A method is described for measuring internal ocular surface (posterior cornea, anterior and posterior crystalline lens) astigmatism. This involves the use of videokeratography, A-scan ultrasonography, and autorefractometry along with multi-meridional phakometric measurements of Purkinje images I(anterior corneal surface) II(posterior corneal surface) and IV(posterior lens surface). Data was collected from both eyes of 66 subjects. Right and left eyes exhibited similar mean levels of astigmatism from the posterior corneal surface (R + 0.21 DC axis 82 degrees; L + 0.22 DC axis 80 degrees), anterior lens surface (R + 0.52 DC axis 8 degrees; L + 0.49 DC axis 165 degrees) and posterior lens surface (R + 1.48 DC axis 99 degrees; L + 1.16 DC axis 90 degrees). It was generally found that astigmatism arising from the anterior corneal and lens surfaces in conjunction with intraocular distance effectivity are almost completely compensated for by the posterior corneal and lens surface. Repeatability was assessed on 20 subjects. Although the methods is prone to accumulated experimental errors, these are random in nature so that the difference between repeat group averaged data never exceeded +/- 0.27 DC cylindrical component and +/- 6 degrees cylinder axis.

The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R-1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images I (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 +/- 7.0 degrees (Hartinger) and 9.4 +/- 9.8 degrees (Canon). Both values were found to be significantly higher than angle alpha 5.0 +/- 1.2 degrees. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.

The aim of this study was to determine the axis of orientation of residual astigmatism in a sample of human eyes applying the principle of astigmatic decomposition. Calculations were carried out on keratoscopic and refractive data collected from the right and left eyes of 70 subjects (37 male and 33 female students) of mixed race (including 25 Asians and 43 Caucasians). No statistically significant difference was found for mean levels of residual astigmatism measured in the right (0.46 DC x 98.2 degrees) and left (0.50 DC x 99.4 degrees) eyes. Residual astigmatism was predominantly against-the-rule (83% of right eyes and 66% of left eyes) and was within +/- 20 degrees of being perpendicularly disposed relative to the corneal astigmatic power axis in two thirds of the eyes measured. No statistically significant differences were found for either gender or race.

Measurements of the sphero-cylindrical components of the rear corneal surface were taken from 80 healthy right eyes in order to determine normal variations. Dimensions of this surface were strongly influenced by the front corneal surface with the exception that the rear surface exhibited more toricity. Both surfaces tended to be flatter in males compared to females and in myopes compared to hyperopes. The corneal surfaces were also found to be flatter in younger eyes compared to older eyes, but this finding was most likely due to the preponderance of myopes in the young and hyperopes in the older group. The influence of the ratio of anterior: posterior corneal surface radius upon the estimation of total corneal power as required for intraocular lens implant calculations was also considered.

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