Host response - Orthopaedic Biomaterials in Research and Practice - page 364

and magnitude of bone deformation, whereas polarity is driven by the
directionality of deformation. It has been shown that deformed rabbit
tibias were capable of generating bioelectric potentials up to 6 mV, while
in vivo measurement of potentials in the ambulating human has been
recorded up to 300 mV. Piezoelectricity is considered to be driven by
collagen, and potential is decreased with increasing water content. It is
useful to consider that structure and chemical composition of bone will
vary by age, gender, region, nutrition, and especially hydration.
Electromechanical properties of bone are the biophysical basis for
Wolff's law. Bone formation is driven by upregulation and downregula-
tion of cellular signaling molecules, which can be directed by chemi-
cal and electrical cues. Any alteration to mechanical stress and electric
signals increases secretion of growth factors, cell proliferation, intercel-
lular calcium, and, resultantly, bone remodeling. This is accomplished
through affecting the cell membrane. In their natural state, osteoblasts are
asymmetric, secreting extracellular matrix only on one side of the cell.
During electric stimulation, there is a high voltage drop across the cel-
lular membrane and enzymatic activity is increased on the electrode side,
encouraging galvanotaxis, or cellular movement. The change in cellular
homeostasis also encourages the free movement of calcium ions through
the cell membrane. Increased calcium levels regulate hormones that
inhibit signals that prevent the formation of new bone. Meanwhile, mes-
enchymal cells translate to the cathode via galvanotaxis, before deriving
into osteoblasts. Electrical stimulation is also thought to have an angio-
genic effect, stimulating tissue healing by affecting vascular growth.
Although the use of electrical stimulation for bone growth therapy is
recorded back into the middle of the 19th century, its efficacy was consid-
ered controversial well into the 20th century, and there still remains some
controversy today. Two areas of interest in orthopaedics include using
electrical stimulation as an adjuvant therapy to stimulate bone regenera-
tion during fracture healing and early bone ingrowth and anchorage of
arthroplasty components. It has also found some use in spinal fusion treat-
ment. Even moderate success with each of these applications could pro-
vide a great benefit to treatment. For example, between 5% and 10% of
long bone fractures result in delayed or nonunion, while THA patients
may lose up to 14% of their bone mass in the first 3 months after a THA.
The most prominent mechanisms for electric stimulation include
pulsed electromagnetic fields (PEMFs), direct current applied either per-
cutaneously or via an implantable device, or a capacitive coupled electric
field through conductive plates attached through the skin. All methods
use low-level electric currents and are indicated for treatment of fracture
nonunion and spinal fusion.

Inductive
coupling: pulsed
electromagnetic
fields

PEMFs use magnetic coils to convert electric current into a series of pulses
transmitted through the affected region to induce electric signals. A rapid
sequence of pulses results in a magnetic flux density anywhere between
0.1 to 18 G (Gauss: unit of electromagnetic flux to create a low level mag-
netic field, typically below 1 kHz on the frequency spectrum). In vitro
studies suggest that PEMFs encourage mineralization and angiogenesis,