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Computer-Animated Model of Accommodation and Theory of Reciprocal Zonular Action

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Computer-Animated Model of Accommodation and Theory of Reciprocal Zonular Action Clinical Ophthalmology Dovepress open access to scientific and medical research Open Access Full Text Article ORIGINAL RESEARCH Computer-animated model of accommodation and theory of reciprocal zonular action Daniel B Goldberg1,2 Abstract: This report presents a computer-animated model of the structures of accommodation 1Ophthalmology Department, Drexel based on new understanding of the anatomy of the zonular apparatus integrated with current College of Medicine, Philadelphia, PA, understanding of the mechanism of accommodation. Analysis of this model suggests a new, 2 Eye Physicians, Little Silver, NJ, USA consolidated theory of the mechanism of accommodation including a new theory of reciprocal zonular action. A three-dimensional animated model of the eye in accommodation and disac- commodation was produced in collaboration with an experienced medical animator. Current understanding of the anatomy of the zonule and the attachments of the vitreous zonule to the anterior hyaloid membrane is incomplete. Recent studies have demonstrated three components of the vitreous zonule: (1) anterior vitreous zonule (previously “hyalocapsular” zonule), which attaches the ciliary plexus in the valleys of the ciliary processes to the anterior hyaloid mem- brane in the region medial to the ciliary body and Weiger’s ligament; (2) intermediate vitreous zonule, which attaches the ciliary plexus to the anterior hyaloid peripherally; and (3) posterior vitreous zonule, which creates a sponge-like ring at the attachment zone that anchors the pars plana zonules. The pars plana zonules attach posteriorly to the elastic choroid above the ora serrata. Analysis of the computer-animated model demonstrates the synchronized movements of the accommodative structures in accommodation and disaccommodation. Utilizing model- based reasoning, it is shown that the posterior zonules attach to and provide traction to the anterior vitreous membrane and Weiger’s ligament. This model supports the concept that the ciliary body/zonule/anterior hyaloid complex contributes to the changes in the posterior lens capsule during accommodation, supporting an extralenticular component to accommodation and demonstrating an alternative to the “vitreous support” theories. There is a reciprocal action of the anterior zonules and the posterior zonules. During ciliary body contraction, the anterior zonules lose tension while the posterior zonules stretch and exert force on the posterior lens capsule playing a role in shaping the posterior lens thickness and curvature. During ciliary body relaxation, the posterior zonules lose tension as the lens flattens and is pulled back by the increasing tension of the anterior zonules. Keywords: zonule, posterior zonule, vitreous zonule, accommodation, mechanism of accommodation, theory of accommodation, model of accommodation, animated model of accommodation Introduction Correspondence: Daniel B Goldberg After more than 200 years of scientific research and debate, the technologic advances Atlantic Eye Physicians, 180 White Road, Little Silver, NJ 07739, USA and research of the past decade have provided the necessary basis to solve some of the Tel +1 732 219 9220 mysteries surrounding the mechanism of accommodation, and integrate this knowledge Fax +1 732 219 9557 Email [email protected] into a new consolidated theory of accommodation. submit your manuscript | www.dovepress.com Clinical Ophthalmology 2011:5 1559–1566 1559 Dovepress © 2011 Goldberg, publisher and licensee Dove Medical Press Ltd. This is an Open Access article http://dx.doi.org/10.2147/OPTH.S25983 which permits unrestricted noncommercial use, provided the original work is properly cited. Goldberg Dovepress Accommodation is widely accepted to occur in capsule. In the 1970s, Farnsworth and Burke,21 Streeten and accordance with the lenticular theory described by von Pulaski,22,23 and Rohen24 demonstrated scanning electron Helmholtz1 in 1855. The work of Fincham2 and Fisher3 microscopy (SEM) of the zonular architecture. Study of the support the lenticular theory. Fincham demonstrated the posterior zonular anatomy has been hindered by the collapse of elasticity of the lens capsule and the ability of the lens spaces surrounding the posterior zonules and the need to remove capsule to “round up” the lens after release of tension on the adherent vitreous membrane for SEM which has prevented the anterior zonules during ciliary body contraction.2 Fisher full understanding of the anatomy of the posterior zonule. used new technologies to measure the accommodative system As many investigators have noted, the architecture in the laboratory and demonstrated that in vitro lens move- of the zonules determines the way the forces of ciliary ments are “solely inherent in the zonule lenticular complex body contraction are distributed in shaping the lens as no vitreous was present in the apparatus.”3 Glasser and during accommodation and disaccommodation. Glasser25 Kaufman4 demonstrated relaxation of zonules due to ciliary demonstrated the circa-2000 understanding of the anatomy muscle contraction and felt that zonular relaxation, together of the zonule in a schematic diagram of the accommodative with the elastic lens capsule, allows the lens to assume the structures (Figure 1). In 2006, Bernal et al12 developed an accommodative state. Their findings support the Helmholtz environmental SEM to image the posterior zonule in human theory with modifications. eyes, and they showed that posterior zonular fibers originate Another major theme in accommodative theory relates to from the ciliary body and anchor in the anterior hyaloid mem- the role of the extralenticular components. This theme has its brane and to Weiger’s ligament, which attaches the anterior origins in the “vitreous support” theories of Cramer (1851),5 hyaloid membrane in a band around the posterior lens capsule. Danish ophthalmologist Marius Tscherning6 (1900), and, This confirmed the findings of Albrecht and Eisner,26 who, in more recently, Koretz and Handleman (1982).7 From 1970 1982, identified these fibers as “hyalocapsular” and “hyalocili- through 2001, Coleman8–10 developed a catenary/hydraulic ary” zonule, now considered to be the elements of the anterior support model propounding that the vitreous compartment division of the vitreous zonule. Other elements of the vitreous evidenced by differential pressure measurement between zonule include the intermediate and posterior fibers of the aqueous and vitreous compartments pushes the posterior vitreous zonule and the pars plana zonules (Figure 2). lens anteriorly and alters the shape of the lens during accom- The three-dimensional (3D) architecture of the anterior modation. The anatomic basis of Coleman’s theory, as well and posterior zonules was studied with SEM by Farnsworth as that of Schachar et al,11 conflict with more recent studies and Burke21 in 1977. They identified four tracts of zonules by Glasser and Kaufman,4 Bernal et al,12 Nankivil et al,13 but believed that most zonules passed directly through the and Croft et al.14 Finite element analysis15 demonstrates ciliary valleys to the pars plana. They noted some zonules that refractive power changes are in accordance with the inserting into the basement lamina of the ciliary processes Helmholtz theory and not in accordance with the Coleman and some zonular bundles originating in the basement lamina theory. Despite the continuing controversy, the vitreous sup- port concept survives, modified by recent advances in the understanding of the vitreous zonule and its attachments to Cornea the vitreous membrane and posterior lens capsule. The role of the ciliary body, which is the engine of accom- Anterior Sclera chamber modation, has been extensively researched and reviewed by Croft et al,14 Stachs et al,16,17 Strenk et al,18,19 and others with Ciliary muscle less controversy. Thus, the components of accommodation, Capsule including the lens, lens capsule, ciliary body, and vitreous, Posterior zonular Ciliary have been extensively investigated. The persistence of accom- fibres body modative amplitudes following vitrectomy and in eyes with Anterior zonular fibres Lens aniridia is evidence of the nonessential role of the iris and Artwork by Adrian Glasser the formed vitreous in accommodation.20 Figure 1 Schematic diagram showing anatomy of accommodation (Glasser25 circa The gap in the knowledge of accommodative structures 2000). Glasser shows the pars plana zonule but does not show vitreous zonule. relates to the anatomy of the posterior zonule and their attach- Note: Reproduced with permission of John Wiley & Sons Ltd from Glasser A. Restoration of accommodation: surgical options for correction of presbyopia. Clin ments to the anterior vitreous membrane and posterior lens Exp Optom. 2008;91(3):279–295. 1560 submit your manuscript | www.dovepress.com Clinical Ophthalmology 2011:5 Dovepress Dovepress Computer-animated model of accommodation Anatomy of zonular apparatus 1. Anterior zonules(zinn) 2. Three elements of posterior vitreous zonule - Anterior - Intermediate - Posterior 3. Pars plana zonule Goldberg Model 2011 Figure 2 Schematic diagram showing elements of zonular apparatus. All elements of the zonular anatomy and attachments to anterior hyaloid and posterior lens capsule are shown. of the ciliary process and extending posteriorly to the pars of the anatomy of the posterior zonule. They identified the plana. They believed that there are two types of connections presence of a cleft between the vitreous membrane and the between
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