THE MEASUREMENT OF ELONGATION OF ANTERIOR CRUCIATE-LIGAMENT GRAFTS IN-VIVO

Many investigators who have studied the mechanical behavior of anterio r cruciate-ligament grafts have attributed the increase in anterior tr anslation of the tibia relative to the femur (an increase in the anter ior laxity of the knee joint) to the temporal changes in the material behavior (strength and elastic properties) of the graft that occur thr oughout the process of remodeling. However, with the onset of motion o f the joint, it is unclear whether the repeatable mechanical behavior of the graft remains unchanged immediately after fixation, if the fixa tion slips, or if the length of the graft changes and produces an incr ease in anterior translation of the tibia relative to the femur. It is also unknown if procedures performed by different surgeons, using sim ilar graft material and similar operative techniques, can produce simi lar mechanical behavior of the graft, or if the behavior of the graft is similar to that of the normal anterior cruciate ligament. In an eff ort to address these questions, two surgeons performed a reconstructio n of the anterior cruciate ligament on ten patients each (groups 1 and 2) with use of a bone-patellar ligament-bone graft. Immediately after fixation of the graft, a Hall-effect transducer was implanted to meas ure the changes in the length of the mid-substance of the graft while the knee was moved through twenty cycles of passive flexion-extension. Unlike the length pattern of the normal anterior cruciate ligament, t he length pattern of the graft changed during the initial cycles of pa ssive motion of the knee. We defined this phenomenon as the cyclic res ponse of the graft and characterized it by calculation of the changes in the length of the graft at fixed positions of the knee across the m ultiple cycles of passive motion. In some patients, the length of the graft increased through the initial passive-motion cycles, while in ot hers, it decreased. With the knee nearly extended, the predicted incre ase in anterior translation of the tibia relative to the femur, result ing from the increase in the length of the graft, was a maximum of 1.0 millimeter. This indicates that increases in anterior translation of the tibia relative to the femur can occur immediately after reconstruc tion of the anterior cruciate ligament and that changes in the length of the graft occur after fixation at loads that are less than the ulti mate failure load of the graft or of the fixation. These findings sugg est that the cyclic response of the graft, and not just the structural properties of the graft or the ultimate failure load of the graft or of the fixation, should be considered during rehabilitation. The graft demonstrated reproducible behavior by the fifteenth cycle of passive motion of the knee in all patients. Therefore, for the repeatable twen tieth cycle of passive motion, a comparison of the local elongation be havior of the graft (calculated with use of the data obtained with the Hall-effect transducer) was made between the two groups of patients. With the numbers studied, we detected no significant differences in th e local elongation behavior between the two groups. When the local elo ngation data for the bone-patellar ligament-bone graft for the twentie th cycle of passive motion were compared with data obtained previously for the normal anterior cruciate ligament in vivo, the results were s imilar. This suggests that it may be possible to restore the local elo ngation behavior of the normal anterior cruciate ligament at the time of reconstruction.