The Popliteus Bypass Graft is biomechanically superior to the isometric Larson Technique for posterolateral instabilities of the knee

AIMS AND OBJECTIVES: Injuries of the posterior cruciate ligament (PCL) are most often accompanied by injuries of the posterolateral corner (PLC) and the ligamentum collaterale fibulare (LCL). This leads to a combined dorsal- and external rotational instability. In Germany most of these types of injuries are treated by an arthroscopic reconstruction of the PCL and combined posterolateral augmentation in the way, of an isometric “Larson-Procedure”. Techniques, which reconstruct the LCL and PLC in a more anatomic way provide better clinical results according to current literature. Comparative studies of these two types of reconstructions do not exist, biomechanical studies are inhomogeneous and the isometric Larson-technique has not been adequately tested so far. In this study the isometric (extra anatomic) Larson-technique was tested against a more anatomic reconstruction of the LCL and PLC with a popliteus bypass graft. MATERIALS AND METHODS: 20 human knee specimen were divided into Group A and B. The PCL, as well as the PLC and LCL were dissected and kinematic measurements were determined using a robotic and optical tracking system. The knee kinematics were determined for 134 N posterior loads, 10 Nm varus loads and 5 Nm external rotational loads in 0°, 20°, 30°, 60° and 90° of knee flexion. All specimens underwent a double bundle PCL reconstruction. In Group A the reconstruction of the PLC was accomplished by a popliteus bypass graft and an additional anatomic LCL graft. In Group B the reconstruction was performed using a Larson-technique with a single femoral tunnel and a sling through the head of the fibular. The protocol was repeated and the differences in the two groups where analyzed (2W-Anova and t-Test, p=0,05) RESULTS: Both groups gained a significant increase of instability after dissecting the ligamentous structures (deficient) in all algorithms tested. Postoperatively both groups achieved a significant improvement in all these parameters. Group A with the popliteus bypass technique revealed significant 37±8% less external rotational instability compared to group B (p<0,03). In contrast to group A, group B presents a remaining increase of external rotation compared to the intact knee. The popliteus bypass in group A reduced the external rotational instability to less than 1,3 degree difference compared to the intact state on average. No significant results were found between the two groups in terms of varus instability and posterior loads. CONCLUSION: In this study we showed for the first time, that the most frequently used posterolateral reconstruction technique (isometric Larson) has limited biomechanical properties compared to a more anatomic reconstruction (popliteus bypass) in terms of reducing external rotation. Whether this is the cause of the remaining posterolateral instability in patients treated with the Larson technique and whether it can be prevented by using a popliteus bypass graft is subject of future clinical studies.