In vivo strain measurements from hardware and lamina during spine fusion

Currently, spine fusion is determined using radiography and clinical evaluation. There are discrepancies between radiographic evidence and direct measurements of fusion, such as operative exploration and biomechanical or histological measurements. In order to facilitate the rapid return of patients to normal activities, a monitoring technique to accurately detect fusion in vivo and to prevent overload during the postoperative period would be useful. The objectives of this study were to develop an implantable monitoring system consisting of CPC-coated strain gauges and a radio transmitter to detect the onset of fusion and measure strain during postsurgical activities. A patient underwent anterior release and fusion, followed by posterior instrumentation and fusion with segmental spinal instrumentation. Four strain gauges were placed during surgery. One was attached to the left-side rod and one to each of the lamina at T9, T10, and T11. An externally powered implanted radio transmitter attached to the gauges was placed in a subcutaneous pouch. Strains were monitored weekly and tabulated during various activities for 7 months. Peak strains during twisting and bending were tabulated to detect the onset of fusion. Strains were also recorded during activities such as climbing off an examination table, rising from a chair, and climbing stairs. Strains collected from the left rod indicated that, immediately postoperatively, it was loaded at acceptable levels. The largest and most consistent strain changes measured from the lamina were recorded during twisting.