CAN AN ISOMETER PREDICT THE TENSILE BEHAVIOR OF A DOUBLE-LOOPED HAMSTRING GRAFT DURING ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

An isometer, a highly compliant spring-scale device for measuring sutu re displacement, has been used intraoperatively by surgeons to select the optimal placement of the femoral tunnel for an anterior cruciate l igament graft. The isometer measures the displacement of a suture cent ered in a tibial tunnel and attached to an intraarticular location on the femur before the femoral tunnel is drilled. Because the placement of the femoral tunnel strongly impacts the tensile behavior of an ante rior cruciate ligament graft and because surgeons have used the amount of suture displacement to guide the placement of the femoral tunnel, the objective of this study was to determine the ability of an isomete r to predict graft tension. In 14 patients undergoing reconstructive s urgery of the anterior cruciate ligament, an isometer was used to meas ure suture displacement during passive knee motion for a provisional f emoral tunnel location. An electrogoniometer recorded the flexion angl e of the knee. The femoral tunnel was drilled. A double-looped semiten dinosus and gracilis autograft was inserted around a post in the femor al tunnel, and the tension in the four limbs of the graft exiting the tibial tunnel was measured during passive knee motion. Graft-tension v ersus knee-flexion-angle curves revealed that each knee exhibited one of two distinct curve shapes: L-shaped, characterized by the maximum t ension occurring at full extension and a nearly flat profile from 35 t o 90 degrees of flexion, or U-shaped, with elevated tensions at 80-90 degrees of flexion (p < 0.001) reaching at least half of the tension i n full extension. Because the shapes of the suture-displacement versus flexion-angle curves were more consistently L-shaped, the intraoperat ive measurement of suture displacement was not a useful predictor of e ither the increase in tension in the graft with flexion or the maximum tension in the graft.