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A device to non-invasively assess spinal fusion was prototyped and bench tested. The device performs real-time calculations of the separation distance between the vertebrae involved in the fusion, postoperatively. The device is comprised of a passive implantable sensor and external interrogator. Sensor implantation would occur during spinal fusion surgery. During postoperative follow-ups, the physician will use the external interrogator to measure the vertebral separation during motion. The performance of the prototype device was investigated using an experimental bench that simulated vertebral motion parameters. A randomized study of performance factors including sensor displacement, interrogator-sensor separation, and the presence of bovine skeletal muscle was conducted and the results statistically analyzed. The bench testing verified that the displacement of sensor components could discriminate displacements to ±0.5 mm. In addition, the distance between the interrogator and sensor does not affect the device response, indicating that the interrogator position is not critical. The technical concepts behind this prototype have been shown to be effective and feasible.