K 208Lens thickness and equivalent power of the lens in different refractive states of the eye
M. Bechmann, M. Schaumberger, C. L. Schönfeld, K. Ludwig
Purpose: In order to achieve and maintain an emmetropic refractive state of the eye, several ocular components have to be coordinated, the most important ones of them being the refractive power of the cornea and of the crystalline lens, and the axial length of the globe. Whether these main variables are independent entities or whether they can influence each other, is not fully understood. The aim of the present study is to investigate, if any correlation between the axial length of the globe and the axial thickness of the lens respectively its equivalent power can be found.
Methods: 66 healthy subjects, aged between 20 to 30 years, were enrolled in this study. Ocular dimensions including anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were measured by A-scan ultrasonography. Corneal radius and corneal refractive power were determined by ophthalmometry. All measurements were made under cycloplegia and refractive error was determined as the spherical equivalent of objective refraction. The equivalent power of the lens was calculated by the method of Bennett and Rabbetts based on the Gullstrand schematic eye.
Results: A significant correlation between the refractive state and the axial length could be shown. No significant difference in lens thickness could be seen between the emmetropic, the myopic and the hyperopic group. In addition the equivalent power of the lens was independent of the refractive state of the eye. The ratio between lens thickness and axial length showed however a significant correlation to the refractive state.
Conclusion: In proportion to the axial length of the globe, the relative lens thickness was significantly increased in hyperopic eyes and decreased in myopic eyes. The corresponding absolute parameters, axial length (thickness) and refractive power of the lens, do, however, not show a significant correlation with the axial length of the eye, despite a broad variation in axial length of the globe in the patient collective investigated. In that respect, absolute crystalline lens thickness and crystalline lens power seem to be variables independent from the axial length of the globe. Further studies will have to evaluate if these findings are of importance in determining the refractive state of the eye.
Department of Ophthalmology, Ludwig-Maximilians-University, Munich, Mathildenstr. 8, D-80336 Munich