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N. E. Knox Cartwright, J. R. Tyrer, P. D. Jaycock, J. Marshall; Strain in the Human Cornea: The Effect of Surgical Incisions and Change During Wound Healing. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3483.
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Longterm follow up studies of refractive surgery have shown that there is a trend towards refractive instability following LASIK. In a minority of cases ectasia can be induced. In contrast, surface procedures are biomechanically stable but result in a higher incidence of corneal haze. This study aimed to determine the effect of intrastromal incisions on the biomechanical integrity of the human cornea and establish the changes that occur during wound healing.
Human eyebank corneas were mounted in artificial anterior chambers. The corneas were divided into six groups (Table 1) and treated with a femtosecond laser (Intralase Corp, CA, USA). A radial shearing speckle interferometer was used to measure corneal strain before and after treatment and again following three weeks in organ culture. This instrument records whole-field, non-contact, real-time measurements of the change in out of plane corneal strain. In these experiments the corneas were stressed by a 0.75 mmHg increase in intraocular pressure above a baseline of 15 mmHg.
All incision types resulted in an increase in strain. At a given depth subsurface incisions resulted in the least and flap formation the greatest weakening. There was no significant recovery during wound healing. Incisions at 80 µm had less of a biomechanical effect (14 % reduction in strength) than incisions at 160 µm (33 %).
All types of incision weakened the cornea, deeper incisions more so than superficial incisions and incisions with side cuts more so than subsurface incisions. The reduction in strength appeared to correlate with the number of collagen lamellae severed and persisted during wound healing. It is likely that the greater reduction strength caused by deeper incisions results in the greater incidence of biomechanical instability and ectasia following LASIK compared to surface procedures. To maintain the optical advantages of LASIK but minimise post-operative corneal weakening the shallowest possible intrastromal flap should be created.
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