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.

• 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