Biomedical Engineering Reference
In-Depth Information
TABLE 7.6
Typical Parameters Used in Bone Growth Stimulation Instruments
Clinical
Method of
Typical
Typical Current
Application
Current Delivery
Waveform
or Voltage
Accelerate bone
Noninvasive capacitive coupling:
Ac constant-current, 60-Hz
5 to 10 mA to produce
fracture healing
ac signal delivered between gelled
sine-wave signal
electrical
fi
field strength at
or bone fusion
skin surface electrodes
the desired fusion site of
2.0 V/m with a current
density of
300
µ
A/cm
2
Noninvasive electromagnetic:
Inductively coupled
fi
field 15-Hz
Maximum magnetic
fi
eld
ac magnetic
fi
field applied through
burst, 20 pulses per burst,
amplitude at the fusion
coils over desired bone fusion site
and a pulse frequency of 4.3 kHz
site
25 G to induce an
electrical
fi
field of
0.2 V/m
Semi-invasive:
cathode electrode is
Dc
20
µ
A
implanted within the fragments of
bone graft at the fusion site; anode
is a skin surface electrode
Fully implanted:
cathode electrode
Dc
20
µ
A
is implanted within the fragments
of bone graft at the fusion site;
anode is the enclosure of the
implanted dc power source
Invasive bone growth stimulators require surgical implantation of a current generator in
a subcutaneous pocket. A cathode electrode is implanted within the fragments of bone
graft at the fusion site. The generator is often no more than a battery and a 20
A constant-
current diode to the cathode electrode via a silicone-coated lead. The device's metallic
enclosure acts as the anode electrode. The implant delivers dc continuously for six to nine
months and is then removed by a simple procedure and is done most often under local
anesthesia.
The precise mechanism of how dc speeds bone healing is not clearly understood.
However, it is believed that the localized increase in pH and lowered oxygen tension at the
cathode favor the activity of bone-forming cells and inhibit bone-absorbing cells. Bone
growth takes place with a current between 5 and 20
µ
µ
A. Currents below 5
µ
A do not
enhance growth, and levels above 20
µ
A cause cell necrosis and bone death.
Electrical Stimulation for the Treatment of Chronic Wounds
Electrical stimulation has been studied as a possible therapy for accelerating wound heal-
ing (Table 7.7). In vitro as well as in vivo animal studies have shown that externally
applied electrical currents can increase ATP concentrations in tissues, increase DNA
synthesis, promote healing of soft tissue or ulcers, cause epithelial and
fibroblasts to
migrate into wound sites, accelerate the recovery of damaged neural tissue, reduce edema,
and inhibit the growth of some pathogens.
Research has been done to evaluate the e
fi
ectiveness of dc, pulsed, and ac currents in
promoting the healing of wounds. However, a technology assessment study conducted by
the Emergency Care Research Institute (ECRI) [1996] concluded that whereas electrical
stimulation does facilitate the complete healing of chronic ulcers compared to the use of
plain dressings, it is no better than conventional wound care involving debridement, clean-
ing agents, antibiotics (systemic or local), and bandages.
ff
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