It will probably also include evidence that the anterior cornea is stronger than the posterior.
Example:
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LASIK studies do not report flaps of less than 160 ?m, which means that the flap always includes the most stable part of the cornea and is connected at the temporal side of the cornea by a small hinge. In a histological study in humans in which LASIK was performed before enucleation of one eye it was demonstrated that mainly fibronectin and tenascin were formed at the border between flap and stroma. These adhesive glycoproteins are present between epithelium and Bowman's layer and can easily be disrupted to perform additional laser treatments. There is no need to wonder why various patients suffer from wrinkles within the flap and epithelial ingrowth.
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PRK is different from LASIK because a great part of the most anterior stroma is ablated. Bowman's layer does not recover and irregularities on the surface are compensated by ingrowth of epithelial cells (personal observation). Interference with the most tightly interwoven part of the cornea may result in visual problems. A significant proportion of treated patients may show refractive regression, haze, or astigmatism after both types of treatment. Astigmatism is a clinical complication related to irregularities in the corneal curvature. Formation of these irregularities may be due to a reduction in cohesiveness of the collagen bundles in the central corneal stroma. Our results indicate that the most rigid part of the stroma is ablated (PRK) or intersected (LASIK), thus weakening the stability of the cornea. This emphasises that people who underwent refractive surgery may have an increased risk of optical problems.
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The biomechanical strength of the posterior stroma is less than that of the anterior stroma. Because the load-bearing function of the anterior stroma is disabled after keratotomy, only the weaker deep stroma is left to maintain corneal integrity.
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Increased severing of corneal lamellae Dr Jaycock noted that the likely cause of the increased biomechanical instability following LASIK compared to PRK was the increased number of collagen lamellae that
are severed in the intrastromal procedure. Collagen lamellae are more densely interwoven in the superficial third of the stroma than in the deeper two thirds. In addition, X-ray diffraction studies indicate that collagen fibres cross perpendicularly in the centre and cross increasingly obliquely towards the periphery of the cornea. (Meek et al, Exp Eye Res;2004:78;503-512). Thus the cornea is stronger anteriorly than
posteriorly and stronger peripherally than centrally. In a 6.0 dioptre PRK correction, approximately five million collagen fibres are severed, whereas for a corresponding LASIK procedure, 230 million fibres are
severed, Dr Jaycock pointed out. ?It is somewhat unfortunate that the standard microkeratome flap incision severs the cornea at the strongest part in both the antero-posterior and radial planes. PRK and LASIK clearly have very different postoperative implications for the structural integrity of the cornea,? Dr Jaycock added.
http://www.escrs.org/PUBLICATIONS/EUROT ... nsflap.pdfQuote:
All the studies in the published literature tell us that the anterior one-third of the cornea is exceptionally strong. In this part, the collagen fibres are interwoven and it is very difficult to pull them apart. There is a lot of biomechanical strength there. The deeper two-thirds of the cornea, by contrast, are actually weak.
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Explaining the implications of this, Dr. Marshall said that when a 6 dioptre PRK or LASEK correction is carried out, around five million supporting collagen fibrils are severed. This increases 40-fold in a LASIK procedure to over 230 million collagen fibrils. He added that lowering the flap in a LASIK procedure will not restore the biomechanical integrity of the cornea.
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"We can see that with some of the early LASIK flap depths of around 140 to 160 microns we are taking between one quarter and one third of the strength of the cornea away. It doesn?t matter at what point you get wound healing ? this tensile strength will never come back,? he said.
Anterior Segment OCT Analysis of Thin IntraLase Femtosecond Flaps
Journal of Refractive Surgery Vol. 23 No. 6 June 2007
Jason E. Stahl, MD; Daniel S. Durrie, MD; Frank J. Schwendeman, OD; Allen J. Boghossian, DO
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The importance of corneal biomechanics in refractive surgery outcomes has become apparent with recent publications.11,12 The corneal stroma consists of lamellae (organized collagen fibers), which run from limbus to limbus. Traditional LASIK, using a mechanical microkeratome, creates a fl ap approximately 160 μm thick, which severs a significant number of collagen fibers compared to PRK. The loss of lamellar integrity following LASIK results in compromised corneal biomechanical integrity due to minimal biomechanical loading distributed throughout the flap. Hence, there is no contribution from the flap to the biomechanical stability of the cornea.1 Cohesive tensile strength studies demonstrate that Bowman?s layer is the strongest structural component of the cornea followed by the anterior third of the corneal stroma.1,13 In fact, the peripheral anterior third of the corneal stroma is stronger than the paracentral and central anterior third.12 These findings are supported by morphologic studies that demonstrate more collagen lamellar interweaving and collagen lamellae orientations that were transverse to the anterior surface of the cornea.14-16 These studies suggest that a thin, uniform flap would leave more of the strong anterior stroma untouched, which should provide greater corneal biomechanical strength than the thicker traditional LASIK flap that severs more of these strong anterior fibers.
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In addition, we speculate that flaps made deep to this level (traditional LASIK), in the weaker posterior cornea where the lamellae lie more parallel and less compact, create weaker corneal biomechanics.
http://www.lasikflap.com/forum/viewtopic.php?t=628
