The influence of quadriceps muscle forces and tibial plateau geometry on anterior cruciate ligament strain during in-vitro simulated jump landing



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A frequent injury in both sport and recreational activities is a rupture of the anterior cruciate ligament or ACL. The ACL is a ligament in the knee that connects the femur to the tibia. It prevents excessive anterior tibial translation and contributes to overall knee joint stability. However, when excessively strained the ligament may rupture. One frequently debated theory of ACL injury is the quadriceps pull mechanism. The principle of this theory is that quadriceps muscles can be applied quickly enough and to such high force levels that the muscles themselves causes injury. To provide insight into this mechanism, as it pertains to jump landing tasks, an in-vitro simulation study was conducted. Additionally, tibial plateau geometric parameters (TPGPs) were evaluated to assess their potential effects on ACL strain during jump landing. A purpose built dynamic knee loading simulator was previously designed and constructed for the testing of human cadaver knees. This machine allows for the simulation of two key phases of jump landing: (1) flight phase, in which anticipatory muscle forces are applied in preparation for landing, and (2) landing phase, where ground contact is made and ground reaction forces are generated. Testing protocols were developed to measure strain in the anterior cruciate ligament using a differential variable reluctance transducer (DVRT) under a variety of pre-landing muscle force levels and kinematic constraints. Quadriceps and hamstrings muscle forces were used as they comprise the two major muscle groups that act on the knee. Nine human cadaver knees (age: 55.1 ± 11 years; 5 male, 4 female) were tested under unopposed quadriceps pre-activation forces (QPFs), low hamstrings forces coupled with QPFs, a restricted hip motion condition, and two different pre-landing valgus angles. After testing, a photographic technique was used to measure three tibial plateau geometric parameters; the slopes of the medial and lateral regions of the tibial plateau and the depth of the medial tibial plateau. Correlation analyses and multiple linear regression techniques were used to assess the effect of muscle forces, kinematic constraints, and TPGPs on ACL strain. In all tests, pooled results showed that ACL strain due to QPF application correlated positively with the level of QPF (p < 0.001). Additionally, ACL strain during the simulated landings showed a significant negative correlation with QPF (p = 0.035). Lastly, total strain (pre-landing + landing) showed no significant correlation with QPF (p = 0.685). Multiple regression models including TPGPs showed that they were all significant predictors of at least one type (pre-landing, landing, or total) of ACL strain. This study yielded two major findings: (1) the quadriceps-pull mechanism may not be a viable mechanism of non-contact ACL injury during jump landing, and (2) tibial plateau geometry was shown to influence ACL strain behavior during dynamic activities. The results of this study show that quadriceps muscle forces do not appreciably affect the level of total ACL strain during jump landing. This finding opposes much of the current literature on the quadriceps-pull mechanism, however the combined effects of QPF increasing pre-landing ACL strain and decreasing landing ACL strain illustrate how this is possible. ACL strain increases under increasing QPF prior to landing, but the increase in muscle forces also increases joint reaction forces in the knee, promoting stability during the landing phase of a jump.