Massive increase in the stiffness of the human lens nucleus with age: the basis for presbyopia?

PURPOSE: To determine the stiffness of different regions of human lenses as a function of age, and to correlate the biophysical measurements in the lens center with nuclear water content.

METHODS

A custom made probe fitted to a dynamic mechanical analyzer was employed to measure stiffness values at 1 mm increments across equatorial sections of individual human lenses. Thermogravimetric analysis was used to determine the percentage water content in the nuclei of human lenses.

RESULTS

There was a pronounced increase in lens stiffness over the age range from 14 to 78. In the nucleus, stiffness values varied almost 1,000 fold over this age range, with the largest change observed in lenses between the ages of 20 to 60. Nuclear stiffness values increased on average by a factor of 450. By contrast, in the cortex the average increase in stiffness was approximately 20 fold over this same time period. In lenses younger than age 30, the nucleus was found to be softer than the cortex. This was true for all six lenses examined. In contrast all lenses older than 30 were characterized by having nuclear values higher than those of the cortex. In lenses over the age of 50, the lens nucleus was typically an order of magnitude more rigid than that of the cortex. The crossover age, when the cortical and nuclear stiffness values were similar, was in the 30s. There was no significant change in the water content of the human lens nucleus from age 13 to age 82.

CONCLUSIONS

There is a marked increase in the stiffness of the human lens with age. This is most pronounced in the nucleus. Since in vivo data indicate that the nucleus must change shape significantly during accommodation, it is highly likely that these measured changes in physical properties will markedly diminish the ability of the lens to accommodate, and thus may be a major contributing factor to presbyopia. Since there was no measurable difference in the water contents of the nuclear regions of the lenses, this marked increase in stiffness is not due to compaction of the lens nucleus.