The elastic constants of the human lens

Abstract

SUMMARY1. When the lens is spun around its antero-posterior polar axis in an apparatus designed for the purpose, high speed photography can be used to record its changing profile. By this method a variable radial centrifugal force can be applied to the lens which mimics the pull of the zonule.2. If the lens is not stressed at its centre beyond 100 Nm-2 it behaves as a truly elastic body. When stressed beyond this limit visco-elastic strain is produced at its poles.3. The human lens has isotropic elastic properties at the extremes of life, but at the other times Young's Modulus of Elasticity varies with the direction in which it is measured.4. Young's Modulus of Elasticity of the lens varies with age, polar elasticity and equatorial elasticity, at birth being 0 75 x 103 and 0-85 x 103 Nm2 respectively, while at 63 years of age both are equal to 3 x 103 Nm2. 5. A comparison of Young's Modulus of the young human lens with that of the rabbit and cat shows that the polar elasticity of the lenses of these animals was 5 times greater in the young rabbit, and 21 times greater in the adult cat. Equatorial elasticities of the rabbit and human lens were equal, while in the cat the equatorial elasticity was four times greater.6. A mathematical model showing the lens substance possessing a nucleus of lower isotropic elasticity than that of the isotropic elastic cortex surrounding it, accounts for the difference between polar and equatorial elasticity of the intact adult lens.7. The implications of these findings are discussed in relation to: (i) accommodation and the theological properties of the lens; (ii) possible differences in the physical state of the lenticular proteins in the cortex and nucleus which may account for the senile variations in Young's Modulus of Elasticity in these regions of the lens;(iii) the loss of accommodation due solely to an increase in Young's Modulus of Elasticity of the lens between the ages of 15 and 60. This would amount to 44 % of the total observed in vivo.

Stiffness gradient in the crystalline lens

“…The equatorial stiffness of the oldest lens was 10 2 times higher than the youngest lens. For the older lenses, the centre was 5.8-210 times stiffer than the periphery, as opposed to earlier results described by Fisher (1971), who found that the periphery was up to 3 times softer than the centre for lenses younger than 70-years-old. For the three youngest lenses (19 to 49 years), the periphery was 2.2-16.6 times stiffer than the centre.…”

“…Although the fibre cell structure in the crystalline lens suggests anisotropy of the mechanical properties, rabbit lens experiments by Itoi [21] and human lens experiments by Fisher [15] indicate that the anisotropy is relatively small compared to the large differences of shear modulus with position and with age found in the current study.…”

“…Comparisons with existing stiffness data Fisher [15] measured the quasi-static Young's modulus of the crystalline lens using a lens-spinning technique. He then calculated separate Young's moduli for nucleus and cortex stiffness to explain the macroscopic anisotropy.…”

“…Numerous studies have been conducted to measure crystalline lens stiffness [2,4,10,12,15,16,[19][20][21][22]28]; however, only a few of these studies consider differences between the nucleus and cortex. Several studies have demonstrated that during accommodation, the axial deformation in the lens takes place predominantly in the nucleus [7,11,23,29].…”

“…Fisher [15] determined the stiffness of human lenses using lens-spinning experiments. Nuclear and cortical stiffness were calculated, based on the geometric changes of the lens during spinning.…”