Mechanical testing of isolated senile human eye lens nuclei.

Phacoemulsification is a cataract surgery technique during which the eye lens nucleus is carefully dissected by an oscillating hollow needle simultaneously serving as a suction line for lens fragments. As the success of this procedure depends greatly on the mechanical properties of the nucleus, the basic mechanical properties of extracted senile human eye lens nuclei were studied. For this purpose a novel measuring system had to be developed enabling the study of the biomaterial's responses to any chosen excitation function. Either force- or displacement-controlled testing signals can be selected serving as a one-dimensional input. The resultant quantity can be recorded in the time or frequency domain. An almost linear viscoelastic behavior of lens nuclei is observed. Typical phenomena such as hysteresis, creep, relaxation, and a frequency-dependent compliance are evaluated. The ultimate strength of the specimen is studied by penetration tests. Finally the possible correlation between mechanical and optical parameters of lens nuclei is discussed. If correlated, a surgeon could use this information when deciding whether an extracapsular extraction or a phacoemulsification should be the preferred approach.