Biomedical Engineering Reference
In-Depth Information
minimizing absorption of laser energy through expulsion of
blood led to further enhancement in developing the Lightshear
High Speed Duet hair removal laser (Lumenis). Here, there is no
sapphire window to attenuate the energy; instead, the vacuum
draws the targeted hair-bearing skin into a chamber surrounded
by six diode laser arrays, which stretches epidermal melanocytes
and melanin particles to further decrease the epidermal absorp-
tion targets (melanin) so that the laser energy can more selec-
tively penetrate into the melanin-containing follicular hair,
allowing the use of decreased fl uence while maintaining effi cacy.
Ross and Kilmer have confi rmed both signifi cant pain reduction
and enhanced treatment effi cacy in laser hair removal using this
novel “pain-reducing” treatment technique (23).
topical anesthetics
Lidocaine/Prilocaine (EMLA)
EMLA cream (Astra Pharmaceutical Products, Inc., Westbor-
ough, Massachusetts, USA) is a 5% eutectic mixture of two
local anesthetics, lidocaine (107 mmol/L = 25 mg/L) and pri-
locaine (113 mmol/L = 25 mg/L), with Arlatone 289 (emul-
gent), Carbopol 934 (thickener), and distilled water in 1 mL
cream. The pH is adjusted to 9.4 with sodium hydroxide. One
gram of cream is applied to a 10-cm
2
area.
EMLA cream has been found to be nontoxic, with plasma
levels in the circulation measured at 100 times lower than
those associated with toxicity (24,25). In 22 infants, ranging in
age from 3 to 12 months, 2 mL of EMLA was applied to 15 cm
2
of skin surface for 4 hours. In each case, the plasma concentra-
tion of lidocaine and prilocaine was below toxic levels (24).
The only adverse sequelae reported in the literature include
mild local reactions, such as pallor, erythema, or edema, observed
in treated skin areas (24,26-32). Cutaneous blanching is thought
to be secondary to vasoconstriction, which raises the possibility
of decreasing the efficacy of laser treatment. Decreased efficacy
was not found in one clinical study (33), nor have we noted a
decreased efficacy in our patients treated with EMLA.
EMLA is recommended to be applied thickly under occlu-
sion with a polyurethane dressing (Tegaderm) for 60 minutes
(Fig. 16.3). A maximum of 10 g should not be exceeded at
any time in any patient. Below the age of 1 year, the maxi-
mum application quantity is 2 g. A prospective double-blind
randomized study assessing pain during laser treatment of
PWS skin occluded with either EMLA or placebo showed
that pain scores for EMLA-treated sites were significantly
lower (
p
< 0.0001) (34).
A study with the argon laser compared injectable lidocaine
hydrochloride with EMLA cream. Experimental subjects were
adults with normal skin. The argon laser was used at a dura-
tion of 200 ms, beam diameter of 3 mm, and maximal inten-
sity of 2.4 W. Interestingly, after 60 minutes of EMLA
application, pain blockade was obtained in 9 of 12 subjects,
with total sensory blockade in the remaining three subjects. In
addition, after a 60-minute application of EMLA, analgesia did
not immediately occur after removal of the cream, but reached
a maximum extent of 15 minutes after cream removal. The
analgesic efficiency of EMLA cream increased with longer
application time (60-120 minutes), approaching an analgesic
state similar to conventional lidocaine infiltration. Lack of
anesthetic effect of the EMLA returned to baseline 90 minutes
after removal of the cream (35).
Figure 16.2
Hyperpigmentation developed after the use of alexandrite laser
hair removal with cryogen spray; this lasted over 2 years.
pneumatic skin flattening
Pain can be reduced by applying a vacuum to the treated skin
area under the laser. The associated vacuum provides a decrease
in pain through various mechanisms. The skin compression
stimulates tactile and pressure neural receptors in the skin to
inhibit afferent pain transmission in the dorsal horn (gate the-
ory) (11,12). According to the gate theory, nerve impulses from
nociceptors (pain) and their sensory fi bers, which are slower
and thinner, arrive at synapses in the spinal cord on their way
to the brain. The myelinated sensory neurons - of larger diam-
eter and thus faster - carry pressure and tactile sensation from
the surrounding skin, thereby activating secondary neurons,
which secrete endogenous opioids into the pain synapse, thus
suppressing the pain sensation.
Pneumatic skin fl attening (PSF) was fi rst demonstrated to
reduce pain from IPL treatment, which was given through a PSF
vacuum chamber Inolase (Netanya, Israel), Candela/Syneron
(Wayland, Massachusetts and Irvine, California) (21). In this
study, three different IPL devices were tested and a signifi cant
pain reduction was seen with all the treatments (21). An addi-
tional PSF study was performed on hair removal using the Light-
shear 800-nm diode laser (Lumenis, Santa Clara, California,
USA) (22). This study also demonstrated a reduction in pain.
Regarding hair removal, an additional benefi t suggested was
the blood expelled from the skin being sucked up into the cham-
ber, which eliminates a secondary target that would further
reduce nonspecifi c absorption of laser light. The authors recog-
nized that effi cacy was preserved despite a 5% decrease in attenu-
ation by the sapphire window. This concept of using the gate
theory through vacuum to minimize pain with the addition of
