Posterior shear force and posterior tibial displacement using a sling bridge in patients with posterior cruciate ligament insufficiency

Background Rehabilitation training of hamstring muscle with deep knee flexion should be restricted following the posterior cruciate ligament (PCL) reconstruction because it stresses the PCL graft. To perform rehabilitation training of the hamstring muscle without the stress to the PCL, we have developed a sling bridge exercise (SB), which is a hamstring exercise with a sling at a lower leg to lift the buttocks in a supine position. Our hypothesis is that anterior traction force generated by a sling decrease strain on the PCL during SB. Objective To analyze posterior shear force and amount of posterior tibial displacement by the SB and compare with those by the conventional bridge (CB) exercise. Design Quasi-experimental study. Setting Controlled laboratory research. Participants Five healthy male volunteers and one patient with PCL insufficiency. Intervention Position of physical segments and force on floor were analyzed by a 3D motion capture system and force plate. Posterior tibial displacement was measured by lateral XP view of the knee during the exercises. Main outcome measurements Posterior shear force of the knee was calculated by using a biomechanical musculoskeletal model. Posterior tibial displacement was measured by digitised XP. Results Posterior shear force of the healthy volunteers was 849.0 ± 322.5 N during CB and 360.6 ± 75.5 N during SB. In a patient with the PCL insufficiency, side-to side difference in posterior tibial displacement by SB (1.2 mm) was smaller than that of CB (3.3 mm). Conclusion The SB hamstring exercise causes less posterior shear force and resulted in less posterior tibial displacement than a CB exercise. Therefore, a sling hamstring exercise is safer than a CB exercise in rehabilitation training for the patients with PCL reconstruction of the knee.