The implanted electrical resistance strain gauge: in vitro studies on data integrity.

Area of Science:

• Biomedical Engineering
• Materials Science

Background:

• Electrical resistance strain gauges are crucial for measuring in vivo loading on spinal implants.
• Tissue fluid ingress can adversely affect strain gauge performance and accuracy.
• Accurate in vivo loading data is essential for assessing spinal fusion and implant success.

Purpose of the Study:

• To investigate and mitigate the effects of tissue fluid ingress on strain gauges used for spinal implants.
• To ensure the long-term accuracy of strain measurements for monitoring bone fusion.
• To develop signal compensation methods for persistent errors.

Main Methods:

• Developed a strain gauge package with maximum metallic coverage (stainless steel foil on epoxide) to minimize moisture absorption.
• Conducted cyclic strain wet endurance testing in saline to simulate the in vivo environment.
• Performed stepped-load and quasi-static (passive) studies to evaluate strain measurement accuracy over time.

Main Results:

• The protected strain gauge showed a fall in indicated quasi-dynamic strain of less than 1.5% at 24 weeks.
• Stiffening of the fusion mass can be deduced with similar accuracy, with creep as a secondary effect.
• Quasi-static studies revealed a total signal error exceeding 16% by 24 weeks due to grid corrosion and long-term drifts.

Conclusions:

• The developed packaging effectively minimizes moisture ingress, enabling accurate quasi-dynamic strain measurements for up to 24 weeks.
• While dynamic measurements are reliable, quasi-static measurements require signal compensation due to cumulative errors.
• An empirical characterization of total error versus time was derived to aid in signal compensation strategies.