Biomedical Engineering Reference
In-Depth Information
[57], while the stroma is almost transparent at this wavelength. ICG is a
frequently used ophthalmic dye, with a history of safety in humans. It is being
used increasingly as an intraocular tissue stain in cataract and vitreoretinal
surgery, as well as in staining of the retinal internal-limiting membrane [58-60].
Furthermore, ICG is commonly used as a chromophore in laser welding or
laser soldering [1], to induce differential absorption between the dyed region
and the surrounding tissue. Photothermal activation of stromal collagen is
thus induced by laser radiation only in the presence of ICG, resulting in a
selective welding effect, which produces an immediate sealing of the wound
edges and good mechanical strength. In addition, with the use of ICG, very low
laser power is required (below 100 mW), and this generally means much safer
operation with respect to the use of other laser types without the association
of dyes.
The procedure used to weld human corneal tissue is as follows: the chro-
mophore solution is placed inside the corneal cut, using an anterior chamber
cannula, in an attempt to stain the walls of the cut in depth. A bubble of air
is injected into the anterior chamber prior to the application of the staining
solution, so as to avoid perfusion of the dye. A few minutes after application,
the solution is washed out with abundant water. The stained walls of the
cut appear greenish, indicating that the concentration of ICG absorbed by
the stroma is much lower than that of the applied solution. Lastly, the whole
length of the cut is subjected to laser treatment. Laser energy is transferred
to the tissue in a noncontact configuration, through a 300-
m core diameter
fibre optic terminating in a hand piece, which enables easy handling under
a surgical microscope. A typical value of the laser energy density is about
13 W cm
−
2
in humans, which results in a good welding effect. During irradia-
tion, the fiber tip is kept at a working distance of about 1 mm, and at a small
angle with respect to the corneal surface (side irradiation technique). This par-
ticular position provides in-depth homogenous irradiation of the wound and
prevents from accidental irradiation of deeper ocular structures. The fiber tip
is continuously moved over the tissue to be welded, with an overall laser irra-
diation time of about 120 s for a 25-mm cut length (the typical perimeter of
a transplanted corneal button).
Experimental studies on laser-induced heating effects on ocular tissues
were performed both ex vivo on animal models [21] and in vivo during surgery.
An IR thermo-camera provided information on the heating of the external
surface of the irradiated tissues and on heat confinement, during the treat-
ment. A partial differential equation modeling of the process enabled us to
investigate temperature dynamics inside the tissue. From this study, it was
possible to point out that the optimal welding temperature is about 60
◦
C
inside the treated wound, i.e., in correspondence with a thermally induced
phase transition of stromal collagen. The heating effect was found to be se-
lectively localized within the cut, with no heat damage to the adjacent tis-
sue, and to induce a controlled welding of the stromal collagen. After laser
welding, collagen fibrils appeared differently oriented among themselves in
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