The study aimed to quantify the tensile strength of the LASIK interface by separating the flap and stromal bed in 25 human corneas that underwent LASIK. The central and paracentral LASIK regions had significantly lower tensile strength than the wound margins or normal corneas. In LASIK wounds, the old collagen network is diminished due to loss of keratocytes and their extracellular matrix, resulting in long-term weakening of the interface. The wound margins exhibited the highest strength due to hypercellular scarring.

1. 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 University
04/17/13

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

3. 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:

4. Five age-matched and time-in-preservation-matched
normal corneoscleral SPECIMENS from 5 patients
served as controls

5. Diagram shows how
the LASIK
corneoscleral
specimens
were sectioned to
obtain 4-mm-wide
STRIPS
and then
subsequently
processed.

6. 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.

7. 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.

8. THE MOTORIZED
PULLING ARM
with a central 4-mm
LASIK corneal strip
(white asterisk) in
place. THE FORCE
REQUIRED FOR
SEPARATION IS
RECORDED

9. 5 years after surgery
TIME

10. 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)

11. 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

12. 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)

13. • Keratocytes COMBINE TO FORM A CELLULAR
NETWORK WHICH CREATES THE collagen and
proteoglycans OF AN EXTRACELLULAR
NETWORK
CORNEAL ANATOMY PHYSIOLOGY
BACKGROUND INFORMATION

14. 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

15. Proteoglycans are space fillers that
hold the water that resists
compression
(NOTE: DOES NOT CONTRIBUTE TO
TENSILE STRENGTH)
CORNEAL ANATOMY PHYSIOLOGY
BACKGROUND INFORMATION

16. LASIK
central/paracentral
regions contain a
hypocellular/
hypokeratocytic scar
consisting of
proteoglycan. (WEAK
BACK TO LASIK STUDY

17. LASIK wound margins consist
of an irregular network of
collagen fibrils, many
interspersed keratocytes,
and occasional
myofibroblasts.
(GREATER TENSILE
BACK TO LASIK STUDY

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

19. In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)

20. Between black arrows = surface of hypercellular
fibrotic stromal scar. Between arrowheads = surface
of hypocellular primitive stromal scar.
In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)

21. Between black arrows = surface of hypercellular
fibrotic stromal scar. Between arrowheads = surface
of hypocellular primitive stromal scar.
In light microscopy (HYPERCELLULARITY = ↑TENSILE STRENGTH)

22. Central
Control
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER
TENSILE STRENGTH)

23. Peripheral
Control
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER
TENSILE STRENGTH)

24. Central
mechanical
flap
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER
TENSILE STRENGTH)

25. Peripheral
mechanical
bed
In electron microscopy = (GREATER PROTRUDING COLLAGEN FIBRILS = GREATER
TENSILE STRENGTH)

26. Retrospective Review
• 144 pts’ FLAPS LIFTED ~1yr after LASIK
• PURELY QUALITATIVE
RESULTS
•the flap removed with
minimal resistance
regardless of the
postoperative time

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

28. Conclusion
QUANTITATIVELY 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

29. Confocal Microscopy and Histopathological
Examination of Diffuse Lamellar Keratitis
in an Experimental Animal Model
• a keratocyte-free layer corresponding to the
interface (arrows).

30. 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