Biomedical Engineering Reference
In-Depth Information
intensities are achieved). Second, the multiphoton absorption coe
cient has
a high value when compared with the linear absorption value: the disruptive
effect may be achieved with very low intensity levels (
Jcm
−
2
or nJ cm
−
2
).
These nonlinear effects have been used to ablate and to modify corneal tissue,
with very high spatial precision and minimal side effects.
Currently, FS laser cutting cells are characterized by high numerical aper-
tures (NA) (
>
0.9 in water and glass) that make possible submicrometrical
precision. Long series of pulses from FS lasers at 80 MHz repetition rate im-
ply accumulative effects, which may cause tissue ablation at pulse energy be-
low the optical breakdown threshold in presence of low density plasmas [64].
The advantages are that nonlinear propagation effects are reduced, highly lo-
calized energy deposition occurs, and subsequently nanosurgery on a cellular
and subcellular level is possible. Moreover, the optical breakdown threshold
weakly depends on the target absorption coe
cient. Thus, any arbitrary cel-
lular structure may be manipulated. For these reasons, near-infrared FS lasers
have been considered the innovation for overcoming problems associated with
the use of UV nanosecond lasers in refractive surgery: UV mutation effects
on cells, low light penetration depth, collateral damage outside the focal vol-
ume, the risk of photon-damage to living cells due to absorption, and prob-
able induction of oxidative stress leading to apoptosis. Moreover, a focused
nanosecond-pulsed laser beam can cause thermal damage and denaturation
around the laser focus.
Typically, the systems proposed for performing corneal manipulation
against refractive problems are based on mode-locked diode-pumped FS
Nd:Glass lasers providing pulses of 500-800 fs duration and a few
µ
Jenergy,
at repetition rates of some tens of kilohertz. Each individual laser pulse is fo-
cused on a specific location inside the cornea, which is fully transparent at the
laser wavelength. A micro-plasma is created, and this generates a microcav-
itation bubble of 5-15
µ
m in diameter, which separates the corneal lamellae.
Thus, a resection plane can be created by delivering, in a prescribed pattern,
thousands of laser pulses connected together. The cut can be performed with
micrometrical precision at different depths inside the stroma, thus allowing
for corneal flaps with a preset, constant thickness. Laser pulses can also be
stacked on the top of each other, to create a vertical or angled cleavage plane
to precisely sculpture the border of the lamellar flap. The possibility of per-
forming the same resection procedure on the donor cornea as well as on the
patient's recipient eye, allows to match the transplanted flap precisely with
the recipient corneal bed.
By exploiting previous clinical experiences in diode laser welding of corneal
wounds, some of us have recently designed a new laser-assisted technique
for lamellar keratoplasty (i.e., a corneal transplant involving replacement of
only the anterior corneal stroma). It is performed by using a FS laser to
prepare donor button and recipient corneal bed, and then suturing the edges
of the wound by means of diode laser-induced corneal welding, without the
application of conventional suture material. This minimally invasive procedure
µ
