A survey of corneal curvature changes from corneal lens wear

The fact that variations in corneal curvature occur as a result os wearing PMMA contact lenses has been reported many times (1-8). The nature of these curvature changes and the factors influencing them are, however, still not well defined. Estimates of the frequency of corneal curvature changes in contact lens wearers vary from 24% (9) TO 100% (5) with a full range of intermediate values. Similar uncertainty is evident in the findings on the extent and direction of these curvature changes. Values for the magnitude of the change range from zero to 2.25D (10) while changes up to 6.00D in corneal aastigmatism have been reported.(11,12)With regard to the direction fo the corneal curvature change, th most commonly expressed view in the early contat lens literature was that contat lens mold the corneal curvature to its own shape. (13-18) A flat lens would flatten the apical curve, a steep lens would steepen the corneal curvature, and in all cases a reduction in corneal astigmatism would result. Reports of surveys of keratometric changes give only partial support to the molding hypothesis (1-9). They show that the curvature changes are more complex than a simple molding of the the corneal curvature, since a lens fitted in any particular way can produce either corneal steepening or flattening, and either an increase or decrease in astigmatism. there are several reasons for this uncertainty in describing the curvature variation. The studies used different types of lenses, particularly in the fitting relationship of the lens to the cornea, and both wearing and measurement times differ from study to study. As well, different criteria for the presence of a change in curvature have been used. Analysis of tthe results of these surveys is further confused by the universal difficulty in adequately describing the curvature changes. The influence of these variables is made harder to ascertain because of ommission of some of the relevant information by almost all studies