A donor cornea with metastatic cells from a cutaneous malignant melanoma.

PURPOSE: To describe the case of a donor cornea that showed hematogenous metastatic spread of cutaneous melanoma to the sclerocorneal limbus. METHODS: Corneal tissue obtained from a donor with cutaneous malignant melanoma was evaluated for endothelial cell density, corneal transparency, and epithelial morphology. Subsequently, hematoxylin and eosin staining and immunohistochemical characterization using S100, HMB45, Melan-A, and CD34 antibodies were performed on the corneal sections. RESULTS: The corneal tissue was transparent with high endothelial cell density; it was graded as being suitable for transplantation according to the current eye bank criteria. However, the aggressiveness of the donor's cancer and the diffuse melanosis of the sclera led to the suspicion of malignant melanoma metastasis to the cornea. Histochemical analysis of the corneoscleral rim showed small aggregates rich in pigmented cells that were localized in cleft-like structures in the sclera, at the sclerocorneal interface and in the peripheral avascular portion of the cornea. The aggregates were positive for the melanocytic tumor markers S100, HMB45, and Melan-A; the rims of the clefts expressed the panvascular CD34 antigen, which was suggestive of neovascularization. CONCLUSIONS: Corneal tissue from a donor with malignant cutaneous melanoma displayed neoplastic lesions of melanocytic origin that had spread from a primitive melanoma through hematogenous routes to the sclerocorneal limbus. On the basis of this finding, we believe that having a metastatic cutaneous malignant melanoma could in some cases be reviewed as an exclusionary criterion for undergoing cornea transplantation.

Protective effects of deswelling on stromal collagen denaturation after a corneal femtosecond laser cut.

PURPOSE: To evaluate the effect of corneal deswelling prior to a microkeratome and femtosecond (fs) laser cut and the impact of the fs laser energy settings on the tissue surface and collagen quality. METHODS: Porcine and human corneas were incubated in THIN-C (deswelled) or EUSOL-C (control) at 4°C for 4 hours. Porcine corneas were cut using a microkeratome or an Intralase fs laser 150 Hz and 0.75 µJ energy. Human corneas were cut using the laser at different energy settings. The tissue thickness was measured using Visante OCT and the endothelial cell density (ECD) was evaluated by an Eye Bank KeratoAnalyzer or trypan blue staining. The tissue was analyzed using scanning electron microscopy (SEM). RESULTS: In porcine corneas, tissue deswelling resulted in a reduction of 14% in the central corneal thickness (CCT), a regular surface, and increased tissue stiffness without affecting ECD. After fs laser cutting, the control corneas showed superficial collagen denaturation that was absent in the deswelled corneas. The deswelled human corneas showed a CCT reduction of 20%, better surface smoothness, preserved collagen, and increased stiffness after fs laser cutting at 1.0 to 1.2 µJ, compared with controls. Surface smoothness decreased and collagen fibers showed a melted-like aspect both in deswelled and control corneas at greater than 1.4 µJ. CONCLUSIONS: Fs laser cutting parameters can reduce corneal surface smoothness and increase collagen melting damage. The use of a deswelling medium resulted in tissues for Descemet stripping automated endothelial keratoplasty (DSAEK) with increased stiffness, smooth surfaces, and no thermal damage to the collagen matrix.

Characterization of age-related variation in corneal biomechanical properties.

An experimental study has been conducted to determine the stress-strain behaviour of human corneal tissue and how the behaviour varies with age. Fifty-seven well-preserved ex vivo donor corneas aged between 30 and 99 years were subjected to cycles of posterior pressure up to 60 mm Hg while monitoring their behaviour. The corneas were mechanically clamped along their ring of scleral tissue and kept in physiological conditions of temperature and hydration. The tissue demonstrated hyper-elastic pressure-deformation and stress-strain behaviour that closely matched an exponential trend. Clear stiffening (increased resistance to deformation) with age was observed in all loading cycles, and the rate of stiffness growth was nonlinear with bias towards older specimens. With a strong statistical association between stiffness and age (p < 0.05), it was possible to develop generic stress-strain equations that were suitable for all ages between 30 and 99 years. These equations, which closely matched the experimental results, depicted corneal stiffening with age in a form suitable for implementation in numerical simulations of ocular biomechanical behaviour.

Regional variation in the biomechanical properties of the human sclera.

The study aimed to determine the variation in thickness and biomechanical properties between the different regions of the human sclera. Thickness measurements were carried out along eight meridian lines extending from the posterior pole to limbus in 36 human donor scleras aged 52-96 years. Strip specimens were extracted from areas close to the limbus, equator and posterior pole, and tested under cycles of uniaxial tension. Two strain rates were considered to assess the viscoelasticity effects on the regional variation in material behaviour. The results were used to derive the stress-strain behaviour of each specimen and to calculate the tangent modulus at each stress level. The scleras had a variable thickness from maximum at the posterior pole to minimum close to the equator, and increasing again towards the limbus. All scleral specimens demonstrated nonlinear behaviour with an initially low tangent modulus (a measure of stiffness) increasing gradually under higher stresses. With reference to specific stress levels, the behaviour comparisons between regions showed a gradual growth in material tangent stiffness with progression from the posterior region towards the limbus. The viscoelasticity of the tissue, which was evident with significant increases in stress (157-203%) and tangent modulus (30.3-38.8%) with strain rate rise (from 8% to 200% per min), was associated with reductions in the regional variation in stiffness. The considerable variation in biomechanical behaviour found in this study should be useful in improving the accuracy of representing the sclera's real-life conditions in numerical simulations.

Experimental assessment of human corneal hysteresis.

PURPOSE: Hysteresis is a viscoelastic property characterized by the difference in behavior under loading and unloading. The aim of the study was to determine corneal hysteresis using experimental means. METHODS: Twenty-nine human corneas with 50-95 year age were subjected to cycles of pressure loading and unloading. Two pressure application rates were adopted to approximate static and dynamic loading conditions. RESULTS: The behavior under both loading and unloading was found to stiffen with increased age. The unloading behavior appeared to be largely independent of the pressure level at which unloading started. The difference between the behavior patterns under loading and unloading was quantified and used as a measure of corneal hysteresis. The hysteresis area was significantly larger with faster loading and with decreased age. CONCLUSIONS: The trend for hysteresis to decrease with age is in agreement with previous clinical observations. Hysteresis was also found to increase with faster pressure application.

Experimental assessment of corneal anisotropy.

PURPOSE: To determine the variation of corneal biomechanical properties with anatomical orientation. METHODS: Strip specimens extracted from fresh porcine corneas were tested under uniaxial tension with strain rates representing static and dynamic loading conditions. The specimens were extracted from the vertical, horizontal, and 45 degrees diagonal directions. The load elongation results were used to derive the stress-strain behavior of each specimen. The average behavior for specimens taken in each anatomical direction was determined along with the effect of strain rate. Specimens from a small number of human corneas were included in the study to verify the findings. RESULTS: Specimens extracted from the vertical direction of porcine and human corneas demonstrated the highest strength (fracture stress) followed by horizontal then diagonal specimens. Vertical specimens were 10% to 20% stronger than horizontal specimens in porcine and human corneas. At low strain rates (1%/min), vertical specimens displayed similar stiffness (resistance to deformation) to horizontal specimens but greater stiffness than diagonal specimens. On increasing the strain rate to 500%/min, the stiffness behavior matched that of strength with vertical specimens being 10% to 20% stiffer than horizontal specimens in porcine and human corneas. CONCLUSIONS: The corneal anisotropic behavior is compatible with the preferential orientation of stromal fibrils in the vertical and horizontal directions. Quantifying the effect of this nonuniform fibril organization on corneal anisotropic behavior will be useful in developing numerical models of the cornea for applications where its integrity is compromised such as in simulating refractive surgery procedures.

Assessment of corneal biomechanical properties and their variation with age.

PURPOSE: The aim of this study was to determine the stress-strain behavior of corneal tissue and how the behavior was affected by age. METHODS: Human corneal specimens ranging in age between 50 and 95 years were tested under inflation conditions to determine their stress-strain behavior. The corneas were subjected to two load rates that represent dynamic and static loading conditions. The pressure-deformation results were analyzed using shell theory to derive the stress-strain behavior. RESULTS: The corneas demonstrated clear nonlinear behavior with an initial low stiffness stage and a final high stiffness stage. The transition between the two stages coincided with intraocular pressures between 12 and 20 mmHg. There was a considerable increase in stiffness associated with both age and load rate. Equations were derived to describe the nonlinear stress-strain relationship of corneal tissue for any age between 50 and 95 years, and these equations are presented in a form suitable for use in numerical simulations. CONCLUSIONS: The cornea demonstrates considerable stiffening with age with the behavior closely fitting an exponential power function typical of collagenous tissue. The increase in stiffness could be related to the additional age-related nonenzymatic cross-linking affecting the stromal collagen fibrils.