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The IOL is generally constituted of two parts:
- optical zone
- support elements
The optical zone is by now universally composed of the material discovered by Ridley: Perspex C.Q. (= P.M.M.A. Clinical Quality). The lenses made with this material need to be refined by hand. Some IOL producers, however, use a different type of PMMA, that allows for shaping by injection; but, the lens manufactured this way turns out to have deforming elements of support and, with cutting margins, it is today preferred to use those molated or levigated by hand. The diameter of the optical zone varies for several of the types of lenses between 5mm and 6mm. Today it is possible to decide from a vast range of powers: from + 6,00 to + 27,00D. In the IOL in the posterior chamber the optical zone has 20 or 4 holes (of the diameter of 0,4 - 0,5 milimeter) that serve as points of attachment for the surgical instruments to position the lens in its seat. The support elements are defined in this way according to how much support is offered to the lens on the ocular structures. They have various denominations according to the IOL group in consideration.
They can be composed of the same material (e.g., IOL of Choyce and IOL of Kelman), or be constituted of other materials in order to obtain a better flexibility: es. Polypropylene (previously also metals like plastic tungsten, platinum etc and other materials like nylon, which turns out to be inferior) were used. The elements of support of the lenses for iridea fixation, for irido-capsular fixation and for the posterior chamber are called loops or handles.

CLASSIFICATION OF THE IOL
Based on the anatomical center of insertion we can distinguish 4 classes of IOL:
1) IOL for the front chamber
2) IOL for iridea fixation
3) IOL for irido-capsular fixation
4) IOL for the posterior chamber
The appraisal of the corrected power of the IOL system is based on two methods:
1) On the use of the Tooriche formulae, based on the principles of the optical geometry applied to the schematic eye;
2) On the use of emperical formulae, based on the retrospective analysis of the data.
In both cases, essential parameters are constituted by the measurement of the corneal refractional power (measured with a oftalmometer or cheratometer), and by the corrected measurement of the axial length of the eye (usually calculated with one of the many biometric ultrasonic To or B-Scan, in commerce).

Related article: Pseudoaphakia: Contraindications to IOL implantation

Da Optometria e Oftalmologia del Prof. Sergio Villani (Volume Uno).

The most severe contraindications to the employment of IOL is for one-eyed patients as advised by authorative experts in the sector. Other contraindications are:

• Diabetic retinopatia whether or not associated with a rubeosis of the iris
  
• Distrophy or corneal degeneration, i.e., the risk that the surgical trauma causes corneal discompensations
  
• Chronic uveitis
  
• Microphtalmos
  
• Patients that were already subject to retinal dehiscence or separation
  
• Glaucoma, only that today it is not an absolute contraindication
  
• Aniridia or pre-existent sectorial iridectomia
  
• Irreversible damage of the pupilar sphyncster
  
• Ambilopia
  
• Acute ocular or peripheral infections
  
• Very low front chamber; flat cornea
  
• Heavy loss of vitreo during the intervention
  
• Ocular blood loss between operations
  
• Retinal or neurological damages
  
• Children of less than 12 years of age
  

Obviously subjects that are contrary to the implantation or result psichologically inadequate should not be considered.

Related article: Pseuodoaphakia: Indications for IOL implantation

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).

The ocular surgeon must evaluate each case of IOL implantation with great care because multiple factors must be considered prior to the implantation of an IOL: age, occupation and the patient’s will are important factors.
The minimum age of the patient, at the beginning of the 80’s, was set at 65 but has recently been lowered due the increased reliability of the lenses and refinement of the surgical techniques. If the ocular situation of the patient permits the implantation, the IOL is necessary in all cases where there is a need for a good binocular vision. Therefore, advanced monolateral and bilateral cataract constitutes the main indication for the implantation, especially if the patients propends to that solution. In these conditions the implantation has to be programmed, especially if the patient is unable to wear corrective contact lenses because of handicaps or environmental conditions (e.g. dusty environments).
Another case in which IOL must be considered is for children (up to 7-8 years) with traumatic or congenital cataract, because it is the only way to prevent functional ambliopia.

Related article: Pseudoaphakia: Intraocular lenses

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).

From the considerations made in the posts on contact lenses one can deduce that intraocular lenses offer a solution that is comparible with the natual vision, provided that the power of the lens, to be inserted in the eye to replace the natural chrystal, has been correctly calculated. The OIL, especially those placed in the rear chamber give, for purely physical reasons, the following advantages:

• Don’t induce a visibile increase the size of the immeages;
  
• Don’t cause a noticeable aniseconia (The IOL in the front chambre generate an image that is ca 4% larger);
  
• Cause scarse aspheric aberrations;
  
• The visual field is not altered;
  
• The fusional activity is resumed.
  

Related article: Pseudoaphakia - Contact Lenses

Da Optometria e Oftalmologia del Prof. Sergio Villani (Volume Uno).

An eye is called pseudoaphakic when the natural chrystal is replaced by an artificial one (INTRA OCULAR LENS, I.O.L.)
As we have seen the aniseiconia obtained with contact lenses has a value of 6-9%; therefore, contrary to common opinion, it needs to be emphasized that the only possibility of sensorial fusion is close to the closest point of fixation and only for small objects that may be classified in the “area of Panum”.
Outside this area the fusional capabilities decrease rapidly and considering that also the perifery is important, one can understand how, even with contact lenses, it is impossible to reconcile the monolateral aphakia with a good fusion. Also new problems may arise with contact lenses especially when they have to be worn only during the day. In practice the patient must be able to manage the entire process. In reality, especially elder people, are unable to do that. Not all cases of aphakia can be treated with contact lenses, for example after a surgery (high astigmatism with lens decentralization), constitutional or ophtalmic problems (reduction of the lacrimal film), exposure to heat or dust, etc…

Related article: Aphakia (5/5).

From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One)

Let’s remember that there exists a compensation between the effects of aberration of the cornea and that of the chrystal (if one undercompensates while the other overcompensates) but the individual difference is such that it makes no sense to determine an average value. Anyway, the influence of spheric aberration of the chrystal is proven by experiments that have demonstrated how, with varying accomodation states, one goes from undercorrection to overcorrection of the retinal image. There exists also a phenomenon that originates from the prismatic effect caused by the rim of the lens. This effect increases with the angle of the prism, i.e., the lens strength. This phenomenon is called “Jack in the box” and also “Roving ring scotoma”.

The dimensions of this blindness “ring” surrounding the visual field depends on several factors: strength of the lens, its dimensions, pupil size, distance from the cornea, the configuration and thickness of the lens.

One of the inconveniences of the above described scotoma is that it tends to move independently from the eye’s rotation. If the bulb rotates laterally by 30 degrees, the scotoma rotates inwards 20 degrees and comes close to the fovea. When the patient sees an object, he sees a dark spot coming closer to him. Without thinking about the consequences he moves his eye in order to focus on the object. Aspherical lenses, also called “panoramic”, have been developed but with modest success.

A (partial) solution can be found correcting the aphakia with contact lenses.

From Optometria e Oftalmologia by Prof. Sergio Villani

Aphakia Art. 4/5

The aphakic eye is unable to focus on objects at different distances and vision is worsened also by the fact that a so strong ametropia reduces field depth; therefore, in theory, one should use a corrective lens for every distance; in practice only 3 types of corrections are required: one for far away, one for medium distance of 80-90 cm and one for reading. If the aphakia is binocular one can obtain an excellent correction also with glasses and, even better, with aspheric lenses. Many aphakics, in order to increase the space in which their vision is sharp, close their eyelids in order to reduce the pupilar area and thus reduce the disc of confusion formed on the retina and increase field depth.

From Optometria e Oftalmologia by Prof. Sergio Villani

Related article: Afakia (3/5)

When the afakia is monocular, the strong imbalance will not permit a normal binocular vision; the cause being the amplication of the retinal image by the corrective lens which is at a distance of the eye. In fact, contact lenses can almost totally eliminate the amplification and return a normal single binocular vision.
This effect is possible because the flaw of the aphakic eye is of a refractive nature. Removing the chrystal, the measurements that first were taken from its principal point to the retina, will now be taken from the corneal apex to the retina; this new reference point creates a lengthening of the ocular dioptric system (of the axial myopic type). For the axial ametropia this lengthening is negative to the point that if one could eliminate this effect, the hypermetropia would be 6 diopters more than it really is.
If the monolateral aphakia is not corrected, the eye without the chrystal tends to move to the outside. This exotropia starts to show itself 8-12 months after the loss of the chrystal and is one of the causes that make it impossible to correct an aphakic eye belatedly.

From Optometria e Oftalmologia by Prof. Sergio Villani

Related article: Afakia (2/5)

The absence of the chrystal can be recognized by the depth and the conical aspect of the front camera, the trembing of the iris which, not supported by the chrystal, varies with the movement of the eye: or also by holding a lamp in front of the eye one can notice the lask of reflexes (Purkinje images) that would have been caused by the front and upper surfaces of the chrystal. The trauma suffered by the cornea after surgery a certain astigmatism remains which, in the healing progress, is reduced by ca. .5 - 1.5 diopters.
In this transition period (lasting approx. 20 days) one must wait for the completion of the healing process before taking corrective action, in the mean time a temporary correction may be given to the patient that gives him his independence. Ignoring the astigmatism, the aphakic eye distinguishes itself by its high degree of hypermetropia (i.e., if the eye before the intervention did not have a myopia > 15.0 D) caused by the absence of the chrystal which when in place has a power of ca. 12 diopters); the spherical refractive flaw of the eye results to be on average 12 - 14 diopters when there are no other complications, the well corrected aphakic eye may be up to 10/10.

From Optometria e Oftalmologia by Prof. Sergio Villani.

Related article: Afakia art. 1/5