Does transient increase in axial length during accommodation attenuate with age?

Background: The aim was to profile transient accommodative axial length (AXL) changes from early adulthood to advanced presbyopia and to determine whether any differences exist between the responses of myopic and emmetropic individuals. Methods: Ocular biometry was measured by the LenStar biometer (Haag-Streit, Switzerland) in response to 0.00, 3.00 and 4.50 D accommodative stimuli in 35 emmetropes and 37 myopes, aged 18 to 60 years. All results were corrected to reduce errors arising from the increase in crystalline lens thickness with accommodation. Accommodative responses were measured sequentially by the WAM 5500 Auto Ref/Keratometer (Grand Seiko, Japan). Results: AXL increased significantly with accommodation (p<0.001), with a mean corrected AXL elongation of 2 ± 18 µm and 8 ± 16 µm observed at 3.00 D and 4.50 D, respectively. The magnitude of accommodative AXL change was not dependent on refractive error classification (p=0.959), however a significant reduction in the magnitude and variance of AXL change was evident after 43-44 years of age (p<0.002). Conclusion: The negative association between transient AXL elongation and age, in combination with reduced variance of data after age 43-44 years, is consistent with a significant increase in posterior ocular rigidity, which may be influential in the development of presbyopia

The importance of parameter choice in modelling dynamics of the eye lens

The lens provides refractive power to the eye and is capable of altering ocular focus in response to visual demand. This capacity diminishes with age. Current biomedical technologies, which seek to design an implant lens capable of replicating the function of the biological lens, are unable as yet to provide such an implant with the requisite optical quality or ability to change the focussing power of the eye. This is because the mechanism of altering focus, termed accommodation, is not fully understood and seemingly conflicting theories require experimental support which is difficult to obtain from the living eye. This investigation presents finite element models of the eye lens based on data from human lenses aged 16 and 35 years that consider the influence of various modelling parameters, including material properties, a wide range of angles of force application and capsular thickness. Results from axisymmetric models show that the anterior and posterior zonules may have a greater impact on shape change than the equatorial zonule and that choice of capsular thickness values can influence the results from modelled simulations

A review of the surgical options for the correction of presbyopia

Presbyopia is an age-related eye condition where one of the signs is the reduction in the amplitude of accommodation, resulting in the loss of ability to change the eye's focus from far to near. It is the most common age-related ailments affecting everyone around their mid-40s. Methods for the correction of presbyopia include contact lens and spectacle options but the surgical correction of presbyopia still remains a significant challenge for refractive surgeons. Surgical strategies for dealing with presbyopia may be extraocular (corneal or scleral) or intraocular (removal and replacement of the crystalline lens or some type of treatment on the crystalline lens itself). There are however a number of limitations and considerations that have limited the widespread acceptance of surgical correction of presbyopia. Each surgical strategy presents its own unique set of advantages and disadvantages. For example, lens removal and replacement with an intraocular lens may not be preferable in a young patient with presbyopia without a refractive error. Similarly treatment on the crystalline lens may not be a suitable choice for a patient with early signs of cataract. This article is a review of the options available and those that are in development stages and are likely to be available in the near future for the surgical correction of presbyopia

Ageing vision and falls: a review

Background: Falls are the leading cause of accidental injury and death among older adults. One of three adults over the age of 65 years falls annually. As the size of elderly population increases, falls become a major concern for public health and there is a pressing need to understand the causes of falls thoroughly. Main body of the abstract: While it is well documented that visual functions such as visual acuity, contrast sensitivity, and stereo acuity are correlated with fall risks, little attention has been paid to the relationship between falls and the ability of the visual system to perceive motion in the environment. The omission of visual motion perception in the literature is a critical gap because it is an essential function in maintaining balance. In the present article, we first review existing studies regarding visual risk factors for falls and the effect of ageing vision on falls. We then present a group of phenomena such as vection and sensory reweighting that provide information on how visual motion signals are used to maintain balance. Conclusion: We suggest that the current list of visual risk factors for falls should be elaborated by taking into account the relationship between visual motion perception and balance control

Sport policy convergence: a framework for analysis

This is an Accepted Manuscript of an article published by Taylor & Francis Group in European Sport Management Quarterly on 30th April 2012, available online at: http://www.tandfonline.com/10.1080/16184742.2012.669390The growth in the comparative analysis of sport management processes and policy has led to an increased interest in the concept of convergence. However, the concept is too often treated as unproblematic in definition, measurement and operationalisation. It is argued in this paper that a more effective framework for examining claims of convergence is one that analyses the concept in terms of seven dimensions which can be explored through a mix of quantitative and qualitative methods of data collection. It is also argued that a deeper understanding of the process of convergence can be gained by operationalising the concept in the context of a selected range of meso-level theories of the policy process or of specific aspects of the process. The proposed analytic framework provides not only a definition of convergence but also the basis for a more nuanced investigation of hypotheses of convergence

Spatial modulation of water ordering in lecithin bilayers. Evidence for a ripple-ripple phase transition.

Intense motional averaging effects on the 2H nuclear magnetic resonance (NMR) spectrum of 2H2O that occur in aqueous dispersions of dimyristoyl-sn-glycero-3-phosphocholine (Myr2-PtdCho) are explained by a spatial modulation in the orientational order of the water induced by ripplelike structures. The ratio of the amplitude to the periodic length of the ripples, A/lambda, at a molar ratio of water/Myr2-PtdCho of 9.5:1, is measured by 2H NMR and found to be consistent with x-ray measurements of this ratio in the P beta phase of dipalmitoyl-sn-glycero-3-phosphocholine (Pam2-PtdCho) bilayers. The sensitivity of 2H NMR allows us to report the presence of two distinct ripple phases mediated with a discontinuous change in the value of A/lambda. This result suggests that the two ripple structures observed for several phospholipid systems in excess water by freeze-fracture electron microscopy may be associated with two different phases instead of the same phase as previously assumed

Mathematical Models for Describing the Shape of the Invitro Unstretched Human Crystalline Lens

We developed orthogonal least-squares techniques for fitting crystalline lens shapes, and used the bootstrap method to determine uncertainties associated with the estimated vertex radii of curvature and asphericities of five different models. Three existing models were investigated including one that uses two separate conics for the anterior and posterior surfaces, and two whole lens models based on a modulated hyperbolic cosine function and on a generalized conic function. Two new models were proposed including one that uses two interdependent conics and a polynomial based whole lens model. The models were used to describe the in vitro shape for a data set of twenty human lenses with ages 7–82 years. The two-conic-surface model (7 mm zone diameter) and the interdependent surfaces model had significantly lower merit functions than the other three models for the data set, indicating that most likely they can describe human lens shape over a wide age range better than the other models (although with the two-conic-surfaces model being unable to describe the lens equatorial region). Considerable differences were found between some models regarding estimates of radii of curvature and surface asphericities. The hyperbolic cosine model and the new polynomial based whole lens model had the best precision in determining the radii of curvature and surface asphericities across the five considered models. Most models found significant increase in anterior, but not posterior, radius of curvature with age. Most models found a wide scatter of asphericities, but with the asphericities usually being positive and not significantly related to age. As the interdependent surfaces model had lower merit function than three whole lens models, there is further scope to develop an accurate model of the complete shape of human lenses of all ages. The results highlight the continued difficulty in selecting an appropriate model for the crystalline lens shape