The Ticklish Gene
125
around the negative electrode but none around the positive. In the con-
trols, there was no growth around either electrode.
ELECTRICAL STIMULATION OF BONE GROWTH
The results were exciting, but in retrospect I believe we made a
serious error when we published this study. In our own minds and in
print as well, we confused the negative potentials of the salamander's
current of injury, the negative potentials that stimulated bone growth,
and the negative potentials from the piezoelectric study. We proceeded
as though they were equivalent, but they were not. The piezoelectric
potentials were measured on the outside of the bone and appeared only
when mechanical stress was applied. They were transitory, and most
likely the periosteum was their target tissue. In the implant study, we
used continuous direct current applied to the inside of the bone, the
marrow cavity. What we were stimulating was the DC control system of
regenerative fracture healing, not the piezoelectric control system of
Wolff's law. We didn't clearly indicate the difference to the scientific
reader, and this led to much confusion, some of which still persists
twenty years later. As a result, many scientists think electricity stimu-
lates bone growth because bone is piezoelectric. Most of these people
don't realize that bone itself doesn't grow when a fracture heals. More-
over, everyone who has proceeded from our technique—and it's being
used today to heal nonunions (see Chapter 8)—has done basically the
same thing, continuously stimulating the bone marrow. No one has
tried to stimulate the periosteum, as the pulsed piezoelectric signal does.