10/13/2017
This week we met with our client to discuss our preliminary report and some of the mechanical attachment ideas we came up with last week. Although he was intrigued by our suggestion of creating a sleeve of resorbable circuitry around the rod, he brought to our attention the need for a way to reliably and safely connect the stimulator on the rod to the screw without stimulating through the rod itself. Several possibilities were discussed:
We discussed the location of ground for the circuit. Possibilities include:
The power solution was also discussed briefly.
This week we met with our client to discuss our preliminary report and some of the mechanical attachment ideas we came up with last week. Although he was intrigued by our suggestion of creating a sleeve of resorbable circuitry around the rod, he brought to our attention the need for a way to reliably and safely connect the stimulator on the rod to the screw without stimulating through the rod itself. Several possibilities were discussed:
- Bare part of the rod (scrape off anodization) and use something like conductive epoxy to make the electrical connection
- Secondary screw with via connection to microwire (mp35n) embedded in pedicle screw
- This smaller screw would go directly into the rod, holding in place the connection from the circuit to the screw.
- Would require revalidation of the rod (structural integrity for safety and efficacy)
- Does not eliminate the problem of stimulating through the rod. Rough calculations suggest that the titanium rod with a 5mm diameter would have a lower electrical resistance than that of the mp35n microwire. Thus, current would pass through the rod instead of the wire
- Mp35n microwire: 1033 μohm/mm (Fort Wayne Metals)
- Estimated resistance of titanium rod: 21.5 μohm/mm using resistivity coefficient of 43E-8 ohm m for titanium and 20 mm cross sectional area.
- Additionally, with two circuits on a single rod (one for each screw), you could potentially create a significant leak current if you only wanted one screw to be on but the rod was included in the current loop, thus stimulating the second screw as well.
We discussed the location of ground for the circuit. Possibilities include:
- On the circuit itself (by the coil) as long as there are a few cm of separation between the tip of the screw and the circuitry, which there would be
- Pigtail wire embedded in soft tissue
- This presents an additional step for the surgeon, and thus seems undesirable.
- Presents an issue for resorbable circuitry because preventing premature degradation would be difficult and eliminating this ground from the circuit while it is still active would be problematic.
- On the cap of the rod
- This would potentially require unnecessary components being added to the rod just to serve as ground. Thus, this seems inefficient.
The power solution was also discussed briefly.
- Dr. MacEwan has successfully tested a transient inductive coil capable of generating sufficient power for nerve stimulation. As such, we know that transient inductive power is viable. Our task will be designing a storage mechanism so that constant charging of the inductor is unnecessary for functionality.
- We need to get more details on the transient coil that has been successfully used, but Dr. MacEwan has not had a chance to reach out to the Northwestern lab on our behalf yet.