The PKR (surface laser technique)
The PKR is an eye operation using laser excimer (photoablation) and intended to improve vision, by changing the shape (optical power) of the cornea to correct nearsightedness or astigmatism.
The PKR or procedures to Refractive sight is based on the issuance of a refractive correction on the surface of the corneal stroma, after removal of superficial corneal epithelium. It requires no cut of flapunlike LASIK. It is performed under local anesthesia (drops). Both eyes are operated on the same day. There are two variants depending on how the epithelium is removed:
- in "manual" technique, the epithelium is removed (peeled) with a sponge after application of a diluted solution
- in "all-laser" or "transepithelial" technique: the epithelium is photo-ablated by the excimer laser.
The choice between "manual" or "all-laser" technique depends on the preferences of the surgeon and the patient. The initial clinical results do not allow to attribute to one or the other of these variants a particular benefit in terms of visual result. The "transepithelial" procedure is faster in its execution, and could be particularly suitable for the correction of small myopias. In cases caseof higher myopia (e.g. -5 or -6D), it may be preferable to perform manual debridement of the epithelium, to limit the total laser dose delivered to the corneal tissue.
PKR: principles and technique
It should be remembered that the cornea consists of two main tunics: the stroma (tissue made up of intertwined collagen fibrils and representing 90% of the corneal thickness), and the epithelium, located on the surface, which is a tissue made up of several layers of cells called "epithelials". This arrangement is similar to that of the dermis and epidermis at the cutaneous level. In PKR, the epithelium is separated from the underlying stroma and then removed. The correction is delivered to the superficial stroma, immediately below the corneal epithelium. The epithelium then grows back in a few days to cover the corneal surface.
The principle of refractive correction (corrective effect) in PKR is similar to LASIK, both processes using an excimer laser to reshape the profile of the cornea to change its optical power (LASIK and PKR have in common the so-called "photoablation" operation time of stromal corneal tissue).
The main difference of the PKR with LASIK is based on the removal of the epithelium and the absence of flap cutting. In LASIK we preserve the superficial epithelial layer, which is not exposed to the beam of the laser: the cut is recline: this component includes the epithelial layer and part of the stromal layer). During the PKR, theepithelium is gently peeled After administration of drops anesthetics to numb the cornea: the superficial part of the stroma, called layer of Bowman, is then exposed to the excimer laser.
The beam of the excimer laser, which is controlled by a computer coupled with the licensing system, is then projected onto the corneal surface stripped to sculpt the fabric superficial corneal stromal.
The classic PKR in video:
The following video shows the entire of a procedure of PKR (surface laser) bilateral, started by the left eye. The treatment is optimized for wave-front (wavefront optimized) and use iris recognition for alignment and optimal balance of treatment. The technique used for the removal of the epithelium is thrifty and not traumatic (unlike the classic PKR and the EpiLASIK, who uses a machine to separate the epithelium of uncontrolled way). Done this way, the technique of PKR is without risk intraoperative. Anesthesia is purely topical (drops).
The transepithelial PKR in video (close-up):
This video, made by filming the eye operated on as closely as possible (with a slow motion x4), concerns a transepithelial PKR procedure (Streamlight mode, EX500 laser, Alcon Wavelight). The first phase (epithelial removal) is performed using laser impacts that are delivered to photoablate the epithelial layer over a surface at least equal to the programmed optical zone. The thickness of the central layer of the epithelium is about 50 microns (this thickness can be measured using high-resolution corneal OCT technology).
If the PKR procedure is painless during its performance, a feeling of discomfort, burning occurs in the hours that follow and usually lasts less than 24 hours. Most surgeons cover the surface of the eye with a Contact lensspecial at the end of the operation, which makes it possible, in addition to taking painkillers, to make the postoperative pain completely bearable. It Contact lensis kept a few days before withdrawal.
The PKR induces a slightly longer than LASIK healing phase, because we expect that the epithelium grows back on the surface of the reshaped cornea, which takes a few days. The pose of a non corrective soft contact lens on the right cornea after laser correction helps speed up the re-growth of the epithelium. Taking Painkiller to alleviate pain (whose duration does not exceed 24 hours, usually).
Why remove the epithelium of the cornea in PKR?
It is necessary to remove the epithelial layer to access the surface of the corneal stroma (the stroma is the main corneal tissue, which represents 90% of the corneal thickness, and which is largely composed of collagen fibrils). If the photo-ablative laser treatment was performed directly on the corneal surface without removing the epithelium, this epithelial layer would certainly be "planed", but the shape printed by the laser on the epithelial layer would only be temporary. The epithelium is indeed a stratified multi-cellular layer (the cells are "stacked one above the other"), whose shape can be modulated by the multiplication of superficial epithelial cells (epithelial hyperplasia is used to describe this proliferation). In other words, if the laser treatment were applied directly to the epithelial surface of the cornea, the epithelial regrowth would erase the effect in a few weeks. The correction printed in the stroma is more durable: the epithelium sometimes tends to thicken compared to the thinned stroma, which can induce a regression of the refractive effect. This occurs especially after correction of hyperopia, for which LASIK is more suitable. On the other hand, low and medium myopias are perfectly eligible for surface laser correction.
Is the PKR less modern than LASIK?
This common misconception is widespread, as LASIK was introduced after the first corrections were made by PKR (early 1990s). PKR and LASIK are based on a similar principle: the remodelling of the cornea, and both benefit from the extraordinary precision of the excimer laser. The "transepithelial" version of PKR has been introduced more recently.
LASIK is not a "revolution" with respect to PKR, it simply consists in delivering laser correction (identical whatever the technique) within the stromal tissue of the cornea, i.e. in depth. This allows a faster recovery, with a reduction in post-operative sensations. However, the use of the LASIK technique is not possible in all patients. Those with thin or irregular corneas, or with certain abnormalities (scars, etc.) are better candidates for PKR. It is up to the surgeon to determine the best technique(s) to correct the patient's optical defect. When both techniques are possible, the choice may be that of the patient, and linked to personal preference or the practice of certain activities.
See : the PKR by steps (video)
Consult : the postoperative after PKR