Putting the pieces together of a neural prosthesis

What happened during those 6 hours under the knife in the OR? How did the research team rebuild a paralyzed body?

A lot can happen in six hours. The delicate balance for this surgical team was to preserve the existing implanted system that works while removing and replacing the broken pieces. Sounds simple, but not so fast. One slip and a wire or connector could be damaged.

The task at hand was this; of the 24-channel system the implanted receiver for 16 of those channels needed to be replaced. For each channel there is an affiliated electrode. The muscle and nerve electrodes did not need to be repaired but the implanted receiving unit did. As a modular system, the implanted electrodes are not wired directly to the receiver unit instead they go to connection points.

The 16-channel unit has connection points in the pelvic region. The elegant design was logical. In the event that an electrode needed to be replaced, the surgeons did not need to disturb the mothership, receiving unit. Conversely if the receiving unit needed to be replaced, then the electrodes would not be disturbed.

In this case, the surgeons dissected the 16-channel receiver unit out of the upper right side of my abdomen. The receiving unit is slightly larger than a cardiac pacemaker and of similar shape. To unplug the receiving unit, they also needed to access the connection points grouped under four incisions, two located in the lower abdomen near the belly button and one located near each hip. With the delicate hand and coordination of the surgeons they carefully scraped away the scarred tissue and unplugged the unit from the connection points.

Using the same ‘pocket’ in the abdomen, they installed the new IST-16 receiving unit and plugged the electrode connections into the new unit. Each channel was then tested. But it was not that simple.

Each stimulus channel is allocated an electrode contact. When the original IST-16 was implanted, it had the following electrode assignments: four for each nerve cuff electrode to the femoral nerve (controlling the quad muscles), two for the left and right quadratus muscles in the back, two for left and right gluteus medius, two for the adductor muscles on the left and right side, one for the right hamstring and one for the right gluteus maximus.

The nerve cuff electrode is a little different from the basic muscle electrode. The cuff is a spiral design; if the spiral is unrolled there are four contacts within that one electrode. The cuff is then wrapped around a nerve and thus it is a four contact electrode using four channels of the stimulating receiver.

Over time, we discovered that my body did not need four channels for each femoral nerve. In fact, when we fired all four channels controlling the group of muscles within the quad, they would fatigue quickly since we were over-stimulating those muscles. Through optimization analysis we were able to select two of the four channels that would provide the best performance of the muscles. In the end we were only using two of the four channels of each cuff electrode.

When plugging in the replacement IST-16, why plug in the two channels of each femoral nerve cuff electrode that are not being used? Could we use those channels for some other function? The answer is yes.

The electrodes currently in place provide use of the muscles above the knees and in the trunk. What about looking below the knee? To gain ankle function of dorsiflexion and plantar flexion, we would need to excite the nerves that control the tibialis anterior and the gastrocnemius muscles that flex and point the foot. But installing wires from the nerve electrodes below the knee all the way up to the IST-16 in the pelvic region is a long distance, so an extension cord needs to be used, otherwise referred to as a jumper. That jumper would be placed on the outer side of each thigh.

Time is of the essence in the operating room. Contrary to what they practice on TV shows like Grey’s Anatomy, the longer a human body is open the increase susceptibility to infection. Staph infections are life threatening and many are antibiotic resistant.

With the original IST-16 removed and the new one inserted, we began to approach six hours in the OR. At that point, it was a safety decision. The team decided to install the jumpers, close up and come back for Act 2, a second surgery. Seven incisions were stitched and cleaned up. The unconscious subject was sent off to the recovery room.

Note: The statements and views posted here are of my own and do not reflect those of Case Western Reserve University, the Department of Veteran Affairs, Metro Health Medical Center or the National Institutes of Health along with their representives involved with this program. If you are interested in this clinical trial, please visit https://clinicaltrials.gov and search NCT00623389 or NCT01923662