The swelling pressure of the human corneal stroma as determined by a new method

The swelling pressure of 115 human corneas was determined using a modified electronic balance modified to simultaneously record the swelling force and the thickness of the stroma. The swelling force was found to follow a straight-line dependence on the stromal thickness when plotted in a double logarithmic scale, which means that the swelling pressure of each cornea could be expressed by a power fit of the form SP = aTb, where SP is swelling pressure, T stromal thickness, and a and b are constants of the cornea. In 45 control corneas swollen in 0.9% NaCl, pH 7.4, 0.01 M Hepes buffer, the mean value (+/- S.D.) of 'a' and 'b' were 7.09 mmHg mm-1 (+/- 2.96) and -3.48 (+/- 0.20), respectively. This corresponded to a mean swelling pressure of 84.0 mmHg at a standard stromal thickness of 0.5 mm. In paired experiments, the swelling pressure was found to be influenced insignificantly (P greater than 0.05) by a number of conditions, including lowering the pH to 4.0, increasing the temperature to 37 degrees C, and increasing the NaCl concentration to 9%. A significant correlation was found between the swelling pressure and the dry weight of the specimen (P less than 0.05), indicating a considerable biological variation of the swelling pressure. It is shown, that this variation may explain the normal variation in human corneal thickness in vivo.