Why should we remove contact lenses for several days before the consultation for refractive surgery?
Contact lenses are in contact with the cornea, which is operated in tissue LASIK or PKR. Regardless of the correction they provide, these lenses have a geometry (diameter, thickness), and a special "stiffness" (eg: soft lens, semi-rigid lens, hard lens, etc.).
The permanent contact with the anterior face of the cornea (epithelial face), the friction induced by the blinking induce with time a modification of the geometry of the cornea; We talk about the distortion induced by contact lenses (' Corneal warpage ' in English). The term "contact lens-induced deformation" therefore refers to the transient changes in the corneal topography induced by contact lenses. These modifications are more frequent after the prolonged use of rigid gas-permeable contact lenses. The topography corneal allows to highlight these deformations, which affect the anterior face of the cornea (interest of the topography Placido). The majority of the lens-induced deformations concern the corneal epithelium, which is "remodeled" according to the mechanical stresses induced by the lens. For example, the epithelium tends to be thinner in relation to areas with strong support.
It interesting to note that theOrthokeratology, which consists in making a (transient) correction of myopia by adapting rigid lenses that are worn only at night during sleep, is based on the induction of a "warping" characterized by a central flattening (allowing to correct the Low myopia and some of the average myopia). When the port of the lenses is interrupted, the corrective effect disappears in 36 to 48h, but the realization of a more remote corneal topography often reveals the presence of residual corneal deformation.
Topographical aspects of the corneal warpage (corneal warpage):
Topographic anomalies may consist of a astigmatism irregular Central, the reduction of the negative corneal Asphericity (reduction of the prolate Asphericity), an appearance in 'asymmetric bow' ('bow - tie asymmetry') or localized areas of camber. Rigid lenses usually produce a higher kurtosis and a relative lower camber due to the flattening of the underlying cornea to lens which is often "hung" or "lifted" in superior by the upper eyelid.
It is important to distinguish between transient abnormalities induced by lenses and permanent anomalies likely to alter the topography of the cornea. Here are the reasons:
-a false appearance of beginner Keratoconus can be induced by the lenses (asymmetry)
-deformation induced by the lenses can hide, or at least blur one aspect of Keratoconus beginner (risk of ectasia if LASIK)
-a deformation of the cornea can alter the optical correction: in general, the rigid lenses induce central flattening, and this flattening results in a reduction of the optical power of the cornea. Native myopia is partially reduced, and this causes a risk of slot correction if one takes measurements while the corneal deformation remains.
Note that this property of 'deformability' of the front of the cornea is put to good use by the technique of Orthokeratology, adapting rigid lenses whose geometry is designed to induce a "crushing" of the central part of the anterior cornea. This technique is little practiced in France, and was mainly developed by Anglo-Saxon optometrists. These lenses are worn overnight, and withdrawn wake up: central corneal flattening a few hours during which a myopia understood between - 1 and-3.5O D can be temporarily corrected.
Type of lens and corneal deformation duration:
Wang et al. (2002) prospectively studied the eyes of 165 consecutive patients with lenses evaluated for refractive surgery. A significant corneal deformation induced by contact lenses was detected by corneal topography in 20 eyes of 11 patients (12%). In patients with contact lens-induced deformation, the mean anterior port duration of lentils was 21.2 years (extremes: 10-30 years). The average recovery time before refraction stabilization, kératométriques values, and topographic profile was 7.8 ± 6.7 weeks. Recovery rates were different depending on the type of lens: extended-port flexible lenses 11.6 ± 8.5 weeks, soft-o-rings 5.5 ± 4.9 weeks, daily port soft lenses 2.5 ± 2.1 weeks and rigid lenses permeable to Gas 8.8 ± 6.8 weeks. This study showed the need for repeated refractions, kératométries, and documented stable contact topographies before planning refractive surgery in patients with suspicion of corneal deformation.
When a patient consults for refractive surgery (e.g., myopia surgery), the topographic assessment should ideally be performed at least a few days to 1 week after the port of soft lenses is stopped, although in most case modifications related to the wearing of soft lenses are very minor and do not interfere very significantly with measured visual correction and corneal topography.
After stopping the wearing of rigid lenses, it is recommended to wait longer, i.e. at least 3 weeks to a month before proceeding with the evaluation. If corneal anomalies are detected by the topography, a monthly assessment should be carried out until the topographic profile is stabilised. At least two topographies with the same profile are recommended before planning a refractive surgery: this is because it is necessary to ensure a return to the "stable" state of the cornea to confirm the operability (topographic normality) and to ensure To make a suitable correction.
Reference: Wang X, what JP, Bowman RW, HD Cavanagh. Time to resolution of contact lens-induced corneal warpage prior to refractive surgery. OALCF J. 2002; 28 (4): 169-71.