9/29/2017
This week began with a conference call with our client. On this call, he clarified that his data suggests that the upper limit of safe DC stimulation lies between 100-200 μA, contrary to one of the review papers which stated that current above 20 μA results in osteonecrosis. Devices currently on the market utilize currents of 40, 60 or 100 μA. Other subjects of discussion included:
We began to discuss options for the mechanical attachment of the circuit and power solution to the pedicle screw. We need to determine if Dr. MacEwan would like a quantitative assessment of the strength of the attachment method and, if so, how to quantify the strength of our chosen attachment.
This week began with a conference call with our client. On this call, he clarified that his data suggests that the upper limit of safe DC stimulation lies between 100-200 μA, contrary to one of the review papers which stated that current above 20 μA results in osteonecrosis. Devices currently on the market utilize currents of 40, 60 or 100 μA. Other subjects of discussion included:
- Resorbable battery: Determined to be outside the scope of our project. We have decided to focus on an alternate power solution—either a hybrid resorbable circuit with a non-resorbable battery or a capacitive charging solution.
- Capacitive charging: We discussed the option of using inductance to charge a subcutaneous capacitor. This capacitor would then discharge slowly, either throughout the course of a day or a week. The hope is that this would be resorbable, and then the entirety of our circuit design including the power solution could resorb. Dr. MacEwan agreed to email the lab at Northwestern to determine what components they are currently able to make (ie. what capacitors and inductors) and what their charging properties may be.
- Acknowledge the safety tradeoff involved with requiring patient compliance. A wireless solution is attractive, but it requires the patient to charge the circuit either daily or weekly. What are the safety implications if they forget to do so? Dr. MacEwan told us to move forward with this line of thinking despite the tradeoff because it had potential.
- Northwestern collaborator: Dr. MacEwan informed us that the turn-around time for resorbable circuits from the Northwestern lab is usually about 2-3 weeks. However, for new components or a brand-new circuit, it may take them 3-4 weeks or more. This is an important consideration, as we will have to have finalized and submitted our final circuit design at least a month before the scheduled April deadline.
We began to discuss options for the mechanical attachment of the circuit and power solution to the pedicle screw. We need to determine if Dr. MacEwan would like a quantitative assessment of the strength of the attachment method and, if so, how to quantify the strength of our chosen attachment.
- We believe the two options for attachment areas are either on the top of the tulip (head of the pedicle screw) to the component which secures the rod between the screws or on the rod itself, as this would provide more surface area.
- Considered a “snap button-like” attachment. Natalie O. has observed instruments with similar attachment faces used in surgery and believes they might be strong enough to withstand any shearing forces during the placement of the screw. This snap attachment could either exist on the cap of the tulip or on the rod.
- Safety concern: How do we make sure that the component does not resorb in such a fashion that a circuit component is floating within the body, no longer attached to the pedicle screw?
- Considered a “sleeve” for the rod which could contain the miniaturized circuity. This would require modification of the rod itself (slight indentation to keep the sleeve from sliding around).
- It appears this solution would allow for safe resorption, as the component would resorb from the inside out, meaning it would remain attached to the rod throughout the process.