The value of intraoperative isometry measurements in anterior cruciate ligament reconstruction: an in vivo correlation between substitute tension and length change.

With the objective to evaluate an "isometry" measurement in an anterior cruciate ligament (ACL) substitute, in vivo measurements were taken on 10 patients undergoing reconstruction due to chronic ACL deficiency. Change in intraarticular length of a 1.2-mm test ligament, measured with an isometer, was correlated to the tension created in the same test ligament after fixation, measured with a piezoelectric load cell. The knee was passively moved through the 0-100 degrees range with the patient under general anaesthesia. Good statistical and visual correlations between length change and tension curves were found in individual knees. A correlation between total length change and maximum tension, for all knees grouped, was also found. Individual knees showed large variation in ability of the tissue to absorb load, resulting in a wide range of N/mm ratios between length and tension. Three knees with an isolated ACL injury all showed the least length change, implying a better restoration of kinematics in the absence of associated injuries. In nine of 10 knees the length change pattern could be used to identify the location of the femoral drill channel, as determined on an intraoperative lateral projection of the knee. Intraoperative isometry measurement can be used to predict the tension pattern in the reconstructed knee, but not the magnitude of tension. It will be useful to the surgeon in avoiding an anterior femoral ligament insertion site, which might threaten the integrity of the graft by tension rise in flexion.