Neuroprosthetic Surgical Strategies for Neuromuscular Stimulation

Introduction

The provision of well-engineered, well-implanted neuroprostheses represents a major advance in the acceptability, safety, and durability of electronic controllers for paralyzed persons. Well-thought-out strategies and progressive understanding of the risks, benefits, and service and maintenance requirements of these devices have led to some experience-based criteria and advice.

A stable neurologic physiology and functional level are requirements for a predictable functional reconstructive improvement. A changing responsiveness of the muscles and nerve below the level of injury creates a moving target of control and outcome. A physical examination with manual muscle testing to assess voluntary and controlled muscle groups is performed. The motor units and muscles are classified with surface and intramuscular electromyography and screening. We identify which muscles are under voluntary control and then separate the paralyzed segments into those that are electrically excitable and maintain their innervation below the lesional level and those that cannot electrically activated and are thus denervated below the lesional level. The denervation can be a result of the stratification within the cervical segment lesion or in the peripheral nervous system, such as ulnar nerve compression at the elbow. We often find that contracture or posturing is caused by a denervated segment. Using electrical stimulation, we identify reflex intact upper motor neuron (UMN) lesion muscles, suitable for functional electrical stimulation (FES) control with a neuroprosthesis. FES control is not possible within the denervation motor unit. Spinal column and spinal cord mechanical stability are reexamined with the use of radiographic, magnetic resonance imaging (MRI), and computed tomography (CT) studies and consultation.

It is important to assess social and family support for care of wounds; skin surveillance; reporting of problems of function; implant performance; mood; protection from burns, sharp injury, falls, and electrical safety; and assurance of nutrition, shelter, and transportation.

Inclusion Criteria: Better Clinical Outcomes and Lessening the Risk for Complications

The goal of avoiding infection has led to rituals of screening and clinical practice but without scientific evidence of reduction in the rate of infection because the numbers of cases are small and we are hesitant to discard best general clinical practice.

We have relatively excluded from clinical trials those persons with systemic immunologic diseases or known risk factors related to their chronic disease. Examples include patients with a history of diabetes mellitus, human immunodeficiency viral infection, or nonhealing pressure sores.

Another relative exclusion from clinical studies are persons with other implanted electronic systems whose safe interaction with a neuroprosthesis is unknown. We have monitored the electrocardiogram in early cases with devices turned on and off without seeing abnormalities. In establishing interaction with an emerging electronic environment with a chance of conflicting operating frequency of implanted devices, possible electrical current interactions with applied and measured signals is always a reason to distance devices or to not implant.

Advance procedure planning of device location and patterns of use will help prevent problems later.

The pulse generator devices should not be placed in areas where skin loss would be difficult or impossible to manage, such as the axilla or perineum. Abdominal skin folds that cause electrode lead kinking near the device during a seated position should be taken into consideration. When transferring a patient from bed to chair using a Hoyer Lift, the location of the lifting straps and internal components is important in planning device position. As such, the most common safer locations are in the chest 6–8 cm below the clavicle or the abdomen, lateral to the umbilicus and between the lower ribs and superior pelvis. In the abdominal locations, it important to have the patient seated when determining device position.

The location of leads and electrodes in the hand considers the pressure applied during weight transfers, propelling wheelchairs, and handling objects. The lead and electrode route to the hand follow existing nerve pathways and avoid weight-bearing skin.

We assess skin contact preferences through observation and applying ink to the skin and observe patterns of ink rub-off by activities of daily living (ADL) contact. To determine areas of use, coat the skin with an erasable marker and observe how patient and caregivers rub away the marks. Videos taken during ADL are helpful with special attention to the palms of the hand and elbows and patient strategies to maintain balance. Examine all skin for pressure sores and areas crossed by seatbelts. Observe the patient during an induced involuntary spasm to determine which parts of the limbs and chairs come in contact. Try to have a strategy for glove protection or pads at elbows.

The tunneling of the leads at the subcutaneous level is for safety and surveillance. Tunneling over bony prominences or pressure contact areas used by the patient or caregivers during daily care should be avoided. We locate power and control modules in areas that do not have skin buckling and avoid incisions and devices in waist creases, axillary areas, or locations where attendants will lift the person.

In performing implantation, we have a plan for body areas that includes future intravenous catheter placement for illness or emergency. A map of the buried stimulation wires and devices for the patient must be available to provide in case of emergency, and advise patients, their caregivers, and treating physician about MRI and other diagnostic compatibilities. The procedure should be timed when patients are neurologically, urologically, and emotionally stable.

Surgical Procedure: Details of Implantation for a Networked Neuroprosthesis for Hand Function

Once the subject has completed the muscle conditioning with an electrical exercise program and the surgical planning is complete, the NNP System is implanted in one or two procedures, typically lasting 4–7 h each ( Fig. 41.1 ).

The surgical implantation procedure will closely correspond to the procedure that has been used for similar systems and has been described in detail by and proved effective to date ( ). The surgery is performed under general anesthesia in a sterile field.

Surgical Preparation

The skin surfaces are prepared by using chlorhexidine or iodine soaps. Surgical sites are clipped rather than shaved to avoid wound contamination. The skin is sealed with an adhesive iodinated plastic membrane to reduce contamination from the skin bacteria into the surgical site. We do not routinely use a clean air system in the operating room. We minimize the number of members in the surgical team and others who rotate into the operating room. Wound irrigations are performed frequently and incisions closed as soon as possible to avoid airborne exposure. The patient’s body or limb temperature is maintained by covering exposed skin, the use of heating blankets and pads, and the heating of infused solutions.

Prophylactic Antibiotics

Antibiotics are administered 1 h before surgery and continued for 24 h postoperatively.

Surgical Procedure

Incisions for access and passage of electrodes and leads vary from case to case depending on the number of devices in the network. Planning is done ahead of time in consultation with the patient, family, therapists, and engineering team. The three types of muscles—voluntary, paralyzed with innervation, and paralyzed with denervation—are taken into consideration for the electrode mapping. Tendon transfers are planned to overcome some of vital denervated segments such as elbow and wrist extensors. Exposure of the power module implantation site in the abdomen is performed for the networked neuroprosthetic (NNP). The power module is placed at the level of the Scarpa fascia for mechanical stability while not excessively deep to interfere with coupling or charging. Access to the subcutaneous plane is required for the implant, connectors, and muscles controlled and for the possibility of service or revision. For electrodes and system components, incisions are made to allow surgical access to the chest overlying the pectoralis major muscle, axilla, anterior deltoid muscle, antecubital fossa, volar forearm, dorsal wrist, and palm of the hand with dedicated incisions and exposures. Stimulating electrode locations are mapped using a temporary monopolar epimysial or intramuscular electrode probe with direct visual observation of stimulation of the muscle. Optimal electrode position is determined by observation of the muscle response with stimulation, with particular attention paid to the maximal force output, maximal selectivity (minimal stimulus spread to adjacent muscles), minimal change in force with changes in muscle length, and modest recruitment gain ( ). The reference electrode (anode) is placed in contact with the tissue in the pocket prepared to receive the nearest stimulator module.