Factors influencing the output of an implantable force transducer

The objective of this study was to evaluate the performance of the Arthrosc opically Implantable Force Probe (AIFP; MicroStrain, Burlington VT) for mea suring force in a patellar tendon graft. Transducer drift, reproducibility of output due to the number of loading cycles and device location, and sens itivity to the tendon cross-sectional area were investigated. The AIFP was initialized, and then implanted into five human patellar tendon grafts thre e times. twice within the same location and once in a different location. T he tendons were cyclically loaded in uniaxial tension for 500 cycles in eac h insertion site. The AIFP was then removed from the tendon and the baselin e output was remeasured. It was determined that transducer drift was neglig ible. The relationship between the tensile load applied to the graft and AI FP output was quadratic and specimen dependent. The cyclic load response of the tendon-AIFP interface demonstrated a 24.9% decrease over the first 20 loading cycles, and subsequent cycling yielded relatively reproducible outp ut. The output of the transducer varied when it was removed from the tendon and then reimplanted in the same location (range 3.7-109.4% error), as wel l as in the second location (range 1.5-202.8% error). No correlation was ob served between the cross-sectional area of the tendon and transducer output . This study concludes that implantable force probes should be used with ca ution and calibrated without removing the transducer from the graft. (C) 20 00 Elsevier Science Ltd. All rights reserved.