THEORIES OF ACCOMMODATION

The mechanism of accommodation has been one of the most studied aspects of visual physiology over the last two centuries.
On accommodation the pupil constricts with contraction of the ciliary muscle, the lens becomes more spherical, and the vitreous pushes forward, moving the lens forward, increasing its effective power.
Although lenticular-based focusing was first proposed by Descartes, it was Thomas Young who in 1793 demonstrated changes in the crystalline lens that occurred on changing focus from distance to near, and Hermann von Helmholtz who in 1856 advanced the first widely accepted explanation of the accommodative process.

VON HELMHOLTZ
The most widely held theory of accommodation was proposed by von Helmholtz in 1856: “When viewing a far object, the circularly arranged Müller’s ciliary muscle is relaxed, allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it in the periphery. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera.” 
According to von Helmholtz, when viewing a near object, the ciliary muscles contract (resisting the outward pull of the sclera) causing the lens zonules to relax which allows the lens to spring into a thicker form. 

MÜLLER
In 1854 Heinrich Müller described the circular muscle of the ciliary body – theorising that the contraction of the ciliary muscle pulled vitreous forward, forcing the lens forward, with resulting power increase. 
Müller’s theory is described by Frans Donders, as follows: “Müller's theory is based on his anatomical investigations of the ciliary muscle… he sees in the action of the most external layers of the ciliary muscle a means of augmenting the pressure of the vitreous humour, of pushing the lens forwards, of diminishing the increased convexity of the posterior surface, and, by the resistance of the simultaneous contraction of the iris, of increasing that of the anterior surface.”

TSCHERNING
Marius Tscherning is best known for his theory regarding the mechanism of accommodation. 
In 1894 Tscherning proposed that accommodation occurred through an increase of zonular tension at the lens equator with contraction of the ciliary muscle, and therefore a bulging of the lens in accommodation was created by compression rather than by passive relaxation. Furthermore, he stated that during accommodation, while the central part of the anterior surface of the lens is bulged, the peripheral portion of the lens is flattened (this theory was first proposed by Antonie Cramer in 1851). At the suggestion of Donders, Cramer used a microscope to demonstrate that accommodation should be ascribed to an increase in the curvature of the lens (in the 1990s Schachar (qv) took up this theory as his own).

DONDERS
Frans Donders favoured von Helmholtz when he wrote On the Anomalies of Accommodation and Refraction of the Eye in 1864. They were colleagues and friends, and no one dared question von Helmholtz for years because of Donders’ influence.

JOHNSON
Dr Lindsay Johnson of Durban, South Africa, in 1924 questioned the rationality of von Helmholtz's theory, because it ignored the action of the circular muscle of Müller. Von Helmholtz had theorised that the lens stayed compressed in the eye when it was not accommodating, because the ciliary muscles were under constant tension, stretching the zonules. Johnson pointed out the lack of logic in this theory, pointing out that muscle tension in a relaxed state is not normal, and the theory not rational. 
Johnson described compression of fluid in the circumlental space on accommodation, with bulging of anterior lens surface and anterior movement of the lens. Aqueous under pressure is forced into the spaces of Fontana during accommodation, and flows back into the chamber upon relaxation of accommodation. Johnson concluded that the increased curvature of the lens was producd by hydraulic pressure, not by relaxation of the ciliary muscle’s tension on the zonules as von Helmholtz claimed.

COLEMAN
D Jackson Coleman, in 1970 and again in 1986, showed that contraction of the ciliary body produced a rise in vitreous pressure, with hydraulic effect on lens deformation producing anterior displacement, confirming Johnson’s theory. He described the interface of vitreous and lens-ciliary body complex as a catenary (the curve formed by a perfectly flexible, inextensible cable suspended from its endpoints).
In 1970 Coleman proposed the Catenary theory, that the lens, zonules and anterior vitreous comprise a hammock-like diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape in the mechanically reproducible state of a steep radius of curvature in the centre of the lens with slight flattening of the peripheral anterior lens, i.e the shape, in cross section, of a catenary, not a parabola. 
The anterior capsule and the zonules form a trampoline or hammock-shaped surface that is reproducible depending on the circular dimensions, i.e the diameter of the ciliary body (Müller’s muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need an equatorial traction force to flatten the lens.
Coleman’s 1970 study showed that contraction of the ciliary body brought about a rise in vitreous pressure, which in turn had a hydraulic effect on crystalline lens with anterior displacement, confirming Johnson’s findings. Again in 1986 Coleman verified the anterior displacement of the lens as a component of accommodation.

THORNTON 
In 1985 and 1986 Spencer P Thornton published real-time A-scan ultrasonography showing anterior movement of the vitreous (and intraocular lens/IOL) on accommodation. This was the first photographic documentation of power increase on anterior movement with an IOL rather than increase in sphericity of the natural lens on accommodation. The accommodating IOL (patent 4,718,904) was based on Johnson’s and Coleman’s observations.
Many investigators have verified that the anterior movement of the lens is a component of accommodation, and most report restored accommodation with IOL forward movement. Most accommodating IOLs depend on this movement for their accommodative effect. Langenbucher and colleagues demonstrated the forward shift of an implanted posterior chamber lens optic, showing that its accommodation is measurable subjectively by usual methods (retinoscopy, videorefractometry, push-up and defocusing) and objectively by measuring anterior chamber (AC) depth decrease with paraxial geometric optics. Their study showed similar theoretical and measured amplitude increase with decreased AC depth.

SCHACHAR
Ronald Schachar in 1992 proposed a theory similar to that of Tscherning (qv) which indicates that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. 

RANA, MILLER, NAWA
Rana A, Miller D and colleagues at Cornea Consultants of Boston, USA, in 2003 demonstrated that good distance and near vision could be achieved with movable IOLs. They state: “The stronger the power of... the IOL... the smaller the amount of movement needed to achieve +2.5 diopters of pseudoaccommodation.”
Nawa and associates in Nara, Japan, in a 2003 article titled Accommodation Obtained With IOL Forward Movement, showed the power increase with 1mm IOL forward movement to vary with the pre-op length of the eye and steepness of the cornea, varying from 0.8D in a long eye to 2.3D in a short eye, varying inversely with corneal power. They concluded that “short eyes with high power IOLs would obtain relatively large accommodation with any given amount of forward IOL movement”. 
A number of studies show that anterior movement of the lens is at least partially responsible for accommodation. 

CONCLuSION
In summary, in contrast to the von Helmholtz theory that is limited to the “natural” lens of the eye, it may be that all theories, including those of von Helmholtz, Tscherning, Müller, Johnson, Coleman and Thornton, are involved both in the phakic and IOL implanted eye.

References available on request

Spencer P Thornton: 
spthornton@comcast.net