Novel handheld sensors for in-vivo soft tissue tension measurement during orthopedic surgery

University of Minnesota Ph.D. dissertation. December 2013. Major: Mechanical Engineering. Advisor: Professor Rajesh Rajamani. 1 computer file (PDF); xii, 99 pages.This thesis focuses on the development of handheld sensors for in vivo measurement of soft tissue tension. The sensors will aid the surgeon in balancing forces in soft tissues during total knee arthroplasty, ACL repair, hip replacement surgeries, shoulder stabilization and other orthopaedic procedures by providing real-time measures of tension in soft tissue. The proposed method utilizes the application of an unknown transverse force on the soft tissue using a handheld probe. An array of miniature sensors on the probe is used to measure the resulting curvature of the soft tissue and tissue tension is estimated from this measurement.The first generation sensor developed in the project utilized capacitive sensing units to measure the forces required to displace the ligament by a fixed amount determined by the pattern of bumps in the sensor. These force values were used to estimate the tension in the ligament. The sensing concept was experimentally demonstrated; however it was not found to be suitable for hand held applications due to restrictions involved with the point of the contact along the ligament and also due to unreliability associated with estimates in the presence of noise.A second generation sensor design was developed to estimate the tension from displacements of three points on the sensor under three transverse loads. A sensor was fabricated using soft rubber bumps. The sensor works reasonably well for controlled orientations; however it is not suitable for hand held applications due to its sensitivity to orientation errors. Several challenges related to micro-fabrication also cause imperfections in the sensor and introduce variability in the results.The third generation sensor utilized changes in magnetic field to measure the displacements and curvature of the soft tissue. Linear coil springs were used in the sensor to ensure accurate calculation of forces from force-displacement relations. The design allowed for higher displacements within the sensor and hence was found to be significantly less sensitive to orientation errors as compared to the second generation sensor. The experimental results both during controlled orientations and handheld operation show that the sensor can measure tension values up to 100 N with a resolution of 10 N or better. The feasibility of the sensor to measure tension in biological tissues is also demonstrated using experimental tests with a turkey tendon.The developed magnetic sensor was also reconfigured for use in two other medical diagnosis applications. The sensor was able to measure tissue elasticity with five times better resolution and four times the range of other elasticity sensors previously proposed in the literature. The sensor could also be used to measure compartment pressure for non-invasive diagnosis of compartment syndrome. In-vitro results using both a pneumatic compartment and an agarose-gel compartment showed that the sensor could accurately measure compartment pressure and could be a non-invasive alternative to invasive catheter based measurements for diagnosing compartment syndrome