Laser Tissue Welding

1. Laser Tissue Welding
P. Ravikumar

2. Background Principles and Theory
• Lasers have been demonstrated to be extremely
useful in many medical and surgical applications
as both diagnostic and therapeutic tools.
• The use of laser energy to induce thermal
changes in connective-tissue proteins is of
particular interest for the joining of severed
tissue, where proteins within the target tissue are
coagulated to form a bond between the two
adjoining edges.

3. Conventional Closure Methods
• The conventional methods for tissue repair use sutures,
staples, or clips.
• Sutures are favored because they are cost-effective,
reliable, and, more importantly, suitable for almost any
type of tissue.
• However, due to their mechanical intrusion, the use of any
of these conventional fasteners causes tissue injury.
• Tissue injury and foreign-body reaction can give rise to
inflammation (nonspecific immune response of tissue to
injury resulting in redness and swelling), granuloma
formation (development of a grain-like tissue lesion),
scarring (permanent marking of tissue), and stenosis
(abnormal narrowing or constriction of tissue lumens).

4. Conventional Closure Methods
• Sutures become difficult or tedious to execute in
microsurgical or minimally invasive endoscopic
applications, where staples or clips are better
suited.
• Staples and clips are not easily adapted to
different tissue dimensions, however, and
maintaining precision of alignment of the tissue is
difficult because of the relatively large force
required to deploy them.
• Finally, none of these fasteners produces a
watertight seal over the repair.

5. Other Closure Methods
• Besides sutures, staples, and clips, other closure techniques,
including both biological and nonbiological adhesives as well as
various coupling devices, have been tested.
• Fibrin glue has long been used as a biological surgical adhesive.
• Fibrin glue imitates the final step of blood coagulation and thus
has been used effectively for hemostasis. However, repairs formed
with fibrin glue alone are typically weak in comparison with
suture repairs.
• Consequently, the glue is almost always used in conjunction with
stay sutures.
• Among the various synthetic adhesives available, cyanoacrylates
are the most popular.
• However, they are toxic to the tissues, not absorbed in the normal
wound healing process, and cause foreignbody granulomas and
allergic reactions.

6. Development of Laser Tissue Welding
• Wide range of lasers have been used for laser tissue welding.
• Infrared sources include carbon dioxide (CO2), thulium-
holmium-chromium, holmium, thulium, and neodymium rare-
earth doped garnets (THC:YAG, Ho:YAG, Tm:YAG, and Nd:YAG,
respectively), and gallium aluminum arsenide diode (GaAlAs)
lasers.
• Visible sources include potassium-titanyl-phosphate (KTP)
frequency-doubled Nd:YAG, and argon lasers.
• The laser energy is absorbed by water at the infrared
wavelengths and by hemoglobin and melanin at the visible
wavelengths, thereby heating proteins within the target tissue.
• As temperatures rise and heating times are prolonged, cellular
and tissue structural proteins undergo denaturation and
conformational changes, a process defined as coagulation

8. Benefits of Laser Tissue Welding
• While laser tissue welding is unlikely to replace sutures in all
applications, it has been shown to achieve functionality comparable
to that of conventional suturing techniques, with the added
advantage of moderately reduced operation times, reduced skill
requirements, reduced suture and needle trauma, reduced foreign-
body reaction and reduced bleeding.
• Repairs formed using laser tissue welding tend to heal faster, have
the ability to grow, and exhibit better cosmetic appearances.
• Welding also has the potential to form complete closures, enabling
an immediate watertight anastomosis intraoperatively in the case
of vascular, genitourinary-tract, and gastrointestinal repairs.
• A watertight closure is also advantageous for neural repairs as it
discourages the exit of regenerating axons and the entry of
fibroblasts

9. Limitations of Laser Tissue Welding
• Two disadvantages of the laser assisted
procedure
– The low strength of the resulting anastomosis,
especially in the acute-healing phase up to 5 d
postoperative
– Thermal damage of tissue by direct laser heating
and heat transfer.
• Technical difficulties with the ambiguity of the
endpoint for the procedure, tissue apposition,
and poor reproducibility are also concerns.

10. • Different steps of laser-assisted microvascular anastomosis
technique using the diode laser (λ = 1.9 μm, spot diameter =
500 μm, P = 125 mW, t = 1.2 sec: 10 to 15 spots on each face)
for digital replantation in humans.
• laser anastomosis of blood vessels, the genitourinary tract,
and the gastrointestinal tract includes the possibility of
stenosis at the irradiated sites.
• Dissolvable stents have been used to successfully separate the
anterior and posterior walls of such tissues, allowing for an
easier and faster welding procedure

11. Photochemical Welding
• Photochemical welding of tissue has been investigated as an
alternative method for tissue repair without the use of heat
and its associated tissue damage.
• The technique utilizes chemical cross-linking agents that,
when light-activated, produce covalent cross-links between
the collagen fibers contained within the tissue.
• In theory, this technique should produce stronger bonds than
the noncovalent bonds produced by photothermal welding.
• Agents used for photochemical welding include 1,8-
naphthalimide, rose bengal (RB), riboflavin-5-phosphate (R-5-
P), fluorescein (Fl), methylene blue (MB), and N-
hydroxypyridine-2-(1H)-thione (N-HPT).

13. Laser Tissue Soldering
• Two advances have been useful in addressing
the issues of low repair strength and thermal
damage associated with laser tissue welding:
– (1) the addition of endogenous and exogenous
materials to be used as solders and
– (2) the application of laser-wavelength-specific
chromophores.

14. Laser Tissue Soldering
• The addition of endogenous and exogenous
materials helps to maintain edge alignment
and to strengthen the wound, particularly
during the acute postoperative healing phase,
while shielding the underlying tissue from
excessive thermal damage caused by direct
absorption of the laser light.
• Useful materials include blood, cryoprecipitate
(typically plasma), fibrinogen and albumin.

15. Laser Tissue Soldering
• The application of wavelength-specific chromophores provides for
differential absorption between the dyed region and the surrounding
tissue.
• An advantage of this technique is that the area may be bathed by the
laser radiation while energy is absorbed selectively only by the target.
• Hence the requirement for precise focusing and aiming of the laser
beam may be removed
• Lower laser irradiances may be used to achieve the required effect,
increasing the safety of the technique.
• Examples of dyes that have been used to assist laser tissue-welding
procedures include carbon black and Fen 6 for use with Nd:YAG lasers,
indocyanine green (ICG) for use with ~800-nm diode lasers, iron oxide
and fluorescein for use with KTP frequency-doubled Nd:YAG lasers,
and basic fuchsin, methyl violet, crystal violet, chlorin(e6), and
fluorescein isothiocyanate for use with argon lasers

16. Mechanisms of Laser Tissue Welding and
Laser Tissue Soldering
Structural Properties of Collagen
• Collagen proteins are a major constituent of all tissue
extracellular matrices.
• They are involved in both structural maintenance and tissue
growth and are thought to be the principal agents involved in
thermal repair procedures
• There are more than 20 types of collagen present in the body.
• Although the basic structure of all types is the same, there are
variations from type to type, depending on the function of the
collagen.
• Collagen constitutes about 20% of the dry weight of the large
elastic arteries such as the aorta (up to 50% in smaller vessels),
about 80% of that of skin, and about 50% of that of peripheral
nerves.