The relationship between quadriceps muscle force, knee flexion, and anterior cruciate ligament strain in an in vitro simulated jump landing

Background: An instrumented cadaveric knee construct was used to quantify the association between impact force, quadriceps force, knee flexion angle, and anterior cruciate ligament relative strain in simulated unipedal jump landings. Hypothesis: Anterior cruciate ligament strain will correlate with impact force, quadriceps force, and knee flexion angle. Study Design: Descriptive laboratory study. Methods: Eleven cadaveric knees (age, 70.8 [19.3] years; 5 male; 6 female) were mounted in a custom fixture with the tibia and femur secured to a triaxial load cell. Quadriceps, hamstring, and gastrocnemius muscle forces were simulated using pretensioned steel cables (stiffness, 7 kN/cm), and the quadriceps tendon force was measured using a load cell. Mean strain on the anteromedial bundle of the anterior cruciate ligament was measured using a DVRT With the knee in 25 degrees of flexion, the construct was vertically loaded by an impact force initially directed 4 cm posterior to the knee joint center. Tibiofemoral kinematics was measured using a 3D optoelectronic tracking system. Results: The increase in anterior cruciate ligament relative strain was proportional to the increase in quadriceps force (r(2) = 0.74; P < .00001) and knee flexion angle (r(2) = 0.88; P < .00001) but was not correlated with the impact force (r(2) = 0.009; P = .08). Conclusion: The increase in knee flexion and quadriceps force during this simulated 1-footed landing strongly influenced the relative strain on the anteromedial bundle of the anterior cruciate ligament. Clinical Relevance: These results suggest that even in the presence of knee flexor muscle forces, the increase in quadriceps force required to prevent the knee from flexing during landing can place the anterior cruciate ligament at risk for large strains.