The significance of the shape of the lens and capsular energy changes in accommodation

Abstract

SUMMARY1. A method for the estimation of the energy released by the anterior part of the lens capsule during accommodation is described. This includes (i) A determination of the pressure required to distend the capsule by a standard volume.(ii) The calculation from the photographed lens profiles of the degree of capsular contraction which occurs when the lens changes from the unaccommodated to the accommodated form.(iii) Capsular volume changes in vitro are then related to the surface area changes calculated for the lens in vivo.2. A correlation exists between the stored capsular energy per unit area or surface tension and the accommodation power of different species. The human lens capsule releases 1170 ergs/cm2 while the more spherical lenses of the cat and rabbit release 520 and 485 ergs/cm2 respectively for a 10 % change in lens diameter. The amount of energy which can be stored depends on the degree of flatness of the lens and the volume of the anterior segment. The flatter the lens and the smaller the volume of the anterior segment, the greater the capsular surface tension.3. The anterior surface of the human lens remains ellipsoidal throughout life. The changes of accommodation which occur in presbyopia may therefore be related to the lens profiles at various ages. It is found that a coefficient obtained by dividing the anterior volume of the lens by the 5th power of the equatorial radius of the lens modifies the degree of accommodation for a given change of lens diameter.4. The loss of accommodation is proportional to the effective capsular surface energy until about the age of 45. The effective capsular surface energy can be defined as the energy which gives the same change in lens dioptric power per erg regardless of the lenticular profile changes which occur with age. It is obtained by multiplying capsular surface tension at a given age by a ratio. This is obtained by dividing the profile coefficient mentioned in paragraph 3 of the given lens, by the profile coefficient of the reference lens aged 15 (0.068). The effective surface energy of the entire lens falls from 110 ergs at the age of 15 to 50 ergs at 60. Assuming that ciliary power remains unaltered 55 % of the loss of accommodation is accounted for solely by the fall in Young's Modulus of elasticity of the capsule and the changing shape of the lens with age.

Evaluation of the change in accommodation amplitude in subjects with pseudoexfoliation

“…It is known that with increasing age, age-related changes are seen in ciliary apparatus, zonular fibres, and lens capsule, lens elasticity decreases, lens thickness and stiffness increases, ability to disaccommodate decreases and ciliary muscle remodelling decreases independent of PEX. 3,[16][17][18][19][20] In our opinion, the reason why PEX did not affect accommodation amplitude in the second age group is that as age-related changes have already been happened in ciliary apparatus, zonular fibres and lens capsule in this age group, and the measured accommodation amplitude in this group is not active accommodation, and tonic accommodation is independent from lens elasticity, zonular fibres, and ciliary muscle. So, accommodation amplitude declined significantly only in group 1 in which active accommodation is still present with a small amount.…”

Presbyopia and the changes with age in the human crystalline lens

“…Calculation of the change in dioptric power of the lens from the lens number and lens shape It has been shown that changes in the dioptric power of the lens, provided it changes from one ellipsoidal profile to another during accommodation, could be related to changes in the shape of the lens (Fisher, 1969b). From the previous paragraphs the change in shape of the lens is proportional to the lens number while the change in dioptric power is calculated from Appendix eqn.…”

“…Calculation of relative capsular strain when differences of pressure within the lens are equalized In a previous paper Fisher (1969b) the concept of an equivalent spherical segment, representing the anterior portion of the lens, was employed to estimate the energy stored in the lens capsule. The change in radius (8Rm) of this theoretical sphere can in like manner be used to give a proportionate estimate of the capsular strain necessary to change pressure within the capsule.…”

“…If the capsular stretching and hence the movement of the capsulo-zonular attachments be assumed to remain constant with age, the relative movement of the lens substance can be estimated at every age. Fisher (1969b) showed that the accommodative power of the lens could be determined from its polar or equatorial movement, so the decrease in accommodative power with age could be deduced from the decrease in polar movement of the ageing lens. Thus an estimate of presbyopia from lenticular causes entails measurement of elasticity of the capsule and lens substance.…”

Inverse elastographic method for analyzing the ocular lens compression test

“…2 Presbyopia is the progressive loss of accommodation amplitude with age and appears to arise due to an alteration in the biomechanical coupling of the lens and its capsule. 3,4 The clinical presentation of presbyopia coincides with the massive sti®ening of the lens¯ber cells, but it is unclear whether this is purely correlation, cause, or e®ect. 5 While it remains unclear whether this relationship is causal or correlative, accurate measurement of lens' mechanical properties may give important insights into potential mechanisms of lens sti®ening and possibly presbyopia.…”