oscillating micro-compression while the deformation was recorded with a total of 256 subsequent B-scans via optical coherence tomography. The corresponding axial strain was retrieved via phase-sensitive image processing and subsequently used as input for an inverse finite element analysis (iFEA) to retrieve the visco-hyperelastic material properties of the lenses.

Results

After freezing at temperatures of -80°C and -20°C, the cortical strains increased by 14% (p = 0.01) and 34% (p < 0.001), and the nuclear strains decreased by 17% (p = 0.014) and 36% (p < 0.001), compared to the lenses tested immediately after postmortem, respectively. According to iFEA, this resulted from an increased ratio of the nuclear: cortical E-modulus (4.06 and 7.06) in -80°C and -20°C frozen lenses compared to fresh lenses (3.3). Decapsulation had the largest effect on the material constant C10, showing an increase both in the nucleus and cortex. Preservation of the intact eye in the refrigerator induced the least mechanical alterations in the lens, compared to the intact fresh condition.

Discussion

Combining iFEA with optical coherence elastography allowed us to identify important changes in the lens mechanics induced after different preserving ex vivo methods.

Controlled Drug Delivery Device for Cornea Treatment and Novel Method for Its Testing

A new solution for local anesthetic and antibiotic delivery after eye surgery is presented. A contact lens-shaped collagen drug carrier was created and loaded by Levofloxacin and Tetracaine with a riboflavin crosslinked surface layer, thus impeding diffusion. The crosslinking was confirmed by Raman spectroscopy, whereas the drug release was investigated using UV-Vis spectrometry. Due to the surface barrier, the drug gradually releases into the corneal tissue. To test the function of the carrier, a 3D printed device and a new test method for a controlled drug release, which mimics the geometry and physiological lacrimation rate of the human eye, were developed. The experimental setup with simple geometry revealed that the prepared drug delivery device can provide the prolonged release profile of the pseudo-first-order for up to 72 h. The efficiency of the drug delivery was further demonstrated using a dead porcine cornea as a drug recipient, without the need to use live animals for testing. Our drug delivery system significantly surpasses the efficiency of antibiotic and anesthetic eyedrops that would have to be applied approximately 30 times per hour to achieve the same dose as that delivered continuously by our device.

Importance of the pig as a human biomedical model

[Figure: see text].

Normative Data of Ocular Biometry, Optical Coherence Tomography, and Electrophysiology Conducted for Cynomolgus Macaque Monkeys

Purpose

To present normative data of optical coherence tomography (OCT) parameters, electrophysiological tests, and optical biometry conducted for cynomolgus monkeys.

Methods

Multimodal examinations were performed for 11 adult cynomolgus monkeys (Macaca fascicularis, weighing 2.6–7.5 kg, aged 45–99 months). A-scan biometry was performed to measure ocular biometry. OCT images were obtained at 30° and 55°. After the pupils were fully dilated, electroretinogram (ERG) and visual evoked potentials (VEP) were recorded with a commercial system using a contact lens electrode.

Results

All cynomolgus monkeys were males. The mean axial length was 17.92 ± 0.34 mm. The central total retinal layer (TRL) and subfoveal choroidal thicknesses were 286.27 ± 18.43 and 234.73 ± 53.93 µm, respectively. The TRL and nerve fiber layer thickness was greater in the nasal than in other quadrants in the Early Treatment Diabetic Retinopathy Study circle in the macula. Peripheral TRL and ganglion cell complex thickness on the temporal outside the vascular arcades were lower than on the other sides. The peak latency of a-wave and b-wave in scotopic and photopic 3.0 ERG was 14.78 ± 1.00 and 32.89 ± 1.81 ms, and 12.91 ± 1.03 and 31.79 ± 2.16 ms, respectively. The n2 wave peak latency of VEP was 15.21 ± 8.07 ms. The a-wave peak latency of ERG and the n2 wave peak latency of VEP negatively correlated with age.

Conclusions

The normative ocular biometric, electrophysiological test, and OCT parametric data of cynomolgus monkeys could serve as reference values for further preclinical studies.

Translational Relevance

We present normative data of cynomolgus monkeys’ eyes, an adequate animal model for preclinical studies.

Changes After a Month Following Micropulse Cyclophotocoagulation in Normal Porcine Eyes

Purpose

To analyze the effects on the uvea, including the pars plicata and ciliary muscle, and retina in normal porcine eyes after performing micropulse transslceral cyclophotocoagulation (MP-TSCPC) with the different energy levels, and conventional continuous wave transslceral cyclophotocoagulation.

Methods

MP-TSCPC was performed in a total of 15 eyes at the different energy levels of 60 J, 120 J, 180 J, 240 J, and 300 J, respectively. Continuous wave transslceral cyclophotocoagulation was performed in three eyes and the other three eyes were controls. The eyes were enucleated after a month following the laser treatment and the uvea and retina were analyzed using hematoxylin and eosin staining and immunohistochemistry staining.

Results

After MP-TSCPC 60 J, the expression of α-smooth muscle actin (α-SMA) and glial fibrillary acidic protein in the ciliary muscle was increased, although there was no structural change in pars plicata. After MP-TSCPC 120 J, partial destruction of the ciliary epithelium was observed in pars plicata, and the retinal thickness was increased. After MP-TSCPC 240 J and 300 J, the structural destruction of the pars plicata and ciliary muscle was observed, and the expression of α-SMA and glial fibrillary acidic protein in pars plicata and the expression of α-SMA in ciliary muscle were increased.

Conclusions

Histologic changes in the uvea and peripheral retina were different based on the energy levels of MP-TSCPC. In particular, MP-TSCPC with low energy levels mainly affected the ciliary muscle, while MP-TSCPC with high energy levels affected both the ciliary muscle and pars plicata. Our results may imply a possibility of intraocular damage with MP-TSCPC in humans.

Translational Relevance

Based on our research, it is possible to infer the possibility of intraocular damage in humans according to the different levels of energy in the clinic.

Advanced Retinal Imaging and Ocular Parameters of the Rhesus Macaque Eye

Purpose

To determine the range of normal ocular biometry and perform advanced retinal imaging and functional assessment of the rhesus macaque eye.

Methods

We performed ocular phenotyping on rhesus macaques at the California National Primate Research Center. This process consisted of anterior and posterior segment eye examination by ophthalmologists, advanced retinal imaging, and functional retinal electrophysiology.

Results

Full eye examinations were performed on 142 animals, consisting of pupillary light reflex, tonometry, external examination and photography, anterior slit lamp examination, and posterior segment examination by indirect ophthalmoscopy. Ages of the rhesus macaques ranged from 0.7 to 29 years (mean, 16.4 ± 7.5 years). Anterior segment measurements such as intraocular pressure (n = 142), corneal thickness (n = 84), lens thickness (n = 114), and axial length (n = 114) were acquired. Advanced retinal imaging in the form of fundus photography (n = 78), optical coherence tomography (n = 60), and quantitative autofluorescence (n = 44) was obtained. Electroretinography (n = 75) was used to assay retinal function. Quantitative analyses of the macular structure, retinal layer segmentation, and rod and cone photoreceptor electrical responses are reported. Quantitative assessments were made and variations between sexes were analyzed to compare with established sex changes in human eyes.

Conclusions

The rhesus macaque has an ocular structure and function very similar to that of the human eye. In particular macular structure and retinal function is very similar to humans, making this species particularly useful for the study of macular biology and development of therapies for cone photoreceptor disorders.

Translational Relevance

Rhesus macaques are an ideal model for future vision science studies of human eye diseases.

Biomimetic models of the human eye, and their applications

Replicating the functionality of the human eye has been a challenge for more than a century, creating a great wealth of biomimetic and bioinspired devices, and providing ever improving models of the eye for myriad research purposes. As improvements in microelectronics have proceeded, individual components of the eye have been replicated, and models of the optical behaviour of the eye have improved. This review explores both work developed for improving medical components, with an ultimate aim of a fully functioning prosthetic eye, and work looking at improving existing devices through biomimetic means. It is hoped that this holistic approach to the subject will aid in the cross pollination of ideas between the two research foci. The review starts by summarising the reported measurements of optical parameters of various components of the eye. It then charts the development of individual bionic components. Particular focus is put on the development of bionic and biomimetic forms of the two main adaptive components of the eye, namely the lens and the iris, and the challenges faced in modelling the light sensitive retina. Work on each of these components is thoroughly reviewed, including an overview of the principles behind the many different approaches used to mimic the functionality, and discussion of the pros and cons of each approach. This is concluded by an overview of several reported models of the complete or semi-complete eye, including details of the components used and a summary of the models' functionality. Finally, some consideration is given to the direction of travel of this field of research, and which existing approaches are likely to bring us closer to the long term goal of a fully functional analogue of the eye.

A promising approach in laser vitrectomy executed by plasma-mediated removal of vitreous body via a diode-pumped Q-switched Nd:YAG laser

To examine the applicability of plasma-mediated vitreous body removal, a diode-pumped Q-switched Neodymium:YAG laser was used for a possible application in eye surgery/vitrectomy. On a total of 1500 porcine vitreous bodies, removal rates were evaluated by comparing different LaserVit-tip designs (Mark I/II Gauge 19 and Mark III Gauge 22). The Nd:YAG laser, operating at a wavelength of 1064 nm and a pulse duration of 4 ns, was utilized for vitreous body removal with respective settings of 2, 3 and 4 mJ and pulse repetition rates (cut rates) from 5 to 25 Hz (300–1500 /min) in 5 Hz-steps as well as for 100 Hz (6000 cuts/min). The exposure times were selected at 10, 20, 40 and 60 s, respectively. Comparative measurements were carried out with mechanical cutters (Gauge 20 and Gauge 23), applying a fixed cut rate of 800 /min (13.33 Hz) at identical exposure times. The LaserVit-tips showed successful vitreous body removal for all laser settings and exposure times (Mark I: 6.2 g/min, Mark II: 8.2 g/min at 1500 cuts/min and 3 mJ, Mark II: 10.1 g/min, Mark III: 3.6 g/min at 6000 cuts/min at 3 mJ). Similar tip-dimensions (Gauge 22_laser and Gauge 23_cutter) showed comparable removal rates of 3.6 g/min_laser and 1.3 g/min_cutter with settings of 6000 cuts/min at 3 mJ (laser) and 800 cuts/min for the mechanical cutter. A diode-pumped Q-switched Nd:YAG laser can successfully and gently remove vitreous body. The efficiency of the laser was comparable to that of mechanical cutters in terms of quantity of material removed per time unit.

Changes in temperature inside an optomechanical model of the human eye during emulated transscleral cyclophotocoagulation

Currently, many diseases of the eye are treated by laser surgery. An understanding of light propagation and the heating of eye tissue during laser exposure is crucial to improving the outcome of these procedures. Here, we present the development of physical and computational models of the human eye by combining optical light propagation and thermal characteristics. For the physical model, all parts of the eye, including cornea, lens, ciliary body, sclera, aqueous and vitreous humors, and iris, were fabricated using a 3D printed holder and modified polydimethylsiloxane. We also present a computational model based on finite element analysis that allows for a direct comparison between the simulation and experimental measurements. These models provide an opportunity to directly assess the rise in temperature in all eye tissues. The simulated and physical models showed good agreement for the transmission of light at varying incident angles. The heating of optical components was investigated in the retina and the ciliary body during simulated laser surgery. Temperature increases of 45.3°C and 30.6°C in the retina and ciliary bodies, respectively, were found in the physical model after 1 minute of exposure to 186 mW of 850 nm laser radiation. This compared to 29.8°C and 33.9°C increases seen under the same conditions in the simulation model with human eye parameters and 48.1°C and 28.7°C for physical model parameters. These results and these models are very promising for further investigation of the impact of laser surgery.