Physiological state of the rabbit cornea following 4 degrees C moist chamber storage

Measurement of corneal tangent modulus using ultrasound indentation

Biomechanical properties are potential information for the diagnosis of corneal pathologies. An ultrasound indentation probe consisting of a load cell and a miniature ultrasound transducer as indenter was developed to detect the force-indentation relationship of the cornea. The key idea was to utilize the ultrasound transducer to compress the cornea and to ultrasonically measure the corneal deformation with the eyeball overall displacement compensated. Twelve corneal silicone phantoms were fabricated with different stiffness for the validation of measurement with reference to an extension test. In addition, fifteen fresh porcine eyes were measured by the developed system in vitro. The tangent moduli of the corneal phantoms calculated using the ultrasound indentation data agreed well with the results from the tensile test of the corresponding phantom strips (R(2)=0.96). The mean tangent moduli of the porcine corneas measured by the proposed method were 0.089±0.026MPa at intraocular pressure (IOP) of 15mmHg and 0.220±0.053MPa at IOP of 30mmHg, respectively. The coefficient of variation (CV) and intraclass correlation coefficient (ICC) of tangent modulus were 14.4% and 0.765 at 15mmHg, and 8.6% and 0.870 at 30mmHg, respectively. The preliminary study showed that ultrasound indentation could be applied to the measurement of corneal tangent modulus with good repeatability and improved measurement accuracy compared to conventional surface displacement-based measurement method. The ultrasound indentation can be a potential tool for the corneal biomechanical properties measurement in vivo.

Dimensions and morphology of the cornea in three strains of mice

PURPOSE

To use a histologic approach to obtain dimensional and morphologic information on the cornea in three commonly used strains of mice.

METHODS

Adult mice (three each of 129/SVJ, C57BL/6, and BALB/c) were euthanatized, and the eyes were enucleated, immersed in 2% glutaraldehyde fixative, and prepared for light and transmission electron microscopy. The full corneal, epithelial, stromal, and posterior limiting lamina (PLL) with endothelium thicknesses were measured at the same location centrally and peripherally.

RESULTS

All three strains showed a statistically significant (P < 0.001) decrease in overall thickness in the peripheral compared with the central cornea. The decrease was due to a reduced thickness of both the epithelium and the stroma. The stroma and epithelium contributed to approximately two thirds and one third of the total corneal thickness, respectively. The epithelium had the classic stratified layout and consisted of 13.00 +/- 1.41 layers centrally versus 10.33 +/- 1.37 peripherally. Some adaptation of stromal tissue was found immediately adjacent to the epithelial basement membrane, but a clearly defined anterior limiting lamina did not exist. The stroma was organized into lamellae but lacked the anterior branching and interweaving reported in humans and had unmyelinated nerve fibers within micrometers of the endothelium. The PLL was 2.17 +/- 0.3 microm thick and was divided into pre- and postnatal layers, with striated bodies in the postnatal portion.

CONCLUSIONS

This study demonstrated that in the three strains of mice examined, the cornea becomes significantly thinner toward the periphery. Dimensionally, proportionally, and anatomically the three strains used appeared to be similar. However, morphologic differences were observed compared with other mammals, and awareness of these differences is important when using the mouse as an animal model applicable to the human.

An assessment of regional differences in corneal thickness in normal human eyes, using the Orbscan II or ultrasound pachymetry

BACKGROUND

In recent years, there has been increasing interest in the characteristics of the peripheral cornea close to the limbus, in both tonometry measures and refractive surgery, but there is relatively little information on these characteristics as provided by modern day pachymetry instruments such as the Orbscan (Bausch & Lomb, Rochester, New York). The current study was therefore undertaken to assess the corneal thickness profile along the horizontal meridian by this scanning slit light method, comparing the data with that obtained with an ultrasound pachymeter.

METHODS

Noncontact specular microscopy was first performed on 17 adults (aged between 20 and 64 years) to check that the corneas were normal. Then, 3 assessments of the corneal thickness profile across the horizontal meridian were taken using the Orbscan II, and both the regional map data (7-mm-diameter measurement ring, 1-mm sample zones) and the point data from the numerical pachymetry output were used to extract data at specific locations nominally 0.5 mm apart. Ultrasound pachymetry (under topical anesthesia with benoxinate 0.4%) was then used to obtain thickness values at central, mid-peripheral (2.75 mm), and peripheral locations close to the limbus (4.5 mm).

RESULTS

Specular microscopy yielded mean thickness of 0.529 +/- 0.032 mm, whereas single-point Orbscan readings at the geometric center of the cornea averaged 0.579 +/- 0.037 mm. Orbscan readings around the 7-mm-diameter measurement zone along the horizontal meridian averaged 0.681 +/- 0.034 mm (i.e., were 0.101 mm or 17.6% greater; P < 0.001). Mid-peripheral readings taken from the numerical maps at 2.5 to 3.0 mm averaged 0.645 mm (or 11% higher than central point readings), whereas peripheral readings between 4.0 and 4.5 mm averaged 0.727 mm (i.e., 26% higher than central point values). In marked contrast, ultrasound readings in the mid-periphery (2.75 mm) averaged just 0.553 mm (or 5.5% greater than the central corneal thickness [CCT]) and just 0.612 mm (i.e., 16.6% higher) in the periphery (4.5 mm). CCT profiles generated from the Orbscan numerical output across the horizontal meridian showed a predictable progressive increase in thickness from the center to the 4.5-mm location on both the temporal and nasal side. The mean differences between the Orbscan II and ultrasound pachymetry measures were thus not constant across the cornea. Without correction, these differences were close to 0.05 mm at the center but more than 0.100 mm at the peripheral sites, and proportional differences persisted after application of the default acoustic factor of 0.92 for the Orbscan readings.

CONCLUSIONS

Orbscan II and ultrasound pachymetry measures generate a rather different profile for corneal thickness. The data from the 2 techniques should be considered as reporting different characteristics, rather than attempts being made to align Orbscan measurements to those of the ultrasound method. A single acoustic correction factor cannot be logically applied to all corneal thickness measures made with an Orbscan II.

On the use of Orbscan II to assess the peripheral corneal thickness in humans: a comparison with ultrasound pachometry measures

PURPOSE

To compare the measures of corneal thickness measurements obtained by an optical scanning slit method with those obtained by an ultrasound (US) pachometer, with special interest in the mid-peripheral (2.5 mm from centre) and peripheral (4.5 mm from centre) region of the cornea.

METHODS

Three measures of corneal thickness were taken using Orbscan II and then by US pachometry (under topical anaesthesia with benoxinate 0.4%) on 24 adults, aged 20-58 years and with up to 8.5 D of myopia. The full Orbscan topography maps were used to extract single point data along the horizontal corneal meridian for the geometric centre, 2.5 mm from centre (nasal and temporal) and 4.5 mm (nasal and temporal) from centre. No correction factor was used for the Orbscan data. The same set of measures were made with the US pachometer. In all cases, the averages of three (centre) or six (mid-periphery and periphery) readings were taken as the measurements from each cornea.

RESULTS

Orbscan readings on the right eyes averaged 0.576, 0.632 and 0.712 mm for central, mid-peripheral and peripheral sites with average values for emmetropic subjects (<1 DS, n = 12) being marginally higher than for myopic subjects (average - 4.00 DS, n = 12). For US pachometry, the average values were however 0.522, 0.554 and 0.606 mm. Similar results were obtained on left eyes. Combining data from both eyes also showed that the mean difference between Orbscan II and US measures was not constant across the cornea, being 0.055 +/- 0.014 mm at the centre, 0.080 +/- 0.019 mm at mid-peripheral locations and 0.107 +/- 0.046 mm at the peripheral sites. These differences persisted after application of the generally recommended acoustic factor (x0.92) to all of the Orbscan readings.

CONCLUSIONS

A single acoustic factor correction cannot be applied to all corneal thickness measures made with an Orbscan II to equate the measures to those made with an US pachometer.

Correcting cell density measurements for tissue hydration changes in scanning electron microscopy--application to the rabbit corneal endothelium

Chemical fixation with glutaraldehyde (followed by osmium tetroxide treatment, methanol dehydration and critical point drying) of biological tissue can result in a reduction in tissue size. As a result, cell density estimates can be much higher than in the original tissue. For rabbit corneal endothelium (cell density of 3300 cells/mm2); preparation of fresh tissue (73% hydrated) results in a net 2-fold increase in apparent endothelial cell density unless the dimensions of the tissue before and after processing for scanning electron microscopy (SEM) are taken into consideration. The increase in endothelial cell density (ECD) is the same regardless of whether the fresh tissue is subjected to primary fixation for 1-14 days or if the fixed (and osmium tetroxide treated) tissue is stored in methanol for up to 80 weeks. However, if the tissue hydration is acutely altered during ex vivo perfusion techniques, the cell density, as assessed by SEM, is found to increase proportionately to the tissue hydration (as measured by corneal thickness); the increase was 600 cells/mm2 for each 100 microns increase in corneal thickness. The latter phenomenon does not appear to be the result of a differential reduction of tissue size associated with either primary fixation or secondary processing. For some tissues, therefore, a change in the tissue hydration level prior to fixation, may produce secondary effects in measures of cell densities.

Identification of topographical variations in the sizes of cells in a monolayer--application to corneal endothelium

The cells in the corneal endothelium, do not have a constant geometric arrangement but rather can be organized into widely different patterns or arrays. Conventional statistical analyses of variance in cell size can be used to report that the endothelial mosaic is homogeneous (homomegethous) or heterogeneous (polymegethous) but do not identify the source of the homogeneity or its loss. We present methods that allow for both identification and graphical presentation of regional homogeneity or heterogeneity in the endothelial cell monolayer. Single electron micrographs (containing 180 cells) were taken at the central region of the rabbit corneal endothelium and cell areas measured by planimetry. Special computer programs allow progressive evaluation of variance in all cell areas across the set of cells. Several examples are presented in detail to illustrate different degrees and types of variance for sets of endothelial cells that have small to extremely large ranges of areas (i.e. coefficient of variation values ranging from 0.12 to over 1.0). There also appears to be a mathematically predictable relationship between cell surface area and the number of cell sides. The methods developed should be applicable to quantitative comparisons of cells in monolayers or tissue sections.

Effect of bicarbonate-free balanced salt solutions on fluid pump and endothelial morphology of rabbit corneas in-vitro

Corneas from young adult albino rabbits were exposed, in-vitro, to various bicarbonate-free balanced salt solutions under an applied hydrostatic pressure of 20 cmH2O at 35 degrees C for up to 4.5 h. The solutions were buffered with a HEPES-MOPS mixture, phosphate salts or acetate-citrate salts (pH 7.1-7.4). All of these solutions support a net fluid pump activity attributed to the corneal endothelium although the net fluid pump rates were less than those measurable with a bicarbonate-buffered, CO2-equilibrated Ringer solution. Evaluation of the endothelia by scanning electron microscopy revealed no evidence of any acute toxic effect of the balanced salt solutions. Morphometric analyses of cell size and shape revealed a normal mosaic for endothelia exposed to commercial balanced salt solution for 5 h. The results further indicate that exogenous bicarbonate is not required for the net fluid pump function of the mammalian corneal endothelium.

Concerning the symmetry of the 'hexagonal' cells of the corneal endothelium

Modeling studies are presented to show how the figure coefficient and hexagon deviation index values decrease as a six-sided endothelial cell is systematically distorted from a completely symmetrical geometric shape (a regular hexagon). By use of scanning electron micrographs of rabbit corneal endothelium and an analysis of published specular micrographs of rabbit corneal endothelium, it is shown that the six-sided cells are not necessarily very symmetrical. Such cells should thus be reported as six-sided cells rather than as 'hexagons' which carries the inherent risk of implying regularity (and thus supposed stability in a tesselated mosaic) of such cells.

Long-term effects of a single dose of ultraviolet-B on albino rabbit cornea--II. Deturgescence and fluid pump assessed in vitro

Both eyes of female albino rabbits (1.9 kg: 9-10 wk old) were exposed to a single dose of UV-B (300 +/- 9 nm; 0.125 J/cm2 total dose) between 13.30 and 15.00 h. At various time periods thereafter (every 12 h for 3 days, 6, 7, 14, 28, 42, 56, 112, 224 and 336 days post-irradiation), animals were sacrificed, samples of aqueous humor taken for analysis and stroma-endothelium preparations obtained from the corneas. Following such threshold irradiation, small increases in aqueous humor tonicity and protein levels were observed. The preparations were mounted in a specular microscope assembly (for measuring the rate and magnitude of corneal deturgescence) or between two half chambers (for measuring fluid pump) and equilibrated for 2 h with a CO2-equilibrated glucose-adenosine-glutathione-supplemented Ringer solution at 37 degrees C and a hydrostatic pressure of 20 cm H2O. After equilibration the stromal thickness showed large variation with large reductions in both the rate and amplitude of deturgescence function observed by 36 h. Large reductions in fluid pump activity were also observed by 36 h. The magnitude of the effects on fluid pump were somewhat greater than the effects on deturgescence. Both functions recovered to pre-irradiation levels by 112 days post-irradiation.