On the correlation of mechanical and optical properties of cataractous eye lens nuclei

It is investigated whether mechanical properties of human cataractous eye lens nuclei can be assessed by their optical ones. This question is of importance as the success of phacoemulsification surgery among other criteria depends on the mechanical properties of the lens nucleus. The mechanical and optical properties of senile cataractous human eye lens nuclei were tested in vitro after extracapsular cataract extraction. A new measurement system was developed to determine their mechanical response. Hereby, a force is applied to the specimen and the resultant displacement is measured or vice versa. Stiffness and ultimate strength of the material are determined. Optical transmission is detected in dependence of light wavelength by a spectrophotometer. In addition the nuclear colour is determined. The correlation of mechanical stiffness and strength via optical transmission at 550 nm and nuclear colour is analyzed. Human eye lens nuclei show a viscoelastic mechanical behaviour. Measurement of stiffness is reproducible after 5-6 loading cycles when a steady-state is reached. Mechanical and optical properties correlate weakly but significantly. Stiffness and strength increase with increasing nuclear coloration. Alterations of mechanical and optical properties are caused by the same structural changes of the molecular components of the ageing lens. Especially lens crystallines are responsible for this process. Thus, it is concluded that optical properties can be useful to assess the mechanical response of cataractous eye lens nuclei.