The mechanics of accommodation in relation to presbyopia.

The cause of presbyopia is closely related to the force of contraction of the ciliary muscle and the resistance to deformation of the crystalline lens. Two views are currently in conflict. The view of Donders (1864) that presbyopia is caused by a decrease in the force of contraction of the ciliary muscle with age, and the opposing view of Helmholtz (1855) that the lens becomes more difficult to deform with age due to lenticular sclerosis. The present paper shows that, in fact, the ciliary muscle undergoes a compensatory hypertrophy as accommodative amplitude decreases with age. The force of contraction is about 50% greater at the onset of presbyopia than in youth. However, because of increased lenticular resistance its effect on the amplitude of accommodation is small. It is shown that the reason the lens becomes more difficult to deform is not because of lenticular sclerosis, since the lens substance does not lose water. The increased difficulty of deformation is because the capsule loses its elastic force with age and the lens fibres, particularly in the nucleus, become more compacted.