Factors affecting sensitivity of a transducer for measuring anterior cruciate ligament force

In order to determine the measurements and calibration methods necessary to accurately measure in vivo forces in the anterior cruciate ligament (ACL) of the goat, an in vitro study was conducted to evaluate the effect of several factors that could influence the sensitivity of a transducer implanted within the ligament. Four factors were studied in six specimens: flexion angle [0°, 10°, 30°, 50°, and 70° from full extension (FFE)]; tibial rotation (0° and 10° of internal rotation at 30°, 50°, and 70° flexion FFE); loading rate (cycling frequencies of 0.2, 0.5, 1.0, and 2.0 Hz); and temperature (22°C and 37°C). Anteroposterior tibial displacements were applied to the specimens following tissue resection to isolate the ACL. The resultant ACL force magnitude was measured with a multi-component load cell, and transducer sensitivity was calculated as the slope of the output vs force curve in the linear response region. Transducer sensitivity varied with joint position in each specimen, but there was no consistent trend from specimen to specimen in how the sensitivity changed. As a result, there were no statistically significant mean differences (p>0.05). There were no significant differences and little variation in sensitivity due to changes in either loading rate or tissue temperature, although the latter produced a voltage offset. The results show that the transducer output with zero force on the ligament must be determined in vivo, after which in vitro calibrations may be conducted at room temperature. The variation in sensitivity suggests that the transducer is best calibrated on a specimen by specimen basis and at multiple joint positions.