Biomedical Engineering Reference
In-Depth Information
disadvantages
Several drawbacks exist in performing LAL. For large areas,
LAL alone may be insuffi cient for proper correction; therefore,
although LAL has been used successfully as a sole procedure
for body contouring, some physicians assert that LAL is not a
substitute for conventional liposuction but a complement to it
(12,18). Undercorrection following LAL may result from inad-
equate cumulative energies used, as many studies do not cal-
culate this parameter (12,21). As with any new technology,
there is a signifi cant learning curve associated with LAL (12),
although the slope is relatively steep in experienced hands
(18). For this reason, results following LAL vary, with some
studies demonstrating no improvement over traditional lipo-
suction (22). In addition, thermal injury is a possible compli-
cation, since there is a relatively narrow therapeutic window
between heat accumulation that stimulates collagen contrac-
tion and thermal injury resulting in dermal-epidermal burns
and potential scarring (27).
As earlier generation and most contemporary LAL devices
require two steps—fi rst for the tissue to be treated with the
laser and second a separate aspiration step—procedure time is
increased (12,22). The innovation of dual functioning cannu-
las, allowing simultaneous laser fi ring and suction, resolves
this issue (28). Finally, the cost of additional laser equipment is
a barrier to entry for some practitioners (18).
damage is diffi cult to evaluate histologically. Likewise, photo-
stimulatory effects on tissue are secondary to photothermal
effects (14).
Thus, the favored mechanism of action in LAL is a purely
thermal effect (17,24,32,33), which is coined as “photohyper-
thermia” (17). Thermal effects following LAL include the fol-
lowing: coagulation of collagen fi bers, thrombosis of vessels,
damage to nerve endings, and reversible (tumefaction) and
irreversible damage (lysis) to adipocytes depending on the
energy employed (5,17,25). At low laser energy, intra- and
extracellular sodium and potassium balance is altered, result-
ing in adipocyte tumefaction (5,25). Eventually, heat gener-
ated in the tissue from laser energy results in cellular membrane
degradation (adipolysis) secondary to protein denaturation
(25). Some authors believe a thermomechanical effect also
plays a role in LAL, as laser treatment on fat tissue results in
adipocyte rupture (5,34).
A mathematical model of LAL using two systems—one with
a 980-nm wavelength and the other 1064 nm—demonstrated
skin contraction due to a heating effect. Mordon and cowork-
ers demonstrated that bioheat transfer initiated from laser
light resulted in collagen remodeling (34). In other words,
laser light energy is converted into heat energy within the adi-
pose layer. This diffuses to the dermis and eventually to the
skin surface. According to the mathematical model, tempera-
tures of 48-50°C must be reached within the dermis to induce
collagen contraction and resultant skin tightening (22,26,34).
Depending on the study, a dermal temperature between
approximately 50°C and 70°C translates to a skin surface tem-
perature of approximately 40-41°C (22,26,34).
Damage is energy dependent (5,18). Compared with an area
treated by lower energy (1000 J), 3000 J of energy resulted in
signifi cant irreversible damage (25). The dose-dependent rela-
tionship between laser energy and thermal damage has been
duplicated by other histologic studies (5,26,31).
Increasing energy creates not only adipocyte changes but
also tissue fi brosis (24). Collagen damage from thermal dam-
age promotes collagen remodeling, leading to increases in
skin tone and texture. These effects continue to improve for
3-6 months following the procedure (12,26).
mechanism of action
Two properties must be considered in determining the effi cacy
of LAL given a particular device—the wavelength employed
and the energy delivered (14). Unique chromophores are more
selectively targeted depending on the wavelength, and the
energy used will determine the thermal effect on tissues.
Different wavelengths have been selected for LAL in an
attempt to specifi cally target fat, collagen (water), and blood
vessels. According to the theory of selective photothermolysis,
these chromophores will preferentially absorb laser energy
according to their absorption coeffi cients at specifi c wave-
lengths. Various wavelengths, including 924, 968, 975, 980,
1064, 1319, 1320, 1344, and 1440 nm, alone or in combinations
have been evaluated for interactions within the subcutaneous
compartment. Some wavelengths have unique advantages. The
924-nm wavelength has the highest selectivity for fat melting
(22) but may not be as effective for skin tightening unless com-
bined with another wavelength. The 1064 nm targets oxyhe-
moglobin providing vessel coagulation (24) has superior heat
distribution and therefore skin tightening effect (27), whereas
the 1320 nm has greater fat absorption with less tissue penetra-
tion and scatter, decreasing the chance for collateral tissue
damage (22). The 1440 nm has demonstrated to be preferen-
tially absorbed by subcutaneous fat compared with other
wavelengths previously utilized. This implies that the laser
beam will be absorbed in a very small primary tissue volume,
potentially inducing high temperatures (29).
Photoacoustic (17), photomechanical, photostimulatory,
and photothermal effects are theorized mechanisms of action
in LAL (14,30). Most of these hypothesized actions are either
secondary to or have been replaced by the idea that heat-gen-
erated effects on tissue are the primary mode of action in LAL.
For example, Khoury et al. asserted that photoacoustic abla-
tion lends to thermal damage (31) although photoacoustic
histologic findings
The histologic fi ndings from human tissue models support the
photothermal effect of LAL, regardless of laser wavelength. In
2002, Badin and colleagues found the immediate cellular
changes following laser treatment of subcutaneous tissue to
include ballooning and rupture of fat cells with reduced bleed-
ing due to vessel coagulation (12). At 3 months following
treatment, histologic studies demonstrated new collagen for-
mation and remodeling (12).
Additional histologic studies using and comparing wave-
lengths of 980, 1064, and 1320 nm all provided similar
evidence for the dose-dependent nature of cellular changes
following LAL (5,25,26,31,35). Cadaveric studies as well as ex
vivo and in vivo studies fi rst showed reversible cellular damage
seen as cellular ballooning (tumefaction). Further energy
delivery resulted in cell membrane destruction and irrevers-
ible cell lysis. Laser treatment also caused thermal damage to
collagen fi bers, vessel thrombosis, and reduced areas of bleed-
ing compared with liposuction-treated areas (5,25,35).
