Method for setting total graft force and load sharing in augmented ACL grafts

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Abstract
The objective of this study was to develop a method for obtaining a controllable and reproducible immediate postoperative mechanical state in a knee with an anterior cruciate ligament (ACL) reconstruction. This method, called the force‐setting technique, was demonstrated using a composite graft consisting of the middle third of the patellar tendon with bone blocks (PT) and the ligament augmentation device (LAD). The total graft force was set to match the force in the intact ACL at 30° flexion with the knee under the same standardized external load, while at the same time the load sharing between the biologic and augmentation components was controlled. The total graft force was set to match the ACL force three separate times in each knee, with ratios of load sharing set at the following levels: 50% PT‐50% LAD, 25% PT‐75% LAD, and 75% PT‐25% LAD. ACL, PT, LAD, and collateral forces were measured using buckle transducers, and three‐dimensional knee motion was measured using an instrumented spatial linkage as 90 N anteriorly directed tibial loads were applied to eigth specimens at 0°, 30°, 60°, and 90° flexion with an intact ACL, an excised ACL, and the three load‐sharing reconstruction states. The total graft force could be consistently set to within an average of 2% of the intact ACL force at 30° flexion, and load sharing between the graft segments could be set to within an average of 5.1% of the desired ratio at 30°. The load sharing was maintained at the other flexion angles except 90° when the PT was to have carried 25 and 50% of the total graft force, and extension when the PT was to have carried 75% of the graft force. When averaged over all specimens and flexion angles, the total graft force was 15% greater than the force in the ACL, and this was not statistically significant.