Host response - Orthopaedic Biomaterials in Research and Practice - page 365

increase DNA synthesis, accelerate apoptosis of osteoclasts, and alter the
cellular calcium content in osteoblasts and their eventual proliferation and
differentiation. Calcium ion activity in osteoblasts is known to be driven
by the frequency and duration of pulse waveforms.
PEMF has been applied to stimulate osteogenesis in patients with
nonunion of bone fractures, foot and ankle arthrodesis, and spinal fusion,
all with limited success. It has also found some success in encouraging
callus formation and maturation in bone distraction procedures and for
the treatment of osteonecrosis of the femoral head. It is important to get
a daily dosage for successful treatment, and compliance can be a chal-
lenge. More information is necessary in order to optimize the magnetic
intensity and dosage. Clinical success in treating fracture nonunion has
been reported from 64% to 87%.
PEMF has been less well studied in orthopaedic applications. Similar
to other modalities, PEMF studies have shown inconsistent results owing
to varied treatments (pulse frequency, duration, size), but at least one
study has shown an effect of PEMF in reducing osteolysis after THA. It
has been suggested that PEMF is effective in treating patients with asep-
tic loosening of THA, but not necessarily in severe cases.

Capacitive
coupling

Capacitive coupling utilizes two surface electrodes applied on the skin
across a region of interest, for example, a fracture site. A battery is used
to generate a 60 kHz sinusoidal wave signal that creates an internal field
of 0.1 to 20 mV/cm, with a current density in the region of 300 μA/cm 2 .
Outside of the application mechanism, there are many similarities between
capacitive coupling and inductive coupling treatments, such as PEMF.
Like PEMF, capacitive coupling is noninvasive, but there are issues with
patient compliance. Also, it is thought that the molecular mechanisms and
the pathways by which bone responds to electric stimulus are similar for
the two methods. Reported clinical success in healing delayed and non-
union fractures is reported from 70% to 77% for capacitive coupling use.

Direct electrical
stimulation

Direct electrical stimulation, approved by the FDA in 1979 for the treat-
ment of established nonunions, utilizes an implanted cathode near the
area of interest with a battery-based anode located subcutaneously to
administer a constant current to the affected region. Current is often in
the range of 20 μA. This method is thought to affect healing differently
from capacitive or inductive coupling because of the simulative effect of
electrochemical reactions at the electrode sites. Localized pH is altered
at the electrode sites owing to reduction oxidation reactions that generate
hydrogen and hydroxide ions. The environment near the cathode becomes
slightly alkaline, which has been shown to be favorable for bone growth.
Oxygen tension in the vicinity is also lowered, which is thought to stimu-
late mesenchymal stem cells and encourage osteoinduction.
Clinically, there are challenges with using direct stimulation. Electrode
implantation and removal has device and infection risks associated with
the transcutaneous leads, such as device failure and infection risks. There
is often the formation of a fibrous capsule at the electrodes, which can
progress to encapsulation under chronic stimulation treatment, potentially