Cohesive tensile strength of human lasik wounds

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

The purpose of the current study was to QUANTIFY the strength of the LASIK interface.

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

Five age-matched and time-in-preservation-matched

normal corneoscleral SPECIMENS from 5 patients served as controls

Diagram shows how the LASIK corneoscleral specimenswere sectioned to obtain 4-mm-wide

STRIPS and then subsequently processed.

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.

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.

THE MOTORIZED PULLING ARM with a central 4-mm LASIK corneal strip (white asterisk) in

place. THE FORCE REQUIRED FOR SEPARATION IS RECORDED

5 years after surgery

TIME

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)

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

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)

• Keratocytes COMBINE TO FORM A CELLULAR NETWORK WHICH CREATES THE collagen and proteoglycans OF AN EXTRACELLULAR NETWORK

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

Proteoglycans are space fillers that hold the water that resists compression

(NOTE: DOES NOT CONTRIBUTE TO TENSILE STRENGTH)


LASIK central/paracentral regions contain a hypocellular/ hypokeratocytic scar consisting of proteoglycan. (WEAK TENSILE STRENGTH)

BACK TO LASIK STUDY

LASIK wound margins consist of an irregular network of collagen fibrils, many interspersed keratocytes, and occasional myofibroblasts.

(GREATER TENSILE STRENGTH)

BACK TO LASIK STUDY

• IN LASIK WOUNDS THE OLD CELLULAR NETWORK IS DIMINISHED AND THEREFORE THE OLD EXTRACELLULAR NETWORK IS DIMINISHED AS WELL.

BACK TO LASIK STUDY

In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)

Between black arrows = surface of hypercellular fibrotic stromal scar. Between arrowheads = surface of hypocellular primitive stromal scar.

In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)

CentralControl

In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER TENSILE STRENGTH)

Peripheral Control


Central mechanical

flap


Peripheral mechanical

bed


Retrospective Review• 144 pts’ FLAPS LIFTED ~1yr after LASIK

• PURELY QUALITATIVE RESULTS

• the flap removed with minimal resistance regardless of the postoperative time

Conclusion

NORMAL CORNEAS HAVE HIGH TENSILE STRENGTH DUE TO THEIR COLLAGEN NETWORK. LASIK CORNEAS ARE WEAKER BECAUSE THEY HAVE A DIMINISHED COLLAGEN NETWORK.

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

Confocal Microscopy and HistopathologicalExamination of Diffuse Lamellar Keratitis

in an Experimental Animal Model

• a keratocyte-free layer corresponding to the interface (arrows).

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,

Cohesive tensile strength of human lasik wounds

Health & Medicine

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