Letter to the Editor: Effect of Sagittal Plane Mechanics on ACL Strain during Jump Landing

Abstract

We read with great interest the article entitled, “Effect of Sagittal Plane Mechanics on ACL Strain During Jump Landing” by Bakker et al.¹ We congratulate the authors for their complex study design that utilized the in sim approach² that is designed to cross-correlate the complex results of in vivo, in vitro, and in silico research approaches. The published study adds information in regards to potential models of the mechanisms of ACL injury to the body of literature.

Dear Editor of The Journal of Orthopaedic Research (JOR)

The in sim technique is designed to integrate a multifaceted methodology to overcome research limitations and utilize an anti-reductionistic approach.² This brings up a few concerns regarding the aforementioned manuscript:

1. The methodology utilized is not an actual in sim approach where there are cross-correlations between the various methodologies. Rather, the approach taken in this manuscript was unidirectional; in vivo kinetics and kinematics developed simulations within OpenSim that then calculated knee moments that were applied to cadaveric specimens. As a result, the computational modeling with OpenSim did not provide detail about ACL strain. Consequently, the results from the in vitro simulation lack the ability to relate back to in vivo application and derivation.

2. The authors referenced their previous study for the methods detailed in this manuscript that demonstrated utilization of in vivo kinetics and kinematics to the knee joint in a single cadaveric specimen. However, the method referred to lacks a simulated ground reaction impulse being applied to the leg. This ground reaction impulse is the factor that generates enough force to rupture the ACL upon ground impact and the musculature further activates in response to control and dampen this potentially injurious impulse. For the authors to suggest that precision muscle actuators can reproduce the impulse force that leads to ACL rupture, as demonstrated in the presented methodology, fails to maintain physiologic relevance as muscle activations are intended to constrain the knee, not disrupt the joint. Consequently, the results reported on five cadaveric specimens that resulted in only one ACL rupture (of the smallest female), one tibial fracture, one tibial plateau failure, and two non-failures demonstrate a lack of clinical relevancy as most ACL ruptures are femoral-sided avulsions or mid-substance tears. Clearly, the simulated model does not accurately develop clinical ACL ruptures and is severely limited due to the lack of gravity-driven impulse force.

3. The authors alluded to their supposition that ACL strain is caused by a range of dynamic muscle forces and resultant knee kinematics and that cadaveric simulations need to replicate the full time history profiles of potential injury-causing activities such as jump landing (multiplanar). Although they adequately described the limitation of their study by confinement of knee joint motion to the sagittal plane, they effectively reduced the cadaveric simulations to solely uniplanar motion and contradicted the aim of their study to find correlations between maximum ACL strain and time to maximum strain with whole-body kinematics and kinetics during jump-landing (multiplanar). As a result, this joint motion restriction violates the proposed in sim approach of an anti-reductionist design and raises concern for reliability of the data as ACL injury is a multiplanar event. Cadaveric robotic simulations have utilized multiplanar motions for measurement of ACL strain and loads throughout dynamic physiologic tasks even though they similarly lack ground reaction impulse.^3–6 The study of a multiplanar mechanism in a uniplanar system will certainly induce error into the results which limits the ability to extract meaningful conclusions to the real-life scenario.

4. Although the in vivo motion capture utilized in this study for resultant data for cadaveric simulation was multiplanar in approach to generate kinematics and kinetics to feed into the computational and in vitro models, error induction was likely to occur as the OpenSim and in vitro models were not developed with respect to the limitations of the initial data acquisition. If in vitro and OpenSim modeling is only desired in the sagittal plane, then the in vivo subjects should have also been confined to the sagittal plane to reduce errors in simulation.

5. The authors mentioned that according to their knowledge, no other study has correlated the external kinetic and kinematic parameters to measured ACL strain. However, this has been performed previously and the results should be acknowledged in the literature.^7–9

In conclusion, we congratulate the authors for their work focused on the development of an in vitro modeling system that can be utilized to accurately simulate muscles as actuators in close approximation to in vivo scenarios. This methodology has applications in many aspects of biomechanics. However, as muscles do not apply the impulse force in athletic activity but rather respond to the impulse, this method is inappropriate to simulate the landing or twisting scenarios that athletes encounter. This modeling system approach lacks the ability to cross-correlate data among methodologies as proposed in the in sim approach due to its reduction approach; extrapolation of a uniplanar model to a multiplanar injury presents significant errors in derived results and conclusions.