Experimental equipment: Model spherical eyeballs and corneas have been constructed of rubber with stress-strain characteristics similar to those of selera. A model tonometer has simulated a Schiotz tonometer with a 7·5 g weight. Three sizes of ball corresponded with eyes of 22, 24 and 26 mm axial lengths (dimensions commonly found in emmetropia): three corneal curvature scorresponded to radii of 7·17, 7·75 and 8·23 mm (a wider range than would be expected in emmetropia—ametropias have been included). Tonometry: The pressure rise produced by a tonometer is greatest in the smallest ball and least in the largest ball, probably because of differences in volume—or surface area, if nonspherical shapes are considered. Rather surprisingly, indentation by the plunger at any given pressure (corneas the same) is least for the smallest ball and greatest for the largest ball, presumably because the steeply curved (small) ball will provide more support for its cornea than a ball with a flatter curve (i.e. larger). Both these effects, i.e. volume and curvature of wall of eyeball excluding cornea, would tend to be additive in producing erroneous estimates of intraocular pressure by Schiotz tonometry, e.g. a smaller-than-normal eyeball would give an erroneously high estimate. A steep cornea provides more support for the plunger of the tonometer than a flat one so that an erroneously high pressure reading from the former and a low one from the latter would be obtained. Since a steep cornea tends to be associated with a small ball, its error will be additive to that produced by the effect of volume and wall curvature. These effects, especially of total volume, must be important factors in “ocular rigidity”, although others probably exist. Applanation tonometry is hardly affected by them. Tonography: A large variation exists between eyes in normal and glaucoma patients, and the dividing line between these two groups is not very clear. The change in volume of the model seleral envelope, due more to distortion (“ squashing ”) than expansion-contraction , has been shown to be much greater than that attributable to corneal indentation. If standard pressure-indentation-volume curves calibrated on “normal” (model) eyeballs and a Friedenwald nomogram are used, a smaller than average eyeball will appear to have a greater outflow facility than in reality it has, and a larger eyeball will appear to have a smaller, other things being equal; the reason is that a given “volume out” will be associated with a greater fall in pressure and therefore a greater increase in indentation in the former than in the latter. The chances of an erroneous conclusion from tonography because of variations in eye volume would be less if the weight of the footplate were reduced and if its curvature were less flat, i.e. more like that of the cornea, or if changes in pressure were eliminated during indentation (isotonography).
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