Background Little is known about how much Anterior cruciate ligament (ACL) strain is increased by a valgus moment versus an internal axial tibial torque loading during a unipedal jump landing.

Objective To test the (null) hypothesis that the anteromedial (AM) bundle ACL relative strain would not be significantly different under a simulated one-foot jump landing imposing an internally-directed tibial torque versus one imposing a valgus knee moment.

Design 12 cadaveric knees (mean (SD) age: 65.0 (10.5) years) were tested to study the effect of internally-directed tibial torque on the AM-ACL relative strain. Pretensioned quadriceps, hamstring and gastrocnemius muscle-tendon unit forces maintained an initial knee flexion angle. A compound impulsive test load (2*BW compression, flexion moment and internal tibial torque or valgus knee moment) was applied to the tibia while recording the 3-D knee loads and tibofemoral kinematics. AM-ACL relative strain was measured using a 3 mm DVRT. Each loading condition (ie, ‘internal’ or ‘valgus’ loading) was compared against ‘baseline’ loading condition under compression and flexion moment. Results were compared with those from Withrow et al. (2005) for 11 cadaveric knees of similar age tested on the same apparatus to investigate the effect of valgus knee loading on AM-ACL relative strain. The Wilcoxon Signed-Rank test was used to test the null hypotheses (p<0.05 being considered significant).

Setting A cross-sectional, repeated measures, in vitro laboratory study.

Assessment of risk factors Independent variables included the magnitude of the impulsive compressive force, flexion moment, and internal tibial torque or valgus knee moment.

Main outcome measurement AM-ACL relative strain.

Results Normalised peak AM-ACL relative strain increased 117% under internally-directed tibial torque (17 ± 4 N-m), whereas the corresponding value increased 30% under valgus knee loading (133 ± 29 N-m) (p=0.016).

Conclusion Internal axial tibial torque increases AM-ACL relative strain more than a valgus load under a simulated jump landing.