Lumbar spinal fusion surgery continues to experience major growth in the United States and worldwide. The surgery is performed by implanting spinal rods and screws within an incision on the lumbar region of the spine. This implanted hardware provides the initial mechanical stiffness until the morselized bone and bone growth factors generate new bone and can provide long term fixation. After surgery, development of the fusion is evaluated with radiographs, but determination of this fusion takes many months as the bone must first mineralize. The early stages are not visible on radiographs; however, this non-mineralized bone does provide substantial mechanical stiffness that could be measured with a sensor. As the spine moves and flexes, it creates a bending moment in the spinal rod, which could be measured as a strain. When initially implanted, this rod would experience its peak strain, but this would decrease as the bone shared some of the load. By periodically sampling the strain with a sensor, a curve could be generated that showed the overall progress of the fusion.
To maximize the output signal, an interdigitated capacitor design was implemented as the most effective way to maximize the capacitance measurement. A design using 51 free-standing, interdigitated fingers resulted in 50 parallel plate capacitors. The interdigitated capacitor was connected to a Low-Z Amplifier circuit and attached to a spinal rod. The rod was then flexed to simulate spinal bending, and the capacitance changed as expected under physiological loads.