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Biomedical subjects

Ronald A Schachar

Publications and source records attributed to Ronald A Schachar.

At least 19 recordsLinked to original sources
PubMed2007

The effect of human in vivo accommodation on crystalline lens stability.

AIM: To determine the effect of human in vivo accommodation on the stability of the crystalline lens. METHODS: Using a dual Purkinje image (DPI) eyetracker, the phase-time difference and amplitudes of Purkinje images I (P(I)) and IV (P(1V)) were measured in 37 normal emmetropic subjects (34 women and 3 men; mean age 19.8, range 18-22 years) when they changed focus from 70 to 15 cm and simultaneously rotated their heads horizontally from side to side or made horizontal saccades between two targets 6.8 degrees apart. RESULTS: When the subjects changed focus from 70 to 15 cm and rotated their heads or made eye saccades, the phase-time difference between P(I) and P(IV) decreased. During saccades, the amplitude of both P(I) and P(IV) overshoots significantly increased with focus at 15 cm; however, their ratio (P(IV) overshoot amplitude/P(I) overshoot amplitude) significantly declined. CONCLUSIONS: The lens is stable during accommodation. The implications of these findings on the mechanism of accommodation are discussed.

Accommodation, Ocular↗
PubMed2006

The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes.

AIM: To determine the relationship between accommodative amplitude and central lens thickness/equatorial lens diameter (CLT/ELD) ratio in vertebrates. METHODS: Midsagittal sections of lenses from fixed, post mortem eyes from 125 different vertebrate species were photographed. Their CLT/ELD ratios were correlated with independently published measurements of their accommodative amplitudes. Using the non-linear finite element method (FEM), the efficiency of zonular traction (the absolute change in central radius of curvature per unit force [|DeltaCR|/F]) for model lenses with CLT/ELD ratios from 0.45 to 0.9 was determined. RESULTS: Vertebrates with CLT/ELD ratios < or =0.6 have high accommodative amplitudes. Zonular traction was found to be most efficient for those model lenses having CLT/ELD ratios < or =0.6. CONCLUSIONS: Vertebrates with lenses that have CLT/ELD ratios < or =0.6--i.e. "long oval" shapes--have the greatest accommodative amplitudes; e.g. primates, diving birds and diurnal birds of prey. Vertebrates that have oval or spherical shaped lenses, like owls and most mammals, have low accommodative amplitudes. Zonular traction was found to be most efficient when applied to model lenses with CLT/ELD ratios < or =0.6. The implications of these findings on the mechanism of accommodation are discussed.

Accommodation, Ocular↗
PubMed2006

Viscoelastic shear properties of the fresh porcine lens.

AIM: To determine the viscoelastic properties of the porcine lens METHODS: Linear viscoelastic shear properties of the stroma of four porcine lenses were measured within 5 hours post-mortem, using sinusoidal oscillatory shear deformation. The elastic shear modulus, viscous shear modulus, dynamic viscosity, damping ratio, and phase shift of the lenses were quantified by a controlled-strain, linear simple-shear rheometer at frequencies of 10-50 Hz. RESULTS: The mean viscoelastic properties and their standard deviations across the frequencies examined were: the elastic shear modulus, G' = 6.2+/-4.0 Pa, the viscous shear modulus, G'' = 19.2+/-2.5 Pa, the dynamic viscosity, eta' = 0.16+/-0.1 Pa x sec, the damping ratio zeta = 4.06+/-1.25, and the phase shift, delta = 76 degrees +/- 5.6 degrees. CONCLUSIONS: The measured viscoelastic shear properties of the porcine lens reflect a low dynamic viscosity with a high damping ratio. The porcine lens is viscoelastic and is more viscous than elastic. The magnitude of the complex shear modulus of the porcine lens, |G*|, is similar to the shear modulus of the young human lens. Understanding these viscoelastic properties of the natural lens may provide guidance in developing a lens substitute capable of accommodation in the post cataract patient.

Animals↗
PubMed2006

Analysis of human crystalline lens accommodation.

The behavior of the human crystalline lens during accommodation is analytically studied. The lens is modeled as a closed axisymmetrical membrane shell of varying thickness enclosing an incompressible liquid. To simulate zonular tension associated with lenticular accommodation, an axisymmetrical radial force or displacement is imposed around the shell equator. Two second-order, simultaneous, nonlinear governing differential equations are derived. Numerical results, obtained from the investigation of human lens profiles of three independently published MRI images and a drawing of a microphotograph, demonstrate that when zonular traction within the physiological force range of the ciliary muscle is exerted, both central lens thickness and central optical power increase. Qualitatively, these increases are independent of lens shape. However, the magnitude of these changes is dependent on the initial profile of the lens and is enhanced by the "natural" variation in capsular thickness. Only when a pulling force significantly exceeds the force capacity of the ciliary muscle does the lens flatten and its central thickness and optical power decrease.

Accommodation, Ocular↗