Anterior Cruciate Ligament Fatigue Failures in Knees Subjected to Repeated Simulated Pivot Landings

Background: It is not known whether the human anterior cruciate ligament (ACL) is susceptible to fatigue failure as a result of repetitive loading or whether certain knee morphologic characteristics increase that risk. Hypotheses: The number of knee loading cycles required to fail an ACL by fatigue failure is unaffected by the magnitude of the external load delivered to the knee joint. Furthermore, sex, ACL cross-sectional area, and lateral tibial slope will not affect the number of loading cycles to ACL failure. Study Design: Controlled laboratory study. Methods: Knee pairs from 10 cadaveric donors (5 female) of similar age, height, and weight were imaged with 3-T magnetic resonance imaging to measure lateral tibial slope and ACL cross-sectional area. One knee from each pair was then subjected to repeated application of a load of 3 times body weight (3*BW), while the other knee was subjected to a 4*BW load, both involving impulsive compression force, knee flexion moment, and internal tibial torque combined with realistic trans-knee muscle forces. The resulting 3-dimensional tibiofemoral kinematics and kinetics were recorded, along with ACL relative strain and quadriceps, hamstring, and gastrocnemius muscle forces. The loading cycle was repeated until the ACL ruptured, a 3-mm increase in cumulative anterior tibial translation occurred, or a minimum of 50 trials was reached. Results: Eight of 10 knees failed under the 4*BW load (mean +/- SD cycles to failure, 21 +/- 18), while 5 of 10 knees failed under the 3*BW load (mean +/- SD cycles to failure, 52 +/- 10). Four knees exhibited a 3-mm increase in anterior tibial translation, 7 knees developed partial or complete visible ACL tears, and 2 knees developed complete ACL tibial avulsions. A Cox regression showed that the number of cycles to ACL failure was influenced by the simulated landing force (P = .012) and ACL cross-sectional area (P = .022). Donor sex and lateral tibial slope did not influence the number of cycles to ACL failure. Conclusion: The human ACL is susceptible to fatigue failure when pivot landings of 3*BW or more load the knee repeatedly within a short time span. An ACL with a smaller cross-sectional area is at greater risk for this type of failure. Clinical Relevance: The results show that the human knee can only withstand a certain number of 3*BW or greater jump loading cycles within a short time period before the ACL will fail. Therefore, limiting the increase in the number and severity of pivot landing maneuvers performed over a week of training would make sense from an injury prevention viewpoint.