PubMed · 6724820
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H E Kaufman. 1984. Keratitis sicca.. https://pubmed.ncbi.nlm.nih.gov/6724820/
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A theoretical model is developed in which the force that restores a soft hydrogel lens to its centered position is ascribed to flexure of the lens. It is shown that flexure makes the dominant contribution to the strain energy in the displaced position. The restoring force is calculated explicitly. The return speed is estimated explicitly, assuming that the dominant resisting force is the viscous drag offered by the postlens tear film. Satisfactory agreement with observations is found.
The assessment of soft contact lens fit by observation of the lens behavior on the eye is one of the most common procedures undertaken in contact lens practice. Nevertheless, the guidelines for doing so have been derived empirically with little or no research into the usefulness of these observations. Over 2000 evaluations of soft contact lens fit were analyzed to determine the predictive value of various indicators of contact lens fit, namely: subjective comfort, lens centration, postblink movement (PBM), version and upgaze lag, and tightness assessed by the push-up test. The receiver-operator characteristic (ROC) curves were prepared for each variable by plotting sensitivity values against false positive rates (FPRs) for various cut-off points. Sensitivity was defined as the proportion of "bad" fits correctly identified by a given variable; FPR was defined as the proportion of "good" fits incorrectly identified as bad fits. With each fitting variable, the optimum cut-off point and its corresponding sensitivity and FPR were determined from the ROC curves. Subjective comfort was found to be of little value in identifying tight fitting lenses but was of some limited value in identifying loose fitting lenses, particularly high water content (HWC) lenses. The push-up test was the most accurate single test of lens fit acceptability having a high sensitivity for both tight (92%) and loose (80%) lens fits. Assessment of PBM was a sensitive indicator of tight fitting lenses (95%), but was a poor indicator of loose fitting lenses, having a low sensitivity (51%). Assessment of version lag showed better sensitivity than upgaze lag in assessing loose fits, whereas upgaze lag showed better sensitivity for tight lens fits. Lens centration was of no predictive value in identifying tight fitting lenses but, as expected, was of some value in identifying loose fitting lenses. The results of this study enable a critical reassessment of the procedures that practitioners follow when fitting soft contact lenses.
PURPOSE: Previous studies have described transient corneal endothelial changes in non-contact lens wearers after a short period of soft contact lens wear by means of contact and noncontact specular microscopy and modified slit lamp biomicroscopy, which provide magnifications from 60 to 100x. In this investigation, we documented and characterized these contact lens-related corneal changes using the white light, real-time confocal microscope, which is capable of cellular resolution imaging of all layers within the cornea at magnifications of 100 to 500x. METHODS: We used a clinical confocal microscope to study corneal changes in three patients wearing a high water content soft contact lens for the first time. RESULTS: In one patient, endothelial changes consisting of irregularly shaped, round or oval, dark regions were observed within the endothelial mosaic. Scattered hyper-reflective keratocyte nuclei were seen in the posterior stroma. The keratocytic and endothelial changes were most evident 20 minutes after placement of the lens. By 30 minutes, the changes were fewer and less prominent, and the brightness of the highly reflective keratocyte nuclei had decreased. CONCLUSIONS: These studies show, for the first time, that the transient changes associated with contact lens wear occur not only in the endothelium, but also in the corneal stroma. It has been suggested that the changes result from an increase in CO2 and lactic acid, which causes a transient reduction in the corneal pH. We hypothesize that the resulting acidic environment may induce gene expression that causes changes in the involved nuclei, which in the keratocytes become hyper-reflective, and in the endothelium become enlarged, resulting in posterior displacement of the cell membrane and producing the dark "blebs" and irregular lines observed at this level of the posterior cornea.