Back pain relief from linear sensors

June 8, 2006
THE SMALLEST magnetostrictive sensor available has been designed into a surgical tool used in a non-invasive treatment for degenerative disc disease. The Dascor disc arthroplasty procedure is minimally invasive, so the patient requires only a small incision for the surgeon to gain access to the affected area. The nucleus material of a disc is removed and replaced with an artificial nucleus that restores the proper gap height between the vertebrae with a patient-specific implant.

Initial versions of the surgical tool delivered the two-part polymer artificial nucleus to the site pneumatically and hade control issues that made it difficult to ensure the desired amount of polymer was injected. Disc Dynamics, Eden Prairie, Minn., the developers of the technique and tools; and Devicix (Chaska, Minn., a medical design firm, then incorporated an MTS C-Series linear sensor into a microprocessor-based solution that has allowed the procedure and equipment to be approved for use in Europe and to undergo clinical trials in the U.S.

During a two-year development cycle, Devicix integrated software, electrical and mechanical disciplines into a microprocessor-based tool that allows closed-loop control of polymer injection via feedback from linear position and pressure sensors. Instead of a pneumatic system, the system now uses a motor driving a ball screw monitored by the linear position sensor to deliver an accurately measured quantity of the two-part polymer.

Forward and rearward travel limits are calibrated from the C-Series position feedback, eliminating the need for redundant limit switches. Originally installed to provide power-up position information to calibrate the position of rotary incremental encoders, the absolute output of the sensor proved resolute and reliable enough to eliminate the encoders. Calculated required volume from a CT scan is compared with filling volume in the balloon, calculated from the sensor’s feedback, ensuring the correct final size while preventing the balloon from becoming too large.