Internal bone growth stimulation for spine fusion
How internal bone growth stimulation works
The internal or surgically implanted bone growth stimulator,
utilizing direct current electrical stimulation,
requires that the device cathode be placed directly at
the spine fusion site and the generator be placed in a soft
tissue pocket under the skin. The length of the device
generator is as small as a half-dollar coin. Depending
on how much soft tissue the patient has and/or how deep
the generator (with batter pack) is implanted, most patients
cannot feel or palpate the generator while under the skin.
During a typical posterolateral gutter spine fusion surgery,
the device is implanted just prior to placement of the
bone graft material, and the electrical wires are placed
in the posterolateral gutter with the goal of maximizing
contact with as much viable bone as possible. Before
closing, the generator is implanted. An effort is make
to place the generator in a comfortable tissue pocket
so that the patient’s skin contour is not raised
or is minimally raised, thus enhancing the patient’s
comfort.
The implantable device typically remains functional,
delivering the therapeutic signal, for a minimum of
six to nine months after implantation and may or may
not be removed after fusion occurs. Removing the generator/battery
pack is a simple procedure and is done most often under
local anesthesia.
The precise mechanism of how the direct current (DC)
technology works is not clearly understood. However,
a number of physiologic changes have been documented.
The fusion process is basically a race between the bone-forming
cells (osteoblasts) and the bone-absorbing cells
(osteoclasts). Direct current (DC) is known to
enhance and the promotion and formation of cells (e.g.
raises the pH and lowers the oxygen tension), thus changing
the local acidity of the bone fusion bed which favors
the bone-forming cells and inhibits the bone absorbing
cells.3,4
General study results
Over the last five years, a number of clinical studies
have been published that demonstrate the impact of direct
current electrical stimulation on lumbar spinal fusions.
A 1996 study reported the results of patients undergoing
anterior and posterior lumbar interbody fusion with
and without direct current electrical stimulation. Overall,
successful fusion rates were found to be 95% in the
direct current electrical stimulation group compared
to 75% in the non-stimulated group. Direct current electrical
stimulation patients had higher success rates in all
patient subgroups. It is important to note that patients
who were smokers had significantly higher fusion rates
with direct current electrical stimulation (93%) compared
to the success rate of non-stimulated patients (71%)
who were smokers.5
A 1988 multi-center study of patients reported the
results of 82 patients undergoing posterior spinal fusion
with direct current electrical stimulation compared
to an historical control group of 150 patients fused
without direct current electrical stimulation. The direct
current electrical stimulation group was found to have
a statistically higher success rate (rate of fusion)
of 91% compared to 81% in the non-stimulated control
group.6,7
Study results for “difficult to fuse”
population
A 1988, a randomized prospective controlled study
examined results from a specifically defined “difficult
to fuse” spine fusion population of patients with
(1) one or more previous fusion attempts (2) multi-level
fusion (3) diagnosis of grade II or worse spondylolisthesis,
and (4) other risk factors consisting of obese patients,
smokers, diabetics, etc. This study compared 28 patients
undergoing posterior spinal fusion without stimulation
and 31 patients with direct current electrical stimulation.
The stimulated group was found to have a successful
fusion rate of 81% compared to 54% in the non-stimulated
group (p=0.026).7
Another 1988 study examined 116 patients in an uncontrolled
trial of posterior spinal fusion with direct current
electrical stimulation in the same “difficult to
fuse” population. The overall fusion rate was 93%.7
Study results for primary spine fusion
There are a number of studies on internal bone stimulators
used to enhance success rates of posterolateral spine fusions
that have also showed encouraging results.
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In 1996, one study reported a success rate of 96%
in patients undergoing posterior spinal fusion with
pedicle screw instrumentation and adjunctive direct
current electrical stimulation as opposed to an 85%
success rate in those patients fused with pedicle
screw instrumentation alone.8
-
In 1999, a similar study on the adjunctive use of
direct current electrical stimulation in patients
undergoing posterior spinal fusion with pedicle screw
instrumentation found a success rate in the stimulated
group of 95% compared to 87% in the non-stimulated
group. In this study, patients who were smokers had
significantly higher rates of fusion with direct current
electrical stimulation than smokers without direct
current electrical stimulation (83% versus 66% respectively).9
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