Effect of Tibial Rotation at Time of Graft Fixation in Single and Double Bundle ACL reconstructed Knees

Introduction: Even though, the interest of scientific research is focussed on the effect and results of DB reconstruction, several aspects of how to improve the rotational stability of the human knee after single bundle reconstruction remain unsolved. The effect of tibial rotation at the time of graft fixation in ACL reconstruction remains unknown. Theoretically, different tibial rotation when the graft is fixed may influence the effect of ACL reconstruction limiting the rotational instability. Aim of the current study was to elucidate the effect of internal, neutral and external tibial rotation on the resulting knee kinematics in the ACL reconstructed knee. We hypothesize that under a simulated pivot shift test external tibial rotation at time of graft fixation of single bundle and double ACL reconstruction will restore the anterior tibial translation of the intact knee joint more closely when compared to internal tibial rotation. Methods: In ten fresh-frozen human cadaveric knees (range 62-77 years) the knee kinematics were examined using robotic/UFS testing system (KR 125, KUKA Robots, Augsburg, Germany). After the system determined the passive path, the knee kinematics were evaluated under an anterior tibial load of 134-N (to simulate a clinical KT-1000 test) and a combined rotatory load of 10 N-m valgus and 4 N-m internal tibial torque (to simulate a pivot shift test). Using this approach, the same specimen the knee kinematics can be assessed under simulated pivot shift and KT 1000 test in different conditions: intact, ACL-deficient, SB and DB ACL reconstructed and fixed in internal, neutral and external tibial rotation thereby increasing the statistical power. At the tibial side, a custom made jig was used for the fixation of the grafts. For the fixation, a standardized tension of 80 N was used and grafts were then fixed in different tibial rotation positions: 30° internal, neutral and 30° external tibial rotation. To exclude any interference the order of rotation at fixation was randomized. Statistical analyses were performed using a two-factor repeated-measures analysis of variance (ANOVA, p < 0.05). Results: Under 134 N anterior tibial load, anterior tibial translation (ATT) of the intact knee was a mean of (± standard deviation) 7.1 (±1.7) mm, 11.0 (± 1.87) mm, 11.6 (± 1.9) mm, and 10.2 (± 1.6) mm at full extension, 30, 60, and 90° of knee flexion, respectively. After the ACL was sectioned, the translations increased significantly at all flexion angles tested (p< 0.05). The resulting ATT under 134 N anterior tibial load was a mean of 14.2 (± 2.6) mm, 25.8 (± 3.0) mm, 24.0 (± 3.1) mm, and 19.4 (± 2.8) mm. After SB ACL reconstruction with the graft fixed in neutral tibial rotation, the ATT was a mean of 6.5 (± 3.0) mm at full extension, 13.6 (± 3.1) mm at 30°, 13.3 (± 3.6) mm at 60°, and 10.8 (± 2.6) at 90° of knee flexion (Fig.5). This difference was statistically not significant when compared to intact knee (p>0.05). SB reconstruction with internal tibial rotation at time of fixation resulted in a mean ATT of 7.3 (± 3.0) mm, 14.8 (± 3.3) mm, 14.5 (± 3.1) mm, and 11.6 (± 3.0) mm at full extension, 30, 60, and 90° of knee flexion, respectively. The ATT at 30 and 60° was statistically higher when compared to the intact knee ((p<0.05). External tibial rotation at time of fixation showed no statistically significant difference when compared to the intact knee. When the resulting ATT after fixation at neutral, internal and external tibial rotation were compared to each other, no significant different was to be found (p>0.05). After DB ACL reconstruction with the graft fixed in neutral, internal and external tibial rotation, ATT was statistically not significant when compared to intact knee (p>0.05). When the different tibial rotations were compared to each other, there was also statistical significant difference (p>0.05). In response to a combined rotatory load, the anterior tibial translation (ATT) for the intact knee was 5.7 (±1.3) mm, 13.0 (±1.9) mm, 12.9 (±1.7) mm, and 11.8 (±2.3) mm for 0°, 30°, 60°, and 90° of knee flexion, respectively. The values increased after sectioning of the ACL up to 9.2 (±1.9) mm at 0°, 17.8 (±2.0) mm at 30°, 18.0 (±3.5) mm at 60°, and 17.9 (±3.2) mm at 90°. The increase in ATT was statistical significant at all flexion angles (p<0.05). After SB ACL reconstruction with the graft fixed in neutral tibial rotation, the ATT showed no statistically significant difference when compared to intact knee (p>0.05). SB reconstruction with internal tibial rotation at time of fixation resulted in ATT of 16.8 (±2.8) mm at 30° of knee flexion. This difference to the intact knee was statistical significant. At all other flexion angles, there was no statistical significant difference. SB reconstruction fixed in external tibial rotation showed no significant differences in ATT when compared to the intact knee. However, the ATT at 30° was 13.2 (±1.8) mm and was significantly lower compared to SB reconstructed knee fixed in internal tibial rotation (p<0.05). At 60 and 90° flexion there was no significant difference to be found. After DB ACL reconstruction with the graft fixed in neutral tibial rotation, ATT was restored to the intact knee showing no statistically significant difference (p>0.05). DB reconstruction fixed in internal tibial rotation resulted in 8.5 (±1.6) mm and 15.1 (±2.0) mm at 0 and 30° of knee flexion significantly higher when compared to the intact knee (p<0.05). ATT after DB reconstruction fixed in external tibial rotation resulted in no significant differences compared to the intact knee. However, ATT of 5.3 (±1.4) mm at 0° and 12.4 (±1.8) mm at 30° of knee flexion was significantly lower compared to DB reconstruction fixed in internal tibial rotation. Discussion: The results support our initial hypothesis that the tibial rotation has significant influence on the resulting knee kinematics. Under a simulated Lachman test, anterior tibial translation (ATT) after SB reconstruction fixed in neutral and external tibial rotation showed no significant difference when compared to the intact knee. However, ATT after SB reconstruction in internal tibial rotation was significantly higher when compared to the intact knee at 30 and 60° of knee flexion (p<0.05). When the different tibial rotations were compared to each other, there was no significant difference. Alteration of the tibial rotation when fixing the PL bundle in DB ACL reconstruction showed no statistically significant effect on the resulting knee kinematics. It is well know that knee flexion angle at time of fixation plays a significant role for the resulting knee kinematics (Vercillo 2007, Miura et al. 2007). To the best of our knowledge, the impact of knee rotation on the resulting knee kinematics has not been well described. Our results show, that at Lachman and pivot shift test fixation of SB or DB grafts in internal tibial rotation failed to restore the intact knee kinematics and resulted in significantly higher ATT compared to reconstructions that were fixed in external tibial rotation. Due to the oblique course of the ACL and the anatomic ACL reconstructed graft, the tibia is placed in position with respect to the femur that has decreased capacity of resisting coupled anterior motion.