Oxygen consumption in the developing chick cornea

Embryonic chick corneas at different stages of development were evaluated for O2 consumption. Some embryos were treated with thyroxine or thiouracil. In untreated animals, corneal QO2 (oxygen consumption/hr./mg. dry weight) decreased from 3.60 at stage 38 to 1.58 after hatching. The temperature coefficient Q10 increased from 1.55 at stage 40 to 2.03 after hatching. If O2 consumption is calculated as microliters of O2 consumed per hour per corneal pair, it increases between stage 38 (3.20) and hatched chicks (6.20) with a plateau between stages 40 and 45. Thiouracil treatment reduced O2 consumption by the cornea at stages 42 and 45, and thyroxine treatment elevated it at stage 40.     

Oxygen consumption of the superior cornea following eyelid closure

Oxygen depletion curves were recorded from the superior and central cornea with a micropolarographic system immediately following eyelid closure in 10 adult human subjects. Superior corneal oxygen uptake rates for the closed-eye state ranged from 13% below to 58% above, with a mean 15% greater than equivalent central corneal oxygen uptake rates. Chronic superior corneal hypoxia induced by upper eyelid coverage when the eyes are open is therefore accompanied by a deeper hypoxia relative to the central cornea of some individuals when their eyes are closed. This hypoxia adds to the physiological compromise endured by the superior cornea and is especially emphasized when contact lenses are worn.     

The etiology of transient endothelial changes in the human cornea

To investigate the etiology of contact lens-induced transient endothelial changes (blebs) in the human cornea, the effects of five different stimuli on corneal thickness and the appearance of the corneal endothelium were assessed. The stimuli included: (1) a silicone contact lens; (2) a silicone contact lens in combination with anoxia; (3) anoxia alone; (4) a thick hydroxyethyl methacrylate (HEMA) contact lens; and (5) a gas mixture of 9.8% carbon dioxide, 20.5% oxygen, and the balance nitrogen. The silicone lens alone produced no significant alteration in endothelial appearance and little change in corneal thickness. However, when nitrogen gas was passed in front of the lens, a typical bleb response was observed. This indicates that the physical presence of a contact lens is insufficient by itself to produce transient endothelial changes. Anoxia alone induced corneal swelling and endothelial bleb formation, indicating a metabolic component in the bleb response. The gas mixture containing 9.8% carbon dioxide also altered the endothelial appearance but had no significant effect on corneal thickness. The thick HEMA lens produced changes in both the appearance of the endothelium and corneal thickness. The only factor common to the stimuli which induced blebs would appear to be their ability to change the pH in or near the corneal endothelial layer.     

In vivo oxygen uptake into the human cornea

Purpose. We provide a new procedure to quantify in situ corneal oxygen uptake using the micropolarographic Clark electrode. Methods. Traditionally, upon placing a membrane-covered Clark microelectrode onto a human cornea, the resulting polarographic signal is interpreted as the oxygen partial pressure at the anterior corneal surface. However, the Clark electrode operates at a limiting current. Hence, oxygen flux is directly detected rather than partial pressure. We corrected this misunderstanding and devised a new analysis to quantify oxygen uptake into the cornea. The proposed analysis is applied to new polarographic data for 10 human subjects during open-eye oxygen uptake. Results. Average open-eye corneal oxygen uptake over 10 subjects is approximately 11 μL/(cm(2) h), approximately five times larger than the average reported by researchers who invoke the original mathematical analysis. Application of the classical interpretation scheme to our experimental data also garners uptake values that are approximately a factor of three to five times smaller than those obtained with our new procedure. Conclusions. The classical procedure originally developed by Fatt and colleagues misinterprets the behavior of the Clark microelectrode. We corrected the analysis of the in situ polarographic technique to provide a simple yet rigorous procedure for analyzing both previous data in the literature and those newly obtained. Our proposed interpretation scheme thus provides a reliable tool for in vivo assessment of corneal oxygen uptake.     

正常国人全角膜数字模型的建立

目的探讨国人个体角膜数学模型的二次曲线方程模式及初步结果。方法建立角膜顶点为原点的笛卡儿空间三维坐标,绕z轴旋转坐标,建立新的三维空间坐标系,并明确新旧坐标的转换关系;采集ORBSCANⅡ角膜地形图系统测量的角膜顶点、0°、30°、60°、90°、120°、150°、180°、210°、240°、270°、300°、330°子午线上距角膜顶点分别为1．0、1．5、2．0、2．5、3．0、3．5、4．0、4．5mm处点的前、后表面曲率和角膜厚度值d0,代人一套方程组,解出二次曲线公式x^2=a2x^2＋a1x（前表面截痕）、^2=a1（z-d0）^2＋a2（z—d0）（后表面截痕,d0为中央角膜厚度）,确定各切面截痕特性及偏心率Q值;从各子午线的截痕的曲线特征归纳角膜前、后表面曲面空间形态的数学表达式。结果角膜前、后表面各子午线截痕的数学表达式均为椭圆二次曲线轨迹。角膜前、后表面曲面符合椭球二次曲面数学表达式。结论本研究报告了二次曲线公式的人眼角膜前、后表面数学模型新表达式;本小样本正常国人角膜前后表面各子午线截痕均符合椭圆二次曲线的形态特征,角膜前、后表面的曲面空间形态均为椭球面。     

正常个体人眼角膜空间形态数学建模路线研究

目的通过数学方法分析角膜的三维数据,并以数学表达式的方式归纳和反映正常成年个体人眼角膜空间形态特征。方法以Orbscan-Ⅱ角膜地形图系统测量和采集角膜顶点0°、30°、60°、90°、120°、150°、180°、210°、240°、270°、300°、330°子午线上距角膜顶点距离分别为1.0 mm1、.5 mm、2.0 mm、2.5 mm、3.0 mm、3.5 mm、4.0 mm、4.5 mm处的前、后表面曲率,角膜厚度值利用空间三维坐标旋转、曲率公式、曲率半径公式及二次曲线公式等进行数据分析和归纳,推导出角膜前、后表面各子午线截面的曲线数学表达式及前、后表面的曲面的数学表达式。并通过个体角膜实际测得的数据对该数学表达式进行具体推导。且验证角膜前、后表面的曲率分布符合Toric光学面特性。结果正常个体角膜前表面数学表达式:x2/7.76762+y2/7.66182+(z-7.8629)2/7.86292=1;后表面数学表达式:x2/7.52082+y2/6.68952+(z-9.3944)2/8.86442=1。结论正常个体角膜的前、后表面各子午线的截面曲线为椭圆,因此其前、后表面均为椭球面。     

The diffusion of fluorescein in the stroma of rabbit cornea

The diffusion rate and the distribution ratio of fluorescein in the stroma was studied in rabbit cornea. A strip of corneal stroma was mounted in a chamber and fluorescein in a phosphate buffer solution was circulated across the end of the strip for 24hr. The change in fluorescence intensity was measured along the strip and the diffusion coefficient was calculated using Fick's diffusion equation. The mean coefficient of diffusion was 1·21±0·24 (±s.d.) × 10^?6 cm²/sec at 19 °C. The ratio of fluorescence between the stroma and the solution was 1·34 to 1·33 for the concentrations ranging from 0·1 to 10 g/ml.     

Human cornea: Superior and central oxygen demands

Corneal oxygen demands, expressed as ratios of uptake rate relative to baseline rates derived from unstressed corneas, were determined with a micropolarographic system for central (closed eye) and superior (open eye) locations of one cornea of each of seven human subjects. The closed-eye central corneal measurements were repeated during two additional experimental sessions. Intrasubject variability of the three mean closed-eye central corneal rate ratios ranged as high as 23%, possibly representing effects of homeostatic mechanisms on the palpebral conjunctival capillaries of some subjects. For the open-eye superior cornea, which was covered by the upper eyelid of every subject prior to measurement, oxygen demand was found to have a greater intersubject range, but was diminished in magnitude relative to the demand associated with the closed-eye central cornea. Superior corneal oxygen demand was not found to be predictable from closed-eye central corneal oxygen demand or extent of eyelid overlap onto the cornea and thus indicated localized open-eye superior corneal environments that were significantly different from those of the corresponding closed-eye central corneas. Such localized environments may be critically important when gauging the susceptibility of particular eyes to superior corneal pathology during contact lens wear.     

Development and clinical assessment of an artificial cornea

Keratoprosthesis research has been a gradual, rather fragmentary process with advances being made by isolated groups of researchers. This has arisen partly because of poor funding in the area; research groups which have achieved commercial support have often had constraints upon the full disclosure of their findings. Despite these difficulties there has been real progress over the last decade by several independent groups. This article concentrates upon our own development of a hydrogel core-and-skirt keratoprosthesis, the Chirila KPro, in order to illustrate the scientific and clinical problems common to keratoprosthesis research. Pilot data from a clinical trial is presented and the priorities for future research are discussed.     

A model of inhomogeneous expansion of the cornea and stability of its focus

A simple derivation of an analytical condition for expanding the cornea to get the stable position of (he corneal focus is given. The corneal thickness increases from the apex till the limbus and the corneal shell expands inhomogeneously as a result of changes in intraocular pressure conditions. It expands more in the centre of the cornea and less at its apex. As a result the expansion changes the paraxial radius of the corneal curvature by the value of δr as well as displaces the position of the centre of the central corneal curvature by the value of s. Presented paraxial calculations show that the position of the corneal focus is stable and insensitive on the expansion of the cornea if the ratio of both values amounts to about 0.34, Comparison of analytical results with published results of experiments on expanding intact cornea in vitro indicates their high accordance. This suggests a new. important role of the thickness distribution of the cornea.     

Mathematical model of the rotation of cornea

The aim of our study is to informatise the procedure, ie, design the computer system and software necessary to automatically define the parameters of our problem. To achieve this, we have used an image analyser which permits us to acquire and digitalise a photograph of the lesioned cornea. Digitalisation of the image allows us to determine the co-ordinates (x, y) of the points which lie on the lesion periphery as well as the co-ordinates of the corneal centre, ie, geometric centre of the cornea. One calculates the centre of trephination and the optimal diameter of trephination as a function of a point situated on the lesion edge.     

Critical oxygen requirements to avoid oedema of the central and peripheral cornea

A randomized double-masked experiment was performed to compare the critical oxygen concentrations required to avoid corneal thickening at the central and peripheral cornea. Pachometry was performed on 10 subjects before and after 3 h of corneal exposure to the following gasmixtures: 0.00, 1.01, 2.65, 5.13 and 10.3% oxygen, balance nitrogen. Widely ranging critical values (from 4.4 to 11.6% for the central cornea and 1.7 to 15.8% for the peripheral cornea) could be obtained depending upon the analytical procedure used to treat the data; that is, the choice of reference baseline, the curve fitting procedure adopted and the method of comparing the reference baseline with the fitted curve. The estimates of the critical oxygen requirement of the central cornea are consistent with previous studies when compared using the same analytical procedure. Similar critical values were derived for the central and peripheral cornea for any given analytical procedure (e.g. 9.8% for the central cornea, and 12.3% for the peripheral cornea for zero mean change in corneal thickness). Our findings indicate that anatomical and physiological differences between the central and peripheral cornea do not have clinically significant effects on the minimum oxygen requirements at these respective sites. This result has implications with respect to lens design considerations for alleviating physiological stress of the cornea during contact lens wear.