Zonular Tension Research Articles

Accommodation‐ presbyopia: mechanism etiology

The purpose of the presentation is to elucidate the mechanism of accommodation and etiology of the age‐related decline in accommodative amplitude that results in presbyopia in the fifth decade of life. Multiple physical, mathematical, clinical, in vitro, and in vivo experiments demonstrate that the human crystalline lens develops an unusual shape during accommodation. Central lens thickness increases; the curvatures of the central surfaces of the lens steepen; while the peripheral surfaces of the lens flatten. This ‘steep profile’ also occurs in other biconvex objects that have an aspect ratio ≤ 0.6 (minor axis to major axis ratio) in response to a small equatorial displacement when the volume of the object only minimally changes. The ‘steep profile’ even occurs when the major axis of an ellipse with an aspect ratio ≤ 0.6 is increased and the area enclosed by the ellipse is held constant or permitted to decrease by 2%. The universality of the occurrence of a ‘steep profile’ implies that the lens is under increased tension during accommodation. This hypothesis was confirmed by using high‐resolution anterior segment optical coherence tomography (OCT) to measure the change of stress on the anterior lens capsule in patients who had undergone a phakic refractive intraocular lens (PRL) at least 1 year prior to the study. The PRL served as an internal control. It was found that during a mean of 8‐diopters of in vivo accommodation, the stress on the lens was increased during accommodation, p < 0.001. The increased stress on the lens capsule during accommodation occurs as a consequence of increased equatorial zonular tension. Normal equatorial lens growth predicts both the age‐related decline in accommodative amplitude and the age‐related increase in intraocular pressure (IOP). The rapid decline in accommodative amplitude and rapid increase in IOP that occur during childhood and their slower changes thereafter, both match the logarithmic pattern of equatorial lens growth, R2 = 0.90. In summary, the lens is under increased stress during accommodation as a consequence of increased equatorial zonular tension. Normal equatorial lens growth is the etiology of both the age‐related decline in accommodative amplitude that results in presbyopia in the fifth decade of life and the increase in IOP that occurs with age.

VISUAL EFFETCS OF HUMAN EYE WITH LASIK AND ARTIFICAL LENS IMPLANTATION

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.

Edinger-Westphal and pharmacologically stimulated accommodative refractive changes and lens and ciliary process movements in rhesus monkeys

During accommodation, the refractive changes occur when the ciliary muscle contracts, releasing resting zonular tension and allowing the lens capsule to mold the lens into an accommodated form. This results in centripetal movement of the ciliary processes and lens edge. The goal of this study was to understand the relationship between accommodative refractive changes, ciliary process movements and lens edge movements during Edinger-Westphal (EW) and pharmacologically stimulated accommodation in adolescent rhesus monkeys. Experiments were performed on one eye each of three rhesus monkeys with permanent indwelling electrodes in the EW nucleus of the midbrain. EW stimulated accommodative refractive changes were measured with infrared photorefraction, and ciliary process and lens edge movements were measured with slit-lamp goniovideography on the temporal aspect of the eye. Images were recorded on the nasal aspect for one eye during EW stimulation. Image analysis was performed off-line at 30 Hz to determine refractive changes and ciliary body and lens edge movements during EW stimulated accommodation and after carbachol iontophoresis to determine drug induced accommodative movements. Maximum EW stimulated accommodation was 7.36 ± 0.49 D and pharmacologically stimulated accommodation was 14.44 ± 1.21 D. During EW stimulated accommodation, the ciliary processes and lens edge moved centripetally linearly by 0.030 ± 0.001 mm/D and 0.027 ± 0.001 mm/D, with a total movement of 0.219 ± 0.034 mm and 0.189 ± 0.023 mm, respectively. There was no significant nasal/temporal difference in ciliary process or lens edge movements. 30–40 min after pharmacologically stimulated accommodation, the ciliary processes moved centripetally a total of 0.411 ± 0.048 mm, or 0.030 ± 0.005 mm/D, and the lens edge moved centripetally 0.258 ± 0.014 mm, or 0.019 ± 0.003 mm/D. The peaks and valleys of the ciliary processes moved by similar amounts during both supramaximal EW and pharmacologically stimulated accommodation. In conclusion, this study shows, for the first time, that the ciliary processes and lens edge move centripetally, linearly with refraction during EW stimulated accommodation. During pharmacological stimulation, the ciliary processes move to a greater extent than the lens edge, confirming that in adolescent monkeys, lens movement limits the accommodative optical change in the eye.

Magnetic resonance imaging of aging, accommodating, phakic, and pseudophakic ciliary muscle diameters

To quantify in vivo accommodative changes in the aging human ciliary muscle diameter in phakic and pseudophakic eyes. Department of Surgery/Bioengineering, UMDNJ-Robert Wood Johnson Medical School, Piscataway, and the Institute of Ophthalmology and Visual Science UMDNJ-New Jersey Medical School, Newark, New Jersey, USA. Images were acquired from 48 eyes of 40 people between the ages of 22 and 91 years, 1 eye of 32 phakic volunteers and both eyes of 8 patients who had monocular implantation of a single-piece AcrySof intraocular lens (IOL) (Alcon Laboratories). Images were acquired during physiological accommodation and with accommodation at rest, and the diameter of the ciliary muscle ring was measured. Results show the ciliary muscle remains active throughout life. The accommodative change in its diameter (mean 0.64 mm) (P<.00001) was undiminished by age or IOL implantation. Preliminary data showed that the accommodative decrease in muscle diameter in phakic and pseudophakic eyes was statistically identical. The phakic eyes had a marked decrease in ciliary muscle diameter with advancing age for both accommodative states (P<.000001 and P<.000001), which did not appear to be altered by IOL implantation. The lens equator was constant with age in the unaccommodated human eye, resulting in decreased circumlental space with advancing age in the phakic eyes. Although the undiminished ability of the ciliary muscle to decrease its diameter with accommodation can be relied on in strategies for presbyopia correction, even in advanced presbyopia, the decreasing circumlental space and its potential effects on zonular tension must also be considered.

Sensitivity study of human crystalline lens accommodation

A nonlinear axisymmetric finite element method (FEM) analysis was employed to determine the critical geometric and material properties that affect human accommodation. In this model, commencing at zero, zonular traction on all lens profiles resulted in central lenticular surface steepening and peripheral surface flattening, with a simultaneous increase in central lens thickness and central optical power. An age-related decline in maximum zonular tension appears to be the most likely etiology for the decrease in accommodative amplitude with age.

Implantation of foldable intraocular lens with anterior optic capture in isolated posterior capsule rupture

We would like to raise several pertinent points regarding isolated posterior capsule rupture occurrence and its management described in the article by Pushker et al.1 Isolated posterior capsule rupture is a well-recognized clinical entity. In our literature review, we came across 22 cases reported prior to the authors' case.2–9 Previous authors suggest that the absence of a formed nucleus in young patients allows contrecoup forces to be transmitted through the lens substance.3 We theorize that its occurrence in young patients following blunt trauma is due to localized pressure buckling the cornea and deforming the soft nucleus (with posterior bulging against the posterior capsule). In addition, zonular tension pulls the posterior capsule centrifugally. This combination results in a blow-out rupture of the posterior capsule. We recently managed a similar case. Two months following blunt ocular trauma, a 19-year-old man had lens aspiration with implantation of an Alcon MA60BM intraocular lens (IOL) in the capsular bag. We aspirated the lens through a superior 2.75 mm clear corneal incision using automated irrigation/aspiration with an anterior chamber maintainer (ACM). With low-flow settings and the ACM, no vitreous prolapsed and anterior vitrectomy was not required. The posterior chamber IOL was placed in the capsular bag, with no optic capture, and has remained stable with over 1 year of follow-up. Even without grossly fibrotic edges, these posterior capsular defects are relatively stable and resistant to enlargement like posterior capsulorhexis. In essence, we feel that vitreous loss should be avoided with the attendant reduction in the risk for retinal detachments and cystoid macular edema. Plain capsular bag fixation is also possible in most cases and is ideal because prolapsing the optic anteriorly brings it closer to the iris and increases the risk for iris chafing and pigment dispersion. If the defect is extremely large or the edges of the posterior capsule defect are not visualized, the posterior chamber IOL haptics should be placed in the ciliary sulcus (with posterior optic capture through an intact anterior capsulorhexis). In fact, anterior optic capture was recommended by Gimbel and DeBroff10 in cases in which the posterior capsule tear occurred during or after posterior chamber IOL insertion rather than in cases with preexisting breaks. In cases of posterior capsule tear during or after posterior chamber IOL insertion, leaving the haptics in the capsular bag, rather than trying to dial them out into the sulcus, minimizes vitreous disturbance. Finally, we would like to point out that the Alcon MA30BA has polymethyl(methacrylate) haptics not polypropylene haptics as mentioned by the authors.

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.

Central surface curvatures of postmortem- extracted intact human crystalline lenses: Implications for understanding the mechanism of accommodation

Purpose To measure the radii of curvature of postmortem, whole, encapsulated human crystalline lenses, free of all zonular attachments, and to calculate their corresponding optical powers. Design Experimental study. Participants Thirty human crystalline lenses from donors with a mean age of 33.6±14.4 years. Methods Intact clear human crystalline lenses were obtained within an average of 21 hours of death. The lenses were removed from the eye by the contributing eye bank and shipped in Optisol-GS, a physiologic preservative storage medium. These lenses, with intact capsules, were freed of all zonular attachments. The lenses were stored at 7° C and were maintained in the same storage medium during the period that they were held for evaluation. Using a portable Keratron Scout corneal topographer (Eyequip, Ponte Vedra Beach, FL) fixed to an optical bench, the radii of curvatures of the anterior and posterior surfaces of the crystalline lens were measured daily for 10 days after receipt of the tissue. The capsules of the crystalline lenses remained intact, and the lenses were clear throughout the study. Measurements were made at room temperature after removing the lens from storage. Eight repetitions of the topography were made from each surface on each day to determine the accuracy and stability of the measurement. Profile photographs were taken daily to establish the central crystalline lens thickness. The corresponding optical power of each physiologically maintained crystalline lens was calculated. Main outcome measures The main outcome measures were the central anterior radius of curvature, the central posterior radius of curvature, the central thickness of each crystalline lens, and the amount of change in these parameters over 10 days. Results The means ± standard deviations of the central anterior and posterior radii of curvatures of the 30 adult lenses were 10.5±0.6 mm and 7.1±1.0 mm, respectively. The mean ± standard deviation of the central thickness, as measured from profile photographs, was 3.9±0.5 mm. These mean measurements remained stable for up to 5 days after receipt of the tissue. The mean and standard deviation of the calculated optical power of these postmortem, physiologically maintained, intact human crystalline lenses from donors with a mean age of 33.6 years was 19.8±1.7 diopters. Conclusions Lenses free of zonular tension are at an optical power that would be associated with optimal distance acuity.

Qualitative effect of zonular tension on freshly extracted intact human crystalline lenses: implications for the mechanism of accommodation.

To determine the topographic effects of zonular tension on the anterior surface of the human crystalline lens. Real-time topography of the anterior surface of seven fully relaxed, freshly extracted intact, clear, human crystalline lenses aged 3, 17, 45, 54, 54, 56, and 56 years was qualitatively obtained before, during, and after the application of zonular traction. Zonular traction was applied manually either by grasping a group of zonules 180 degrees apart with tying forceps (three lenses), or with micrometers by clamping four portions of the ciliary body that were 90 degrees apart (four lenses). Zonular tension began with the lenses in the fully relaxed, baseline state. As zonular tension was increased across one meridian of all seven lenses, the center of the anterior surface steepened while the periphery of the anterior surface flattened across that meridian of traction. When the tension was reduced across that meridian of traction, the center of the lens flattened while the periphery steepened in that meridian. Four-point zonular traction applied 90 degrees apart produced symmetrical central steepening (four lenses). Reduction of zonular tension across both orthogonal meridians caused symmetrical central flattening. These observations reveal that when zonular tension is applied to the fully relaxed lens, the center steepens and its periphery flattens in the meridian (or meridians) in which zonular tension is applied. The reverse of this process demonstrates that as tension is reduced, the center of the lens flattens while the periphery steepens either in the meridian of relaxation or symmetrically when zonular tension is released from two orthogonal meridians. These results are opposite to what would have been predicted on the basis of Helmholtz's theory of accommodation.

Magnetic resonance imaging study of the effects of age and accommodation on the human lens cross-sectional area.

To evaluate the effect of age and accommodation on lens cross-sectional area (CSA). High-resolution magnetic resonance images of the eye were acquired from 25 subjects ranging in age from 22 to 50 years during accommodation and with accommodation at rest. The images were analyzed to obtain the total lens CSA and the CSAs of the anterior and posterior portions of the lens. The total lens CSA and the CSA of the anterior portion increased with age in both accommodative states. With accommodation, the CSA was larger in these portions of the lens; however, this difference decreased with age. Conversely, the CSA of the posterior portion of the lens remained statistically independent of both age and accommodative state. This preliminary study documents, in vivo, that the lens grows with age. This growth appears to be confined to the anterior portion. A quite unexpected finding is that both the total lens CSA and the CSA of the anterior portion are greater during accommodation when zonular tension is minimized. This accommodative change in CSA, which decreases with age, may be due to compression of the lens material during relaxed accommodation when zonular tension is greatest. That both age and accommodative changes in CSA appear to be limited to the anterior portion of the lens may be related to properties of the anterior capsule and lens material, the position of the zonular attachments, and the location of the fetal nucleus.

Immunohistochemical Study of the Extracellular Matrices Related to Accommodation in Monkeys

The mechanism of accommodation was studied by immunohistochemical staining of extracellular matrices in monkey eyes. Frozen sections from five pairs of post-mortem eyes donated by physiologists were used. The antigens listed in Table 1 were selected, and a 2- or 3-step staining technique was used. Confocal laser microscopy was used to evaluate the staining and take photographs. Many frames were reconstituted for the Figures. a) The ciliary muscle was divided into circular and longitudinal portions by arrangement of alpha-smooth muscle actin and surrounded by dense nets of alpha-elastin at the inner border under pigmented ciliary epithelium. Elastin nets extended into the stroma of the ciliary processes. In the lens, elastin was observed in the equatorial and central subcapsular area. b) Zonules were heavily stained by fibrillin-1 as bundles. Continuous fibers connected the surface of ciliary processes with the lens capsule, and were also anchored along the ciliary epithelium toward the pars plana. Type IV collagen was seen in circumferential lens capsule and between the lens fibers, especially in the equator and central cortex. c) There was more alpha-1A adrenoceptor in the longitudinal than the circular portion of the ciliary muscle. Our results provide an argument against Helmholtz's theory, but in favor of Tsherning's theory. The accommodation to near might be accomplished by contraction of circular ciliary muscle held by elastin nets pulling the zonules centripetally. The zonular tension puts pressure on the equator and also pulls the lens capsule anterocentrally. This coordination of the matrices will shorten the equatorial plane of the lens and expand the inter-lens-fiber space, making the polar axis wider. As to the far accommodation, longitudinal muscle will pull the zonules peripherally, narrowing the inter-fiber space.

Mechanics of accommodation of the human eye

The classical Helmholtz theory of accommodation has, over the years, not gone unchallenged and most recently has been opposed by Schachar at al. (1993) (Annals of Ophthalmology, 25 (1) 5–9) who suggest that increasing the zonular tension increases rather than decreases the power of the lens. This view is supported by a numerical analysis of the lens based on a linearised form of the governing equations. We propose in this paper an alternative numerical model in which the geometric non-linear behaviour of the lens is explicitly included. Our results differ from those of Schachar et al. (1993) and are consistent with the classical Helmholtz mechanism.

Controlling the capsular shape in lens refilling.

To investigate control of the capsular shape by determining the ability of the lens capsule to mold injected silicone and to evaluate the relationship among the volume of silicone injected, refraction, and the amplitude of accommodation. After endocapsular phacoemulsification following an upper, minicircular capsulorhexis, the lens capsule of a pig eye was refilled with a silicone mixture that polymerizes in vitro in 2 hours. The minicircular capsulorhexis opening was sealed by a small silicone plug to prevent leakage. The anterior capsule curvature and refraction of the lens were measured by a Scheimpflug camera and lensometer, respectively, with and without zonular tension. Zonular tension was created using a ciliary ring sutured to the ciliary bodies and expanded. The mean (+/- SD) anterior curvature of the lenses without zonular tension was 6.50 +/- 0.07 mm after 17 hours and 6.54 +/- 0.04 mm after 42 hours; with zonular tension it was 7.01 +/- 0.11 mm and 7.23 +/- 0.24 mm, respectively. The curvature became flatter when zonular tension was applied or steeper when zonular tension was abolished momentarily during measurements after 17 hours, but after 42 hours the curvature was unaffected by the application or removal of zonular pressure. The mean (+/- SD) amplitude of accommodation (the difference between refraction without zonular tension and that with it) was 3.2 +/- 0.5 diopters (D), 6.1 +/- 1.8 D, 4.8 +/- 0.8 D, and 2.8 +/- 1.3 D, when the lens was refilled with a silicone volume corresponding to 45%, 55%, 75%, and 95%, respectively, of the mean normal lens volume. The lens capsule possesses some ability to mold the injected silicone during its polymerization. When the eye is atropinized, the lens capsule may conform to its nonaccommodated state. Accommodation could be obtained by various degrees of refilling. Moderate refilling yields a greater amplitude of accommodation than does more complete refilling. Using a silicone plug to seal the capsular opening facilitates lens refilling with excellent reproducibility and seems to be useful in research.

In vivo increase of the human lens equatorial diameter during accommodation.

The movement of the human lens equator during accommodation was examined in vivo. High-resolution ultrasound images of the lens equator were obtained from young human subjects whose amplitude of accommodation was controlled with 1% tropicamide and 2% pilocarpine. To avoid errors that otherwise arise from eye rotation or other movement, the cornea and sclera were used as positional references in comparative studies of the video images obtained from the unaccommodated and accommodated states. During accommodation, the movement at the lens equator involved small displacement; i.e., < 100 microns, and the equator did not move anteriorly or posteriorly but peripherally toward the sclera. These results indicate that the lens equator is under increased zonular tension during accommodation, in contradiction to Helmholtz's widely accepted theory of accommodation.

Model of the accommodative mechanism in the human eye

The crystalline lens of the age 11 human eye has been modelled mathematically, using simplified assumptions about lens curvature, internal organization and elasticity. From this representation, expressions for description of strain and stress during accommodation have been obtained. Solution of these equations indicates that the lens capsule acts as a force distributor, spreading tension applied by the suspensory apparatus evenly over the surface of the underlying lens material. It also becomes clear that the vitreous body provides an essential support function during the accommodative process. Finally, the relative contribution of lens-associated structures has been determined for five different values of the Poisson ratio. In order for accommodation to occur by relaxation of zonular tension, this value must be greater than 0.38; with an additional constraint of the net axial force equalling zero during a small accommodative change, the Poisson ratio equals 0.46.

The force of contraction of the human ciliary muscle during accommodation.

1. Apparatus has been designed to alter the shape of the human lens by tensile forces applied to the zonular fibres indirectly through the ciliary body. The changes in dioptric power of the lens for monochromatic sodium light were measured at the same time. Simultaneous serial photography, and direct measurement enabled one to relate a change in shape of the lens to the change in dioptric power. Subsequently, the same lens was isolated and spun around its antero-posterior polar axis and high speed photography recorded its changing profile.2. By comparing the changes in lens profile due to zonular tension and centrifugal force respectively, the force developed in the zonule for a given change in the shape of the lens could be calculated. Changes in dioptric power associated with those of shape can thus be related directly to the force of contraction of the ciliary muscle necessary to reduce the initial tension of the zonule in the unaccommodated state.3. The force of contraction of the ciliary muscle as measured by radial force exerted through the zonule and the change in dioptric power of the lens were not linearly related. The relationship is more exactly expressed by the equation [Formula: see text] where D = amplitude of accommodation in dioptres (m(-1)), F(CB) = force of contraction of the ciliary muscle as measured by changes in tension of the zonule (N), K(df) = dioptric force coefficient and is constant for a given age (m(-1)N(-(1/2)) x 10(2.5)). This coefficient is 0.41 at 15 yr and 0.07 at 45 yr of age.4. In youth for maximum accommodation (10-12 D) the force is approximately 1.0 x 10(-2) N while to produce sufficient accommodation for near vision (3.5 D) the force is less than 0.05 x 10(-2) N.5. After the age of 30 yr the force of contraction of the ciliary muscle necessary to produce maximum accommodation rises steadily to about 50 yr of age and thereafter probably falls slightly. At about 50 yr of age the ciliary muscle is some 50% more powerful than in youth.6. Even if hypertrophy of the muscle did not occur the amplitude of accommodation would be reduced at the most by only 0.8 D of that observed at the onset of presbyopia.

Early diagnosis of ciliary body melanomas.

In the clinicopathologic analysis of seven cases of circumscribed malignant melanoma of the ciliary body, the earliest and most consistent symptom was a progressive decrease in vision, not correctable with lenses and without apparent clinical cause. This symptom was related to refractive aberrations caused by tumor-induced alterations in the noncataractous lens. Mechanisms responsible for these optical effects included localized changes in lens shape, caused by asymmetric zonular tension or by direct compression, and changes in lens position related to decentration or tilting. Other important findings included segmentally prominent episcleral vessels, slight ocular hypotony, and significant changes in the anterior chamber depth.

MECHANISM OF ACCOMMODATION

The Helmholtz 1 theory of accommodation appeared in 1855, and since that time the refutation or substantiation of this hypothesis has concerned students of ocular physiology. The difficulty of observing the interior of the anterior segment without mutilating the eye has permitted the theory to persist in explanations of this ocular function. There are two firmly entrenched schools of thought on the mechanism of accommodation: One supports Helmholtz, and the other favors the Tscherning 2 theory. The two agree that the fundamentals of accommodation are contraction of the ciliary muscle and increased convexity of the crystalline lens but differ on the method by which the former accomplishes the latter. The followers of Helmholtz believe that contraction of the ciliary muscle reduces zonular tension, permitting the elastic capsule of the lens to shape the lens. In opposition, Tscherning's disciples think the force of the ciliary muscle acting through the zonule and

MECHANISM OF ACCOMMODATION

The Helmholtz 1 theory of accommodation appeared in 1855, and since that time the refutation or substantiation of this hypothesis has concerned students of ocular physiology. The difficulty of observing the interior of the anterior segment without mutilating the eye has permitted the theory to persist in explanations of this ocular function. There are two firmly entrenched schools of thought on the mechanism of accommodation: One supports Helmholtz, and the other favors the Tscherning 2 theory. The two agree that the fundamentals of accommodation are contraction of the ciliary muscle and increased convexity of the crystalline lens but differ on the method by which the former accomplishes the latter. The followers of Helmholtz believe that contraction of the ciliary muscle reduces zonular tension, permitting the elastic capsule of the lens to shape the lens. In opposition, Tscherning's disciples think the force of the ciliary muscle acting through the zonule and