ABSTRACT:The relationships between non-contact anterior cruciate ligament injuries and the underlying biomechanics are still unclear, despite large quantities of academic research. The purpose of this research was to study anterior cruciate ligament strain during jump landing by investigating its correlation with sagittal plane kinetic/kinematic parameters and by creating an empirical model to estimate the maximum strain. Whole-body kinematics and ground reaction forces were measured from seven subjects performing single leg jump landing and were used to drive a musculoskeletal model that estimated lower limb muscle forces. These muscle forces and kinematics were then applied on five instrumented cadaver knees using a dynamic knee simulator system. Correlation analysis revealed that higher ground reaction force, lower hip flexion angle and higher hip extension moment among others were correlated with higher peak strain (p < 0.05). Multivariate regression analyses revealed that intrinsic anatomic factors account for most of the variance in strain. Among the extrinsic variables, hip and trunk flexion angles significantly contributed to the strain. The empirical relationship developed in this study could be used to predict the relative strain between jumps of a participant and may be beneficial in developing training programs designed to reduce an athlete's risk of injury. Keywords: ACL; muscle force; musculoskeletal modeling; risk factor; knee injuryDespite the large quantity of research available on non-contact anterior cruciate ligament (ACL) injuries, the contributing factors and their relative contribution to the injury is still under debate. 1 This is in part due to the difficulty of measuring ACL strain in vivo 2 and inability to relate the ACL strain to the possible contributing factors. Unless the relationships between body kinematics, muscle forces and ACL strain is understood, the mechanism of ACL injury will remain unclear. Understanding the mechanics behind these injuries is crucial for injury prevention. Injuries may be prevented if screening and training programs are created for athletes who display at-risk mechanics. [3][4][5] Sagittal plane factors have been identified as important contributors to ACL injury mechanisms. [6][7][8] In addition to these extrinsic biomechanical factors, ACL strain is also dependent on a number of intrinsic anatomic factors such as tibial slope, 9,10 femoral notch width, 11 and ACL size. 12 Although these factors are known correlates with ACL strain, the relative contribution of extrinsic biomechanical and intrinsic anatomical factors is unknown.Pioneering efforts have been made to understand the relationship between knee kinematics, kinetics and ACL strain by surgically placing strain gauges on ligaments in live participants. 13 However, for ethical reasons, such approaches have not been extended to activities that are dynamic in nature. Numerical modelling approaches have been used to address this gap [14][15][16] ; however, model validation is complicated by the lack...

Prophylactic knee brace could reduce the strain in the anterior cruciate ligament of high-risk subjects during drop-landing through altered muscle firing pattern associated with brace wear. This could help reduce the anterior cruciate ligament injury risk.

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Estimation of muscle forces through musculoskeletal simulation is important in understanding human movement and injury. Unmatched filter frequencies used to low-pass filter marker and force platform data can create artifacts during inverse dynamics analysis, but their effects on muscle force calculations are unknown. The objective of this study was to determine the effects of filter cutoff frequency on simulation parameters and magnitudes of lower-extremity muscle and resultant joint contact forces during a high-impact maneuver. Eight participants performed a single-leg jump landing. Kinematics was captured with a 3D motion capture system, and ground reaction forces were recorded with a force platform. The marker and force platform data were filtered using 2 matched filter frequencies (10-10 Hz and 15-15 Hz) and 2 unmatched filter frequencies (10-50 Hz and 15-50 Hz). Musculoskeletal simulations using computed muscle control were performed in OpenSim. The results revealed significantly higher peak quadriceps (13%), hamstrings (48%), and gastrocnemius forces (69%) in the unmatched (10-50 Hz and 15-50 Hz) conditions than in the matched (10-10 Hz and 15-15 Hz) conditions (P < .05). Resultant joint contact forces and reserve (nonphysiologic) moments were similarly larger in the unmatched filter categories (P < .05). This study demonstrated that artifacts created from filtering with unmatched filter cutoffs result in altered muscle forces and dynamics that are not physiologic.

Unloader knee braces are prescribed for patients with unicompartmental osteoarthritis of the knee. These braces aim to reduce pain in patients by applying a coronal moment to the knee to unload the symptomatic knee compartment. However, existing unloading mechanisms use straps that go directly behind the knee joint, to apply the needed moment. This can impinge on the popliteal artery and peroneal nerves thereby causing discomfort to the patient. Hence, these braces cannot be worn for prolonged periods of time. This research focused on developing a new knee brace to improve comfort while unloading the osteoarthritic knee. A new knee brace was developed that uses a four-point bending approach to unload the knee. In this brace, unloading can be adjusted, and the unloading mechanism is away from the joint. The new brace was tested on a cadaver specimen to quantify its capability to unload the knee compartment. The brace was also worn by a patient with osteoarthritis who subjectively compared it to his existing unloader brace. During cadaver testing, the new brace design could reduce the force exerted on the medial condyle by 25%. Radiographic images of the patient's knee confirmed that the brace unloaded the medial condyle successfully. The patient reported that the new brace reduced pain, was significantly comfortable to wear and could be used for a longer duration in comparison to his existing brace.

Two unloader knee braces effectively reduced strain in the medial meniscus with an intact anterior cruciate ligament during dynamic activities. Neither brace made a significant reduction in strain for anterior cruciate ligament-deficient knees. Clinical relevance Unloader knee braces could be used to reduce the medial meniscus strain following meniscal surgery and during rehabilitation in patients with an isolated medial meniscus injury. However, these braces cannot be recommended for this purpose in patients with an anterior cruciate ligament deficiency.

Letter to the Editor: Effect of Sagittal Plane Mechanics on ACL Strain During Jump Landing"]Dear Editor:We thank the authors of the letter 1 for their comments regarding our paper "Effect of Sagittal Plane Mechanics on ACL Strain during Jump Landing." 2 Breaking apart the fundamental influencers of ACL strain and injury is difficult, in part due to the multitude of variables involved. Research studying these interconnected variables increases the experimental complexity required for biofidelic experimental setups. Our paper investigated many interconnected variables by applying full-time profiles of measured sagittal plane kinematics and kinetics, which we believe is an antireductionist approach. 3 The authors of the letter have brought up several issues regarding our current publication. We believe that many of these issues are in large, due to misunderstandings of how our experimental setup works. Hopefully, a more indepth explanation will relieve these issues.In Response to Concern 1 First, the current paper does not suggest that the methodology employed is the in-sim approach coined in Quatman et al., but suggests that the combined in vivo/computational/in vitro approach is in the spirit of the antireductionism principle Quatman et al. outlines. 4 To this effect, we have termed our approach as a "novel approach" in the paper.The authors of the letter suggest, that because our combined approach did not use the computational (OpenSim) modeling to provide detail about the ACL strain, our in vitro simulations lack the ability to relate back to the in vivo application. We respectfully disagree and believe that our use of computational modeling improves our ability to relate the in vitro simulations with in vivo applications. Our computational models were used to generate biomechanically plausible sagittal kinematic and kinetic time series constraints to apply to the in vitro models. We believe this approach is a better approximation to realistic loading compared to existing in vitro approaches in which neither the muscle forces nor the knee kinematics are matched to in vivo conditions. The closer that in vitro modeling matches in vivo scenarios, the more reliable its application is. Therefore, we have taken great care to perform all aspects of the in vivo/ computational/in vitro approach to the standards and guidelines outset by the creators of such technologies. This allows us to more comfortably make connections between in vivo and in vitro conditions. In Response to Concern 2 A common misconception exists that our Dynamic Knee Simulator does not apply ground reaction forces (GRF) during the in vitro simulations. This is not true. GRF forces are naturally developed through the applications of the time series kinematics and muscle forces of the said movement.Researchers using in vitro simulation techniques have several different options available to replicate biofidelic dynamic conditions. Typically, this includes a combination of kinematics, muscles forces, and ground reaction forces. A frequently employed comb...