Phakic lenses
Phakic lenses is an alternative concept for the correction of myopia, farsightedness and / or astigmatism that keeps the ocular structure intact.
An intraocular implant with particular design characteristics is placed depending on whether it is positioned in front or behind the iris, thus adding another diopter to the system, configuring a polyphakic / multiphakic optical system.
Phakic lenses
Phakic lenses is an alternative concept for the correction of myopia, farsightedness and / or astigmatism that keeps the ocular structure intact.
An intraocular implant with particular design characteristics is placed depending on whether it is positioned in front or behind the iris, thus adding another diopter to the system, configuring a polyphakic / multiphakic optical system.
Why wear phakic lenses?
Surgery LASIK (Laser Assisted in Situ Keratomileusis) main representative of keratorefractive surgeries not only modifies geometric parameters of the cornea (eg anterior curvature of the cornea, asphericity and degree of corneal aberrations) but also reduce its thickness and with this it affects the corneal biomechanics thus reducing its stiffness much more accentuated in patients with susceptibility to corneal ectasia.
Several studies on the results of excimer laser surgery have shown that treatments that induce changes in the corneal diopter, leaving curvatures below 34 diopters or above 47 diopters, are susceptible to induce poor visual quality due to loss of contrast sensitivity or decrease in best corrected visual acuity.
It is also known that a residual stroma below 250 um, retreatments, high ametropia corrections increase the risk of suffering from corneal ectasia with secondary detriment to vision.
That is why the good use of phakic lenses incorporating them in daily practice as a state-of-the-art alternative to cases of high ametropia and in whom keratorefractive surgery is biomechanically contraindicated, this last parameter today increasingly better and more evaluated in the most important centers dedicated to refractive surgery state-of-the-art through the use of the Ocular Response Analyzer (ORA). This instrument is responsible for measuring "in vivo" the ability to respond to corneal deformity, that is, it measures corneal hysteresis.
are recognized international results with phakic lenses as very safe and with high final visual quality for patients even with gain in lines of vision.
What is intraocular lens refractive surgery used for?
Phakic intraocular lens refractive surgery is used to correct refractive vision defects such as la myopia, la farsightedness I el astigmatism. In this way, the person avoids dependence on glasses or contact lenses.
This correction is achieved by implantation of a lens in the eye. To carry out the intervention, a detailed study of the morphology of the eye is made, which is what will determine the characteristics of the lens in each of the patients.
One of the main and advantages of refractive surgery with lens implant is that the morphology of the cornea is not modified.
Myopia
Myopia is a refractive error that occurs when a person sees distant objects blurry but can see nearby objects correctly.
Farsightedness
Farsightedness manifests itself when a person cannot see objects that are close by clearly but can see those that are at a certain distance.
Astigmatism
Astigmatism occurs when the surface of the cornea is irregular. Resembling the wavy mirrors of amusement parks.
Who is recommended?
Among the main indications that we find in our daily practice are:
- Fine corneas with pachymetries with thicknesses less than 500 um, with the exception that the use of pancorneal pachymetry by tomographic methods is very important since the punctual ultrasonic method lacks sensitivity to detect corneas with anomalous thickness profiles where the central point of the cornea does not coincide with the finest point on its surface. Ex. Oculus Pentacam Ocular Tomograph
- High myopia with spherical equivalent greater than -7.00 diopter in those under 48 years of age
- High hyperopia with spherical equivalent greater than + 5.00 diopter in those under 48 years of age
- Any surgery that leaves us with corneas with final curvatures less than 34 diopters or greater than 47 diopters
- Special case: the Keratoconus refractive surgery correction Mild – Moderate, isolated or as a combined modality to ring segment implants.
Variables to consider
The good evaluation of the anterior segment of the eye is of the utmost importance and for this nothing better than the use of the consolidated diagnostic tool the Anterior Segment Tomography nowadays well represented by Tomographs based on the Scheimpflug Image such as the Pentacam of Oculus or the Galilei of Ziemer, the use of the image of Optical Coherence Tomography (OCT) and High Frequency Ultrasound Biomicroscopy (UBM) of the anterior segment.
Both providers of sufficient morphometric information of the anterior segment (eg volume of the anterior chamber, depth of the anterior chamber, angle of the anterior chamber, central pachymetry of the cornea, map of the depth of the anterior chamber, etc.) and of the iris profile to be able as well assess the correct selection of the anterior chamber or posterior chamber phakic implant model.
It should also be remembered that the endothelial count through specular microscopy It is essential mainly when morphologically the anterior segment is a good candidate for phakic implant of the anterior chamber since there is a minimum corneal endothelial density of 2800 cells/mm2 that should be respected for these models.
In the same way the mesopic pupil size It is also important to know that the candidate should not ideally exceed the size of the optical zone in the anterior chamber models and be somewhat more tolerant with the posterior chamber models.
According to the white outer measure –horizontal white, lacks sensitivity and specificity to ensure good adaptation of a posterior chamber implant, so the use of the high frequency ultrasonic biomicroscopy (UBM) to better select the length of these types of implants based on best approximation to the real size of the site (ciliary groove) where they will be housed behind the iris.
Otherwise it increases the risk of acute angle-closure glaucoma by implantation of a larger lens at the appropriate diameter corresponding to that of the ciliary groove.
Types of phakic lenses
There are three models of intraocular phakic lens each It is distinguished by how it is fixed to the internal structures of the eye and the position it occupies with respect to the pupil-iris plane.It is in this order that we are going to discuss them.
As we have mentioned before, the highly satisfactory results with these implants depend on adequate knowledge of their safety profile and knowledge of the rate of complications associated with their use.
Phakic lenses with iris fixation
Models phakic implants with fixation in the iris by means of an advanced ¨clip¨ system are those that are based on a philosophy that can be defined by the phrase in English "one size fits all" from where the independence of the size of these implants is rescued to the morphometry of the anterior segment, whereby the rate of complications secondary to poor or insufficient estimation of the appropriate implant measurements for each eye is diminished.
They are also the only group of implants that allow correct centering in the independent pupil if there is a slight displacement of the center of the pupil, thus reducing the halos that could occur due to the disparity in centering between the optical zone of the implant and the position of the pupil.
Although intraocular inflammations could occur somewhat more frequently with these implants, they are easy to manage and control.
Posterior chamber phakic lenses
The posterior chamber intraocular implants as ICL lenses, are implanted between the iris and the lens They have two versions, one with a larger diameter is fixed in the ciliary sulcus and requires the precise measurement of this ocular structure, so the maximum precision and reliability of this measurement is mandatory for decrease the possibility of contact with the lens and secondary cataract, in cases of implants shorter than the ideal length or otherwise acute glaucoma as discussed above.
This version of collamero material can correct myopia, hyperopia and/or astigmatism.
The other version of posterior chamber phakic implant it is made of silicone of shorter length than the previous one, it is not fixed in the ciliary groove rather floats in the rear chamber and the complications do not come from the correct measurement but from the possible injury of the ciliary zonule due to the shearing movement of its edges when it rotates freely in this space, therefore dislocations of these implants in the vitreous chamber have been described.
Recommendations for the use of phakic lenses
- Corneas with pachymetry with thicknesses less than 500 µm.
- Myopia with spherical equivalent greater than -7.00 diopters.
- Farsightedness with spherical equivalent greater than + 5.00 diopters.
- Curvature Corneas post op less than 34 diopters or greater than 47 diopters.
- Corneal topography/tomography suspicious for keratoconus.
- Corneal hysteresis decreased (eg CH 8,1 CRF 7,0).
Pre-operative tests for phakic implants
- Tomography/Ocular Topography (Scheimpflug Image).
- Optical Coherence Tomography (OCT of Anterior Segment).
- Endothelial microscopy.
- Axial Biometrics (Optical or Ultrasonic).
- Ultrasonic Biomicroscopy (UBM).
