Integrating Multibody Dynamics and Computational Modeling
PROBLEM STATEMENT AND SIGNIFICANCE
Over 100,000 reconstructions of the anterior cruciate ligament (ACL) occur in the U.S. each year
(Owings and Kozak 1998). Reconstructive knee surgery is often successful at restoring static joint
stability in the short term, but the elevated risk for developing osteoarthritis (OA) in the long term
remains. Abnormal kinematics following surgery may contribute to the onset of OA by altering
tibiofemoral cartilage loading patterns in a way that contributes to cartilage degeneration. Computational
models provide a mechanism by which to assess the influence of surgical factors, such as tunnel
placement, on cartilage contact during functional movement. In this study, we will first empirically
investigate the relevance of accounting for load-dependent variations in knee kinematics on muscle
moment arms. We will then investigate the use of cosimulation techniques to describe this inherent
coupling between functional movement and joint mechanics. Finally, we will use cosimulation models to
predict the influence of ACL tunnel placement on tibiofemoral mechanics.
Aim 1. Investigate the influence of functional joint loading on knee muscle moment arms.
Rationale: Musculoskeletal models often assume that joints are simply kinematic constraints. However in
reality, joint kinematics vary with loading (Westphal 2009), which can influence the line of action and
moment arm of a muscle about the knee. Hypothesis: The three-dimensional patellar tendon moment arm
will be significantly reduced with quadriceps loading in knee flexion, due to anterior translation of the
tibia. Methods: Dynamic MRI imaging techniques will be used to measure three-dimensional knee
kinematics under load. Eight subjects will perform cyclic knee flexion/extension tasks against inertial
and elastic loads, which will induce quadriceps activity with knee flexion and extension, respectively. For
both loading conditions, the instantaneous axis of rotation will be calculated as well as the patellar
tendon’s moment arm with respect to this axis (Sheehan 2007; Sheehan 2007). Anticipated Outcomes:
This study will demonstrate the importance of accounting for load-dependent variations in muscle
moment arms, which directly influence the magnitude of muscle and joint contact loads.
Aim 2. Investigate the use of cosimulation to describe soft tissue deformations during movement.
Rationale: Injury and surgery can alter soft tissue material properties and lines of action, which in turn
affects cartilage loading patterns. Current serial simulations do not capture this inherent coupling, which
may be relevant to consider in the context of joint degeneration. Hypothesis: A cosimulation method will
predict significantly different kinematics and cartilage contact patterns than an uncoupled approach.
Methods: We will implement a cosimulation approach to simultaneously solve a multibody
musculoskeletal dynamics model and a finite element model of the knee. The cosimulation approach will
be used to simulate the knee flexion-extension tasks from Aim 1, with model predictions of knee
kinematics compared to experimental measures. Anticipated Outcomes: This aim will establish a
realistic computational framework to predict knee cartilage loading during movement.
Aim 3. Use the model to investigate how femoral tunnel placement affects cartilage contact.
Rationale: The position of the ACL graft is the most critical parameter during surgery (Beynnon,
Johnson et al. 2005), with recent studies suggesting it may substantially influence tibiofemoral rotation
during movement. Hypothesis: A femoral tunnel placed midway between the anteromedial and
posterolateral bundle origins of the natural ACL will produce contact pressure points that agree most
closely with normal. Methods: The ACL attachment to the femur will be modified in the FE model to
attach at 10, 10:30, and 11 o’clock. The method developed in Aim 2 will then be used to predict contact
points on the tibiofemoral cartilage. Anticipated Outcomes: This aim will give better insight into the
sensitivity of cartilage loading patterns to femoral tunnel placement, which is important to consider when
evaluating potential for developing OA.
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