Originally Published MPMN October 2006

PROFILE

Magnetorestrictive Technology Improves Control in Disk Arthroplasty Treatment

A small sensor is useful for high-volume products with constrained space

After initial versions of a surgical tool for minimally invasive treatment of degenerative disk disease showed significant control issues, a microprocessor-based linear sensor was incorporated into the system. Disc Dynamics (Eden Prairie, MN; www.diskdyn.com), the developers of the technique and tool, and Devicix (Chaska, MN; www.devicix.com), a medical design firm, used the C-Series linear sensor from MTS Systems Corp. (Cary, NC; www.mts.com)

The procedure, called the Dascor disk arthroplasty system, is minimally invasive, so only a small incision in the patient is needed for the surgeon to gain access to the affected area. The nucleus material of a disk 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 used to install the artificial nucleus delivered the two-part polymer to the site pneumatically. However the device had control issues that made it hard to ensure the desired amount of polymer was injected.

The technique’s developers, Disc Dynamics, worked with Devicix to integrate the procedural requirements into a viable surgical tool that solved these problems and could be easily operated by physicians and trained technicians. The firm 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 pneumatics, the system now uses a motor driving a ball screw monitored by a C-Series Temposonics magnetorestrictive sensor to deliver an accurately measured quantity of the two-part polymer.

“Temposonics was chosen because of its no-wear nature, no drift over time, no required calibration, and ease of connection to the mechanical system,” says Jesse Russell, commercial product marketing manager at MTS.

Forward and rearward travel limits are calibrated from the C-Series position feedback, eliminating the need for redundant light switches. The absolute output of the C-Series proved so resolute and reliable that the encoders were eliminated. Calculated required volume from a CT scan is compared with fill volume within the delivery balloon, calculated from the sensor’s feedback, ensuring the correct amount of polymer while preventing the balloon from becoming too large.

Measuring 35 mm, the C-Series head is 45% smaller than its predecessors, and the 4-mm sensor shaft diameter, 18-mm dead zone, and 21-mm null zone are at least 50% smaller. This improves the overall length-to–active zone ratio, which is key in achieving the small, desktop-sized devices that are in increasing demand in the healthcare market.

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