J Cataract Refract Surg. 2005 Jan;31(1):175-84.
Interferometric technique to measure biomechanical changes in the cornea induced by refractive surgery.
Jaycock PD, Lobo L, Ibrahim J, Tyrer J, Marshall J.
Department of Ophthalmology, Rayne Institute, St. Thomas' Hospital, London, United Kingdom.
philipjaycock@hotmail.com
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Given that the corneal stroma consists of lamellae thought to run from limbus to limbus across the corneal arc and that the lamellae consist of organized collagen fibers, loss of lamellar integrity may compromise corneal strength. A significant number of collagen fibers are severed in LASIK compared with a corresponding PRK procedure.4 Hence, the microkeratome flap in LASIK uncouples a significant proportion of the corneal biomechanics, which may affect refractive stability. With the significant reduction in the biomechanical integrity of the cornea, at worst LASIK has the risk for inducing iatrogenic keratectasia5, 6, 7, 8, 9, 10, 11 and at best, a propensity for long-term instability.
Although the incidence of iatrogenic keratectasia appears low at present, LASIK has only been used in clinical practice in recent years and the long-term results are unknown. Despite this potentially serious complication, the popularity of LASIK is growing at a fast pace and more than 5.5 million excimer laser refractive procedures have been performed. Although iatrogenic keratectasia is most common in eyes in which thin beds have been left, cases have been observed when the corneal bed after surgery is thicker than 250 μm, which is regarded by some as an arbitrary safe minimum corneal thickness.
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When the microkeratome flap was replaced to cover the stromal bed, the out-of-plane surface movement was of the same order as the movement when the flap was removed, exposing the stromal bed. This indicates that although tissue bulk had been restored, the biomechanical properties remained altered relative to preoperatively because of uncoupling of the collagen fibril array.
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The study showed that measurable changes occur in corneal displacement subsequent to microkeratome incisions and such changes are hardly influenced by relocation of the flap. Furthermore, such variations were measured with pressure changes of 0.15 mm Hg (20 Pa), corresponding to a 1% change in IOP. The apparent small difference in out-of-plane displacement (0.3 μm) between the operated and unoperated eyes should be considered in relation to the precision of the measuring technique; that is, 0.01 μm. Thus, these are real effects and should be of concern to the ophthalmic community.
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Our results can be summarized as showing (1) a measurable change in out-of-plane forward movement or ?bulging? of the cornea with increasing pressure and (2) a region of disharmony coincident with the plane of the microkeratome transsection.
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In contrast, biomechanical considerations would predict instability with LASIK. This concept is derived from the finding that no repair transgresses the plane of the microkeratome incision and integrity in this region is established by deposition of ground substances such as fibronectin and tenascin. It is further supported by histological observation of corneas that have keratoplasty after LASIK. In such samples, although the bed remains intact, the collagen fibers within the flap show disorganization and atrophy. This finding supports the concept of reduced strain in the flap because of uncoupling from the stromal bed and atrophy of the fibers tectonically isolated from biomechanical movements engendered by processes such as accommodation. It is unfortunate that few studies have been published on the long-term effects of LASIK. The 1 study that reports 6-year data gives graphic information that is contrary to written claims in that there is a trend toward regression with time. If this trend becomes significant, it further emphasizes the need for a better understanding of the biomechanics of the cornea.32
