Wireless Metamaterial Interbody Cage for Real-Time Assessment of Lumbar Spinal Fusion In Vivo

PROJECT SUMMARY
Current technology is constrained by a lack of specificity and sensitivity for wireless force sensing
during spinal fusion. The conventional methods for acquiring force-sensing data from smart wireless spinal
implants require various bulky modules for signal generation, power supply, signal modulation, and transmission.
Consequently, there is a growing demand for wireless force sensing techniques for spinal implants characterized
by their compact form, self-powered operation, and precise data transmission. We propose to determine the
utility of the first-of-its-kind, personalized, wireless, electronic-free metamaterial interbody fusion cage
that can precisely measure forces during spinal fusion in-vivo and transmit the data wirelessly in real
time. We hypothesize that the electrical signals generated by a metamaterial fusion cage under spine motions
correlate with the applied forces, change with spinal fusion progression, and can wirelessly propagate through
animal tissue. In addition, we hypothesize that these electrical signals can be detected using electrodes placed
on the skin, enabling accurate, reliable, and focal assessments of spinal fusion progress in vivo. This is clinically
significant as it allows clinicians to consistently monitor spinal fusion progress through recorded data
in evolution curves. Our first aim will be to design and thoroughly evaluate the packaged wireless-sensor-
nanogenerator metamaterial cages to ensure they can withstand in vivo conditions. Our approach involves
tailoring the overall dimensions and stiffness of the metamaterial cages to match the 3D anatomy of each
individual sheep precisely. The cages will undergo mechanical testing and a specific hermetic sealing process
to maintain optimal sensing and signal transmission performance in vivo. Our second objective will be to validate
the hermetically sealed metamaterial cages for the wireless monitoring of posterior lumbar spinal fusion in ovine
lumbar spines. We will consider one spinal fusion cohort of six adult sheep undergoing posterior lumbar interbody
fusion at the L4/5 level. An unplated model will be considered to prevent supplemental fixation from influencing
the construct stiffness. The voltage generated and transmitted by the metamaterial cages will be wirelessly
interrogated daily using the validated electrode patches. We anticipate a decrease in the reference baseline
voltage signal measured wirelessly post-procedure as the bone fuses and the functional spinal unit (FSU) gains
stiffness. After termination, operative motion segments will be taken for mechanical testing, MicroCT analysis,
and histological assessment. The retrieved voltage recordings will be matched with the FSU stiffness and Denver
sheep fusion scale to determine the relationship between the voltage and bone growth. The proposed wireless
force sensing concept extends beyond orthopedic and spinal implants, thanks to the scalable nature of
metamaterial systems. This innovation lays the foundation for the development of cutting-edge wireless
metamaterial implants in diverse fields, including but not limited to cardiac, dental, and gastrointestinal implants.

Grant Number: 1R21AR085218-01
NIH Institute/Center: NIH
Principal Investigator: Amir Alavi