MT3 Lecture 16: Implanted Walking Assist Systems for Foot Drop Flashcards
Explain slide 20 schematic b
Recorded action potential length increases with how much you lift the nerve out of the saline with a hook electrode.
The nerve signal amplitudes recorded depend non-linearly on length of nerve between electrode and ground reference.
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.
Compare 3 methods of closing nerve cuffs
self-coiling variable lumen cuff
* connective tissue will invade the coil if the cuff is not sutured shut. This can result in electrical shunting/leakage through the connective tissue bridge.
* cuff can flip itself open if not sutured shut aswell.
* can result in too loose or too tight of a fit
sutured closed fixed lumen cuff
* cuff made of stiff silicone tubing that can be sealed with sutures on the outside.
* not good for stimulation or recording, signals can be contaminated
* suture sites can become sites for excessive connective tissue formation, adhesions, and growth of infectious agents
**Neurocuff **
* an inside-sealing flap with piano-hinge interlocking system.
* no sutures needed, rapid closure, leak proof seal, minimal adhesions, easily removable
* to close permanently, you can use a monofilament nylon suture which will result in minimal adhesions. suture is sealed with local heat application. but can be reopened easily if needed.
In the following experiment what was demonstrated?
Slide 22: Use a controlled motor to apply force on skin of cat’s foot. measure response of tibial nerve using implanted cuff electrodes
- demonstrates you can track stance phase of gait using nerve cuff on tibial nerve. lays foundation to build prosthetic device to use this set up as a sensor
Note: nerve resonds vigorously in changes of force from 0 to something.
nerve doesn’t respond much to smlal changes in force decline but will respond vigorously when force decreases completely.
note: skin indentation increases doesn’t linearly follow with force increases. As the skin gets squeezed more and more, you get less and less change in skin indentation for similar amounts of force increase
Describe the set up and results of this study: cats gait analysis after paralysis.
purpose: develop FES assisted walking system by paralyzing cat extensors and trying to apply stimulation when you “estimate” gait via recording of radial and median nerve EMG
set up:
distal median recording cuff and distal and proximal radial recording cuff.
- records nerve activity
inject short acting anesthetic into catheter connected to proximal median blocking cuff. This will temporarily paralyze the nerve and leg distal to this nerve
results (before paralysis):
- nice EMG bursts - you can tell when they start and end. provides markers of start and end of stance and swing phase
- you can see early onset sharp peak in median and ulnar nerve that mark the stance and swing phase
- temporarily paralyzed the extensors of the upper limb by blocking nerve activity with anesthetic. this will cause the wrist to yield. the cat doesn’t place their wrist down when they walk (they tip toe). when you block activity to extensors, you have this yielding of the wrist. blocked motor but not sensory
applied electrical activity to assist cats gait based on gathering palmaris longus activity before paralysis.
Describe the design of hte
why did the neurocuffs have ridges?
allowed for nice snug fit at time of implant that provided insulation while allowing for some swelling (edema - 1 week later) typical postoperative of the implantation and allow the nerve to grow into the cuff w/o compression (encapsulation in thin layer of connective tissue at 6-12 months)) (squeeze from all sides = compression; squeeze from some parts = allow it to change shape)
List the parts of the neurostep system
neurocuff
neurolink - implantable nerv signal amplifier
What are the benefits of the multichamber aspect of neurocuffs?
- improved stimulation and sensory recording selectivity (each set records or stimulates from a specific portion of nerve)
- snug fit at time of implantation + allows for expansion and protects against nerve compression during swelling phase
What was the purpose of the implantable signal amplifier
- what were the highlighted requirements for such a device?
amplify signal for electroneurograhic recording
Requirements:
input noise level less of a microvolt
power consumption less than 12 mW @ 5V
- since it must be on all the time
Describe the neurostep pilot study
* patient background
* surgical implantation procedure of neurocuff
In the first patient implanted with a Neurostep system, how long did it take to restore substantial force and endurance to the disused ankle flexor muscles? What do you think was happening at the muscle level to mediate these observed changes in force and endurance?
What was noticed about foot contact time based on tibial nerve recording?
Ex firefighter - 3 years post stroke with left foot drop and severe hypotonia
* knee brace + ankle foot orthroses + cane and contact guard assistance
* Could only walk 5-10 m without fatiguing.
Implantation
- 6-cm longitudinal incision in the posterior thigh
- The Common Peroneal (CP) and Tibial nerves were cleared over 6 cm.
* measure circumference of nerve to determine cuff size using flexibile silicone ruler. select appropriate size neurocuff. 2 common peroneal nerve (1 for each branch) cuffs and 1 tibial nerve cuff implanted.
* suture Neurocuff closed with cautery tip. protect nerves with Teflon heat shield strip during this part.
* implant control unit in medial thigh subcutaneous pocket
- neurostep was turned on or off with a telemetry programming interface or a magnet (patient was given magnet)
- patient underwent electrical exercising protocol
results:
* stimulation of channels 1 and 3 (out of 4 channels) gave the best combination of moving the foot forward and not everting or inverting. foot could be lifted enough for walking!
* reversed disuse atrophy in paralyzed muscles
* ankle dorsiflexor force increased
* took 30 days before substantial dorsiflexor force was measured with stimulation. by day 50, forces were substantial enough for toe clearance and walking
measured fatigue resistance as well (time to reach 70% of initial averaged force when stimulating nerve every second)
* 30 days in: took a minute to decline to 70%.
* 2 months in: 75s to decline to 70%. endurance improved
- conversion from fast twitch, fast fatiguing fibres to MHC-2a which are better for endurance and power. Muscle atrophy reverses –> increase in force.
Tibial nerve signals during walking. Noted that patient spent more time on good foot (right). left foot had a longer swing phase since it was weaker
What did the subject say was the benefits of the neurostep?
- 5 weeks after implant, he no longer needed a knee brace.
- 10 weeks after implant, his gait had improved markedly.
- 6 months after implant, he regularly walked the length of his driveway and back
(250m) without fatiguing. - His balance control was greatly improved.
- Using Exercise Mode at home proved very useful for strengthening muscle
- The nerve stimulation was never painful, nor unpleasant. he liked the tingling as it proved that it was on and working
- He was appreciative of how much using the device had improved his mobility,
balance and lifestyle.
What happened after the battery ran out after 8 months?
therapeutic effect, could still walk for a bit but then he slowly deteriorated as muscle atrophied due to lack of drive
unfortunately device didn’t get commercially approved so he never got a new one
however he went to alberta and stein fitted him for an external FES. not as convenient. old device was still in there but did not interfere with external FES
For neurostep cuff implantation for functional electrical stimulation systems, what is a requirement that must be seen on MRI?
As a safety precaution, the distance between natural separation level of the 2 branches of sciatic nerve and the axis of the knee should at minimum be 9cm to put a 3cm nerve cuff. Establish this using MRI before cuff implanatation.
This is because you cannot separate connective tissue too close to knee joint as nerve must bend around knee. Thus, the cuff must be placed high. if too far distal (and near knee joint) will cause loss of function
In 33 subjects, found that
knee to branch separation level (SK) could vary between 5cm to more than 15cm. Thus, only half the people tested would be eligible for nerve cuff. Shorter people are less likely to be eligible
Compare 4 methods to treat foot drop in hemiplegia in the following factors
* benefits
* risks
* limitations
ankle foot orthrosis
- benefits: non-invasive, lowest cost
- risks: disuse atrophy, skin damage, peroneal palsy
- limitations: must don/doff, pain, discomfort, interferes with plantarflexion
external FES
- benefits: non-invasive, moderate cost
- risks: skin abrasion, skin burns
- limitations: must don/doff, pain, discomfort
partially implanted FES
- benefits: more stable stimulation; multichannel stimulation
- risks: require surgery, possible nerve damage
- limitations: must don/doff, may be inconvenient
fully implanted FES
- benefits: most stable stimulation, multichannel, always ready to use, invisible, comfortable
- risks: requires surgery, possible nerve damage
- limitations:battery life, not commercially available
What is the reason for needing a very good hermetic seal along the whole length of a longitudinally slit nerve recording cuff, since the cuff is open and in contact with body fluids at either end, anyway?
Maintaining the stability of the recording environment within the cuff by protecting it against bodily fluids.
Preventing the growth of fibrous tissue into the cuff, which can interfere with recording quality.
Improves recording signals
