Lecture 16: Implanted Walking Assist Systems for Foot Drop Flashcards
How do partially implanted stimulation systems differ from external stimulation systems in target location?
targets peripheral nerve
external system targets intramuscular and skin surface
Pros of a partially implanted FES system
* compare it against external stimulation
Partially implanted stimulation has more stable stimulation of the peripheral nerve as the electrodes are implanted near, inside, or around the peroneal nerve. External systems stimulation can be affected by humidity, moisture, and user error (such as inaccurate placement of electrodes).
Partially implanted systems have similar functional results to external devices except there is no skin irritation, better comfort, and better convenience.
- 6 min gait endurance test increased significantly
- can walk 20m faster
- subjects reported improvements in quality of life
Which parts are implanted vs external on a partially implanted FES?
stimulation electrodes and implanted RF receivers are implanted
power source (battery), RF transmitter, and sensors (i.e. foot switch) are external
What are the components of the neuromuscular assist.
- an implanted bipolar electrode wrapped on the peroneal nerve
- implanted RF receiver connected to bipolar electrode
- external RF transmitter and power source
- a heel switch worn inside the shoe.
In a 2-year follow up study completed on neuromuscular assist system patients, what were the findings?
- Correction of foot drop was obtained in 13 of 16 subjects.
- Failures were due to infection (1), peroneal nerve palsy (1) or
discontinued use (1). - Stimulation voltages were stable after 6 weeks and nerve conduction velocity was normal throughout. [nerve compression can cause changes of the voltage and nerve conduction velocity]
- Dorsiflexion torque (supramaximal stimulation) remained above
immediate post-operative values (Waters, 1977).
What were the given limitations for the partially implanted neuromuscular assist system?
Patients must have intact cognitive function and adequate motivation to wear and operate the system. It is difficult to obtain balanced dorsiflexion with a single channel of stimulation. Often there is malalignment between internal and external components like the receiver and control box. **The nerve cuff must err on the side of too loose to avoid nerve compression and thus nerve damage. Loose cuffs often result in poor electrical contact. As such, a secondary procedure may be needed to reduce cuff size. **
thus
- broad acceptance of a footdrop correction system would
require a fully implanted system, as for the heart pacemaker. - multi-channel peroneal stimulation would be necessary to
obtain more balanced dorsiflexion.
Odstock STIMuSTEP
* describe the implantation process
An incision is made into the epineurium of the deep and superficial branch of the common peroneal nerve. This is where the foil electrodes will be placed.
The foil electrodes are made of 90% platinum (good conductor and soft) and 10% iridium (for some stiffness). The electrodes measure 10mm long and 1.6mm wide. The electrodes are held in place by a tag that sutures it in place.
The wires of the electrode are coiled to allow for flexibility and are wrapped in silicone rubber (silicone is often not rejected by the human body).
With foot drop systems like the ActiGait and Neurostep that use nerve cuffs, what must be ensured to
minimize risks to a cuffed nerve?
Suboptimal positioning of ActiGait stimulator body in relation to position of nerve cuff electrode.
If the maximal allowable distance between the nerve cuff electrode and stimulator body is not correctly determined, it may result in a pull of the nerve cuff electrode and can damage the common peroneal nerve
orthopedic surgeon must leave enough slack between nerve cuff and antenna during implantation
How does actigait differ from other partially implanted systems
Implanted portions of the system are implanted above the knee as opposed to below. The control unit can be worn as a belt.
Multiple electrodes can be placed around the peroneal nerve to stimulate different portions separately.
Discuss the components of the fully implanted FES system
- All components were implanted in the thigh
- Two 30-mm long NeurocuffsTM
o 4-channel CP nerve stimulation cuff
o Tibial nerve recording cuff - Standard pacemaker battery and titanium case
- Proprietary nerve signal amplification circuitry
- Proprietary gait phase detection algorithm
why 2 nerve cuffs
- one on peroneal (multichannel)
- one is tibial nerve cuff (single channel sensing cuff); record nerve activity. monitored foot follow (heel contact and toe lift. stance phase). skin sensors in sole of foot supplied by tibial nerve. if you step in cold water, you lose balance because you lose sensation in your foot.
What is the reason for wanting to use fairly long (30mm) nerve cuffs to record nerve activity?
The amplitude of the signal recorded depends non-linearly on the length of the cuff. The longer the nerve cuff, the greater the nerve signal amplitude that can be recorded. To obtain maximal signal amplitudes, a cuff of length comparable to the wavelength of the action potential source should be used. As the cuffs are placed on large myelinated axons, a 30mm long cuff is appropriate.
With foot drop systems like the ActiGait and Neurostep that use nerve cuffs, what must be ensured to minimize risks to a cuffed nerve?
When implanting the electrode and stimulator body, the distance between the cuff electrode and stimulator body must lead to a slack, tension free electrode cable path. Suboptimal positioning can result in the nerve cuff electrode being tensioned/pulled and damaging the common peroneal nerve.
Surgeon resonsible for implanation must mark the leg and measure before implanting stimulator body and cuff.
If in the proximal direction: With the knee extended, measure and mark 28cm, 12cm, and 8cm from the knee fold on the thigh. Then, mark the thigh and knee incision. The 12cm and 8cm marks will guide where the stimulator body will tunnel through.
If in the distal direction: measure 24cm instead of 28cm.
