Comparison of the behavior of natural and refilled porcine lenses in a robotic lens stretcher

The mechanism by which the eye dynamically changes focal distance (accommodation), and the mechanism by which this ability is lost with age (presbyopia), are still contested. Due to inherent confounding factors in vivo, in vitro measurements have been undertaken using a robotic lens stretcher to examine these mechanisms as well as the efficacy of lens refilling - a proposed treatment for presbyopia. Dynamic forces, anterior and posterior curvatures, and lens thickness are all correlated for young natural and refilled porcine lenses. Comparisons are made to lenses refilled with a homogeneous polymer system. The amplitude of accommodation of the young porcine lens is very small such that it may be a suitable model for presbyopia. The behavior of refilled lenses was highly dependent on the refill volume. The volume could be tuned to maximize accommodative amplitude in the refilled lens.     

Surgical treatment of presbyopia: scleral, corneal, and lenticular

PURPOSE OF REVIEW: Having solved most of the problems concerning myopia, hyperopia, and astigmatism, it is perfectly understandable that the surgical treatment of presbyopia should be next on the agenda. It is a new challenge where every possible aspect is explored. RECENT FINDINGS: The past five years have been troubled by the debate over von Helmholtz theory on accommodation. Numerous investigations have been carried out on the primate and humans using various procedures. The more we learn about this mechanism, the nearer we will be to finding a solution to presbyopia. It appears essential to refer to recent works confirming von Helmholtz theory. Therefore, understanding presbyopia requires a great deal of optical ingenuity such as monovision, scleral modifications, which still remain controversial, or clear lens exchange or refilling. SUMMARY: In 2003, with all the different techniques available, the surgeon has a wide choice to offer patients that are satisfactory from a practical, theoretical, and ethical point of view. However, these techniques must only be proposed once the patients have been carefully informed.     

老视理论及治疗进展

老视是一种常见的眼部退行性病变,表现为年龄相关性调节力下降,它影响到每一个老年人。随着全新的Schachar调节理论的提出以及传统理论的发展,近年来出现了多种老视治疗方法。目前除了通过各种框架眼镜和接角镜来矫正老视,还可以应用手术方法如前睫状区巩膜切开样(AGS)以及巩膜扩张(SRP)来恢复调节能力。采用人工晶状体植入形成单眼视,多焦点人工晶状体,可调节型人工晶状体,LASIK手术也都是可行的老视治疗措施。本文总结了目前有关老视的各种机制和治疗方法。     

老視理論及治療進展

老視是一種常見的眼部退行性病燮,表現為年齡相關性調節力下降,它影響到每一個老年人.隨着全新的Schachar調節理論的提出以及傳統理論的發展,近年來出現了多種老視治瘵方法.目前除了通過各種框架眼鏡和接觸鏡來矯正老視,還可以應用手術方法如前睫狀區鞏膜切開術(ACS)以及鞏膜擴張(SRP)來恢復調節能力.采用人工晶狀體植入形成單眼視,多焦黠人工晶狀體,可調節型人工晶狀體,LASIK手術也都是可行的老視治瘵措施.本文總結了目前有關老視的各種機制和治瘵方法.     

The IOL horizon: accommodative intraocular lenses.

BACKGROUND: For more than half a century, intraocular lenses have been used effectively to correct vision after cataract extraction. New developments in intraocular lens design have led to the creation of lenses that may have a significant role in the restoration of accommodation as well. PURPOSE: Accommodative lens technology is constantly evolving. Many different lens designs are currently under development. It is important for optometrists to be familiar with this technology and the various lenses being developed, as accommodative intraocular lenses stand to become the mainstay for cataract surgery, as well as for the surgical treatment of presbyopia. METHOD: The literature is reviewed in order to summarize the developments in accommodative lens technology.     

Accommodating intraocular lenses: a critical review of present and future concepts

Background Significant efforts have been made to develop lens implants or refilling procedures that restore accommodation. Even with monofocal implants, apparent or pseudoaccommodation may provide the patient with substantial though varying spectacle independence. True pseudophakic accommodation with a change of overall refractive power of the eye may be induced either by an anterior shift or a change in curvature of the lens optic. Materials and methods Passive-shift lenses were designed to move forward under ciliary muscle contraction. This is the only accommodative lens type currently marketed (43E/S by Morcher; 1CU by HumanOptics; AT-45 by Eyeonics). The working principle relies on various hypothetical assumptions regarding the mechanism of natural accommodation. Dual-optic lenses were designed to increase the dioptric impact of optic shift. They consist of a mobile front optic and a stationary rear optic which are interconnected with spring-type haptics. With active-shift lens systems the driving force is provided by repulsing mini-magnets. Lens refilling procedures replace the lens content by an elastic material and provide accommodation by an increase of surface curvature. Results Findings with passive-shift lenses have been contradictory. While uncorrected reading vision results were initially reported to be favorable with the 1CU, and excellent with the AT-45 lens, distant-corrected near vision did not exceed that with standard monofocal lenses in later studies. Mean axial shift from laser interferometric measurements under stimulation with pilocarpine showed a moderate anterior shift with the 1CU, while the AT-45 paradoxically exhibited a small posterior shift. With the 1CU, the shift-induced accommodative effect was calculated to be less than +0.5 D in most cases, while +1 D was achieved in a single case only. Ranges and standard deviations were very large in relation to the mean values. Under physiological near-point stimulation, however, no shift was seen at all. Prevention of capsule fibrosis by extensive capsule polishing did not enhance the functional performance. Dual optic lenses are under clinical investigation and are reported to provide a significant amount of accommodation. However, possible long-term formation of interlenticular opacifications remains to be excluded. Regarding magnet-driven active-shift lens systems, initial clinical experience has been promising. Prevention of fibrotic capsular contraction is crucial, and it has been effectively counteracted with a special capsular tension ring, or lens fixation technique, together with capsule polishing. Lens refilling has been extensively studied in the laboratory and in primates. Though it offers great potential for fully restoring accommodation, a variety of problems must be solved, such as achieving emmetropia in the relaxed state, adequate response to ciliary muscle contraction, satisfying image quality over the entire range of accommodation and sustained functioning. The key problem, however, is again after-cataract prevention. Conclusions As opposed to psychophysical evaluation techniques, laser interferometry measures what shift lenses are designed to provide: axial shift on accommodative effort. While under pilocarpine some movement was recorded, no movement at all was found under near-point stimulation with any of the lenses currently marketed. In contrast, magnetic-driven active-shift lens systems carry the potential of sufficiently topping up apparent accommodation to provide for clinically useful accommodation while using conventional lens designs with proven after-cataract performance. Dual optic implants significantly increase the impact of axial optic shift. The main potential problem, however, is delayed formation of interlenticular regenerates. Lens refilling procedures offer the potential of fully restoring accommodation due to the great impact of increase in surface curvature on refractive lens power. However, various problems remain to be solved before clinical use can be envisaged, above all, again, after-cataract prevention. The concept of passive single-optic shift lenses has failed. Concomitant poor capsular bag performance makes these lenses an unacceptable trade-off. Magnet-assisted systems potentially combine clinically useful accommodation with satisfactory after-cataract performance. Dual optic lenses theoretically offer substantial accommodative potential but may allow for interlenticular after-cataract formation. Lens refilling procedures have the greatest potential for fully restoring natural accommodation, but will again require years of extensive laboratory and animal investigations before they may function in the human eye. The author has no proprietary interest in any of the materials or equipment mentioned in this study.     

Accommodation and presbyopia : part 1: physiology of accommodation and development of presbyopia

Accommodation is a dynamic change in the dioptric power of the eye. According to the widely accepted and experimentally confirmed theory of Helmholtz, it is achieved by release of zonular tension with contraction of the ciliary muscle and consecutive modelling of the shape of the crystalline lens by the elastic lens capsule. The ability to accommodate is gradually lost with age (presbyopia). Because of difficulties in examining the accommodative apparatus in vivo, many theories, in part contradictory, about the mechanism of accommodation and the origin of presbyopia have been developed. In recent years experimental studies have greatly increased the knowledge about the acommodative apparatus and suggest a multifactorial aetiology of presbyopia. A better understanding of the physiology of accommodation and presbyopia can contribute to the development of effective treatments.     

Akkommodation und Presbyopie

Accommodation is a dynamic change in the dioptric power of the eye. According to the widely accepted and experimentally confirmed theory of Helmholtz, it is achieved by release of zonular tension with contraction of the ciliary muscle and consecutive modelling of the shape of the crystalline lens by the elastic lens capsule. The ability to accommodate is gradually lost with age (presbyopia). Because of difficulties in examining the accommodative apparatus in vivo, many theories, in part contradictory, about the mechanism of accommodation and the origin of presbyopia have been developed. In recent years experimental studies have greatly increased the knowledge about the acommodative apparatus and suggest a multifactorial aetiology of presbyopia. A better understanding of the physiology of accommodation and presbyopia can contribute to the development of effective treatments.     

Accommodating intraocular lenses

PURPOSE OF REVIEW: With the advent of interest in accommodative lenses as a solution for presbyopia and the growing baby-boomer demographic, ophthalmic surgeons will have the opportunity to provide this technology to facilitate near, intermediate, and distance vision for their patients. RECENT FINDINGS: At present, six corporate entities and lens designs are attempting to commercialize accommodative intraocular lens devices. One Food and Drug Administration (FDA) clinical trial has been completed and the first FDA-approved accommodating lens is available. SUMMARY: For the first time, ophthalmic surgeons will be able to provide a full range of visual focus in each eye of a patient to maintain binocular function while also avoiding the unwanted mesopic and scotopic visual disturbances that are experienced with monovision and multifocal lens technologies. Accommodative intraocular lenses could revolutionize not only cataract visual rehabilitation but also the surgical approach to presbyopia.     

Polymer refilling of presbyopic human lenses in vitro restores the ability to undergo accommodative changes

PURPOSE: Because presbyopia is thought to be accompanied by increased lens sclerosis this study was conducted to investigate whether refilling the capsule of the presbyopic human lens with a soft polymer would restore the ability of the lens to undergo accommodative changes. METHODS: Accommodative forces were applied to natural and refilled lenses by circumferential stretching through the ciliary body and zonular complex. Nine natural lenses and 10 refilled lenses from donors ranging in age from 17 to 60 years were studied. Two refill polymers with a different Young's modulus were used. The lens power was measured by a scanning laser ray-tracing technique, and lens diameter and lens thickness were measured simultaneously while the tension on the zonules was increased stepwise by outward pull on the ciliary body. RESULTS: In the natural lenses the older lenses were not able to undergo power changes with stretching of the ciliary body, whereas in the refilled lenses, all lenses showed power changes comparable to young, natural lenses. The refilled human lenses had a higher lens power than the age-matched natural lenses. The Young's modulus of the polymers influenced the lens power change when measured with the ciliary body diameter increased by 4 mm. CONCLUSIONS: Refilling presbyopic lenses with a soft polymer enabled restoration of lens power changes with mechanical stretching. Because sclerosis of the lens is an important factor in human presbyopia, refilling the lens during lens surgery for cataract could enable restoration of clear vision and accommodation in human presbyopia.     

Artificial crystalline lens

If presbyopia is caused by hardening of the crystalline lens, replacing it with a material with mechanical properties similar to the young crystalline lens should restore accommodative ability. Such a silicone material has been developed. Refilling the capsular bag with this material results in 3 to 5 D of accommodation in primates in response to pilocarpine.     

Accommodation: mechanism and measurement

A clear understanding of the anatomy of accommodative structures, the mechanism of accommodation, and the causes of presbyopia is necessary to understand whether accommodation may be restored to the presbyopic eye. Not withstanding the significant challenges that are faced owing to age-related changes in the eye, theoretically, it may be possible to restore accommodation to a presbyopic eye with an artificial accommodative intraocular lens.     

Presbyopia toward the end of the 20th century

Recent advances in our understanding of the anatomy and physiology of accommodation have contributed to current concepts of the possible constituent factors in presbyopia, largely supplanting the long-held belief that presbyopia is due to sclerosis of the crystalline lens. In this review, the author examines epidemiologic, basic scientific, and clinical evidence for a multifactorial decrease in accommodative amplitude with age. Methods of measurement, oculomotor effects of presbyopia, and approaches to correcting it are also considered.     

Accommodation,convergence and aging

It is generally believed that the ratio of accommodative convergence to accommodation (AC/A ratio) is relatively constant with age. This is based on studies of what is known as the AC/A stimulus ratio in which the denominator is the accommodative stimulus value. However, if the objective accommodative response to each stimulus is used instead in the denominator, then the ratio, called the AC/A response, is not constant but shows an increase beginning just prior to presbyopia. This suggests that greater effort is necessary for a unit of lens accommodation when one gets older or conversely a given effort produces less and less accommodation with age. The disparity between AC/A stimulus and AC/A response is discussed with reference to clinical presbyopia.Supported in part by GRANT #EY00191 of the National Eye Institute.A preliminary report was presented at the New York Academy of Medicine, Ophthalmology Section, May 20, 1968.     

The mechanism of presbyopia

Accommodation in humans refers to the ability of the lens to change shape in order to bring near objects into focus. Accommodative loss begins during childhood, with symptomatic presbyopia, or presbyopia that affects one's day to day activities, striking during midlife. While symptomatic presbyopia has traditionally been treated with reading glasses or contact lenses, a number of surgical interventions and devices are being actively developed in an attempt to restore at least some level of accommodation. This is occurring at a time when the underlying cause of presbyopia remains unknown, and even the mechanism of accommodation is occasionally debated. While Helmholtz' theory regarding the mechanism of accommodation is generally accepted with regard to broad issues, additional details continue to emerge. Age-related changes in anterior segment structures associated with accommodation have been documented, often through in vitro and/or rhesus monkey studies. A review of these findings suggests that presbyopia develops very differently in humans compared to non-human primates. Focusing on non-invasive in vivo human imaging technologies, including Scheimpflug photography and high-resolution magnetic resonance imaging (MRI), the data suggest that the human uveal tract acts as a unit in response to age-related increasing lens thickness and strongly implicates lifelong lens growth as the causal factor in the development of presbyopia.     

Questioning our classical understanding of accommodation and presbyopia

The development of precise instrumentation, electron microscopy, and enhanced analytical capabilities have renewed interest in understanding the mechanisms of accommodation and presbyopia. The properties of the ciliary muscle, the zonule, the lens capsule, and the crystalline lens are being reevaluated, suggesting, for example, that these components' elasticities change significantly with age and that the biochemical properties of the crystalline lens may be altered as the lens ages. The recent findings, mainly during the past decade, are contrasted with or incorporated into our classical understanding of the accommodative mechanism and presbyopia as stated originally by Helmholtz and Fincham.     

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.     

Analysis of human crystalline lens curvature as a function of accommodative state and age

Slit-lamp photographs from four human subjects, aged 11, 19, 29, and 45 were reanalyzed using computer-based digitization and curve-fitting methods in order to obtain more complete information on internal lens curvature changes during accommodation. All discernible curves (N = 742) could be fit to parabolas with chi 2 less than or equal to 0.001 irrespective of lens age, accommodative state, or curve location within the lens. For each lens, the coefficients of the parabolas, when displayed in graphic form, exhibit a linear relationship between location within the lens and the coefficient of the chi 2 term. The slope of this line remains unchanged over accommodation for a given lens, but is shifted in position. The slope changes as a function of age. The age 45 lens exhibits these characteristics to a limited extent only, the differences possibly related to the development of presbyopia. The further a given curve is located from the lens surface, the smaller the region of its arc that can be considered approximately circular. A roughly hourglass figure is generated by these circular bounds; the waist of the hourglass decreases with increasing accommodation, since changes in radius of curvature with accommodation are more pronounced internally. Calculations of arc lengths as a function of increasing accommodation indicate that these lengths change very little over the entire accommodative range.     

The mechanism of accommodation in primates.

OBJECTIVE: To study the accommodative mechanism in primates using monkeys, in light of a recently proposed novel accommodative mechanism in primates and a concomitant controversial surgical procedure for the reversal of presbyopia, DESIGN: Experimental study. METHODS: Accommodation was induced by stimulation of an electrode surgically implanted in the midbrain and by topical ocular application of muscarinic agonists. Pharmacologic disaccommodation was achieved by topical application of a muscarinic antagonist. Movements of the lens equator and the ciliary body were imaged during accommodation and disaccommodation using ultrasound biomicroscopy and goniovideography, and the images were analyzed to determine the direction and the extent of the movements. RESULTS: Despite the systematic eye movements occurring with electrical stimulation and the nonsystematic eye movements occurring with pharmacologic stimulation, in all instances the ciliary body and the lens equator moved away from the sclera during accommodation. CONCLUSIONS: Movement of the accommodative structures is consistent with the classic mechanism of accommodation described by Helmholtz, and contrary to that recently proposed by Schachar.     

Accommodation and presbyopia

The mechanism of accommodation has been studied for at least four hundred years. The most interesting aspect of accommodation is that its time course is well in advance of other physiological functions – it begins to decline by adolescence and is lost about two-thirds of the way through the normal line span. The state of presbyopia is reached when accommodation has declined sufficiently to interfere with close tasks requiring acute vision. Presbyopia is generally considered to originate with the 'plant' of the accommodative system, either within the lens and its capsule or within their support structures. One of the lenticular theories, the Hess–Gullstrand theory, is distinguished from other theories by its claim that as age increases there is an increasing excess amount of ciliary muscle contraction beyond the ability of the lens and capsule to respond to it. For all other theories, the maximum possible amount of ciliary muscle contraction is always necessary to produce maximum accommodation, at least beyond the age at which it reaches its peak. From my review of the present understanding of the mechanisms of accommodation and the theories of the development of presbyopia, I conclude that there is overwhelming evidence against the Hess–Gullstrand theory and that it is unlikely that changes in the ciliary muscle contractility contribute significantly to the development of presbyopia.