LASIK and corneal thickness
Thickness of the cornea and LASIK possibility
The thickness of the cornea is important in surgery LASIK, especially for short-sighted. Some patients, surgical candidates, are left out of the LASIK technique because "their cornea is too thin. Others may benefit from LASIK, even for a high myopia, because the cornea is thick enough. The technique of PKR (surface laser) is available to some ineligible myopic LASIK (low and medium myopia).
Legitimate questions arise from this type of verdict: why is the thickness of the cornea in some insufficient case? Is there a limit of thickness below which LASIK is impossible? To appreciate the role of corneal thickness in the choice of the refractive surgery technique, it is important to understand the mode of action of LASIK, and familiarize themselves with certain data about the biomechanical resistance of the cornea: this page is dedicated to this goal.
What is the thickness of the cornea?
The cornea is a generally hemispherical dome whose thickness decreases from the edges toward the Center. When we're interested in the thickness of the cornea, it is usually the thickness of its "central region".
We can measure the corneal thickness in different ways:
-corneal product: this is rather imprecise (it does measure the thickness that at a given point)
-Topographical maps in thickness (corneal tomography): these maps allow to analyse thickness corneal "point by point", and get a map "point by point", which notably allows to determine the value of minimum thickness and contact information.
-OCT cards, which provide the same information as the tomographic maps, and will in the near future to achieve specific maps of the epithelial thickness. OCT 'online' technique allows to measure the thickness of the cornea during LASIK surgery real-time: it equips the latest platforms (ex: Alcon Wavelight EX500)
Here is an example of map of thickness (CT) corneal:
In the population known as "caucaseienne", free corneal pathology, theminimum thickness of the cornea is close to an average 550 microns (see:) http://www.IOVs.org/content/51/11/5546/T2.expansion.html), with a standard deviation of 35 microns about.
Thus, close 70% of the myopic patients have a central corneal thickness of at least 515 microns, and only 2.5% of the population of 'healthy' myopic eyes have less than 480 microns thick (95% of the eyes of a representative sample of healthy eyes have a corneal thickness between the average more or less two standard deviations, is between 480 and 620 microns about (these figures are of course rounded).) These benchmarks are specific to the caucaseienne population; native populations of the Maghreb and the Indian continent have a corneal thickness lower than average. Patients with Keratoconus, even beginner (ex: forms frustrated), have a reduced average corneal thickness (525 microns).
The cornea is composed of several tissue layers, which are the three principal: (on the surface) epithelium, stroma and endothelium.
The thickness at the center of the corneal epithelium is understood between 40 and 60 microns (or nearly 10% of the total corneal thickness). Unlike the stroma, the epithelium is a purely cellular tissue, which has the ability to "push back" after debridement. That is why it would be unwise to issue the correction laser directly on the epithelium, because the printed effect would disappear in a few days as epithelial regrowth. The endothelium has a thickness of only a few microns. 90% of the thickness of the cornea is so filled by tissue stromal (or stroma) of the cornea, which is made up of collagen fibers intertwined.
What are these fibers which are subject to certain biomechanical constraints: intraocular fluid (aqueous) pressure exerted on the internal face of the cornea, so that its outer side is the contact of air and is under atmospheric pressure. In addition to the importance of the natural stiffness of the collagen fibers of the stroma of the cornea, the thickness of the "lattice" they are involved in maintaining the harmonious shape of this transparent fabric.
Correction of myopia and thickness of the cornea
The correction of myopia (excess of optical power of the eye) in LASIK involves a sculpture to sight refractive (laser photoablation), designed to change the corneal profile: reduction of the curvature of the cornea causes a reduction in the power of the cornea, which must be equal to preoperative myopia.
The characteristics of the profile of laser photoablation cause one photoablation maximum Center of the cornea. However, we have seen that the center of the cornea is naturally the finest region. By technique LASIK, the laser correction is issued under a previously realized flap (to the) femtosecond laser in modern centers) then recline, before be repositioned after the operation. The flap, whose thickness is chosen by the surgeon (around a value of 110 microns in general) is composed of a bilayer tissue: the epithelial layer on the surface, and the layer stromal shallow in depth.
To correct a dipotrie of myopia, thin the center of the cornea about 14 microns on a 6 mm optical zone (this value is approximate and depends on the corneal Asphericity initial and final target): http://www.IOVs.org/content/43/4/941/F3.expansion.html. During the programming of the correction in the excimer laser ablation maximum depth is displayed (ex: 70 microns for a correction of 5 diopters). Some lasers have tissue saving program: these programs are actually software changes designed to reduce tissue cost that was initially higher with these lasers to dictate what the laws of physics with first generation lasers. Modern lasers use by default the most fuel-efficient ablation strategies in tissue (corneal)to learn more about the strategy of economy of fabric / fabric saving claimed with the Zyoptix correction and compare "tissue cost" with some platforms lasers)
The thickness of the tissue removed is not the only one given position to determine the biomechanical impact of the laser photoablation. The volume occupied by the stromal to the flap contingent is relatively consistent, and often far exceeds the volume of cornea removed by laser excimer for the correction of myopia. This arises from the constant thickness of the edges stromal flap toward the Center, and its diameter (8-9 mm). In comparison, the lenticule of tissue removed by the excimer laser has a thickness that decreases to its edges, and a near 6 to 7 mm diameter.
We have established a simplified formula that provides the volume removed by a correction of myopia laser:
V = D (S/9) ^ 4 (V: in millimeter cube, D diopters, and S = diameter of the optical zone in mm). A diopter of myopia corrected on a 6 mm box "consumes" a volume of 0.20 stromal tissue about mm3. A LASIK flap in diameter 8.5 mm and a thickness of 120 microns (including 70 microns of stroma) represents in comparison a close volume 4 mm3 (equivalent to a correction of 20 diopters). Even if this flap is back in place, the fibers that make her volume no longer participate in the stability of the corneal Vault.
The rest of flap of LASIK is done on what is called the "residual stromal rear wall", whose thickness is equal to that of the cornea before the operation under the flap, and less of the photoablation. The predicted this posterior residual wall thickness value is an important factor in operative planning.
Residual wall posterior stromal
It is considered as the flap of LASIK, even rested, no longer significantly involved to the biomechanical strength properties the corneal dome after the operation. The posterior residual wall only helps maintain the rigidity and the curvature of the cornea.
It is logical and intuitive to apply that This rear wall must not be too thinto resist the constraints that represent intra ocular, to potential eye rubbing digital repeated pressure, etc.
Some experimental studies and clinical data accumulated over time suggest to actually maintain a sufficient residual thickness at the center of the cornea, to avoid that eventual permanent progressive deformation of it occurs: we call this (fortunately rare) complication thecorneal ectasia. This permanent deformation is a result of a phenomenon akin to a "creep". even if the permanent constraints suffered by the cornea (mainly related to the intra ocular pressure, which corresponds to the force exerted on a surface unit) does not reach the "yield strength" (stress from which a material's deforming in a reversible manner - elastic), she can start to deform irreversibly over time in the same way as a shelf bearing many books, can "bend" gradually.
There is a consensus among the community of refractive surgeons to consider that the Central wall thickness residual posterior stromal in LASIK must be at least equal to 250 microns (nearly half of the initial thickness of the corneal stroma). This value is empirical, and stems from observations of case of "decompensation" more or less late of the biomechanical resistance of the cornea for less than that value residual thicknesses (thirty years ago, a now abandoned technique was to voluntarily induce a 'ectasia' to correct hyperopia, by cutting a very thick flap, of the order of 350 microns or more) ", leaving a very thin residual wall). Some surgeons recommend a residual thickness of posterior wall of 300 microns, in particular for corneas with less than average initial thickness.
This video shows the creation of a constant monitoring of the product (Central corneal thickness measured by OCT 'online pachymetry' technology laser excimer Alcon Wavelight EX500) during LASIK surgery. The treatment of myopia of-6.75 D imposes Central removal of 99 microns deep. The initial thickness of the cornea at the Center's 530 microns. A posterior residual wall more than 300 microns is preferred for this eye: scheduled flap of LASIK with femtosecond FS200 laser thickness is 120 microns. The first measurement is done just after the creation of the interface with the femtosecond laser and found about 550 microns thick (this thickening is related to the diffusion of water in the interface during the creation of the flap). After the uprising of the flap, a new measurement is performed, until the photoablation is issued. At the end of it, and before the rest of the flap, a final measure of the residual rear wall is completed.
However, things would be too simple if the feasibility of LASIK depended only on the residual thickness of the posterior stromal wall... The lift of a shelf does not depend only on its thickness: the quality of the material put into play in its construction is obviously guarantor of its resistance to the weight carried. The same goes for the cornea: the thickness of the corneal wall isn't the only component to be considered to determine operability in LASIK : the intrinsic "quality" of the corneal tissue (stromal fibers, they even made up of collagen fibrils) is intuitively a given importance. Even if the calculation of the residual thickness of posterior stroma provides a reassuring figure, he must eliminate the presence of a biomechanical intrinsic fragility.
Thickness and biomechanical quality of the cornea
Keratoconus is an exemplary pathology to illustrate the importance of the integrity of the structural properties of the cornea. Any of the causes of Keratoconus remains unknown. Biomechanical measures (ex: Ocular Response Analyser) show an early modification of the biomechanical properties of the cornea in Keratoconus. the hysteresis, i.e. the ability to absorb the energy of a constraint as a jet of air pressure is reduced. This is related to a reduction in the 'strength' of the corneal tissue; Collagen fibers are disorganized, disunited, and lose their arrangement harmonious because they "slide" more easily on top of each other. This causes a progressive deformation spontaneous, responsible for a astigmatism irregular, and the resurgence of progressive myopia.
Keratoconus is a contraindication for LASIK, whatever the central cornea thickness (the thickness is usually reduced over the course of the disease). Them Beginner forms of Keratoconus must absolutely be tracked in the preoperative consultation, because thinning caused by a LASIK would reduce even the resistance of the corneal dome and would accelerate the strain of it, realizing the clinical picture of post LASIK ectasia. Corneal topography, supplemented by biomechanical measures (ORA), is systematic; It must include an analysis of the posterior side of the cornea, as well as its variations of thickness of the edges to the Center (CT). The use of modern testing indices is indicated (ex:) SCORE Analyzer)
It is important to note that the main risk factor for the ectasia is the presence of a form no diagnosed of Keratoconus, and no excessive thinning of the residual wall rear induction ! The appreciation of this point is complicated because the early forms of Keratoconus are usually associated with a lesser thickness of corneal tissue! However, many case of ectasia are seen on eyes which the posterior residual wall has a close or even superior to 300 microns thickness, but the retrospective analysis of the topographic maps obtained before the procedure reveals the presence of a beginner not tracked Keratoconus.
The indication of a LASIK depends on many parameters and respect for the contraindications of this technique.
The initial thickness of the central cornea imposes certain limits, since it must ensure that the degree of correction to carry out will not lead to a depth of ablation laser which, added to that of the flap, will leave a residual rear wall at least equal to 250 microns.
Even if this condition is satisfied, it is crucial to eliminate the presence of a fledgling form of Keratoconus (beginner infra clinical Keratoconus). Regardless of the corneal thickness and the degree of correction, need not perform LASIK, and possibly direct the patient to a surface technique (PKR), without cutting of flap. The fact that the corneal ectasia is essentially observed after LASIK (the case of ectasia after PKR are exceptional in comparison) accredits the assumption that the cutting of the flap is an important causal factor (remember that the volume of the flap exceeds the cornea removed by the laser to correct nearsightedness).
In case thin cornea (ex 510 microns), and unscathed from a beginner Keratoconus type pathology, it is not possible to correct as many short-sightedness in thick cornea case (580 microns).
For example, if a myopia of 6 diopters, corrected on an area of 6 mm photoablation implies a central tissue ablation of 85 Micron and to maintain a wall of residual stromal thickness of 300 microns, with a LASIK flap of 120 microns, it will take that preoperative corneal Central thickness is at least 505 microns. The thickness of the flap is today determined with precision with femtosecond lasers: Unfortunately, the mechanical microkeratomes, still used today in some centres, are less accurate and sometimes induce cuts of greater than expected thickness (ex: 170 microns instead of 130 microns).