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Osseointegrated Neural Interface (ONI): Rethinking a Conventional Surgical Treatment for Amputation Neuromas in the Digital Age.
Aaron M. Dingle, PhD1, Joseph Novelo, MS1, Jared P. Ness, MS1, Jacqueline S. Israel, MD1, Brett Nemke, BS2, Yan Lu, MD1, Sarah Brodnik, BS1, Jane A. Pissanello, BS1, Lisa Krugner-Higby, DVM, PhD2, Mark D. Markel, DVM, PhD2, David Goodspeed, MD2, Justin C. Williams, PhD2, Samuel O. Poore, MD, PhD2.
1University Wisconsin, Madison, WI, USA, 2University Wisconsin, Madiosn, WI, USA.

PURPOSE:
Modern prosthetic limbs have reaped the benefits of the Digital Age, with improvements in materials, degrees of freedom and computational power. What has lagged behind these advances, is the ability of the recipient to control these devices. Neural interfaces are devices that aim to bridge the gap between the biological tissues and the robotic prosthetic. In most cases, the neural interface is placed on the skin, actuated by myoelectric signals highly susceptible to motion artifact and muscle signal crosstalk, ultimately preventing widespread clinical application.
In 1943 Edwin Boldrey first published the transposition of nerve in bone to treat amputation neuromas. This method is still in use today, under the fundamental principal that placing the nerve in bone protects the neuroma from the mechanical and electrical stimuli that causes neuropathic pain. By re-directing transected nerves into the medullary cavity of long bones, the terminal end of the nerve is protected from external stimuli, whilst also providing direct access to the highly vascular stem cell niece. This already established surgical model presents the perfect in vivo bioreactor for the potential interfacing of transected nerves and electronic prosthetic devices.
The research objectives of this pilot study were to create an animal model -termed the Osseointegrated Neural Interface (ONI), utilizing histology to demonstrate the stability and health of the nerve and surrounding tissues and electrophysiology to demonstrate nerve conductivity.
METHODS:
Transfemoral amputation was performed in New Zealand white rabbits. Briefly, the sciatic nerve was isolated and severed above the point of bifurcation. The femur was amputated at the midpoint and the nerve passed through a corticotomy. The terminal end of the nerve was sutured into a PDMS nerve sleeve, representing a mock electrode, which was pressed back into the opening of the medullary cavity forming a tight seal. The muscle and skin were closed over the femur. Animals were explored at 5 weeks via histology and electrophysiology.
RESULTS: Gross examination of the ONI limb demonstrates that the nerve is stable at 5 weeks. Healthy nerve morphology can be identified by Schwann cells (S100+) along the length of the transected nerve. Cross sections of proximal portions of the nerve demonstrate the ONI nerve contains smaller myelinated axons when compared to the contralateral healthy sciatic nerve. Electrophysiology demonstrates that the nerve is alive within the bone, as demonstrated by compound action potentials. The transected nerve demonstrated action potentials equivalent to half that of the contralateral healthy nerve, which correlates with the smaller diameter of the myelinated axons in the ONI nerve.
CONCLUSION:
Terminal ends of amputated nerves are functional following being re-directed into the medullary cavity of the femur at 5 weeks. This result provides proof of principle for the ONI model and its ability to house functional prosthetic interfaces. Work is currently underway to test various electrodes in this model.


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