Measurement of astigmatism arising from the internal ocular surfaces.

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.