Intra ocular implant in cataract surgery
During the cataract surgery, remove the lens that is clouded and causes Visual discomfort. Monofocal implant (monofocal implant) are intended to compensate for the strong farsightedness which would be induced if you did not compensate the reduction of optical power ' vergence) related to the removal of the lens. The monofocal implant is comparable to a biconvex converging lens with a single optical power. This is expressed in diopters, and is calculated through biometric examination (biometrics). For each model of implant, the surgeon has a wide range of powers (from-10 to + 30 diopters diopters by 0.50 no D for the more extensive ranges). More the dioptre power is high, more implant focus almost the light; as a result, eyes operated initially farsighted (short) generally receive implants of Dioptric power higher than the nearsighted eyes (long eyes).
The lens is a natural lens that has (at rest, without accommodation) optical power close to 20 to 22 D in the eye: this power is used to supplement that of the cornea for converging the incident light on the retina. However, this power can vary slightly from one patient to the next, and tends to increase in case of cataract. The cataract surgery to remove the lens, the power of the eye is "weakened" if one does not replace the implant by an artificial (implant) of appropriate power.
An operated eye cataracts without implant placement is an eye said "aphake" (aphakia). The optical power of the cornea is not enough to make converge the incident light rays on the retina: measured in terms of the glasses (1.2 cm in front of the eye), this farsightedness is averaging close to 12 diopters.
Calculation of the power of the implant - ocular biometry
The optical power of the monofocal implant is chosen according to the results of the biometric calculation (biometrics). Biometrics is based on measurement of the length of the eye (axial length), and the measurement of the optical power of the cornea (keratometry). Broadly speaking, when we know the length of the eye and the keratometry, we can predict the means of formulas (ex: formula SRK - T, Haigis formula, etc.) the power of the implant that will allow the 'Emmetropic' eye (naturally corrected for distance vision), if the patient wishes to see from a distance without glasses (emmetropisation). In this case, the power is calculated so that once the implant is placed in the eye, the light rays emitted by a distant source are focused on the retina.
For patients who wish to read without glasses, one computes the power of the implant in order to this that once inserted into the eye, it misleads a slight myopisation, allowing the rays from a close source (ex: book) focus on the retina.
Optimize the optical quality of the operated eye is logical when it comes to patient independent optical correction by glasses for distance vision, thus monofocal implants are today aspherical and generate negative spherical aberration to compensate for all or part of the corneal positive spherical aberration. The optical part of the first implants (artificial Crystal) pseudophakes was classically a little biconvex lens (near 6 mm diameter) to spherical surfaces, including the curvature (and refractive index) determines the optical power. This spherical geometry was dictated by industrial constraints but induced no profit in functional terms. Ideally, the artificial lens implant should return to the patient operated a visual quality equivalent to that of the young subjects, many studies have highlighted a reduction in the optical quality of the eyes pseudophakes with respect to the phakes and healthy eyes.
The optical quality of the result depends on obtaining a superior visual acuity or equal to 10/10 without correction from a distance, but to maintain it for the weakly contrasting and/or night vision objects.
In the case of an emmetropisation in both eyes, the port of a correction still required to close fine vision (reading). Some (initially myopic) patients wish to retain a slight myopia in order to be able to read without glasses; they will then wear a correction in glasses glasses to see far away.
THEastigmatism eyepiece is mostly original corneal and its correction may be considered during the cataract operation by conducting additional corneal incisions, or the insertion of an implant of toric artificial lens. A toric implant has a generating of astigmatism: when positioning the implant to ensure astigmatism that it induces is oriented at 90 ° with respect to that of the cornea. The biometric calculation of toric implant requires to take into account the corneal astigmatism. The placement of the o-ring implant requires special care during surgery (positioning and orientation of the implant).