Method for establishing and measuring in vivo forces in an anterior cruciate ligament composite graft: Response to differing levels of load sharing in a goat model

In order to determine the appropriate load history for optimal remodeling of an anterior cruciate ligament graft, methods for establishing and measuring graft forces in vivo are required. Our objectives with this study were to (a) develop a method in which the graft force due to an external load could be set to a preselected value in a living animal, (b) show that this force could be maintained after fixation, and (c) determine what happens to the forces after the animal has functioned for as long as 2 weeks postoperatively, when differing levels of load sharing between the segments had been set at surgery. The anterior cruciate ligament was reconstructed in 12 goats with use of a bone‐patellar tendon‐bone graft and a synthetic augmentation device. The forces in the graft segments were established, at the time of surgical fixation, with use of a force‐setting technique. In five animals, the tendon segment was set to carry 90% of the total graft force; in the remaining seven animals, the augmentation segment was set to share 90% of the total graft force. Graft forces were measured, with the use of buckle transducers mounted extra‐articularly over the anterior tibia, under a 67 N anterior tibial load at 60° of knee flexion before and after fixation and at 2 weeks postoperatively. When the tendon was the high force‐carrying member, the force in the tendon decreased by an average of 25% during the 2 weeks after surgical fixation, while the force in the augmentation segment showed a significant increase of 111%. When the augmentation segment was the high force‐carrying segment, the force in that segment decreased by an average of 10% during the 2 weeks and the tendon force increased by 47%. These data suggest that when the tendon was set to carry high force, it may have stretched, due to biological changes in the tissue, and that the augmentation segment was not as greatly affected over the 2 postoperative weeks when it was set to carry most of the total load. Because of the frictional effects of the graft segments passing around the osseous contours of the anterior tibia and the fact that the calibrations of the buckle transducer from surgery were used to reduce the data on force at 2 weeks postoperatively, there are uncertainties related to the measured graft segment forces in this study. However, if the animals had been killed and the buckle transducers calibrated immediately after the test at 2 weeks, these uncertainties would not exist.