AN IMPLANTABLE TRANSDUCER FOR MEASURING TENSION IN AN ANTERIOR CRUCIATE LIGAMENT GRAFT

The goal of this study was to develop a new implantable transducer for measuring anterior cruciate ligament (ACL) graft tension postoperativ ely in patients who have undergone ACL reconstructive surgery. A uniqu e approach was taken of integrating the transducer into a femoral fixa tion device. To devise a practical in vivo calibration protocol for th e fixation device transducer (FDT), several hypotheses were investigat ed: (I) The use of a cable versus the actual graft as the means for ap plying load to the FDT during calibration has no significant effect on the accuracy of the FDT tension measurements; (2) the number of flexi on angles at which the device is calibrated has no significant effect on the accuracy of the FDT measurements; (3) the friction between the graft and femoral tunnel has no significant effect on measurement accu racy. To provide data for testing these hypotheses, the FDT was first calibrated with both a cable and a graft over the full range of flexio n. Then graft tension was measured simultaneously with both the FDT on the femoral side and load cells, which externally. Measurements were made with both standard and overdrilled tunnels. The that neither the means of applying the calibration lend, the number of flexion angles u sed for calibration, nor the tunnel size had a significant effect on t he accuracy of the FDT. Thus a cable may be used instead of the graft to transmit lends to the FDT during calibration, thus simplifying the procedure. Accurate calibration requires data from just three flexion angles of 0, 45, and 90 deg and a curve fit to obtain a calibration cu rve over a continuous range of flexion within the limits of this angle group. Since friction did not adversely affect the measurement accura cy of the FDT, the femoral tunnel can be drilled to match the diameter of the gl aft and does not need to be overdrilled. Following these pr ocedures, the error in measuring graft tension with the FDT averages l ess than 10 percent relative to a fill-scale load of 257 N.