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H. D. Patel, B. Gilmartin, R. P. Cubbidge, N. S. Logan; In vivo Measurement of Regional Variations in Scleral Rigidity in Humans. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3947.
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© ARVO (1962-2015); The Authors (2016-present)
The principal method of measuring ocular rigidity (Ko) in vivo has been via corneal indentation using the Schiotz tonometer. We estimate via scleral indentation, regional variations in scleral rigidity (Ks) using raw data from Schiotz scale readings and their transformation using algorithms employed to calculate Ko. Given the relevance of scleral biomechanics to the aetiology of myopia, variation in Ks is compared for subjects with and without myopia.
Data were collected from both eyes of 26 normal young adult subjects [MSE (D) 14 without myopia (≥-0.50) +0.48±1.22, range (-0.50:+4.38), 12 with myopia (<-0.50) -4.44±3.35, range (-0.75:-10.56)]. IOPs were determined using Schiotz and Goldmann tonometry. Schiotz tonometry (5.5g & 7.5g loads) was randomly assigned to four scleral quadrants (approx 8mm from limbus); supero temporal (ST) and nasal (SN), infero temporal (IT) and nasal (IN) and the scale readings used to calculate Ks using Friedenwald’s tables. Biometric data (refraction, corneal radius, anterior chamber depth and axial length) were compiled using autorefraction and the Zeiss IOLMaster.
Data for RE only are presented. Values of Ko were consistent with those previously reported (mean 0.0101±0.0082, range 0.0019-0.0304) and within- and between- observer repeatability (n=2) was robust with typical coefficients of variation of 9% and intra-class correlation coefficients of 0.682. Goldmann and Schiotz measures of IOP were significantly correlated. For Ks differences in refractive error, quadrants and post-hoc contrasts were non-significant. As calculation of Ko uses empirical corneal data regional variations were examined for raw scale data. Mean refractive group differences were insignificant. Mean scale readings were 5.5g: cornea 5.93±1.14, ST 8.05 ±1.58, IT 7.03±1.86, SN 6.25 ±1.10, IN 6.02 ±1.49; 7.5g: cornea 9.26±1.27, ST 11.56±1.65, IT 10.31±1.74, SN 9.91±1.20, IN 9.50±1.56. There were significant differences (p<0.0001) for both weights between means for the cornea and ST; ST and SN; ST and IT, ST and IN. These differences were concordant with the LE data.
There is scope for the use of Schiotz tonometry in estimating relative regional variations in Ks but no evidence for characteristic variations occurring in the myopic eye.
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