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In Vitro Lens Stretching Experiments Apply Non-Physiological Forces to the Lens
Author Response: In Vitro Lens Stretching Experiments Apply Non-Physiological Forces to the Lens
Opponent Theories of Accommodation Reconciled?
Author Response: Opponent Theories of Accommodation Reconciled?
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Susan A. Strenk
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sstrenk{at}wowway.com Susan A. Strenk
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Dr. Schachar1 describes his lens stretching experiment as qualitative. Specifically, he states that he has not quantified the applied tension and that only qualitative topography data was obtainable. A fatal flaw exists, not only in Dr. Schachar's qualitative lens stretching experiment, but in all in vitro lens stretching experiments2-5: in vivo MRI images clearly show the ciliary muscle is anterior to the crystalline lens equator in the human eye (see for example the IOVS cover May 1999). Any stretching experiments that disregard this relative geometry by placing the ciliary muscle at the lens equator apply an anteroposterior force on the lens that does not exist in vivo, resulting in an artificial distortion of the surface of the lens during resting accommodation (see Glasser and Campbell3 Fig. 2). Furthermore, Strenk and colleagues6 show that the average accommodative change in ciliary muscle diameter is 0.661 ± 0.062 mm; any stretching experiments using larger changes are applying non-physiologic forces on the lens. Of course these stretching experiments also disregard the effects of intraocular pressure, the vitreous and the iris. Thus any findings and conclusions derived from such experiments are necessarily of limited value. Susan A. Strenk1 1Department of Surgery, University of Medicine and Dentistry of New
Jersey–Robert Wood Johnson Medical School, Piscataway, New Jersey References 1. Schachar RA. Qualitative effect of zonular tension on freshly extracted intact human crystalline lenses: implications for the mechanism of accommodation. Invest Ophthalmol Vis Sci. 2004;45:2691-2695. |
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Ronald A. Schachar
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ron{at}2ras.com Ronald A. Schachar
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Zonular Traction Applied to the Human Crystalline Lens Results in Central Surface Steepening and Peripheral Flattening We acknowledge the letter of Strenk et al., which addresses some of the caveats associated with in vitro research on the lens. These issues were carefully considered before undertaking and in evaluating our study. In this qualitative, in vitro, research study, we demonstrated highly reproducible responses in the lens curvature to uniformly applied zonular traction.1 Whether our in vitro observations reflect the actual in vivo lens surface changes during accommodation is subject to correlation with the results of human in vivo studies. Both Tscherning2 and Fincham3 have independently demonstrated that the central anterior surface of the lens steepens while its peripheral surface flattens during in vivo human accommodation, just as we demonstrated in vitro. The position of the ciliary muscle has been clearly demonstrated, extending posteriorly from the trabecular meshwork to beyond the equatorial plane of the human lens. In vitro4,5,6 and in vivo techniques,7,8 with a resolution of more than 10 times that of MRI images, have all shown that the ciliary muscle extends posteriorly beyond the lens equator. Ultra-fast optical coherence tomography has demonstrated that the ciliary muscle in vivo lies below the insertion of the rectus muscle,9 which is posterior to the lens equator4 (see Fig. 8 in Radhakrishnan et al.9). Ultrasound biomicroscopy of human accommodation in vivo has characterized that zonular traction in response to ciliary muscle contraction is equatorially directed.7,8 The anatomic attachments of the anterior, equatorial, and posterior zonules from the ciliary body to the lens capsule provide for direct parallel transduction of the force generated by ciliary muscle contraction to the equatorial meridian of the lens.5,6 Many investigators have demonstrated in vivo the independence of the amplitude of human accommodation from those tissues adjacent to the lens. Neither a total iridectomy,10 mydrasis,11 nor vitrectomy12 has any effect. Moreover, the negligible compressibility of the lens13,14 results in the absence of any effect of intraocular pressure15 or vitreous pressure16,17 on lenticular accommodation.18,19 Experimentally, a change in intraocular pressure of up to 6 mm Hg did not alter human accommodative amplitude.15 In summary, we believe that the qualitative surface curvature changes induced by the in vitro application of equatorial zonular traction to fresh human lenses reflect the in vivo lenticular surface changes that occur during human accommodation. Ronald A. Schachar References 1. Schachar RA. Qualitative effect of zonular tension on freshly extracted intact human crystalline lenses: implications for the mechanism of accommodation. Invest Ophthalmol Vis Sci. 2004;45:2691-2695. |
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Barbara K. Pierscionek, Professor of Optometry and Vision Science University of Ulster
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b.pierscionek{at}ulster.ac.uk Barbara K. Pierscionek
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Helmholtz's theory has retained its status as the most widely accepted explanation for accommodation. It has stood the test of time and withstood dissension.1,2 However, a theory only lasts until it is disproved, and durability is not always indicative of accuracy. The latest opposition to Helmholtz, the theory of Schachar deserves consideration, and Schachar's latest paper shows clear evidence in support of his theory.3 Although it is unfortunate that the results are presented as qualitative, from the images shown measurements can be made. The diameters of successive rings from the center outwards were measured from magnified images of Figure 4A and B and are shown in Table 1.
It is clear that when traction is applied, the diameters of the central rings decrease (Columns 2 and 4), indicating a steepening of the central surface. It was not possible, from the magnified images, to discern rings in the outer lens with sufficient clarity. A challenge to orthodox reasoning should be considered objectively. Some years ago I developed an instrument for simulating accommodation, which was the first to incorporate optical components for ray tracing.4,5 Recognizing the limitations of working with in vitro samples, I set out to find no more than whether older lenses were less malleable than younger samples. Among the results there was a finding which showed that the curvature of an unstretched lens was flatter than its curvature after application of the first stage of traction; thereafter traction caused progressive flattening of the surface (27-year-old lens4). Bearing in mind that in vitro samples must be treated with caution, the results of initial traction support Schachar's theory. The conflict which has been created by the seemingly opponent theories of Helmholtz and Schachar has been founded on the assumption that only one theory must be correct and the other must therefore be wholly in error. This very assumption may be mistaken. It is possible that both theories are applicable depending on the age, size, shape, and density of the lens, positions of zonular attachment, the relative strengths of the various parts of the zonule, and the amount and direction of applied force. In our haste to harmonize systems, find trends, and present unified explanations for physiological mechanisms and anatomical structures, we often ignore the possibility that individual variations may exist and may mask any general trend. Moreover, with age there are changes in the dimensions of the component structures, attachment positions of the zonule,6 and therefore in the geometry of the accommodative system.7 One further point with relation to accommodation in the living eye: Schachar mentions that the cornea does not change with accommodation. This is also orthodoxy and findings have shown otherwise.8,9 [The study of Buehren et al.10 relied on mathematical assumptions to make corrections for eye movements. These were not appropriate for the measurement of subtle changes]. This notwithstanding, the magnitude of change in corneal shape was insufficient to invalidate Schachar's conclusions. Barbara K. Pierscionek References 1. Grossman K. The mechanism of accommodation in man. Ophthalmic Review. 1904;16:1-21. |
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Ronald A. Schachar
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ron{at}2ras.com Ronald A. Schachar
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Reassessing the Mechanism of Accommodation We appreciate Dr. Pierscionek's thoughtful letter, reflecting her attempt to incorporate the often disparate results of published studies. Her task has been made more challenging because of the recently published articles on this subject that lack the adequate controls required of rigorous research.1-5 Their publication may reflect the predisposition of some reviewers to accept works of lesser quality that support established interpretations. It is reassuring to know that discerning scientists, like Dr. Pierscionek, are approaching this subject with an open mind and critically evaluating the data presented. Ronald A. Schachar References 1. Levy NS. Comparing MRIs with movement artifact (letter). Invest Ophthalmol Vis Sci [serial online]. Available at http://www.iovs.org/cgi/eletters/40/6/1162#7. Accessed on April 1, 2005. |
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