cohesive tensile strength of human lasik wounds
TRANSCRIPT
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Cohesive Tensile Strength of Human LASIK Wounds With
Histologic, Ultrastructural, and Clinical Correlations
Ingo Schmack, MD; Daniel G. Dawson, MD; Bernard E. McCarey, PhD;
George O. Waring III, MD, FACS, FRCOphth; Hans E. Grossniklaus, MD; Henry F. Edelhauser, PhD
Northeastern State University04/17/13
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The purpose of the current study was to QUANTIFY the strength of the LASIK interface.
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25 corneoscleral
SPECIMENS from 13 donors with a history of LASIK (23 mechanical microkeratome, 2 laser microkeratome) were obtained from various eye banks in North America.
Avg Age: 44 years
Avg Time after LASIK: 3 years
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Five age-matched and time-in-preservation-matched
normal corneoscleral SPECIMENS from 5 patients served as controls
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Diagram shows how the LASIK corneoscleral specimenswere sectioned to obtain 4-mm-wide
STRIPS and then subsequently processed.
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Cross-sectional diagram of THE MIDDLE STRIP in (A) showing how the manual lamellar dissection was connected to the LASIK interface wound at the hinge. C = central wound, P = paracentral wound, M = peripheral flap wound margin.
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Diagram demonstrates HOW THE FLAP AND RESIDUAL STROMAL BED WERE SEPARATED using the motorized pulling device. Blue lines = LASIK wound, red lines = manual lamellar dissection, red dots = point where blunt tipped cannula was inserted.
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THE MOTORIZED PULLING ARM with a central 4-mm LASIK corneal strip (white asterisk) in
place. THE FORCE REQUIRED FOR SEPARATION IS RECORDED
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5 years after surgery
TIME
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Symbol Key: 1. Square (at arrow) = mean of 5 control corneas; 2. LASIK wound margin hypercellular scar (black solid circles); 3. Central/paracentral hypocellular LASIK scar (open circles)
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The cohesive tensile strength of these three groups were found to be different from each other in a statistically significant way.
3. LASIK paracentral & central regions
2. LASIK wound margin, which plateaus in strength after 3.5 yrs
1. Normal cornea
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A GUIDE TO INTERPRETATION OF RESULTS
Microscopy• Light (hypercellularity = greater tensile strength) (>
EVIDENCE IN ANOTHER STUDY)• Electron (Transmission and Scanning) (greater
protruding collagen fibrils = greater tensile strength) (GESTALT IMPRESSION OF SMOOTHNESS)
Retrospective Review • 144 eyes that received a LASIK flap-lift retreatment
(easier the flap lift = less tensile strength)
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• Keratocytes COMBINE TO FORM A CELLULAR NETWORK WHICH CREATES THE collagen and proteoglycans OF AN EXTRACELLULAR NETWORK
CORNEAL ANATOMY PHYSIOLOGY BACKGROUND INFORMATION
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CORNEAL ANATOMY PHYSIOLOGY BACKGROUND INFORMATION
Collagen resists FORCES THAT PULL THE TISSUE APART. (NOTE REGULARITY and DIRECTIONALITY)
Krachmer Chpt 1: Electron microscopy of the corneal stroma showing lamellar structure of collagen fibers
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Proteoglycans are space fillers that hold the water that resists compression
(NOTE: DOES NOT CONTRIBUTE TO TENSILE STRENGTH)
CORNEAL ANATOMY PHYSIOLOGY BACKGROUND INFORMATION
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LASIK central/paracentral regions contain a hypocellular/ hypokeratocytic scar consisting of proteoglycan. (WEAK TENSILE STRENGTH)
BACK TO LASIK STUDY
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LASIK wound margins consist of an irregular network of collagen fibrils, many interspersed keratocytes, and occasional myofibroblasts.
(GREATER TENSILE STRENGTH)
BACK TO LASIK STUDY
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• IN LASIK WOUNDS THE OLD CELLULAR NETWORK IS DIMINISHED AND THEREFORE THE OLD EXTRACELLULAR NETWORK IS DIMINISHED AS WELL.
BACK TO LASIK STUDY
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In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)
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Between black arrows = surface of hypercellular fibrotic stromal scar. Between arrowheads = surface of hypocellular primitive stromal scar.
In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)
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Between black arrows = surface of hypercellular fibrotic stromal scar. Between arrowheads = surface of hypocellular primitive stromal scar.
In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)
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CentralControl
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER TENSILE STRENGTH)
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Peripheral Control
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER TENSILE STRENGTH)
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Central mechanical
flap
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER TENSILE STRENGTH)
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Peripheral mechanical
bed
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER TENSILE STRENGTH)
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Retrospective Review• 144 pts’ FLAPS LIFTED ~1yr after LASIK
• PURELY QUALITATIVE RESULTS
• the flap removed with minimal resistance regardless of the postoperative time
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Conclusion
NORMAL CORNEAS HAVE HIGH TENSILE STRENGTH DUE TO THEIR COLLAGEN NETWORK. LASIK CORNEAS ARE WEAKER BECAUSE THEY HAVE A DIMINISHED COLLAGEN NETWORK.
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ConclusionQUANTITATIVELY CENTRAL
PARACENTRAL LASIK CORNEAS HAVE ONLY 2.4% OF THE TENSILE STRENGTH OF NORMAL CORNEAS AND STAY THAT WAY FOR AT LEAST 6.5 YEARS
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Confocal Microscopy and HistopathologicalExamination of Diffuse Lamellar Keratitis
in an Experimental Animal Model
• a keratocyte-free layer corresponding to the interface (arrows).
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Interface Corneal Edema Secondary to Steroid-induced Elevation of Intraocular Pressure Simulating Diffuse
Lamellar Keratitis
LASIK creates a lifelong lamellar corneal potential space.
This potential space could be a site for: 1. fluid accumulation,
2. inflammation, or 3. infection,