FACTORS AFFECTING SENSITIVITY, OF A TRANSDUCER FOR MEASURING ANTERIORCRUCIATE LIGAMENT FORCE

In order to determine the measurements and calibration methods necessa ry to accurately measure in vivo forces in the anterior cruciate ligam ent (ACL) of the goat, an in vitro study was conducted to evaluate the effect of several factors that could influence the sensitivity of a t ransducer implanted within the ligament. Four factors were studied in six specimens: flexion angle [0 degrees, 10 degrees, 30 degrees, 50 de grees, and 70 degrees from full extension (FFE)]; tibial rotation (0 d egrees and 10 degrees of internal rotation at 30 degrees, 50 degrees, and 70 degrees flexion FFE); loading rate (cycling frequencies of 0.2, 0.5, 1.0, and 2.0 Hz); and temperature (22 degrees C and 37 degrees 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 transduc er sensitivity was calculated as the slope of the output vs force curv e in the linear response region. Transducer sensitivity varied with jo int position in each specimen, but there was no consistent trend from specimen to specimen in how the sensitivity changed. As a result, ther e were no statistically significant mean differences (p>0.05). There w ere 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 transduce r output with zero force on the ligament must be determined in vivo, a fter which in vitro calibrations may be conducted at room temperature. The variation in sensitivity suggests that the transducer is best cal ibrated on a specimen by specimen basis and at multiple joint position s.