Biomechanical analysis of knee hyperextension and of the impingement of the anterior cruciate ligament: a cinematographic MRI study with impact on tibial tunnel positioning in anterior cruciate ligament reconstruction

This study analyzed the interaction between the anterior cruciate ligament (ACL) and the intercondylar notch roof (INR) in hyperextension of the knee using magnetic resonance cinematography. Cinematographic image series of 15 knees were investigated. Two independent observers identified the image th at displayed the beginning of contact between the ACL and the INR. They det ermined knee extension on this image and on the image that displayed maximu m hyperextension of the knee. Correlations between a variable representing impingement and the inclination angle of the INR, the anterior laxity of th e knee, and full hyperextension were examined. Theoretical, impingement-fre e tibial tunnel positions for the knees were calculated as a percentage of the anteroposterior tibial width. All ACLs of the knees in this study made contact with the INR. The average extension angle at the beginning of impin gement was -6.3 +/- 3.8 degrees. There were significant correlations betwee n impingement and maximum manual displacement as measured with the arthrome ter (r = 0.77; P < 0.001), maximum hyperextension (r = 0.67; P = 0.007), an d notch roof angle (r = -0.73; P = 0.002). There were biomechanically accep table tunnel positions for all knees but one. Hyperextension is physiologic ally associated with impingement of the ACL. In uninjured knees there was a correlation between ACL impingement and hyperextension, inclination of the INR, and maximum manual displacement of the tibia. Impingement free tibial tunnel positioning is possible in most knees without notchplasty.