An aphakic eye is usually strongly hypermetropic; in tae absence of the lens, parallel rays of light are brought to a focus 31mm behind the cornea, while the average antero-posterior diameter of the eye is only 23 to 24mm. The dioptric system must therefore be supplemented by a strong converging lens, usually, if the eye were originally emmetropic, of about +10 D.
The aphakic eye suffers from several disadvantages. Astigmatism, due to operative interference with the cornea, is frequently present.
The accommodation is abolished, and consequently the patient should theoretically be provided with a pair of glasses for every distance at which he desires to see clearly. In practice it is usually sufficient to provide a glasses for distant vision, one for reading distance, with in -addition, in many cases, one for an intermediate position.
A further considerable disadvantage is the limitation of the visual field, for the spectacles -do not move with the eyes, and the prismatic effects produced by their periphery are great.
The patient must therefor learn to move his head rather than his eyes, especially on looking -downwards, so that he always uses the central portion of his glasses.
The optical conditions are completely changed. The image in a lensless eye, corrected by spectacles, is about 33% larger than when the lens is present. The visual acuity is therefore theoretically worse than that is indicated by the usual clinical tests. This enlargement introduces -a false spatial orientation of familiar objects, which, being of unusual size, are judged to be .nearer than they actually are.
Partly owing to the total lack of accommodation, and partly, and in greater measure, owing to the increased size of the retinal image, an aphakic eye can rarely be used in association with a normal one. Aphakia thus becomes an extreme and accentuated example of anisometropia. A unilateral cataract is removed, it is rarely wise to attempt to the correction of the aphakic eyse´by glasses.
In addition, two optical defects inseparable from the use of strong lenses exist - the distortion -of peripheral objects due to. spherical aberration aid existance of a blind area due to prismatic deviation at the periphery of the lens which give rise to a ring-scotoma, both of which phenomena are much more accentuated on moving the eyes.
As a result of these optical conditions the aphakic is rarely comfortable initially if the .refractive condition is corrected by spectacles.
It is true that some patients accept their new visual world rapidly and almost without complaints; but it is equally true that others of a less adaptable or more neurotic type are never able to reconcile themselves to them, remain completely disoriented and distressed. The majority, however, are faced with a real problem in visual rehabilitation which can only be solved by a considerable amount of intelligent determination over, some months.
Many of these disadvantages the impossibility of binocular vision, the limitation of the field, and so on can be overcome, with some risk, if a plastic lens of a suitable strength is inserted intra-ocularly, or, more safely and easily, if contact glasses are employed instead of spectacle lenses; these however, are often not easily manipulated or worn by the average patient requiring surgery for senile cataract.
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