The scleral rigidity of eyes with different refractions.

BACKGROUND

The weakened biomechanical properties of the sclera is an important feature of myopic eyes. The quantitative evaluation in vivo of posterior scleral resistance to the elongation remains a challenge.

METHODS

This study comprised 172 eyes from 86 subjects with a mean age of 20.6 years (range, 18-28 years). Ultrasound biometry was performed using an immersion technique and the A-scan device (the Biometer AL-1000 -TOMEY). The axial length of the eye was measured twice: before and during the application of an external pressure of 30 g on the eye. The difference between two mean values of AL measurements before and during the pressure application was considered as a degree of change in the axial length that resulted from the IOP elevation. The data were entered into an Excel spreadsheet (Microsoft Corp.) for subsequent analysis. Statistical analysis was performed using SigmaPlot software (version 11.0, Systat Software, Inc.). A value of 0.05 or less was considered statistically significant.

RESULTS

The means ± SD of axial changes before and during the external pressure for hyperopia, emmetropia, myopia 0.5-3.0 D, myopia 3.25-6.0 D, myopia 6.25-12.0 D and myopia over 12.0 D were as follows: 0.03 ± 0.01 mm, 0.05 ± 0.01 mm, 0.18 ± 0.07 mm, 0.31 ± 0.02 mm, 0.38 ± 0.07 mm, and 0.51 ± 0.9 mm, respectively. The difference among groups was statistically significant.

CONCLUSIONS

In conclusion, our study indicates that the biomechanical properties of the scleral coat, in terms of stretching and AL elongation, are measurable. The hypermetropic and emmetropic eyes possessed stiff sclera. The extent of AL remained practically unchanged during IOP elevation in these eyes. The absolute majority of the myopic eyes revealed a biomechanical weakness of the scleral shell. A higher degree of myopia was associated with increased AL elongation. Our approach to measuring the biomechanical properties of the sclera may have clinical significance in the future.